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Harvard CS50 (2023) – Full Computer Science University Course

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Core Theme: This content introduces CS50, Harvard University's foundational computer science course, emphasizing algorithmic thinking, problem-solving, and the principles of programming through a series of lectures. It aims to empower students to apply these concepts across various fields, not just computer science.
Key Points:
- CS50 teaches algorithmic thinking and efficient problem-solving, applicable beyond computer science.

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this is a Harvard University course

taught by Dr David Meen and we are proud

to bring it to the freeo camp YouTube

channel throughout a series of lectures

Dr men will teach you how to think

algorithmically and solve problems

efficiently and make sure to check the

description for a lot of extra resources

[Music] course

[Music]

all right this is cs50 Harvard

University's introduction to the

intellectual Enterprises of computer

science and the Arts of programming my

name is David man and I actually took

this course myself back in 1996 I was a

sophomore at the time I was actually

concentrating in government because a

year prior as a first year I'd come into

Harvard thinking that I liked high stre

and constitutional law and sort of

similar classes in high school and so

when I got here I rather gravitated

toward that which was familiar I figured

if I liked and if I were good at that

particular subject in high school then

that's presumably who I'm supposed to be

here but it wasn't until sophomore year

that I got up the nerve to step foot in

the cs50 classroom and even then it was

only out of curiosity like I had no

intention of studying computer science

of even taking cs50 when I got to campus

but people were talking about it and

there was a lot of like beware and it

was it perhaps for the initiated only

and I didn't really know ultimately what

computer science was but for me the sort

of light bulb went off and I found that

contrary to what I had seen in high

school where I saw friends of mine like

programming away in the computer lab

heads down sort of anti-socially just

doing whatever it was they were doing it

really wasn't that once I got to this

particular class in this particular

place it was much more about problem

solving more generally and just learning

how to express yourself in code in

different languages so that you can

actually solve problems of interest to

you even if you have no intention of

being a computer scientist or an

engineer but just want to be able to

solve problems analyze data do

interesting things in the Arts Humanity

social sciences physical sciences or

really any other field and indeed this

particular path led me to computer

science but the hope for cs50 more

generally is that indeed you just find

your way to applying principles that

you'll learn over the coming months to

whatever field is of interest to you

with that said it was definitely a lot

of work and not without its frustrations

for me but there was no better feel

then like banging your head proverbial

against the wall for some number of

hours even days trying to fix a bug a

mistake in your code and then oh my God

the rush of emotion of accomplishment of

pride of exhaustion when you finally

solve some problem that's really been

weighing on you it's just so incredibly

gratifying but also empowering because

unlike a lot of fields the computer

science was built by humans themselves

and so if a human built this surely you

another human can understand it as well

and so even though there's going to be

some distractions along the way you're

going going to see what looks incredibly

cryptic if you've never programmed

before over time and with practice

everything just starts to make more

sense and with time and with practice

you just get better at this particular

field and indeed really the key to

success in programming in general is

just to allow yourself enough time and

so at least thankfully I quickly got

into the habit of starting early in the

week for instance when writing actual

code why because you're going to like

run up against a wall you're not going

to see some bug something's not going to

jump out at you and that's fine that's

when you sort of call it a day take a

break move move on to something else and

then just come back to it and that's

what keeps programming fund for me even

all of these years later whether it's

teaching or actually applying it but

there's uh down the road a history of uh

an MIT hack and it looked a little

something like this in yester year and

there was a little sign the MIT students

when they made this hack uh on the wall

that says getting an education from MIT

is like drinking from a fire hose which

indeed they have connected to what

should have been otherwise just a water

fountain and that's going to be what it

feels like sometimes not just in

computer science per se but just in UNAM

familiar field if you're not from stem

if you're not from CS that's fine but so

much of it ultimately is going to be

absorbed by you and going to be uh

within your grasp by term Zen so just

keep in mind that's that's very much the

intent but you'll be amazed what you're

able to create to accomplish uh just

three or so months hence indeed 2third

of you contrary to what you might think

or assume have never taken a CS class

before so it's absolutely not the case

that the person to the left and to the

right surely must know more than you

indeed it's quite the opposite and as

you'll see in the coming weeks as you

write your own code and solve your own

problems what ultimately matters in this

course is not so much where you end up

relative to your classmates but where

you end up relative to yourself when you

began and it really is all about that

Delta whether you've programmed or not

just getting something out of a class

like this and and if it does take time

and if you do feel those frustrations

but you simultaneously eventually feel

that sense of accomplishment that just

means it's all working and indeed

hopefully all the more worthwhile and

gratifying ultimately as a result so

what are we going to do in the coming

week so here we are in week zero we'll

soon see why computers and computer

scientists start counting if you will

from zero but week zero is which is one

in which we explore computational

thinking sort of thinking like a

computer and starting to clean up your

thought processes getting you to think

to solve problems more methodically and

then ultimately Translating that into

code and some of you might recognize

this environment here AKA scratch

coincidentally also from MIT you might

have used it in in grade school we'll

use it today and a little bit this

weekend in the course's first homework

assignment or problem set but not so

much to kind of play around in a way

that you might have if you did use it in

yester year but to explore ideas of

computer science and programming that

we're going to use and reuse every week

Hereafter as well uh thereafter we're

going to transition just next week to

week one so to speak whereby we'll

introduce you to a more traditional

language a lower level language an older

language called C and in C you're going

to use your keyboard not so much your

mouse and pointing and clicking but

you're going to write code that soon is

going to look a little something like

this and if you programmed before can

probably glean what this is going to do

if you've never programmed before which

is the case for most of you this too

will soon make sense but this is the

most canonical program that most any

programmer ever writes called hello

world and indeed that and all of the

surrounding syntax above and below just

that sentence hello world will soon make

all the more sense you'll learn how to

use industry standard tools so to speak

pictured here something called Visual

Studio code or vs code you'll use a

cloud-based version of it initially so

you don't have to suffer with any

technical difficulties or headaches like

that it'll just work right off the bat

but we'll use that tool and others

ultimately to then explore ideas in

computer science principles that you can

apply and we'll take a look underneath

the hood so to speak of your computer at

your memory or Ram Random Access Memory

where all of the data is ultimately

going to be stored we'll also take a

look thereafter at bugs a bug is a

mistake in a program here is an actual

bug in an actual computer in yester year

but we'll teach you how to debug

programs find your own mistakes find

others mistakes and improve that code as

well we'll transition then to algorithm

step-by-step instructions for solving

some problems which we'll touch on today

too and if you picture here this is

actually a pretty representative

problems odds are you haven't had to

deal with something like this but it's

representative sorting for instance if

you think of each of these small bars as

being a small number each of the bigger

bars as being a bigger number you might

wonder well how is you could do as a

human sort all of these bars like get

all the short bars over here all the big

bars over there well odds are if you're

like me you would probably kind of

eyeball it and if you could physically

interact you might just start grabbing

the smallest elements first put them

over on the left maybe grab the biggest

elements put them over on the right but

what's your algorithm there like how

would you teach someone younger than you

who's never done that before how to do

it how would you compel your Mac or PC

or phone to do something like that you

can't just kind of wave your hand and

say oh you know figure it out move

things around you have to express

yourself more methodically so we'll

translate even ideas like this into code

to and that's what the Googles and

others of the world are doing constantly

as they sort and organize the world's

information we'll use metaphors along

the way if it helps we'll talk about

your computer's memory is kind of being

like a postal address like every mailbox

in the world has some form of postal

address Street city state country and

the like and it turns out that's how

your Mac your PC and your phone also

work you've got a whole bunch of memory

like the picture before but you can

think of it really as individual

mailboxes and you can put anything you

want in those mailboxes and you can go

to a mailbox to grab information that

from it so at the end of the day that's

really all your computer's doing with

information it's just kind of organizing

it not into mailboxes per se but a term

you probably know called bites for

instance instead we'll talk about

problems that arise even nowadays in

fact most of you are familiar with your

Mac PC even phone like spontaneously

rebooting sometimes crashing the little

Annoying spinning beach ball or

Hourglass Icon that Happ like what is

with that well those are just bugs in

programs that humans at Apple and Google

and Microsoft and others they screwed up

and they wrote buggy code and your

computer when it encounters those

mistakes doesn't know what to do and so

nine times out of 10 so to speak it just

crashes or freezes or the like but that

kind of stuff will make more sense so

even the real world will make sense and

pictured here or some lower level terms

will eventually get to Mid semester but

generally speaking when something is

going this way as per this arrow and

something is going this way as per this

Arrow like that does not end well and

that often is what happens when your

computer crashes someone's using memory

up here but someone else is using memory

down here here and then they're not

really talking left hand and right hand

so that is just a high level overview of

some of the problems we'll encounter but

we'll focus to on data ultimately so

picture here is something fairly

technical called a hash table it's an

amalgam of something we're going to soon

call an array and also something we call

a linked list and these are just fancy

terms for describing how you can

organize information even more flexibly

than just putting individual ma values

in mailboxes like how could you build

structures like actual data structures

so to speak speak two-dimensional

structures at that and so what you're

seeing here is a glimpse uh as some of

you might have recognized of some Harry

Potter Universe names but they're

organized somewhat alphabetically and

notice that anytime there's multiple

people with a name that starts with H

like hermion Harry and Hagrid well they

can't all fit in that mailbox if each of

these squares along the left is that

same mailbox so you have to kind of

chain them together well you'll learn

how to do that in code so that even if

you get more data than you expect if

your business is booming and you're some

web-based business how do you keep

adding and adding information to your

software to actually keep up with it but

this again is what codee's going to soon

look like as soon as next week in week

one this here being C but we'll

transition in a few weeks to a more uh

modern higher level language so to speak

called python indeed the course Very

deliberately back in my day and now this

introduces you first to C which funny

enough most many people don't tend to

program and certainly every day I you

see generally September October November

December when teaching C is 50 itself

but it's everywhere nonetheless in fact

even today's other languages with which

you might be familiar like Python and

Java and uh yet others still you see

this same primitive language underneath

the hood because it's so darn fast and

as you'll learn over the coming weeks it

really gives you access to and an

understanding of what's going on

conceptually down here so that

thereafter after cs50 when you're

writing code you can sort of think at a

very high level what's actually going on

so in fact in just a few weeks what

looks like this C is going to look

instead like this in Python and you'll

better understand what's going on

underneath the hood and odds are after

this class you'll reach for a language

like python more frequently than C but

you're going to benefit from that bottom

up understanding thereof thereafter and

toward terms end we'll introduce you to

a few other ideas like where do you put

large amounts of data in things called

databases not things like spreadsheets

like here but actual databases where

using those same kinds of data

structures you lay things out in an

interesting way in memory thereafter

will transition to a very familiar

environment that you and I use every day

the web like the web has become rather

the user interface or UI that we use

everywhere on the uh laptops desktops

and even mobile devices nowadays well

pictured here is a language called HTML

it's not a programming language it's a

markup language and some of you might

have made homepages or portfolios in the

past but you'll understand what's going

on here but more powerfully you'll

understand how the computer sees that

same kind of code builds up a

hierarchical family tree type structure

in memory and then you can manipulate

that tree with code to actually add more

and more information chat messages

anything on the screen that you like and

finally we'll tie all of this together

by introducing what are called

Frameworks and libraries third-party

code that makes it a lot easier to solve

problems of interest to you and so in

particular here this is the very first

web app that I myself made back in like

1997 I was part of the first year

intramural sports program not as an

athlete but as the programmer and I was

teaching myself how to build web

applications I only knew C and maybe a

little bit of something else at the time

but this became for Harvard at least the

very first website for the first year uh

intramural sports program and it wasn't

just a static website with links and

images and the like it was interactive

you could register for sports we could

input exactly who is in a tournament

bracket or the leg and it could actually

automatically keep track of this data so

there two after just three months of a

class like this you'll go from writing

quite simply this week in next hello

world to building things like this for

whether it's web mobile uh or other

platforms as well if you so choose but

we'll get you off of the courses

infrastructure by the end of the term

you won't be using any toy environments

along the way will Empower you

ultimately to write code after cs50

especially if this is the only CS class

you ever take on your own Mac or PC

using the same software but not the

cloud-based version thereof but all of

this software is itself free and can be

used by you uh powerfully after the

course's own end but along the way as

you may know uh is there this tradition

within the class particularly in healthy

times of a number of events that really

brings people get together not just

collaboratively academically but to just

solve problems and generally uh engage

with each other as well coming up first

cs50 puzzle day which is meant to be not

jigsaw puzzles but logic puzzles that

require no prior experience with

computer science or programming but it's

just an opportunity to sort of quietly

work on a packet of puzzles with some

number of friends for for prizes and

more later the semester once you tackle

your final projects the Capstone of the

course where we don't give you a

homework to write you yourself come up

with something to build we'll get

together generally around 7 a uh 700

p.m. in the evening uh wrap up around 7

a.m. if you so choose and it's an

evening a 12-hour opportunity to

collaborate with classmates on your very

own final project in a large space on

campus uh that ends if you're awake with

us at 5:00 a.m. uh we can hop on some

cs50 shuttles and go down the road for

some pancakes at IHOP around 6 uh of

course uh of course this is 6 7 a.m. at

that point but it's an opportunity

finally to lead into What's called the

cs50 fair which is an end of semester

celebration an exhibition of everything

that you'll accomplish over the coming

months and in fact pictured here are

some of your predecessors and healthy

times um the cs50 fair allows you to

come with your laptop or phone and

exhibit a students faculty and staff

across campus put together something in

person and on video that people can

Delight in seeing as you exhibit what it

is you create in what you learned over

the course of the several weeks and

ultimately a chance to just share and

Inspire others as well um and you'll all

walk home ultimately with your own I to

cs50 t-shirt saying as much as well so

with that highlevel overview of the

course I propose that we begin to take a

look at what computer science itself is

and what it is we're going to be doing

over the next several weeks at this

lower level than two so what is computer

science right if you're maybe like me or

new people like my friends and High

School you probably assume that it means

programming and that's absolutely a big

part of it for a lot of people because

with code you can write you can express

ideas and solve actual problems

especially involving data but computer

science itself is really the study of

information if you will how do you

represent it and how do you actually

process it and in that sense

computational thinking is just the

application of ideas from computer

science a course like this to problems

of interest to you again in the Arts

Humanities Sciences social sciences

whatever the domain interest is to you

so with that if computer science is all

about information and with it the

solving of problems well what does it

actually mean to solve a problem let's

see if we can't propose a model into

which all of the Lessons Learned will

ultimately follow and I'd propose that

this is problem solving you've got some

input which is like the problem you want

to solve the goal is to solve it so

that's the so-called output and then

somewhere in here the proverbial black

box is some kind of Secret Sauce that

gets the work done and in the coming

months have to decide well how are we

going to represent these inputs and

outputs and really how do we code up how

do we write solutions for what it is

that's solving the problem of interest

to us so when it comes to representation

of information like there's a lot of

ways we can do this and for instance if

the problem at hand quite simply is to

like take attendance at the beginning of

class on the first day of school well

how could we go about doing this well we

could actually use a system called unary

well what is that well that's a fancy

way of saying 1 2 3 4 5 maybe 6 7 8 9 10

and I can use my digits pun intended on

my fingers to actually count everyone up

and eventually you need toes and whatnot

if you have to count so high but unary

is a very simple system of using a

single symbol a human finger in this

case to just solve some problem like

counting the number of people in the

room let's make this slightly more

technical for a moment a little more

mathy that's just called base one where

the base under which you're operating

has one digit in it like literally a

human finger and maybe multiple such

fingers if you need to count higher but

of course of course all most of you if

not all of you generally vaguely know

that computers use something other than

unary and even you and I probably don't

use this that often they use what

language or alphabet

instead yeah so binary so binary is

indeed the system that computers somehow

use so in this case by implying two and

so computers have two digits it turns

out at their dispos one in fact if

you've ever heard the technical term bit

which is like a smaller version of a

bite more on that soon well a binary

digit is the origin of that term bit

because if you get rid of some of the

letters and you're left from binary

digit with just bit thus is a bit a bit

is just a zero and one it's two more

digits than you might have on your own

finger and of course it's fewer though

than you and I have you and I typically

use as humans the decimal system deck

meaning 10 because you and I generally

use zero through nine so on the one hand

another pun intended you've got unary

computers use binary we humans generally

think and talk in terms of De

but at the end of the day these are

fundamentally going to be the same thing

which is to say that it's all pretty

accessible to us even if you're not a

computer person I dare say you're about

to be so what is a bit well a bit then

is a zero or a one that is a so-called

binary digit but how do computers only

speak in binary how do they solve

problems represent information using

only binary well at the end of the day

if they want to represent zero and one

we need to do so physically somehow and

I dare say that maybe the simplest way

to think about a bit a zero or one is

like a light bulb and so by human

convention let's just assume that if you

are a computer be it a laptop desktop

phone or the like and you want to

represent the number zero you know what

you just keep a light switch off you

keep a light bulb off if by contrast

you're that same computer and you want

to represent the number one you take

that same switch that same light bulb

and just turn it on so a light bulb

that's on represents a one and a light

bulb that's off represents a zero so why

is this relevant into computers well at

the end of the day you and I are

charging our laptops or phones at night

so there's some like physical resource

being replenished there whether you're

on battery or some power cord and so a

inside of a computer are just thousands

millions of tiny little switches

nowadays you can think of them

metaphorically as light bulbs but they

don't yet it's actually shine light but

there are tiny tiny little switches and

those switches if you've ever heard the

term are just called transistors so like

computers have millions of transistors

that can either be flipped on to

represent ones or flipped off to

represent zeros and from that very

simple mechanism electricity is there or

it's not a one or a zero computers can

actually count obviously from zero to

one but it turns out even higher if they

use a little more electricity as well so

how might I do this well let me go ahead

and propose that I just grab one of our

own light bulbs here on stage this one's

off so for instance If This Were

miniaturized inside of your Mac PC or

phone this would be a transistor and

indeed here's the little switch on the

bottom and if your computer wants to

represent a zero it just leaves the

switch off and the light is not shining

if you want to represent a one well now

I've counted as high as one because the

switch is now on I've grabbed a little

electricity I'm holding on to it inside

of the computer and so now I see that

this is a one all right but

unfortunately with just one switch one

light bulb I can only count from zero to

one how do I count out higher might you think

think

intuitively say again yeah so more light

bulbs so let me do this let me just put

grab something to put these on so I can

use a few of them at a time and let me

propose that here instead of having just

one light bulb let me give myself maybe

three in total so all of them are

initially off and if you think of this

in miniature form in your mind's eye

this is like a computer with three

transistors three switches representing

now the number you and I know as zero

why they're just all off so how does a

computer go about representing the

number one well it turns on one of these

light bulbs and how does the computer

represent the number two well you might

think if I by may you just turn on a

second light bul and if you might think

how does a computer represent three you

just turn on the third light bulb and so

as such with three bits a computer would

seem to be able to count from zero on up

to 1 two three but it turns out if I'm a

little smarter here I can actually count

higher than that why well I'm just

considering like the combination of

bulbs being on here what if I do

something like this this is still zero I

will claim but what if I propose now

that this will be how a computer

represents one on on off off this though

will be how the computer represents two

notice I didn't turn on the same two I'm

just turning on the one in the middle

this I now claim will be how a computer

represents three this is going to be in

just a second how a computer represents

the number we know as four and yet I'm

still only using three bulbs this is

going to be the number the computer

represents as five this is going to be

how the computer represents the number

six and then lastly it turns out with

three light bulbs if you're smart about

it you can count it seems as high as

seven now even if you kind of lost track

of like what I was turning on and Y I

claimed there were eight different

patterns from all of them off to all of

them on but notice that I started to

permute them I took into account which

ones were on and which ones were off why

though do these represent the numbers we

know as 0 through 7even well let me go

ahead and maybe let's let's do this

instead of just considering there to be

light bulbs let's assign some special

significance to each of them based on

where it is and maybe for this could we

get uh maybe three volunteers three

volunteer okay yeah who's you're being

volunteered okay come on up if you want

to go over to the stage there yeah you

want to come on up as well and over here

as well so there's some stairs on either

end maybe a round of applause for our

term all right so you want to be you are

number one and if you want want to go

ahead and stand roughly right here how

about you want to be number two okay

come on over and right uh to the right

of here and you'll be number four it

turns out and if you want to come over

here on this end let's give you all a

moment to uh introduce yourselves uh

briefly to your classmates if you'd like

hi I'm Ellie I'm a senior nice to meet

I'm rahana and I'm a first year welcome

hi I'm Joseph and I am a first year

welcome all right so so glad to have all

you let me propose now that we'd like

you you three to represent how about the

number zero and I claim now that if each

of you now represents a switch you have

sort of fancier light bulbs now one is a

one one is a two one is a four but each

of you still just has a switch on the

bottom in fact of your plastic devices I

claim these three volunteers are

representing the number zero let me ask

you all now how might you represent the

number one how should you cooperate here

okay so so we would have on off off

which I think is matches what I did with

my three light bulbs as well if you want

to go ahead and turn yours off how might

you three represent the number

two okay so off on off now from right to

left how would you3 represent the number

three ah so that's why my two light

bulbs went on at the end how would you

four perfect number

five number six

and number

seven all right and give us one more how

would you represent

eight okay you can't uh how about then

one more volunteer one more volunteer

up all right what's your name my name is

Moen if you want to say it there my name

is Moen all right Moen you're GNA be

number eight and if now you all actually

let's make this uh how would you

represent number eight all collectively

as four bits or four

switches okay eight and now lastly give me

15 everyone's awkwardly doing arithmetic

unar hey yeah is

everyone yes okay Round of Applause okay

thank you

all if you want to leave your numbers

over here we have a 650 stress ball for

you but thank you for volunteer you can

turn those numbers off and leave them

over here so thank

you so how do we go about how do we go

from there to uh creating these patterns

well even though we still had three bits

initially and three switches later four

bits and four switches ultimately we

still used it used the same approach

fundamentally to actually representing

information and now why were they those

patterns and why did I very deliberately

have our volum volunteers line up in

that way well I wanted them using base 2

AKA binary but with binary there come

certain rules and even if you're not

familiar with binary beyond that it

exists and relates somehow to computers

it's actually pretty much identical to

the system you and I use every day known

as base 10 AKA decimal so let's consider

if you will by rewinding to primary

school for just a moment like how

decimal works and you'll see that even

if you're not a computer person you

actually are you just have to tweak your

mental model ever so slightly so here is

the number that you're probably viewing

as 123 but why is that well it's not

really 123 these are just uh this is

just a pattern of three symbols on the

screen 1 2 three and your mind is sort

of rapidly assigning mathematical

meaning to them 123 but why is that well

if you're like me you probably learned

back in the day when you have a

three-digit number like this the

rightmost number is in the ones place

the middle digit is the 10 place the

leftmost digit is in the hundreds place

and why is that relevant well if you

then quickly do some mental math as you

and I just do uh instantly nowadays that

just means 100 * 1 plus 10 * 2 plus 1 *

3 of course 100 + 20 + 3 gives us the

number you and I know as

123 but beyond that how do we get to

just two digits instead of as many as

nine in the decimal system well let's

generalize this in the decimal system

you and I know if we've got three digits

represented by these hashes here yes

it's the ones place T Place hundreds

place and if we keep going thousands 10

thousands and so forth but why is that

well base terminology is now a little

more gerine that's technically the 10 to

the zero column the 10 to the 1 10 to

the two so these are powers of 10 where

10 is your base computers just simplify

things a little bit because computers at

the end of the day only have access to

electricity on or off they don't have

access to like 10 different types of

electricity just two on or off if you

will well they just use a different base

and the rightmost digit would be in the

so-called 2 to the zero then the middle

digit is 2 to the one the leftmost is 2

to the 2 AKA one's Place two's Place

Four's place and as we kept going 8 and

if we keep going 16 32 64 128 and so

forth but the idea is fundamentally the

same so why is this how the computer

represents the number you and I know is

zero well off off off from right to left

or in this case left to right is just

zero why CU that's 4 * 0 + 2 * 0 + 1

time 0 is of course 0o this is why 0 01

represents one this is why 0 1 0

represents two and three and four and

five and six and seven on up and why did

we need a fourth bit to represent eight

well we kind of needed to carry the one

so to speak using our human familiar uh

familiar human terminology but for that

we need a fourth bit another transistor

and this now represents the number eight

and that's why we ended with on from

left to right off off off so I keep

saying on and off or the light bulb is

on or off but really I just mean one or

zero and so computers and we humans

think of things digitally as just being

zeros and ones but mechanically you can

think of it indeed as these light bulbs

now a bit not very useful even three

bits four bits not that useful you can

count to seven or 15 generally speaking

bites are a more useful unit of measure

and anyone familiar how many bits is in

a bite yeah so eight bits are in a bite

can think of it as an octet equivalently

uh in some context there are nuances

there but think of a bite as just being

eight bits and that's just a more useful

measure so what does this mean in real

terms well if you've ever downloaded

like a music file or a photograph or a a

video those are measured in bytes

probably not small numbers of bytes

probably kilobytes for thousands of

bytes megabytes for millions of bytes

gigabytes for billions of bytes

especially for video that just means you

have a lot of patterns of 8 Bits some

combination of zeros and ones on your

computer's hard drive here then with a

bite of bits eight bits is how a

computer would typically represent the

number zero and if that same computer

uses all eight of its bits it's full

bite to rep uh to change it to one

anyone who's quick with math or have

seen this before how high can a computer

count with eight bits or one yeah 255

why is that well we're not going to turn

this into a constant math exercise

indeed after today we're not really

going to think about or talk about bits

at this low level but this is the ones

place 2os 4S 8 16 32 64 128 and if I do

all of that math from left to right that

indeed gives me

255 it ignores how we might represent

negative numbers but perhaps more on

those some other day but computers of

course do so much more than numbers and

math and all this lowlevel stuff we send

text messages right documents emails and

the like so how might a computer

represent something like the letter A I

claim at the end of the day your Mac

your PC your phone just has lots of

transistors lots of switches that it can

use in units of eight in units of bytes

how though if it's already using those

patterns of zeros and ones apparently to

represent numbers from zero on up how do

you go about representing letters of the

alphabet might you think yeah okay so we

could assign a number to every letter

okay so let me just conjecture well

let's just call a0 for Simplicity B1 1

C2 and now let me play Devil's Advocate

okay how do I now represent zero or one

or two well we've maybe created a

problem for oursel if now we have to

steal some numbers to represent letters

we kind of have to pick a lane but

there's a solution to that too that

we'll see and it turns out the world is

not quite as simple as a being zero a

typically is represented by computers

everywhere phones everywhere with the

number 65 the decimal number 65 using 8

Bits if we turn some of the zeros to

ones let me just stipulate you can

represent the letter A Using eight bits

by turning certain ones on and certain

ones off but we will try not to focus on

the that binary level too much so if a

is 65 it turns out that b is going to be

66 and C is going to be 67 and so forth

and so where does that get us well it

turns out there's a whole system that

Maps uh numbers to letters and here uh

as I alluded to verbally a moment ago is

the pattern of zeros and ones via which

you'd represent 65 and just quick check

here we won't constantly do math uh

one's place that's easy 2os Four's 8 16

32 64's place so 64 + 1 gives us

65 so once I do that how do I get to all

of the others well it turns out a bunch

of Americans years ago came up with this

asky the American Standard code for

information interchange now what does

that mean well it's just an acronym

describing what really you proposed a

mapping between numbers and letters not

not quite as simple as 012 starts at 65

66 67 for capital letters but here are

most of the letters in use today at

least with this system so this is just a

big chart from online and you'll see it

in the middle of this chart here here's

my 65a here's my 66 b c and let's see 72

is H 73 is I and so forth so there's a

mapping at least for English between all

of these numbers and all of these

letters and if we focus here those are

the beginning of our up case alphabet so

suppose then that today tomorrow you

receive a text message from someone and

underneath the hood now that you're a

computer person you figure out a way to

see what pattern of zeros and ones was

sent in this case it's wireless as

opposed to wired but it's still some

pattern of zeros and ones and your phone

is turning some switches of its own on

and off to represent that message from a

friend suppose that the three patterns

you received were these three btes from

left to right spelling out a

three-letter word well if if we do out

the math one's Place two's place and so

forth I'll spoil it for you suppose that

you received a text message that doesn't

literally say 72 73 33 but you've

received a pattern of 8 plus 8 plus 8 24

bits that if you do out the math

represent the decimal number 72 73 33

anyone recall what message you might

have received from the green and white charts

charts

yeah hi but yes hi is the message but

7273 gives H and I what's 33 any any

guesses to

33 yeah over here yeah so it's an

exclamation point how would you know

that well you really do need some kind

of cheat sheet AKA aski in this case and

if we look elsewhere let me highlight

the left of the chart you can see that

next to 33 in decimal is indeed the

exclamation point so back in the day a

bunch of humans got in a room decided

that hey when we start building PCS and

later Macs and phones we all just have

to agree on this form of representation

of letters of the the English alphabet

in this case we just need to agree on

this mapping but somewhat curiously

notice this it turns out that once you

paint yourself into this corner and

start using 65 for a 66 for B well how

do you represent 65 the number and 66

the number if you want to do math or use

Excel or something like that does anyone

kind of see the solution perhaps how do

you represent the number one in asy yeah

in the

middle yeah so this is getting a little

maybe Inception or something but you

could represent numbers with other

numbers and so if you want to represent

the number you and I know as one like

when you type it on your keyboard turns

out the computer stores that as the

decimal number 49 if you hit two on your

keyboard the computer is not storing two

per se it's storing the decimal number

50 now thankfully the the uh the uh the

the Paradox kind of stops there we just

have a mapping now of numbers to to

numbers but really at the end of the day

and you're going to learn this when we

start writing code in that other

language C next week it's just context

dependent at the end of the day inside

of your Mac PC and phone there's just

all of these permutations of bits all of

these patterns of zeros and ones and

generally speaking when you open up a

text message that you've received from

someone it's zeros and ones but

obviously if it's a text message the

whole point of text messages is to send

text and so those patterns of zeros and

ones by default will typically be

interpreted as letters of the alphabet

so you won't see zeros and ones you

won't see decimal numbers you'll see the

English message that your friend

intended by contrast if you open up

something like Excel that same pattern

of zeros and ones might indeed work out

to be 6 uh 72 73 33 you might see cells

in your spreadsheet with literally those

three numbers why because spreadsheets

are all about numbers and number

crunching and math uh in many cases if

by contrast you open up Photoshop and

try to look at that same pattern of

zeros and ones it's not going to be 727

73 33 it's not going to be zeros and

ones it's not going to be high it's

going to be some color of the rainbow

you're going to use those patterns of

zeros and ones it turns out too to

represent colors and indeed so long as

you and I just agree as humans long have

what these patterns are going to be all

of our systems many of our systems

nowadays are indeed interoperable but

I'm being very biased here and indeed

the a in ASI is very American Centric

what do you not see in this chart if you

speak any other language in English odds

are you're not seeing characters you

know and love and need every day to type

or send messages well there's a huge

character set that's not supported here

whether it's accented characters and a

lot of Asian alphabets you have many

more symbols than can fit even on this

screen here and so humans kind of

painted themselves into a corner early

on or really Americans did but on a

typical keyboard us English keyboard

yeah you have A's and B's and C's

uppercase and lowercase but you also

have accented characters here and

nowadays not sure if this is maybe

necessary but nowadays you have other

characters on your keyboard like these

and these are kind of a playful

incarnation of what's actually a

technical solution to this problem if I

claim for the moment that ASI

historically used seven bits to

represent letters and let's just round

that up to a bite eight bits to

represent letters aski can represent as

many as 255 or really

256 total characters why 256 well if you

have them all zero that's zero and the

highest number I claimed a moment AG go

was 255 so that's 256 total

possibilities that's not many letters

it's fine for English but not a lot of

human languages so what might the

intuitive solution be if you want to

represent accented characters um Asian

characters Emoji even like these which

are just keys on a keyboard

nowadays what's the intuitive solution

if a bite's too few

yeah yeah so add another digit just like

we had a fourth volunteer come on up to

give us a fourth bid let's just throw

throw Hardware at the problem and use a

few more bits so maybe instead of one

bite let's use two or heck let's use

three or four bytes even though it's

getting a little expensive we're going

from 8 to 16 to 24 or 32 bits that's how

computers do things these days and

thankfully we have so much memory inside

of our computers and phones we can

certainly spare a few to represent these

things and the solution then to aski is

what we'll call Unicode so Unicode is

also just a mapping of letters to num of

numbers to letters but in many many

different languages and indeed the

Unicode Consortium is a bunch of people

from all different companies and and uh

and a lot of different uh companies and

countries and cultures whose mission as

an organization is to capture digitally

all forms of human language in this case

and to ensure that especially smaller

demographics of humans speaking lesser

known languages are nonetheless

represented and preserve digitally using

some mapping of these zeros and ones it

turns out though if you start using 32

bits as many as 32 bits to represent

characters on a keyboard that's 4

billion possible permutations of zeros

and ones that's way more than we need

for most human languages so there's a

little bit of room in there for some of

those more playful things like those

Emoji so for instance suppose you got a

text message with this pattern of zeros and

and

ones or if we do out the math suppose

you receive a text message that if you

do out the math in decimal is 4 B 36

m991 106 anyone know what emoji you're looking

looking

at this would be weird if you do

but what is this Well turns out that as

of this past year this is the most

popular Emoji to be sent uh by many

measures face with tears of joy so that

is the pattern that a bunch of humans in

the Unicode Consortium decided would

represent this but you'll notice some

many of you might have iPhones some of

you might have Android devices too and

sometimes these don't actually look

quite the same this happens to be the

current version of face with tears of

joy on iOS on Android it tends to look a

little something more like this and

here's kind of a curiosity even though

you and I look at these things and they

look like images they're not images

they're characters at least as we've

defined them now in Unicode and a IOS

and Android and Windows and Facebook and

other companies and uh apps nowadays

really just have different fonts if you

will so just like fonts with English and

other languages can give you different F

characters with seraps or not emoji are

themselves yes drawings that someone

made but they're really just a font and

so that same pattern of zeros and ones

might just render slightly differently

on someone's phone or another if you've

ever gotten like an icon on your phone

that's broken and you've been sent an

emoji but it's like a square or

something arbitrary and not sensical

sensible it might just mean that you

have not updated to the latest version

of iOS or Android which just updates the

font of supported Emoji because those

Folks at Unicode pretty much every year

nowadays are adding more and more Emoji

to that that particular character said

now I went down the rabbit hole figuring

out the other day just which are the

most popular Emoji these days on Twitter

specifically this past year the most

popular Emoji by contrast uh was loudly

crying face I don't know if that says

more about 20121 or about Twitter but

you'll see different Trends certainly in

how these are used but even humans

themselves didn't necessarily think two

steps ahead and now a lot of the Emoji

are the sort of default yellow color but

there's a lot of emoji that aren't sort

of these cartoon characters but they're

meant to represent humans in various

professions or gestures or the like and

nowadays too you've probably noticed on

your phone and Macs and PCs there are

different skin tones that you can assign

to certain emojis if it's supported by

the company and by Unicode you can

actually like touch and hold on a

certain emoji and then you can choose

the appropriate skin tone to represent

yourself or someone else and that then

modifies the display well let's just

think for a moment here how did Apple

and Google and Microsoft and others go

about implementing support for emoji

with different skin tones how could you

do this if you want to represent some

smiling Emoji but in five in this case

different skin tones you could come up

with what five different patterns that

are identical structurally except for

the skin tone used in places in that

image but that's a little inefficient

right to kind of just do copy paste

paste paste paste and like change the

color in Photoshop if you will that's

going to use more bits more information

than you might need to how else if you

now start to think a little bit more

like a computer scientist if at the end

of the day all you have are zeros and

ones how else could you implement skin

tones might you think

yeah okay so RGB and we'll come to that

in just a moment that stands for red

green blue that's one way in this case

though I'm seeking an alternative to

just using five different patterns of

zeros and ones to represent the same

Emoji but different skin tones so not

quite RGB

yeah okay so store one copy of the Emoji

and then store different variants of the

color that you want to assign to that

emoji yeah so this is actually an

example of do you want to

elaborate okay so you can use a loop to

actually output these things more on

that in a moment let me go down this

this road for just a moment this would

be in some sense sort of a a better

design if you will uh but why yeah

filter okay so filter if we think sort

of in the Instagram sense you can sort

of change the color of something and

that could be related here too could it

be oh interesting so maybe it could be

just a completely different font and you

have five different fonts that are

almost identical except for the various

interpretations of skin tone for those

same Emoji let me spoil I think if we go

down this one particular Road the way

the uh the Unicode folks decided to do

this some years ago were the first uh

bite or bites that you receive via text

or email just represent like the

structure of the Emoji the default

Yellow Version thereof but if it's

immediately followed by a certain

pattern of bits that these humans

standardize to represent each of these

different shades of skin tone then the

phone the Mac the PC will change that

default color yellow in most cases to

whatever the more apt human tone is so

you just use twice as many bits but you

don't use five times as many bits so

what do I mean you don't have uh five

completely distinct patterns per se uh

for each of these possible variants you

have a representation of just the Emoji

itself structurally and then uh reusable

pattern patterns for those five skin

tones un um unfortunately that wasn't

quite versatile enough for other

features that were in the pipeline and

nowadays too and there's a double

meaning now to representation emojis had

tended to focus on certain professions

and early on too were certain

professions associated with certain

genders and vice versa and you couldn't

necessarily be one gender or another in

a certain profession or another there

were these combinatorics that just

weren't possible but nowadays as you

might have seen you can have couples in

love for instance that actually look a a

little more like three emojis but just

kind of combined into one and indeed

this is just one key press on your phone

and you can combine different emoji on

the left and in the right with the Emoji

in the middle and so it turns out how

computers nowadays represent these

patterns are one set of bits for the

character on the left one set of bits

for a character on the right one set of

bits for whatever Emoji you want in the

middle and then you assemble more

complicated compositions of emoji by

just reusing those same patterns and

bits and bits Emoji doesn't the Unicode

folks don't have to come up with a whole

new representation for some very

specific Incarnation they can create one

for person for male for female for other

uh characters that you might want to

display and reuse those same patterns of

zeros and ones and so here you see sort

of the imperfection of or lack of

foresight of humans for building a

system early on that was entirely

American Centric no characters Emoji or

the like that's evolved too and so

that's an important detail in Computing

nowadays um it too is evolved in and the

languages you're about to learn in the

coming days those twoo are evolving as

well and new features are getting added

and even programming languages have

version numbers you might have a

different version of an app on your

phone programming languages too have

different versions as well questions

then thus far and how information is

represented using asy or

Unicode or anything in between yeah so

you can use a string of a good question

so to recap why can't you just um well

let me summarize that as why can't you

similarly use different patterns to

change the context of what these

patterns of bits represent whether it's

a number or a letter or a graphic in

actuality that's kind of what's

Happening underneath the hood it's not

standardized in quite the same way but

starting next week when we transition

from scratch to C you'll learn about

types data types where the onus

initially is going to be on you the

programmer to tell the program whether

or not this pattern of bit should be

interpreted as a number or as a letter

or as a color or something else nowadays

though in toward the end of the semester

you'll use languages like python where

the computer just figures it out for you

by context which makes it even easier

and faster to program as well other

questions on Unicode asky or the

like all right well how about just a few

other forms of information RGB was

called out earlier red green blue How do

images get represented in computers well

in fact it's typically an assembly of

some amount of red some amount of green

some amount of blue but there are other

representations if you're a graphic

designer you might know them but RGB is

still pretty common what does this mean

this means to represent every dot on

your phone or every dot on your TV or

your laptop or desktop there is a number

representing how much red that dot

should show a number representing how

much green and a number representing how

much blue it should show red green blue

respectively so for instance if a DOT on

your screen were using these three

numbers these three values or bytes 72

73 33 in a text message email that would

be interpreted as I claimed high but in

Photoshop or in some graphical program

that same pattern would be repres would

be interpreted as let's call it a medium

amount of red a medium amount of green

and a little bit of blue and why medium

and little turns out that each of these

are bites the smallest value you can

have in a bite we said is zero the

largest value you can have in a bite is

255 so I'm just kind of spitballing here

this is like medium medium and a low

amount of red green blue uh specifically

those three colors um like uh

wavelengths of light are combined in

such a way that you would have this dot

on the screen a sort of murky shade of

yellow or brown that is how a computer

would store precisely that color and in

fact we've seen this color when you type

in face with tears of joy generally on

your screen it looks like this typically

much smaller but let's zoom in or let's

zoom in a little more what do you

starting to see if you know the term so

pixels it's getting very pixelated a

pixel is just a DOT on the screen and if

you really zoom in on it you can

literally see all of the dots that

compose in emoji in this case on iOS in

the font that Apple's using to represent

this particular pattern of zeros and

ones so one of those yellow dots and

there's many of them all that kind of

blend together here each dot on the

screen I claim is three bytes how much

red green blue for this dot how much red

green blue for this dot how much red

green blue for this Dot and you'll

notice too that when it gets to be sort

of brownish here the dots really stand

out the three values the three bytes AKA

24 bits are just slightly different and

so underneath the hood this is why

images photographs that you take or GIFs

that you download get so darn big

potentially because you have a number

representing every dot on the screen

well if this I claim is indeed how

images are typically represented using

pattern of bits that are assigned to

some amount of red green or blue how do

you get video what is a video if at the

end of the day all we have are zeros and ones

ones

what's a video perhaps yeah oh let's go

here weighing back yeah pixel's really

changing values over time and do you

want to confirm or deny the hand that

went up here yeah or equivalently a

sequence of images that over time are

changing on the screen so both of those

are valid interpretations and you know

just for fun if you uh grew up with

these sort of picture books you might

remember a little something like this if

we could dim the [Music]

[Music] lights

lights [Music]

[Music]

so that's sort of the old school analog

way to implement a video in the sense

that um that artist wrote out like

hundreds of pieces of paper with almost

identical images but where the ink from

their pencil or pen was slightly moving

and if you digitize that such that each

of those Strokes are represented with

dots instead that's really what you're

seeing as a sequence of all these images

flying across the screen and if we dive

into the the real world if you've ever

watched a film a Hollywood movie is

typically 24 FPS frames per second that

really means you're seeing 24 images per

second or on TV or in soap operas it's

often 30 frames per second that makes

things look a little more smooth so it's

not actual motion picture if you will it

sequences of pictures and your brain and

mind are kind of interpolating that oh

this is smooth movement even though

we're just seeing a lot of pictures

really fast now that gets really big and

we'll talk later in the semester how you

can compress information so that you're

not using way more bits than you

actually need to and there's fancy

algorithms that folks have developed but

at the end of the day that's really all

a video might be is a sequence of images

conversely if you want to represent the

music that accompanies that or something

else if any of you play an instrument

and can read sheet music How Could You

digitize this like how could you

represent musical notes in a computer

you and I hear them when we play files

but what's really going on underneath the

the

hood any any musicians piano players

anyone yeah Val okay so Hertz value so

some frequency so sound is some

frequency and it's kind of hitting your

eardrum and that's what makes it sound

low or high or somewhere in between so

maybe we could assign just like there's

letters a through G here maybe we could

assign specific frequency values which

are just going to be numbers measured in

something called Hertz something per

second and maybe we could have a few

other numbers for each of these notes

not just the note or the frequency maybe

we could represent the loudness of it

like how hard or how softly a human

might equivalently press it maybe a

third number like duration like how long

is their finger on the keyboard so you

could imagine quantizing something like

music that in the real world is

perfectly continuous as something more

discret by representing each note over

time as just some sequence of values and

there's so many different ways to do

this midi if you've heard uh MP3s AAC I

mean almost all the file extensions you

see on your Mac or PC if you see them at

all ultimately just mean there's a

different form of representation for in

this case something like sound so let me

just stipulate there are these and many

more ways to represent inputs and

outputs and thankfully humans have

standardized a lot of this they don't

always dis agree and this is why we have

different file formats for Apple numbers

and Microsoft Excel and Google

spreadsheets and sort of stupid

incompatibilities like that but

generally speaking humans have

standardized how we represent the inputs

and outputs to and from problems but

let's now focus on this black box so to

speak in the middle this abstraction so

abstraction is technically a term that

you'll see all over the place in

computer science and really problem

solving that just refers to the

simplification of something so that you

don't focus on the lower level

implementation details you really just

focus on the high level goals or the

process itself uh Therefore your car if

youve uh if you have a license and have

driven or have been in a car a car so

far as you're concerned is probably an

abstraction most of us if you're like me

probably don't really know or care how

the engine works and all the parts that

are moving to you it's just a way of

getting from point A to point B it's an

abstraction but someone hopefully the

mechanic does know those lower level

implementation details if you had to

understand how a car works every time

you want to go to school or to the store

it's probably going to be a pretty slow

process you just want to think and

operate at this higher level of

abstraction and we're going to do this

all the time when writing code and

solving problems

so what then is in this black box this

abstraction at the moment well generally

it's what a computer scientist would

call an algorithm stepbystep

instructions for solving some problem

now let's consider the implementation

details that is to say how you might

solve certain problems and let's take a

a sort of old school example but in

modern form this icon if you have an

iPhone is of course for your contacts

application and if you've got a whole

bunch of family members or friends or

colleagues in your phone book you have

some kind of contacts pictured here here

and it's alphabetized typically by first

name and last name and odds are you and

I are in the habit if they're not

already a favorite of like clicking on

search and then using autocomplete and

what happens when you start typing

autocomplete well if you type in the

letter H you'll see only presumably

Hagrid Harry Hermione and so forth if

you type in ha that shows you only

Hagrid and Harry and it all happens

super fast so how is that happening well

typically you could just start at the

top and look to the bottom searching for

all the hes or all of the ha but for

larger data sets that's going to get

slow for the Googles of the world that's

going to get really slow and even on our

phones when you have hundreds thousands

of contacts eventually even that kind of

approach that algorithm step by step but

it might be slow so how might we go

about searching for someone in a phone

book like this uh like say uh John

Harvard well here's an old school

incarnation of this and uh odds are you

might not have had occasion to even

physically use this thing nowadays and

in fact this is a bit of a white lie cuz

this is the Yellow Pages which means

this is a book of companies not people

uh but for this is all you can find and

at that it's even hard to find this but

this is the same thing in analog form

physical form so if I wanted to search

for someone like John Harvard how could

I do that well I could start on page one

and I could start searching for page two

page three page four page five little

hard to do physically especially since

no one's used this phone book in a lot

of years but uh is this algorithm

correct Turning Page by Page

very inelegantly is this correct will I

find John Harvard if if he's in here all

right so yes I mean this is a little

stupidly tedious because if there's like

a thousand Pages he might be a few

hundred pages into this but it's correct

at some point I will find him and if

he's on the page I'll be able to call

why because presumably the names are

alphabetized in here and there's no like

cheat sheet on the edge so I have to

search for John Harvard from left to

right searching for H if it's

alphabetized by last name well what

would be marginally better well how

about two pages at a time it's hard to

do with a 20-year-old old phone book

where the pages are kind of uh grown

together but 2 4 6 8 10 12 this algorithm is this correct all right so

algorithm is this correct all right so no

no why yeah so I'm skipping every other

why yeah so I'm skipping every other page so if I don't consider that and I

page so if I don't consider that and I find myself in like the I section or the

find myself in like the I section or the J section well I might accidentally

J section well I might accidentally conclude nope I haven't found John

conclude nope I haven't found John Harvard yet just because I skipped them

Harvard yet just because I skipped them because it was sandwiched between two

because it was sandwiched between two pages now I can fix this I think if I do

pages now I can fix this I think if I do hit the I section well let me just

hit the I section well let me just double back one page just in case he was

double back one page just in case he was in that last page so it's recoverable

in that last page so it's recoverable but it's almost twice as fast minus that

but it's almost twice as fast minus that that hiccup there but what most of us

that hiccup there but what most of us would do and what your phones are doing

would do and what your phones are doing albeit digitally is they open up roughly

albeit digitally is they open up roughly to the middle of the phone book and they

to the middle of the phone book and they look down and they say oh I'm in roughly

look down and they say oh I'm in roughly the M section so I'm roughly halfway

the M section so I'm roughly halfway through this thousand page phone book

through this thousand page phone book but what do I now know about John

but what do I now know about John Harvard where is he to my left or to my

Harvard where is he to my left or to my right all right so alphabetically he's

right all right so alphabetically he's presumably to my left and so here I get

presumably to my left and so here I get can both uh met uh metaphorically and

can both uh met uh metaphorically and physically tear the problem in

physically tear the problem in half you don't need to be impressed it's

half you don't need to be impressed it's really easy down the the spine that way

really easy down the the spine that way but uh I know that John Harvard is to

but uh I know that John Harvard is to the left here but now I can throw

the left here but now I can throw unnecessarily dramatically half and page

unnecessarily dramatically half and page one out of the way and what do I now

one out of the way and what do I now know I've gone from a thousand pages to

know I've gone from a thousand pages to like 500 I can kind of repeat roughly

like 500 I can kind of repeat roughly the same algorithm go to the half of

the same algorithm go to the half of this and so this time I went back a

this and so this time I went back a little too far I'm in now the um e

little too far I'm in now the um e section so what do I know is John

section so what do I know is John Harvard to my left or to my

Harvard to my left or to my right to my right so I can again tear

right to my right so I can again tear the problem in half throw this half away

the problem in half throw this half away and now I'm really flying I'm toing it

and now I'm really flying I'm toing it verbally slowly but that went from a

verbally slowly but that went from a th000 pages to 500 to now 250 and now I

th000 pages to 500 to now 250 and now I can do it again 125 i' do it again

can do it again 125 i' do it again roughly like 67 and keep doing it again

roughly like 67 and keep doing it again and again and again until I get left

and again and again until I get left with hopefully just one single page or

with hopefully just one single page or in this case an ad for ironically a

in this case an ad for ironically a mechanic okay so

mechanic okay so what is the implication for our

what is the implication for our performance well let's just do this sort

performance well let's just do this sort of in the abstract if you will if that

of in the abstract if you will if that first algorithm were to be plotted just

first algorithm were to be plotted just quickly on a chart without even numbers

quickly on a chart without even numbers here's my x-axis size of problem on the

here's my x-axis size of problem on the x-axis so the bigger the problem the

x-axis so the bigger the problem the farther out that way time to solve the

farther out that way time to solve the problem the tire you go up on the y-

problem the tire you go up on the y- axis the uh more time you're taking to

axis the uh more time you're taking to solve it how would we draw the running

solve it how would we draw the running time The amount of time taken to run

time The amount of time taken to run that first algorithm well it's going to

that first algorithm well it's going to be a straight line why cuz if you add

be a straight line why cuz if you add one more page next year because more

one more page next year because more people move to Cambridge you're going to

people move to Cambridge you're going to add one more page turn potentially so

add one more page turn potentially so one more second one more unit of time so

one more second one more unit of time so it's a straight line and we'll abstract

it's a straight line and we'll abstract it away as n if there's n pages in the

it away as n if there's n pages in the phone book the slope of this line is

phone book the slope of this line is essentially n the second algorithm

essentially n the second algorithm wherein I was doing two pages at a time

wherein I was doing two pages at a time was twice as fast but it's still a

was twice as fast but it's still a straight line and in fact let me just

straight line and in fact let me just draw some dotted lines here if the phone

draw some dotted lines here if the phone book is this big with my first algorithm

book is this big with my first algorithm it might take this many step this many

it might take this many step this many units of time this many steps this many

units of time this many steps this many page turns but with that that second

page turns but with that that second algorithm notice that the intersection

algorithm notice that the intersection is with much lower on the yellow line

is with much lower on the yellow line than on the red so n/2 means there's

than on the red so n/2 means there's half as many pages here if n is the

half as many pages here if n is the number of pages so indeed that algorithm

number of pages so indeed that algorithm the second one is twice as fast minus

the second one is twice as fast minus the little hiccup that I have to double

the little hiccup that I have to double back one page but that's not a big deal

back one page but that's not a big deal if I'm still doing things twice as fast

if I'm still doing things twice as fast but the third algorithm looks

but the third algorithm looks fundamentally different it looks like

fundamentally different it looks like this logarithms if you recall from high

this logarithms if you recall from high school or prior if you don't that's fine

school or prior if you don't that's fine too it's just a fundamentally different

too it's just a fundamentally different function a different shape and notice

function a different shape and notice that the green line is going up and up

that the green line is going up and up and up but a much slower rate of

and up but a much slower rate of increase which means crazy things are

increase which means crazy things are possible if two towns in Massachusetts

possible if two towns in Massachusetts like Cambridge and Austin across the

like Cambridge and Austin across the river merge next year for instance in

river merge next year for instance in terms of their phone book their phone

terms of their phone book their phone book just got twice as big for the first

book just got twice as big for the first algorithm that's going to take me twice

algorithm that's going to take me twice as many steps to go through the second

as many steps to go through the second algorithm almost Twi it's going to take

algorithm almost Twi it's going to take me 50% more steps to go through two at a

me 50% more steps to go through two at a time but the third algorithm that I

time but the third algorithm that I ended with tearing things again and

ended with tearing things again and again divid and conquering if you will

again divid and conquering if you will in half and in half and in half how many

in half and in half and in half how many more steps will my third algorithm take

more steps will my third algorithm take if Cambridge and Austin merge into a

if Cambridge and Austin merge into a phone book that's twice as

phone book that's twice as big just one more step right no big deal

big just one more step right no big deal you just take a really big bite out of

you just take a really big bite out of the problem once you decide if John

the problem once you decide if John Harvard is to the left or to the right

Harvard is to the left or to the right and so you've made much faster progress

and so you've made much faster progress and so this in essence is what your

and so this in essence is what your computer your phone is probably doing

computer your phone is probably doing underneath the hood when searching for

underneath the hood when searching for Harry or Hermione or Hagrid or anyone

Harry or Hermione or Hagrid or anyone else because it's that much faster

else because it's that much faster especially when you have large data if

especially when you have large data if you don't have that many contacts

you don't have that many contacts probably doesn't matter if you search

probably doesn't matter if you search from top to bottom or more uh more in

from top to bottom or more uh more in the form of this divide and conquer

the form of this divide and conquer algorithm but if you're the Googles of

algorithm but if you're the Googles of the world or you're analyzing large data

the world or you're analyzing large data sets indeed this is going to add up

sets indeed this is going to add up quite quickly so where do we go with

quite quickly so where do we go with this well we're going to introduce next

this well we're going to introduce next something called pseudo code how can I

something called pseudo code how can I translate what I did verbally there sort

translate what I did verbally there sort of intuitively to actual code well this

of intuitively to actual code well this won't be scratch this won't be C or

won't be scratch this won't be C or python just yet it's just going to be an

python just yet it's just going to be an english-like syntax and this is how many

english-like syntax and this is how many programmers would start solving a

programmers would start solving a problem they don't start typing out code

problem they don't start typing out code in C or python or the like they use

in C or python or the like they use English or whatever their human language

English or whatever their human language is to jot down an outline for their

is to jot down an outline for their ideas my first step really was picking

ideas my first step really was picking up the phone book my second step was

up the phone book my second step was opening to the middle of the phone book

opening to the middle of the phone book my third step was somewhat different

my third step was somewhat different look at the page because why my fourth

look at the page because why my fourth step was if person I'm looking for is on

step was if person I'm looking for is on the page I then do what never happened

the page I then do what never happened in my example but I call the person so

in my example but I call the person so I'm done else if the person is earlier

I'm done else if the person is earlier in the book alphabetically as John

in the book alphabetically as John Harvard was in the case of my H then I

Harvard was in the case of my H then I should search to the middle of the left

should search to the middle of the left of the phone book and then I should go

of the phone book and then I should go back to step three step three is look at

back to step three step three is look at the page thereby repeating the same

the page thereby repeating the same process again and again step nine though

process again and again step nine though might be else if the person is later in

might be else if the person is later in the book then let's go ahead and open to

the book then let's go ahead and open to the middle of the right half of the book

the middle of the right half of the book and then go back to line three else

and then go back to line three else there's a fourth scenario we should

there's a fourth scenario we should probably consider lest my search process

probably consider lest my search process freeze or crash or give me one of those

freeze or crash or give me one of those spinning beach balls with a bug

spinning beach balls with a bug yeah yeah what if John Harvard isn't in

yeah yeah what if John Harvard isn't in the phone book I'd prefer that my

the phone book I'd prefer that my algorithm my phone not just reboot or

algorithm my phone not just reboot or freeze I should handle that with some

freeze I should handle that with some kind of catchall else so to speak let's

kind of catchall else so to speak let's just quit the program so there's

just quit the program so there's welldefined behavior for every possible

welldefined behavior for every possible scenario of the four now let's call out

scenario of the four now let's call out a few of these Salient terms it turns

a few of these Salient terms it turns out if I highlight in yellow here

out if I highlight in yellow here there's a pattern to what I've been

there's a pattern to what I've been doing here these are all of my English

doing here these are all of my English verbs and we're in a moment we're going

verbs and we're in a moment we're going to start calling those verbs functions

to start calling those verbs functions when you program or write code and you

when you program or write code and you want the program or the computer to do

want the program or the computer to do something for you some action or verb

something for you some action or verb we're going to refer to those actions or

we're going to refer to those actions or verbs as these things called functions

verbs as these things called functions like those here by contrast I've just

like those here by contrast I've just highlighted instead my if my El if my

highlighted instead my if my El if my Els if and Els this is going to

Els if and Els this is going to represent what we're going to start

represent what we're going to start calling a conditional a proverbial fork

calling a conditional a proverbial fork in the road where you can either go this

in the road where you can either go this way or that way do this thing or this

way or that way do this thing or this other thing and you're going to decide

other thing and you're going to decide which of those things to do based on

which of those things to do based on what I've now highlighted here which are

what I've now highlighted here which are going to be called Boolean Expressions

going to be called Boolean Expressions bull referring to a mathematician last

bull referring to a mathematician last named bull a Boolean expression is just

named bull a Boolean expression is just a question with a yes no a true false a

a question with a yes no a true false a one or a zero answer if you will and it

one or a zero answer if you will and it governs whether you do this thing or

governs whether you do this thing or this thing or this thing or that the

this thing or this thing or that the indentation in this case is important

indentation in this case is important the fact that I've indented line five

the fact that I've indented line five implies by convention in programming

implies by convention in programming that I should only do line five if the

that I should only do line five if the answer to line four is a yes or true and

answer to line four is a yes or true and same for these other indented lines as

same for these other indented lines as well and the last characteristic here is

well and the last characteristic here is this here uh someone called this out

this here uh someone called this out earlier in fact these lines eight and 11

earlier in fact these lines eight and 11 are now highlighted and represent

are now highlighted and represent what what might we call these in code if

what what might we call these in code if you've done that yeah so these are Loops

you've done that yeah so these are Loops some kind of cycles that result in my

some kind of cycles that result in my doing the same thing again and again but

doing the same thing again and again but there's a key detail with this algorithm

there's a key detail with this algorithm in pseudo code even though it's telling

in pseudo code even though it's telling me to go back to line three why is this

me to go back to line three why is this algorithm event going to stop why do I

algorithm event going to stop why do I not constantly keep looking for John

not constantly keep looking for John Harvard Forever by nature of these Loops

Harvard Forever by nature of these Loops telling me to keep going back to line

telling me to keep going back to line three good eventually he'll be on the

three good eventually he'll be on the page or or to your point earlier he

page or or to your point earlier he won't be at all and we're out of pages

won't be at all and we're out of pages and so we just quit and that's the key

and so we just quit and that's the key about going to the left half or the

about going to the left half or the right half it doesn't matter if you do

right half it doesn't matter if you do the same thing again and again you're

the same thing again and again you're not going to get stuck in a so-called

not going to get stuck in a so-called infinite Loop so long as you keep

infinite Loop so long as you keep dividing the problem and shrinking it

dividing the problem and shrinking it into something small smaller smaller

into something small smaller smaller eventually there's going to be no

eventually there's going to be no problem left to solve so even if you

problem left to solve so even if you don't think of yourself as a computer

don't think of yourself as a computer person even if you've never written code

person even if you've never written code what you'll find in the coming days is

what you'll find in the coming days is that these ideas that we've just kind of

that these ideas that we've just kind of harnessed from real life are at your

harnessed from real life are at your fingertips already and a lot of the

fingertips already and a lot of the process of learning to code is yes it's

process of learning to code is yes it's going to be a bumpy Ro uh some bumps in

going to be a bumpy Ro uh some bumps in the road because you can't quite see the

the road because you can't quite see the new syntax in a familiar way but you'll

new syntax in a familiar way but you'll find that the ideas in fact are going to

find that the ideas in fact are going to be more familiar than you might

be more familiar than you might otherwise think and so we'll see in a

otherwise think and so we'll see in a bit and we'll take a break in a moment

bit and we'll take a break in a moment uh to take a breather that you will see

uh to take a breather that you will see these same ideas in a moment in the

these same ideas in a moment in the context of scratch an actual programming

context of scratch an actual programming language via which we drag and drop

language via which we drag and drop puzzle pieces to make actual code work

puzzle pieces to make actual code work we'll see some variant of these ideas

we'll see some variant of these ideas things called arguments and return

things called arguments and return values and variables but we'll

values and variables but we'll ultimately convert it into this somehow

ultimately convert it into this somehow anyone want to wager what this program

anyone want to wager what this program will do if fed to your Mac or PC or

will do if fed to your Mac or PC or phone here's just a massive pattern of

phone here's just a massive pattern of zeros and

zeros and ones it will indeed say rather

ones it will indeed say rather disappoint ly apparently just hello

disappoint ly apparently just hello world and indeed baked into all of these

world and indeed baked into all of these zeros and ones are not just the h l l o

zeros and ones are not just the h l l o but also the verbs the action of

but also the verbs the action of printing something to the screen and

printing something to the screen and there's other stuff too so that the

there's other stuff too so that the program knows how to start and how to

program knows how to start and how to stop a lot of stuff that we won't have

stop a lot of stuff that we won't have to worry about that whoever designed the

to worry about that whoever designed the computer or the language did but at the

computer or the language did but at the end of the day you're never going to be

end of the day you're never going to be writing these zeros and ones yourselves

writing these zeros and ones yourselves though our ancestors Once Upon a Time

though our ancestors Once Upon a Time did in some form we'll be using a much

did in some form we'll be using a much higher l level language like this in C

higher l level language like this in C or better yet in just a moment like in

or better yet in just a moment like in scratch like this and indeed this is why

scratch like this and indeed this is why today we focus with focus on and begin

today we focus with focus on and begin with scratch this graphical programming

with scratch this graphical programming language so we have a way of expressing

language so we have a way of expressing ourselves with functions conditionals

ourselves with functions conditionals loops and more but in a way that doesn't

loops and more but in a way that doesn't have stupid parentheses and curly braces

have stupid parentheses and curly braces and all these visual distractions in the

and all these visual distractions in the way and we'll translate that thereafter

way and we'll translate that thereafter to this lower level language but for now

to this lower level language but for now that was a lot that was definitely a

that was a lot that was definitely a fire hose let's go ahead and take a

fire hose let's go ahead and take a 10-minute break feel free to get up or

10-minute break feel free to get up or stay here and we'll resume in a bit with

stay here and we'll resume in a bit with some actual

some actual code uh so this then is scratch a

code uh so this then is scratch a graphical programming language from a

graphical programming language from a friends down the road at mit's media lab

friends down the road at mit's media lab that indeed some of you might have used

that indeed some of you might have used in grade score the like for playing and

in grade score the like for playing and writing code and the like but you maybe

writing code and the like but you maybe didn't necessarily think about how some

didn't necessarily think about how some of these Primitives ultimately worked

of these Primitives ultimately worked and in fact everything you've done if

and in fact everything you've done if you've used scratch before and

you've used scratch before and everything you'll see today is going to

everything you'll see today is going to apply to all of the weeks to come as we

apply to all of the weeks to come as we explore these things called functions

explore these things called functions and loops and conditionals Boolean

and loops and conditionals Boolean expressions and more with scratch

expressions and more with scratch because it's so graphical and animated

because it's so graphical and animated can create can you create animations

can create can you create animations like this one interactive art and

like this one interactive art and software more generally but you'll do so

software more generally but you'll do so by dragging and dropping puzzle pieces

by dragging and dropping puzzle pieces that only lock together if it makes

that only lock together if it makes logical sense to do so and what you

logical sense to do so and what you won't have to deal with in this first

won't have to deal with in this first week of class is Curly braces

week of class is Curly braces parentheses all of the weird symbology

parentheses all of the weird symbology that you might recall seeing when we

that you might recall seeing when we just wanted to say hello world now this

just wanted to say hello world now this particular program um riging men was

particular program um riging men was written by a former cs50 teaching fellow

written by a former cs50 teaching fellow Andrew bar who's actually now the

Andrew bar who's actually now the general manager of the Cleveland Browns

general manager of the Cleveland Browns the American football team and so these

the American football team and so these are just some of the programs that some

are just some of the programs that some of your predecessors in the class have

of your predecessors in the class have created and you'll see in the remainder

created and you'll see in the remainder of class here a couple of others as well

of class here a couple of others as well and more in the course's first

and more in the course's first assignment namely problem set zero so

assignment namely problem set zero so how do we get there well first a quick

how do we get there well first a quick tour of what it is we're going to do

tour of what it is we're going to do this in scratch is perhaps the simplest

this in scratch is perhaps the simplest program you can write and even if you've

program you can write and even if you've never seen scratch or any programming

never seen scratch or any programming language before can probably guess that

language before can probably guess that this just says on the screen somehow

this just says on the screen somehow hello world but what you don't have to

hello world but what you don't have to do is type esoteric commands and weird

do is type esoteric commands and weird syntax those curly braces and

syntax those curly braces and parentheses I keep alluding to you just

parentheses I keep alluding to you just drag this yellow puzzle piece you drag

drag this yellow puzzle piece you drag this purple puzzle piece let them

this purple puzzle piece let them magnetically lock together so to speak

magnetically lock together so to speak click a button and boom with those same

click a button and boom with those same building blocks and several others can

building blocks and several others can you make what exactly the sorts of

you make what exactly the sorts of things that Andrew brought to life as

things that Andrew brought to life as well so here's what we're about to see

well so here's what we're about to see at scratch.mit.edu is a cloud-based

at scratch.mit.edu is a cloud-based programming environment on MIT servers

programming environment on MIT servers you can also download it offline on your

you can also download it offline on your own Mac or PC and it gives you an

own Mac or PC and it gives you an interface like this on the left hand

interface like this on the left hand side of the screen you'll see a blocks

side of the screen you'll see a blocks pette these puzzle pieces AKA blocks

pette these puzzle pieces AKA blocks come in different colors which rather

come in different colors which rather categorize them so pictured here for

categorize them so pictured here for instance in blue are a whole bunch of

instance in blue are a whole bunch of motion related blocks so Andrew used a

motion related blocks so Andrew used a whole bunch of those to have the singer

whole bunch of those to have the singer and the men moving around on the screen

and the men moving around on the screen um in synchronicity with the song that

um in synchronicity with the song that was playing in the background meanwhile

was playing in the background meanwhile in the middle of this interface is going

in the middle of this interface is going to be the code area and this is where

to be the code area and this is where Andrew and sunu will drag and drop some

Andrew and sunu will drag and drop some of those puzzle pieces and other colors

of those puzzle pieces and other colors as well and lock them together to get

as well and lock them together to get your character soon to be invented to do

your character soon to be invented to do something on the screen indeed at the

something on the screen indeed at the bottom right here will you see

bottom right here will you see ultimately a Sprite area where a Sprite

ultimately a Sprite area where a Sprite is a technical term for like a character

is a technical term for like a character in a video game or a programming

in a video game or a programming environment like this by default

environment like this by default historically scratch uh is the cat the

historically scratch uh is the cat the mascot if you will for this programming

mascot if you will for this programming environment and so here we see by

environment and so here we see by default just one Sprite selected because

default just one Sprite selected because on the top right of the screen is the

on the top right of the screen is the stage for that Sprite and you can click

stage for that Sprite and you can click in Click and zoom in to make it full

in Click and zoom in to make it full screen but this is the world in which

screen but this is the world in which Scratch by default the cat will live but

Scratch by default the cat will live but you can change scratch's costume so that

you can change scratch's costume so that it looks like a singer or a man falling

it looks like a singer or a man falling from the sky or the like or anything

from the sky or the like or anything else either creating the art yourself or

else either creating the art yourself or importing some of the things that come

importing some of the things that come with it or elsewhere online so what is

with it or elsewhere online so what is this world that scratch rather lives in

this world that scratch rather lives in well generally speaking we won't have to

well generally speaking we won't have to care too much about numbers because

care too much about numbers because we'll be able to ask questions like

we'll be able to ask questions like intera active ones like is scratch the

intera active ones like is scratch the cat or any character otherwise touching

cat or any character otherwise touching the edge of the screen touching

the edge of the screen touching something else but scratch does exist in

something else but scratch does exist in this

this two-dimensional uh coordinate system

two-dimensional uh coordinate system world so when the cat or any character

world so when the cat or any character is dead center in the middle that would

is dead center in the middle that would be XY location 0 comma 0 if you will

be XY location 0 comma 0 if you will meanwhile over here is 240 pixels or

meanwhile over here is 240 pixels or dots all the way to the right so this

dots all the way to the right so this would be 240 comma 0 where Y is z

would be 240 comma 0 where Y is z because it's right on that midline so

because it's right on that midline so it's neither up or below

it's neither up or below over here to the left of course would be

over here to the left of course would be 240 and 0 above the cat would be xal 0

240 and 0 above the cat would be xal 0 cuz it's right on that vertical midline

cuz it's right on that vertical midline and 180 and then down here as you might

and 180 and then down here as you might guess would be 0 comma netive 180

guess would be 0 comma netive 180 generally speaking we don't have to care

generally speaking we don't have to care about those precise pixel coordinates

about those precise pixel coordinates but it's helpful ultimately if you do

but it's helpful ultimately if you do want the cat to move up down left or

want the cat to move up down left or right having some sense of direction

right having some sense of direction according to the x- axis and y AIS as

according to the x- axis and y AIS as well can help you express your ideas

well can help you express your ideas ultimately so what might some of those

ultimately so what might some of those ideas be well let's do this I'm going to

ideas be well let's do this I'm going to go ahead and create on

go ahead and create on scratch.mit.edu just an empty screen

scratch.mit.edu just an empty screen like this one here and so this is the

like this one here and so this is the exact same interface but now I'm in my

exact same interface but now I'm in my browser uh full screen so that I can

browser uh full screen so that I can start writing some code and let's get

start writing some code and let's get that cat to say something actually on

that cat to say something actually on the screen now this takes a little bit

the screen now this takes a little bit of practice but honestly just by

of practice but honestly just by scrolling through these puzzle pieces

scrolling through these puzzle pieces can you quickly get a sense of what's

can you quickly get a sense of what's possible not just categorically but

possible not just categorically but specifically and I'll jump around

specifically and I'll jump around because I've done this of course but I'm

because I've done this of course but I'm going to go to events in yellow first

going to go to events in yellow first and I'm going to drag and drop this

and I'm going to drag and drop this first block called when green flag

first block called when green flag clicked and I've zoomed in there just to

clicked and I've zoomed in there just to make it a little more legible and notice

make it a little more legible and notice that the shape of this green flag just

that the shape of this green flag just so happens to mirror this green flag

so happens to mirror this green flag here at top next to this red stop sign

here at top next to this red stop sign of sorts and the green flag is going to

of sorts and the green flag is going to mean go and the red stop sign's going to

mean go and the red stop sign's going to mean stop to start or stop our program

mean stop to start or stop our program next week you're going to be writing a

next week you're going to be writing a textual command at your keyboard to do

textual command at your keyboard to do the exact same idea but for now it's a

the exact same idea but for now it's a button so when green flag clicked what

button so when green flag clicked what do I want scratch to do well how about

do I want scratch to do well how about we have scratch just initially say

we have scratch just initially say something like Hello World which indeed

something like Hello World which indeed historically is the first program that

historically is the first program that most any programmer might write so

most any programmer might write so anything related to what the cat looks

anything related to what the cat looks like it's doing is actually going to be

like it's doing is actually going to be under looks here in purple so I'm going

under looks here in purple so I'm going to drag over say hello and you'll notice

to drag over say hello and you'll notice something curious and different about

something curious and different about this purple block it says of course say

this purple block it says of course say in purple but then there's this white

in purple but then there's this white oval and some text that by default is

oval and some text that by default is hello cuz MIT just decided that by

hello cuz MIT just decided that by default the placeholder will be hello

default the placeholder will be hello but anytime you see this white oval it's

but anytime you see this white oval it's an opportunity to provide an input into

an opportunity to provide an input into the function called say and so here I'm

the function called say and so here I'm borrowing terminology from before

borrowing terminology from before problem solving again is all about

problem solving again is all about inputs producing outputs and in between

inputs producing outputs and in between there is some algorithm in a moment

there is some algorithm in a moment we're going to start referring to

we're going to start referring to algorithms quite frequently as functions

algorithms quite frequently as functions why because it's the implementation of

why because it's the implementation of some algorithm so let me override the

some algorithm so let me override the default with hello comma World I'll zoom

default with hello comma World I'll zoom out and now if I go to the top right of

out and now if I go to the top right of the screen and click the green flag

the screen and click the green flag we'll see hopefully my very first

we'll see hopefully my very first program in code now it wasn't a huge

program in code now it wasn't a huge lift right it only was a matter of

lift right it only was a matter of dragging and dropping puzzle pieces but

dragging and dropping puzzle pieces but what has now happened well it turns out

what has now happened well it turns out that two things have happened when I the

that two things have happened when I the human clicked on that green flag I

human clicked on that green flag I triggered what we're going to start

triggered what we're going to start calling now an event an event is

calling now an event an event is generally something graphical or

generally something graphical or interactive that just happens in a

interactive that just happens in a computer program you and I trigger

computer program you and I trigger events on our phones all day long

events on our phones all day long whenever you tap or drag or long press

whenever you tap or drag or long press or pinch or any of those gestures in

or pinch or any of those gestures in Vogue nowadays on phones you are

Vogue nowadays on phones you are triggering events and people at Apple

triggering events and people at Apple and Google and elsewhere have written

and Google and elsewhere have written code that listen for those events and do

code that listen for those events and do something when that event happens that's

something when that event happens that's what I just did when green flag is

what I just did when green flag is clicked I want something to happen

clicked I want something to happen namely I want this purple function this

namely I want this purple function this verb this action called say to do

verb this action called say to do something what do I want it to do I want

something what do I want it to do I want it to say what this input is and I'm

it to say what this input is and I'm going to introduce another vocabulary

going to introduce another vocabulary term the white ovals here are yes inputs

term the white ovals here are yes inputs very generically but in the programmer's

very generically but in the programmer's terminology they're called arguments

terminology they're called arguments otherwise known as parameters and that

otherwise known as parameters and that just means an input to a function that

just means an input to a function that modifies Its Behavior in some way when I

modifies Its Behavior in some way when I click stop that's just another event and

click stop that's just another event and that one is just built into scratch

that one is just built into scratch scratch knows that when you click the

scratch knows that when you click the green stop sign uh everything should

green stop sign uh everything should just stop automatically I don't have to

just stop automatically I don't have to write code to support that feature so

write code to support that feature so that's all fine and good hello world but

that's all fine and good hello world but if I keep doing stop and start and stop

if I keep doing stop and start and stop and start it's going to do the same

and start it's going to do the same thing again and again and it's really

thing again and again and it's really not that interesting at the end of the

not that interesting at the end of the day maybe gratifying once but it'd be

day maybe gratifying once but it'd be nice if this were a little more

nice if this were a little more interactive so it turns out that we can

interactive so it turns out that we can do that too but we need a different

do that too but we need a different mental model instead so in this case

mental model instead so in this case here when we think about this function

here when we think about this function say in this input hello world this

say in this input hello world this actually Maps pretty cleanly to this

actually Maps pretty cleanly to this model earlier that I propose is problem

model earlier that I propose is problem solving is computer science if you will

solving is computer science if you will the input to the current problem is

the input to the current problem is going to be in white here hello world

going to be in white here hello world the algorithm is the say algorithm now I

the algorithm is the say algorithm now I don't know how MIT got it to print out

don't know how MIT got it to print out the little pretty speech bubble on the

the little pretty speech bubble on the screen but they wrote those underlying

screen but they wrote those underlying low-level implementation details and

low-level implementation details and they gave me and you a purple function

they gave me and you a purple function called say that just does that for you

called say that just does that for you you and I don't have to reinvent that

you and I don't have to reinvent that wheel the output of SE is another

wheel the output of SE is another technical term now called a side effect

technical term now called a side effect a side effect is usually something

a side effect is usually something visual that happens like as a side

visual that happens like as a side effect of you calling a function and so

effect of you calling a function and so the side effect here is that the cat has

the side effect here is that the cat has this speech bubble magically appear

this speech bubble magically appear inside of which is hello world so we

inside of which is hello world so we have an input we have an output we have

have an input we have an output we have an algorithm but now we're talking about

an algorithm but now we're talking about these ideas in the context of

these ideas in the context of programming so now the input is an

programming so now the input is an argument the algorithm is a function and

argument the algorithm is a function and the output in this case is a side effect

the output in this case is a side effect terminology that you'll just hear more

terminology that you'll just hear more and more and it'll eventually sink in

and more and it'll eventually sink in but not to worry if the terminology

but not to worry if the terminology doesn't come naturally early on so what

doesn't come naturally early on so what more might I do with this let me go back

more might I do with this let me go back to scratch here and make this maybe

to scratch here and make this maybe perhaps more interactive and actually

perhaps more interactive and actually get the cat to say something a little

get the cat to say something a little more dynamically so instead of hello

more dynamically so instead of hello world why don't I get it to say hello to

world why don't I get it to say hello to me or to you or anyone else so let me do

me or to you or anyone else so let me do this let me go under say uh let me get

this let me go under say uh let me get rid of this first and you'll notice this

rid of this first and you'll notice this neat trick as soon as you start dragging

neat trick as soon as you start dragging on a block if it gets close to it it

on a block if it gets close to it it kind of goes gray and it can be

kind of goes gray and it can be magnetically snapped together you don't

magnetically snapped together you don't have to do it very precisely conversely

have to do it very precisely conversely if I want to get rid of a puzzle piece I

if I want to get rid of a puzzle piece I just drag it anywhere on the left let go

just drag it anywhere on the left let go and that deletes it or you can right

and that deletes it or you can right click or control click in a little menu

click or control click in a little menu will let you delete it as well well let

will let you delete it as well well let me do this instead under sensing which I

me do this instead under sensing which I know is there because I've done this

know is there because I've done this before are a whole bunch of things

before are a whole bunch of things related to sensing whereby the cat can

related to sensing whereby the cat can kind of feel out its World in some sense

kind of feel out its World in some sense it can do things like ask this question

it can do things like ask this question am I touching the mouse pointer like the

am I touching the mouse pointer like the user's cursor am I touching a specific

user's cursor am I touching a specific color that you can override to be

color that you can override to be anything you want is the distance to the

anything you want is the distance to the mouse pointer some specific value but

mouse pointer some specific value but for now I'm going to focus this on this

for now I'm going to focus this on this this blue puzzle piece that asks a

this blue puzzle piece that asks a question which itself is this white oval

question which itself is this white oval that I can apparently change and then

that I can apparently change and then it's going to wait for a response but

it's going to wait for a response but this puzzle piece is a little different

this puzzle piece is a little different it's a little special it comes with a

it's a little special it comes with a freebie it comes with what we're going

freebie it comes with what we're going to call technically a return value so

to call technically a return value so some functions don't just do something

some functions don't just do something on the screen they hand you back so to

on the screen they hand you back so to speak a value that you can do anything

speak a value that you can do anything that you want with nothing happens

that you want with nothing happens immediately unless you do something with

immediately unless you do something with that so-called return value so let me go

that so-called return value so let me go ahead and drag this thing over here ask

ahead and drag this thing over here ask what's your name and I'll use the

what's your name and I'll use the default question that seems a reasonable

default question that seems a reasonable place to start I'm not going to override

place to start I'm not going to override that default and now let me go ahead and

that default and now let me go ahead and zoom out let me go back to looks let me

zoom out let me go back to looks let me go to say and let me just form the

go to say and let me just form the English sentence I want so let me zoom

English sentence I want so let me zoom in here and type in hello maybe comma

in here and type in hello maybe comma space I could do David but that's that's

space I could do David but that's that's obviously not right because I'm asking

obviously not right because I'm asking for a name and then I'm like in advance

for a name and then I'm like in advance hardcoding my name that's not what I

hardcoding my name that's not what I want I just want hello comma and now let

want I just want hello comma and now let me zoom out and grab one more say block

me zoom out and grab one more say block Let Me Maybe say here okay I don't want

Let Me Maybe say here okay I don't want to say hello hello I I don't want to

to say hello hello I I don't want to just type in my own name CU again then

just type in my own name CU again then what's the point of asking the user for

what's the point of asking the user for their name but notice this if I go back

their name but notice this if I go back to the sensing block this is where that

to the sensing block this is where that oval that's blue called answer is useful

oval that's blue called answer is useful this will be the so-called return value

this will be the so-called return value of that function so I'm just going to go

of that function so I'm just going to go ahead and do this and drag and drop even

ahead and do this and drag and drop even though it's not the right size it is the

though it's not the right size it is the right shape and so scratch will be smart

right shape and so scratch will be smart about it and grow to fill that puzzle

about it and grow to fill that puzzle piece for you let me zoom out now and

piece for you let me zoom out now and now let me click the green flag you'll

now let me click the green flag you'll see that scratch is indeed prompting me

see that scratch is indeed prompting me with the speech bubble what's your name

with the speech bubble what's your name notice the little text box below the cat

notice the little text box below the cat is asking what's your name so I'm going

is asking what's your name so I'm going to type in da a v d and hit enter or I

to type in da a v d and hit enter or I can click the blue check

can click the blue check enter okay it's a little weird I wanted

enter okay it's a little weird I wanted him to say hello not just my name so let

him to say hello not just my name so let me stop let me start it again all right

me stop let me start it again all right hello what's your name da a v

hello what's your name da a v enter huh kind of rude uh why is there

enter huh kind of rude uh why is there this bug like I wanted to say hello

this bug like I wanted to say hello David not just David and and yet twice

David not just David and and yet twice it has failed to do so uh

it has failed to do so uh yeah yeah the computer is processing my

yeah yeah the computer is processing my directions my actions really quickly and

directions my actions really quickly and so it actually is doing it it's just you

so it actually is doing it it's just you and I in the room are just way too slow

and I in the room are just way too slow to notice that it said hello David it

to notice that it said hello David it just seems to have just said David so

just seems to have just said David so all right how can I fix this well here's

all right how can I fix this well here's where you start to poke around and think

where you start to poke around and think about how you might solve this let me go

about how you might solve this let me go back under looks maybe there's a smarter

back under looks maybe there's a smarter way to do this maybe I could do okay I

way to do this maybe I could do okay I could do this how about instead of just

could do this how about instead of just say hello there's apparently another

say hello there's apparently another puzzle piece where I can time it so I

puzzle piece where I can time it so I can maybe slow things down a little bit

can maybe slow things down a little bit so let me do this let me throw away all

so let me do this let me throw away all of this let me drag a say hello for two

of this let me drag a say hello for two seconds let me drag another say hello

seconds let me drag another say hello for two seconds let me change the first

for two seconds let me change the first one to indeed hello comma and then let

one to indeed hello comma and then let me go back to sensing let me grab that

me go back to sensing let me grab that same answer because I threw it away a

same answer because I threw it away a second ago and I'll just change it I

second ago and I'll just change it I don't even have to delete hello I can

don't even have to delete hello I can just overwrite it like this so now I

just overwrite it like this so now I think we'll kind of pump the brakes and

think we'll kind of pump the brakes and see things more slowly let me stop let

see things more slowly let me stop let me start da ID enter

me start da ID enter hello David okay so it's better like it

hello David okay so it's better like it seems to be working I think your

seems to be working I think your hypothesis was right just looks kind of

hypothesis was right just looks kind of stupid right like the fact that it's

stupid right like the fact that it's saying

saying hello David like we can do better and

hello David like we can do better and like literally every piece of software

like literally every piece of software on your phone or Mac or PC is better

on your phone or Mac or PC is better than that it adds words together in the

than that it adds words together in the user interfaces you and I are familiar

user interfaces you and I are familiar with so let's go a little more fishing

with so let's go a little more fishing here let me throw away these let me go

here let me throw away these let me go back to look and just get the simpler

back to look and just get the simpler say I want this to say hello comma name

say I want this to say hello comma name where name comes from that answer return

where name comes from that answer return value so how can I do this well let me

value so how can I do this well let me go under operations which we haven't

go under operations which we haven't been before there's a lot of stuff in

been before there's a lot of stuff in here some of it's mathematically related

here some of it's mathematically related adding subtracting and so forth you can

adding subtracting and so forth you can Generate random numbers which might be

Generate random numbers which might be useful and if I keep scrolling down

useful and if I keep scrolling down there's this join apple and banana but

there's this join apple and banana but that's just placeholder text you can

that's just placeholder text you can join one piece of text with another

join one piece of text with another piece of text by default apple and

piece of text by default apple and banana but let's change it to hello and

banana but let's change it to hello and my name name so this to wrong size but

my name name so this to wrong size but right shape so let me let it snap into

right shape so let me let it snap into place let me go ahead now and do hello

place let me go ahead now and do hello comma and now I think I just want to go

comma and now I think I just want to go grab that answer return value let me

grab that answer return value let me drag the same oval as before clobber

drag the same oval as before clobber that is overwrite banana so now I'm kind

that is overwrite banana so now I'm kind of composing functions the output of one

of composing functions the output of one function join is going to be the input

function join is going to be the input of another function say so let's see

of another function say so let's see what happens now that they're kind of

what happens now that they're kind of stacked on top of each other or nested

stacked on top of each other or nested so to speak click the Green Arrow a

so to speak click the Green Arrow a green flag DAV ID enter hello David all

green flag DAV ID enter hello David all right that was pretty fast let's just

right that was pretty fast let's just just do it once more stop start here we

just do it once more stop start here we go daavid enter okay right it's not the

go daavid enter okay right it's not the most exciting program in the world but

most exciting program in the world but it's more correct it's better design

it's more correct it's better design just because that's what you would kind

just because that's what you would kind of expect the software to do and not be

of expect the software to do and not be some kind of lame user interface that's

some kind of lame user interface that's just inserting random delays to just

just inserting random delays to just make it kind of work like that's a

make it kind of work like that's a workaround a hack if you will but

workaround a hack if you will but there's some cool things you can do with

there's some cool things you can do with scratch and we won't really go down the

scratch and we won't really go down the rabbit hole of all of the fun and

rabbit hole of all of the fun and familyfriendly features that it has but

familyfriendly features that it has but there is one that's kind of cool here

there is one that's kind of cool here let me go into the extensions button at

let me go into the extensions button at the bottom left of my screen and this

the bottom left of my screen and this one's kind of cool let me go to text to

one's kind of cool let me go to text to speech and you'll notice that this one

speech and you'll notice that this one requires internet because it's

requires internet because it's cloud-based but this just gave me some

cloud-based but this just gave me some new puzzle pieces in a new category text

new puzzle pieces in a new category text to speech and these green ones do

to speech and these green ones do exactly what they say so let me do this

exactly what they say so let me do this let me zoom out again let me keep the

let me zoom out again let me keep the join block and I'm just going to

join block and I'm just going to temporarily toss it over here it's not

temporarily toss it over here it's not going to delete itself cuz I didn't drag

going to delete itself cuz I didn't drag it over to the other side but I'm going

it over to the other side but I'm going to get rid of the say block in purple

to get rid of the say block in purple I'm going to do the speak block here in

I'm going to do the speak block here in green and let it snap into place and

green and let it snap into place and then I'm going to drag and drop this

then I'm going to drag and drop this onto the input to speak and now perhaps

onto the input to speak and now perhaps a little more adorably let's try this

a little more adorably let's try this green flag what's your name d ID enter

green flag what's your name d ID enter and hello

and hello David okay it's a little it's a little

David okay it's a little it's a little robotic but at least now it has

robotic but at least now it has synthesized speech and I've kind of got

synthesized speech and I've kind of got my own like Siri or Google assistant or

my own like Siri or Google assistant or Alexa thing going on here now where it's

Alexa thing going on here now where it's now recognized whatever text it is and

now recognized whatever text it is and it's played it well let's make this an

it's played it well let's make this an actual cat that doesn't talk in that

actual cat that doesn't talk in that weird human voice let me go ahead and

weird human voice let me go ahead and get rid of most of this stuff and let's

get rid of most of this stuff and let's get the cat to actually meow like a cat

get the cat to actually meow like a cat tends to and let me go under the sounds

tends to and let me go under the sounds block now MIT gives you a few sounds for

block now MIT gives you a few sounds for free because it's designed around a cap

free because it's designed around a cap by default and I'm going to go ahead and

by default and I'm going to go ahead and grab this one play sound meow until done

grab this one play sound meow until done and now and we saw I heard a teaser for

and now and we saw I heard a teaser for this earlier in the

this earlier in the crowd

crowd it's a little piercing admittedly we can

it's a little piercing admittedly we can lower the volume a little bit there but

lower the volume a little bit there but notice if I want the cat to meow a

notice if I want the cat to meow a second time I'll just click it

second time I'll just click it again okay and over there too I

again okay and over there too I hear okay all right so it's kind of cute

hear okay all right so it's kind of cute now right so it's just meow okay yes

now right so it's just meow okay yes echo echo so it's meowing now every time

echo echo so it's meowing now every time I hit the green flag now that's great

I hit the green flag now that's great but even a kid is probably going to like

but even a kid is probably going to like would prefer that it just meow perhaps

would prefer that it just meow perhaps like again and again without having to

like again and again without having to keep hitting the button so well how

keep hitting the button so well how might we do this all right well if I

might we do this all right well if I want it to meow multiple times why don't

want it to meow multiple times why don't I just like grab it another time and

I just like grab it another time and another time alternatively you can right

another time alternatively you can right click or control click a puzzle piece

click or control click a puzzle piece and just duplicate it from a little menu

and just duplicate it from a little menu that drops down so here we go three

meows all right that's not really a happy cat it sounds maybe hungry so can

happy cat it sounds maybe hungry so can we slow that down well maybe in fact if

we slow that down well maybe in fact if I poke around let me go under control

I poke around let me go under control looks like there's a weight block wait 1

looks like there's a weight block wait 1 second by default and notice scratch

second by default and notice scratch will be pretty accommodating if you just

will be pretty accommodating if you just hover in between blocks it will grow to

hover in between blocks it will grow to fill that too so I could change it to

fill that too so I could change it to one or two or anything seconds I'll just

one or two or anything seconds I'll just leave it at the default for now one and

leave it at the default for now one and now I'll go ahead and do

this okay so cuter and less hungry and just more friendly but this isn't the

just more friendly but this isn't the best design it is correct and let's use

best design it is correct and let's use that as a term of art correct means the

that as a term of art correct means the code does what you want it to do I want

code does what you want it to do I want the cat to meow three times slow slly

the cat to meow three times slow slly and it did so iager this is correct but

and it did so iager this is correct but it's not the best design and this is

it's not the best design and this is where things get more subjective right

where things get more subjective right like you could write accurate sentences

like you could write accurate sentences in an essay for an English class but

in an essay for an English class but otherwise just it's just completely a

otherwise just it's just completely a mess like your arguments here and there

mess like your arguments here and there and you don't say anything wrong but you

and you don't say anything wrong but you don't say it well in the context of code

don't say it well in the context of code we can do better than this and copy

we can do better than this and copy paste or repeating yourself again and

paste or repeating yourself again and again tends to be bad practice why

again tends to be bad practice why suppose that you want to change the

suppose that you want to change the weight to two seconds instead of one

weight to two seconds instead of one it's aditt not a big deal F I click

it's aditt not a big deal F I click there I change it to two I click there I

there I change it to two I click there I change it to two but what if you Ma five

change it to two but what if you Ma five times 10 times now I have to change the

times 10 times now I have to change the weight like in five 10 different places

weight like in five 10 different places like that's just stupid it's taking

like that's just stupid it's taking unnecessary human time and you're going

unnecessary human time and you're going to screw up eventually especially if

to screw up eventually especially if your program is getting longer you're

your program is getting longer you're going to miss one of the inputs you're

going to miss one of the inputs you're going to leave the number wrong and

going to leave the number wrong and you're that's a bug so just based on

you're that's a bug so just based on what you've seen already or if you've

what you've seen already or if you've programmed before which a few of you

programmed before which a few of you have what's the term of art here that

have what's the term of art here that will solve this how can we design this

will solve this how can we design this better I heard it here here yeah so a

better I heard it here here yeah so a loop a loop some kind of cycle that says

loop a loop some kind of cycle that says do that again do that again not

do that again do that again not infinitely many times necessarily but

infinitely many times necessarily but some finite number well you can perhaps

some finite number well you can perhaps see a spoiler on the screen under the

see a spoiler on the screen under the same uh orange control category is a

same uh orange control category is a repeat block and by default it's

repeat block and by default it's proposing 10 but we can change that so

proposing 10 but we can change that so let me do this I'm going to throw away

let me do this I'm going to throw away most of this copy paste as redundant I'm

most of this copy paste as redundant I'm going to detach this temporarily just to

going to detach this temporarily just to make room for something else and I'm

make room for something else and I'm going to drag a repeat block over here

going to drag a repeat block over here and let that snap into place and I'm

and let that snap into place and I'm going to change it for now just to be

going to change it for now just to be three for consistency and this is the

three for consistency and this is the correct shape even though it's too small

correct shape even though it's too small but scratch will accommodate that for us

but scratch will accommodate that for us and now same uh same output but arguably

and now same uh same output but arguably better designed why because if I want to

better designed why because if I want to change the number of meows I change it

change the number of meows I change it in one place no copy paste messiness if

in one place no copy paste messiness if I want to change the waiting one place I

I want to change the waiting one place I don't have to change it in multiple

don't have to change it in multiple places and not screw up so let me hit

places and not screw up so let me hit the green

flag all right so nice now it would have been nice if MIT had just given us a

been nice if MIT had just given us a meow block that just automates all of

meow block that just automates all of this for us let me wager they gave us

this for us let me wager they gave us the low-level implementation details

the low-level implementation details they gave us the play sound meow but I

they gave us the play sound meow but I had to implement like a decent number of

had to implement like a decent number of blocks just to get a cat to meow again

blocks just to get a cat to meow again and again I feel like we should have

and again I feel like we should have gotten that for free from MIT well they

gotten that for free from MIT well they don't have to be the only ones that

don't have to be the only ones that invent blocks for us to use you can

invent blocks for us to use you can write your own functions your own verbs

write your own functions your own verbs or actions so how can we do this let's

or actions so how can we do this let's make our own puzzle piece called now

make our own puzzle piece called now that uses this code but creates it in

that uses this code but creates it in such a way that it's reusable elsewhere

such a way that it's reusable elsewhere so let me do this under my blocks in

so let me do this under my blocks in pink here I'm going to go ahead and

pink here I'm going to go ahead and click literally make a block now here's

click literally make a block now here's an interface via which I can give the

an interface via which I can give the block a name Meo W will be the name of

block a name Meo W will be the name of this block and I'm just going to go

this block and I'm just going to go ahead and quickly click okay that just

ahead and quickly click okay that just gives me a very generic pink puzzle

gives me a very generic pink puzzle piece that starts with the word Define

piece that starts with the word Define because scratch is asking me to Define

because scratch is asking me to Define that is implement or create this new

that is implement or create this new puzzle piece for me well what does it

puzzle piece for me well what does it mean to meow I'm going to claim that it

mean to meow I'm going to claim that it means to do these two steps to play the

means to do these two steps to play the sound meow and then just wait for one

sound meow and then just wait for one second but what's powerful about this

second but what's powerful about this idea is look at this up top now that

idea is look at this up top now that I've made a block it exists in scratch D

I've made a block it exists in scratch D MIT didn't need to create this for me I

MIT didn't need to create this for me I created it for myself and even you if we

created it for myself and even you if we end up sharing code so I can now drag

end up sharing code so I can now drag meow up in here and what's nice about

meow up in here and what's nice about meow is that itself is yes a function

meow is that itself is yes a function but it's also an abstraction like never

but it's also an abstraction like never again do I or even you need to worry or

again do I or even you need to worry or care about what it means to meow or

care about what it means to meow or implement it I can sort of drag it out

implement it I can sort of drag it out of the way I didn't delete it drag it

of the way I didn't delete it drag it out of the way out of sight out of mind

out of the way out of sight out of mind why because my code is now even better

why because my code is now even better designed in some sense because it's more

designed in some sense because it's more readable what is it doing when the green

readable what is it doing when the green flag is clicked repeat three times meow

flag is clicked repeat three times meow it just says what it means and so it's a

it just says what it means and so it's a lot easier to read it and it's a lot

lot easier to read it and it's a lot easier to think about it especially if

easier to think about it especially if you're using Meow in other uh projects

you're using Meow in other uh projects too now let me go ahead and right hit

too now let me go ahead and right hit click play same thing so it's not really

click play same thing so it's not really fundamentally any different but I can

fundamentally any different but I can make this custom puzzle piece this own

make this custom puzzle piece this own function of M meow even more powerful

function of M meow even more powerful let me kind of rewind a bit and go to my

let me kind of rewind a bit and go to my meow puzzle piece and I am going to

meow puzzle piece and I am going to control click or right click on my pink

control click or right click on my pink puzzle piece and I'm going to edit it so

puzzle piece and I'm going to edit it so I kind of regret making meow so simple

I kind of regret making meow so simple wouldn't it be nice if meow took an

wouldn't it be nice if meow took an input AKA an argument that tells meow

input AKA an argument that tells meow how many times to meow then I can get

how many times to meow then I can get rid of that Loop and just tell meow how

rid of that Loop and just tell meow how many meows I actually want so I'm going

many meows I actually want so I'm going to click on another button here called

to click on another button here called literally add an input and it's going to

literally add an input and it's going to have placeholder here so I'm just going

have placeholder here so I'm just going to put a placeholder there I keep using

to put a placeholder there I keep using n for number which is a go-to in

n for number which is a go-to in computer scientist terms um and I'm

computer scientist terms um and I'm going to add some descriptive text just

going to add some descriptive text just so that it's a little more

so that it's a little more self-explanatory I'm just going to say

self-explanatory I'm just going to say meow n times but there's only one oval

meow n times but there's only one oval times is just going to be explanatory

times is just going to be explanatory text and now notice what has happened

text and now notice what has happened now my puzzle piece takes an input AKA

now my puzzle piece takes an input AKA an argument that will tell that function

an argument that will tell that function to meow some number of times but it's

to meow some number of times but it's not just going to going to work

not just going to going to work magically I need to implement that lower

magically I need to implement that lower level detail so let me zoom out I have

level detail so let me zoom out I have to remind myself what this function was

to remind myself what this function was so I'm going to drag it higher up just

so I'm going to drag it higher up just so they're on the screen at the same

so they're on the screen at the same time I'm going to go ahead now and

time I'm going to go ahead now and temporarily move this over here I'm

temporarily move this over here I'm going to temporarily detach this over

going to temporarily detach this over here why because what I think I want to

here why because what I think I want to do is move my Loop into the function

do is move my Loop into the function itself move the play and the weight into

itself move the play and the weight into the loop but I don't want to hardcode

the loop but I don't want to hardcode three notice that n here is its own oval

three notice that n here is its own oval I can drag a copy of N and just let it

I can drag a copy of N and just let it go there so now I have a new version of

go there so now I have a new version of meow that takes an argument in that

meow that takes an argument in that tells meow how many times to meow and

tells meow how many times to meow and now let me again drag this out of sight

now let me again drag this out of sight out of mind because who cares how I

out of mind because who cares how I implemented it once it's implemented

implemented it once it's implemented it's sort of done now my program is even

it's sort of done now my program is even better designed in some sense why

better designed in some sense why because now it really just says what it

because now it really just says what it means there's no Loop there's no repeat

means there's no Loop there's no repeat no like implementation details when

no like implementation details when green flag clicked meow three times and

green flag clicked meow three times and so functions indeed let you implement

so functions indeed let you implement algorithms like they're just code that

algorithms like they're just code that do something for you but they're also

do something for you but they're also themselves abstractions why because once

themselves abstractions why because once a function exists it has a name and you

a function exists it has a name and you can think about it in that term and you

can think about it in that term and you can use it by its name you don't have to

can use it by its name you don't have to care or remember how the function itself

care or remember how the function itself was built whether it's by you or even

was built whether it's by you or even MIT so again here I'll click the green

MIT so again here I'll click the green flag it's the same

thing so still correct but better and better designed and so anytime here and

better designed and so anytime here and out with scratch or soon C and

out with scratch or soon C and eventually python when you find yourself

eventually python when you find yourself doing anything resembling copy paste or

doing anything resembling copy paste or again and again grabbing the same code

again and again grabbing the same code probably an opportunity to say wait a

probably an opportunity to say wait a minute let me refactor this so to speak

minute let me refactor this so to speak that is rip out the code that seems to

that is rip out the code that seems to be repeated again and again and put it

be repeated again and again and put it in its own function so you can give it a

in its own function so you can give it a descriptive name and use and reuse it

descriptive name and use and reuse it any questions just yet on now saying or

any questions just yet on now saying or these Loops or these functions that

these Loops or these functions that we're using yeah

we're using yeah [Music]

[Music] how did I make it so it meows three

how did I make it so it meows three times so I originally only had a puzzle

times so I originally only had a puzzle piece called meow and I decided to

piece called meow and I decided to improve it so I held down control and I

improve it so I held down control and I right clicked or control clicked on the

right clicked or control clicked on the pink puzzle piece at top left and I

pink puzzle piece at top left and I clicked edit and that brought back the

clicked edit and that brought back the original interface that lets me add some

original interface that lets me add some arguments to the puzzle piece itself and

arguments to the puzzle piece itself and I clicked add an input on the left here

I clicked add an input on the left here and then I clicked on add a label over

and then I clicked on add a label over here so that just lets you customize it

here so that just lets you customize it even further all right so we've done

even further all right so we've done this let's add one of those other

this let's add one of those other Primitives too to do something

Primitives too to do something optionally so how about we make the cat

optionally so how about we make the cat meow only if it's being petted by a

meow only if it's being petted by a human as by moving the mouse to hover

human as by moving the mouse to hover over the cat like a human would pet a

over the cat like a human would pet a cat well let me go ahead and throw uh

cat well let me go ahead and throw uh away the meowing uh for now and let me

away the meowing uh for now and let me simplify it by just using a sound I'm

simplify it by just using a sound I'm going to go ahead and do this I'm going

going to go ahead and do this I'm going to go ahead and have a control block

to go ahead and have a control block that says if because I want to implement

that says if because I want to implement the idea of if the cursor is touching

the idea of if the cursor is touching the Cal

the Cal then play sound meow or I could use my

then play sound meow or I could use my same pink puzzle piece but I'm going to

same pink puzzle piece but I'm going to throw that away and focus only now on

throw that away and focus only now on the sounds and I'm going to do this uh

the sounds and I'm going to do this uh if uh touching Mouse pointer so I need

if uh touching Mouse pointer so I need to sense something about the world and

to sense something about the world and we saw this earlier so if touching Mouse

we saw this earlier so if touching Mouse pointer so notice this shape here way

pointer so notice this shape here way too big but it is the right shape so if

too big but it is the right shape so if I hover just right it'll snap into place

I hover just right it'll snap into place and this now in blue is my Boolean

and this now in blue is my Boolean expression a yes no question true false

expression a yes no question true false uh if is a conditional and what do I

uh if is a conditional and what do I want to do well if the Cat is being uh

want to do well if the Cat is being uh is touching the mouse pointer I want to

is touching the mouse pointer I want to go ahead and play sound meow until done

go ahead and play sound meow until done so let's do this I'm going to hit green

so let's do this I'm going to hit green flag click now nothing's happened yet

flag click now nothing's happened yet because it's a conditional right it's

because it's a conditional right it's only supposed to do something if I'm

only supposed to do something if I'm touching the cat let me move the cursor

touching the cat let me move the cursor over to the cat and and and wait for

over to the cat and and and wait for it h another bug why is the cat not

it h another bug why is the cat not meowing even though I very explicitly

meowing even though I very explicitly said if touch in Mouse pointer meow yeah

said if touch in Mouse pointer meow yeah in the

middle yeah this is again my computers are so darn fast like yours I click the

are so darn fast like yours I click the green flag it asked the question am I

green flag it asked the question am I touching the mouse pointer well no cuz

touching the mouse pointer well no cuz my cursor was up there not touching the

my cursor was up there not touching the cat it's too late the cat's out of the

cat it's too late the cat's out of the bag and so we have to instead solve this

bag and so we have to instead solve this some other mean by some other means how

some other mean by some other means how can we fix this how do we fix that sort

can we fix this how do we fix that sort of race yeah yeah so why don't we just

of race yeah yeah so why don't we just keep asking the question until I

keep asking the question until I eventually am I'm not actually petting

eventually am I'm not actually petting the cat so let me detach this

the cat so let me detach this temporarily let me go under control let

temporarily let me go under control let me go under instead of repeat some

me go under instead of repeat some finite number of times let's just do it

finite number of times let's just do it forever so sometimes Loops that do work

forever so sometimes Loops that do work forever are a good thing like the clock

forever are a good thing like the clock on your phone that's in a loop forever

on your phone that's in a loop forever because you want it to always tell time

because you want it to always tell time and not stop at the end of the day so

and not stop at the end of the day so sometimes you do want code to Loop

sometimes you do want code to Loop forever as in this case so let me go

forever as in this case so let me go ahead and drag and drop it there let me

ahead and drag and drop it there let me again click the green flag nothing's

again click the green flag nothing's happening yet but notice the program's

happening yet but notice the program's still running and so if if I move my

still running and so if if I move my cursor move my cursor move my cursor

cursor move my cursor move my cursor and okay so maybe we could add some

and okay so maybe we could add some waiting but the cat does not want to be

waiting but the cat does not want to be pet in this case but it's indeed

pet in this case but it's indeed conditional so there we have an

conditional so there we have an incarnation in scratch of doing

incarnation in scratch of doing something conditionally now we can make

something conditionally now we can make this really cool really fast if you will

this really cool really fast if you will let me stop this version let me go ahead

let me stop this version let me go ahead and do this uh let me go ahead and throw

and do this uh let me go ahead and throw all of this away let me go into my

all of this away let me go into my little uh extensions Bucket over here

little uh extensions Bucket over here and let me do video sensing since most

and let me do video sensing since most uh laptops or phones these days have

uh laptops or phones these days have cameras and there indeed I am with

cameras and there indeed I am with Sanders behind me and let me do this um

Sanders behind me and let me do this um when video motion and let me get out of

when video motion and let me get out of the way when video motion is greater

the way when video motion is greater than some value so 10 is the default

than some value so 10 is the default this is just a number that measures how

this is just a number that measures how much motion there is or isn't so small

much motion there is or isn't so small number is like no motion big number is

number is like no motion big number is lots of motion so I'm going to choose 50

lots of motion so I'm going to choose 50 somewhat arbitrarily here so 50 this is

somewhat arbitrarily here so 50 this is not normal to program off to the side

not normal to program off to the side but I'm now going to say this when video

but I'm now going to say this when video motion is 50 go ahead and play sound

motion is 50 go ahead and play sound meow like this so the cat is still in

meow like this so the cat is still in that world I'm going to stop the program

that world I'm going to stop the program and rerun it so here we go green flag

and rerun it so here we go green flag and

and now here come all right this is a little

now here come all right this is a little creepy the way I'm petting the cat

creepy the way I'm petting the cat but

and ah okay there we go okay so 50 was too big

okay there we go okay so 50 was too big of a number I have to pet the cat

know yeah so okay so you can make things even more

so okay so you can make things even more interactive in this way by just

interactive in this way by just assembling different puzzle pieces and

assembling different puzzle pieces and honestly there are so many different

honestly there are so many different puzzle pieces in here we're not going to

puzzle pieces in here we're not going to even scratch the surface of a lot of

even scratch the surface of a lot of them but they generally just do what

them but they generally just do what they say and indeed when you see on the

they say and indeed when you see on the screen here um this pallet of puzzle

screen here um this pallet of puzzle pieces really a lot of programming

pieces really a lot of programming especially early on when learning a

especially early on when learning a language is just trying different things

language is just trying different things and try and fail and if it doesn't work

and try and fail and if it doesn't work quite look for doesn't work quite right

quite look for doesn't work quite right look for an alternative solution there

look for an alternative solution there too as even I just had to do a moment

too as even I just had to do a moment ago well let's go ahead and use actually

ago well let's go ahead and use actually how about another example of something a

how about another example of something a predecessor of yours made let me go

predecessor of yours made let me go ahead and grab a program I opened in

ahead and grab a program I opened in advance here uh called wacka um might we

advance here uh called wacka um might we get a brave volunteer to come up who is

get a brave volunteer to come up who is willing to whack a mole with their head

willing to whack a mole with their head virtually maybe okay let's see how about

virtually maybe okay let's see how about in way back you want to come on down all

in way back you want to come on down all right come on down and in just a sure

right come on down and in just a sure Round of Applause for our

have come on down there what's your name I'm Josh oh actually say it into the

I'm Josh oh actually say it into the microphone hi I'm Josh

microphone hi I'm Josh okay nice welcome Josh come on

okay nice welcome Josh come on over all right so same idea here I'll

over all right so same idea here I'll take the mic back you can you'll have to

take the mic back you can you'll have to stand in front of the camera in just a

stand in front of the camera in just a moment you're going to have to position

moment you're going to have to position your head in a box that your classmate

your head in a box that your classmate from yester year

created and we'll start with beginner okay so line your head up in the Box in

okay so line your head up in the Box in a moment all right all

a moment all right all [Music]

[Music] [Applause]

[Music] nice 12

seconds 5 Seconds notice the score is up to 18 already pretty

to 18 already pretty good all right a round of applause for

good all right a round of applause for Josh if you

can so notice how using some fairly simple Primitives things do get

simple Primitives things do get interesting pretty fast and how was that

interesting pretty fast and how was that implemented well there were probably at

implemented well there were probably at least four Sprites so you're not re

least four Sprites so you're not re confined to just one cat you can create

confined to just one cat you can create more and more Sprites change what they

more and more Sprites change what they look like so they actually look like a

look like so they actually look like a mole in this case there's probably some

mole in this case there's probably some conditionals in there Some Loops for 30

conditionals in there Some Loops for 30 seconds that's checking if Josh's head's

seconds that's checking if Josh's head's movement is exceeding some value over

movement is exceeding some value over this way or over this way then increment

this way or over this way then increment something called a variable we'll see

something called a variable we'll see those two just like in algebra you might

those two just like in algebra you might have X and Y and Z storing values like

have X and Y and Z storing values like numbers so can computer programs have

numbers so can computer programs have variables called X or Y or Z or more

variables called X or Y or Z or more descriptively called score as in this

descriptively called score as in this case at top right or another variable

case at top right or another variable called countdown typically one word in

called countdown typically one word in code but in this case two words that

code but in this case two words that just store some value so there's

just store some value so there's probably some math going on in there

probably some math going on in there whereby the author of this program just

whereby the author of this program just is incrementing that is adding one in

is incrementing that is adding one in one every time it detected that uh a

one every time it detected that uh a mole had been whacked in this case with

mole had been whacked in this case with movement so back in the day I myself

movement so back in the day I myself actually implemented my very first

actually implemented my very first program in scratch when I was at a

program in scratch when I was at a graduate student actually at MIT um cost

graduate student actually at MIT um cost registered at MIT taking a class from

registered at MIT taking a class from mit's media lab specifically the

mit's media lab specifically the lifelong kindergarten Group which is the

lifelong kindergarten Group which is the group that created scratch itself and

group that created scratch itself and the program I wrote all those years ago

the program I wrote all those years ago and still rather cling to is a little

and still rather cling to is a little something here called Oscar time that I

something here called Oscar time that I thought I'd play just a quick excerpt of

thought I'd play just a quick excerpt of my myself here so in this case consider

my myself here so in this case consider as the music starts playing how this

as the music starts playing how this program which is much more sophisticated

program which is much more sophisticated certainly than the earliest say hello

certainly than the earliest say hello examples we just did might also be

examples we just did might also be implemented let me go ahead now and

implemented let me go ahead now and click the green

flag so some trash is moving presumably in some kind of loop from the

in some kind of loop from the Top If I'm touching the mouse cursor it

Top If I'm touching the mouse cursor it follows

follows me if I hover over the trash can it

me if I hover over the trash can it responds if I let

responds if I let go in some kind of loop Oscar pops out

go in some kind of loop Oscar pops out creates a variable with the current

creates a variable with the current score and it happens

but I don't need to keep playing this up on stage in front of everyone so my

on stage in front of everyone so my score is already now up to some six or

score is already now up to some six or so but in a moment two you'll see that

so but in a moment two you'll see that it's going to escalate so I'm taking

it's going to escalate so I'm taking into account some time apparently so

now so more and more Sprites are suddenly appearing and notice that each

suddenly appearing and notice that each time they're appearing from a different

time they're appearing from a different part of the screen that's an illusion

part of the screen that's an illusion perhaps do that pick a random number

perhaps do that pick a random number between X and Y so you can actually pick

between X and Y so you can actually pick some range of values to have the game

some range of values to have the game constantly changing and indeed I'm going

constantly changing and indeed I'm going to go ahead and click stop since i'

to go ahead and click stop since i' spent like 8 hours plus years ago making

spent like 8 hours plus years ago making this and I can never listen to the song

this and I can never listen to the song again not that I should be anyway at

again not that I should be anyway at this point in my life but this song is

this point in my life but this song is uh synchronized in with a lot of the

uh synchronized in with a lot of the actions that's happening and ultimately

actions that's happening and ultimately there's just a lot of building blocks

there's just a lot of building blocks but I didn't sit down and Implement

but I didn't sit down and Implement Oscar time as I called it all at once I

Oscar time as I called it all at once I really did take baby steps so to speak

really did take baby steps so to speak and I figured out well how could I

and I figured out well how could I decompose this Vision I had at the time

decompose this Vision I had at the time to create this game ultimately and how

to create this game ultimately and how do I bite off maybe the easiest Parts

do I bite off maybe the easiest Parts first and honestly the first thing I did

first and honestly the first thing I did was I found this image and I just like

was I found this image and I just like dragged and dropped it into scratch okay

dragged and dropped it into scratch okay done like lamp post is installed it

done like lamp post is installed it doesn't do anything it's not interactive

doesn't do anything it's not interactive but I at least set the stage so to speak

but I at least set the stage so to speak for the program then what else might I

for the program then what else might I have done well let me do this let me go

have done well let me do this let me go ahead and open up uh in another editor

ahead and open up uh in another editor here a early incarnation of Oscar Time

here a early incarnation of Oscar Time by doing this let me go into Oscar time

by doing this let me go into Oscar time here let me full screen this and here

here let me full screen this and here you have let me hide the trash for just

you have let me hide the trash for just a moment is what I might call the second

a moment is what I might call the second uh version of my program wherein at the

uh version of my program wherein at the top right of the stage here I had the

top right of the stage here I had the lamp post which I just dragged and

lamp post which I just dragged and dropped and got going but then I added

dropped and got going but then I added an actual Sprite and it has to be a

an actual Sprite and it has to be a Sprite if you wanted to do things

Sprite if you wanted to do things interactively the lamp post not a Sprite

interactively the lamp post not a Sprite it's just an image a costume if you will

it's just an image a costume if you will for the whole stage itself a backdrop

for the whole stage itself a backdrop but this thing is indeed a Sprite

but this thing is indeed a Sprite because it needs to respond to code and

because it needs to respond to code and events like dragging and dropping so

events like dragging and dropping so what might I have done early on with

what might I have done early on with that code well maybe the first version

that code well maybe the first version would have been something like this

would have been something like this where by my very first version of Oscar

where by my very first version of Oscar time might have said something like oh

time might have said something like oh this how about let me control the

this how about let me control the program as before or rather events when

program as before or rather events when the green flag is clicked what do I want

the green flag is clicked what do I want to do well I want to go ahead and

to do well I want to go ahead and forever do something like this uh

forever do something like this uh forever so I want the lid to open up if

forever so I want the lid to open up if I touch it so if the cursor gets near

I touch it so if the cursor gets near the lid I want the lid to open up and

the lid I want the lid to open up and then if I move away I want it to close

then if I move away I want it to close so how can I do that I want an if but I

so how can I do that I want an if but I just don't want one question I really

just don't want one question I really want two a fork in the road that goes

want two a fork in the road that goes left or right so to speak and let me

left or right so to speak and let me grab this puzzle piece here as I did

grab this puzzle piece here as I did long ago so notice it grows to fill

long ago so notice it grows to fill what's the question I want to ask well

what's the question I want to ask well under sensing I'm going to go ahead here

under sensing I'm going to go ahead here and say if this trash can is touching

and say if this trash can is touching the mouse pointer what do I want to do

the mouse pointer what do I want to do well I want to change what the trash can

well I want to change what the trash can looks like and this part I did in

looks like and this part I did in advance of class if you go up here to

advance of class if you go up here to costumes this is where all the graphical

costumes this is where all the graphical stuff happens and you'll see that I

stuff happens and you'll see that I imported a whole bunch of different

imported a whole bunch of different costumes that effectively much like a

costumes that effectively much like a video when you play them quickly creates

video when you play them quickly creates the illusion of movement some animation

the illusion of movement some animation but it's really just dot dot dot dot dot

but it's really just dot dot dot dot dot different images showing on the screen

different images showing on the screen well some of these costumes are called

well some of these costumes are called like Oscar 1 Oscar 2 Oscar 1 is closed

like Oscar 1 Oscar 2 Oscar 1 is closed Oscar 2 is open so let's just deal with

Oscar 2 is open so let's just deal with those first so if I'm touching the mouse

those first so if I'm touching the mouse pointer let me go under how about looks

pointer let me go under how about looks and we didn't use this before but

and we didn't use this before but there's this block switch costume to

there's this block switch costume to something else I'm going to drag and

something else I'm going to drag and drop this inside of the if and notice

drop this inside of the if and notice it's a little bit indented I'm going to

it's a little bit indented I'm going to change it not to Oscar 8 but Oscar 2

change it not to Oscar 8 but Oscar 2 otherwise if not touching the mouse

otherwise if not touching the mouse pointer this is the other direction in

pointer this is the other direction in the fork and the road let's go ahead and

the fork and the road let's go ahead and switch the costume back to what I

switch the costume back to what I described as Oscar 1 so let me run this

described as Oscar 1 so let me run this program and not much of interest is

program and not much of interest is happening yet but notice if I move the

happening yet but notice if I move the cursor up down but how is that working

cursor up down but how is that working it's just changing the costume that's

it's just changing the costume that's being overlaid on the Sprite so it looks

being overlaid on the Sprite so it looks like interactivity but you're really

like interactivity but you're really just changing the Aesthetics and we

just changing the Aesthetics and we humans are just kind of you know

humans are just kind of you know assuming oh it's opening up well no it's

assuming oh it's opening up well no it's just changing a costume so here's the

just changing a costume so here's the difference the high level abstraction

difference the high level abstraction trash can opening the lower level

trash can opening the lower level implementation detail

implementation detail costume changing creating that illusion

costume changing creating that illusion and if I wanted to look prettier I could

and if I wanted to look prettier I could just have many other costumes and go

just have many other costumes and go boom boom boom boom boom to create more

boom boom boom boom boom to create more frames per second if you will so I need

frames per second if you will so I need to do um one other thing maybe if I

to do um one other thing maybe if I accidentally leave the trash can open

accidentally leave the trash can open let me make one change here let me make

let me make one change here let me make sure that the very first thing I do when

sure that the very first thing I do when the green flag is clicked is always

the green flag is clicked is always start with the trash can closed because

start with the trash can closed because otherwise you might accidentally leave

otherwise you might accidentally leave it open so this gets me into some

it open so this gets me into some default state so now it's always closed

default state so now it's always closed until I manually hover over it instead

until I manually hover over it instead well what might I have done next well if

well what might I have done next well if I wanted to introduce something like the

I wanted to introduce something like the trash I need a second Sprite and here in

trash I need a second Sprite and here in advance I grabb the image already let me

advance I grabb the image already let me pretend that this never happened let me

pretend that this never happened let me drag this away here and now I have

drag this away here and now I have nothing in my code area for this piece

nothing in my code area for this piece of trash but it is a second Sprite and

of trash but it is a second Sprite and all I did was I clicked on the little

all I did was I clicked on the little cat plus icon here created a second

cat plus icon here created a second Sprite I named it trash I added an

Sprite I named it trash I added an costume for it sort of the aesthetic

costume for it sort of the aesthetic stuff I did in advance but here I'll do

stuff I did in advance but here I'll do now the code how do I want to do this

now the code how do I want to do this well how about when the green flag is

well how about when the green flag is clicked for the trash can I want the

clicked for the trash can I want the trash can in parallel to do or I want

trash can in parallel to do or I want the trash the piece of trash to do its

the trash the piece of trash to do its own thing so what I wanted to do is

own thing so what I wanted to do is maybe let's do motion how about and

maybe let's do motion how about and let's go to a specific coordinate now

let's go to a specific coordinate now there's a lot of options here there's

there's a lot of options here there's turning go to a random position go to x

turning go to a random position go to x comma y Glide more elegantly there's a

comma y Glide more elegantly there's a lot of different ways to implement

lot of different ways to implement movement I just wanted to go to a very

movement I just wanted to go to a very specific location first so I'm just

specific location first so I'm just going to go to x comma y first and I'm

going to go to x comma y first and I'm going to say x how about will be um uh

going to say x how about will be um uh let's not hardcode this let's just have

let's not hardcode this let's just have it be well let's do it at zero initially

it be well let's do it at zero initially and then 240 so whoops let's do 0 comma

and then 240 so whoops let's do 0 comma 240 so that this piece of trash always

240 so that this piece of trash always starts at the top middle of the screen

starts at the top middle of the screen if you think back to that coordinate

if you think back to that coordinate system 0 0 is in the middle 240 is

system 0 0 is in the middle 240 is straight above it all right now after I

straight above it all right now after I do that what do I want to do well how

do that what do I want to do well how about I control this thing by forever

about I control this thing by forever falling now how do I make the trash move

falling now how do I make the trash move we haven't seen this puzzle piece yet

we haven't seen this puzzle piece yet but under motion the very first thing is

but under motion the very first thing is called move some number of steps by

called move some number of steps by default it's 10 but we'll do it more

default it's 10 but we'll do it more simply let me go ahead and

simply let me go ahead and move uh oh sorry move is going to move

move uh oh sorry move is going to move it uh in whatever Direction it's facing

it uh in whatever Direction it's facing I only want it to move down so here even

I only want it to move down so here even I'm getting confused as to how many

I'm getting confused as to how many different ways there are to do things

different ways there are to do things what I think I want to do is this let me

what I think I want to do is this let me only change my y AIS as follows so

only change my y AIS as follows so here's another puzzle piece called

here's another puzzle piece called change y so again Y is the vertical so

change y so again Y is the vertical so let me just change y by one pixel

let me just change y by one pixel downward at a time so Nega one one pixel

downward at a time so Nega one one pixel at a time so it's kind of slow and I

at a time so it's kind of slow and I think now I think that's it let me hit

think now I think that's it let me hit stop notice that my trash can is still

stop notice that my trash can is still going to be interactive I haven't

going to be interactive I haven't changed or deleted that code I've just

changed or deleted that code I've just added now code for my trash piece of

added now code for my trash piece of trash if I click the green flag notice

trash if I click the green flag notice that after I enable it let me start that

that after I enable it let me start that again I had it hidden for before class

again I had it hidden for before class but let me enable it now green flag

but let me enable it now green flag notice it starts dead center at xal 0 y

notice it starts dead center at xal 0 y = 240 and it's dropping one pixel at a

= 240 and it's dropping one pixel at a time if that seems a little boring we

time if that seems a little boring we can change it to negative 10 pixels at a

can change it to negative 10 pixels at a time and boom it's done so that's how

time and boom it's done so that's how you might change the speed of a program

you might change the speed of a program but I'm going to leave it more simply as

but I'm going to leave it more simply as negative 1 and honestly it would be nice

negative 1 and honestly it would be nice if it doesn't always start from the top

if it doesn't always start from the top otherwise this game is not going to be

otherwise this game is not going to be very interactive I'm literally going to

very interactive I'm literally going to be grabbing the trash from the same

be grabbing the trash from the same place every time so why don't I instead

place every time so why don't I instead stop this let me go under operators and

stop this let me go under operators and let's pick our random number so let me

let's pick our random number so let me change the hardcoded the manually

change the hardcoded the manually inputed zero and let's make X be

inputed zero and let's make X be somewhere between zero so in the middle

somewhere between zero so in the middle and all the way over to uh what was it

and all the way over to uh what was it one oh I got my numbers wrong 240 and my

one oh I got my numbers wrong 240 and my y will be 180 sorry I got my X and my my

y will be 180 sorry I got my X and my my y confused so let me play this again and

y confused so let me play this again and now we have a game that's more like

now we have a game that's more like games you might have played growing up

games you might have played growing up or even now like there's some Randomness

or even now like there's some Randomness to it so the CPU so to speak is doing

to it so the CPU so to speak is doing something more interesting let me run it

something more interesting let me run it again now it's a little to the left let

again now it's a little to the left let me run it again now it's a little more

me run it again now it's a little more to the left again now it's back to the

to the left again now it's back to the right so Randomness just makes games

right so Randomness just makes games more interesting and this is why when

more interesting and this is why when you play any video game if different

you play any video game if different things are happening there's probably

things are happening there's probably just some Randomness and it's quantized

just some Randomness and it's quantized is just a simple number now I think I

is just a simple number now I think I just need one final flourish here if I

just need one final flourish here if I may let me go ahead and add this how

may let me go ahead and add this how about uh events or rather yes events

about uh events or rather yes events when green flag is clicked I can do

when green flag is clicked I can do multiple things within the same Sprite

multiple things within the same Sprite they don't all have to be attached to

they don't all have to be attached to the same one let me go ahead and forever

the same one let me go ahead and forever go ahead and do something else how about

go ahead and do something else how about whenever the trash is how about touching

whenever the trash is how about touching the trash can so forever if let's see I

the trash can so forever if let's see I need a sensing block so how about is

need a sensing block so how about is touching uh not the mouse pointer this

touching uh not the mouse pointer this time but touching Oscar himself there

time but touching Oscar himself there now let's see what happens all right so

now let's see what happens all right so let's go ahead and click the green flag

let's go ahead and click the green flag now I go down over here and let

now I go down over here and let go okay that's I kind of want it to go

go okay that's I kind of want it to go into the trash can how do I make it go

into the trash can how do I make it go into the trash can how can we take this

into the trash can how can we take this high level idea put trash into the trash

high level idea put trash into the trash can and make it seem to

can and make it seem to disappear logically what could we do

disappear logically what could we do yeah okay so when it touches it let's

yeah okay so when it touches it let's have it disappear so I could hide it or

have it disappear so I could hide it or honestly if the game's going to be

honestly if the game's going to be ongoing like it was letting me Mo drop

ongoing like it was letting me Mo drop more and more trash let me just have it

more and more trash let me just have it go ahead and pick a new random location

go ahead and pick a new random location so let me do this let me go ahead and

so let me do this let me go ahead and copy this puzzle piece up here and

copy this puzzle piece up here and duplicate and I don't want the whole

duplicate and I don't want the whole thing sorry let me get rid of this let

thing sorry let me get rid of this let me just do this let me go back to some

me just do this let me go back to some random location at the top so now notice

random location at the top so now notice what happens if I click and drag on it

what happens if I click and drag on it here it goes and I let go it looks like

here it goes and I let go it looks like it's going into the trash can because it

it's going into the trash can because it snaps back up to some random location

snaps back up to some random location now the only thing I'm not doing really

now the only thing I'm not doing really is keeping track of any kind of score

is keeping track of any kind of score and it turns out if I full screen this

and it turns out if I full screen this it's not going to be draggable by

it's not going to be draggable by default so just as a corner case so to

default so just as a corner case so to speak something that you might trip over

speak something that you might trip over otherwise let me go ahead and under uh

otherwise let me go ahead and under uh let's see uh sensing it turns out I also

let's see uh sensing it turns out I also need this for the piece of trash there's

need this for the piece of trash there's this way of setting and scratch a puzzle

this way of setting and scratch a puzzle a Sprite to be draggable or not

a Sprite to be draggable or not draggable I need to explicitly make it

draggable I need to explicitly make it draggable so that when I do full screen

draggable so that when I do full screen this thing now it Still Remains

this thing now it Still Remains draggable and someone like myself can

draggable and someone like myself can play it again and again well how about

play it again and again well how about we supplement this with one final

we supplement this with one final flourish why don't we keep track now of

flourish why don't we keep track now of the user score so how about when the

the user score so how about when the user actually drags the piece of trash

user actually drags the piece of trash to the trash can let me go under

to the trash can let me go under variables here where in advance I've

variables here where in advance I've already made myself a variable called

already made myself a variable called score I could have called it X or Y or Z

score I could have called it X or Y or Z or ABC but that's not very descriptive

or ABC but that's not very descriptive in programming you typically give things

in programming you typically give things a more descriptive English or some other

a more descriptive English or some other language name so I called this one score

language name so I called this one score so how do I want to do this in my score

so how do I want to do this in my score well let me go ahead and initially set

well let me go ahead and initially set this game score to zero at the very top

this game score to zero at the very top of one of these Scripts

of one of these Scripts or one of these programs up here and

or one of these programs up here and then anytime my cursor my piece of trash

then anytime my cursor my piece of trash is touching Oscar let's not just jump to

is touching Oscar let's not just jump to the top let's change the score by one up

the top let's change the score by one up here so now notice if touching Oscar

here so now notice if touching Oscar change the score that is add one to the

change the score that is add one to the score and then pick a new random

score and then pick a new random location and now green flag let's do

location and now green flag let's do this slowly here it goes it's the trash

this slowly here it goes it's the trash can opens I let go and now notice at the

can opens I let go and now notice at the top left of my program notice the score

top left of my program notice the score is now two notice the score if I do this

is now two notice the score if I do this again is about to become three and so

again is about to become three and so here we have building blocks literally

here we have building blocks literally of making this program better and better

of making this program better and better and better and so indeed that's how you

and better and so indeed that's how you generally approach solving any large

generally approach solving any large program uh any problem with code be it

program uh any problem with code be it in scratch or C or python or some other

in scratch or C or python or some other you take this Vision you might have or

you take this Vision you might have or some Vision you've been assigned in a

some Vision you've been assigned in a homework assignment and try to break it

homework assignment and try to break it down into these constituent parts and

down into these constituent parts and just pluck off the easy ones first put

just pluck off the easy ones first put the lamp post there first and at least

the lamp post there first and at least feel like you're making some progress

feel like you're making some progress then pluck off something like the trash

then pluck off something like the trash can and just make it do a little thing

can and just make it do a little thing and it doesn't have to be in some same

and it doesn't have to be in some same order here I could have done this in a

order here I could have done this in a million different ways but figure out

million different ways but figure out what the small pieces are that

what the small pieces are that ultimately like a few of the problems

ultimately like a few of the problems we've solved today assemble into a

we've solved today assemble into a greater solution there too uh so that

greater solution there too uh so that you have now a mental model for these

you have now a mental model for these types of blocks and others let's return

types of blocks and others let's return for a moment to this we saw a moment ago

for a moment to this we saw a moment ago that when I started saying hello David

that when I started saying hello David and nesting those puzzle pieces we had a

and nesting those puzzle pieces we had a whole different Paradigm Al together my

whole different Paradigm Al together my input for that second version of hello

input for that second version of hello world was to now pass in for instance

world was to now pass in for instance what's your name into my function called

what's your name into my function called ask that gave me not a side effect but

ask that gave me not a side effect but what I called again a return value

what I called again a return value called answer by default in scratch and

called answer by default in scratch and now notice and recall when I had that

now notice and recall when I had that same output become the input to my next

same output become the input to my next block it looked a little something like

block it looked a little something like this say so how does this type of block

this say so how does this type of block and this nesting the stacking of blocks

and this nesting the stacking of blocks fit into the same mental model well same

fit into the same mental model well same idea my input for that part of the story

idea my input for that part of the story is now taking in not one input but two

is now taking in not one input but two two arguments hello and the answer from

two arguments hello and the answer from before the function in this case is that

before the function in this case is that new block called join the output thereof

new block called join the output thereof is hello David which itself became if we

is hello David which itself became if we sort of animate this the input to my

sort of animate this the input to my final function which indeed was still

final function which indeed was still say and this is only to say no pun

say and this is only to say no pun intended that almost everything that

intended that almost everything that that you do with these puzzle pieces be

that you do with these puzzle pieces be in the context of Oscar time or the mole

in the context of Oscar time or the mole whacking or even just something simple

whacking or even just something simple like hello world will ultimately fit

like hello world will ultimately fit into that relatively simple mental model

into that relatively simple mental model there now I thought we'd End by taking a

there now I thought we'd End by taking a look at just a couple final examples

look at just a couple final examples these ones two made by some of your

these ones two made by some of your predecessors and for this I thought we

predecessors and for this I thought we would not write code together but read

would not write code together but read it instead and so allow me to open up

it instead and so allow me to open up one other example here that will show us

one other example here that will show us a few different versions of a program

a few different versions of a program that a predecessor made give me just a

that a predecessor made give me just a moment here and we'll see how we might

moment here and we'll see how we might build up to something even more

build up to something even more interactive and in just a moment we'll

interactive and in just a moment we'll see something they called Ivy's uh

see something they called Ivy's uh hardest game focused here on these

hardest game focused here on these particular mechanics so here is version

particular mechanics so here is version zero so to speak of this program where

zero so to speak of this program where in the goal is to create a game where

in the goal is to create a game where you have to like get out of some kind of

you have to like get out of some kind of Maze and you have to get out in this

Maze and you have to get out in this case the Harvard Crest from this maze

case the Harvard Crest from this maze let me go ahead and just hit play on

let me go ahead and just hit play on this green flag so you can see what what

this green flag so you can see what what the first building block for this

the first building block for this program might have been notice that my

program might have been notice that my hand here is actually on the arrow keys

hand here is actually on the arrow keys of my keyboard and it seems that by

of my keyboard and it seems that by moving up down left or right this little

moving up down left or right this little Crest on the screen responds in exactly

Crest on the screen responds in exactly that way now let's hypothesize for just

that way now let's hypothesize for just a moment even though we've not done

a moment even though we've not done anything quite like this before how

anything quite like this before how might this code be implemented how do

might this code be implemented how do you get a Sprite be it a cat or a Crest

you get a Sprite be it a cat or a Crest to respond to keys on a keyboard might

to respond to keys on a keyboard might you think intuitively yeah there could

you think intuitively yeah there could be something that's sensing what key

be something that's sensing what key you're pressing on yeah there could be

you're pressing on yeah there could be something sensing what key you're

something sensing what key you're pressing on and if you do it again in a

pressing on and if you do it again in a forever Loop you'll just constantly be

forever Loop you'll just constantly be listening for keystrokes and this is how

listening for keystrokes and this is how like every piece of software nowadays

like every piece of software nowadays works it's constantly waiting for your

works it's constantly waiting for your phone to be tapped or something to be

phone to be tapped or something to be typed on the screen so let me go ahead

typed on the screen so let me go ahead and look inside of this existing program

and look inside of this existing program here and there's more going on but we'll

here and there's more going on but we'll take a quick glance what's actually

take a quick glance what's actually going on well up here at top left notice

going on well up here at top left notice we just have go to x equals 0 and Y

we just have go to x equals 0 and Y equals 0 that means put the Harvard

equals 0 that means put the Harvard Crest dead center in the middle of the

Crest dead center in the middle of the stage then we have forever two functions

stage then we have forever two functions that we made in advance as custom

that we made in advance as custom functions uh listen for keyboard feel

functions uh listen for keyboard feel for walls so it's doing two things at

for walls so it's doing two things at once it's forever listening for the

once it's forever listening for the keyboard up down left right and feeling

keyboard up down left right and feeling for the walls in the sense that if I get

for the walls in the sense that if I get too far to the left I don't want it to

too far to the left I don't want it to keep moving past that black wall and if

keep moving past that black wall and if it moves too far to the right I don't

it moves too far to the right I don't want it to blow through that wall either

want it to blow through that wall either so it's going to do two things

so it's going to do two things constantly listening for keyboard and

constantly listening for keyboard and feeling for walls so to speak and how

feeling for walls so to speak and how are those implemented well this one's a

are those implemented well this one's a bit long but on the left here is listen

bit long but on the left here is listen for keyboard so this pink puzzle piece

for keyboard so this pink puzzle piece listen for keyboard first checks if the

listen for keyboard first checks if the key up arrow is pressed question mark

key up arrow is pressed question mark Boolean expression in a conditional

Boolean expression in a conditional change y by one that means move it up

change y by one that means move it up one else if the key down arrow is

one else if the key down arrow is pressed then change y by negative one

pressed then change y by negative one and similar for left Arrow similar for

and similar for left Arrow similar for right arrow and even though there's not

right arrow and even though there's not a loop in this pink function there is

a loop in this pink function there is where I'm using it so it's constantly

where I'm using it so it's constantly being asked again and again how about

being asked again and again how about feeling for walls well over here to the

feeling for walls well over here to the right it's a little cut off but here you

right it's a little cut off but here you have if touching left wall change X by

have if touching left wall change X by one so if you hit the wall it's too late

one so if you hit the wall it's too late you're kind of blowing through it

you're kind of blowing through it already so I want to move it wh one

already so I want to move it wh one pixel so it's no longer touching that

pixel so it's no longer touching that wall similarly if it's touching the

wall similarly if it's touching the right wall I want to back it up one

right wall I want to back it up one pixel so it's no longer touching that

pixel so it's no longer touching that wall so it's kind of like bouncing off

wall so it's kind of like bouncing off ever so slightly so that it doesn't slip

ever so slightly so that it doesn't slip through that actual wall and what are

through that actual wall and what are those walls well noce down here it's

those walls well noce down here it's just a simple Sprite with a black line

just a simple Sprite with a black line that I've oriented hor vertically

that I've oriented hor vertically instead of horizontally and that's just

instead of horizontally and that's just so that I can ask questions of these

so that I can ask questions of these other two Sprites now that gives me that

other two Sprites now that gives me that form of interactivity what more can I

form of interactivity what more can I now do well what if we make things a

now do well what if we make things a little more interactive here let me go

little more interactive here let me go ahead and see inside version one our

ahead and see inside version one our second and let me propose what's going

second and let me propose what's going to happen here well how might we add a

to happen here well how might we add a little something like Yale into the mix

little something like Yale into the mix well what's Yale going to do when I I

well what's Yale going to do when I I hit the green flag now based on this

hit the green flag now based on this code any

code any hunches here is the code for my Yale

hunches here is the code for my Yale Sprite yeah yeah it's kind of got to be

Sprite yeah yeah it's kind of got to be an adversary by blocking my path

an adversary by blocking my path theoretically if I keep writing more

theoretically if I keep writing more code so why it to goes to the middle of

code so why it to goes to the middle of the screen it points in Direction 90 de

the screen it points in Direction 90 de so similarly there's a whole degree

so similarly there's a whole degree system as well and it forever asks this

system as well and it forever asks this if touching the left wall or notice the

if touching the left wall or notice the green block touching the right wall then

green block touching the right wall then just turn around 180° and in if you

just turn around 180° and in if you think this through logically that just

think this through logically that just means you're bouncing this way and this

means you're bouncing this way and this way by just flipping yourself around

way by just flipping yourself around 180° for just this Yale Sprite so if I

180° for just this Yale Sprite so if I go ahead and zoom in on this and click

go ahead and zoom in on this and click the green flag I can still move up and

the green flag I can still move up and down but Yale is just kind of doing this

down but Yale is just kind of doing this all day long back and forth and back and

all day long back and forth and back and forth forever nothing bad happens if I

forth forever nothing bad happens if I try to go through it but we could add

try to go through it but we could add that certainly to the mix in fact let's

that certainly to the mix in fact let's add one final feature before we play

add one final feature before we play this particular game and let me go ahead

this particular game and let me go ahead and open up the final version of these

and open up the final version of these building blocks that adds MIT to the mix

building blocks that adds MIT to the mix so here's

so here's MIT someone want to explain what this

MIT someone want to explain what this code does and this is what we're doing

code does and this is what we're doing this itself is a skill reading someone

this itself is a skill reading someone else's code and understanding it is half

else's code and understanding it is half of the part of programming besides

of the part of programming besides writing yeah yeah it's chasing down the

writing yeah yeah it's chasing down the Harvard logo outline so this is

Harvard logo outline so this is apparently the name of the costume that

apparently the name of the costume that this student made Harvard logo outline

this student made Harvard logo outline outline and apparently it goes to a

outline and apparently it goes to a random position first but then it

random position first but then it forever points to Har so no matter where

forever points to Har so no matter where I'm moving it up down left or right MIT

I'm moving it up down left or right MIT is being a little more strategic than

is being a little more strategic than Yale bouncing back and forth like this

Yale bouncing back and forth like this so let's go ahead and play this one in

so let's go ahead and play this one in full screen and here we have a green

full screen and here we have a green flag so if I move up

flag so if I move up MIT rather strategically is following me

MIT rather strategically is following me no matter where I go all right so still

no matter where I go all right so still nothing bad happens but and now it's

nothing bad happens but and now it's struggling right it's going up down up

struggling right it's going up down up down it's trying to uh follow me even

down it's trying to uh follow me even though I'm not moving so we need some

though I'm not moving so we need some final flourishes and so I think for this

final flourishes and so I think for this we need perhaps one final volunteer

we need perhaps one final volunteer after this cake awaits for everyone

after this cake awaits for everyone outside as is an end of first lecture

outside as is an end of first lecture cs50 tradition would you like to come up

cs50 tradition would you like to come up and be our our vinyl of

volunteer all right and so this will be the actual version bit written by one of

the actual version bit written by one of your predecessors that I'll full screen

your predecessors that I'll full screen here it's going to stitch together all

here it's going to stitch together all of these same Primitives in more but add

of these same Primitives in more but add the notion of score and lives so that

the notion of score and lives so that there's actually a goal which in this

there's actually a goal which in this case is to move the Harvard Crest to

case is to move the Harvard Crest to constantly pursue the character on the

constantly pursue the character on the right hand side so that your Sprite

right hand side so that your Sprite touches that one would you like to

touches that one would you like to introduce yourself uh hi my name is

introduce yourself uh hi my name is Muhammad all right wonderful welcome

Muhammad all right wonderful welcome aboard and here we come with some

aboard and here we come with some instructions and final flourish if we

instructions and final flourish if we want to keep the lights up but perhaps

want to keep the lights up but perhaps increase the

music [Music]

[Music] all right this is cs50 and this is week

all right this is cs50 and this is week one wherein we continue programming but

one wherein we continue programming but we do it in a different language because

we do it in a different language because recall last time we focused on this

recall last time we focused on this graphical language called scratch but we

graphical language called scratch but we use scratch uh not only because it's

use scratch uh not only because it's sort of fun and accessible but because

sort of fun and accessible but because it allows us to explore a lot of these

it allows us to explore a lot of these Concepts here Nam functions and

Concepts here Nam functions and conditionals Boolean Expressions Loops

conditionals Boolean Expressions Loops variables and more and so indeed even if

variables and more and so indeed even if today's syntax as we transition to this

today's syntax as we transition to this new language called C feels a little bit

new language called C feels a little bit cryptic maybe a little intimidating at

cryptic maybe a little intimidating at first and you don't quite see all of the

first and you don't quite see all of the meaning of the symbols beyond the syntax

meaning of the symbols beyond the syntax itself realized that the ideas are

itself realized that the ideas are ultimately going to be the same in fact

ultimately going to be the same in fact as we transition from what was last week

as we transition from what was last week a hello world program that looked a

a hello world program that looked a little something like this this week of

little something like this this week of course it's going to now look a little

course it's going to now look a little more cryptic it's going to look a little

more cryptic it's going to look a little something like this and now even if you

something like this and now even if you can't quite distinguish what all of the

can't quite distinguish what all of the various symbols mean in this code turns

various symbols mean in this code turns out that at the end of the day it's

out that at the end of the day it's indeed going to do what you expect it's

indeed going to do what you expect it's just going to say hello world on the

just going to say hello world on the screen just like we did in scratch so

screen just like we did in scratch so let's start to apply some terminology to

let's start to apply some terminology to these to these uh tokens first so what

these to these uh tokens first so what we're about to see what we're about to

we're about to see what we're about to write henceforth we're going to start

write henceforth we're going to start calling source code code that you the

calling source code code that you the human programmer write is just

human programmer write is just henceforth called source code doesn't

henceforth called source code doesn't matter if it's scratch doesn't matter if

matter if it's scratch doesn't matter if it's C doesn't matter if it's python

it's C doesn't matter if it's python before

before source code is the general term for

source code is the general term for really what you and I as human

really what you and I as human programmers will ultimately write of

programmers will ultimately write of course computers don't understand source

course computers don't understand source code it turns out computers don't

code it turns out computers don't understand scratch and puzzle pieces per

understand scratch and puzzle pieces per se or C code like we're about to see

se or C code like we're about to see they only understand this which we

they only understand this which we called what last week yeah so this is

called what last week yeah so this is binary zeros and ones but really it's

binary zeros and ones but really it's just information represented in binary

just information represented in binary and in fact the technical term now for

and in fact the technical term now for patterns of zeros and ones that a

patterns of zeros and ones that a computer not only understands

computer not only understands understands how to interpret as letters

understands how to interpret as letters or numbers or colors or images or more

or numbers or colors or images or more but knows how to execute as well

but knows how to execute as well henceforth is going to be called machine

henceforth is going to be called machine code to contrast it with source code so

code to contrast it with source code so whereas you and I the humans write

whereas you and I the humans write source code it's the computer that

source code it's the computer that ultimately only understands machine code

ultimately only understands machine code and even though we won't get into the

and even though we won't get into the details of exactly what pattern of

details of exactly what pattern of symbols means what you'll see that in

symbols means what you'll see that in this kind of pattern of zeros and ones

this kind of pattern of zeros and ones there's going to be numbers there's

there's going to be numbers there's going to be letters but there's also

going to be letters but there's also going to be instructions because indeed

going to be instructions because indeed computers are really good at doing

computers are really good at doing things addition subtraction moving

things addition subtraction moving things in and out of memory and suffice

things in and out of memory and suffice it to say that the Macs the PCS the

it to say that the Macs the PCS the other computers of the world have just

other computers of the world have just decided as a society what certain

decided as a society what certain patterns of zeros and ones mean when it

patterns of zeros and ones mean when it comes to operations as well so not just

comes to operations as well so not just data but instructions but those patterns

data but instructions but those patterns are not something we're going to focus

are not something we're going to focus on in a class like this we're going to

on in a class like this we're going to focus on the higher level software side

focus on the higher level software side of things simply assuming that we need

of things simply assuming that we need to somehow output machine code so it

to somehow output machine code so it turns out then that this problem we have

turns out then that this problem we have to solve getting from source code to

to solve getting from source code to machine code actually fits into the same

machine code actually fits into the same Paradigm as last time but the input in

Paradigm as last time but the input in this case is going to be source code on

this case is going to be source code on the one hand like that's what you and I

the one hand like that's what you and I ideally will write so that we don't have

ideally will write so that we don't have to write zeros and ones but we need to

to write zeros and ones but we need to somehow output machine code because

somehow output machine code because that's what your Macs PCS phones are

that's what your Macs PCS phones are actually going to understand well it

actually going to understand well it turns out there's special programs in

turns out there's special programs in life whose purpose is to do exactly this

life whose purpose is to do exactly this conversion convert the source code you

conversion convert the source code you and I write to the machine code that our

and I write to the machine code that our phones and computers understand and that

phones and computers understand and that type of program is going to be called a

type of program is going to be called a compiler so indeed today we'll introduce

compiler so indeed today we'll introduce you to another piece of software and

you to another piece of software and these come in many forms we'll use a

these come in many forms we'll use a popular one here that allows you to

popular one here that allows you to convert source code in C to machine code

convert source code in C to machine code in uh zeros and ones now you didn't have

in uh zeros and ones now you didn't have to do this with scratch in the world of

to do this with scratch in the world of scratch it was as simple as clicking the

scratch it was as simple as clicking the green flag because essentially MIT did

green flag because essentially MIT did all of the heavy lifting there figuring

all of the heavy lifting there figuring out how to convert these graphical

out how to convert these graphical puzzle pieces to the underlying machine

puzzle pieces to the underlying machine code but now starting today as we begin

code but now starting today as we begin to study programming and computer

to study programming and computer science proper now that power moves to

science proper now that power moves to you and it's up to you now to do that

you and it's up to you now to do that kind of conversion but thankfully the

kind of conversion but thankfully the fact that these compilers exist means

fact that these compilers exist means that you and I don't have to program in

that you and I don't have to program in machine code like our ancestors Once

machine code like our ancestors Once Upon a Time did be it virtually or with

Upon a Time did be it virtually or with physical Punch Cards like pieces of

physical Punch Cards like pieces of paper with holes in them you and I get

paper with holes in them you and I get to focus uh on our keyboard as such but

to focus uh on our keyboard as such but it's not just going to be a matter today

it's not just going to be a matter today of like writing code it's going to be a

of like writing code it's going to be a matter ultimately today onward of good

matter ultimately today onward of good code as well and this is the kind of

code as well and this is the kind of thing that you don't just learn

thing that you don't just learn overnight it takes time it takes

overnight it takes time it takes practice just like writing an essay in

practice just like writing an essay in any subject might take time and practice

any subject might take time and practice and iteration over time but in a

and iteration over time but in a programming class like cs50 we're going

programming class like cs50 we're going to Aspire to evaluate the quality of

to Aspire to evaluate the quality of code along these three axes generally is

code along these three axes generally is it correct first and foremost like does

it correct first and foremost like does the code do what it's supposed to do

the code do what it's supposed to do after all if it doesn't well what was

after all if it doesn't well what was the point of writing it in the first

the point of writing it in the first place so it sort of goes without saying

place so it sort of goes without saying that you want code you write to be

that you want code you write to be correct and it's obviously not always

correct and it's obviously not always again anytime you're Mac or PC or phone

again anytime you're Mac or PC or phone has crashed some human somewhere wrote

has crashed some human somewhere wrote buggy that is code with mistakes but C

buggy that is code with mistakes but C correctness is going to be the first and

correctness is going to be the first and foremost goal but then there's a more

foremost goal but then there's a more subjective goal we'll see in time a

subjective goal we'll see in time a matter of design and we saw a little bit

matter of design and we saw a little bit of this last week when I proposed that

of this last week when I proposed that we could design even scratch programs

we could design even scratch programs better maybe by using Loops instead of

better maybe by using Loops instead of just by copying and pasting the same

just by copying and pasting the same blocks again and again so design is more

blocks again and again so design is more subjective it's more of a a learned art

subjective it's more of a a learned art whereby two people might ultimately

whereby two people might ultimately disagree as to which version of a

disagree as to which version of a program is better designed but we'll

program is better designed but we'll give you building blocks and principles

give you building blocks and principles over the coming weeks so that you can

over the coming weeks so that you can have a better sense for yourself if your

have a better sense for yourself if your own code is well designed and why is

own code is well designed and why is that valuable well the better design

that valuable well the better design your code is often the faster it's going

your code is often the faster it's going to run the more maintainable it's going

to run the more maintainable it's going to be by you or colleagues if you're

to be by you or colleagues if you're working with others in the real world so

working with others in the real world so good design is a good thing it helps you

good design is a good thing it helps you communicate your ideas just like an

communicate your ideas just like an typical English essay and then lastly

typical English essay and then lastly we'll talk this week onward about style

we'll talk this week onward about style and this is really just the Aesthetics

and this is really just the Aesthetics of your code it turns out that computers

of your code it turns out that computers often don't care how sloppy your actual

often don't care how sloppy your actual code is um where uh in the world of code

code is um where uh in the world of code it turns out that you don't really need

it turns out that you don't really need to indent things in a beautiful way you

to indent things in a beautiful way you don't need to paginate things like you

don't need to paginate things like you might in an essay the computer generally

might in an essay the computer generally does not care but the human does the

does not care but the human does the teaching assistant does you will care

teaching assistant does you will care the next day when you're just trying to

the next day when you're just trying to understand what your code does so we'll

understand what your code does so we'll focus lastly on Style the Aesthetics of

focus lastly on Style the Aesthetics of the code that you're writing so where

the code that you're writing so where are we going to write code where are we

are we going to write code where are we going to compile code so for this class

going to compile code so for this class not only with C but the other languages

not only with C but the other languages we use later in the term we're going to

we use later in the term we're going to use a free text editor that is program

use a free text editor that is program called Visual Studio code AKA vs code

called Visual Studio code AKA vs code it's super popular nowadays not just for

it's super popular nowadays not just for C but for C++ and Python and Java and

C but for C++ and Python and Java and any number of other languages it's a

any number of other languages it's a text editor in the sense that it lets

text editor in the sense that it lets you edit text and that all that's all

you edit text and that all that's all code is going to be now strictly

code is going to be now strictly speaking you could write code on paper

speaking you could write code on paper pencil in fact in high school if you

pencil in fact in high school if you took a class you might have done that

took a class you might have done that one or more times as sort of an in-class

one or more times as sort of an in-class exercise you can't run it on paper of

exercise you can't run it on paper of course but you could write it certainly

course but you could write it certainly you could use something like Microsoft

you could use something like Microsoft Word or notepad.exe or text edit on the

Word or notepad.exe or text edit on the Mac but none of those programs are

Mac but none of those programs are really designed to format the code in

really designed to format the code in the best way for you nor are they

the best way for you nor are they designed to let you compile and run the

designed to let you compile and run the code so VSS code is going to be a tool

code so VSS code is going to be a tool via which you can do all that and more

via which you can do all that and more write the code compile the code run the

write the code compile the code run the code so that you all don't have to

code so that you all don't have to wrestle with stupid technical support

wrestle with stupid technical support headaches at the beginning of the course

headaches at the beginning of the course by installing this software and that on

by installing this software and that on your Macs or PCS we'll use a cloud based

your Macs or PCS we'll use a cloud based version of VSS code at code. cs50. and

version of VSS code at code. cs50. and that's going to be the exact same tool

that's going to be the exact same tool and the goal then is by the end of the

and the goal then is by the end of the semester to sort of uh migrate you off

semester to sort of uh migrate you off of that cloud-based environment to your

of that cloud-based environment to your own Mac and PC so that even if cs50 is

own Mac and PC so that even if cs50 is the only CS class you ever take you're

the only CS class you ever take you're 100% equipped to continue writing code

100% equipped to continue writing code after the class using not something

after the class using not something that's even cs50 specific but a de facto

that's even cs50 specific but a de facto industry standard at least for some time

industry standard at least for some time so what's this program VSS code going to

so what's this program VSS code going to look like be it on your Mac PC or

look like be it on your Mac PC or initially in your browser and it's going

initially in your browser and it's going to look a little something like this and

to look a little something like this and there's going to be several different

there's going to be several different regions to the screen and picture here

regions to the screen and picture here is that very same code I keep proposing

is that very same code I keep proposing is the simplest program you can write in

is the simplest program you can write in C and what are these different regions

C and what are these different regions of the screen well there's essentially

of the screen well there's essentially these four here so first highlighted up

these four here so first highlighted up top is going to be one or more tabs

top is going to be one or more tabs where you're going to actually write

where you're going to actually write code so much like in Google Docs or

code so much like in Google Docs or Microsoft Word you can have tabs open

Microsoft Word you can have tabs open with files similarly in VSS code or

with files similarly in VSS code or really any programming environment do

really any programming environment do generally nowadays have tabs of some

generally nowadays have tabs of some sort and this is going to be a tab

sort and this is going to be a tab containing a file it seems called

containing a file it seems called hello.c and that's going to be the very

hello.c and that's going to be the very first file we write in just a moment uh

first file we write in just a moment uh down here though is going to be an

down here though is going to be an interface that many of you might not

interface that many of you might not know this is what's called a terminal

know this is what's called a terminal window and a terminal window provides

window and a terminal window provides what's generally called a commandline

what's generally called a commandline interface or CLI and this is in contrast

interface or CLI and this is in contrast with a graphical user interface or guey

with a graphical user interface or guey now you and I every day are using guies

now you and I every day are using guies on our phones on our PCS and a guy is

on our phones on our PCS and a guy is literally graphical so menus and buttons

literally graphical so menus and buttons and icons and you generally use your

and icons and you generally use your finger or a trackpad or a mouse or

finger or a trackpad or a mouse or something like that to interact with it

something like that to interact with it but it turns out that many programmers

but it turns out that many programmers dare say most programmers at least over

dare say most programmers at least over time come to prefer not a guey but a CLI

time come to prefer not a guey but a CLI a command line interface where you

a command line interface where you actually do everything somewhat uh

actually do everything somewhat uh somewhat arcely via keyboard alone why

somewhat arcely via keyboard alone why well it turns out there's just more

well it turns out there's just more features built in to most computers if

features built in to most computers if you can access them with a keyboard

you can access them with a keyboard turns out you can most of us can type

turns out you can most of us can type faster than you can point and click and

faster than you can point and click and so that ends up being an efficient gain

so that ends up being an efficient gain over time so in time will you get

over time so in time will you get comfortable using this terminal window

comfortable using this terminal window to do things like compile your code or

to do things like compile your code or make your program as well as run it so

make your program as well as run it so you won't be in the habit initially of

you won't be in the habit initially of just double clicking icons like we do in

just double clicking icons like we do in our typical real world you'll do it sort

our typical real world you'll do it sort of the programmer's way but it's not to

of the programmer's way but it's not to the exclusion of adding icons and

the exclusion of adding icons and clickability and more on the left hand

clickability and more on the left hand side of vs code there's going to be a

side of vs code there's going to be a somewhat familiar file explorer some

somewhat familiar file explorer some kind of hierarchical tree like on your

kind of hierarchical tree like on your Mac or PC where you can see all of the

Mac or PC where you can see all of the files in your account pictured here for

files in your account pictured here for instance is just hello.c which I'll

instance is just hello.c which I'll create myself in a moment and then far

create myself in a moment and then far away on the left is the so-called

away on the left is the so-called activity bar and this is where you just

activity bar and this is where you just get a lot of traditional menus and

get a lot of traditional menus and buttons so VSS code itself gives you

buttons so VSS code itself gives you both a guey and a CLI but it's within

both a guey and a CLI but it's within the CLI the terminal window the bottom

the CLI the terminal window the bottom region of the screen that we're actually

region of the screen that we're actually going to type most of our commands and

going to type most of our commands and in general in class I'm going to hide

in general in class I'm going to hide all of the graphical stuff that's just

all of the graphical stuff that's just not of all that uh that much interest so

not of all that uh that much interest so with that said let me actually change

with that said let me actually change over to a live version of vs code and

over to a live version of vs code and I've indeed HD in the activity bar I've

I've indeed HD in the activity bar I've indeed hid in the file explorer so what

indeed hid in the file explorer so what I have here for visibility sake is a

I have here for visibility sake is a really big area for writing code and a

really big area for writing code and a really big terminal window at the bottom

really big terminal window at the bottom you'll see in the terminal window

you'll see in the terminal window there's a dollar sign and this doesn't

there's a dollar sign and this doesn't mean any form of currency this is just

mean any form of currency this is just the standard symbol that represents type

the standard symbol that represents type commands here so the fact that there's

commands here so the fact that there's just a dollar sign in a cursor means

just a dollar sign in a cursor means eventually that's where I'm going to

eventually that's where I'm going to type commands but first i'm going to

type commands but first i'm going to actually create some code so how might I

actually create some code so how might I program using vs code be it on my Mac PC

program using vs code be it on my Mac PC or in this cloud-based environment that

or in this cloud-based environment that you'll get set up for problem set one go

you'll get set up for problem set one go about writing my first file well perhaps

about writing my first file well perhaps the easiest way is this literally run

the easiest way is this literally run the command code and then the name of

the command code and then the name of the file you want to create notice that

the file you want to create notice that I deliberately end the file with C in

I deliberately end the file with C in lowercase notice that I've deliberately

lowercase notice that I've deliberately lowercased the whole file name and these

lowercased the whole file name and these are just conventions you could use a

are just conventions you could use a capital H you kind of could use a

capital H you kind of could use a Capital C but just don't do that follow

Capital C but just don't do that follow best practices so that it's consistent

best practices so that it's consistent with what most everyone else would do

with what most everyone else would do when I hit enter I just get an empty tab

when I hit enter I just get an empty tab just like the screenshot a moment ago

just like the screenshot a moment ago and it's in this tab where I can now

and it's in this tab where I can now write my very first program in C

write my very first program in C unfortunately it's not quite as user

unfortunately it's not quite as user friendly as scratch where you drag and

friendly as scratch where you drag and drop a couple of puzzle pieces and boom

drop a couple of puzzle pieces and boom it's done so I'm going to do this for

it's done so I'm going to do this for memory but this too will become familiar

memory but this too will become familiar to you over time I'm going to include

to you over time I'm going to include something called standard io. I'm going

something called standard io. I'm going to type int main void and parentheses on

to type int main void and parentheses on a new line I'm going to insert some

a new line I'm going to insert some curly braces as we'll call them and then

curly braces as we'll call them and then I'm going to type print F and then some

I'm going to type print F and then some parentheses and then in quotes hello

parentheses and then in quotes hello comma world then a backslash then a

comma world then a backslash then a lowercase n then a close quote and then

lowercase n then a close quote and then a semicolon at the very end of the line

a semicolon at the very end of the line so all I've done is recreate just from

so all I've done is recreate just from memory that very first program in a

memory that very first program in a little bit we'll make clear what most of

little bit we'll make clear what most of this does but for now let's just

this does but for now let's just actually run this thing and just like I

actually run this thing and just like I click the green flag last week for the

click the green flag last week for the first time let's actually compile and

first time let's actually compile and run this program if it were your Mac PC

run this program if it were your Mac PC and Google or Microsoft or someone else

and Google or Microsoft or someone else had made the software at this point in

had made the software at this point in the story we'd be double clicking an

the story we'd be double clicking an icon but we can't do that yet this is

icon but we can't do that yet this is still source code so I'm going to click

still source code so I'm going to click back down in my terminal window notice I

back down in my terminal window notice I have a second dollar sign below the

have a second dollar sign below the first which just means it's ready for a

first which just means it's ready for a second command and now the command via

second command and now the command via which to make this an actual program to

which to make this an actual program to compile it from source code to machine

compile it from source code to machine code is going to be quite simply make

code is going to be quite simply make and then the name of the program I want

and then the name of the program I want to make slight subtlety I'm omitting

to make slight subtlety I'm omitting deliberately. C because the program I

deliberately. C because the program I want to make I just want to call hello

want to make I just want to call hello so don't write make hello. C just write

so don't write make hello. C just write make hello and this program make is

make hello and this program make is essentially our compiler technically

essentially our compiler technically speaking it's a program that automates

speaking it's a program that automates the compilation of my program for me but

the compilation of my program for me but it is going to see that I've typed the

it is going to see that I've typed the word hello it's going to automatically

word hello it's going to automatically look now for a file on the hard drive

look now for a file on the hard drive called hello.c and convert it from

called hello.c and convert it from source code in C to machine code in

source code in C to machine code in zeros and ones so if I didn't make any

zeros and ones so if I didn't make any typos

typos enter nothing seems to happen and that's

enter nothing seems to happen and that's a good thing almost always if nothing

a good thing almost always if nothing gets outputed on the screen like you did

gets outputed on the screen like you did good like you didn't make any mistakes

good like you didn't make any mistakes you didn't get yelled at there's no

you didn't get yelled at there's no error messages so this is actually a

error messages so this is actually a good thing how do I now run this program

good thing how do I now run this program well notice I've got a third dollar sign

well notice I've got a third dollar sign which just means I'm ready for a third

which just means I'm ready for a third command and now I'm going to go ahead

command and now I'm going to go ahead and run dot slash hello and this is

and run dot slash hello and this is admittedly a little weird that you have

admittedly a little weird that you have to do dot slash but for now just take on

to do dot slash but for now just take on faith that this is how you run a program

faith that this is how you run a program called hello in your current folder in

called hello in your current folder in your current directory in this

your current directory in this cloud-based environment all right

cloud-based environment all right crossing my fingers again hitting enter

crossing my fingers again hitting enter and voila my very first program in see

and voila my very first program in see hello world and now let me go ahead and

hello world and now let me go ahead and reveal the file explorer that I proposed

reveal the file explorer that I proposed exist earlier I'm just going to use a

exist earlier I'm just going to use a keyboard shortcut to reveal that and

keyboard shortcut to reveal that and generally I keep it close because I

generally I keep it close because I don't really need to know constantly

don't really need to know constantly what files are in my account but you'll

what files are in my account but you'll see now in the file explorer similar

see now in the file explorer similar INSP to a Mac or PC but graphically a

INSP to a Mac or PC but graphically a little different here's my file hello.c

little different here's my file hello.c it's highlighted because I have that tab

it's highlighted because I have that tab open but now there's a second file here

open but now there's a second file here called just hello that's the name of my

called just hello that's the name of my program so if you were on a Mac or PC

program so if you were on a Mac or PC you would ideally double click that

you would ideally double click that thing you can't do that in a command

thing you can't do that in a command line environment you have to run it down

line environment you have to run it down here but that's all we've done we've

here but that's all we've done we've created a file called hello.c and then

created a file called hello.c and then my compiler made the program from that

my compiler made the program from that let me pause here and see if there's any

let me pause here and see if there's any questions because it's a lot of magical

questions because it's a lot of magical phrases

phrases yeah yeah so if you're currently

yeah yeah so if you're currently following along uh playing along at home

following along uh playing along at home and you're getting some kind of error

and you're getting some kind of error message part of today will be for me to

message part of today will be for me to deliberately induce some of those error

deliberately induce some of those error messages for now let me just propose

messages for now let me just propose that if you literally did what I did you

that if you literally did what I did you must have made a typo somewhere and

must have made a typo somewhere and notice that it's indeed standard IO

notice that it's indeed standard IO stdio.h uh maybe you type

stdio.h uh maybe you type studio. okay super common mistake if I

studio. okay super common mistake if I don't if I could call you out um like it

don't if I could call you out um like it is not studio. it is standard io. so

is not studio. it is standard io. so common but this is exactly

common but this is exactly representative of like the kind of

representative of like the kind of stupid headaches you're going to run

stupid headaches you're going to run into this week probably for a few weeks

into this week probably for a few weeks probably honestly for a few years but

probably honestly for a few years but you start to see past these sort of

you start to see past these sort of stupid mistakes over time and it just

stupid mistakes over time and it just gets easier and easier because the

gets easier and easier because the computer is going to be so regimented

computer is going to be so regimented like you it will only do what you tell

like you it will only do what you tell it to do and if you say because it's

it to do and if you say because it's verbally sounds like studio. it's not

verbally sounds like studio. it's not going to know what the file is so

going to know what the file is so actually thank you for tripping over

actually thank you for tripping over that so early that's super common to

that so early that's super common to happen yeah have two hello so I why do I

happen yeah have two hello so I why do I have two hello files so why do I have

have two hello files so why do I have two hello files one is the one I created

two hello files one is the one I created as the human called hello.c and it's

as the human called hello.c and it's pictured right here but then when I ran

pictured right here but then when I ran make hello that process compiled my

make hello that process compiled my source code into machine code so this

source code into machine code so this second file just called hello is the

second file just called hello is the file that contains all of those zeros

file that contains all of those zeros and ones that the server actually

and ones that the server actually understands all right so yeah

question if you try clicking on the Hello file you'll see in this

Hello file you'll see in this environment of vs code quote unquote the

environment of vs code quote unquote the file is not displayed in the editor

file is not displayed in the editor because it is either binary AKA zeros

because it is either binary AKA zeros and ones or uses an unsupported text

and ones or uses an unsupported text encoding in this case it's binary it's

encoding in this case it's binary it's zeros and ones now you could use

zeros and ones now you could use software to see those zeros and ones it

software to see those zeros and ones it won't be intellectually enlightening to

won't be intellectually enlightening to most any human so VSS code just takes

most any human so VSS code just takes the choice of not showing it to you at

the choice of not showing it to you at all so that would be a common mistake

all so that would be a common mistake too clicking on a file you don't intend

too clicking on a file you don't intend but the source code is indeed going to

but the source code is indeed going to be editable by us all right all right so

be editable by us all right all right so I've written this program it seems to

I've written this program it seems to sort of magically work at least with

sort of magically work at least with some effort if you get every single

some effort if you get every single keystroke right well what is it that's

keystroke right well what is it that's going on and how is this working well

going on and how is this working well first of all notice that even without my

first of all notice that even without my highlighting things or choosing buttons

highlighting things or choosing buttons from menus notice that it's already kind

from menus notice that it's already kind of colorcoded and yet I wasn't

of colorcoded and yet I wasn't highlighting along the way in sort of

highlighting along the way in sort of Google doc style changing the color

Google doc style changing the color certainly well it turns out what VSS

certainly well it turns out what VSS code and most programming environments

code and most programming environments nowadays do for you automatically is

nowadays do for you automatically is syntax highlighting so syntax

syntax highlighting so syntax highlighting is just this feature of

highlighting is just this feature of typical text editors nowadays that

typical text editors nowadays that analyzes the code that you've typed and

analyzes the code that you've typed and when it notices certain types of

when it notices certain types of keystrokes things that represent

keystrokes things that represent functions or conditionals or Loops or

functions or conditionals or Loops or variables a lot of the vocab from last

variables a lot of the vocab from last week it just highlights it ever so

week it just highlights it ever so differently for you so main for instance

differently for you so main for instance which we'll soon see is in purple here

which we'll soon see is in purple here int and void and include are in red

int and void and include are in red hello world is in blue my parentheses

hello world is in blue my parentheses are in green this will totally vary by

are in green this will totally vary by programmer too in fact if you do want to

programmer too in fact if you do want to change these colors for problem set one

change these colors for problem set one for your own environment you can poke

for your own environment you can poke around vs code settings via the gear

around vs code settings via the gear icon you can change a different a change

icon you can change a different a change to a different color theme syntax

to a different color theme syntax highlighting isn't some specific color

highlighting isn't some specific color scheme like it is in scratch it just

scheme like it is in scratch it just generally is to each human their own

generally is to each human their own preference but that's all that's

preference but that's all that's happening here is this notion of syntax

happening here is this notion of syntax highlighting at the moment well what

highlighting at the moment well what more is going on in this code before I

more is going on in this code before I run it but rather write it well it looks

run it but rather write it well it looks a little something like this if I take

a little something like this if I take away all of the colors and then just for

away all of the colors and then just for discussion sake let me ahead and color

discussion sake let me ahead and color it a little more like scratch recall

it a little more like scratch recall that our very first scratch program that

that our very first scratch program that just said hello world on the screen had

just said hello world on the screen had a when green flag clicked icon uh puzzle

a when green flag clicked icon uh puzzle piece roughly an orange and then a

piece roughly an orange and then a purple uh sa block beneath it so whereas

purple uh sa block beneath it so whereas this is the C version if we rewind to

this is the C version if we rewind to last week this was the same program in

last week this was the same program in scratch but what's happening now is

scratch but what's happening now is exactly the same so if you think back to

exactly the same so if you think back to last week and you've got some function

last week and you've got some function like the say function in purple that

like the say function in purple that might take one or more arguments like

might take one or more arguments like inputs that influences what it says on

inputs that influences what it says on the screen and then functions recall can

the screen and then functions recall can sometimes have side effects right like

sometimes have side effects right like the speech bubble appears on the screen

the speech bubble appears on the screen so last week when we used the sa block

so last week when we used the sa block and we passed in an argument of hello

and we passed in an argument of hello world at left we got this visual side

world at left we got this visual side effect on the screen that says now hello

effect on the screen that says now hello world in the speech bubble and that's

world in the speech bubble and that's exactly what just happened in VSS code

exactly what just happened in VSS code but much much more textually and let's

but much much more textually and let's look a little closer now at the code

look a little closer now at the code itself let me wave my hand at the

itself let me wave my hand at the equivalent of the when green flag

equivalent of the when green flag clicked part of my code and let's focus

clicked part of my code and let's focus only on the say block in scratch and the

only on the say block in scratch and the corresponding function in C so if I step

corresponding function in C so if I step through this and I wanted to convert

through this and I wanted to convert what we did last week with the say block

what we did last week with the say block to C I would first use the print

to C I would first use the print function although that's actually a bit

function although that's actually a bit of a white lie it's actually the print F

of a white lie it's actually the print F function print f means formatted and

function print f means formatted and it's just a function that allows you to

it's just a function that allows you to format text on the screen There Is No Sa

format text on the screen There Is No Sa function in C there's a print F function

function in C there's a print F function what MIT did down the road years ago go

what MIT did down the road years ago go was they took what existed historically

was they took what existed historically as print F and they simplified it for a

as print F and they simplified it for a broader audience by just calling it

broader audience by just calling it essentially say instead but notice that

essentially say instead but notice that now if I want to convert the scratch

now if I want to convert the scratch code at left to C code at right it's

code at left to C code at right it's sort of the same shape so MIT

sort of the same shape so MIT deliberately used this white oval if

deliberately used this white oval if only because it kind of conjures these

only because it kind of conjures these uh this idea of having parentheses too

uh this idea of having parentheses too so on the right if I want to pass an

so on the right if I want to pass an argument or an input to the print F

argument or an input to the print F function I use an open parenthesis and a

function I use an open parenthesis and a CL parenthesis in those parentheses I

CL parenthesis in those parentheses I then type whatever it is I want to print

then type whatever it is I want to print on the screen in this case hello comma

on the screen in this case hello comma world but notice I've deliberately left

world but notice I've deliberately left some room because you need some extra

some room because you need some extra keystrokes in the world of C anytime you

keystrokes in the world of C anytime you type out some text otherwise known as a

type out some text otherwise known as a string of text to use computer science

string of text to use computer science jargon you need to quote it in this case

jargon you need to quote it in this case with double quotes double quote at the

with double quotes double quote at the left double quote at the right and

left double quote at the right and notice to I'm going to include some

notice to I'm going to include some slightly cryptic symbol here too back

slightly cryptic symbol here too back sln which I also typed and said verbally

sln which I also typed and said verbally earlier and then one last nuisance at

earlier and then one last nuisance at the end of this which is a semicolon so

the end of this which is a semicolon so suffice it to say this is why we start

suffice it to say this is why we start with scratch this drag and drop you're

with scratch this drag and drop you're good to go in a language like C print F

good to go in a language like C print F parenthesis double quotes the text you

parenthesis double quotes the text you want backs slash and semicolon at the

want backs slash and semicolon at the end there's just so much syntactic

end there's just so much syntactic overhead but at the end of the day it's

overhead but at the end of the day it's just a function and you'll get used to

just a function and you'll get used to these sort of nuisances like the

these sort of nuisances like the parentheses the quotes the semicolon and

parentheses the quotes the semicolon and the like but things can very easily go

the like but things can very easily go wrong and it's very easy to make

wrong and it's very easy to make mistakes even with lines of code like

mistakes even with lines of code like this so let me do this let me go back to

this so let me do this let me go back to vs code where I have the exact same code

vs code where I have the exact same code notice that on line five is exactly that

notice that on line five is exactly that line of code so this is the equivalent

line of code so this is the equivalent of the sa block and let's consider what

of the sa block and let's consider what mistakes I may make early on or even now

mistakes I may make early on or even now 20 years later after learning this that

20 years later after learning this that are quite common um in general suppose I

are quite common um in general suppose I forget the semicolon there so easy to do

forget the semicolon there so easy to do you will do this eventually let's see

you will do this eventually let's see what happens now when I go back to my

what happens now when I go back to my terminal window and try to compile my

terminal window and try to compile my code again just to keep things tidy I'm

code again just to keep things tidy I'm going to clear my screen but that's just

going to clear my screen but that's just for lecture sake so that we can focus

for lecture sake so that we can focus only on the most recent command but I'm

only on the most recent command but I'm going to go ahead now and rerun make

going to go ahead now and rerun make Hello this will ensure that my program

Hello this will ensure that my program is recompiled and this is a manual

is recompiled and this is a manual process I changed my code the zeros and

process I changed my code the zeros and ones on the hard drive have not changed

ones on the hard drive have not changed I need to recompile it to Output the

I need to recompile it to Output the latest machine code so here we go I'm

latest machine code so here we go I'm going to hit enter crossing my fingers

going to hit enter crossing my fingers as before but again I remove the

as before but again I remove the semicolon by

semicolon by accident oh my God there's like more

accident oh my God there's like more lines of Errors now than there are of

lines of Errors now than there are of actual code and this too takes them

actual code and this too takes them getting used to um the programs we're

getting used to um the programs we're using were not necessarily written with

using were not necessarily written with the least comfortable audience in mind

the least comfortable audience in mind but really professional programmers back

but really professional programmers back in the day but through practice and

in the day but through practice and through experience and through mistakes

through experience and through mistakes you'll start to notice patterns here too

you'll start to notice patterns here too so here's what I typed make hello after

so here's what I typed make hello after the dollar sign prompt now I get yelled

the dollar sign prompt now I get yelled at with as follows hello.c colon

at with as follows hello.c colon 529 well what's that referring to I've

529 well what's that referring to I've screwed up somewhere on line five on the

screwed up somewhere on line five on the 29th character on that line generally

29th character on that line generally the specific character is not that

the specific character is not that useful unless you actually want to count

useful unless you actually want to count it out but line five is a good clue why

it out but line five is a good clue why it means I screwed up somewhere on line

it means I screwed up somewhere on line five here all right well what is the

five here all right well what is the error expected a semicolon after

error expected a semicolon after expression and this error is actually

expression and this error is actually pretty obvious now that I see it and I

pretty obvious now that I see it and I realize oh wait a minute all right

realize oh wait a minute all right here's my line of code here in sort of

here's my line of code here in sort of asky art so to speak textual uh text

asky art so to speak textual uh text representing Graphics it wants me to put

representing Graphics it wants me to put in green here this semicolon at the end

in green here this semicolon at the end of that line one error generated built

of that line one error generated built in so some esoteric stuff there but my

in so some esoteric stuff there but my program did not compile when you see an

program did not compile when you see an error like this it means it did not work

error like this it means it did not work so what's the fix well obviously the fix

so what's the fix well obviously the fix is to go back up here put the semicolon

is to go back up here put the semicolon there and now if I recompile my code

there and now if I recompile my code with make hello I won't clear my screen

with make hello I won't clear my screen just yet just to show you the difference

just yet just to show you the difference now it just worked so we're back in

now it just worked so we're back in business as before all right let me

business as before all right let me pause here though and ask if there's any

pause here though and ask if there's any questions about what I just did

questions about what I just did [Music]

[Music] these error messages will become

these error messages will become frequent initially

yeah really good question do you need a semicolon after every line or just some

semicolon after every line or just some it turns out just some uh this is

it turns out just some uh this is something you'll learn through practice

something you'll learn through practice through demonstrations and examples

through demonstrations and examples today generally you put a semicolon

today generally you put a semicolon after a statement so to speak and this

after a statement so to speak and this is the technical term for this line of

is the technical term for this line of code it's a statement and think of it is

code it's a statement and think of it is it's kind of the code equivalent of like

it's kind of the code equivalent of like an English sentence so the semi colon

an English sentence so the semi colon and code is sort of like a period in

and code is sort of like a period in English when you're done with that

English when you're done with that particular thought you don't need

particular thought you don't need semicolons for now anywhere else and

semicolons for now anywhere else and we'll see examples of where else you put

we'll see examples of where else you put them but it usually is at the end of a

them but it usually is at the end of a line of code that hasn't that isn't

line of code that hasn't that isn't purely syntactic like uh curly braces

purely syntactic like uh curly braces instead other questions on the mistake I

instead other questions on the mistake I just fixed and created for myself

just fixed and created for myself [Music]

[Music] yeah uh correct so line five is where

yeah uh correct so line five is where the error is most likely character 29

the error is most likely character 29 means it's sort of 29 characters that

means it's sort of 29 characters that way and then it's actually in this case

way and then it's actually in this case giving me a suggestion the compiler

giving me a suggestion the compiler won't always know how to advise me

won't always know how to advise me especially if I've made a real mess of

especially if I've made a real mess of my code but often it will do its best to

my code but often it will do its best to give you the answer like this

yeah ah so how come I first typed code space hello.c and now I'm typing make

space hello.c and now I'm typing make hello two different processes so when I

hello two different processes so when I typed code space hello.c that was

typed code space hello.c that was because I wanted to open vs code and

because I wanted to open vs code and create a new new file called hello.c

create a new new file called hello.c it's like going to file new in in a a

it's like going to file new in in a a Mac or PC thereafter though once the

Mac or PC thereafter though once the file exists and is actually open here

file exists and is actually open here and it does autosave you don't need to

and it does autosave you don't need to hit command s or control s all the time

hit command s or control s all the time I can now compile it with make hello

I can now compile it with make hello again and again so theoretically I

again and again so theoretically I should never need to type code space

should never need to type code space hello.c again unless I want to create a

hello.c again unless I want to create a brand new file called the same thing all

brand new file called the same thing all right so what about this other piece of

right so what about this other piece of syntax here let me clear my terminal

syntax here let me clear my terminal window here you can also hit control L

window here you can also hit control L just to throw everything away just to

just to throw everything away just to clean it up aesthetically suppose that I

clean it up aesthetically suppose that I omit whatever this sequence of symbols

omit whatever this sequence of symbols is back sln since I'm not really sure at

is back sln since I'm not really sure at first glance why that's even there does

first glance why that's even there does anyone want to conjecture especially if

anyone want to conjecture especially if you've never programmed before what

you've never programmed before what might happen now if I recompile and

might happen now if I recompile and rerun this version of the program I left

rerun this version of the program I left the semicolon but I took away the backs

the semicolon but I took away the backs slash

slash n any

n any instincts all right well yeah

instincts all right well yeah will the next dollar sign appear

will the next dollar sign appear straight after your it will the next

straight after your it will the next dollar sign will appear right after my

dollar sign will appear right after my hello world but what makes you think

hello world but what makes you think that back creates a line exactly back

that back creates a line exactly back sln is actually a special sequence of

sln is actually a special sequence of symbols that creates a new line and so

symbols that creates a new line and so to your point if I recompile this

to your point if I recompile this program make hello enter no syntax error

program make hello enter no syntax error so it did compile this time so you don't

so it did compile this time so you don't need the backs slash in you do need the

need the backs slash in you do need the semicolon but if you don't have the back

semicolon but if you don't have the back sln watch what happens when I do/ hello

sln watch what happens when I do/ hello this time now indeed I see hello comma

this time now indeed I see hello comma world and then a weird dollar sign and

world and then a weird dollar sign and this is still a prompt I can still type

this is still a prompt I can still type commands at it like clear and everything

commands at it like clear and everything gets cleaned up but it just looks kind

gets cleaned up but it just looks kind of stupid if I run it again here with

of stupid if I run it again here with hello you know it's just not very user

hello you know it's just not very user friendly it is convention that when

friendly it is convention that when you're done running your program you

you're done running your program you should ideally clean things up move the

should ideally clean things up move the cursor to the next line for the user and

cursor to the next line for the user and so the backs slash n is simply the

so the backs slash n is simply the special symbol otherwise known as an

special symbol otherwise known as an escape

escape sequence that c knows means move the

sequence that c knows means move the cursor to the next line in other

cursor to the next line in other languages python among them uses the

languages python among them uses the same symbology as well now if I go back

same symbology as well now if I go back to the code here and for instance I try

to the code here and for instance I try to do this differently like suppose I

to do this differently like suppose I don't put the backs slash N I just hit

don't put the backs slash N I just hit enter like a normal person would in

enter like a normal person would in Google Docs or Microsoft Word let me go

Google Docs or Microsoft Word let me go ahead and try compiling this program and

ahead and try compiling this program and this you would hope would work right you

this you would hope would work right you would hope this would print out hello

would hope this would print out hello world and then a blank line because I

world and then a blank line because I move the cursor to the next line but no

move the cursor to the next line but no if I run make hello now and try to

if I run make hello now and try to compile that c does not like this now I

compile that c does not like this now I get a different error still on line five

get a different error still on line five this time starting at character 12 uh

this time starting at character 12 uh error missing terminating double quote

error missing terminating double quote character and then some other esoteric

character and then some other esoteric stuff and then this does not sound good

stuff and then this does not sound good fatal error this time too many errors

fatal error this time too many errors admitted stopping now so I really

admitted stopping now so I really screwed up here so why can't I do this

screwed up here so why can't I do this just because like the humans who

just because like the humans who designed C decided that if you have a

designed C decided that if you have a string of text it must stay on the same

string of text it must stay on the same line it can get really long it can soft

line it can get really long it can soft wrap that is without you hitting enter

wrap that is without you hitting enter but you can't hit enter to create a new

but you can't hit enter to create a new line if you deliberately want a new line

line if you deliberately want a new line you have to indeed use this back slash

you have to indeed use this back slash and Escape character so let me go ahead

and Escape character so let me go ahead and do this let me put it back let me go

and do this let me put it back let me go back to my terminal window I'll clear

back to my terminal window I'll clear the screen again let me go ahead now and

the screen again let me go ahead now and do make Hello to recompile to that

do make Hello to recompile to that version do/ hello and voila we're back

version do/ hello and voila we're back in business with uh hello all right so

in business with uh hello all right so now let's tease apart some some other

now let's tease apart some some other aspects of this code because there's a

aspects of this code because there's a lot going on just to get us to say hello

lot going on just to get us to say hello world on the screen for today we're

world on the screen for today we're largely going to ignore this int main

largely going to ignore this int main void and these curly braces here we'll

void and these curly braces here we'll come back to that before long as to why

come back to that before long as to why it's there but for now just think of int

it's there but for now just think of int main void and these curly braces here as

main void and these curly braces here as really being the C equivalent of when

really being the C equivalent of when green flag clicked like why you just

green flag clicked like why you just need it there that's how you get your

need it there that's how you get your program going and Maine is indeed going

program going and Maine is indeed going to be some special function but more on

to be some special function but more on that another time but why do I have this

that another time but why do I have this line of code here the cor spelling is

line of code here the cor spelling is indeed standard io.

indeed standard io. stdio.h and they're angled brackets this

stdio.h and they're angled brackets this time so that's a little new there's a

time so that's a little new there's a hash and then an include keyword you

hash and then an include keyword you know if you don't know what something is

know if you don't know what something is you know there's not really that much

you know there's not really that much harm in just getting rid of it and see

harm in just getting rid of it and see what happens so let me delete that line

what happens so let me delete that line let me go back to my terminal window

let me go back to my terminal window clear the screen and then run make hello

clear the screen and then run make hello again and let's try compiling this

again and let's try compiling this program now without that first line why

program now without that first line why I don't understand it so let's see what

I don't understand it so let's see what happens all right here's yet another

happens all right here's yet another error but let's see how hello. C line

error but let's see how hello. C line five character 5 so it's pretty early on

five character 5 so it's pretty early on error implicitly declaring Library

error implicitly declaring Library function printf with type int and then

function printf with type int and then dot dot dot so implicitly declaring

dot dot dot so implicitly declaring Library function print F so this is very

Library function print F so this is very cryptic sounding you'll get better at

cryptic sounding you'll get better at understanding phrases like these but

understanding phrases like these but apparently I do need the include line

apparently I do need the include line for standard

for standard i.h but why based on this symptom what

i.h but why based on this symptom what might Your Instinct be for what that

might Your Instinct be for what that first line of code is doing for us in

first line of code is doing for us in the first place why intuitively must it

the first place why intuitively must it be

be there exactly it's like importing a

there exactly it's like importing a library so that you can do things like

library so that you can do things like print things out on the screen now in

print things out on the screen now in scratch you didn't have to do this for

scratch you didn't have to do this for most of the puzzle pieces but you might

most of the puzzle pieces but you might recall that partway in through week zero

recall that partway in through week zero I went to the extensions button at the

I went to the extensions button at the bottom left of the scratch screen and I

bottom left of the scratch screen and I imported some extra puzzle pieces for

imported some extra puzzle pieces for text to speech that gave us the sort of

text to speech that gave us the sort of creepy uh humanized voice that actually

creepy uh humanized voice that actually came out of the cat's mouth well that

came out of the cat's mouth well that was like adding a library code that

was like adding a library code that someone else wrote in that case it was a

someone else wrote in that case it was a third party then but I gave myself

third party then but I gave myself access to it same here turns out that

access to it same here turns out that you don't really get printf

you don't really get printf automatically in C you have to include a

automatically in C you have to include a so-called header file that declares that

so-called header file that declares that function to exist now the reason for

function to exist now the reason for this historically is just efficiency

this historically is just efficiency back in the day when computers were

back in the day when computers were really slower and resource constrained

really slower and resource constrained you don't want to just give yourself

you don't want to just give yourself access to the entire kitchen sync of

access to the entire kitchen sync of functionality you only want to include

functionality you only want to include only the functions you actually care

only the functions you actually care about nowadays it's sort of a sort of a

about nowadays it's sort of a sort of a a copy paste step because you almost

a copy paste step because you almost always want to print something out on

always want to print something out on the screen at least when writing

the screen at least when writing programs like these but these so-called

programs like these but these so-called header files contain enough information

header files contain enough information about all of the functions in What's

about all of the functions in What's called the standard IO library and

called the standard IO library and standard IO just means standard input in

standard IO just means standard input in output and that's appropriate right

output and that's appropriate right because printing is pretty basic output

because printing is pretty basic output turns out there's other functions for

turns out there's other functions for getting input from the human's keyboard

getting input from the human's keyboard more on that in a bit but anytime you

more on that in a bit but anytime you want to print something on the screen

want to print something on the screen and see you indeed need to include this

and see you indeed need to include this header file at the top of your code and

header file at the top of your code and that's going to essentially inform the

that's going to essentially inform the compiler hey compiler I want to use

compiler hey compiler I want to use functionality from the standard IO

functionality from the standard IO Library including printf in this case

Library including printf in this case and if you omit the header file by

and if you omit the header file by accident it's just not going to work

accident it's just not going to work because it doesn't know what print f is

because it doesn't know what print f is it's sort of some unrecognized symbol in

it's sort of some unrecognized symbol in that case all right questions then about

that case all right questions then about this line of

this line of code this line of code here or what

code this line of code here or what these header files

these header files are all right you might wonder well how

are all right you might wonder well how do you know what functions exist how do

do you know what functions exist how do you know what files you might indeed

you know what files you might indeed want to include well it turns out that c

want to include well it turns out that c is a many-year old language and it has

is a many-year old language and it has ample documentation a caveat is that its

ample documentation a caveat is that its documentation isn't necessarily all that

documentation isn't necessarily all that userfriendly but what we have for the

userfriendly but what we have for the course is a simplified version of the

course is a simplified version of the official documentation for C at this URL

official documentation for C at this URL here manual. cs50. so in the world of c

here manual. cs50. so in the world of c and other languages too there are what

and other languages too there are what are called manual pages and these are

are called manual pages and these are just like text-based documentation that

just like text-based documentation that honestly is typically written in a voice

honestly is typically written in a voice that you kind of have to be an

that you kind of have to be an experienced programmer to understand

experienced programmer to understand some of it so what we've done it this

some of it so what we've done it this version of the same documentation is

version of the same documentation is we've imported all of the original

we've imported all of the original official documentation but we've added

official documentation but we've added sort of less comfortable translations in

sort of less comfortable translations in English for a lot of the functionality

English for a lot of the functionality that you might use in class just to help

that you might use in class just to help onboard you so at the end of the day you

onboard you so at the end of the day you don't need this documentation long term

don't need this documentation long term but just to get started we'll translate

but just to get started we'll translate it into terminology uh that you might

it into terminology uh that you might appreciate from a teaching assistant for

appreciate from a teaching assistant for instance as opposed to the original

instance as opposed to the original author of these documents and so for

author of these documents and so for instance if you were interested in

instance if you were interested in reading up on what functions exist in

reading up on what functions exist in the standard io. um header file well you

the standard io. um header file well you could go to a URL like this or you could

could go to a URL like this or you could search for it at manual. cs50. that

search for it at manual. cs50. that would show you a list of all of the

would show you a list of all of the available functions in that library and

available functions in that library and print F indeed would be one of them and

print F indeed would be one of them and then you could click further on that

then you could click further on that reaching a URL like this that's just

reaching a URL like this that's just going to give you all of the

going to give you all of the documentation for how to use print def

documentation for how to use print def it turns out you can do even more than

it turns out you can do even more than it uh than just printing out hello world

it uh than just printing out hello world and we'll scratch the surface of that

and we'll scratch the surface of that today but it turns out that the

today but it turns out that the documentation will always be your

documentation will always be your authoritative Source ultimately for

authoritative Source ultimately for questions like what can I do and how can

questions like what can I do and how can I do it meanwhile it turns out that cs50

I do it meanwhile it turns out that cs50 has its own library in A Accessible via

has its own library in A Accessible via header file called cs50.h it turns out

header file called cs50.h it turns out in C that output is actually pretty easy

in C that output is actually pretty easy relatively speaking once you get used to

relatively speaking once you get used to all the curly braces parentheses quote

all the curly braces parentheses quote marks and the like but input is a little

marks and the like but input is a little more difficult and if you have

more difficult and if you have programmed before input's not that hard

programmed before input's not that hard to do in Python it's not that hard to do

to do in Python it's not that hard to do in Java it's more difficult to do in C

in Java it's more difficult to do in C and we'll see why in a couple of weeks

and we'll see why in a couple of weeks but for the first couple of weeks of the

but for the first couple of weeks of the class we actually provide you with some

class we actually provide you with some training wheels of sorts whereby we have

training wheels of sorts whereby we have a number of functions that are declared

a number of functions that are declared in this file cs50.h it lives its

in this file cs50.h it lives its documentation at a URL like this this

documentation at a URL like this this and in a moment we'll use a few of these

and in a moment we'll use a few of these you'll see that cs50 provides you with

you'll see that cs50 provides you with some functions like get Char forget a

some functions like get Char forget a single character from the user's

single character from the user's keyboard uh get int to get an integer

keyboard uh get int to get an integer from the user's keyboard uh get string

from the user's keyboard uh get string to get a sequence of text from the

to get a sequence of text from the user's keyboard and a bunch of others as

user's keyboard and a bunch of others as well so let's actually use some of these

well so let's actually use some of these functions how about by revisiting really

functions how about by revisiting really the second program we wrote in scratch

the second program we wrote in scratch last time which adds some input to the

last time which adds some input to the output so first version of scratch was

output so first version of scratch was just hello world said the same thing

just hello world said the same thing every time you click the green flag

every time you click the green flag version two recall though did this it

version two recall though did this it asked the user what's your name and then

asked the user what's your name and then that somehow gave it back a a return

that somehow gave it back a a return variable return value we called it and

variable return value we called it and we then joined hello in that name to say

we then joined hello in that name to say something a little more interesting on

something a little more interesting on the screen so what did that model look

the screen so what did that model look like same thing as before we've got a

like same thing as before we've got a function in the Middle where function is

function in the Middle where function is like the code implementation of our

like the code implementation of our algorithm that takes in one or more

algorithm that takes in one or more arguments like what is it you want to uh

arguments like what is it you want to uh say on the the screen ultimately and

say on the the screen ultimately and return value in this case is going to be

return value in this case is going to be actually a value that comes back so in

actually a value that comes back so in the case of getting input we can

the case of getting input we can consider this ask block again like last

consider this ask block again like last week the input to it is whatever words

week the input to it is whatever words of English you want to ask the user and

of English you want to ask the user and then it returns a value and this was

then it returns a value and this was called by default in mit's World answer

called by default in mit's World answer that we'll see and see you can call

that we'll see and see you can call these return values anything you want

these return values anything you want ultimately in variables but this is

ultimately in variables but this is different from a side effect a side

different from a side effect a side effect is just something visual often

effect is just something visual often that happens on the screen like the

that happens on the screen like the speech bubble or hello world a return

speech bubble or hello world a return value is actually a value you get back

value is actually a value you get back from a function that you can use or

from a function that you can use or reuse so how do we convert this scratch

reuse so how do we convert this scratch block from last week to C code this week

block from last week to C code this week well if you want to ask the user for

well if you want to ask the user for something like their name you can do

something like their name you can do this you use a cs50 function called get

this you use a cs50 function called get string and you use the parentheses to

string and you use the parentheses to represent here comes the inputs there

represent here comes the inputs there too you can then put the sentence you

too you can then put the sentence you want to ask the user quote unquote

want to ask the user quote unquote what's your name but you do indeed need

what's your name but you do indeed need the quotes literally in C so I'll go

the quotes literally in C so I'll go ahead and add those as well well subtle

ahead and add those as well well subtle but I've deliberately included a space

but I've deliberately included a space after the question mark but before the

after the question mark but before the double quote just so that the cursor

double quote just so that the cursor moves one step over because in this case

moves one step over because in this case we're not going to get a special speech

we're not going to get a special speech box like we did in scratch it's just

box like we did in scratch it's just going to leave the cursor where it is so

going to leave the cursor where it is so we'll see that aesthetically this just

we'll see that aesthetically this just moves the blinking cursor one space

moves the blinking cursor one space after the sentence on the screen all

after the sentence on the screen all right but the catch is with scratch we

right but the catch is with scratch we just automatically got back the answer

just automatically got back the answer from the user in a special variable

from the user in a special variable called answer in C you're going to have

called answer in C you're going to have to be a little more specific in C if you

to be a little more specific in C if you want to get back a return value from a

want to get back a return value from a function like get string you have to use

function like get string you have to use an equal sign and then the name of a

an equal sign and then the name of a variable on the left the choice of

variable on the left the choice of variables is up to you I could have

variables is up to you I could have called this anything X YZ I'm going to

called this anything X YZ I'm going to more descriptively call it answer for

more descriptively call it answer for parody with what MIT did with scratch

parody with what MIT did with scratch but notice that this doesn't represent

but notice that this doesn't represent equality per se this is assignment in

equality per se this is assignment in this case so in C when you use a single

this case so in C when you use a single equal sign that means means copy the

equal sign that means means copy the value on the right over to the value on

value on the right over to the value on the left from right to left so what does

the left from right to left so what does this do for us well if get string is a

this do for us well if get string is a function that prompts the user with

function that prompts the user with quote unquote what's your name and it

quote unquote what's your name and it has I claim a return value that means it

has I claim a return value that means it kind of hands me back some value but

kind of hands me back some value but it's up to me in C to do something with

it's up to me in C to do something with that value so if I want to copy that

that value so if I want to copy that value into a variable that I can use and

value into a variable that I can use and reuse I use an equal sign and I invent

reuse I use an equal sign and I invent on the left hand side of that equal sign

on the left hand side of that equal sign any variable name I want there's certain

any variable name I want there's certain rules certain conventions but generally

rules certain conventions but generally if you use a single word with all

if you use a single word with all lowercase you're in good shape but C is

lowercase you're in good shape but C is a little more pedantic than that and

a little more pedantic than that and those of you who have have programmed

those of you who have have programmed before might not be used to this for

before might not be used to this for instance in Python which is a world

instance in Python which is a world we'll get to in a few weeks you also

we'll get to in a few weeks you also have to tell see what type of value

have to tell see what type of value you're storing so if I do want a string

you're storing so if I do want a string of text from the user so not an integer

of text from the user so not an integer not a single character I want a whole

not a single character I want a whole string of text like a phrase a sentence

string of text like a phrase a sentence a name in this this case I have to tell

a name in this this case I have to tell C that this variable is of type string

C that this variable is of type string so it's a little wordy but you get used

so it's a little wordy but you get used to it and you just have to be precise

to it and you just have to be precise you're informing the computer what type

you're informing the computer what type of value is going in this

of value is going in this variable all right it's so close to

variable all right it's so close to being correct but I have omitted

being correct but I have omitted something that's annoyingly important

something that's annoyingly important still what's missing still yeah so

still what's missing still yeah so semicolon this is a statement this is

semicolon this is a statement this is like a a a full thought if you will in

like a a a full thought if you will in code I do need to end It ultimately with

code I do need to end It ultimately with the semicolon at the end there all right

the semicolon at the end there all right so this was more of a mouthful but let's

so this was more of a mouthful but let's try using this in now my code let me go

try using this in now my code let me go back to vs code where I have version

back to vs code where I have version zero of my code here let me go ahead and

zero of my code here let me go ahead and include one other file at the top of

include one other file at the top of hello.c namely include cs50.h so that I

hello.c namely include cs50.h so that I have access now to get string and

have access now to get string and anything else I might want now let me go

anything else I might want now let me go ahead and add a line of cod here inside

ahead and add a line of cod here inside of these curly braces and let me go

of these curly braces and let me go ahead and do this string answer equals

ahead and do this string answer equals get string quote unquote what's your

get string quote unquote what's your name question mark I'm going to add an

name question mark I'm going to add an extra space before the double quote I'm

extra space before the double quote I'm going to indeed end my thought with a

going to indeed end my thought with a semicolon and now let me deliberately

semicolon and now let me deliberately make a mistake just to make a point here

make a mistake just to make a point here let me now try changing hello world to

let me now try changing hello world to hello comma answer all right now perhaps

hello comma answer all right now perhaps even though this is some new lines of

even though this is some new lines of code you can see where I've aired

code you can see where I've aired already but let me try making this

already but let me try making this program now so far so good so no error

program now so far so good so no error messages so that's a good thing let me

messages so that's a good thing let me go ahead and run /hello and you'll see

go ahead and run /hello and you'll see the prompt what's your name question

the prompt what's your name question mark and notice the cursor indeed one

mark and notice the cursor indeed one space to the right just because I

space to the right just because I thought it would look prettier to put a

thought it would look prettier to put a little Blank Space there as opposed to

little Blank Space there as opposed to leaving it right after the question mark

leaving it right after the question mark let me type my name but even if you've

let me type my name but even if you've never programmed before I have screwed

never programmed before I have screwed up here what are we going to see on the

up here what are we going to see on the screen when I hit

screen when I hit enter yeah hello answer most likely why

enter yeah hello answer most likely why cuz the computer is going to take me

cuz the computer is going to take me literally and if I say quote unquote

literally and if I say quote unquote hello answer that is the string of text

hello answer that is the string of text followed by a new line that's going to

followed by a new line that's going to be outputed to the screen so we need

be outputed to the screen so we need some way of actually plugging answer

some way of actually plugging answer into this line of code it's not quite as

into this line of code it's not quite as simple as scratch where you could just

simple as scratch where you could just grab like a second say block and drag

grab like a second say block and drag and drop the variable there we actually

and drop the variable there we actually need a new syntax and it's going to look

need a new syntax and it's going to look weird at first but it is everywhere in

weird at first but it is everywhere in software nowadays especially in the

software nowadays especially in the world of c and certain other languages

world of c and certain other languages so let me go ahead and propose that I

so let me go ahead and propose that I solve it as follows well back when we

solve it as follows well back when we did this in scratch remember that the

did this in scratch remember that the most elegant solution was this here we

most elegant solution was this here we used the say block still which is going

used the say block still which is going to be analogous to print F today but I

to be analogous to print F today but I use the join puzzle piece in scratch to

use the join puzzle piece in scratch to combine hello comma space and then the

combine hello comma space and then the name of the human so how do we translate

name of the human so how do we translate this code to C well it's going to look a

this code to C well it's going to look a little different now I'm going to start

little different now I'm going to start with print f with some parth C's and a

with print f with some parth C's and a semicolon representing the say block but

semicolon representing the say block but how do I now do this joining this is

how do I now do this joining this is where the puzzle pieces don't quite

where the puzzle pieces don't quite translate perfectly this would be the

translate perfectly this would be the way to do this you put hello comma and

way to do this you put hello comma and then a placeholder so this is what's

then a placeholder so this is what's known as a format code in C specifically

known as a format code in C specifically for printf and it just means this is a

for printf and it just means this is a placeholder for a string again a string

placeholder for a string again a string is just text so this means hey computer

is just text so this means hey computer print out literally hello comma space

print out literally hello comma space and then not literally percent s percent

and then not literally percent s percent s is uh treated specially to mean plug

s is uh treated specially to mean plug in some value here all right so what

in some value here all right so what else do I still need well this is still

else do I still need well this is still some text so I'm still going going to

some text so I'm still going going to surround the whole thing with double

surround the whole thing with double quotes I'm still going to include my

quotes I'm still going to include my back sln just to keep things tidy and

back sln just to keep things tidy and move the cursor to the next line so the

move the cursor to the next line so the last step here in C is to somehow join

last step here in C is to somehow join the answer with that word hello and the

the answer with that word hello and the way you do this is with printf passing

way you do this is with printf passing it not one argument which is what I keep

it not one argument which is what I keep doing I keep passing it one string of

doing I keep passing it one string of text quote unquote I'm going to now add

text quote unquote I'm going to now add a comma and then the name of the value

a comma and then the name of the value that I want want printf to go back and

that I want want printf to go back and plug into that percent s and printf is

plug into that percent s and printf is just smart about this if you have 1% s

just smart about this if you have 1% s and one additional argument after a

and one additional argument after a comma it just does from right to left it

comma it just does from right to left it plugs it in if you have two percent S's

plugs it in if you have two percent S's and two variables after the comma that's

and two variables after the comma that's okay too if you separate them with

okay too if you separate them with commas it'll plug the first into the

commas it'll plug the first into the first percent s and the second variable

first percent s and the second variable into the second percent s so it's just

into the second percent s so it's just left to right order of operations it's

left to right order of operations it's not as pretty or as uh simple as this

not as pretty or as uh simple as this but this is how it's done in C all right

but this is how it's done in C all right let me pause because this is a lot of

let me pause because this is a lot of symbology any questions on this

symbology any questions on this technique here

technique here [Music]

[Music] yeah yeah really good question why did I

yeah yeah really good question why did I exclude the backslash n a moment ago

exclude the backslash n a moment ago really just my sense of Aesthetics if

really just my sense of Aesthetics if you will No Good Reason beyond that so

you will No Good Reason beyond that so if I look back at my code you quite

if I look back at my code you quite rightly noticed that I didn't have a

rightly noticed that I didn't have a backslash in there that's just because

backslash in there that's just because for whatever sense of style that I have

for whatever sense of style that I have I wanted the name to be typed right

I wanted the name to be typed right after the question I totally could have

after the question I totally could have added a back sln there instead of a

added a back sln there instead of a space that would have just allowed me to

space that would have just allowed me to type down here totally fine just wanted

type down here totally fine just wanted to show you something different good

to show you something different good good catch

good catch yeah can I show an example with 2% s's

yeah can I show an example with 2% s's surely so let me uh in vs code do this

surely so let me uh in vs code do this let me clear my terminal window to clean

let me clear my terminal window to clean things up and let me do this instead of

things up and let me do this instead of calling the variable answer all over the

calling the variable answer all over the place let me call it uh first and I'll

place let me call it uh first and I'll ask two questions what's your first name

ask two questions what's your first name and now let me do string last equals get

and now let me do string last equals get string quot whoops capitalization

string quot whoops capitalization matters so let me fix my capital S there

matters so let me fix my capital S there quote unquote what's your last name

quote unquote what's your last name question mark semicolon and now we'll

question mark semicolon and now we'll plug in one percent s and a second

plug in one percent s and a second percent s and now I'm going to plug in

percent s and now I'm going to plug in first first and last last coincidentally

first first and last last coincidentally and now I'm going to go back to the

and now I'm going to go back to the terminal window make hello crossing my

terminal window make hello crossing my fingers

fingers all good/ hello here's my first question

all good/ hello here's my first question David here's my second question me and

David here's my second question me and again hello David Ma so it just inserts

again hello David Ma so it just inserts them left to right all I was doing for

them left to right all I was doing for parody with scratch though and let me go

parody with scratch though and let me go ahead and undo this again I'll go back

ahead and undo this again I'll go back to answer like this I'll go back to just

to answer like this I'll go back to just asking for the person's name I'm going

asking for the person's name I'm going to delete mention of last I'm going to

to delete mention of last I'm going to delete mention of the second perc s and

delete mention of the second perc s and now if I recompile this simpler

now if I recompile this simpler version I did screw up didn't intend it

version I did screw up didn't intend it what did I do

wrong yeah so just newbie mistakes so I changed my variable back to answer just

changed my variable back to answer just to be consistent with week zero but I

to be consistent with week zero but I didn't change it here so I have an use

didn't change it here so I have an use of Undeclared identifier first it's

of Undeclared identifier first it's Undeclared in the sense that I declared

Undeclared in the sense that I declared answer align prior I didn't declare

answer align prior I didn't declare first so indeed intuitively I want to

first so indeed intuitively I want to just change that to that let me now do

just change that to that let me now do make hello again /hello type in just my

make hello again /hello type in just my first name this time and there it is

first name this time and there it is hello David questions on this then

hello David questions on this then syntax with print F

syntax with print F yeah uh the placeholder I'll zoom in is

yeah uh the placeholder I'll zoom in is just a single percent and then an S so

just a single percent and then an S so inside of my string here is percent s

inside of my string here is percent s and then I have a comma outside the

and then I have a comma outside the quotes and then the name of the variable

quotes and then the name of the variable whose value I want to plug in for that

whose value I want to plug in for that percent s and now notice there's

percent s and now notice there's technically two commas in side of these

technically two commas in side of these parentheses on line seven and yet I

parentheses on line seven and yet I claim that printf at the moment is only

claim that printf at the moment is only taking in two

taking in two arguments why is there then two commas

arguments why is there then two commas but only two arguments if there were two

but only two arguments if there were two commas you would think there would be

commas you would think there would be three arguments

right exactly the comma in between the quotes is just an English thing it's

quotes is just an English thing it's separating the hello from the name so

separating the hello from the name so that's why indeed it's not only in

that's why indeed it's not only in quotes that that's also why programs

quotes that that's also why programs like VSS code tend to syntax highlight

like VSS code tend to syntax highlight it a little differently just so that it

it a little differently just so that it sort of jumps out as different to you

sort of jumps out as different to you even though in this case it's a little

even though in this case it's a little subtle a light blue versus white but

subtle a light blue versus white but indeed it's trying its best other

indeed it's trying its best other questions now on this placeholder

yeah ah good question if I wanted to add an exclamation point after the name

an exclamation point after the name would I have to add another placeholder

would I have to add another placeholder and so forth I could actually do that

and so forth I could actually do that much more simply I can just put the

much more simply I can just put the exclamation point right after the

exclamation point right after the percent sign I don't need an additional

percent sign I don't need an additional placeholder per se if I zoom out now and

placeholder per se if I zoom out now and run make hello again/ hello and type in

run make hello again/ hello and type in just my name no exclamation point now

just my name no exclamation point now you'll see more excitedly hello comma

you'll see more excitedly hello comma David So print f is smart it will figure

David So print f is smart it will figure out where the percent s is and then go

out where the percent s is and then go and replace it now let me propose that a

and replace it now let me propose that a common thing in programming is that as

common thing in programming is that as soon as we make a decision as to how to

soon as we make a decision as to how to design something we often paint

design something we often paint ourselves into a corner and sort of

ourselves into a corner and sort of regret a decision can anyone think of a

regret a decision can anyone think of a problem that arises from using percent S

problem that arises from using percent S as a

as a placeholder in this string to print F

placeholder in this string to print F what could go wrong if we're using

what could go wrong if we're using percent in this special

way yeah if you literally want to say for whatever weird reason percent s on

for whatever weird reason percent s on the screen or honestly even just a

the screen or honestly even just a single percent it turns out that a

single percent it turns out that a percent sign is treated specially inside

percent sign is treated specially inside of of print F strings so what's the

of of print F strings so what's the solution here there's different uh

solution here there's different uh patterns of solutions to problems like

patterns of solutions to problems like these but suppose you wanted to say uh I

these but suppose you wanted to say uh I got 100% for instance let me let me go

got 100% for instance let me let me go ahead and change this completely so I

ahead and change this completely so I got 100% on your test or whatever all

got 100% on your test or whatever all right let me go ahead and run make hello

right let me go ahead and run make hello enter all right so invalid conversion

enter all right so invalid conversion specifier I mean I have no idea what

specifier I mean I have no idea what this means but it's underlining the

this means but it's underlining the percent sign is problematic well it

percent sign is problematic well it turns out that humans years ago decided

turns out that humans years ago decided all right damn it we already Ed percent

all right damn it we already Ed percent well 2% signs will mean 1% literally so

well 2% signs will mean 1% literally so now if I rerun make hello aha hello I

now if I rerun make hello aha hello I got 100% so there's going to be things

got 100% so there's going to be things like that honestly that you have to ask

like that honestly that you have to ask someone you have to Google you have to

someone you have to Google you have to look it up in the documentation but

look it up in the documentation but there's always a solution to those kinds

there's always a solution to those kinds of problems and thankfully they don't

of problems and thankfully they don't come up all that often yeah oh just

come up all that often yeah oh just poting other questions

poting other questions [Music]

[Music] yeah if you have multiple variables it

yeah if you have multiple variables it is in the Left Right order so print F

is in the Left Right order so print F will analyze the first string of text

will analyze the first string of text that you pass in between quotes and

that you pass in between quotes and whatever the first percent is the next

whatever the first percent is the next the first variable that's passed in

the first variable that's passed in after a comma gets plugged in there and

after a comma gets plugged in there and then the second gets plugged into the

then the second gets plugged into the second third and to the third and so

second third and to the third and so forth so it's just based on left to

forth so it's just based on left to right

yeah it's just a placeholder it's called a format code and it just means

a format code and it just means colloquially plug in some value here and

colloquially plug in some value here and printf the like the humans who wrote

printf the like the humans who wrote printf decades ago decided to treat

printf decades ago decided to treat percent s special why just because they

percent s special why just because they needed some placeholder they decided

needed some placeholder they decided that eh no one's ever going to really

that eh no one's ever going to really want to type percent s and if they do

want to type percent s and if they do they can just do percent percent s so

they can just do percent percent s so they decided to implement printf in such

they decided to implement printf in such a way that they have code that look

a way that they have code that look analyzes whatever text comes in looks

analyzes whatever text comes in looks for percent s and then somehow plugs in

for percent s and then somehow plugs in the subsequent values into that

the subsequent values into that placeholder and just this H question

placeholder and just this H question [Music]

[Music] sorry ah so what if you wanted to do

sorry ah so what if you wanted to do single characters like initials like DM

single characters like initials like DM or djm for first middle last absolutely

or djm for first middle last absolutely and that too is a perfect segue from the

and that too is a perfect segue from the two of you to what in general are going

two of you to what in general are going to be called data types in C so it turns

to be called data types in C so it turns out in C there's not only strings as

out in C there's not only strings as text and we'll see in more detail in

text and we'll see in more detail in over the next couple of weeks what a

over the next couple of weeks what a string really is underneath the hood but

string really is underneath the hood but strings of text are not the only thing

strings of text are not the only thing that programs can output they can indeed

that programs can output they can indeed output single characters as for initials

output single characters as for initials they can output integers as well turns

they can output integers as well turns out that printf has different format

out that printf has different format codes for all sorts of different data

codes for all sorts of different data types and just some of the data types

types and just some of the data types we'll see in the coming weeks will be

we'll see in the coming weeks will be this list here which you'll notice it

this list here which you'll notice it almost perfectly lines up with the cs50

almost perfectly lines up with the cs50 functions that I rattled off earlier

functions that I rattled off earlier like get Char get int get string the

like get Char get int get string the reason we called those functions that is

reason we called those functions that is because each of them is designed to

because each of them is designed to return to you a different type of value

return to you a different type of value we've used get string already in this

we've used get string already in this example here we'll soon see get in and

example here we'll soon see get in and we'll see opportunities to use others

we'll see opportunities to use others but these indeed are the menu of

but these indeed are the menu of available data types plus others dot dot

available data types plus others dot dot dot that you can use when writing a

dot that you can use when writing a program in C the onus therefore is on

program in C the onus therefore is on you to decide in advance do I want to

you to decide in advance do I want to store an INT in this variable or a

store an INT in this variable or a string or heck when writing fancier code

string or heck when writing fancier code an image or a sound or a video even

an image or a sound or a video even those can all be different data types

those can all be different data types dot dot dot but for now we'll focus

dot dot dot but for now we'll focus really on just these Primitives that was

really on just these Primitives that was a lot let's go ahead and take a 5minute

a lot let's go ahead and take a 5minute break here no cookies yet but in 5

break here no cookies yet but in 5 minutes we'll come back dive into more

minutes we'll come back dive into more detail and our second break today we'll

detail and our second break today we'll have

cookies all right we are back and so if you have been playing along at home but

you have been playing along at home but hitting some bumps in the road that's

hitting some bumps in the road that's totally normal and indeed the goals of

totally normal and indeed the goals of lecture generally will be to give you a

lecture generally will be to give you a sense uh conceptually of where we'll be

sense uh conceptually of where we'll be going during the course of the week but

going during the course of the week but it's indeed through the Hands-On labs

it's indeed through the Hands-On labs and problem sets that you'll really have

and problem sets that you'll really have an opportunity at your own pace to work

an opportunity at your own pace to work through uh some of those same bumps in

through uh some of those same bumps in the road but for today let me give you a

the road but for today let me give you a few more building blocks and these two

few more building blocks and these two will translate from scratch initially

will translate from scratch initially namely like conditionals like how now

namely like conditionals like how now and see after knowing now how we can use

and see after knowing now how we can use functions at least uh get string and

functions at least uh get string and print F and we can use variables like

print F and we can use variables like the string I created earlier how can I

the string I created earlier how can I now add to the mix things like

now add to the mix things like decisionmaking and conditionals at that

decisionmaking and conditionals at that well with conditionals in scratch we had

well with conditionals in scratch we had this kind of syntax on the left here in

this kind of syntax on the left here in scratch is how you might Express if two

scratch is how you might Express if two variables X and Y have this relationship

variables X and Y have this relationship if x is less than y then say on the

if x is less than y then say on the screen X is less than y well let me

screen X is less than y well let me translate that to the right now in C

translate that to the right now in C code so in C the corresponding code is

code so in C the corresponding code is going to look like this assuming X and Y

going to look like this assuming X and Y already exist more on that later and

already exist more on that later and notice a pattern we're going to see

notice a pattern we're going to see again and again there is going to be

again and again there is going to be parentheses around the X and less than y

parentheses around the X and less than y so parentheses around the Boolean

so parentheses around the Boolean expression recall the Boolean expression

expression recall the Boolean expression is the true false the yes no answer that

is the true false the yes no answer that a question that you're trying to ask in

a question that you're trying to ask in order to decide whether or not to do

order to decide whether or not to do something so you use parentheses there

something so you use parentheses there so similar in functions where we Ed

so similar in functions where we Ed parentheses for printf and parentheses

parentheses for printf and parentheses for get string and this is just a weird

for get string and this is just a weird inconsistency stylistically when using

inconsistency stylistically when using the keyword if you should as a matter of

the keyword if you should as a matter of better best practice put a space after

better best practice put a space after the word if when using a function like

the word if when using a function like print F or get string you shouldn't both

print F or get string you shouldn't both will work but you'll find that these are

will work but you'll find that these are conventions stylistically that most

conventions stylistically that most people adhere to so space when using an

people adhere to so space when using an if here all right now inside of the

if here all right now inside of the curly braces is where the actual code

curly braces is where the actual code goes that you want to execute

goes that you want to execute conditionally so if you want to print

conditionally so if you want to print out X is less than y only if x is

out X is less than y only if x is actually less than y in C you use this

actually less than y in C you use this open curly brace which up until now

open curly brace which up until now you've probably rarely used on your

you've probably rarely used on your keyboard and the Clos curly brace down

keyboard and the Clos curly brace down here and those are kind of hugging if

here and those are kind of hugging if you will the one or more lines of code

you will the one or more lines of code underneath the if very similar in spirit

underneath the if very similar in spirit to how the orange block here kind of

to how the orange block here kind of hugs the purple puzzle piece here so

hugs the purple puzzle piece here so there's no Graphics in C it's all text

there's no Graphics in C it's all text so you can think of those curly braces

so you can think of those curly braces as really representing the same idea as

as really representing the same idea as a side note if you only have one line of

a side note if you only have one line of code inside of the if condition if you

code inside of the if condition if you will you strictly speaking don't need

will you strictly speaking don't need the curly braces but as a matter of good

the curly braces but as a matter of good style do include them it will make more

style do include them it will make more obvious what your intent is how about in

obvious what your intent is how about in scratch if you wanted to express this

scratch if you wanted to express this two ways in the road that you might go

two ways in the road that you might go left or right so to speak well if x is

left or right so to speak well if x is less than y I want to say x is less than

less than y I want to say x is less than y else I want want to say the opposite X

y else I want want to say the opposite X is not less than y in this case so I'm

is not less than y in this case so I'm making a decision based on that Boolean

making a decision based on that Boolean expression in C it's almost the same but

expression in C it's almost the same but you're adding to the mix the keyword Els

you're adding to the mix the keyword Els so MIT borrowed for scratch the same

so MIT borrowed for scratch the same keyword there and a second pair of curly

keyword there and a second pair of curly braces open and close respectively and

braces open and close respectively and you might guess now what goes inside of

you might guess now what goes inside of those well you print out X's less than y

those well you print out X's less than y or you print out X's not less than y all

or you print out X's not less than y all right what if there's a three-way fork

right what if there's a three-way fork in the road in scratch this actually

in the road in scratch this actually gets a little unwieldy graphically if

gets a little unwieldy graphically if you will but notice that in scratch this

you will but notice that in scratch this is how we could express if x is less

is how we could express if x is less than y say x is less than y else if x is

than y say x is less than y else if x is greater than y say x is greater than y

greater than y say x is greater than y else if x equals y then say x is equal

else if x equals y then say x is equal to Y now minor inconsistency here just a

to Y now minor inconsistency here just a little bit ago I claimed in C that a

little bit ago I claimed in C that a equal sign represents what

equal sign represents what operation assignment from right to left

operation assignment from right to left left in so far as scratch is really

left in so far as scratch is really meant for kids and they didn't really

meant for kids and they didn't really want to get into the weeds of this kind

want to get into the weeds of this kind of uh semantic equal sign in scratch

of uh semantic equal sign in scratch means equality however we're going to

means equality however we're going to need to fix this in C in just a moment

need to fix this in C in just a moment in c equal sign means assignment right

in c equal sign means assignment right to left in scratch it literally means

to left in scratch it literally means what you would expect all right let's

what you would expect all right let's translate this code then to C on the

translate this code then to C on the right this code would correspond really

right this code would correspond really to this and you can perhaps see somewhat

to this and you can perhaps see somewhat goofy what the solution was not unlike

goofy what the solution was not unlike the percent percent solution earlier

the percent percent solution earlier when hum painted themselves into one

when hum painted themselves into one other corner you say if you say else if

other corner you say if you say else if and you say else if and how did we

and you say else if and how did we resolve the use of a single equal sign

resolve the use of a single equal sign already in C when you want to repr when

already in C when you want to repr when you want to express equality is the

you want to express equality is the thing on the left equal to the thing on

thing on the left equal to the thing on the right you literally use two equal

the right you literally use two equal signs right next to each other no space

signs right next to each other no space in between them but now this code would

in between them but now this code would be correct on both the left and the

be correct on both the left and the right whether you're doing this in

right whether you're doing this in scratch or C respectively but now we can

scratch or C respectively but now we can kind of nitpick our code specifically

kind of nitpick our code specifically the the design thereof logically can

the the design thereof logically can anyone critique the design of this code

anyone critique the design of this code either in scratch or C like I feel like

either in scratch or C like I feel like we could do better how about in

back perfect logically it's got to be the case that X is less than y or X is

the case that X is less than y or X is greater than y or by conclusion it's got

greater than y or by conclusion it's got to be equal to Y so why are you wasting

to be equal to Y so why are you wasting my time or the computer's time asking a

my time or the computer's time asking a third question you don't need to ask

third question you don't need to ask this final else if because logically as

this final else if because logically as you knowe it should go without saying so

you knowe it should go without saying so it's a minor tweak like you're doing

it's a minor tweak like you're doing extra work potentially in cases where x

extra work potentially in cases where x equals y so we can just refine that and

equals y so we can just refine that and just like in scratch you could just use

just like in scratch you could just use an else block similarly in C could we

an else block similarly in C could we simplify this code to just an else a

simplify this code to just an else a sort of catchall logically that just

sort of catchall logically that just handles the reality that of course

handles the reality that of course that's going to be the final situation

that's going to be the final situation instead all right so we have this

instead all right so we have this ability now to express conditionals with

ability now to express conditionals with Boolean expressions let's actually do

Boolean expressions let's actually do something with this next here so let me

something with this next here so let me go back to vs code I've closed hello.c

go back to vs code I've closed hello.c and I want to create a second file for

and I want to create a second file for the sake of some demos now recall that

the sake of some demos now recall that you can create new files by typing code

you can create new files by typing code space and then the name of the file you

space and then the name of the file you want to create uh for instance I might

want to create uh for instance I might do compare. c I want to write a program

do compare. c I want to write a program that's going to start comparing some

that's going to start comparing some values for demonstration sake but before

values for demonstration sake but before I do that let me just show you by

I do that let me just show you by opening the file explorer at right this

opening the file explorer at right this is similar in spirit to a Mac or PC like

is similar in spirit to a Mac or PC like you can go up here and click on an icon

you can go up here and click on an icon and you can click on the plus icon and

and you can click on the plus icon and you'll get a blue box and I can type in

you'll get a blue box and I can type in compare. C and I can just manually

compare. C and I can just manually create it that way notice that opens the

create it that way notice that opens the tab even without my having typed code so

tab even without my having typed code so again on the left you have a guey a

again on the left you have a guey a graphical user interface albeit a

graphical user interface albeit a simplistic one on the right and at the

simplistic one on the right and at the bottom here you have a command line

bottom here you have a command line interface but they're one and the same

interface but they're one and the same what's nice though is that if I close

what's nice though is that if I close this file accidentally intentionally

this file accidentally intentionally whatnot I can reopen it without creating

whatnot I can reopen it without creating a new one by just running that same

a new one by just running that same command code space compare. c so code is

command code space compare. c so code is a vs code thing it's just a userfriendly

a vs code thing it's just a userfriendly shortcut but it's just creating a file

shortcut but it's just creating a file or opening an existing file like that

or opening an existing file like that I'm going to hide the file explorer just

I'm going to hide the file explorer just to make more room for code here and

to make more room for code here and let's go ahead and do this let's write a

let's go ahead and do this let's write a program that compares two values that

program that compares two values that the human inputs but not strings this

the human inputs but not strings this time let's use some actual integers all

time let's use some actual integers all right I'm going to go ahead and include

right I'm going to go ahead and include the cs-50 librar header file at top

the cs-50 librar header file at top cs50.h I'm going to also include

cs50.h I'm going to also include standard io. why one gives me

standard io. why one gives me userfriendly input via get string get

userfriendly input via get string get int and so forth one gives me user

int and so forth one gives me user friendly output via printf in the case

friendly output via printf in the case of standard i.h now I'm just going to

of standard i.h now I'm just going to kind of blindly type this line of code

kind of blindly type this line of code which we'll come back to in future weeks

which we'll come back to in future weeks but for now that's analogous to the when

but for now that's analogous to the when green flag clicked code in scratch and

green flag clicked code in scratch and now let's go ahead and do this let me go

now let's go ahead and do this let me go ahead and get an INT from the user and

ahead and get an INT from the user and ask the user what's X question mark I'm

ask the user what's X question mark I'm not going to bother with a new line I

not going to bother with a new line I want to keep it all in one line just for

want to keep it all in one line just for Aesthetics sake but when I get back an

Aesthetics sake but when I get back an INT just like I get back a string I get

INT just like I get back a string I get back a return value so if I want to

back a return value so if I want to store the result of get int somewhere I

store the result of get int somewhere I had better put it in a variable and I

had better put it in a variable and I can call the variable anything I want

can call the variable anything I want previously I used answer or first or

previously I used answer or first or last now I'm going to use x but there's

last now I'm going to use x but there's still two things left to do here

still two things left to do here logically even though we haven't

logically even though we haven't technically done this yet what do I

technically done this yet what do I still need to

still need to do so I need the semicolon at the

do so I need the semicolon at the end and the in at the beginning you the

end and the in at the beginning you the programmer starting today kind of need

programmer starting today kind of need to decide what you're going to be

to decide what you're going to be storing in your variables and you just

storing in your variables and you just need to tell the computer that so that

need to tell the computer that so that it knows now as a teaser for languages

it knows now as a teaser for languages like python more modern languages turns

like python more modern languages turns out humans realize well gee this is

out humans realize well gee this is stupid like why can't the computer just

stupid like why can't the computer just figure out that I'm putting an INT there

figure out that I'm putting an INT there why do I have to tell it proactively so

why do I have to tell it proactively so in some languages nowadays like python

in some languages nowadays like python we'll get rid of some of this syntax

we'll get rid of some of this syntax we'll get rid of the semicolons but for

we'll get rid of the semicolons but for now we're looking at really the origins

now we're looking at really the origins of how this all worked all right so I've

of how this all worked all right so I've done this one line ending with semicolon

done this one line ending with semicolon let me do one other and let me get a

let me do one other and let me get a second int asking the user what's why

second int asking the user what's why question mark so almost identical but

question mark so almost identical but different responses from the user

different responses from the user hopefully and let me just ask simply if

hopefully and let me just ask simply if x is less than y in parentheses then

x is less than y in parentheses then some curly braces let me go ahead and

some curly braces let me go ahead and print out quote unquote X is less than y

print out quote unquote X is less than y back sln and now just as a side note I I

back sln and now just as a side note I I seem to be typing kind of fast some of

seem to be typing kind of fast some of that is because vs code is helping me

that is because vs code is helping me let me go back to this

let me go back to this first line with the if hit enter and now

first line with the if hit enter and now I'm only on my keyboard going to type

I'm only on my keyboard going to type the open curly brace this is a feature

the open curly brace this is a feature of many text editors nowadays it

of many text editors nowadays it finishes part of your thought why just

finishes part of your thought why just to save yourself a keystroke to make

to save yourself a keystroke to make sure you don't accidentally forget the

sure you don't accidentally forget the closing one so you'll notice sometimes

closing one so you'll notice sometimes that things are happening that you

that things are happening that you didn't type it's just vs code or future

didn't type it's just vs code or future programs you use trying to be helpful

programs you use trying to be helpful for you I'll go ahead and manually type

for you I'll go ahead and manually type out now printf uh X is less than y back

out now printf uh X is less than y back slend close quote semicolon so let me go

slend close quote semicolon so let me go ahead now and try to run this and we'll

ahead now and try to run this and we'll see let's see so make not hello but make

see let's see so make not hello but make compare because this file is called

compare because this file is called compare. C hitting enter okay no output

compare. C hitting enter okay no output is good because it means I haven't

is good because it means I haven't messed up let me do dot SL compare

messed up let me do dot SL compare instead of/ hello enter what's X how

instead of/ hello enter what's X how about one what's Y how about two x is

about one what's Y how about two x is less than y well let's try it again and

less than y well let's try it again and here I'll save you some keystrokes too

here I'll save you some keystrokes too let me clear my screen instead of

let me clear my screen instead of constantly typing dot slash this and Dot

constantly typing dot slash this and Dot slash that you can also use your

slash that you can also use your keyboard's arrow keys in vs code to

keyboard's arrow keys in vs code to scroll back through time so if I hit up

scroll back through time so if I hit up once there's the last command I wrote If

once there's the last command I wrote If I write uh do it up twice there's the

I write uh do it up twice there's the second to last command I wrote so

second to last command I wrote so sometimes if you see me doing things

sometimes if you see me doing things fast it's just because I'm kind of

fast it's just because I'm kind of cheating and going through my history

cheating and going through my history like that all right let me go ahead

like that all right let me go ahead though and

though and rerun compare enter let's reverse it

rerun compare enter let's reverse it this time two for x one for y and now of

this time two for x one for y and now of course there's no no output all right

course there's no no output all right well that's logically to be expected

well that's logically to be expected because we didn't have an else here so

because we didn't have an else here so let's add that else now let's open my

let's add that else now let's open my curly braces letting vs code do one of

curly braces letting vs code do one of them for me print F quote unquote X is

them for me print F quote unquote X is not less than y back sln semicolon let

not less than y back sln semicolon let me go ahead and try this again/ compare

me go ahead and try this again/ compare enter again two for x one for y and we

enter again two for x one for y and we should see

should see huh what did I do wrong why am I not

huh what did I do wrong why am I not seeing any else output

yeah exactly you got to get into the Habit after you change your code of

Habit after you change your code of recompiling it otherwise the zeros and

recompiling it otherwise the zeros and ones in the server are the old ones

ones in the server are the old ones until you manually compile so let's fix

until you manually compile so let's fix this make compare enter no error

this make compare enter no error messages that's good do/ compare 2 1 and

messages that's good do/ compare 2 1 and now I get back the output so X is not

now I get back the output so X is not less than y how about if I go and add in

less than y how about if I go and add in the third condition well we can do this

the third condition well we can do this either efficiently or inefficiently let

either efficiently or inefficiently let me go ahead head and refine this so else

me go ahead head and refine this so else if x is greater than y let's literally

if x is greater than y let's literally say x is greater than y and now I could

say x is greater than y and now I could do X else if x equals equals y but I

do X else if x equals equals y but I think we already claim that that's

think we already claim that that's unnecessarily inefficient so let's just

unnecessarily inefficient so let's just have our catchall and here I'm going to

have our catchall and here I'm going to say quote unquote X is equal 2 y back

say quote unquote X is equal 2 y back sln close quote there so I think now

sln close quote there so I think now with this code we've handled all three

with this code we've handled all three scenarios let me go ahead and recompile

scenarios let me go ahead and recompile it properly compare do/ compare and now

it properly compare do/ compare and now 1 and 2 x is less than y let me run it

1 and 2 x is less than y let me run it again 2 and One X is greater than y and

again 2 and One X is greater than y and lastly one and one and X is equal to Y

lastly one and one and X is equal to Y so for the most part our code's getting

so for the most part our code's getting longer we're up to like 21 lines of code

longer we're up to like 21 lines of code though some of them are just single

though some of them are just single characters on the screen almost

characters on the screen almost everything else is the same I'm using

everything else is the same I'm using the cs50 libraries header file for my

the cs50 libraries header file for my get int function standard i.h for my

get int function standard i.h for my print F function and the rest of this is

print F function and the rest of this is just now new syntax for conditionals as

just now new syntax for conditionals as well questions then on this C

well questions then on this C implementation of just some basic

implementation of just some basic comparisons like

comparisons like this any questions

this any questions [Music]

[Music] yeah good question do the opening

yeah good question do the opening brackets need to be on a separate line

brackets need to be on a separate line in cs50 yes what you'll see is that as

in cs50 yes what you'll see is that as part of the submission process We

part of the submission process We compare your code against a style guide

compare your code against a style guide which is the norm in Industry a company

which is the norm in Industry a company would have its own sense of style and

would have its own sense of style and how its code should look and there's

how its code should look and there's generally automated tools within a

generally automated tools within a company that help uh give feedback on

company that help uh give feedback on the code or stylize it as such there are

the code or stylize it as such there are alternative Styles than what we use in

alternative Styles than what we use in the class we deliberately keep and ask

the class we deliberately keep and ask that you keep the curly braces on their

that you keep the curly braces on their own line if only because it rather

own line if only because it rather resembles like the hugging nature of

resembles like the hugging nature of scratches blocks and just makes clear

scratches blocks and just makes clear that they're balanced open and closed

that they're balanced open and closed however another common Paradigm in some

however another common Paradigm in some languages and with some programmers is

languages and with some programmers is to do something like this uh on each of

to do something like this uh on each of them so you have the opening curly brace

them so you have the opening curly brace on the same line as here we do not

on the same line as here we do not recommend this this is invogue in the

recommend this this is invogue in the JavaScript world and some others um but

JavaScript world and some others um but ultimately in the real world it's up to

ultimately in the real world it's up to each individual programmer Andor the

each individual programmer Andor the company they're working for if

company they're working for if applicable to decide on those

applicable to decide on those things all right so beyond then these

things all right so beyond then these conditionals what if we want to do

conditionals what if we want to do something that's maybe pretty common so

something that's maybe pretty common so almost every piece of software or

almost every piece of software or website nowadays that you use has you

website nowadays that you use has you like agree to some terms and conditions

like agree to some terms and conditions by typing like yes or no or just y for

by typing like yes or no or just y for yes and N for no so how could we

yes and N for no so how could we Implement some kind of Agreement System

Implement some kind of Agreement System well let me do this let me create a new

well let me do this let me create a new program a third one called agree. C so

program a third one called agree. C so I'm going to write code agree. C just to

I'm going to write code agree. C just to give myself a new tab I'm going to start

give myself a new tab I'm going to start as always now include cs50.h let's

as always now include cs50.h let's include standard i.h and then let me do

include standard i.h and then let me do my int main void which again for today's

my int main void which again for today's purposes we'll take it face value is

purposes we'll take it face value is just copy paste and if I just want to

just copy paste and if I just want to get y or n for instance instead of yes

get y or n for instance instead of yes or no we can just use a simpler variable

or no we can just use a simpler variable here how about just a Char a character A

here how about just a Char a character A Single Character so I can use get Char

Single Character so I can use get Char to ask the user for instance do you

to ask the user for instance do you agree question mark but before as before

agree question mark but before as before I need to store this somewhere so I

I need to store this somewhere so I don't want a string because it's a

don't want a string because it's a single Char I don't want an INT I just

single Char I don't want an INT I just want a Char and it's literally CH h a r

want a Char and it's literally CH h a r and then I could call this thing

and then I could call this thing anything I want it's conventional if you

anything I want it's conventional if you have a simple program with just a single

have a simple program with just a single variable and it's of type Char call it C

variable and it's of type Char call it C if it's an INT call it I if it's a

if it's an INT call it I if it's a string call it s for now I'm just going

string call it s for now I'm just going to keep it simple and call it C and now

to keep it simple and call it C and now I'm going to ask a question so if C

I'm going to ask a question so if C equals equals how about quote unquote

equals equals how about quote unquote y then let me go ahead and print out uh

y then let me go ahead and print out uh agreed back sln as though they agreed to

agreed back sln as though they agreed to my terms and conditions uh otherwise

my terms and conditions uh otherwise let's see else if the character equals

let's see else if the character equals equals quote unquote n

equals quote unquote n then let me go ahead and print out uh

then let me go ahead and print out uh say not agreed as though they didn't

say not agreed as though they didn't quote unquote and let's leave it at that

quote unquote and let's leave it at that I think here initially now you'll notice

I think here initially now you'll notice one curiosity one inconsistency perhaps

one curiosity one inconsistency perhaps does anyone want to call it out though

does anyone want to call it out though it's somewhat

it's somewhat subtle I've done something ever so

subtle I've done something ever so slightly differently without explaining

slightly differently without explaining it yet do you see it single single yeah

it yet do you see it single single yeah so I've suddenly used single quotation

so I've suddenly used single quotation marks for my single characters and

marks for my single characters and double quotes for my actual strings of

double quotes for my actual strings of text this is a necessity in C when

text this is a necessity in C when you're dealing with strings like strings

you're dealing with strings like strings of text like someone's name a sentence a

of text like someone's name a sentence a paragraph anything really more than one

paragraph anything really more than one character you typically use uh double

character you typically use uh double quotes and indeed you must when dealing

quotes and indeed you must when dealing with deliberately single characters like

with deliberately single characters like I am here for y or n you must use single

I am here for y or n you must use single quotes instead why because that makes

quotes instead why because that makes sure that the computer knows that it's

sure that the computer knows that it's indeed a Char and not a string so double

indeed a Char and not a string so double quotes are for Strings single quotes are

quotes are for Strings single quotes are for chars so with that said let me go

for chars so with that said let me go ahead and zoom out let me go ahead in my

ahead and zoom out let me go ahead in my terminal window run make agree enter

terminal window run make agree enter seems to work okay so let me go ahead

seems to work okay so let me go ahead and do/ agree uh let me go ahead now and

and do/ agree uh let me go ahead now and type in y here we go

type in y here we go enter huh uh let me try that again rerun

enter huh uh let me try that again rerun do/ agree how about no

do/ agree how about no enter why is it not behaving as I would

enter why is it not behaving as I would have expected uh because you the capital

have expected uh because you the capital Y yeah I kind of cheated there and I hit

Y yeah I kind of cheated there and I hit the caps lock key just as I started

the caps lock key just as I started typing in input why because I

typing in input why because I deliberately wanted to type in uppercase

deliberately wanted to type in uppercase instead of lowercase which is kind of

instead of lowercase which is kind of reasonable right it's a little obnoxious

reasonable right it's a little obnoxious if you force the user to toggle their

if you force the user to toggle their caps lock key on or off when you just

caps lock key on or off when you just need a simple answer that's not the best

need a simple answer that's not the best user experience or ux but it would work

user experience or ux but it would work if I cooperated let me run this again

if I cooperated let me run this again without caps lock on y lowercase for yes

without caps lock on y lowercase for yes ah that worked and uh lowercase for no

ah that worked and uh lowercase for no that work worked but how could I get it

that work worked but how could I get it to work for both well how about this let

to work for both well how about this let me go ahead and just add two

me go ahead and just add two possibilities so else if C equals equals

possibilities so else if C equals equals quote unquote capital Y then also do

quote unquote capital Y then also do printf agreed back sln and down here

printf agreed back sln and down here else if C equals equals uh single quote

else if C equals equals uh single quote uh capital N then go ahead and print out

uh capital N then go ahead and print out again not agreed okay this I will claim

again not agreed okay this I will claim now is correct and I'll do make agree

now is correct and I'll do make agree real fast do slash agree and I'll use

real fast do slash agree and I'll use Capital it now works I'll use uh Capital

Capital it now works I'll use uh Capital it again works but this is perhaps not

it again works but this is perhaps not the best design let me hide the terminal

the best design let me hide the terminal window and pull this up on the screen

window and pull this up on the screen all at

all at once why might this arguably not be the

once why might this arguably not be the best design even though it's

correct there's another term of art we can toss here like something smells kind

can toss here like something smells kind of funky about this code this is an

of funky about this code this is an actual term of art like there's code

actual term of art like there's code smell here like something smells a

smell here like something smells a little off why what do you

little off why what do you [Music]

[Music] think yeah there's the same output again

think yeah there's the same output again and again I mean I manually typed it but

and again I mean I manually typed it but honestly I might as well have just

honestly I might as well have just copied and pasted most of my original

copied and pasted most of my original code to do it again and again for the C

code to do it again and again for the C two capital letters so if line 10 and 14

two capital letters so if line 10 and 14 are the same and line 18 and 22 are the

are the same and line 18 and 22 are the same and then the rest of these if and

same and then the rest of these if and Els ifs are almost the same like like

Els ifs are almost the same like like there's some code smell there like it's

there's some code smell there like it's not well-designed why because if I want

not well-designed why because if I want to change things now just like last week

to change things now just like last week in scratch I might have to change my

in scratch I might have to change my code in multiple places or copy paste is

code in multiple places or copy paste is never a good thing and go God forbid I

never a good thing and go God forbid I want to add support for yes and no as

want to add support for yes and no as full words it's really going to get long

full words it's really going to get long so how can we solve this well it turns

so how can we solve this well it turns out like we can combine some of these

out like we can combine some of these thoughts so let me try to improve the

thoughts so let me try to improve the yeses first it turns out if I delete

yeses first it turns out if I delete that Clause I can actually or things

that Clause I can actually or things together in scratch there's a couple

together in scratch there's a couple puzzle pieces if you didn't discover

puzzle pieces if you didn't discover them that literally have the word or and

them that literally have the word or and the word and on them which allow you to

the word and on them which allow you to combine Boolean Expressions so that

combine Boolean Expressions so that either this or this is true or this and

either this or this is true or this and this is true in C you can't just say the

this is true in C you can't just say the word or you instead use two vertical

word or you instead use two vertical bars and vertical bars together mean or

bars and vertical bars together mean or logically and so I can say C equals

logically and so I can say C equals equals quote unquote capital Y agreed

equals quote unquote capital Y agreed and now I can get rid of this code down

and now I can get rid of this code down here and let me go ahead and say

here and let me go ahead and say vertical vertical bar twice C equals uh

vertical vertical bar twice C equals uh quote unquote n in all caps and now my

quote unquote n in all caps and now my program's like you know roughly a third

program's like you know roughly a third uh smaller which is good there's less

uh smaller which is good there's less redundancy and if I reopen my terminal

redundancy and if I reopen my terminal window rerun make of agree SL agree now

window rerun make of agree SL agree now I can type little y or Big Y and same

I can type little y or Big Y and same thing for lowercase and uppercase n any

thing for lowercase and uppercase n any questions then on this syntax whereby

questions then on this syntax whereby now you can combine thoughts and just

now you can combine thoughts and just kind of tighten things up

kind of tighten things up and there'll be other such tricks too

yeah a really good question is there not a function to just ignore the case short

a function to just ignore the case short answer there is and we'll see how to do

answer there is and we'll see how to do that in actually just about a week's

that in actually just about a week's time and in other languages there's even

time and in other languages there's even more ways to just canonicalize the

more ways to just canonicalize the user's input throwing away any space

user's input throwing away any space characters they might have accidentally

characters they might have accidentally hit forcing everything to lower case in

hit forcing everything to lower case in C it's going to be a little more work on

C it's going to be a little more work on our part to do that but in fact as early

our part to do that but in fact as early as next week we'll see how we can do

as next week we'll see how we can do that but for now we're comparing indeed

that but for now we're comparing indeed just these literal values other

just these literal values other [Music]

[Music] questions really good question so we are

questions really good question so we are assuming with this program in all of my

assuming with this program in all of my last ones that the human's cooperating

last ones that the human's cooperating when I asked for their name they typed

when I asked for their name they typed in David and not one two three or in

in David and not one two three or in this case they typed in a single

this case they typed in a single character and not a full word so this is

character and not a full word so this is one of the features often of using a

one of the features often of using a library so for instance if I run agree

library so for instance if I run agree again

again and I say something like sure enter it

and I say something like sure enter it rejects it alt together why because s e

rejects it alt together why because s e is a string of characters it's not a

is a string of characters it's not a single character now I could just say

single character now I could just say something like X which is neither y nor

something like X which is neither y nor n of course but it tolerates that

n of course but it tolerates that because it's a single character but

because it's a single character but built into cs50's library is some

built into cs50's library is some built-in rejections of inputs that's not

built-in rejections of inputs that's not expected so if you use get int and the

expected so if you use get int and the user types in not the number one or two

user types in not the number one or two but cat C A it will just prompt them

but cat C A it will just prompt them again prompt them again and this is

again prompt them again and this is where too if you were to do this

where too if you were to do this manually in C you end up writing this

manually in C you end up writing this much code just to check for all of these

much code just to check for all of these errors that's why we use these training

errors that's why we use these training wheels for a few weeks just to make the

wheels for a few weeks just to make the code more robust but in a few weeks time

code more robust but in a few weeks time we'll take the library away and you'll

we'll take the library away and you'll see and understand how it's doing all

see and understand how it's doing all it's indeed doing all that all right so

it's indeed doing all that all right so how about this let's now transition to

how about this let's now transition to something a little more scratch like

something a little more scratch like literally uh by creating how about

literally uh by creating how about another program here called meow so

another program here called meow so meow. C we won't have any audio

meow. C we won't have any audio capabilities for this one will just rely

capabilities for this one will just rely on print and suppose that I wanted to

on print and suppose that I wanted to write a program in C that just simulates

write a program in C that just simulates like a cat meowing so I don't need any

like a cat meowing so I don't need any user input just yet so I'm just going to

user input just yet so I'm just going to use standard i.h I'm going to do my

use standard i.h I'm going to do my usual int main void up here and then I'm

usual int main void up here and then I'm just going to go ahead and do printf

just going to go ahead and do printf meow back sln and let's have this C meow

meow back sln and let's have this C meow three times like last week so I'm going

three times like last week so I'm going to do meow meow meow notice as an aside

to do meow meow meow notice as an aside whenever you highlight the lines you'll

whenever you highlight the lines you'll see little dots appear this is just a

see little dots appear this is just a visual cue to you to let you figure out

visual cue to you to let you figure out how many spaces you've indented vs code

how many spaces you've indented vs code like a lot of editors will automatically

like a lot of editors will automatically indent your code for you I've not been

indent your code for you I've not been hitting the space bar four times every

hitting the space bar four times every time I've not even been hitting tab

time I've not even been hitting tab however in C the convention is indeed to

however in C the convention is indeed to indent lines where appropriate by four

indent lines where appropriate by four spaces so not three not five and these

spaces so not three not five and these dots help you see things so that they

dots help you see things so that they just line up as a matter of good style

just line up as a matter of good style all right so this program I'm just going

all right so this program I'm just going to stipulate right now is indeed going

to stipulate right now is indeed going to work make meow which is kind of cute

to work make meow which is kind of cute and now meow there three times correct

and now meow there three times correct it's meowing three times but of course

it's meowing three times but of course this is not well designed it wasn't well

this is not well designed it wasn't well designed in scratch last week why what

designed in scratch last week why what should I be doing

should I be doing differently

differently yeah

yeah yeah it's a perfect like opportunity for

yeah it's a perfect like opportunity for a loop why because if you wanted to

a loop why because if you wanted to change maybe the capitalization of these

change maybe the capitalization of these words you wanted to change the sound to

words you wanted to change the sound to like woof or a dog or something like

like woof or a dog or something like you'd have to change it one two three

you'd have to change it one two three places and that's just kind of stupid

places and that's just kind of stupid right in code you should ideally change

right in code you should ideally change things in one place so how might I do

things in one place so how might I do that well we could introduce a loop yes

that well we could introduce a loop yes but we're going to need another building

but we're going to need another building block as well that we had in scratch

block as well that we had in scratch namely those things called variables so

namely those things called variables so we're call that a variable like in

we're call that a variable like in algebra x y z whatever can store a value

algebra x y z whatever can store a value for you and a variable in scratch might

for you and a variable in scratch might have looked like this you use this

have looked like this you use this orange puzzle piece to set a variable of

orange puzzle piece to set a variable of any name not just X Y or Z but you could

any name not just X Y or Z but you could call it something more descriptive like

call it something more descriptive like counter and you can set it equal to some

counter and you can set it equal to some value in C the way to do this is similar

value in C the way to do this is similar to to Spirit to some of the syntax we've

to to Spirit to some of the syntax we've seen thus far you start by saying the

seen thus far you start by saying the name of the variable you want a single

name of the variable you want a single equal sign and then the value you want

equal sign and then the value you want to initialize it to copying therefore

to initialize it to copying therefore from right to left why because the equal

from right to left why because the equal sign denotes again assignment from right

sign denotes again assignment from right to left this isn't enough though you

to left this isn't enough though you might have the intuition already what's

might have the intuition already what's missing probably from this line of code

missing probably from this line of code just to create a variable so we need int

just to create a variable so we need int to make sure the computer knows that

to make sure the computer knows that this is indeed an INT and then lastly

this is indeed an INT and then lastly semicolon as well and that now completes

semicolon as well and that now completes the thought so a little more annoying

the thought so a little more annoying than scratch but we're starting to see

than scratch but we're starting to see patterns here so not every piece of

patterns here so not every piece of syntax will be new all right if you

syntax will be new all right if you wanted to increment the counter by one

wanted to increment the counter by one scratch uses the verb change and they

scratch uses the verb change and they mean add the value to counter so if I

mean add the value to counter so if I want to increment an existing variable

want to increment an existing variable called counter this syntax is a little

called counter this syntax is a little more interesting it turns out the code

more interesting it turns out the code looks like this which almost seems like

looks like this which almost seems like a paradox like how can counter equal

a paradox like how can counter equal counter plus one like that's not how

counter plus one like that's not how math works but again a single equal sign

math works but again a single equal sign is assignment from right to left so this

is assignment from right to left so this is saying take whatever the value of

is saying take whatever the value of counter is add one to it and copy that

counter is add one to it and copy that value from right to left into counter

value from right to left into counter itself you still need the semicolon but

itself you still need the semicolon but I claim you do not need to mention the

I claim you do not need to mention the keyword int when updating an existing

keyword int when updating an existing variable so only when you create a

variable so only when you create a variable in C do you use the word string

variable in C do you use the word string or the word int or any of the others

or the word int or any of the others will eventually see only when creating

will eventually see only when creating it or initializing It For the First Time

it or initializing It For the First Time thereafter if you want to change it it

thereafter if you want to change it it just exists it's the word you gave it

just exists it's the word you gave it the computer's smart enough to at least

the computer's smart enough to at least remember what type it is so this line is

remember what type it is so this line is now complete turns out in code as we'll

now complete turns out in code as we'll see it's pretty common to want to add

see it's pretty common to want to add things together increase increment

things together increase increment Things by One so there's actually

Things by One so there's actually different Syntax for the same idea the

different Syntax for the same idea the term of art here is syntactic sugar like

term of art here is syntactic sugar like there's often in code many ways to do

there's often in code many ways to do the same thing even though at the end of

the same thing even though at the end of the day they do exactly the same

the day they do exactly the same functionality so for instance if after a

functionality so for instance if after a few days of cs50 you find this a little

few days of cs50 you find this a little tedious to keep typing and some program

tedious to keep typing and some program you can simplify it to just this this is

you can simplify it to just this this is the syntactic sugar you can use plus

the syntactic sugar you can use plus equals and only mention the variable

equals and only mention the variable name once on the left and it just knows

name once on the left and it just knows that that means the previous thing it's

that that means the previous thing it's just slightly uh more uh more succinct

just slightly uh more uh more succinct this to is such a common thing to add

this to is such a common thing to add one to a value and it doesn't have to be

one to a value and it doesn't have to be one but in this case it is but if it is

one but in this case it is but if it is indeed one you can further tighten the

indeed one you can further tighten the code up to just do this counter Plus+ so

code up to just do this counter Plus+ so anytime in C you see plus plus it means

anytime in C you see plus plus it means literally adding one to that particular

literally adding one to that particular variable there's other ways to do this

variable there's other ways to do this in the other direction if you want to

in the other direction if you want to subtract one from a variable you can use

subtract one from a variable you can use any of the previous syntax using a minus

any of the previous syntax using a minus sign instead of Plus or you can more

sign instead of Plus or you can more succinctly do counter minus minus this

succinctly do counter minus minus this is the way a typical C programmer would

is the way a typical C programmer would do this all right so if we have now

do this all right so if we have now variables let's go and solve the meowing

variables let's go and solve the meowing with loops so in scratch we saw Loops

with loops so in scratch we saw Loops like this this of course had the cat

like this this of course had the cat meow three times how do we do this in C

meow three times how do we do this in C now this this is where things get a

now this this is where things get a little more uh involved codewise but if

little more uh involved codewise but if you understand each and every line it

you understand each and every line it will follow logically what's going on so

will follow logically what's going on so here I claim is one way to implement a

here I claim is one way to implement a loop that iterates three times in C and

loop that iterates three times in C and this is kind of ridiculous right like we

this is kind of ridiculous right like we went from two super simple puzzle pieces

went from two super simple puzzle pieces like this to my God like it's 1 2 3 four

like this to my God like it's 1 2 3 four five six lines of code all of which are

five six lines of code all of which are pretty involved so like that escalated

pretty involved so like that escalated quickly but what's each line doing and

quickly but what's each line doing and we'll see other ways to do this more

we'll see other ways to do this more simply so we're izing a variable called

simply so we're izing a variable called counter to three just like before why

counter to three just like before why well what does it mean to Loop or to

well what does it mean to Loop or to repeat something three times well it's

repeat something three times well it's kind of like doing something three times

kind of like doing something three times and then do it and then count down and

and then do it and then count down and then do it and then count down and then

then do it and then count down and then do it until you're all out of counts so

do it until you're all out of counts so this is declaring a variable called

this is declaring a variable called counter setting it equal to three then

counter setting it equal to three then I'm inducing a loop in C which is

I'm inducing a loop in C which is similar in spirit to repeat three but

similar in spirit to repeat three but you have to do more of the math yourself

you have to do more of the math yourself so I'm asking the question in

so I'm asking the question in parentheses

parentheses while counter is greater than zero what

while counter is greater than zero what do I want to do well Pur the indentation

do I want to do well Pur the indentation inside the curly braces I want to meow

inside the curly braces I want to meow one time and then to be clear what's

one time and then to be clear what's this last line of code doing if counter

this last line of code doing if counter starts off at three this makes it two by

starts off at three this makes it two by subtracting one from it then what

subtracting one from it then what happens by nature of a loop just like in

happens by nature of a loop just like in scratch it kind of knows to go back and

scratch it kind of knows to go back and forth even though there's nice pretty

forth even though there's nice pretty arrow in scratch and there isn't here C

arrow in scratch and there isn't here C knows to do this again and again and

knows to do this again and again and again con stantly asking this question

again con stantly asking this question and then updating this value at the end

and then updating this value at the end so if I highlight just a few of these

so if I highlight just a few of these steps the variable starts off at three

steps the variable starts off at three and actually let me simplify two I

and actually let me simplify two I claimed earlier that uh when using

claimed earlier that uh when using single variables people very often just

single variables people very often just call it I for INT or c for Char or s for

call it I for INT or c for Char or s for string unless you have multiple

string unless you have multiple variables so let me tighten the code up

variables so let me tighten the code up and this already makes it look a little

and this already makes it look a little more tolerable let me actually tighten

more tolerable let me actually tighten it up further and one more step so now

it up further and one more step so now this is about as tight as succinct as

this is about as tight as succinct as you can make this code at the moment so

you can make this code at the moment so what's actually going to happen here

what's actually going to happen here well the first line of code executes and

well the first line of code executes and that initializes I to 3 then we check

that initializes I to 3 then we check the condition while I is greater than

the condition while I is greater than zero is I greater than zero we'll per my

zero is I greater than zero we'll per my three fingers obviously so we print out

three fingers obviously so we print out meow on the screen then we subtract one

meow on the screen then we subtract one from I at which point now we have two as

from I at which point now we have two as the value of I then the code goes back

the value of I then the code goes back to the condition and notice the

to the condition and notice the condition there is in parentheses that's

condition there is in parentheses that's another Boolean expression so Loops can

another Boolean expression so Loops can use Boolean Expressions just like

use Boolean Expressions just like conditionals use Boolean Expressions to

conditionals use Boolean Expressions to make decis ision the loop though is

make decis ision the loop though is deciding not whether to do this thing or

deciding not whether to do this thing or that but whether to do the same thing

that but whether to do the same thing again and again and again and as it

again and again and again and as it ticks through the code one line after

ticks through the code one line after the other it's ultimately going to get

the other it's ultimately going to get down to uh one and then zero and then

down to uh one and then zero and then stop so put another way came with some

stop so put another way came with some props here so suppose uh this bowl here

props here so suppose uh this bowl here is your variable and you initialize it

is your variable and you initialize it to three with like three stress balls

to three with like three stress balls you can do something three times right

you can do something three times right if I want to give out three stress balls

if I want to give out three stress balls here's your chance for free stress ball

here's your chance for free stress ball without having to answer any questions

without having to answer any questions any okay there we go so here we go

any okay there we go so here we go subtracting one from my variable I'm

subtracting one from my variable I'm left with two uh oh my God all right uh

left with two uh oh my God all right uh don't tell

don't tell Sounders oh I'm sorry oh okay that ended

Sounders oh I'm sorry oh okay that ended poorly apologies all right but now the

poorly apologies all right but now the educational point though is that my

educational point though is that my variable has been uh decremented further

variable has been uh decremented further to just have I'm not throwing that far

to just have I'm not throwing that far again I can't do this in here we go all

again I can't do this in here we go all right here we go and one final

right here we go and one final subtraction and now our variable is left

subtraction and now our variable is left so we have three stress balls there and

so we have three stress balls there and that's all a variable is right it's some

that's all a variable is right it's some kind of storage it's actually of course

kind of storage it's actually of course implemented in the computer's memory but

implemented in the computer's memory but metaphorically it's really just like a

metaphorically it's really just like a bow with some values and every time you

bow with some values and every time you add or in this case subtract you're just

add or in this case subtract you're just changing the value of that variable and

changing the value of that variable and then the code meanwhile of course in

then the code meanwhile of course in parenthesis is just checking is the bow

parenthesis is just checking is the bow empty is the bowl empty is the bowl

empty is the bowl empty is the bowl empty AKA is I greater than zero or not

empty AKA is I greater than zero or not any questions on how we've implemented

any questions on how we've implemented Loops in this way

Loops in this way and I I'll you a stress ball after class

and I I'll you a stress ball after class questions on Loops all right so it turns

questions on Loops all right so it turns out this is kind of ugly and like this

out this is kind of ugly and like this really starts to take the fun out of

really starts to take the fun out of programming uh when you have to like

programming uh when you have to like write out this uh sequence of steps so

write out this uh sequence of steps so it turns out there's other ways to do

it turns out there's other ways to do this but first let's see logically how

this but first let's see logically how else you might Express this because it's

else you might Express this because it's a little weird that we keep using zero

a little weird that we keep using zero so the one other way to do this would be

so the one other way to do this would be to invert the logic you could absolutely

to invert the logic you could absolutely start with your variable call it I equal

start with your variable call it I equal to one and then you ask the question is

to one and then you ask the question is I less than or equal to three and notice

I less than or equal to three and notice a bit of new syntax here on your uh

a bit of new syntax here on your uh typical keyboard there is no less than

typical keyboard there is no less than or equal sign or greater than or equal

or equal sign or greater than or equal sign like you would write in math class

sign like you would write in math class with one over the other and so in C you

with one over the other and so in C you use two characters less than followed by

use two characters less than followed by an equal sign or if appropriate greater

an equal sign or if appropriate greater than followed by an equal sign and that

than followed by an equal sign and that logically captures that idea so notice

logically captures that idea so notice that I'm kind of changing my questions

that I'm kind of changing my questions I'm initializing I to one and then I'm

I'm initializing I to one and then I'm going to increment It ultimately to two

going to increment It ultimately to two and then three but because I'm doing

and then three but because I'm doing less than or equal to it's still going

less than or equal to it's still going to go from 1 to three so that works too

to go from 1 to three so that works too we could similarly do this yet another

we could similarly do this yet another way we could initialize I to zero and

way we could initialize I to zero and then we could say well I is less than

then we could say well I is less than three and keep incrementing it and I

three and keep incrementing it and I showed this last form is actually the

showed this last form is actually the most canonical like it might be the most

most canonical like it might be the most humanlike to think in terms of 1 to

humanlike to think in terms of 1 to three it might be the most stress ball

three it might be the most stress ball like to think in terms of three to zero

like to think in terms of three to zero counting down but Ty typically the go-to

counting down but Ty typically the go-to Syntax for most programmers once you get

Syntax for most programmers once you get comfortable counting from zero is to

comfortable counting from zero is to always start counting from zero and

always start counting from zero and count up to less than the value you're

count up to less than the value you're counting up to so it would be incorrect

counting up to so it would be incorrect why to change this to less than or equal

why to change this to less than or equal to three here what would happen if I

to three here what would happen if I Chang the less than to less than or

Chang the less than to less than or equal to it'll me out twice yeah it'll

equal to it'll me out twice yeah it'll out an extra a fourth time in fact total

out an extra a fourth time in fact total right because you'll start at zero then

right because you'll start at zero then one then two then three and less than or

one then two then three and less than or equal to three sorry three will give you

equal to three sorry three will give you the fourth time so we do want it indeed

the fourth time so we do want it indeed to be just a single less than all right

to be just a single less than all right so now that we have those options let me

so now that we have those options let me just give you one other and this one

just give you one other and this one takes a little more getting used to as

takes a little more getting used to as well but it's probably the more common

well but it's probably the more common way to write this let me go ahead and

way to write this let me go ahead and propose that we implement this as

propose that we implement this as follows let me go back to my code here

follows let me go back to my code here let me go into my several printfs

let me go into my several printfs getting rid of all but one of them

getting rid of all but one of them ultimately and let's implement this in

ultimately and let's implement this in code so let's do in I get that's zero

code so let's do in I get that's zero how about then while I is less than

how about then while I is less than three then let's go ahead and say print

three then let's go ahead and say print F quote unquote meow uh mellow meow back

F quote unquote meow uh mellow meow back sln and then we have to do I minus minus

sln and then we have to do I minus minus or plus

or plus plus so plus plus because we're starting

plus so plus plus because we're starting at zero and going up two but not through

at zero and going up two but not through three so let me go ahead now and make

three so let me go ahead now and make meow after clearing my terminal SL meow

meow after clearing my terminal SL meow and it's still just as correct but it's

and it's still just as correct but it's a little more uh it's a little better

a little more uh it's a little better designed why because now if I want to

designed why because now if I want to change it from three to 30 times for

change it from three to 30 times for instance I can change it there I can

instance I can change it there I can recompile my code I can do/ meow and

recompile my code I can do/ meow and done I don't have to copy and paste it

done I don't have to copy and paste it 27 more times to get that effect and I

27 more times to get that effect and I can even change what the word is by

can even change what the word is by changing it in just one location but it

changing it in just one location but it turns out there's other ways to do this

turns out there's other ways to do this too and let me propose that we introduce

too and let me propose that we introduce you to what's called a for loop as well

you to what's called a for loop as well so if you want to repeat something three

so if you want to repeat something three times you can absolutely take the while

times you can absolutely take the while Loop approach that we just saw or you

Loop approach that we just saw or you can do this and this one's a little

can do this and this one's a little takes a little more getting used to but

takes a little more getting used to but it kind of consolidates into one line

it kind of consolidates into one line all of the same logic so notice we have

all of the same logic so notice we have the keyword four here and four is just a

the keyword four here and four is just a a preposition in this case that

a preposition in this case that generally implies here comes a loop

generally implies here comes a loop inside of parenthesis here is not just a

inside of parenthesis here is not just a Boolean expression and this where things

Boolean expression and this where things get a little weird there's three things

get a little weird there's three things to the left of the semicolon in the

to the left of the semicolon in the middle of the two semicolons and to the

middle of the two semicolons and to the right of the semicolon this is really

right of the semicolon this is really the only other context we'll see

the only other context we'll see semicolons and it's weird normally it's

semicolons and it's weird normally it's been at the end of the line now it's two

been at the end of the line now it's two of them in the middle of the line but

of them in the middle of the line but this is the way humans decided years ago

this is the way humans decided years ago to do it so what is this doing almost

to do it so what is this doing almost the same thing it is going to initialize

the same thing it is going to initialize a variable called I to zero it's going

a variable called I to zero it's going to then check if it's less than three

to then check if it's less than three it's then going to do what's ever in the

it's then going to do what's ever in the curly braces is and it's lastly going to

curly braces is and it's lastly going to increment I and repeat so just

increment I and repeat so just highlighting in turn at first I is

highlighting in turn at first I is initialized to zero just like before

initialized to zero just like before then this condition is checked this is a

then this condition is checked this is a Boolean expression yes or no true or

Boolean expression yes or no true or false will be its answer and if I is

false will be its answer and if I is less than three which it should be once

less than three which it should be once it starts at zero well then we're going

it starts at zero well then we're going to go ahead and print out meow then I is

to go ahead and print out meow then I is going to get incremented so it starts at

going to get incremented so it starts at zero it goes now to one at that point

zero it goes now to one at that point the Boolean expression is checked again

the Boolean expression is checked again so you don't keep changing I back to

so you don't keep changing I back to zero that first step happens only once

zero that first step happens only once but now you repeat through those three

but now you repeat through those three other highlights I check if I is less

other highlights I check if I is less than three it is so I print out meow it

than three it is so I print out meow it then increments I I check if I now two

then increments I I check if I now two is less than three it is I print out

is less than three it is I print out meow I gets incremented I now check is I

meow I gets incremented I now check is I less than three no it's not because

less than three no it's not because three is not less than three and so the

three is not less than three and so the whole thing stops and whatever code is

whole thing stops and whatever code is below this curly brace if any starts

below this curly brace if any starts executing instead just like in scratch

executing instead just like in scratch you break out of the loop and the the

you break out of the loop and the the puzzle piece being hugged questions then

puzzle piece being hugged questions then about this alternative Syntax

about this alternative Syntax for Loops AKA a four

Loop sorry say again yeah can I explain again why it

again yeah can I explain again why it doesn't reset to zero honestly just

doesn't reset to zero honestly just because like this was the syntax they

because like this was the syntax they chose this first part before the first

chose this first part before the first semicolon is only executed once just

semicolon is only executed once just because that's how it's designed

because that's how it's designed everything else Cycles again and again

everything else Cycles again and again and this is just an alternative syntax

and this is just an alternative syntax to to using the slightly more lines of

to to using the slightly more lines of code it was like six lines of code using

code it was like six lines of code using the while loop logically it's the same

the while loop logically it's the same thing programmers once they get more

thing programmers once they get more comfortable tend to prefer this because

comfortable tend to prefer this because it just expresses all your same thoughts

it just expresses all your same thoughts more succinctly that's all yeah okay so

more succinctly that's all yeah okay so let's just work this into my meow

let's just work this into my meow example let me go back to the code here

example let me go back to the code here and notice indeed if I highlight all

and notice indeed if I highlight all these lines I think we can tighten this

these lines I think we can tighten this up let me get rid of all of those and

up let me get rid of all of those and instead do four in I equals 0 and I'm

instead do four in I equals 0 and I'm saying equals most programmers would say

saying equals most programmers would say gets so in I gets zero means assignment

gets so in I gets zero means assignment the word get now I'm going to do I is

the word get now I'm going to do I is less than three i+ plus now in here I'm

less than three i+ plus now in here I'm going to do my print F quote unquote

going to do my print F quote unquote meow back sln and so it's indeed a

meow back sln and so it's indeed a little tighter I mean two of the lines

little tighter I mean two of the lines are just curly braces there's really

are just curly braces there's really only two juicy lines of code now let me

only two juicy lines of code now let me go ahead and do make

go ahead and do make meow meow and again we're back in

meow meow and again we're back in business with three of them printing

business with three of them printing only all right there's one last

only all right there's one last structure we should explore just cuz

structure we should explore just cuz it's sometimes useful this was a forever

it's sometimes useful this was a forever block and this would be a little weird

block and this would be a little weird in scratch to just say meow Forever at

in scratch to just say meow Forever at least without waiting but there is

least without waiting but there is indeed a forever Block in scratch which

indeed a forever Block in scratch which means do the following forever and I

means do the following forever and I proposed I think verbally last week's at

proposed I think verbally last week's at least one example where this is useful

least one example where this is useful meowing forever little annoying but can

meowing forever little annoying but can you think of common cases where you

you think of common cases where you might want to write code or use a

might want to write code or use a program that Loops forever

program that Loops forever yeah yeah playing music like Spotify

yeah yeah playing music like Spotify playlist just repeating again and again

playlist just repeating again and again would be some kind of loop for

would be some kind of loop for collisions checking for collisions and

collisions checking for collisions and scratch so seeing if something's

scratch so seeing if something's bouncing off the wall or another Sprite

yeah checking for input so yeah get string is essentially just waiting there

string is essentially just waiting there forever for me to type in some input

forever for me to type in some input until I do the time checking the time

until I do the time checking the time and actually maintaining like human time

and actually maintaining like human time like a wall clock behind you was that

like a wall clock behind you was that the

the same okay checking the time and one more

same okay checking the time and one more detecting a key press too like in

detecting a key press too like in scratch just waiting for some kind of

scratch just waiting for some kind of event to happen just like on a phone or

event to happen just like on a phone or a browser and so there's so many

a browser and so there's so many examples where you might want to do

examples where you might want to do something forever just so you've seen

something forever just so you've seen the corresponding C building block it's

the corresponding C building block it's a little weird but this is probably the

a little weird but this is probably the most canonical way to do it in C if you

most canonical way to do it in C if you want to print meow forever which would

want to print meow forever which would be a little crazy because it literally

be a little crazy because it literally print and take over your computer

print and take over your computer printing forever meow you would

printing forever meow you would generally do it like this why well a

generally do it like this why well a while loop expects in parentheses a

while loop expects in parentheses a Boolean expression and a Boolean

Boolean expression and a Boolean expression is again a yes no a true

expression is again a yes no a true false question but if you want the

false question but if you want the answer to that question always to be yes

answer to that question always to be yes or really always to be true turns out in

or really always to be true turns out in C in a lot of languages well then just

C in a lot of languages well then just say true because true T R is never going

say true because true T R is never going to change magically to false I mean it's

to change magically to false I mean it's just a special word in the programming

just a special word in the programming language so by saying while true it just

language so by saying while true it just means do the following forever another

means do the following forever another common Paradigm before true and false

common Paradigm before true and false became common place would be to do uh

became common place would be to do uh this instead change uh while one you

this instead change uh while one you might see in online examples and texts

might see in online examples and texts and the like while one is really the

and the like while one is really the same thing any value that is zero is

same thing any value that is zero is generally interpreted as false by a

generally interpreted as false by a computer any value that is one or any

computer any value that is one or any other nonzero value is generally

other nonzero value is generally interpreted as true and so this too

interpreted as true and so this too would have the same effect saying while

would have the same effect saying while true or while one generally speaking

true or while one generally speaking while true is perhaps a little clearer

while true is perhaps a little clearer these days now meowing forever is not a

these days now meowing forever is not a good thing but suppose I did that by

good thing but suppose I did that by intent or by accident well let's try

intent or by accident well let's try this so here I'll go into my code I'm

this so here I'll go into my code I'm going to get rid of my for Loop and

going to get rid of my for Loop and change my while loop to uh how about

change my while loop to uh how about true um and in this case here well we'll

true um and in this case here well we'll keep it let's do this make meow enter

keep it let's do this make meow enter and you'll see this use of Undeclared

and you'll see this use of Undeclared identifier or true this is actually

identifier or true this is actually hinting at my mention that the old way

hinting at my mention that the old way was zero and one nowadays you could say

was zero and one nowadays you could say true or false but true and false are

true or false but true and false are themselves special words that you have

themselves special words that you have to include and it turns out if you want

to include and it turns out if you want to use special Boolean values like this

to use special Boolean values like this there's another header file we haven't

there's another header file we haven't seen called standard bu that essentially

seen called standard bu that essentially creates true and false as keywords

creates true and false as keywords alternatively cs50 includes that same

alternatively cs50 includes that same file so it's more common in cs50 to see

file so it's more common in cs50 to see it like this now if I clear my terminal

it like this now if I clear my terminal window and do make meow and then do/

window and do make meow and then do/ meow and hit enter well unfortunately

meow and hit enter well unfortunately this isn't the best thing to do uh

this isn't the best thing to do uh infinitely when you're in the cloud

infinitely when you're in the cloud using a browser this is indeed a a

using a browser this is indeed a a browser full screened here um this means

browser full screened here um this means I'm sending millions of meows over the

I'm sending millions of meows over the internet to my computer here uh so this

internet to my computer here uh so this will happen to you at some point

will happen to you at some point probably not with meow but you'll lose

probably not with meow but you'll lose control over your terminal window why

control over your terminal window why because you screwed up and like you have

because you screwed up and like you have an infinite Loop you didn't really

an infinite Loop you didn't really intend it or maybe you did you were

intend it or maybe you did you were curious to see what happens what do you

curious to see what happens what do you do like when when does the meowing stop

do like when when does the meowing stop what recourse do we have here all right

what recourse do we have here all right well control c will be your friend

well control c will be your friend sometimes you have to hit it a bunch in

sometimes you have to hit it a bunch in a cloud environment but control C for

a cloud environment but control C for cancel with will interrupt a program

cancel with will interrupt a program that's running and I promise that almost

that's running and I promise that almost all of you will at some point

all of you will at some point accidentally introduce an infinite Loop

accidentally introduce an infinite Loop because you're math is slightly off when

because you're math is slightly off when in doubt click in the terminal window

in doubt click in the terminal window and hit contrl C sometimes multiple

and hit contrl C sometimes multiple times and that will indeed cancel

times and that will indeed cancel whatever is happening there in this case

whatever is happening there in this case I might have intended it but sometimes

I might have intended it but sometimes it's not in fact intended all right so

it's not in fact intended all right so we've been taking for granted this whole

we've been taking for granted this whole graphical user interface for some time

graphical user interface for some time and indeed uh the uh commands that I'm

and indeed uh the uh commands that I'm typing and the buttons I'm clicking and

typing and the buttons I'm clicking and let me just give you a better sense of

let me just give you a better sense of what it is we are using underneath the

what it is we are using underneath the hood this whole time um namely an

hood this whole time um namely an operating system called Linux so I keep

operating system called Linux so I keep alluding verbally of course to Macs and

alluding verbally of course to Macs and PCs because almost all of us are running

PCs because almost all of us are running Mac OS or Windows on our desktops or

Mac OS or Windows on our desktops or laptops nowadays but there's lots of

laptops nowadays but there's lots of other operating systems out there and

other operating systems out there and one of the most popular one is called

one of the most popular one is called Linux and Linux is very often used on

Linux and Linux is very often used on servers nowadays companies that host

servers nowadays companies that host email companies that host websites or

email companies that host websites or apps more generally um certain computer

apps more generally um certain computer scientists or computer science students

scientists or computer science students often like to brag that they run Linux

often like to brag that they run Linux just because that's a thing um but it is

just because that's a thing um but it is really just an alternative to Mac OS or

really just an alternative to Mac OS or windows that provides you with both a

windows that provides you with both a guey if you want it but also and

guey if you want it but also and especially a command line environment

especially a command line environment now fun fact Windows and Mac OS do have

now fun fact Windows and Mac OS do have terminal windows or the equivalent

terminal windows or the equivalent thereof and eventually you might uh use

thereof and eventually you might uh use it on your own Mac or PC to solve some

it on your own Mac or PC to solve some problem but Linux is really known for

problem but Linux is really known for along with other operating systems it's

along with other operating systems it's command line environment which again I

command line environment which again I distinguished earlier from Guy as a

distinguished earlier from Guy as a command line interface or CLI and that

command line interface or CLI and that refers really to the terminal window so

refers really to the terminal window so if I go back to VSS code here and let me

if I go back to VSS code here and let me in fact go ahead and close my tab and

in fact go ahead and close my tab and focus entirely on the terminal window

focus entirely on the terminal window this terminal window is really just your

this terminal window is really just your command line interface to your very own

command line interface to your very own server in the cloud the term of art here

server in the cloud the term of art here is you each will have your own container

is you each will have your own container in the cloud which is like your own

in the cloud which is like your own computer running somewhere on the

computer running somewhere on the internet with your own username and

internet with your own username and password to which you have access and

password to which you have access and your own hard drive if you will your own

your own hard drive if you will your own home folder that has all of your files

home folder that has all of your files for the class and it's only accessible

for the class and it's only accessible to you unless you enable live sharing

to you unless you enable live sharing thereof so when you're typing commands

thereof so when you're typing commands here it looks like you're typing them of

here it looks like you're typing them of course on your own Mac or PC but they're

course on your own Mac or PC but they're actually being sent over the browser to

actually being sent over the browser to uh some server in the cloud where you

uh some server in the cloud where you are controlling really your own account

are controlling really your own account therein so it turns out that there are

therein so it turns out that there are other commands that are worth knowing

other commands that are worth knowing and we'll give you just a few of these

and we'll give you just a few of these today and over the coming weeks will you

today and over the coming weeks will you have opport ities to play with others as

have opport ities to play with others as well but these are kind of some of the

well but these are kind of some of the basics and they're all incredibly

basics and they're all incredibly succinct because indeed for things

succinct because indeed for things you're typing at the command line humans

you're typing at the command line humans generally have not wanted to type out

generally have not wanted to type out long commands so a lot of these are

long commands so a lot of these are abbreviations here now perhaps the most

abbreviations here now perhaps the most common one I'll start with first is LS a

common one I'll start with first is LS a lowercase l and a lowercase s that

lowercase l and a lowercase s that stands for succinctly list so if I go to

stands for succinctly list so if I go to my terminal window now where up until

my terminal window now where up until now I've only typed code which is a vs

now I've only typed code which is a vs code thing for creating and opening

code thing for creating and opening files and make which triggers the compil

files and make which triggers the compil ation of my code what if I now type LS

ation of my code what if I now type LS this will list all of the files in my

this will list all of the files in my current folder my hard drive in the

current folder my hard drive in the cloud if you will so if I hit enter

cloud if you will so if I hit enter you'll see a whole bunch of results now

you'll see a whole bunch of results now they're color-coded too the white ones

they're color-coded too the white ones here and in. c those are the source code

here and in. c those are the source code files I've written during class today

files I've written during class today agree. C compare. C hello.c and meow. C

agree. C compare. C hello.c and meow. C and you can perhaps guess the green ones

and you can perhaps guess the green ones here that just by convention have an

here that just by convention have an asterisk on the end to denote that

asterisk on the end to denote that they're special represent what

they're special represent what what are the four others

what are the four others yeah yeah the machine code so those are

yeah yeah the machine code so those are my actual programs that are identically

my actual programs that are identically named minus the C extension and the

named minus the C extension and the asterisk means that they're executable

asterisk means that they're executable that is in the world of Mac OS or

that is in the world of Mac OS or Windows you would double click but in

Windows you would double click but in the world of a command line environment

the world of a command line environment that means you do dot slash and then the

that means you do dot slash and then the name without the asterisk to execute or

name without the asterisk to execute or run the code therein so if I open up my

run the code therein so if I open up my file explorer and I'm hitting command B

file explorer and I'm hitting command B on my computer here just as a keyboard

on my computer here just as a keyboard shortcut you'll see the exact same thing

shortcut you'll see the exact same thing so LS is the command line interface for

so LS is the command line interface for listing the files in your account but

listing the files in your account but here because I'm using vs code or any

here because I'm using vs code or any program like it I also get a graphical

program like it I also get a graphical user interface as well so it's just two

user interface as well so it's just two different places to be you're welcome to

different places to be you're welcome to use whatever you're comfortable with but

use whatever you're comfortable with but over time will you naturally get more

over time will you naturally get more comfortable and capable with the

comfortable and capable with the terminal window alone well what else is

terminal window alone well what else is on this this list here well during the

on this this list here well during the break I saw that at least one of you for

break I saw that at least one of you for instance had created a file called hello

instance had created a file called hello instead of hello. so you were in a

instead of hello. so you were in a situation where you did this

situation where you did this accidentally and hit enter and then you

accidentally and hit enter and then you went ahead and typed in all of your code

went ahead and typed in all of your code like this and then down in your terminal

like this and then down in your terminal window you were trying to do make hello

window you were trying to do make hello enter and this now didn't actually do

enter and this now didn't actually do anything like I can't I'm hitting um I'm

anything like I can't I'm hitting um I'm trying to run the command I got

trying to run the command I got permission denied as at least one of you

permission denied as at least one of you did now why is that well let's just do a

did now why is that well let's just do a quick check if I do LS I see now hello

quick check if I do LS I see now hello but hello has no asteris next to it

but hello has no asteris next to it which means it's not executable that's

which means it's not executable that's my code why well notice the top of my

my code why well notice the top of my tab confirms oh I screwed up I didn't

tab confirms oh I screwed up I didn't name my file hello.c which it just has

name my file hello.c which it just has to be so what do you do well you could

to be so what do you do well you could very hackish like copy this create a new

very hackish like copy this create a new file paste it in or no no no like we

file paste it in or no no no like we know how to rename things now here

know how to rename things now here because that's one of our options let me

because that's one of our options let me do this let me do MV for move hello and

do this let me do MV for move hello and then hello.c and hit enter you'll see

then hello.c and hit enter you'll see the tab closes cuz hello no longer

the tab closes cuz hello no longer exists but if I now now type LS you'll

exists but if I now now type LS you'll see ah there is hello.c and if I open

see ah there is hello.c and if I open that file now there's all of my same

that file now there's all of my same code and now if I do make hello make

code and now if I do make hello make hello now I do get an executable file

hello now I do get an executable file wherein the world is restored so MV is

wherein the world is restored so MV is just a command not just for renaming but

just a command not just for renaming but it also turns out eventually for moving

it also turns out eventually for moving files as well you can also create

files as well you can also create directories or folders so for instance

directories or folders so for instance if I go into vs code again and suppose I

if I go into vs code again and suppose I hover over here and click not on the

hover over here and click not on the plus file icon but plus folder I can

plus file icon but plus folder I can create a folder called for instance like

create a folder called for instance like pset one for problem set one in the

pset one for problem set one in the class and you'll see now that it's empty

class and you'll see now that it's empty CU all of my other files are in the

CU all of my other files are in the default folder of my account but I could

default folder of my account but I could also go in there like this and I could

also go in there like this and I could click on file and now I can create a new

click on file and now I can create a new file called like mario.c which is uh one

file called like mario.c which is uh one of the first problems for instance but

of the first problems for instance but you'll notice now that

you'll notice now that mario.c is inside of the pet one folder

mario.c is inside of the pet one folder so if I zoom out and I type LS at my

so if I zoom out and I type LS at my terminal window

terminal window I won't see mario. C anywhere but I do

I won't see mario. C anywhere but I do see a pet one folder and it's in light

see a pet one folder and it's in light blue followed by a slash which you don't

blue followed by a slash which you don't have to type it just indicates that's a

have to type it just indicates that's a folder now I can visually at top left

folder now I can visually at top left obviously see pet one contains mario.c

obviously see pet one contains mario.c but if I try to do something like make

but if I try to do something like make Mario here no rule to make Target Mario

Mario here no rule to make Target Mario like it just doesn't seem to exist and

like it just doesn't seem to exist and that's because you're in the wrong

that's because you're in the wrong directory so in a command line interface

directory so in a command line interface it's not quite as simple as just

it's not quite as simple as just clicking on a folder and voila it opens

clicking on a folder and voila it opens you have to change into the directory or

you have to change into the directory or folder and CD is going to be the command

folder and CD is going to be the command there so if I want to actually change

there so if I want to actually change into that directory I can do CD space

into that directory I can do CD space pet one enter and now you'll see my

pet one enter and now you'll see my prompt changes and this is just a common

prompt changes and this is just a common convention but it's not the only one out

convention but it's not the only one out there now I still have a dollar sign

there now I still have a dollar sign which indicates where I can type

which indicates where I can type commands but before it I see a reminder

commands but before it I see a reminder constantly what folder I'm in and we put

constantly what folder I'm in and we put that there deliberately like a lot of

that there deliberately like a lot of Linux users do just to remind themselves

Linux users do just to remind themselves where they are cuz unlike Mac OS where

where they are cuz unlike Mac OS where you or Windows where you have a nice big

you or Windows where you have a nice big window telling you where you are at the

window telling you where you are at the command line you kind of need to be

command line you kind of need to be reminded textually but now if I type LS

reminded textually but now if I type LS and hit enter what should I

and hit enter what should I see yeah mario.c and now if I want to

see yeah mario.c and now if I want to open it uh if I want to uh actually

open it uh if I want to uh actually compile it I can run make Mario in this

compile it I can run make Mario in this directory once I actually type out all

directory once I actually type out all the code rest assured that in problem

the code rest assured that in problem sets in Labs we almost always certainly

sets in Labs we almost always certainly in the first weeks of the class give you

in the first weeks of the class give you exactly the commands to type odds are

exactly the commands to type odds are because it's new to many of you view you

because it's new to many of you view you will accidentally type the wrong

will accidentally type the wrong commands no big deal just remember that

commands no big deal just remember that you have different ways to solve these

you have different ways to solve these problems you've got like the graphical

problems you've got like the graphical file explorer which should feel a little

file explorer which should feel a little more familiar but in time you'll start

more familiar but in time you'll start to know and honestly probably prefer

to know and honestly probably prefer commands like these so CD for change

commands like these so CD for change directory CP for copy a file uh LS for

directory CP for copy a file uh LS for list MK dirt to make a directory create

list MK dirt to make a directory create a new folder at the command line instead

a new folder at the command line instead of with the button MV for move or rename

of with the button MV for move or rename RM

RM for remove so be careful with that one

for remove so be careful with that one RM dur remove directory and there's

RM dur remove directory and there's dozens hundreds of other commands you

dozens hundreds of other commands you won't need many of them but we'll start

won't need many of them but we'll start to scratch the surface all the more over

to scratch the surface all the more over time but ultimately this command line

time but ultimately this command line interface is going to be a more powerful

interface is going to be a more powerful mechanism a more capable mechanism and

mechanism a more capable mechanism and ultimately a more efficient mechanism

ultimately a more efficient mechanism for writing code running commands uh

for writing code running commands uh solving problems analyzing data more

solving problems analyzing data more generally even though no there's going

generally even though no there's going to be some uh some Growing Pains early

to be some uh some Growing Pains early on just because it's probably so new for

on just because it's probably so new for many of you so with that said we have

many of you so with that said we have some problems still to solve but we

some problems still to solve but we promised cookies today so let's go ahead

promised cookies today so let's go ahead and take a 10-minute break cookies are

and take a 10-minute break cookies are now served in the transcept and we'll be

now served in the transcept and we'll be back here in

back here in 10 all right we are back and up until

10 all right we are back and up until now each of the code examples in C we've

now each of the code examples in C we've done if been kind of designed to show

done if been kind of designed to show one specific topic but we thought we'd

one specific topic but we thought we'd try to take a step back and solve a more

try to take a step back and solve a more General problem and give you a sense of

General problem and give you a sense of when given a problem set for instance or

when given a problem set for instance or just a programming problem more

just a programming problem more generally like where you even begin and

generally like where you even begin and how you go about approaching it when

how you go about approaching it when it's not obvious what the point of the

it's not obvious what the point of the exercise is so one of my favorite games

exercise is so one of my favorite games from yester year is this one here Super

from yester year is this one here Super Mario Brothers that has come in so many

Mario Brothers that has come in so many different forms since but in this

different forms since but in this original uh two-dimensional Sid scroller

original uh two-dimensional Sid scroller game there was a lot of artwork like

game there was a lot of artwork like this so for instance up here in the sky

this so for instance up here in the sky were four question marks and we'll find

were four question marks and we'll find that in C in a lot of programming

that in C in a lot of programming languages initially it's a lot easier a

languages initially it's a lot easier a lot more accessible to focus really on

lot more accessible to focus really on black and white type interactive

black and white type interactive programs textually as opposed to

programs textually as opposed to full-fledged graphics and the like but

full-fledged graphics and the like but more on the more Gra graphical acoustic

more on the more Gra graphical acoustic uh type of programs before long but for

uh type of programs before long but for now let me go over and propose that we

now let me go over and propose that we try to just Implement an asy art aski

try to just Implement an asy art aski again being the code that Maps numbers

again being the code that Maps numbers to letters at least for English um into

to letters at least for English um into a textual version of these four question

a textual version of these four question marks in the sky so for this let me go

marks in the sky so for this let me go over to vs code I'll create my own

over to vs code I'll create my own version of mario.c that will be

version of mario.c that will be different from the one you're challenged

different from the one you're challenged with in problem set one indeed in

with in problem set one indeed in problem set one you'll be challenged to

problem set one you'll be challenged to build a little something like this

build a little something like this albeit with hashtags uh for asky art

albeit with hashtags uh for asky art instead of graphics and in mario.c I

instead of graphics and in mario.c I want to just solve this simple problem

want to just solve this simple problem first so it's all involving output so

first so it's all involving output so I'll do include standard i.h so I can

I'll do include standard i.h so I can use printf I'll do my int main void more

use printf I'll do my int main void more on why we keep doing that in future

on why we keep doing that in future weeks and I'm just going to do something

weeks and I'm just going to do something simple initially like 1 2 3 4 back sln

simple initially like 1 2 3 4 back sln this is about the simplest way I can

this is about the simplest way I can Implement four question marks in the sky

Implement four question marks in the sky like these here using pure text like

like these here using pure text like this so let me go ahead and do make

this so let me go ahead and do make Mario / Mario and voila we have those

Mario / Mario and voila we have those four question marks but we've seen of

four question marks but we've seen of course that there are better ways to do

course that there are better ways to do this and if you wanted to generalize

this and if you wanted to generalize this to be five question marks six 60

this to be five question marks six 60 different question marks you know Loop

different question marks you know Loop was always the answer for not repeating

was always the answer for not repeating ourselves so maybe I should rewrite this

ourselves so maybe I should rewrite this a little bit more flexibly and say

a little bit more flexibly and say something like this four in I gets zero

something like this four in I gets zero I less than four I plus plus and then

I less than four I plus plus and then inside of the for Loop now I can just do

inside of the for Loop now I can just do a single question mark But I don't think

a single question mark But I don't think what I've just done is correct anyone

what I've just done is correct anyone spot the aesthetic bug

spot the aesthetic bug already yeah why is this why is this

already yeah why is this why is this wrong if I want to print the same thing

wrong if I want to print the same thing [Music]

[Music] yeah yeah so I don't think I want to

yeah yeah so I don't think I want to backs slash in after every question mark

backs slash in after every question mark because the goal is again this like row

because the goal is again this like row of question marks in the sky so if I now

of question marks in the sky so if I now recompile this make

recompile this make Mario Mario okay it's almost there but

Mario Mario okay it's almost there but now I have that regression to where the

now I have that regression to where the dollar sign's not on its own line so I

dollar sign's not on its own line so I think I need a new line but I don't

think I need a new line but I don't think I want it here cuz that was not

think I want it here cuz that was not going to end well where do I want it

going to end well where do I want it instead any Instinct

instead any Instinct yeah yeah so outside the for Loop so

yeah yeah so outside the for Loop so indeed I can just go below line eight

indeed I can just go below line eight and above line nine creating a new one

and above line nine creating a new one and now there's it's totally fine to

and now there's it's totally fine to just print a new line like that you

just print a new line like that you don't have to print anything else with

don't have to print anything else with it it's indeed a character unto itself

it it's indeed a character unto itself so let's do make Mario one last time/

so let's do make Mario one last time/ Mario okay so now we're back in business

Mario okay so now we're back in business there well what if we wanted to do some

there well what if we wanted to do some other scene from Mario uh such as this

other scene from Mario uh such as this one here where there's a lot of vertical

one here where there's a lot of vertical obstacles like the These Bricks here if

obstacles like the These Bricks here if I wanted to print out now a column of

I wanted to print out now a column of three bricks and I'll use hashtags for

three bricks and I'll use hashtags for these instead of anything graphical well

these instead of anything graphical well I think we're almost there right like I

I think we're almost there right like I think I can now it's almost maybe a

think I can now it's almost maybe a little easier I can go back here change

little easier I can go back here change the question mark to something that

the question mark to something that looks more like a brick like this hash

looks more like a brick like this hash symbol and I think now I do want the new

symbol and I think now I do want the new line character because when I now do

line character because when I now do make Mario Mario okay there's my wall of

make Mario Mario okay there's my wall of four oh but wait I didn't want four I

four oh but wait I didn't want four I wanted to be consistent just with this

wanted to be consistent just with this particular scene here so I just want

particular scene here so I just want three so I can still change it in one

three so I can still change it in one place and here again is that Paradigm

place and here again is that Paradigm even whether you're using four or three

even whether you're using four or three if you get into the habit of starting

if you get into the habit of starting counting from zero you go on up to but

counting from zero you go on up to but not through the value you want to count

not through the value you want to count up to so that's why I'm using less than

up to so that's why I'm using less than instead of less than or equals to there

instead of less than or equals to there so this would be the common Paradigm

so this would be the common Paradigm though you could certainly count it like

though you could certainly count it like we saw earlier in different ways but

we saw earlier in different ways but what if things escalate one level

what if things escalate one level further and when you're in the

further and when you're in the underground version of Super Mario

underground version of Super Mario Brothers there's a lot of these

Brothers there's a lot of these underground obstructions including like

underground obstructions including like grids of bricks like this and let me

grids of bricks like this and let me conjecture that if you slice this up

conjecture that if you slice this up it's roughly a 3X3 grid of bricks that

it's roughly a 3X3 grid of bricks that all interlock uh prettily to give us

all interlock uh prettily to give us just one big large brick like this so if

just one big large brick like this so if I want to print out a 3X3 grid now

I want to print out a 3X3 grid now things are getting a little more

things are getting a little more interesting because up until now I

interesting because up until now I printed either one row horizontally or

printed either one row horizontally or one column vertically but we haven't

one column vertically but we haven't really seen any code where I'm sort of

really seen any code where I'm sort of printing or living in two different

printing or living in two different dimensions like the game would imply but

dimensions like the game would imply but let me propose that we could do this let

let me propose that we could do this let me go ahead and say all right suppose I

me go ahead and say all right suppose I want to print a 3X3 grid of bricks it's

want to print a 3X3 grid of bricks it's really that I want to print what three

really that I want to print what three rows of bricks like a grid is three rows

rows of bricks like a grid is three rows so if I take the highle idea and reduce

so if I take the highle idea and reduce it to something a little simpler how do

it to something a little simpler how do I do that well let me get rid of the

I do that well let me get rid of the print for a moment as I did and let me

print for a moment as I did and let me just stipulate that this for Loop even

just stipulate that this for Loop even though it doesn't do anything useful yet

though it doesn't do anything useful yet will do something how many times just by

will do something how many times just by Design all right three times right this

Design all right three times right this for Loop is good to go it will do

for Loop is good to go it will do something three times by just using I to

something three times by just using I to do the counting all right well if I want

do the counting all right well if I want to print out now a row of three bricks

to print out now a row of three bricks all on the same line that's pretty

all on the same line that's pretty similar to what we did earlier when I

similar to what we did earlier when I just wanted to print out four question

just wanted to print out four question marks in the sky so we've kind of seen a

marks in the sky so we've kind of seen a solution there and I dare say we can

solution there and I dare say we can compose one into the other so if I want

compose one into the other so if I want to print out a row of bricks I could

to print out a row of bricks I could just do this four in I gets zero I less

just do this four in I gets zero I less than three I ++ and then inside of this

than three I ++ and then inside of this inner loop if you will let me print out

inner loop if you will let me print out a single brick like this and then I I

a single brick like this and then I I don't like where this is going yet but I

don't like where this is going yet but I think I've taken two ideas and I've

think I've taken two ideas and I've combined them but what might be

combined them but what might be problematic about lines five and seven

problematic about lines five and seven at the moment

at the moment what might be bad here uh yeah and

back yeah I'm using the same integer ey which I feel like could get me into

which I feel like could get me into trouble right if I'm sort of trying to

trouble right if I'm sort of trying to count three things here but then I'm

count three things here but then I'm hijacking this variable and using it

hijacking this variable and using it inside of the loop like I feel like I

inside of the loop like I feel like I should avoid this this Collision of

should avoid this this Collision of names and so what's a good alternative

names and so what's a good alternative to I well a programmer if nesting Loops

to I well a programmer if nesting Loops in this way would pretty commonly go

in this way would pretty commonly go with J you could certainly change this

with J you could certainly change this to be like rows and columns if you want

to be like rows and columns if you want more descriptive variables but I and J

more descriptive variables but I and J is pretty canonical so I'm going to go

is pretty canonical so I'm going to go ahead and do this j++ instead of i++

ahead and do this j++ instead of i++ everywhere and let me try compiling this

everywhere and let me try compiling this so make Mario

so make Mario enter

enter Mario okay so a couple of things are

Mario okay so a couple of things are wrong here this is not a 3X3 grid but if

wrong here this is not a 3X3 grid but if you count these things how many did I

you count these things how many did I indeed print at

indeed print at least can probably just guess logically

least can probably just guess logically yeah there's nine hashes there

yeah there's nine hashes there unfortunately they're all on the same

unfortunately they're all on the same line instead of on three different lines

line instead of on three different lines so where logically can I fix this I'm

so where logically can I fix this I'm definitely printing all the bricks

definitely printing all the bricks they're just not on the right levels

yeah yeah so put a new line after the first Loop this inner loop if you will

first Loop this inner loop if you will the nested Loop if you will so let me go

the nested Loop if you will so let me go ahead and print out just a back slash n

ahead and print out just a back slash n here and what's this doing well I think

here and what's this doing well I think that's going to solve it by just moving

that's going to solve it by just moving the cursor to the next line after you've

the cursor to the next line after you've done one row so let me go ahead and do

done one row so let me go ahead and do make Mario enter SL Mario and now we're

make Mario enter SL Mario and now we're in business so it's a very simplistic

in business so it's a very simplistic version of the same graphic but I'm

version of the same graphic but I'm leveraging two different ideas now the

leveraging two different ideas now the same or the same idea twice rather now

same or the same idea twice rather now I'm using one Loop to kind of control my

I'm using one Loop to kind of control my cursor going row by row by row but then

cursor going row by row by row but then within that Loop I'm doing left to right

within that Loop I'm doing left to right do do dot dot dot with printing out each

do do dot dot dot with printing out each of these individual bricks like this now

of these individual bricks like this now there's a little sloppiness here still

there's a little sloppiness here still like if I want this to always be a

like if I want this to always be a square just because that's what it looks

square just because that's what it looks like in the game well I could change it

like in the game well I could change it to be a 4x4 uh Square by doing this or a

to be a 4x4 uh Square by doing this or a 5x5 grid whoops by doing this why is

5x5 grid whoops by doing this why is this perhaps not the best design to just

this perhaps not the best design to just keep changing the numbers when I want to

keep changing the numbers when I want to change the

change the size where could this go AR

yeah yeah if it's always going to be a square and height is going to be the

square and height is going to be the same as width I'm just inviting trouble

same as width I'm just inviting trouble here right eventually I'm going to screw

here right eventually I'm going to screw up I'm going to change one but not the

up I'm going to change one but not the other then it's going to come out to be

other then it's going to come out to be a rectangle instead of a proper Square

a rectangle instead of a proper Square so I should probably solve this a little

so I should probably solve this a little differently so let me do that at the top

differently so let me do that at the top of my main function here let me go ahead

of my main function here let me go ahead and give myself a variable called maybe

and give myself a variable called maybe uh n for the number of bricks I want

uh n for the number of bricks I want horizontally and vertically and I'll

horizontally and vertically and I'll just initialize that to three initially

just initialize that to three initially and instead of putting three here I'll

and instead of putting three here I'll literally just use n but I'll do it in

literally just use n but I'll do it in both places so that now henceforth if I

both places so that now henceforth if I ever want to change this and change it

ever want to change this and change it to four or five or anything else like

to four or five or anything else like I'm all done like it's better designed

I'm all done like it's better designed because there's a lower probability of

because there's a lower probability of mistakes but I could technically SK

mistakes but I could technically SK still screw up somehow I could

still screw up somehow I could technically accidentally write line of

technically accidentally write line of code like n++ or you know I could just

code like n++ or you know I could just change the value of that variable even

change the value of that variable even though I don't want it to ever change

though I don't want it to ever change and maybe it's because I'm a bad

and maybe it's because I'm a bad programmer I copy pasted wrong I'm

programmer I copy pasted wrong I'm working with someone who doesn't know

working with someone who doesn't know what n represents I can defend myself

what n represents I can defend myself and my code against human error like

and my code against human error like that by going up here to line five and

that by going up here to line five and instead of just declaring a simple

instead of just declaring a simple variable like we did in scratch I can

variable like we did in scratch I can further Harden my code so to speak by

further Harden my code so to speak by declaring it to be a constant using the

declaring it to be a constant using the keyword const now this is just a feature

keyword const now this is just a feature of c and some other languages to protect

of c and some other languages to protect you against Yourself by proactively

you against Yourself by proactively saying N is a constant specifically the

saying N is a constant specifically the number five or previously the number

number five or previously the number three you cannot accidentally write code

three you cannot accidentally write code elsewhere that changes it the computer

elsewhere that changes it the computer will throw an error and catch that error

will throw an error and catch that error so it's just a way of programming a

so it's just a way of programming a little more defensively um some

little more defensively um some languages have this some languages don't

languages have this some languages don't but in general it's a good practice it

but in general it's a good practice it makes your code better designed because

makes your code better designed because it just is less vulnerable to Mistakes

it just is less vulnerable to Mistakes by you colleagues or anyone else using

by you colleagues or anyone else using the code so let me change this back to

the code so let me change this back to three just to be our default but now I'm

three just to be our default but now I'm using n in both places and if I do make

using n in both places and if I do make mario. Mario we're back to where we

mario. Mario we're back to where we originally started but the code's a

originally started but the code's a little more better design and let me

little more better design and let me note this too all this time I've been

note this too all this time I've been mentioning that uh there's correctness

mentioning that uh there's correctness is important design is important there

is important design is important there was also this matter of style I've been

was also this matter of style I've been very deliberately writing pretty code if

very deliberately writing pretty code if you will not just the syntax

you will not just the syntax highlighting which is automatic but

highlighting which is automatic but notice that I keep indenting everything

notice that I keep indenting everything nicely anytime I have curly braces like

nicely anytime I have curly braces like on lines 4 and 14 everything is indented

on lines 4 and 14 everything is indented one level when I have additional curly

one level when I have additional curly braces on line 7 and 13 everything is

braces on line 7 and 13 everything is nicely indented uh as well technically

nicely indented uh as well technically speaking the computer does not care

speaking the computer does not care about that kind of white space so to

about that kind of white space so to speak and you could really make a mess

speak and you could really make a mess of things like this because you have a

of things like this because you have a strange sense of style or just because

strange sense of style or just because you're being a little sloppy but this

you're being a little sloppy but this code is actually still correct if I

code is actually still correct if I recompile it let me open up my terminal

recompile it let me open up my terminal window make uh Mario no errors Mario it

window make uh Mario no errors Mario it works perfectly fine but you can imagine

works perfectly fine but you can imagine just how annoying this now is to read

just how annoying this now is to read like certainly for a TA U but certainly

like certainly for a TA U but certainly for you the next day certainly for a

for you the next day certainly for a colleague who has to read your code this

colleague who has to read your code this is just bad style like it still works

is just bad style like it still works and it's welld designed in that like

and it's welld designed in that like you're uh writing code defensively

you're uh writing code defensively you're using a constant but my God the

you're using a constant but my God the style is atrocious now you'll often find

style is atrocious now you'll often find that there's tools that can help you

that there's tools that can help you format your code for you in a manner

format your code for you in a manner consistent with a courses or a company's

consistent with a courses or a company's uh style but this is the kind of muscle

uh style but this is the kind of muscle memory you'll want to develop over time

memory you'll want to develop over time to take vs code suggestions as it's

to take vs code suggestions as it's outputting lines of code for you because

outputting lines of code for you because it's trying to format your code in a

it's trying to format your code in a readable way and oh my God if and when

readable way and oh my God if and when you do have bugs in your code and things

you do have bugs in your code and things aren't even indented properly there's no

aren't even indented properly there's no way you the human are going to be able

way you the human are going to be able to wrap your mind around what's

to wrap your mind around what's happening and where like you're just

happening and where like you're just making the problem harder for yourself

making the problem harder for yourself so do get into this habit too of

so do get into this habit too of manifesting good style as well all right

manifesting good style as well all right well let me propose that we don't only

well let me propose that we don't only want a 3X3 grid we want this to be a

want a 3X3 grid we want this to be a little more Dynamic so suppose we moved

little more Dynamic so suppose we moved away from a constant to just using an

away from a constant to just using an integer called n and let's ask the user

integer called n and let's ask the user for the size of this grid as by

for the size of this grid as by prompting them with get int as we've

prompting them with get int as we've done before and I'll store it in N here

done before and I'll store it in N here and then I can go ahead and more

and then I can go ahead and more dynamically run make Mario to compile it

dynamically run make Mario to compile it whoops oh I screwed up

whoops oh I screwed up accidentally what is it in suggesting I

accidentally what is it in suggesting I do albeit

cryptically yeah I forgot to include the cs50 header file up top and that's why

cs50 header file up top and that's why it doesn't know that get int is in fact

it doesn't know that get int is in fact valid so that's an easy fix U I'm just

valid so that's an easy fix U I'm just going to go up here and include

going to go up here and include cs50.h now I'm going to clear my

cs50.h now I'm going to clear my terminal and rerun make Mario now we're

terminal and rerun make Mario now we're good Mario and now notice I'm prompted

good Mario and now notice I'm prompted for size so if I type in three it's the

for size so if I type in three it's the same as before if I type in 10 it's even

same as before if I type in 10 it's even bigger but it happens all now

bigger but it happens all now automatically but there are some things

automatically but there are some things that we're not detecting for instance

that we're not detecting for instance suppose I type in cat well that's

suppose I type in cat well that's handled by the get it function as I

handled by the get it function as I claimed earlier that's one of the

claimed earlier that's one of the features of using a library you don't

features of using a library you don't have to deal with erroneous input but we

have to deal with erroneous input but we only designed a function called get int

only designed a function called get int to get you an integer we don't know if

to get you an integer we don't know if you want it to be positive negative zero

you want it to be positive negative zero or some combination thereof and it's

or some combination thereof and it's kind of weird to allow the user to type

kind of weird to allow the user to type in like negative one for the size of the

in like negative one for the size of the Grid or you know ne3 for the size of the

Grid or you know ne3 for the size of the grid and indeed your code does nothing

grid and indeed your code does nothing so at least it's not crashing but that's

so at least it's not crashing but that's kind of stupid right like it'd be nice

kind of stupid right like it'd be nice to force the user if they want a grid to

to force the user if they want a grid to give us a positive value so how could we

give us a positive value so how could we do this well I could go up here and I

do this well I could go up here and I could say something like if n is less

could say something like if n is less than one so if it's zero or negative

than one so if it's zero or negative which I don't want what could I do well

which I don't want what could I do well I could say well prompt the user again

I could say well prompt the user again for the size and now notice I'm not

for the size and now notice I'm not declaring and again because once it

declaring and again because once it exists you don't have to mention the

exists you don't have to mention the data type again we said that earlier but

data type again we said that earlier but this is kind of stupid why because now

this is kind of stupid why because now when You' given the user a second chance

when You' given the user a second chance okay now maybe I'll do all right if this

okay now maybe I'll do all right if this version of n is less than one well let's

version of n is less than one well let's just go and prompt the user a third time

just go and prompt the user a third time I mean you can kind of see where this is

I mean you can kind of see where this is stupidly going like this can't be the

stupidly going like this can't be the right solution to keep typing

right solution to keep typing recursively the same thing again and

recursively the same thing again and again like where would it stop you'd

again like where would it stop you'd have to give them a finite number of

have to give them a finite number of chances or just you know make a mess of

chances or just you know make a mess of your code so what would be intuitively a

your code so what would be intuitively a better solution

better solution here yeah so some kind of loop we've

here yeah so some kind of loop we've seen a while loop we've seen a four Loop

seen a while loop we've seen a four Loop so maybe one of those so let me try this

so maybe one of those so let me try this let me delete this messiness and just go

let me delete this messiness and just go back to the first question and let me do

back to the first question and let me do this so while n is less than one so

this so while n is less than one so while the number is not what we want

while the number is not what we want let's just prompt the user in a loop

let's just prompt the user in a loop this time for the size again now here

this time for the size again now here too this is better because it's only two

too this is better because it's only two requests for information but clearly

requests for information but clearly line six and N are pretty much identical

line six and N are pretty much identical other than the int and if I went in and

other than the int and if I went in and changed the size you know uh if I add

changed the size you know uh if I add this if I change the wording here change

this if I change the wording here change it to a different language like I have

it to a different language like I have to change it in two places that's bad

to change it in two places that's bad copy paste bad so what might be better

copy paste bad so what might be better well it turns out there's another

well it turns out there's another Paradigm in C that you can use that gets

Paradigm in C that you can use that gets around this problem this duplication of

around this problem this duplication of code it would be much nicer if I just

code it would be much nicer if I just write this code once and I can do that

write this code once and I can do that using a third type of loop called a do

using a third type of loop called a do while loop so it turns out in C you can

while loop so it turns out in C you can do this if you want to get the value of

do this if you want to get the value of a variable like n first just decree

a variable like n first just decree create the variable without an initial

create the variable without an initial value so int n semicolon means it we

value so int n semicolon means it we don't know what value it has yes but

don't know what value it has yes but that's okay we're going to add a value

that's okay we're going to add a value to it eventually then I'm going to say

to it eventually then I'm going to say this do literally I'm going to open my

this do literally I'm going to open my curly braces and what do I want to do I

curly braces and what do I want to do I want to assign to n the return value of

want to assign to n the return value of get in prompting the user for size well

get in prompting the user for size well when do you want to do that I want to do

when do you want to do that I want to do that while n is less than one and this

that while n is less than one and this code now achieves the exact same goal

code now achieves the exact same goal but by never repeating myself why well

but by never repeating myself why well notice on these lines of code now I'm

notice on these lines of code now I'm literally saying on line six give me a

literally saying on line six give me a variable called n of type integer it

variable called n of type integer it doesn't have a value initially but

doesn't have a value initially but that's fine you can do that line seven

that's fine you can do that line seven says do the following what do you want

says do the following what do you want to do get int prompting the user with

to do get int prompting the user with the word size and just store that value

the word size and just store that value in N but because code runs top to bottom

in N but because code runs top to bottom left to right now it's reasonable on

left to right now it's reasonable on line 11 to ask that question okay is the

line 11 to ask that question okay is the current value of n which it definitely

current value of n which it definitely got on line a less than one and if the

got on line a less than one and if the user didn't cooperate they typed in zero

user didn't cooperate they typed in zero or Nega 1 or3 what's going to happen

or Nega 1 or3 what's going to happen it's going to go back up here and repeat

it's going to go back up here and repeat repeat repeat everything in the do while

repeat repeat everything in the do while loop so a do while loop in C which is

loop so a do while loop in C which is not something some other languages have

not something some other languages have like python if you know it does not have

like python if you know it does not have a do while loop this is perhaps the

a do while loop this is perhaps the cleanest way to achieve this even though

cleanest way to achieve this even though it's a little weird that you have to

it's a little weird that you have to declare your variable create your

declare your variable create your variable up top and then check it down

variable up top and then check it down below but other wise it's similar to a

below but other wise it's similar to a while loop it just flips the order in

while loop it just flips the order in which you're asking the question any

which you're asking the question any questions on this construct and Doh in

questions on this construct and Doh in general is super useful when you want to

general is super useful when you want to get input from the user and make sure it

get input from the user and make sure it meets certain requirements all right so

meets certain requirements all right so now that we have this building block

now that we have this building block after that interlude how can I go about

after that interlude how can I go about cleaning up this code and then let's

cleaning up this code and then let's conclude by taking a look at things that

conclude by taking a look at things that our code can't do or can't do very well

our code can't do or can't do very well or correctly let me propose that in a

or correctly let me propose that in a final version of Mario let me just add

final version of Mario let me just add what are called now some comments so it

what are called now some comments so it turns out in code in C you can Define

turns out in code in C you can Define what are called comments which are just

what are called comments which are just notes to self some of you discovered

notes to self some of you discovered these in scratch there's little yellow

these in scratch there's little yellow sticky notes you can use to add

sticky notes you can use to add citations or explanations in C there's a

citations or explanations in C there's a couple of ways to write comments and in

couple of ways to write comments and in general comments are notes for yourself

general comments are notes for yourself for your ta for your colleague as to

for your ta for your colleague as to what your code is doing and why or how

what your code is doing and why or how it's a little explanatory note in

it's a little explanatory note in English or whatever your human language

English or whatever your human language might be so for instance what I might do

might be so for instance what I might do here in my implementation of this

here in my implementation of this version of Mario I might first ask

version of Mario I might first ask myself a question like I I might first

myself a question like I I might first make a note to S like this on a new line

make a note to S like this on a new line above this first block of code uh get

above this first block of code uh get size of Grid it's just an explanatory

size of Grid it's just an explanatory remark in any tur English that generally

remark in any tur English that generally explains the next six or so lines the

explains the next six or so lines the next chunk or block of code if you will

next chunk or block of code if you will it would be a little excessive to

it would be a little excessive to comment every single line at some point

comment every single line at some point the programmer should know what

the programmer should know what individual lines of code do but it's

individual lines of code do but it's nice to be able to kind kind of glance

nice to be able to kind kind of glance at this comment on line six that starts

at this comment on line six that starts with two slashes and it gets gray out

with two slashes and it gets gray out because of syntax highlighting it's not

because of syntax highlighting it's not logic it's just a note to self it

logic it's just a note to self it generally gives me a little cheat sheet

generally gives me a little cheat sheet as to what the following lines of code

as to what the following lines of code should be doing and or why and then down

should be doing and or why and then down here well there's a second block of code

here well there's a second block of code that's a bunch of lines but together

that's a bunch of lines but together this just what prints uh grid of bricks

this just what prints uh grid of bricks and so it's another comment to myself

and so it's another comment to myself that just makes it a little more

that just makes it a little more understandable what these 20 some odd

understandable what these 20 some odd lines of code are doing by adding some

lines of code are doing by adding some English explanations thereof but now

English explanations thereof but now that I have these you know wouldn't it

that I have these you know wouldn't it be nice if I could kind of abstract

be nice if I could kind of abstract these pieces of functionality away this

these pieces of functionality away this getting of the size and this printing of

getting of the size and this printing of the Grid in other words suppose that you

the Grid in other words suppose that you didn't know where to begin with this

didn't know where to begin with this problem and the problem at hand were

problem and the problem at hand were literally Implement a program that

literally Implement a program that prints a grid of bricks of some variable

prints a grid of bricks of some variable size three or four or five or whatever

size three or four or five or whatever the human types in if you have really no

the human types in if you have really no idea where to start comments are

idea where to start comments are actually a good way of getting started

actually a good way of getting started because comments can be a approximation

because comments can be a approximation of what we called last week pseudo code

of what we called last week pseudo code pseudo code is tur English that kind of

pseudo code is tur English that kind of gets your point across like for the uh

gets your point across like for the uh phone book searching like last time so

phone book searching like last time so if you didn't really know where to begin

if you didn't really know where to begin you could do something like this I could

you could do something like this I could for instance just say get size of grid

for instance just say get size of grid as my first step and then print grid of

as my first step and then print grid of bricks as my second step and that's it

bricks as my second step and that's it for my program thus far this is now

for my program thus far this is now implemented in pseudo code I have some

implemented in pseudo code I have some massive placeholders there like I still

massive placeholders there like I still have work to be done but at least I have

have work to be done but at least I have a highle solution to the problem in

a highle solution to the problem in comments and now I can even go this far

comments and now I can even go this far I could say well let's suppose that

I could say well let's suppose that there's just a function already that

there's just a function already that exists called get size I could do

exists called get size I could do something like this I could do Inn

something like this I could do Inn equals get size and now I just have to

equals get size and now I just have to assume for the moment that some

assume for the moment that some abstraction called get size exists it

abstraction called get size exists it doesn't this does not come with the

doesn't this does not come with the cs-50 library but I could invent it I

cs-50 library but I could invent it I bet how else might I proceed well let's

bet how else might I proceed well let's just assume for the moment that there's

just assume for the moment that there's also a function called print grid that

also a function called print grid that just prints that a grid of that size n

just prints that a grid of that size n so here too is an abstraction these

so here too is an abstraction these puzzle pieces don't exist these

puzzle pieces don't exist these functions don't yet exist but in C just

functions don't yet exist but in C just like in scratch I can create my own

like in scratch I can create my own functions how do I do that well let me

functions how do I do that well let me go down later in the file and by

go down later in the file and by convention you generally want to leave

convention you generally want to leave main at the top of your code why because

main at the top of your code why because it's the main function and it's just

it's the main function and it's just where the human eye is going to look to

where the human eye is going to look to see what some file of code does and let

see what some file of code does and let me do this I want to create a function

me do this I want to create a function of my own called get size whose purpose

of my own called get size whose purpose in life is to get the size that the user

in life is to get the size that the user wants I want this function to return an

wants I want this function to return an integer and the Syntax for doing that is

integer and the Syntax for doing that is this write similar to a variable the

this write similar to a variable the data type that this function returns I

data type that this function returns I don't need this function to take any

don't need this function to take any inputs and so I'm going to use a new

inputs and so I'm going to use a new keyword that we've actually been using

keyword that we've actually been using thus far more on it another time just

thus far more on it another time just called void which just means this get

called void which just means this get size function does not take any inputs

size function does not take any inputs it does have an output it outputs an INT

it does have an output it outputs an INT and this is just a weird order in which

and this is just a weird order in which you write it you write the output format

you write it you write the output format the name of the function and then the

the name of the function and then the inputs if any inside of parentheses and

inputs if any inside of parentheses and now I can Implement get size but I've

now I can Implement get size but I've already implemented get size or at least

already implemented get size or at least now at this point in the story I at

now at this point in the story I at least know concretely what to do and I

least know concretely what to do and I could figure out eventually with some

could figure out eventually with some trial and error perhaps all right if I

trial and error perhaps all right if I declare a variable and I do the

declare a variable and I do the following n equals get in prompting the

following n equals get in prompting the user for size and I keep doing that

user for size and I keep doing that while n is less than one once that block

while n is less than one once that block of code is done here is a new keyword in

of code is done here is a new keyword in C where you can return that value n so I

C where you can return that value n so I keep referring to these values that some

keep referring to these values that some functions return as return values in C

functions return as return values in C there's literally a keyword called

there's literally a keyword called return that will hand back to any

return that will hand back to any function that uses that function the

function that uses that function the value in question so in a nutshell

value in question so in a nutshell between lines 15 and 21 now here is some

between lines 15 and 21 now here is some code identical to our solution earlier

code identical to our solution earlier that gets a value n from the user that

that gets a value n from the user that is positive it's one or two or higher

is positive it's one or two or higher it's not zero or it's not less than one

it's not zero or it's not less than one and as soon as we've got that value we

and as soon as we've got that value we hand it back as a return value notice

hand it back as a return value notice how I'm using this function on line

how I'm using this function on line 7even just like with get int just like

7even just like with get int just like with get string I'm calling the function

with get string I'm calling the function nothing in the parenthesis in this case

nothing in the parenthesis in this case but then I'm using the assignment

but then I'm using the assignment operator to copy whatever its return

operator to copy whatever its return value is into my variable n and so now I

value is into my variable n and so now I have a function that didn't used to

have a function that didn't used to exist called get size that gets me a

exist called get size that gets me a positive integer no matter what and now

positive integer no matter what and now for the grid how do I do this how do I

for the grid how do I do this how do I invent a function called print grid that

invent a function called print grid that takes a single argument a number and

takes a single argument a number and prints a grid of that size well let's go

prints a grid of that size well let's go down here I'm going to write the name of

down here I'm going to write the name of this function print grid this function

this function print grid this function just needs to print it has a side effect

just needs to print it has a side effect as we keep saying so I'm just going to

as we keep saying so I'm just going to say it has no return value it's just

say it has no return value it's just void it doesn't have an output per se

void it doesn't have an output per se it's just an aesthetic side effect but

it's just an aesthetic side effect but it does take an an argument an argument

it does take an an argument an argument is an input and the Syntax for this in C

is an input and the Syntax for this in C is to name the type of the input it

is to name the type of the input it takes

takes and the name of the variable and I could

and the name of the variable and I could call this anything I want I'll call it

call this anything I want I'll call it size I could call it n and it's okay to

size I could call it n and it's okay to use the same variable in different

use the same variable in different functions but I'll call it size just to

functions but I'll call it size just to be distinct and then in this function

be distinct and then in this function I'm just going to copy from memory the

I'm just going to copy from memory the same code as before for in I get zero I

same code as before for in I get zero I less than size instead of three I ++

less than size instead of three I ++ inside of this four uh int J gets zero J

inside of this four uh int J gets zero J is less than size j++ and inside of that

is less than size j++ and inside of that print print out with print f a single

print print out with print f a single hash print out after that Loop a single

hash print out after that Loop a single new line and that's it now I did this

new line and that's it now I did this fast admittedly but it's the same code

fast admittedly but it's the same code that I wrote earlier but now just like I

that I wrote earlier but now just like I did with scratch let me just arbitrarily

did with scratch let me just arbitrarily hit enter a bunch of times to like move

hit enter a bunch of times to like move the code out of sight out of mind now I

the code out of sight out of mind now I have abstractions I have puzzle pieces

have abstractions I have puzzle pieces that now exist called get size and print

that now exist called get size and print grid Syntax for which takes some getting

grid Syntax for which takes some getting used to but they now just exist except I

used to but they now just exist except I do need to do one thing because C is a

do need to do one thing because C is a little naive if I try to do make Mario

little naive if I try to do make Mario now and hit enter implicit Declaration

now and hit enter implicit Declaration of function get size is invalid and

of function get size is invalid and we've seen that before when I hadn't

we've seen that before when I hadn't included a file right when I hadn't

included a file right when I hadn't included cs50 Library get in didn't work

included cs50 Library get in didn't work but that's not the issue here because

but that's not the issue here because this is not from a library I just

this is not from a library I just invented this C kind of takes you

invented this C kind of takes you literally and if you define these

literally and if you define these functions at the bottom of your file

functions at the bottom of your file they don't exist on line seven or 10 so

they don't exist on line seven or 10 so I could do this I could all right fine

I could do this I could all right fine well let me just kind of highlight all

well let me just kind of highlight all of this cut with to my clipboard and

of this cut with to my clipboard and paste it up here this would solve the

paste it up here this would solve the problem I could just move all of those

problem I could just move all of those functions at the top of my file that's

functions at the top of my file that's kind of annoying because now main is

kind of annoying because now main is like the bottom of the file you're not

like the bottom of the file you're not it's going to take longer to find it

it's going to take longer to find it it's just that's not a clean solution so

it's just that's not a clean solution so let me put it back where it was at the

let me put it back where it was at the bottom and let me do this this is the

bottom and let me do this this is the only time in cs50 and really in C

only time in cs50 and really in C programming where copy paste is

programming where copy paste is reasonable if you copy and paste the

reasonable if you copy and paste the first line of code from each function

first line of code from each function and then end it with a semicolon you can

and then end it with a semicolon you can tease the compiler by giving it just

tease the compiler by giving it just enough of a hint at the top of the file

enough of a hint at the top of the file that okay these functions don't exist

that okay these functions don't exist till down later but here's a hint that

till down later but here's a hint that they will exist this is how you can uh

they will exist this is how you can uh convince the compiler to trust you so

convince the compiler to trust you so those other functions can still be lower

those other functions can still be lower in the file below main but now when I do

in the file below main but now when I do make Mario oh damn it oh I said print

make Mario oh damn it oh I said print instead of print F that's my

instead of print F that's my bad print f

bad print f so if I do make

so if I do make Mario Mario now I can type in three and

Mario Mario now I can type in three and we're back in business now this was a

we're back in business now this was a very heavy-handed way and long way to

very heavy-handed way and long way to get to a much more complicated solution

get to a much more complicated solution but this solution in some sense is

but this solution in some sense is better designed why because now

better designed why because now especially without the comments I mean

especially without the comments I mean look how short my code is my main

look how short my code is my main function is literally two lines of code

function is literally two lines of code why well I kind of factored out the

why well I kind of factored out the juicy stuff into its own functions and

juicy stuff into its own functions and now especially if I'm working with

now especially if I'm working with colleagues or others you could imagine

colleagues or others you could imagine splitting up up large programs into

splitting up up large programs into smaller parts having different people

smaller parts having different people Implement different parts so long as you

Implement different parts so long as you all agree in advance on what those

all agree in advance on what those inputs and those outputs actually are

inputs and those outputs actually are all right so let's now consider what

all right so let's now consider what computers can do well and not so well C

computers can do well and not so well C indeed supports a whole bunch of

indeed supports a whole bunch of operators mathematically via which we

operators mathematically via which we can do addition and subtraction

can do addition and subtraction multiplication division and even

multiplication division and even calculate the remainder when you divide

calculate the remainder when you divide one number by another in fact why don't

one number by another in fact why don't we go ahead and use these in a very

we go ahead and use these in a very simple program and make our very own

simple program and make our very own calculator so let me go over here here

calculator so let me go over here here to vs code let me go ahead and create a

to vs code let me go ahead and create a new file called calculator. C and in

new file called calculator. C and in this file let's go ahead and first

this file let's go ahead and first include a couple of now familiar header

include a couple of now familiar header files

files cs50.h as well as standard i.h let's go

cs50.h as well as standard i.h let's go ahead then and declare main with int

ahead then and declare main with int main void and then inside of main let's

main void and then inside of main let's do something relatively simple let's

do something relatively simple let's declare an INT and call it X and set it

declare an INT and call it X and set it equal to whatever the return value is of

equal to whatever the return value is of get int prompting the user for a value

get int prompting the user for a value for x let's then give ourselves a second

for x let's then give ourselves a second variable we'll call it say y set that

variable we'll call it say y set that equal to the return value of another

equal to the return value of another call to get int prompting the user this

call to get int prompting the user this time for that value Y and then let's

time for that value Y and then let's very simply go ahead at the very end and

very simply go ahead at the very end and just print out say the sum of X Plus y a

just print out say the sum of X Plus y a super simple calculator so I'll use

super simple calculator so I'll use print F quote unquote percent I for

print F quote unquote percent I for integer back sln to give me the new line

integer back sln to give me the new line then I'm going to go ahead and do x + y

then I'm going to go ahead and do x + y to indeed print out the sum let me go

to indeed print out the sum let me go down to my terminal window now let me do

down to my terminal window now let me do make calculator in order to compile the

make calculator in order to compile the C no error messages so that's good let

C no error messages so that's good let me do/ calulator and let's do something

me do/ calulator and let's do something like 2 + 2 which of course should equal

like 2 + 2 which of course should equal 4 and it does but it turns out that

4 and it does but it turns out that sometimes there are going to be

sometimes there are going to be limitations that we bump up against and

limitations that we bump up against and let me get a little more ambitious here

let me get a little more ambitious here let me clear my terminal window and let

let me clear my terminal window and let me go ahead and rerun calculator again

me go ahead and rerun calculator again and this time let's type in

O2 billion for x and let's type in the same for y and of course now the answer

same for y and of course now the answer of 2 billion plus 2 billion should have

of 2 billion plus 2 billion should have of course be 4 billion and yet it's not

of course be 4 billion and yet it's not so curiously we see of all things a

so curiously we see of all things a negative number here which suggests that

negative number here which suggests that somehow the plus operator doesn't quite

somehow the plus operator doesn't quite work as well as we might like now why

work as well as we might like now why might this actually be well it turns out

might this actually be well it turns out that inside of your computer is of

that inside of your computer is of course memory or Ram random access

course memory or Ram random access memory and depending on the size of your

memory and depending on the size of your computer and the type of computer it

computer and the type of computer it might very well look a little something

might very well look a little something like this a little circuit board with

like this a little circuit board with these black little modules on it that

these black little modules on it that actually contain all of the btes of your

actually contain all of the btes of your computer's memory unfortunately you and

computer's memory unfortunately you and I only have a finite amount of this

I only have a finite amount of this memory inside of our computers which

memory inside of our computers which means no matter how high we want to

means no matter how high we want to count there's ultimately going to be a

count there's ultimately going to be a limitation on high how we can count

limitation on high how we can count because we only have a finite amount of

because we only have a finite amount of memory we don't have an infinite number

memory we don't have an infinite number of zeros and ones to play with we have

of zeros and ones to play with we have to actually be bounded ultimately so

to actually be bounded ultimately so what's the implication of this well it

what's the implication of this well it turns out that computers typically use

turns out that computers typically use as many as 32 bits zeros or ones to

as many as 32 bits zeros or ones to represent something like an integer or

represent something like an integer or in C an INT so for instance the smallest

in C an INT so for instance the smallest number we could represent using 32 ins

number we could represent using 32 ins of course using 32 bits of course would

of course using 32 bits of course would be zero 32 zeros like this here and the

be zero 32 zeros like this here and the biggest number we could represent is by

biggest number we could represent is by changing all of those zeros to ones

changing all of those zeros to ones which in this case will ideally give us

which in this case will ideally give us a number that equals roughly 4 billion

a number that equals roughly 4 billion in total it's actually 4 billion 294 mil

in total it's actually 4 billion 294 mil 967 295 maximally if you set all 32 of

967 295 maximally if you set all 32 of those bits to ones and then do out the

those bits to ones and then do out the actual math the catch though is that we

actual math the catch though is that we humans and Compu in general also

humans and Compu in general also sometimes want to and need to be able to

sometimes want to and need to be able to represent negative numbers so if you

represent negative numbers so if you want to represent negative numbers as

want to represent negative numbers as well as positive numbers in zero you

well as positive numbers in zero you can't really just start counting at zero

can't really just start counting at zero and go all the way up to roughly four

and go all the way up to roughly four billion you got to kind of split the

billion you got to kind of split the difference and maybe allocate half of

difference and maybe allocate half of those patterns of zeros and ones to

those patterns of zeros and ones to negative numbers and the other half

negative numbers and the other half roughly to positive numbers so in fact

roughly to positive numbers so in fact in practice when you're using even as

in practice when you're using even as many as 32 bits the highest most

many as 32 bits the highest most computers could count certainly in a

computers could count certainly in a program like this in C using an INT

program like this in C using an INT would be roughly 2 billion that is 2 b47

would be roughly 2 billion that is 2 b47 m

483,000 flow where you want to use more bits than you have available and as a

bits than you have available and as a result you sort of overflow the

result you sort of overflow the available space what does this mean in

available space what does this mean in fact in real terms well let's suppose

fact in real terms well let's suppose that you only have three bits but I'm

that you only have three bits but I'm going to gray out a fourth bit just to

going to gray out a fourth bit just to convey where we'd like to uh put an

convey where we'd like to uh put an additional bit ultimately if this of

additional bit ultimately if this of course is zero per week zero discussion

course is zero per week zero discussion this is 1 2 3 4 5 6 7 now ideally in

this is 1 2 3 4 5 6 7 now ideally in binary if you want to add one more to

binary if you want to add one more to this value seven you're going to have to

this value seven you're going to have to carry the one mathematically and that

carry the one mathematically and that would ideally give you 1 0 0 0 but if

would ideally give you 1 0 0 0 but if you don't have four bits and your

you don't have four bits and your computer's only sophisticated enough to

computer's only sophisticated enough to have three bits not even 32 but three

have three bits not even 32 but three the implication is that you're

the implication is that you're effectively representing not one0 0 but

effectively representing not one0 0 but rather 0 z0 there's just no room to

rather 0 z0 there's just no room to store that fourth bit that I've gray out

store that fourth bit that I've gray out here which is to say that your integer

here which is to say that your integer might overflow and as soon as you get to

might overflow and as soon as you get to seven the next number once you add one

seven the next number once you add one is actually going to be zero or worse as

is actually going to be zero or worse as we've seen here in my code a negative

we've seen here in my code a negative value instead so what could we do to

value instead so what could we do to perhaps address this kind of concern

perhaps address this kind of concern well C does not have just integers or

well C does not have just integers or ins it also has Longs which as the name

ins it also has Longs which as the name suggest or just longer integers which

suggest or just longer integers which means they have more bits available to

means they have more bits available to them so let me go back into my code here

them so let me go back into my code here I'll clear the terminal window and let

I'll clear the terminal window and let me go ahead and change my integers to

me go ahead and change my integers to literally long here long here I'm going

literally long here long here I'm going to have to change my function in

to have to change my function in uh cs50's library to be not get in but

uh cs50's library to be not get in but get long and that's indeed another

get long and that's indeed another function we provide in the library let

function we provide in the library let me change this get in to get long as

me change this get in to get long as well I'll keep my variable names the

well I'll keep my variable names the same but I do need to make one other

same but I do need to make one other change it turns out that print F also

change it turns out that print F also support supports other format codes so

support supports other format codes so not just percent I for integers or

not just percent I for integers or percent s for Strings but also for

percent s for Strings but also for instance percent Li for a long integer

instance percent Li for a long integer as well as percent f for floating Point

as well as percent f for floating Point values with decimals so with that said

values with decimals so with that said let's go ahead and change my print F

let's go ahead and change my print F line to be not perc I but percent Li I

line to be not perc I but percent Li I now let me go ahead and do make

now let me go ahead and do make calculator again enter no apparent

calculator again enter no apparent errors now do/ calulator and 2 + 2 still

errors now do/ calulator and 2 + 2 still equals 4 as before but now if I do

equals 4 as before but now if I do calculator again and let's do

calculator again and let's do two billion again as well as 2 billion

two billion again as well as 2 billion for y previously we overflowed the size

for y previously we overflowed the size of an integer and got some weird

of an integer and got some weird negative number because the pattern was

negative number because the pattern was misinterpreted if you will as a negative

misinterpreted if you will as a negative number instead but along instead of

number instead but along instead of using using 32 bits conventionally uses

using using 32 bits conventionally uses 64 bits which means we have more than

64 bits which means we have more than enough spare bits to go when we add 2

enough spare bits to go when we add 2 billion plus 2 billion and now in fact

billion plus 2 billion and now in fact we get the correct answer of four

we get the correct answer of four billion which does fit inside of the

billion which does fit inside of the size of a long now along can count up

size of a long now along can count up quite high and in fact it can count as

quite high and in fact it can count as high as this nine quintilian and so that

high as this nine quintilian and so that will give us quite a bit more Runway but

will give us quite a bit more Runway but of course it too is ultimately going to

of course it too is ultimately going to be finite so if you have numbers that

be finite so if you have numbers that need to go bigger than that you might

need to go bigger than that you might still very well have a problem now

still very well have a problem now there's another problem that we might

there's another problem that we might run into as well and we can see it in

run into as well and we can see it in the context of even this simple

the context of even this simple calculator computers also suffer from

calculator computers also suffer from potentially what's called truncation

potentially what's called truncation where especially when you're doing math

where especially when you're doing math involving floating Point values that is

involving floating Point values that is numbers with decimals you might

numbers with decimals you might accidentally unknowingly truncate the

accidentally unknowingly truncate the value that is lose everything after the

value that is lose everything after the decimal point so in fact let me go back

decimal point so in fact let me go back to vs code here I'll clear my terminal

to vs code here I'll clear my terminal window and let's still use Longs but

window and let's still use Longs but let's go ahead and use division instead

let's go ahead and use division instead of addition here so let me change this

of addition here so let me change this plus to a divide operator let me go

plus to a divide operator let me go ahead and recompile the code down here

ahead and recompile the code down here with make calculator let me go ahead and

with make calculator let me go ahead and run/ calculator and let me go ahead and

run/ calculator and let me go ahead and do something like 14x and 3 for y and

do something like 14x and 3 for y and we'll see that well wait a minute 1

we'll see that well wait a minute 1 divided 3 I learned should be 13 but in

divided 3 I learned should be 13 but in a floating point value that should be

a floating point value that should be point0 it should be

point0 it should be 0.33333 you know maybe with a little

0.33333 you know maybe with a little line over in grade school but really an

line over in grade school but really an infinite number of Threes And yet we

infinite number of Threes And yet we seem to have lost even one of those

seem to have lost even one of those threes after the decimal point because

threes after the decimal point because the answer is coming back here as just

the answer is coming back here as just zero so why might that be well if I know

zero so why might that be well if I know that two integers when divided one by

that two integers when divided one by the other is supposed to give me a

the other is supposed to give me a fraction a floating point value with a

fraction a floating point value with a decimal point I can't continue to use

decimal point I can't continue to use integers or even in this case Longs

integers or even in this case Longs which do not have support for decimal

which do not have support for decimal points so let me go ahead and change

points so let me go ahead and change this format code here from percent Li I

this format code here from percent Li I to percent F which is again going to

to percent F which is again going to represent a floating point value instead

represent a floating point value instead of a long integer or even an integer and

of a long integer or even an integer and let me go ahead further and Define maybe

let me go ahead further and Define maybe a third variable Z as a float itself so

a third variable Z as a float itself so I'll give myself a variable Z equals x /

I'll give myself a variable Z equals x / Y and now rather than print X ided Y

Y and now rather than print X ided Y let's just go ahead and print Z so now

let's just go ahead and print Z so now I'm operating in a world of floating

I'm operating in a world of floating Point values because I know proactively

Point values because I know proactively that an long or an INT divided by

that an long or an INT divided by another such value if it's meant to have

another such value if it's meant to have a fraction needs to be stored in a

a fraction needs to be stored in a floating point value something with a

floating point value something with a decimal point well let me go down to my

decimal point well let me go down to my terminal window here and rerun make of

terminal window here and rerun make of calculator seems to work okay/ calulator

calculator seems to work okay/ calulator and let's do 1 / 3 again and still here

and let's do 1 / 3 again and still here we see all zeros well so we do at least

we see all zeros well so we do at least see a decimal point so we've made some

see a decimal point so we've made some progress thanks to the percent F in the

progress thanks to the percent F in the float but it seems that we've already

float but it seems that we've already truncated the value 1 / 3 so how do we

truncated the value 1 / 3 so how do we actually get around this issue well if

actually get around this issue well if you the program know that you're dealing

you the program know that you're dealing in a world that's going to give you

in a world that's going to give you floating Point values with decimal

floating Point values with decimal points you might very well need to use

points you might very well need to use what's called a feature known as typ

what's called a feature known as typ casting that is convert one data type to

casting that is convert one data type to another by explicitly telling the

another by explicitly telling the compiler that you want to do so now how

compiler that you want to do so now how do I do this well let's go back to my

do I do this well let's go back to my code here and if the issue fundamentally

code here and if the issue fundamentally is that c is still treating X and Y as

is that c is still treating X and Y as integers or technically Longs with no

integers or technically Longs with no decimal point and dividing one by the

decimal point and dividing one by the other therefore has no room so to speak

other therefore has no room so to speak for any numbers after a decimal point

for any numbers after a decimal point why don't I proactively do this let me

why don't I proactively do this let me using a slightly new syntax with

using a slightly new syntax with parenthesis specify that I want to

parenthesis specify that I want to convert X proactively from a long to a

convert X proactively from a long to a float let me specify proactively that I

float let me specify proactively that I want to convert y from a long to a float

want to convert y from a long to a float as well and now let me go ahead and

as well and now let me go ahead and trust that in Z should be the result of

trust that in Z should be the result of dividing not a long by a long or an INT

dividing not a long by a long or an INT by an INT but rather a float by a float

by an INT but rather a float by a float Let Me Clear My terminal window run make

Let Me Clear My terminal window run make calculator again seems to work okay/

calculator again seems to work okay/ calulator and now 1 3 and hopefully now

calulator and now 1 3 and hopefully now we actually see that my code has

we actually see that my code has outputed

outputed 0.333 333 and I think if we kept showing

0.333 333 and I think if we kept showing more numbers after the decimal point

more numbers after the decimal point we'd theoretically see as many of those

we'd theoretically see as many of those threes as we want but there is still one

threes as we want but there is still one more catch and especially when we're

more catch and especially when we're manipulating numbers in this way in a

manipulating numbers in this way in a computer using a finite amount of memory

computer using a finite amount of memory another challenge we might run up

another challenge we might run up against besides integer overflow besides

against besides integer overflow besides truncation is this known as floating

truncation is this known as floating point in Precision just as we can't

point in Precision just as we can't represent as big of an integer as we

represent as big of an integer as we want using int or long alone because

want using int or long alone because there is going to be an upper bound

there is going to be an upper bound they're similarly going to be a boundary

they're similarly going to be a boundary on just how precise our numbers can be

on just how precise our numbers can be and indeed let's go back to vs code here

and indeed let's go back to vs code here I'll clear my terminal window yet again

I'll clear my terminal window yet again and this time let me use some slightly

and this time let me use some slightly unley syntax to specify that I don't

unley syntax to specify that I don't want to see the default number of

want to see the default number of numbers after the the decimal point

numbers after the the decimal point which percent F gives us automatically

which percent F gives us automatically let's go ahead and show me 20 decimal

let's go ahead and show me 20 decimal point numbers after the decimal point

point numbers after the decimal point and the weird Syntax for this is to do

and the weird Syntax for this is to do not percent F but percent period to zero

not percent F but percent period to zero to indicate to see that I want to see 20

to indicate to see that I want to see 20 digits not the default after now the

digits not the default after now the decimal point let me rerun make

decimal point let me rerun make calculator let me do dot calculat again

calculator let me do dot calculat again and let's do one let's do three and now

and let's do one let's do three and now this is even weirder right from grade

this is even weirder right from grade school you presumably learned that 1

school you presumably learned that 1 divided 3 is of course 1/3 but that

divided 3 is of course 1/3 but that should be

should be 0.33333 infinitely many times or on

0.33333 infinitely many times or on paper with a little line over it but the

paper with a little line over it but the computer is just doing some weird

computer is just doing some weird approximation here it's a whole bunch of

approximation here it's a whole bunch of Threes And then 43 267 44 079

Threes And then 43 267 44 079 590 well what's really happening under

590 well what's really happening under the hood well again is this issue of

the hood well again is this issue of floating point in Precision if you only

floating point in Precision if you only have a finite number of bits and in turn

have a finite number of bits and in turn a finite amount of memory the computer

a finite amount of memory the computer can really only be so precise

can really only be so precise intuitively alter Converse or

intuitively alter Converse or equivalently the computer has decided on

equivalently the computer has decided on some way of representing floating Point

some way of representing floating Point values but the catch is per grade school

values but the catch is per grade school math there's an infinite number of

math there's an infinite number of numbers out there and an infinite number

numbers out there and an infinite number of floating Point values because you can

of floating Point values because you can keep adding more and more digits if you

keep adding more and more digits if you want so the computer given the way it's

want so the computer given the way it's implementing these floating Point values

implementing these floating Point values is essentially giving us the closest

is essentially giving us the closest approximation that it can now how can we

approximation that it can now how can we go about improving the situation well

go about improving the situation well there is one alternative instead of

there is one alternative instead of using float I can use something called a

using float I can use something called a double which as the name suggests uses

double which as the name suggests uses twice as many bits as a float so instead

twice as many bits as a float so instead of 32 typically it will use 64 and

of 32 typically it will use 64 and that's just like the difference between

that's just like the difference between a long and an INT which gave us more

a long and an INT which gave us more bits but in this case this will be used

bits but in this case this will be used for more Precision let's go ahead and

for more Precision let's go ahead and Cast X to a double let's cast y to a

Cast X to a double let's cast y to a double and now let's go ahead and using

double and now let's go ahead and using the same format code percent 2f is still

the same format code percent 2f is still okay for doubles let me do make

okay for doubles let me do make calculator let me do do slash calculator

calculator let me do do slash calculator and now let me do 1 / 3 and we still

and now let me do 1 / 3 and we still have some of that in precision and we'd

have some of that in precision and we'd see even more of it if we looked at more

see even more of it if we looked at more than just 20 digits but now we have more

than just 20 digits but now we have more threes after the decimal point so it's

threes after the decimal point so it's at least more more more precise but it's

at least more more more precise but it's not perfect but it it's at least more

not perfect but it it's at least more precise so these kinds of issues then

precise so these kinds of issues then are going to be necessary to keep in

are going to be necessary to keep in mind anytime you do something

mind anytime you do something numerically scientifically at least with

numerically scientifically at least with a language like C where you're going to

a language like C where you're going to bump up against these real world

bump up against these real world limitations of hardware and intern

limitations of hardware and intern language now later in the semester we'll

language now later in the semester we'll transition to a language called Python

transition to a language called Python and that's actually going to solve at

and that's actually going to solve at least one of these problems for us but

least one of these problems for us but just automatically giving us more bits

just automatically giving us more bits so to speak as we need them at least for

so to speak as we need them at least for integers but even the issue of floating

integers but even the issue of floating point in Precision is going to remain

point in Precision is going to remain now just how real world are these issues

now just how real world are these issues well back in the year 1999 we got a

well back in the year 1999 we got a taste of this when the world realized in

taste of this when the world realized in the Years leading up to that date that

the Years leading up to that date that it might not have been the best idea to

it might not have been the best idea to implement computers and software therein

implement computers and software therein by storing years using just two digits

by storing years using just two digits like instead of storing 1999 to

like instead of storing 1999 to represent the year 1999 a lot of

represent the year 1999 a lot of computers for reasons of space and cost

computers for reasons of space and cost were in the habit of kind of cutting a

were in the habit of kind of cutting a corner and just using two digits to keep

corner and just using two digits to keep track of the year the problem with that

track of the year the problem with that is that if systems were not updated by

is that if systems were not updated by the year 1999 to support the year 2000

the year 1999 to support the year 2000 2001 and so forth is that just like

2001 and so forth is that just like before with integer overflow some

before with integer overflow some computers might add one to the year in

computers might add one to the year in their memory 99 it should be the year

their memory 99 it should be the year 2000 but if they're only using two

2000 but if they're only using two digits to represent years they might

digits to represent years they might mistake the year as some systems may

mistake the year as some systems may very well have for the year 1900 instead

very well have for the year 1900 instead taking literally a big step backwards if

taking literally a big step backwards if you will now you'd like to think that

you will now you'd like to think that kind of issue is behind us especially as

kind of issue is behind us especially as we understand all the more about the

we understand all the more about the limitations of code and Computing but

limitations of code and Computing but we're actually going to run up against

we're actually going to run up against this very same type of issue again in

this very same type of issue again in just a few years on January 19th in the

just a few years on January 19th in the year

year 2038 we will have run out of bits in

2038 we will have run out of bits in most computers right now to keep track

most computers right now to keep track of time it turns out years ago humans

of time it turns out years ago humans decided to use a 32-bit integer to keep

decided to use a 32-bit integer to keep track of how many seconds had elapsed

track of how many seconds had elapsed over time they chose a somewhat

over time they chose a somewhat arbitrary date in the past January 1st

arbitrary date in the past January 1st 1970 and they just started counting

1970 and they just started counting seconds from there on out and so if a

seconds from there on out and so if a computer stores some number of seconds

computer stores some number of seconds that tells the computer how many seconds

that tells the computer how many seconds have passed since that particular date

have passed since that particular date January 1st 1970 unfortunately using a

January 1st 1970 unfortunately using a 32-bit integer as we've seen you can

32-bit integer as we've seen you can only count so high at which point you

only count so high at which point you overflow the size of that variable and

overflow the size of that variable and so potentially if we don't get ahead of

so potentially if we don't get ahead of this as humans as a society as computer

this as humans as a society as computer scientist on the date January 19th

scientist on the date January 19th 2038 that bit might flip over there by

2038 that bit might flip over there by overflowing the size of those integers

overflowing the size of those integers bringing us back computationally to

bringing us back computationally to December 13th

December 13th 1901 so this is to say now with all of

1901 so this is to say now with all of this computational ability and code

this computational ability and code comes a responsibility to actually write

comes a responsibility to actually write correct code next week we'll peel back

correct code next week we'll peel back some of these layers but for now this

some of these layers but for now this was week one and best of luck on problem

was week one and best of luck on problem set

all right this is cs50 and this is this is week two wherein we're going to take

is week two wherein we're going to take a look at a lower level at how things

a look at a lower level at how things work and indeed among the goals of the

work and indeed among the goals of the course isn't this bottom up

course isn't this bottom up understanding so that in a couple of

understanding so that in a couple of weeks time even a few years time when

weeks time even a few years time when you encounter some new technology you'll

you encounter some new technology you'll be able to think back hopefully on some

be able to think back hopefully on some of this week's and this courses basic

of this week's and this courses basic building blocks and Primitives and

building blocks and Primitives and really just deduce how tomorrow's

really just deduce how tomorrow's Technologies work but along the way it's

Technologies work but along the way it's going to seem it's going to be a little

going to seem it's going to be a little hard perhaps to see the forest for the

hard perhaps to see the forest for the tree so to speak and so the goal at the

tree so to speak and so the goal at the end of the day still is going to be

end of the day still is going to be problem solving and so we thought we'd

problem solving and so we thought we'd begin today with a look at some of the

begin today with a look at some of the problems we'll talk about or solve this

problems we'll talk about or solve this coming week uh and for that we have some

coming week uh and for that we have some Brave volunteers who have already come

Brave volunteers who have already come up if we could turn on some dramatic

up if we could turn on some dramatic lighting and meet today's

lighting and meet today's volunteers so on my left here we have hi

volunteers so on my left here we have hi my name is Alex I'm a first year at the

my name is Alex I'm a first year at the college and I'm from Chapo North

college and I'm from Chapo North Carolina Welcome to Alex and to Alex's

Carolina Welcome to Alex and to Alex's right um I'm Sarah I'm from Toronto

right um I'm Sarah I'm from Toronto Canada and I'm also a first year student

Canada and I'm also a first year student at the college wonderful well welcome to

at the college wonderful well welcome to both Al and Sarah so one of the problems

both Al and Sarah so one of the problems you'll perhaps solve this week for

you'll perhaps solve this week for problem set two is to analyze the

problem set two is to analyze the reading level of a body of text whether

reading level of a body of text whether someone reads at a first grade level

someone reads at a first grade level second grade level third grade level all

second grade level third grade level all the way up to 12 or 13 or Beyond but

the way up to 12 or 13 or Beyond but you've perhaps never quite thought about

you've perhaps never quite thought about certainly in terms of code like how you

certainly in terms of code like how you would analyze some text some book and

would analyze some text some book and figure out what reading level is it at

figure out what reading level is it at and yet surely our teachers growing up

and yet surely our teachers growing up kind of knew or had an intuitive sense

kind of knew or had an intuitive sense of this so let's consider some sample

of this so let's consider some sample text for instance Alex what have you

text for instance Alex what have you been reading lately um

been reading lately um One Fish Two Fish Red F Fish Blue Fish

One Fish Two Fish Red F Fish Blue Fish wonderful so given that what grade level

wonderful so given that what grade level would you say Alex is currently reading

would you say Alex is currently reading at feel free to just shout it

at feel free to just shout it out first first so indeed you'll see

out first first so indeed you'll see this week if you run your code on Alex's

this week if you run your code on Alex's text it actually turns out he reads

text it actually turns out he reads below a first grade reading level but

below a first grade reading level but but why might that be what might your

but why might that be what might your intuition be for why we've

intuition be for why we've uh why we've accused Alex of reading at

uh why we've accused Alex of reading at this level feel free to shout

this level feel free to shout out yeah so very few syllables short

out yeah so very few syllables short words short sentences and so there's

words short sentences and so there's some puristic perhaps we can infer from

some puristic perhaps we can infer from that short text that that probably means

that short text that that probably means that it's best for younger children now

that it's best for younger children now Sarah by contrast what have you been

Sarah by contrast what have you been reading Mr and Miss dersley of number

reading Mr and Miss dersley of number four privet Drive were proud to say that

four privet Drive were proud to say that they were perfectly normal thank you

they were perfectly normal thank you very much they were the last people

very much they were the last people you'd expect to be involved in anything

you'd expect to be involved in anything strange or mysterious ious because they

strange or mysterious ious because they just didn't hold with much nonsense all

just didn't hold with much nonsense all right now irrespective of what grade you

right now irrespective of what grade you were in when you might have read that

were in when you might have read that text what grade level does Sarah seem to

text what grade level does Sarah seem to be reading

be reading at so eighth grade second grade okay so

at so eighth grade second grade okay so hearing a bit of everything so that at

hearing a bit of everything so that at least according to code would actually

least according to code would actually be seventh grade and what might the

be seventh grade and what might the intuition there be why is that a higher

intuition there be why is that a higher grade level even though we might

grade level even though we might disagree exactly which grade it is comp

disagree exactly which grade it is comp yeah so complicated sentences longer

yeah so complicated sentences longer sentences so indeed a lot more more

sentences so indeed a lot more more words were being spoken by Sarah because

words were being spoken by Sarah because there was so much more there on the page

there was so much more there on the page so we'll translate these ideas this

so we'll translate these ideas this coming week and problem set two if you

coming week and problem set two if you tackle this one to code so that you can

tackle this one to code so that you can ultimately infer things of these

ultimately infer things of these quantitatively but to do so we're going

quantitatively but to do so we're going to have to understand text so let's

to have to understand text so let's first thank our volunteers and then

first thank our volunteers and then we'll dive in to that lower

we'll dive in to that lower level stress

balls sure you can keep those yeah all right so besides that let's consider one

right so besides that let's consider one another body of text perhaps that you

another body of text perhaps that you might see this week which is namely a

might see this week which is namely a little something like this what I have

little something like this what I have here on the screen is what we'll start

here on the screen is what we'll start calling today Cipher text it's the

calling today Cipher text it's the result of encrypting some piece of

result of encrypting some piece of information and encryption or more

information and encryption or more generally the Art and Science of

generally the Art and Science of cryptography is all around us it's what

cryptography is all around us it's what you're using on the web on your phones

you're using on the web on your phones with your Banks and anything that tries

with your Banks and anything that tries to keep data secure is using encryption

to keep data secure is using encryption but there's going to be different levels

but there's going to be different levels of encryption strong encryption weak

of encryption strong encryption weak encryption and what you see here on the

encryption and what you see here on the screen isn't all that strong but we'll

screen isn't all that strong but we'll see later today how we might decrypt

see later today how we might decrypt this and actually reveal what the plain

this and actually reveal what the plain text is that corresponds to that Cipher

text is that corresponds to that Cipher text but in order to do so we have to

text but in order to do so we have to start taking off some training wheels so

start taking off some training wheels so to speak and believe it or not even

to speak and believe it or not even though your time with C this past week

though your time with C this past week for the first time probably might have

for the first time probably might have been rather in the weeds and much more

been rather in the weeds and much more complicated seemingly than C it turns

complicated seemingly than C it turns out that along the way we have been

out that along the way we have been providing and will continue to provide

providing and will continue to provide certain training wheels for instance the

certain training wheels for instance the cs50 library is one of them and even

cs50 library is one of them and even some of the explanations give up topics

some of the explanations give up topics for now in these early weeks will be

for now in these early weeks will be somewhat simplified abstracted away if

somewhat simplified abstracted away if you will but the goal ultimately is for

you will but the goal ultimately is for you to understand each and every one of

you to understand each and every one of those details so that after cs50 you

those details so that after cs50 you really can stand on your own and

really can stand on your own and understand and wrap your mind around any

understand and wrap your mind around any future Technologies as well so let's

future Technologies as well so let's consider first the very first program

consider first the very first program with which we began last week which was

with which we began last week which was this one so hello world and C at the end

this one so hello world and C at the end of the day it was really the print a

of the day it was really the print a function that was doing the interesting

function that was doing the interesting part of the work but there was a lot of

part of the work but there was a lot of technical stuff above and below it the

technical stuff above and below it the the curly braces the parentheses words

the curly braces the parentheses words like void and include and then of course

like void and include and then of course the angled brackets and more but at the

the angled brackets and more but at the end of the day we needed to convert that

end of the day we needed to convert that source code in C to machine code the

source code in C to machine code the zeros and ones in binary that the

zeros and ones in binary that the computer understood and to do that of

computer understood and to do that of course we ran we compiled the code we

course we ran we compiled the code we ran make and then we were able to

ran make and then we were able to actually run that code there so let me

actually run that code there so let me actually go over here to VSS code and

actually go over here to VSS code and really quickly recreate that hello.c

really quickly recreate that hello.c pretty much by transcribing the same so

pretty much by transcribing the same so I have here uh include standard

I have here uh include standard i.h uh int main void and then in here I

i.h uh int main void and then in here I had quite simply hello comma world with

had quite simply hello comma world with my back slend quotes and more now last

my back slend quotes and more now last time to compile this I indeed ran make

time to compile this I indeed ran make hello followed by enter hopefully you

hello followed by enter hopefully you see no errors and that's a good thing

see no errors and that's a good thing and if you do do/ hello you see in fact

and if you do do/ hello you see in fact the results of that program but it turns

the results of that program but it turns out that make is not actually a compiler

out that make is not actually a compiler as I alluded to last week it's a program

as I alluded to last week it's a program that clearly makes your program but it

that clearly makes your program but it itself just automates the process of

itself just automates the process of using an actual compiler and there's

using an actual compiler and there's lots of different compilers out there

lots of different compilers out there and the one that it's actually using

and the one that it's actually using underneath the hood is a little

underneath the hood is a little something called clang for C language

something called clang for C language and clang is a pretty popular compiler

and clang is a pretty popular compiler nowadays there's another one that's been

nowadays there's another one that's been around for ages called GCC but these are

around for ages called GCC but these are just specific names for types of

just specific names for types of compilers that different people

compilers that different people different companies different groups

different companies different groups have actually created but if you use in

have actually created but if you use in week one a compiler yourself manually

week one a compiler yourself manually you have to know you have to understand

you have to know you have to understand a little more about what's going on

a little more about what's going on because it's even more cryptic than with

because it's even more cryptic than with just make a loan so in fact let me go

just make a loan so in fact let me go back to my terminal window here let me

back to my terminal window here let me go ahead and clear the screen a little

go ahead and clear the screen a little bit and just run really the raw compiler

bit and just run really the raw compiler command so what make is automating for

command so what make is automating for me let me actually do this manually for

me let me actually do this manually for just a moment so if I want to compile uh

just a moment so if I want to compile uh hello.c into an executable program I can

hello.c into an executable program I can run I can do this uh clang space hello

run I can do this uh clang space hello C and then enter and now there's no

C and then enter and now there's no output which is a good thing in this

output which is a good thing in this case no errors but notice this if I go

case no errors but notice this if I go ahead and type LS it turns out there's a

ahead and type LS it turns out there's a uh a file that's been created suddenly

uh a file that's been created suddenly in my current folder weirdly called

in my current folder weirdly called a.out that stands for assembler output

a.out that stands for assembler output and long story short that's actually the

and long story short that's actually the default name of a program that's created

default name of a program that's created when you just run C by itself now that's

when you just run C by itself now that's a pretty uh bad name for a program

a pretty uh bad name for a program because it doesn't describe what it said

because it doesn't describe what it said does so better would be here to perhaps

does so better would be here to perhaps do well instead of a.out which yes still

do well instead of a.out which yes still prints hello. world but isn't really a a

prints hello. world but isn't really a a clearly named program it'd be nice to

clearly named program it'd be nice to name this hello so what could I do I

name this hello so what could I do I could do like we learned last week well

could do like we learned last week well I could rename a.out to Hello by using

I could rename a.out to Hello by using linux's MV command so I'm going to move

linux's MV command so I'm going to move a. out to become hello but that too

a. out to become hello but that too seems kind of tedious now I have three

seems kind of tedious now I have three steps like write my code compile my code

steps like write my code compile my code and then rename it before I can even run

and then rename it before I can even run it like we can do better than that and

it like we can do better than that and so it turns out that certain commands

so it turns out that certain commands like clang support what we're going to

like clang support what we're going to start today calling commandline

start today calling commandline arguments a commandline argument unlike

arguments a commandline argument unlike an argument to a function is just an

an argument to a function is just an additional word or key phrase that you

additional word or key phrase that you type after a command at your prompt in

type after a command at your prompt in your terminal window that just modifies

your terminal window that just modifies the behavior of that command it

the behavior of that command it configures it a little more specifically

configures it a little more specifically so what you're seeing here on the screen

so what you're seeing here on the screen is a summary of a better command with

is a summary of a better command with which to run clang so that now I can

which to run clang so that now I can specify the output of this command this-

specify the output of this command this- o so what do I mean by that well let me

o so what do I mean by that well let me go ahead and clear my terminal window

go ahead and clear my terminal window again and more explicitly type clang d o

again and more explicitly type clang d o hello hello. C and then enter nothing

hello hello. C and then enter nothing again appears to happen but that's a

again appears to happen but that's a good thing when you see no errors and

good thing when you see no errors and now the program I just created is indeed

now the program I just created is indeed called hello so it achieves really the

called hello so it achieves really the same exact effect as make did but what I

same exact effect as make did but what I don't have to do with make is type and

don't have to do with make is type and remember something as long as this

remember something as long as this command and this too is a bit of a white

command and this too is a bit of a white lie it turns out we have preconfigured

lie it turns out we have preconfigured vs code in the cloud for you to also use

vs code in the cloud for you to also use some other features of clang that would

some other features of clang that would be even more tedious for you to write

be even more tedious for you to write yourselves and so really this is why we

yourselves and so really this is why we distill this as ultimately just running

distill this as ultimately just running make so let me pause here to see first

make so let me pause here to see first if there's any questions on what I've

if there's any questions on what I've done by taking my very first program in

done by taking my very first program in C and just now compiling it first with

C and just now compiling it first with make but then starting over and now

make but then starting over and now manually compiling it with clang with

manually compiling it with clang with what we'll call command line arguments -

what we'll call command line arguments - o space hello and then the name of the

o space hello and then the name of the file

file yeah yeah so a.out is a historical name

yeah yeah so a.out is a historical name it refers to assembler output more on

it refers to assembler output more on that soon and it's just the default file

that soon and it's just the default file name that you get automatically if you

name that you get automatically if you just run the compiler on any file so

just run the compiler on any file so that you have just a standard name for

that you have just a standard name for it but it's not a very well-named

it but it's not a very well-named program instead of running Microsoft

program instead of running Microsoft Word on your Mac or PC it would like be

Word on your Mac or PC it would like be like double clicking on a.out so instead

like double clicking on a.out so instead with these command line arguments you

with these command line arguments you can customize the output of clang and

can customize the output of clang and call it hello or anything you want other

call it hello or anything you want other questions on what I've done here with

questions on what I've done here with clang itself the

clang itself the compiler

compiler yeah so- o and you would only know this

yeah so- o and you would only know this from reading the manual taking a class

from reading the manual taking a class means output so- o means change clangs

means output so- o means change clangs output to be a file called hello instead

output to be a file called hello instead of the default which is a.out and this

of the default which is a.out and this too is again a detail you would have to

too is again a detail you would have to uh look it up on a web page read the

uh look it up on a web page read the manual hear someone like me tell you

manual hear someone like me tell you about it and in fact there's even more

about it and in fact there's even more than these options but we'll just

than these options but we'll just scratch the surface here all right so if

scratch the surface here all right so if we now know this what more is actually

we now know this what more is actually happening underneath the hood well let's

happening underneath the hood well let's take a a closer look at not just this

take a a closer look at not just this version of my code but my slightly more

version of my code but my slightly more complicated version last week which

complicated version last week which looked a little something like this

looked a little something like this wherein I added in some Dynamic input

wherein I added in some Dynamic input from the user so I could say not Hello

from the user so I could say not Hello World to everyone but hello David or

World to everyone but hello David or hello to whoever actually runs this

hello to whoever actually runs this program so in fact let me go ahead and

program so in fact let me go ahead and change my code here in vs code just to

change my code here in vs code just to match that same code from last week so

match that same code from last week so no new code yet I'm just going to in a

no new code yet I'm just going to in a moment compile it in a slightly

moment compile it in a slightly different way so I did last week string

different way so I did last week string uh I think answer equals get string

uh I think answer equals get string quote unquote what's your name just like

quote unquote what's your name just like in scratch and then down here instead of

in scratch and then down here instead of doing world I initially wrote answer but

doing world I initially wrote answer but that didn't go well what did I

that didn't go well what did I ultimately do instead to print out hello

ultimately do instead to print out hello David or hello so and so yeah sorry a

David or hello so and so yeah sorry a little louder yeah so percent s the

little louder yeah so percent s the so-called format code that printf just

so-called format code that printf just knows how to deal with and I had to add

knows how to deal with and I had to add one other thing someone else besides

one other thing someone else besides percent F

percent F yeah the name of the variable that I

yeah the name of the variable that I want to plug into that placeholder

want to plug into that placeholder percent s and in this case it's answer

percent s and in this case it's answer now let me make one refinement only

now let me make one refinement only because now we're in week two and we're

because now we're in week two and we're going to start writing more lines of

going to start writing more lines of code even though scratch called the

code even though scratch called the return value of the ask puzzle piece

return value of the ask puzzle piece answer always and see we have full

answer always and see we have full control over what our variables are

control over what our variables are called and now it's probably good not to

called and now it's probably good not to just generically always call my variable

just generically always call my variable answer if I'm using get string let's

answer if I'm using get string let's call it what it is so this is now just a

call it what it is so this is now just a matter of style if you will let me

matter of style if you will let me change the variable to be name just so

change the variable to be name just so that it's a little clear to me to you to

that it's a little clear to me to you to a TF or ta exactly what that variable

a TF or ta exactly what that variable represents instead of more generically

represents instead of more generically answer all right so that said let me go

answer all right so that said let me go down to my terminal window and last week

down to my terminal window and last week again I ran make to compile this exact

again I ran make to compile this exact same program now though let me go ahead

same program now though let me go ahead and just use clang so clang d o I'll

and just use clang so clang d o I'll still call this version hello space

still call this version hello space hello.c so exact same command as before

hello.c so exact same command as before the only thing that's different is I've

the only thing that's different is I've added a couple of more lines of code to

added a couple of more lines of code to get the user's input let me hit enter

get the user's input let me hit enter and now darn it our first error so

and now darn it our first error so output from clang and make is not a good

output from clang and make is not a good thing and here we're seeing something

thing and here we're seeing something particularly cryptic uh so something in

particularly cryptic uh so something in function main undefined reference to get

function main undefined reference to get string and then Linker command failed

string and then Linker command failed with exit code one so there's actually a

with exit code one so there's actually a lot of jargon in there that will tease

lot of jargon in there that will tease apart today but my hint is that clearly

apart today but my hint is that clearly my problem's in Maine although that's

my problem's in Maine although that's not surprising because there's nothing

not surprising because there's nothing else going on here get string is an

else going on here get string is an issue and the uh issue is that it's an

issue and the uh issue is that it's an undefined reference and yet notice I was

undefined reference and yet notice I was pretty good I added the cs50 header file

pretty good I added the cs50 header file and I said last week that that's enough

and I said last week that that's enough to teach the compiler that functions

to teach the compiler that functions exist but the problem is that even

exist but the problem is that even though this does in fact teach clang

though this does in fact teach clang that get string exists it is not

that get string exists it is not sufficient information for clang to go

sufficient information for clang to go find on the hard drive of the computer

find on the hard drive of the computer the zeros and ones that actually

the zeros and ones that actually Implement get string itself so in other

Implement get string itself so in other words this include line per last week is

words this include line per last week is a little bit of a hint it's a teaser to

a little bit of a hint it's a teaser to CLA that you're about to see and use

CLA that you're about to see and use this function somewhere but if you

this function somewhere but if you actually want to use the zeros and ones

actually want to use the zeros and ones that cs50 wrote some time ago and bake

that cs50 wrote some time ago and bake those into your program so your program

those into your program so your program actually knows how to get input from the

actually knows how to get input from the user well then I'm going to have to go

user well then I'm going to have to go ahead and run a slightly different

ahead and run a slightly different command so let me do this let me clear

command so let me do this let me clear my terminal window just to get rid of

my terminal window just to get rid of that distraction and let me propose now

that distraction and let me propose now that we run this command instead almost

that we run this command instead almost the same as before clang - o space hello

the same as before clang - o space hello then hello. C but with one additional

then hello. C but with one additional command line argument at the end and

command line argument at the end and this is a-h L not a number one so- L

this is a-h L not a number one so- L cs50 with no space in between those two

cs50 with no space in between those two now the L is going to result in all of

now the L is going to result in all of those zeros and ones that actually were

those zeros and ones that actually were written by cs50 being linked into your

written by cs50 being linked into your code your few lines of code or mine here

code your few lines of code or mine here but that's the second step that the

but that's the second step that the compiler requires in order to know how

compiler requires in order to know how to actually execute and rather compile

to actually execute and rather compile your code and cs50's and cs50 is not the

your code and cs50's and cs50 is not the only one that does this if you use any

only one that does this if you use any third-party library in C that doesn't

third-party library in C that doesn't come with the language you would do- L

come with the language you would do- L such and such where whoever however

such and such where whoever however they've named their own library but you

they've named their own library but you don't have to do it for built-in things

don't have to do it for built-in things like uh like we've been using thus far

like uh like we've been using thus far all right so let me go ahead and try

all right so let me go ahead and try this I'll go back to vs code here and

this I'll go back to vs code here and let me go ahead now and run clang - o

let me go ahead now and run clang - o hello then hello. C and now instead of

hello then hello. C and now instead of just hitting enter - L cs50 with no

just hitting enter - L cs50 with no space between the L and the cs50 enter

space between the L and the cs50 enter now nothing bad happens and now I can

now nothing bad happens and now I can do/ hello what's your name I'll type in

do/ hello what's your name I'll type in David enter and now we see hello David

David enter and now we see hello David now honestly this is where we're really

now honestly this is where we're really getting into the weeds and now this is

getting into the weeds and now this is taking this is really just adding new to

taking this is really just adding new to the process of compiling and running

the process of compiling and running your code and so the reality is even

your code and so the reality is even though this is indeed what is happening

though this is indeed what is happening this is why we used last week and we're

this is why we used last week and we're going to continue using this week onward

going to continue using this week onward make because it just automates that

make because it just automates that whole process for you but it's ideal to

whole process for you but it's ideal to understand what's going wrong because

understand what's going wrong because any of the error messages you saw for

any of the error messages you saw for problem set one any of the error

problem set one any of the error messages you see for the next few weeks

messages you see for the next few weeks probably aren't coming from make they're

probably aren't coming from make they're coming from clang underneath the hood

coming from clang underneath the hood because make is just automating the

because make is just automating the process but with make you literally just

process but with make you literally just write make and then the name of the

write make and then the name of the program you don't have to worry about

program you don't have to worry about any of those command line arguments

any of those command line arguments questions then on compiling with- l cs50

questions then on compiling with- l cs50 or anything else

or anything else yeah sorry what is the benefit

yeah sorry what is the benefit of what is the benefit of using clang

of what is the benefit of using clang manually none really in fact all main is

manually none really in fact all main is doing is just sa make is doing is saving

doing is just sa make is doing is saving us some keystrokes um if you prefer

us some keystrokes um if you prefer though and you just like to be more in

though and you just like to be more in control you can totally run clang

control you can totally run clang manually if you remember the various

manually if you remember the various command line arguments yeah

exp exactly why did I have to explain that is provide a hint to cs50 with the

that is provide a hint to cs50 with the cs50.h henter file but I didn't have to

cs50.h henter file but I didn't have to do that with standard i.h just because

do that with standard i.h just because standard i.h comes with C just like a

standard i.h comes with C just like a few other libraries come with C that

few other libraries come with C that we'll start seeing today um cs50 though

we'll start seeing today um cs50 though is not built into C everywhere and so

is not built into C everywhere and so you do have to explicitly add that one

you do have to explicitly add that one there

yeah a command line argument is a a word or phrase that you type at the command

or phrase that you type at the command line AKA your terminal in order to

line AKA your terminal in order to influence the behavior of a

influence the behavior of a program for whatever you're doing yeah

program for whatever you're doing yeah it changes the defaults right in our

it changes the defaults right in our guey World graphical user interface you

guey World graphical user interface you and I would probably click some boxes we

and I would probably click some boxes we would select some menu options to

would select some menu options to configure a program to behave in the

configure a program to behave in the same way at a command line interface you

same way at a command line interface you have to just say everything all at once

have to just say everything all at once and that's why we have command line

and that's why we have command line arguments

yeah no make is not just for cs50 it's used globally in any project really

used globally in any project really nowadays using C C++ even other

nowadays using C C++ even other languages as well in fact most every

languages as well in fact most every command you see in this class unless it

command you see in this class unless it has 5 zero at the end of it is globally

has 5 zero at the end of it is globally used only those suffix with 50 are

used only those suffix with 50 are indeed course specific and even those

indeed course specific and even those will gradually take training wheels off

will gradually take training wheels off of so that you know exactly what those

of so that you know exactly what those commands are doing as well all right so

commands are doing as well all right so what is it that we've just done

what is it that we've just done everything we've just done of course I

everything we've just done of course I keep calling compiling but let's just go

keep calling compiling but let's just go down one Rabbit Hole so that you

down one Rabbit Hole so that you understand that when you compile code

understand that when you compile code there's actually a whole bunch of steps

there's actually a whole bunch of steps happening and this is going to enable uh

happening and this is going to enable uh a lot of features like companies can

a lot of features like companies can write code and then convert it to run it

write code and then convert it to run it on Macs and PCs alike or phones or the

on Macs and PCs alike or phones or the like so it's not just a matter of

like so it's not just a matter of converting source code to machine code

converting source code to machine code there's actually four steps involved in

there's actually four steps involved in what you and I as of last week know as

what you and I as of last week know as compiling and these aren't terms that

compiling and these aren't terms that you'll have to keep in mind constantly

you'll have to keep in mind constantly because again we're going to abstract a

because again we're going to abstract a lot of this away but just so we've gone

lot of this away but just so we've gone down the rabbit hole once let's consider

down the rabbit hole once let's consider each of these four steps that have been

each of these four steps that have been happening for you for a week

happening for you for a week automatically uh the first of which is

automatically uh the first of which is called pre-processing so what is this

called pre-processing so what is this mean well let's consider that same

mean well let's consider that same program as before notice that a two of

program as before notice that a two of the lines of code start with a hash mark

the lines of code start with a hash mark that is a special symbol in C and it's a

that is a special symbol in C and it's a so-called pre-processor directive you

so-called pre-processor directive you don't need to memorize terms like that

don't need to memorize terms like that but it just means that it's a little

but it just means that it's a little different from every other line and

different from every other line and anything with a hash symbol here should

anything with a hash symbol here should be pre-processed that is analyzed

be pre-processed that is analyzed initially before anything else happens

initially before anything else happens so let's consider these two lines up top

so let's consider these two lines up top what exactly is happening well it turns

what exactly is happening well it turns out with these two lines you have two

out with these two lines you have two header files of course cs50.h and

header files of course cs50.h and standard. i.h where are those files

standard. i.h where are those files because they've never been in VSS code

because they've never been in VSS code for you seemingly if you type LS if you

for you seemingly if you type LS if you open up the file explorer in the GUI you

open up the file explorer in the GUI you have never seen probably cs50.h or

have never seen probably cs50.h or standard i.h they just work but that's

standard i.h they just work but that's because there's a folder somewhere on

because there's a folder somewhere on the uh the hard drive that you're using

the uh the hard drive that you're using on your Mac or PC or somewhere in the

on your Mac or PC or somewhere in the cloud as in our case and inside of this

cloud as in our case and inside of this folder traditionally called sluser SL

folder traditionally called sluser SL include and user is deliberately

include and user is deliberately misspelled it's just slightly more

misspelled it's just slightly more succinct although it's a little weird

succinct although it's a little weird why we drop that one letter but user SL

why we drop that one letter but user SL include is just a folder on the server

include is just a folder on the server that contains cs50.h standard i.h and a

that contains cs50.h standard i.h and a bunch of other things as well so in fact

bunch of other things as well so in fact if you type in uh VSS code in your

if you type in uh VSS code in your terminal window uh when you're using

terminal window uh when you're using Code spaces in the cloud and type LS

Code spaces in the cloud and type LS space SL user include you can can see

space SL user include you can can see all of the files in that folder but

all of the files in that folder but we've pre-installed all of that stuff

we've pre-installed all of that stuff for you so let's consider what's

for you so let's consider what's actually in those files here where if I

actually in those files here where if I highlight these two lines up top that

highlight these two lines up top that start with hash include well I kind of

start with hash include well I kind of hinted last week that what's in that

hinted last week that what's in that first file is a hint as to what

first file is a hint as to what functions cs50 wrote for you so you can

functions cs50 wrote for you so you can kind of think of these include lines as

kind of think of these include lines as being temporary placeholders for what's

being temporary placeholders for what's going to become like a global find and

going to become like a global find and replace that is the first thing clang is

replace that is the first thing clang is going to do it's pre-process this file

going to do it's pre-process this file it's going to look for any line that

it's going to look for any line that starts with hash include and if it sees

starts with hash include and if it sees that it's going to essentially go into

that it's going to essentially go into that file like cs50.h and then just copy

that file like cs50.h and then just copy and paste the contents of that file

and paste the contents of that file magically there for you you don't see it

magically there for you you don't see it visually on the screen but it's

visually on the screen but it's happening behind the scenes and so

happening behind the scenes and so really what's happening with this first

really what's happening with this first line is that somewhere in

line is that somewhere in cs50.h is the Declaration of get string

cs50.h is the Declaration of get string like we talked last week and it probably

like we talked last week and it probably looks a little something like this and

looks a little something like this and we didn't spend much time on this yet

we didn't spend much time on this yet this past week but we will in time more

this past week but we will in time more notice that this is how the a function

notice that this is how the a function is declared that is it is decreed to

is declared that is it is decreed to exist the name of the function of course

exist the name of the function of course is get string inside of the parenthesis

is get string inside of the parenthesis are its arguments in this case there's

are its arguments in this case there's one argument to get string I claim today

one argument to get string I claim today but you've known this implicitly and

but you've known this implicitly and it's a prompt it's the prompt that the

it's a prompt it's the prompt that the human sees when you use get string what

human sees when you use get string what is that prompt well it's a string of

is that prompt well it's a string of text like quote unquote what's your name

text like quote unquote what's your name or anything else that I asked last week

or anything else that I asked last week mean meanwhile get string as we know

mean meanwhile get string as we know from last week has a return value it

from last week has a return value it returns something to you and that too is

returns something to you and that too is a string so again this is also called a

a string so again this is also called a functions prototype it's the thing

functions prototype it's the thing toward the end of last week that I just

toward the end of last week that I just copied and pasted from the bottom of my

copied and pasted from the bottom of my file to the top just so that it was like

file to the top just so that it was like this teaser for clang as to what would

this teaser for clang as to what would exist later so you can think then of

exist later so you can think then of these include lines as just kind of uh

these include lines as just kind of uh combining all of those function

combining all of those function declarations in some separate file

declarations in some separate file called cs50.h so that you yourself don't

called cs50.h so that you yourself don't have to type them every time you use the

have to type them every time you use the library or worse so that you yourself

library or worse so that you yourself don't have to copy and paste those lines

don't have to copy and paste those lines this is what clang is doing for you in

this is what clang is doing for you in its first step of pre-processing second

its first step of pre-processing second and last in this example what happens

and last in this example what happens when clang pre-processes this second

when clang pre-processes this second include line well the only other

include line well the only other function we care about in this story is

function we care about in this story is printf of course which comes with C so

printf of course which comes with C so essentially you can think of printf's

essentially you can think of printf's prototype or Declaration as just being

prototype or Declaration as just being this print f is the name of the function

this print f is the name of the function it takes a string that you want to

it takes a string that you want to format like hello comma world or hello

format like hello comma world or hello comma percent s and then with dot dot

comma percent s and then with dot dot dot this actually has technical meaning

dot this actually has technical meaning it means of course that you can plug in

it means of course that you can plug in zero variables one variable two or 10 so

zero variables one variable two or 10 so dot dot dot means some number of

dot dot dot means some number of variables now we haven't talked about

variables now we haven't talked about this yet and we won't really in general

this yet and we won't really in general print F actually returns a value a

print F actually returns a value a number that is an integer but more on

number that is an integer but more on that perhaps another time it's generally

that perhaps another time it's generally not something the programmer tends to

not something the programmer tends to look at but that's all we mean by

look at but that's all we mean by pre-processing so that the the end of

pre-processing so that the the end of this process even though there's more

this process even though there's more lines of code in cs50.h and standard i.h

lines of code in cs50.h and standard i.h what's really just happening is that

what's really just happening is that clang in pre-processing the file copies

clang in pre-processing the file copies and pastes the contents of those files

and pastes the contents of those files into your code so that now your code

into your code so that now your code knows about everything get string printf

knows about everything get string printf and anything else any questions then on

and anything else any questions then on that first step

that first step pre-processing

good question when you include a file does it only include what you need or

does it only include what you need or Does it include everything think of it

Does it include everything think of it as including everything so if it's a big

as including everything so if it's a big file that's a lot of code at the very

file that's a lot of code at the very top and that's why if you think back to

top and that's why if you think back to all of the zeros and ones I showed a

all of the zeros and ones I showed a little bit ago as well as last week

little bit ago as well as last week there's a lot of zeros and ones that end

there's a lot of zeros and ones that end up on the screen as a result of just

up on the screen as a result of just writing hello world a lot of those zeros

writing hello world a lot of those zeros and ones are perhaps coming from code

and ones are perhaps coming from code that you didn't actually necessarily

that you didn't actually necessarily need but some of it is perhaps there but

need but some of it is perhaps there but there are ways to optimize that as well

there are ways to optimize that as well all right so step two of compiling is

all right so step two of compiling is confusingly called compiling it's just

confusingly called compiling it's just this is the term that most everyone uses

this is the term that most everyone uses to describe the whole process instead of

to describe the whole process instead of just this one step but once a program

just this one step but once a program has been pre-processed uh behind the

has been pre-processed uh behind the scenes by the compiler for you it looks

scenes by the compiler for you it looks now a little something like this and

now a little something like this and I've put dot dot dots just to imply that

I've put dot dot dots just to imply that yes to your question there's more stuff

yes to your question there's more stuff above it there's more stuff below it

above it there's more stuff below it it's just not interesting right now for

it's just not interesting right now for us so now we have just C code there's no

us so now we have just C code there's no more pre-processor directives at this

more pre-processor directives at this point all of the hash symbols and those

point all of the hash symbols and those lines of code have been pre-processed

lines of code have been pre-processed and convert it to something else and so

and convert it to something else and so now and this is where things get a

now and this is where things get a little spooky looking uh here now is

little spooky looking uh here now is what happens when clang or any compiler

what happens when clang or any compiler literally compiles code like this it

literally compiles code like this it converts it from this in C to this in

converts it from this in C to this in assembly code so this is among the

assembly code so this is among the scarier languages I myself don't really

scarier languages I myself don't really have fond memories this is not language

have fond memories this is not language that many people program in if you take

that many people program in if you take a subsequent class in computer science

a subsequent class in computer science in systems uh a higher level class you

in systems uh a higher level class you might actually learn this or some

might actually learn this or some variant there of but there's at least a

variant there of but there's at least a few people out there that need to know

few people out there that need to know this stuff because this is closer to

this stuff because this is closer to what the computers themselves nowadays

what the computers themselves nowadays understand like the Intel CPUs or the

understand like the Intel CPUs or the AMD CPUs the brains of today's computers

AMD CPUs the brains of today's computers and phones understand stuff that looks

and phones understand stuff that looks more like this and less like C now it's

more like this and less like C now it's completely uh esoteric but let me just

completely uh esoteric but let me just highlight a few phrases there's some

highlight a few phrases there's some stuff that's a little familiar there is

stuff that's a little familiar there is mention of Maine at the top there in

mention of Maine at the top there in yellow there is mention of get string

yellow there is mention of get string toward the bottom there is mention of

toward the bottom there is mention of prf down below so this is just another

prf down below so this is just another programming language called Assembly

programming language called Assembly Language that decades ago humans myself

Language that decades ago humans myself included in school did write code in and

included in school did write code in and absolutely some people still write this

absolutely some people still write this code especially since you can write very

code especially since you can write very very efficient code but it's a lot more

very efficient code but it's a lot more uh Arcane it's a lot more uh lot less

uh Arcane it's a lot more uh lot less user friendly so you'll see in Yello now

user friendly so you'll see in Yello now the these are the so-called instructions

the these are the so-called instructions that a computer's brain or CPU

that a computer's brain or CPU understands pushing values around moving

understands pushing values around moving them subtracting values uh calling

them subtracting values uh calling functions and move move move so really

functions and move move move so really the low-level operations that computers

the low-level operations that computers understand tend to be arithmetic

understand tend to be arithmetic operations subtraction addition and the

operations subtraction addition and the like moving things in and out of memory

like moving things in and out of memory it's just a lot more tedious for folks

it's just a lot more tedious for folks like us to write code like this this is

like us to write code like this this is why you and I tend to write stuff like

why you and I tend to write stuff like this and ideally still people like you

this and ideally still people like you and I tend to drag and drop puzzle

and I tend to drag and drop puzzle pieces that sort of abstract all of that

pieces that sort of abstract all of that away further but for now this is again

away further but for now this is again called Assembly Language it is what

called Assembly Language it is what happens when the compiler literally

happens when the compiler literally compiles your code but of course this

compiles your code but of course this still not zeros and ones so we got two

still not zeros and ones so we got two steps to go so when a compiler proceeds

steps to go so when a compiler proceeds to step three this is where things get

to step three this is where things get converted to machine code and when a

converted to machine code and when a compiler assembles your code for you it

compiler assembles your code for you it converts what we just saw on the screen

converts what we just saw on the screen here to actual zeros and ones the

here to actual zeros and ones the so-called machine code that your phone

so-called machine code that your phone or your computer understands but it's

or your computer understands but it's worth noting that these are not

worth noting that these are not necessarily all of the zeros and ones of

necessarily all of the zeros and ones of your program yes they re uh they are the

your program yes they re uh they are the zeros and ones that correspond to your

zeros and ones that correspond to your hello program or printf and get string

hello program or printf and get string and the like but notice that here we

and the like but notice that here we need one final step in those zeros and

need one final step in those zeros and ones are only your lines of code but

ones are only your lines of code but what about cs50's lines of code that we

what about cs50's lines of code that we wrote to Implement get string what about

wrote to Implement get string what about the lines of code that humans wrote

the lines of code that humans wrote decades ago to implement printf those

decades ago to implement printf those are somewhere on this hard drive like on

are somewhere on this hard drive like on my Mac my PC or somewhere in the cloud

my Mac my PC or somewhere in the cloud but we need to combine all of those

but we need to combine all of those zeros and ones together and Link My code

zeros and ones together and Link My code with cs50's code with uh standard io's

with cs50's code with uh standard io's code all together and so what happens in

code all together and so what happens in the last step ultimately is that if we

the last step ultimately is that if we have my code here in yellow and then the

have my code here in yellow and then the code that cs50 wrote and the code that

code that cs50 wrote and the code that the authors of C itself wrote what

the authors of C itself wrote what really is happening is that somewhere we

really is happening is that somewhere we have not only hello.c which obviously I

have not only hello.c which obviously I wrote and wrote with us live here

wrote and wrote with us live here there's also let's assume somewhere on

there's also let's assume somewhere on the computer a cs50.c file that

the computer a cs50.c file that coincidentally I and cs50 staff wrote

coincidentally I and cs50 staff wrote years ago and also somewhere on the

years ago and also somewhere on the computer there's another file Let Me

computer there's another file Let Me oversimplify by just calling it standard

oversimplify by just calling it standard io. C in practice it's probably

io. C in practice it's probably specifically called print F.C but

specifically called print F.C but there's somewhere these two other files

there's somewhere these two other files and so this last step called linking

and so this last step called linking takes my zeros and ones from the code I

takes my zeros and ones from the code I just wrote namely this code on the

just wrote namely this code on the screen here it then grabs the zeros and

screen here it then grabs the zeros and ones that cs50 wrote and it grabs the

ones that cs50 wrote and it grabs the zeros and ones that the authors of C

zeros and ones that the authors of C wrote In order to implement the standard

wrote In order to implement the standard IO library and lastly voila links them

IO library and lastly voila links them all together and this is the same blob

all together and this is the same blob of zeros and ones that we saw earlier

of zeros and ones that we saw earlier it's just now the result of

it's just now the result of pre-processing your code compiling your

pre-processing your code compiling your code assembling your code linking your

code assembling your code linking your code and my God it's at this point like

code and my God it's at this point like if there were any fun in programming for

if there were any fun in programming for you yet we've just taken it all away we

you yet we've just taken it all away we just call this whole process compiling

just call this whole process compiling why because now that we know those steps

why because now that we know those steps exist and smart people solve that

exist and smart people solve that problem for us you and I can kind of

problem for us you and I can kind of operate at this level of abstraction and

operate at this level of abstraction and just assume that compiling converts

just assume that compiling converts source code to machine code questions

source code to machine code questions though on any of these

though on any of these intermediate steps

a good question so where are all of these zeros in one store because you and

these zeros in one store because you and I we've been using a browser at code.

I we've been using a browser at code. cs50. of course is this web-based user

cs50. of course is this web-based user interface but again recall from last

interface but again recall from last week even though you're using a web

week even though you're using a web browser to access VSS code that

browser to access VSS code that web-based version of vs code is

web-based version of vs code is connected to an actual server somewhere

connected to an actual server somewhere in the cloud and on that server you have

in the cloud and on that server you have your own account and your own file and

your own account and your own file and really your own hard drive virtually in

really your own hard drive virtually in the cloud think of it a little like

the cloud think of it a little like Dropbox or box or Google drive or one

Dropbox or box or Google drive or one drive or something like that so you have

drive or something like that so you have a hard drive somewhere out there that

a hard drive somewhere out there that we've provisioned for you and it's on

we've provisioned for you and it's on that hard drive that we have uh your

that hard drive that we have uh your code that you just wrote or I just wrote

code that you just wrote or I just wrote cs50.c standard I.C and all of the other

cs50.c standard I.C and all of the other code that implements the math functions

code that implements the math functions and everything else that c

and everything else that c supports good question

supports good question yeah

yeah c

good question that uh hash includes cs50.h line at the top of my code if I

cs50.h line at the top of my code if I just replace that with the contents of

just replace that with the contents of cs50.c would that work short answer yes

cs50.c would that work short answer yes that would work you could copy all of

that would work you could copy all of the code there however there's some

the code there however there's some order of operations that might come into

order of operations that might come into play and so it's probably not quite as

play and so it's probably not quite as simple as copy paste but conceptually

simple as copy paste but conceptually yes that's what what's happening now

yes that's what what's happening now with that said in

with that said in cs50.h are only the prototypes of the

cs50.h are only the prototypes of the functions the hints as to how the

functions the hints as to how the functions look what their return type is

functions look what their return type is what their name is and what their

what their name is and what their arguments are it's in the C file that

arguments are it's in the C file that actual code tends to be written and this

actual code tends to be written and this is a little confusing now because you

is a little confusing now because you and I have only written code in C files

and I have only written code in C files but in the next few weeks you'll

but in the next few weeks you'll actually start writing some of your own

actually start writing some of your own files as well just like cs50 just like

files as well just like cs50 just like standard iio but in essence that line of

standard iio but in essence that line of code just makes it easier to use and

code just makes it easier to use and reuse code that's already been written

reuse code that's already been written and that's the whole point of a

louder yes does linking happen when you compile your code yes when you run make

compile your code yes when you run make as we have been doing the past week now

as we have been doing the past week now all four of these steps are happening

all four of these steps are happening pre-processing converts the hash include

pre-processing converts the hash include lines to something else compiling

lines to something else compiling technically converts it to assembly code

technically converts it to assembly code which the Mac the PC the server more

which the Mac the PC the server more closely understands assembly converts

closely understands assembly converts that language to Binary machine code

that language to Binary machine code that this computer actually understands

that this computer actually understands and then linking combines everything

and then linking combines everything together and in fact if you think back a

together and in fact if you think back a few minutes ago to when I did this- L

few minutes ago to when I did this- L cs50 the reason I had to add that and

cs50 the reason I had to add that and the reason my code did not compile at

the reason my code did not compile at first was because I forgot to tell clang

first was because I forgot to tell clang to link in cs50's zeros and ones per

to link in cs50's zeros and ones per that last step I don't need to do- L

that last step I don't need to do- L standard IO because it comes with c so

standard IO because it comes with c so that would just be tedious for everyone

that would just be tedious for everyone in the world but cs50 does not come with

in the world but cs50 does not come with C so we link that in and to be clear too

C so we link that in and to be clear too we won't always use cs50's Library

we won't always use cs50's Library that'll be yet another pair of training

that'll be yet another pair of training wheels we take off in the coming weeks

wheels we take off in the coming weeks but for now it makes a few things

but for now it makes a few things simpler

yeah short answer yes so what do the zeros and ones the machine code

zeros and ones the machine code translate to yes there is a one toone

translate to yes there is a one toone relationship between the machine code

relationship between the machine code and the assembly code assembly code it's

and the assembly code assembly code it's not really English but at least it's

not really English but at least it's symbols I recognize it's not zeros and

symbols I recognize it's not zeros and ones machine code of course is just

ones machine code of course is just zeros and ones so back in the day before

zeros and ones so back in the day before c existed people were programming only

c existed people were programming only in assembly code before assembly code

in assembly code before assembly code existed people were coding in zeros and

existed people were coding in zeros and ones and you can imagine just how

ones and you can imagine just how painful that was and so each of these

painful that was and so each of these languages makes life for us sort of

languages makes life for us sort of easier and easier in a few weeks we'll

easier and easier in a few weeks we'll transition to python which will in turn

transition to python which will in turn make C even uh simpler or coding in

make C even uh simpler or coding in general simpler to do to all right so

general simpler to do to all right so with that said what now can we uh what

with that said what now can we uh what could go wrong with this well it turns

could go wrong with this well it turns out that besides compiling technically

out that besides compiling technically speaking there's decompiling and we've

speaking there's decompiling and we've not done this and we won't do this but

not done this and we won't do this but it's worth considering for just a moment

it's worth considering for just a moment uh if you were to not compile your code

uh if you were to not compile your code but decompile it as the word suggests

but decompile it as the word suggests this just means reversing the process

this just means reversing the process converting it ideally from machine code

converting it ideally from machine code zeros and ones maybe back to C now this

zeros and ones maybe back to C now this would be cool perhaps if all you have is

would be cool perhaps if all you have is a program you can convert it and see the

a program you can convert it and see the actual source code what might a downside

actual source code what might a downside be if if anyone on the Internet is able

be if if anyone on the Internet is able to decompile code on their machine

to decompile code on their machine yeah okay so it's easier to find bugs in

yeah okay so it's easier to find bugs in the code that oh to exploit so it might

the code that oh to exploit so it might be easier to uh hack into the software

be easier to uh hack into the software by finding mistakes you and I made

by finding mistakes you and I made because literally they're staring at you

because literally they're staring at you in code worries the zeros and ones make

in code worries the zeros and ones make it way less obvious other downsides of

it way less obvious other downsides of what I call decompiling

yeah yeah yeah if your code your work is your

yeah yeah if your code your work is your intellectual property copyrighted or

intellectual property copyrighted or otherwise you know that's kind of

otherwise you know that's kind of obnoxious that someone can just like run

obnoxious that someone can just like run a command and boom they can see the

a command and boom they can see the original code that you wrote now it

original code that you wrote now it turns out it's not quite as simple as

turns out it's not quite as simple as that and so even though yes you could

that and so even though yes you could take a program like hello or even

take a program like hello or even Microsoft Word and convert it from zeros

Microsoft Word and convert it from zeros and ones back to some form of source

and ones back to some form of source code be it in C or Java or python or

code be it in C or Java or python or something else whatever it was

something else whatever it was originally written in odds are it's

originally written in odds are it's going to be an utter mess to look look

going to be an utter mess to look look at why because things like variable

at why because things like variable names are not retained in the zeros and

names are not retained in the zeros and ones typically function names might not

ones typically function names might not be retained in the zeros and ones the

be retained in the zeros and ones the code is the logic is but the computer

code is the logic is but the computer doesn't care what pretty variables you

doesn't care what pretty variables you chose and how nicely named your uh

chose and how nicely named your uh functions were it just needs to know

functions were it just needs to know them as Z in ones moreover if you think

them as Z in ones moreover if you think about last week we introduced things

about last week we introduced things like loops and c and besides four Loops

like loops and c and besides four Loops there's what other kind of loop for

there's what other kind of loop for instance so a while loop and even though

instance so a while loop and even though they look different and you have to

they look different and you have to write different code they achieve

write different code they achieve exactly the same functionality which is

exactly the same functionality which is to say when you compile a for Loop or

to say when you compile a for Loop or you compile a while loop if they

you compile a while loop if they logically do the same thing they might

logically do the same thing they might end up looking identical as zeros and

end up looking identical as zeros and ones and so therefore it's not

ones and so therefore it's not necessarily predictable that you'll get

necessarily predictable that you'll get back the original code why cuz because

back the original code why cuz because the zeros and ones might not know so to

the zeros and ones might not know so to speak whether it was a for Loop or a

speak whether it was a for Loop or a while loop so maybe decompiling will

while loop so maybe decompiling will show you one or the other and honestly

show you one or the other and honestly decompiling while possible and it's one

decompiling while possible and it's one way of reverse engineering someone's

way of reverse engineering someone's product odds are if you're good enough

product odds are if you're good enough to start reading code that's been

to start reading code that's been decompiled and reading through the

decompiled and reading through the messiness of it odds are you have the

messiness of it odds are you have the talent probably to just write that same

talent probably to just write that same program from scratch yourself now that's

program from scratch yourself now that's an overstatement perhaps but it's not

an overstatement perhaps but it's not quite as easy or threatening um as you

quite as easy or threatening um as you might first think so in general once

might first think so in general once codee is piled it's pretty challenging

codee is piled it's pretty challenging timec consuming costly to reverse

timec consuming costly to reverse engineer it um much like it would be in

engineer it um much like it would be in the real world right like all of us have

the real world right like all of us have some kind of phone probably nowadays in

some kind of phone probably nowadays in our pocket there's nothing stopping you

our pocket there's nothing stopping you from opening it up somehow poking around

from opening it up somehow poking around recreating what's there that's a huge

recreating what's there that's a huge amount of effort most likely and at that

amount of effort most likely and at that point maybe you should just invent the

point maybe you should just invent the phone instead of trying to reverse

phone instead of trying to reverse engineer it so same kind of idea in the

engineer it so same kind of idea in the physical world any questions then on

physical world any questions then on compiling or even decompiling in these

compiling or even decompiling in these forms

forms all right so odds are at this point not

all right so odds are at this point not only I but you have made mistakes and

only I but you have made mistakes and you've written buggy code a bug in a

you've written buggy code a bug in a code is just a mistake a logical error

code is just a mistake a logical error or otherwise where the code just does

or otherwise where the code just does not behave correctly as you intend and

not behave correctly as you intend and up until now odds are your debugging

up until now odds are your debugging techniques have been to maybe look back

techniques have been to maybe look back at what I did in class maybe ask a

at what I did in class maybe ask a question online or in person but

question online or in person but ultimately it'd be nice if you had some

ultimately it'd be nice if you had some tools of your own with which to debug

tools of your own with which to debug code and this honestly is a lifelong

code and this honestly is a lifelong skill you're going to emerge from cs50

skill you're going to emerge from cs50 and even 20 years from now you're not

and even 20 years from now you're not going to be writing if you're writing

going to be writing if you're writing code at all correct code all of the time

code at all correct code all of the time like all of us on the staff continue to

like all of us on the staff continue to write bugs hopefully they get a little

write bugs hopefully they get a little more sophisticated and not sort of like

more sophisticated and not sort of like oops I missed a semicolon but even those

oops I missed a semicolon but even those kinds of mistakes we make too but

kinds of mistakes we make too but there's tools out there and techniques

there's tools out there and techniques that can make your life easier when it

that can make your life easier when it comes to solving those problems now the

comes to solving those problems now the term bug has actually been around for

term bug has actually been around for decades but a fun story to tell is that

decades but a fun story to tell is that the first documented actual bug was

the first documented actual bug was actually somehow connected to Harvard in

actually somehow connected to Harvard in fact this is the log book relating to

fact this is the log book relating to the Harvard Mark 2 computer from 1947

the Harvard Mark 2 computer from 1947 whereby if you read the notes here and

whereby if you read the notes here and if I zoom in this was an actual moth

if I zoom in this was an actual moth discovered inside of this big Mainframe

discovered inside of this big Mainframe computer that was causing some kind of

computer that was causing some kind of problems and the engineers at the time

problems and the engineers at the time actually thought it was funny that wow

actually thought it was funny that wow physical bug actually explains the issue

physical bug actually explains the issue and it's been forever uh taped to the

and it's been forever uh taped to the sheet of paper which I believe now is on

sheet of paper which I believe now is on display in the Smithsonian uh with that

display in the Smithsonian uh with that said this is just represented two of a

said this is just represented two of a logical bug and that story is actually

logical bug and that story is actually uh that story was often retold by a

uh that story was often retold by a famous mathematician then computer

famous mathematician then computer scientist really uh Dr Grace Hopper who

scientist really uh Dr Grace Hopper who actually worked not only on the Harvard

actually worked not only on the Harvard Mark 2 computer but its predecessor the

Mark 2 computer but its predecessor the Harvard Mark 1 and if you ever spent

Harvard Mark 1 and if you ever spent time yet in the engineering building

time yet in the engineering building across the river here you can actually

across the river here you can actually see much of this computer which is along

see much of this computer which is along the wall when you first walk into the

the wall when you first walk into the science and engineering complex and

science and engineering complex and indeed as you've probably heard growing

indeed as you've probably heard growing up this is a Mainframe computer like

up this is a Mainframe computer like this is what Macs and PCs so to speak

this is what Macs and PCs so to speak looked like back in the day with very

looked like back in the day with very physical things that essentially

physical things that essentially implemented the zeros and ones that you

implemented the zeros and ones that you and I take for granted now being

and I take for granted now being miniaturized in our laptops and phones

miniaturized in our laptops and phones so there's a piece of history there if

so there's a piece of history there if you visit campus that side of Campus

you visit campus that side of Campus sometime do take a look but let's

sometime do take a look but let's consider then how we solve not of course

consider then how we solve not of course physical bugs but logical bugs and let's

physical bugs but logical bugs and let's consider something like this from last

consider something like this from last week whereby we were trying very simply

week whereby we were trying very simply to print like this uh column of three

to print like this uh column of three bricks using hashtags of sorts so let me

bricks using hashtags of sorts so let me go over here in just a moment to VSS

go over here in just a moment to VSS code and I'm going to go ahead and open

code and I'm going to go ahead and open a program I wrote in advance and I'm

a program I wrote in advance and I'm bringing it to class because there's a

bringing it to class because there's a bug in it and I'd like to figure out how

bug in it and I'd like to figure out how to solve this bug so let me open up uh

to solve this bug so let me open up uh buggy z.c which is version zero of my

buggy z.c which is version zero of my code and let's just take a quick peek at

code and let's just take a quick peek at what's here it's pretty short it

what's here it's pretty short it includes only standard i.h it uses

includes only standard i.h it uses printf it uses a for Loop and the goal

printf it uses a for Loop and the goal quite simply is to print out that column

quite simply is to print out that column of three bricks now it's short enough

of three bricks now it's short enough that some of you if you're getting comfy

that some of you if you're getting comfy already with see you might already see

already with see you might already see The Logical bug it's not a syntax error

The Logical bug it's not a syntax error like it will compile and run but there's

like it will compile and run but there's a bug there and suppose that I'm very

a bug there and suppose that I'm very new to see I'm very uncomfortable with C

new to see I'm very uncomfortable with C it's 2 a.m. and I just can't see the bug

it's 2 a.m. and I just can't see the bug what are my recourses here for actually

what are my recourses here for actually finding a mistake like this well first

finding a mistake like this well first let's look at the symptom let me go down

let's look at the symptom let me go down to my terminal window I'm going to use

to my terminal window I'm going to use make buggy zero because again the file

make buggy zero because again the file is called buggy zero. C I'm not going to

is called buggy zero. C I'm not going to use clang in fact I'm never really going

use clang in fact I'm never really going to use clang manually here and out I'm

to use clang manually here and out I'm just going to use make because it makes

just going to use make because it makes our lives easier it does compile no

our lives easier it does compile no errors so it's not syntax it's not

errors so it's not syntax it's not something silly like a missing semicolon

something silly like a missing semicolon but when I runbuggy Z I of course see 1

but when I runbuggy Z I of course see 1 2 3 4 and this of course does not match

2 3 4 and this of course does not match the zero the one two three bricks that I

the zero the one two three bricks that I actually intended for that column and

actually intended for that column and yet I'm starting counting at zero as I

yet I'm starting counting at zero as I usually do I've got three I'm going up

usually do I've got three I'm going up to three so where is my logical error if

to three so where is my logical error if it hasn't obviously jumped out at you

it hasn't obviously jumped out at you already well how can I solve this well

already well how can I solve this well first and foremost perhaps the best

first and foremost perhaps the best technique for solving bugs at least

technique for solving bugs at least early on is just use printf like thus

early on is just use printf like thus far we've used printf to say hello and

far we've used printf to say hello and other things on the screen but print def

other things on the screen but print def is just a function for printing anything

is just a function for printing anything and there's no reason you can't

and there's no reason you can't temporarily use print def to like print

temporarily use print def to like print out the contents of variables what's

out the contents of variables what's going on inside of your program just to

going on inside of your program just to figure out where your mistake is and

figure out where your mistake is and then you can delete that line of code

then you can delete that line of code later it doesn't have to stay there

later it doesn't have to stay there forever so let me do this instead of

forever so let me do this instead of just printing out in vs code the hash

just printing out in vs code the hash symbol let me do a little safety check

symbol let me do a little safety check here and print out the value of I so let

here and print out the value of I so let me go ahead and say something like I is

me go ahead and say something like I is now I want to say I is this but of

now I want to say I is this but of course this is not how I print out the

course this is not how I print out the value of I if I want to print out the

value of I if I want to print out the value of I what should I put

value of I what should I put here so percent I for integer instead of

here so percent I for integer instead of percent s for string so they're still

percent s for string so they're still placeholders but we use percent s for

placeholders but we use percent s for integers and now if I want to print out

integers and now if I want to print out I I just need the comma as the second

I I just need the comma as the second argument and then I all right let me go

argument and then I all right let me go ahead and back to vs uh to my terminal

ahead and back to vs uh to my terminal window let me recompile the program

window let me recompile the program because I've changed it that still works

because I've changed it that still works fine do/ bugy Z and now let me increase

fine do/ bugy Z and now let me increase the size of my terminal window here you

the size of my terminal window here you just see some diagnostic information if

just see some diagnostic information if you will like this is not the goal this

you will like this is not the goal this is not what you should be submitting for

is not what you should be submitting for this homework problem we're at one uh

this homework problem we're at one uh but it is helping us diagnostically know

but it is helping us diagnostically know that okay when I is zero here's a hash

that okay when I is zero here's a hash when I is one here's a hash when I is

when I is one here's a hash when I is two here's a hash when I is three here's

two here's a hash when I is three here's a hash well wait a minute that's one two

a hash well wait a minute that's one two three four so clearly I'm printing it

three four so clearly I'm printing it one too many times so let me look back

one too many times so let me look back at the code here by shrinking my

at the code here by shrinking my terminal window and let me just ask the

terminal window and let me just ask the group where is in fact the mistake or

group where is in fact the mistake or what equivalently would be the solution

what equivalently would be the solution yeah in the

middle yeah instead of less than or equal to use just less than so you got

equal to use just less than so you got to kind of pick a lane here like if

to kind of pick a lane here like if you're going to start counting from zero

you're going to start counting from zero you generally use less than and go up to

you generally use less than and go up to but not through the value or if you

but not through the value or if you prefer like in the human world counting

prefer like in the human world counting from one on up you can use great less

from one on up you can use great less than or equal to but you have to be

than or equal to but you have to be consistent and in general as a

consistent and in general as a programmer just always start counting

programmer just always start counting from zero if you're doing something

from zero if you're doing something canonical like this but the solution is

canonical like this but the solution is indeed just to change this by changing

indeed just to change this by changing the greater less than or equal to to

the greater less than or equal to to less than if I re compile this program

less than if I re compile this program with make buggy zero and then do buggy

with make buggy zero and then do buggy zero again and let me increase the size

zero again and let me increase the size of my terminal window now you see okay

of my terminal window now you see okay almost the same output but indeed I

almost the same output but indeed I starts at zero goes up two but not

starts at zero goes up two but not through three all right so printf in

through three all right so printf in short should be can be your first uh

short should be can be your first uh diagnostic tool instead of just staring

diagnostic tool instead of just staring at the screen or raising your hand I

at the screen or raising your hand I mean use printf to see literally what's

mean use printf to see literally what's going on inside of your program by just

going on inside of your program by just printing out things of interest and then

printing out things of interest and then once you've solved the problem you can

once you've solved the problem you can go back into your code AS I'll do here

go back into your code AS I'll do here by shrinking my terminal window I'll

by shrinking my terminal window I'll delete the print F line and now I'm

delete the print F line and now I'm ready to share this program with the

ready to share this program with the world or submit it as homework or the

world or submit it as homework or the like it's just meant there to be

like it's just meant there to be temporary any questions on printf as a

temporary any questions on printf as a debugging

tool no all right well that only gets us so far and honestly as your programs

so far and honestly as your programs grow and grow and grow it's going to

grow and grow and grow it's going to actually get really annoying to start

actually get really annoying to start going in and adding printfs then

going in and adding printfs then removing them and figuring out if you've

removing them and figuring out if you've got multiple printfs well which one

got multiple printfs well which one printed what like it just gets messy

printed what like it just gets messy eventually to rely on printf alone so

eventually to rely on printf alone so being a computer scientist computer

being a computer scientist computer scientists have written software to make

scientists have written software to make it easier to debug code that software is

it easier to debug code that software is what we would generally call a debugger

what we would generally call a debugger which would be the second tool of the

which would be the second tool of the trade that you can use to actually solve

trade that you can use to actually solve problems in your code now in the world

problems in your code now in the world of VSS code there's actually a debugger

of VSS code there's actually a debugger built in so the graphical user interface

built in so the graphical user interface you're about to see in VSS code isn't

you're about to see in VSS code isn't specific to cs50 it actually comes with

specific to cs50 it actually comes with vs code and it supports C and C++ and

vs code and it supports C and C++ and Java and Python and lots of other

Java and Python and lots of other languages too but it's admittedly a

languages too but it's admittedly a little complicated to just start using

little complicated to just start using the debugger you have to like create a

the debugger you have to like create a configuration file and do like some

configuration file and do like some annoying steps that just get in the way

annoying steps that just get in the way of solving real problems so we have

of solving real problems so we have automated the process for you of just

automated the process for you of just starting the debugger and thereafter

starting the debugger and thereafter it's sort of Industry standard how you

it's sort of Industry standard how you use it but we save you the headache of

use it but we save you the headache of having to create those configuration

having to create those configuration files so suppose I want to do this

files so suppose I want to do this suppose I want to try to debug this

suppose I want to try to debug this program step by step using special

program step by step using special software well how can I do that well let

software well how can I do that well let me propose that if I revert this back to

me propose that if I revert this back to the original version where I was less

the original version where I was less than or equal to three I'm pretty sure

than or equal to three I'm pretty sure that I was printing to hashes so I'm

that I was printing to hashes so I'm going to do this and you might have done

going to do this and you might have done this accidentally or never at all but

this accidentally or never at all but notice if you hover over the gutter so

notice if you hover over the gutter so to speak in vs code the part of it all

to speak in vs code the part of it all the way to the left of the editor you

the way to the left of the editor you see this sort of grayed out uh Red Dot

see this sort of grayed out uh Red Dot if you click there it becomes a brighter

if you click there it becomes a brighter Red Dot and this represents what we're

Red Dot and this represents what we're going to call a break point and this is

going to call a break point and this is just a visual indicator that you've put

just a visual indicator that you've put like a stop sign equivalent there and

like a stop sign equivalent there and you're telling the debugger in a moment

you're telling the debugger in a moment stop running my code there why because I

stop running my code there why because I prefer to step through my code at sort

prefer to step through my code at sort of a human speed and not as computer

of a human speed and not as computer speed where it runs all at once so I've

speed where it runs all at once so I've set my breakpoint which is step one and

set my breakpoint which is step one and then step two is quite simply this

then step two is quite simply this instead of running the program itself

instead of running the program itself run a command called debug 50 and then

run a command called debug 50 and then do/ bugy Z and now this will start your

do/ bugy Z and now this will start your program but inside of the debugger which

program but inside of the debugger which is a special program that smart people

is a special program that smart people wrote that will Empower you to now step

wrote that will Empower you to now step through your code line by line at again

through your code line by line at again at your own Comfort Pace I'm going to

at your own Comfort Pace I'm going to hit enter some stuff's going to happen

hit enter some stuff's going to happen on the screen whoops uh notice this is a

on the screen whoops uh notice this is a common mistake that I made accidentally

common mistake that I made accidentally here looks like I've changed my code I

here looks like I've changed my code I did because I went in and changed the

did because I went in and changed the less than or equal to sign so let me go

less than or equal to sign so let me go ahead and rerun make buggy zero enter

ahead and rerun make buggy zero enter good now let me rerun debug 50 enter and

good now let me rerun debug 50 enter and now some stuff just happened on the

now some stuff just happened on the screen and it takes a moment to get

screen and it takes a moment to get started but once it's started you'll see

started but once it's started you'll see this you'll still see your code but

this you'll still see your code but you'll see this yellow highlight which

you'll see this yellow highlight which you've probably not seen before and

you've probably not seen before and notice that it's specifically

notice that it's specifically highlighted in the same line that I set

highlighted in the same line that I set a breakpoint on why that just means the

a breakpoint on why that just means the program has EXE the debugger has

program has EXE the debugger has executed all of these lines except for

executed all of these lines except for line seven it has broken at not in a bad

line seven it has broken at not in a bad way but it has paused execution on line

way but it has paused execution on line seven so it hasn't yet printed any

seven so it hasn't yet printed any hashes and you can see that no hashes in

hashes and you can see that no hashes in the terminal window yet it's paused

the terminal window yet it's paused execution but what's interesting with

execution but what's interesting with the debugger is the stuff over here on

the debugger is the stuff over here on the left hand side in the debugger here

the left hand side in the debugger here you'll see under variables all of your

you'll see under variables all of your so-called local variables and we haven't

so-called local variables and we haven't really made a distinction between local

really made a distinction between local and something called Global but for now

and something called Global but for now local variables just means all of the

local variables just means all of the variables that exist in your function so

variables that exist in your function so I currently has a value of zero okay and

I currently has a value of zero okay and that makes sense so now how do I step

that makes sense so now how do I step through my code and see what it's doing

through my code and see what it's doing well at the top of the screen here

well at the top of the screen here you'll see some playback icons kind of

you'll see some playback icons kind of like a video player but they have

like a video player but they have special meaning this first one will just

special meaning this first one will just play the rest of your program all the

play the rest of your program all the way to the end so you only click that if

way to the end so you only click that if you sort of solved a problem and you

you sort of solved a problem and you just want to run run it to completion

just want to run run it to completion like before but the next three or next

like before but the next three or next two really are really the juiciest the

two really are really the juiciest the second one here if you hover over it

second one here if you hover over it eventually you'll see that it's called

eventually you'll see that it's called step over step over means that the

step over step over means that the debugger will run this currently

debugger will run this currently highlighted line of code but it's not

highlighted line of code but it's not going to dive into it so if it's a

going to dive into it so if it's a function like print F it's not going to

function like print F it's not going to start stepping through print F line by

start stepping through print F line by line why because I can pretty much

line why because I can pretty much assume print F written decades ago is

assume print F written decades ago is correct problems probably with me but

correct problems probably with me but this next line if I did really want to

this next line if I did really want to step in into the printf code to figure

step in into the printf code to figure out how it works or find some problem in

out how it works or find some problem in it all these years later you can step

it all these years later you can step into printf and then the screen would

into printf and then the screen would change and you'd see each of the lines

change and you'd see each of the lines for print F line by line at least if you

for print F line by line at least if you have the source code for print F

have the source code for print F installed all right I'm going to use the

installed all right I'm going to use the first one step over and watch as the

first one step over and watch as the yellow highlight moves and watch as in

yellow highlight moves and watch as in the terminal window there's a hash

the terminal window there's a hash symbol here we go there's one hash now

symbol here we go there's one hash now notice line five is highlighted that

notice line five is highlighted that means it has paused on line five line

means it has paused on line five line five has not yet been exec executed so

five has not yet been exec executed so what does that mean the value of I per

what does that mean the value of I per the top left hand corner is still zero

the top left hand corner is still zero but as soon as I click step over again

but as soon as I click step over again Watch What Happens at the top left where

Watch What Happens at the top left where I is a variable on the

I is a variable on the screen now I and it flashed briefly has

screen now I and it flashed briefly has a value of one and now if I step over

a value of one and now if I step over again watch the terminal window there's

again watch the terminal window there's my second hash now let me click step

my second hash now let me click step over on the for Loop watch the variable

over on the for Loop watch the variable at top left now one goes to two now let

at top left now one goes to two now let me click it again third hash and Here's

me click it again third hash and Here's Where The Logical error is perhaps

Where The Logical error is perhaps revealed let me go ahead and step over

revealed let me go ahead and step over the loop now I is three wait a minute

the loop now I is three wait a minute I'm still going to print out a hash

I'm still going to print out a hash there it is there's the fourth hash and

there it is there's the fourth hash and at this point hopefully the light bulb

at this point hopefully the light bulb proverbially has gone off I realize oh I

proverbially has gone off I realize oh I screwed up I can either stop the program

screwed up I can either stop the program Al together with the red square or I can

Al together with the red square or I can just let it run all the way to the end

just let it run all the way to the end which just terminates everything at this

which just terminates everything at this point I just want to get back into my

point I just want to get back into my code and start fixing things and you can

code and start fixing things and you can close for instance as I will here the

close for instance as I will here the file explorer just to hide the panel

file explorer just to hide the panel that open so that's debug 50 but it's

that open so that's debug 50 but it's not a cs50 thing that just starts the

not a cs50 thing that just starts the debugger for you which is something

debugger for you which is something you'd find in most any programming

you'd find in most any programming environment nowadays questions on

environment nowadays questions on debugging questions

yeah good question where does it tell you where it went wrong so sadly it does

you where it went wrong so sadly it does not tell you any of that the onus is

not tell you any of that the onus is still on you the human to use this tool

still on you the human to use this tool productively to walk through your code

productively to walk through your code at a a saner pace but your brain is is

at a a saner pace but your brain is is the one that still needs to solve it and

the one that still needs to solve it and I don't doubt down the line with

I don't doubt down the line with artificial intelligence and more

artificial intelligence and more programs like this will get all the more

programs like this will get all the more helpful and start answering questions

helpful and start answering questions like that for us and there are other

like that for us and there are other tools we'll introduce you this semester

tools we'll introduce you this semester that are even more powerful than this

that are even more powerful than this but for now it's just a tool really to

but for now it's just a tool really to slow things down and not have to change

slow things down and not have to change your code the fact that I had that panel

your code the fact that I had that panel on the left that just showed me eyes

on the left that just showed me eyes changing value is just an alternative to

changing value is just an alternative to print F and I can step through it a

print F and I can step through it a little more slowly other questions on

little more slowly other questions on debugging now let me show you one final

debugging now let me show you one final example with this debugger here and this

example with this debugger here and this one too I wrote in advance let me close

one too I wrote in advance let me close buggy z.c and let me open up buggy1.c my

buggy z.c and let me open up buggy1.c my second version thereof let me close my

second version thereof let me close my terminal window for a second and give

terminal window for a second and give you a quick tour of this program which

you a quick tour of this program which similarly has a mistake now at the top

similarly has a mistake now at the top of this program some familiar includes

of this program some familiar includes cs50.h and standard i.h this is not

cs50.h and standard i.h this is not something we've seen before it's

something we've seen before it's specific to this example a function

specific to this example a function called get Negative int takes no

called get Negative int takes no arguments and it returns an integer what

arguments and it returns an integer what does it do it literally gets a negative

does it do it literally gets a negative integer ideally from the user fun fact

integer ideally from the user fun fact though it doesn't correctly that's the

though it doesn't correctly that's the bug get Negative int is broken at the

bug get Negative int is broken at the moment so what does Maine do well main

moment so what does Maine do well main just calls this function passing in

just calls this function passing in nothing in parenthesis no inputs and it

nothing in parenthesis no inputs and it stores the return value in I and then it

stores the return value in I and then it just prints out I on the screen so

just prints out I on the screen so honestly just by eyeballing this you

honestly just by eyeballing this you know I feel comfortable enough with

know I feel comfortable enough with programming and see I think main is

programming and see I think main is correct let me just stipulate main is

correct let me just stipulate main is correct but there is going to be a bug

correct but there is going to be a bug down here now what's the bug down here

down here now what's the bug down here here well let me look at get Negative in

here well let me look at get Negative in implementation notice this first line 12

implementation notice this first line 12 is identical to the Prototype up here

is identical to the Prototype up here the Prototype is sort of stupidly

the Prototype is sort of stupidly required up here because C reads things

required up here because C reads things top to bottom left to right the compiler

top to bottom left to right the compiler technically does so if you reference get

technically does so if you reference get Negative in here but you don't implement

Negative in here but you don't implement it until down here and you haven't told

it until down here and you haven't told C in advance that it will exist again

C in advance that it will exist again you get the error we saw last week all

you get the error we saw last week all right so how does get Negative int work

right so how does get Negative int work we declare a variable called n

we declare a variable called n we've got a do while loop that does what

we've got a do while loop that does what it uses get int which comes with the

it uses get int which comes with the cs50 library per last week it prompts

cs50 library per last week it prompts the user for negative integer quote

the user for negative integer quote unquote and stores the value in n i then

unquote and stores the value in n i then do all of this while n is less than zero

do all of this while n is less than zero right remember we used a do while loop

right remember we used a do while loop last week to make sure the human

last week to make sure the human cooperates and doesn't give us the wrong

cooperates and doesn't give us the wrong type of value be it positive or negative

type of value be it positive or negative or something else and then we return n

or something else and then we return n and there's some subtleties anyone

and there's some subtleties anyone recall or have an intuition for why I've

recall or have an intuition for why I've declared clared in on line 14 instead of

declared clared in on line 14 instead of on line 17 this is a c specific

thing exactly there's this notion of scope in see and we'll continue to see

scope in see and we'll continue to see this over time whereby a variable only

this over time whereby a variable only exists inside of the most recent curly

exists inside of the most recent curly braces that you've opened so if I've

braces that you've opened so if I've declared and here on line 14 I can use

declared and here on line 14 I can use it anywhere between lines 13 and 21

it anywhere between lines 13 and 21 because those are the nearest curly

because those are the nearest curly Braes if by contrast as you note if I

Braes if by contrast as you note if I instead said this int n equals get int

instead said this int n equals get int and so forth and didn't have the current

and so forth and didn't have the current line 14 well n would exist inside of

line 14 well n would exist inside of these curly braces but not here which is

these curly braces but not here which is too late and definitely not here so you

too late and definitely not here so you just have to declare it first and then

just have to declare it first and then use and reuse it as such now let me just

use and reuse it as such now let me just show you how I can debug this but let me

show you how I can debug this but let me show you the symptoms first let me open

show you the symptoms first let me open my terminal window let me run make buggy

my terminal window let me run make buggy one compiles okay so it's not something

one compiles okay so it's not something silly like a semicolon dots SL buggy one

silly like a semicolon dots SL buggy one and I'm asked for a negative integer all

and I'm asked for a negative integer all right let me give it negative one

right let me give it negative one enter well the main function supposed to

enter well the main function supposed to print out what I typed but it clearly

print out what I typed but it clearly didn't it's prompting me again all right

didn't it's prompting me again all right so maybe it'll like -2 no maybe

so maybe it'll like -2 no maybe -3 50 okay so it's definitely broken

-3 50 okay so it's definitely broken right it kind of seems logically to be

right it kind of seems logically to be doing the opposite now you can perhaps

doing the opposite now you can perhaps see why this is happening already these

see why this is happening already these are deliberately simple programs by uh

are deliberately simple programs by uh for demonstration sake but let's do this

for demonstration sake but let's do this let me go ahead and set a break point in

let me go ahead and set a break point in main even though I'm pretty sure main is

main even though I'm pretty sure main is correct but it just helps me start my

correct but it just helps me start my thought process start with Main and then

thought process start with Main and then take it from there let me run now uh

take it from there let me run now uh debug

debug 50 bugy one enter and let's see with

50 bugy one enter and let's see with that breakpoint now the the goey is

that breakpoint now the the goey is going to reconfigure itself it's going

going to reconfigure itself it's going to pause on line eight because that's

to pause on line eight because that's the first interesting line inside of

the first interesting line inside of Maine so I could have just put the

Maine so I could have just put the breakpoint on line eight too it's smart

breakpoint on line eight too it's smart enough to know that if I set it on six

enough to know that if I set it on six eh you really mean line eight because

eh you really mean line eight because that's the first actual line of code and

that's the first actual line of code and watch when now what happens if I step

watch when now what happens if I step over this line notice that I which at

over this line notice that I which at the moment seems to have a default value

the moment seems to have a default value of zero more on that another time but if

of zero more on that another time but if I click step over like before I'm

I click step over like before I'm prompted for a negative integer let me

prompted for a negative integer let me type negative 1 enter and now

type negative 1 enter and now notice there's no additional yellow

notice there's no additional yellow highlight why where am I currently stuck

logically yeah just logically I must be in that D while Loop and even if you

in that D while Loop and even if you don't understand it like that's the only

don't understand it like that's the only explanation if you keep getting prompted

explanation if you keep getting prompted surely there's a loop going on there's

surely there's a loop going on there's only one Loop in my code so there's

only one Loop in my code so there's probably a problem there so okay I can't

probably a problem there so okay I can't just set a breakpoint in Main and then

just set a breakpoint in Main and then wait for this to work so let me just uh

wait for this to work so let me just uh let me stop this with the red square and

let me stop this with the red square and let me think all right instead of I can

let me think all right instead of I can still set my break point in main but let

still set my break point in main but let me rerun the debugger instead and this

me rerun the debugger instead and this time not step over that line of code let

time not step over that line of code let me step into that line of code so Watch

me step into that line of code so Watch What Happens now instead of clicking the

What Happens now instead of clicking the second icon here let me click the third

second icon here let me click the third whose name is indeed step into and watch

whose name is indeed step into and watch as the yellow highlight does not move to

as the yellow highlight does not move to line n it dives into line8 the function

line n it dives into line8 the function on line eight thereby bringing me whoosh

on line eight thereby bringing me whoosh down to line 17 it's kind of going down

down to line 17 it's kind of going down into that next function now it didn't

into that next function now it didn't bother pausing on line 12 or 13 or 14

bother pausing on line 12 or 13 or 14 because there's nothing intellectually

because there's nothing intellectually interesting there happening yet the

interesting there happening yet the juicy part really starts it would seem

juicy part really starts it would seem in line 17 so now notice n is my

in line 17 so now notice n is my variable at the top left if I click I

variable at the top left if I click I don't want to click step into now

don't want to click step into now though what what would go wrong if I

though what what would go wrong if I click on step into or what would it do

click on step into or what would it do that I don't think I want to

that I don't think I want to do

do yeah yeah it would step into get int but

yeah yeah it would step into get int but I'd like to think that the staff's

I'd like to think that the staff's version of get in is correct and that's

version of get in is correct and that's not our problem today so I want to step

not our problem today so I want to step over it and watch now at top left that

over it and watch now at top left that nothing happens yet to the value of n

nothing happens yet to the value of n until I go to the terminal window now

until I go to the terminal window now and type in something like Nega 1 now

and type in something like Nega 1 now notice it jumps to line 19 which is the

notice it jumps to line 19 which is the next interesting line top left n indeed

next interesting line top left n indeed is1 and here's where I can now pause as

is1 and here's where I can now pause as a human and think all right so while n

a human and think all right so while n is less than zero all right n per the

is less than zero all right n per the top left corner is negative 1 so all

top left corner is negative 1 so all right while negative 1 is less than zero

right while negative 1 is less than zero well obviously that's true

well obviously that's true mathematically so what's going to happen

mathematically so what's going to happen it's a do while loop so when I click on

it's a do while loop so when I click on step over again it's going to go to this

step over again it's going to go to this line cuz it's at the end of the inside

line cuz it's at the end of the inside of that Loop and now here it's looping

of that Loop and now here it's looping through again and again all right let me

through again and again all right let me do this once more I'm going to step over

do this once more I'm going to step over all right I'm going to type in -2 and

all right I'm going to type in -2 and it's the exact same thing now is my

it's the exact same thing now is my chance on the yellow line okay wait a

chance on the yellow line okay wait a minute -2 is obviously less than zero

minute -2 is obviously less than zero let me try this one more time click it

let me try this one more time click it once here and now all right let me give

once here and now all right let me give it 50 and now okay while 50 is less than

it 50 and now okay while 50 is less than zero that's not true so the loop is over

zero that's not true so the loop is over because it's not going to do it while 50

because it's not going to do it while 50 is less than zero that's not true so now

is less than zero that's not true so now watch when I click step over once more

watch when I click step over once more it then finishes the loop even though

it then finishes the loop even though there's nothing more to do it's now

there's nothing more to do it's now about to return n it jumps back up to

about to return n it jumps back up to main where I left off on line nine it

main where I left off on line nine it now prints in my terminal window the

now prints in my terminal window the number 50 and hopefully at this point to

number 50 and hopefully at this point to your question earlier my human brain has

your question earlier my human brain has realized oh I'm an idiot like I flipped

realized oh I'm an idiot like I flipped my my sign there so I probably let me

my my sign there so I probably let me stop this I probably want to do

stop this I probably want to do something like this if the goal is to

something like this if the goal is to get a negative integer I probably want

get a negative integer I probably want to say well n is for instance greater

to say well n is for instance greater than or equal to zero would work so

than or equal to zero would work so while n is greater than or equal to zero

while n is greater than or equal to zero keep doing this and that's the logic I

keep doing this and that's the logic I wanted to express so the debugger just

wanted to express so the debugger just saves me from staring at the screen

saves me from staring at the screen raising a hand sort of asking someone

raising a hand sort of asking someone else at least in this case it allows me

else at least in this case it allows me to go through it at a healthier Pace

to go through it at a healthier Pace questions now on debug 50 which should

questions now on debug 50 which should be your new friend even if it's not your

be your new friend even if it's not your first instinct after

printf any questions on debug 50 no all right all right well there's one last

right all right well there's one last Technique we can equip you with here um

Technique we can equip you with here um and that is in addition to printf and a

and that is in addition to printf and a debugger no joke um a rubber duck is

debugger no joke um a rubber duck is actually a reasonably recommended uh

actually a reasonably recommended uh solution to finding bugs in your code to

solution to finding bugs in your code to your question earlier the duck two is

your question earlier the duck two is not going to solve the problem for you

not going to solve the problem for you but if you've wondered why this little

but if you've wondered why this little guy has been here for so long um there's

guy has been here for so long um there's this technique has its own Wikipedia

this technique has its own Wikipedia article of called rubber duck debugging

article of called rubber duck debugging the idea of which is that if you're home

the idea of which is that if you're home in your dorm room wrestling with some

in your dorm room wrestling with some bug in your code printf didn't quite

bug in your code printf didn't quite reveal the source to you debugger isn't

reveal the source to you debugger isn't really helping honestly maybe it would

really helping honestly maybe it would help to just sound out what problem

help to just sound out what problem you're having similar to going to office

you're having similar to going to office hours talking to a a TA or a professor

hours talking to a a TA or a professor just walking through your problems

just walking through your problems because in sort of talking to the duck

because in sort of talking to the duck about you know the fact that you're

about you know the fact that you're doing this while uh n is less than zero

doing this while uh n is less than zero and then if it is I wait a minute I'm an

and then if it is I wait a minute I'm an idiot not just for talking to the rubber

idiot not just for talking to the rubber duck you realize hopefully in expressing

duck you realize hopefully in expressing yourself literally verbally you probably

yourself literally verbally you probably will hear with non-zero probability like

will hear with non-zero probability like some illogic in your statement and just

some illogic in your statement and just by sounding things out you'll realize

by sounding things out you'll realize like oh that's my problem and so frankly

like oh that's my problem and so frankly if you have roommates you can also use a

if you have roommates you can also use a roommate for this but the rubber duck is

roommate for this but the rubber duck is just sort of a go-to when your roommates

just sort of a go-to when your roommates have no interest in your you know C

have no interest in your you know C problem set talking something through

problem set talking something through that um as follow as such and this is an

that um as follow as such and this is an invaluable technique I admittedly tend

invaluable technique I admittedly tend not to do it so much with a rubber duck

not to do it so much with a rubber duck but ideally with colleagues human

but ideally with colleagues human colleagues but just talking through

colleagues but just talking through things often will help you just realize

things often will help you just realize oh I said something logical now I can go

oh I said something logical now I can go back to the code so don't solve problems

back to the code so don't solve problems by staring at your screen endlessly for

by staring at your screen endlessly for minutes for hours at that point it's

minutes for hours at that point it's time for a break time to walk away time

time for a break time to walk away time to talk to the duck if you've already

to talk to the duck if you've already exhausted some of those other tools um

exhausted some of those other tools um as an aside on your way out today at the

as an aside on your way out today at the end of class we have H clearly plenty of

end of class we have H clearly plenty of rubber ducks uh for you um and uh it's

rubber ducks uh for you um and uh it's become a thing over the years at least

become a thing over the years at least among some uh to bring the duck with

among some uh to bring the duck with them when they travel and send us photos

them when they travel and send us photos here for instance is uh cs50's rubber

here for instance is uh cs50's rubber duck debugger AKA ddb for duck debugger

duck debugger AKA ddb for duck debugger which is a pun on a geekier program

which is a pun on a geekier program called GDB the ganu debugger which is an

called GDB the ganu debugger which is an actual piece of software for debugging

actual piece of software for debugging this is cs50's debugger uh in the hills

this is cs50's debugger uh in the hills of Puerto Rico uh also here on the sea

of Puerto Rico uh also here on the sea uh he made its way to San Francisco here

uh he made its way to San Francisco here uh also down by Fisherman's dwarf by the

uh also down by Fisherman's dwarf by the sea lions if familiar uh here at

sea lions if familiar uh here at Stanford where there's a William Gates

Stanford where there's a William Gates computer science building for computer

computer science building for computer science uh down the road in SF at Google

science uh down the road in SF at Google uh and this is the Tre fountain in Rome

uh and this is the Tre fountain in Rome and lastly uh the Coliseum so we'll be

and lastly uh the Coliseum so we'll be curious to see in the coming years where

curious to see in the coming years where your duck two travels so that then was

your duck two travels so that then was quite a bit why don't we go ahead here

quite a bit why don't we go ahead here and take a short five minute break no

and take a short five minute break no snacks yet you're welcome to get up or

snacks yet you're welcome to get up or sit down we'll return in about five all

sit down we'll return in about five all right so we are back and if the goal

right so we are back and if the goal ultimately today is to have a better

ultimately today is to have a better understanding of things like strings so

understanding of things like strings so that we can solve problems with text

that we can solve problems with text let's consider some simpler types of

let's consider some simpler types of data first how we might represent those

data first how we might represent those and then see if that doesn't lead us to

and then see if that doesn't lead us to a discovery as to like how strings in

a discovery as to like how strings in just today's modern software is using

just today's modern software is using things like that so when we talked on

things like that so when we talked on week zero about representation of data

week zero about representation of data we had different ways of doing it in

we had different ways of doing it in terms of binary and decimal and uh unary

terms of binary and decimal and uh unary even when we started talking about the

even when we started talking about the same last week in code we started

same last week in code we started talking about uh data types instead and

talking about uh data types instead and these data types were a way of telling

these data types were a way of telling the computer like do you want an integer

the computer like do you want an integer do you want a character do you want a

do you want a character do you want a floating point value like a real number

floating point value like a real number or even a string as we've seen but it

or even a string as we've seen but it turns out that computers of course only

turns out that computers of course only have finite amounts of resources your

have finite amounts of resources your computer only has a fixed amount of

computer only has a fixed amount of memory or RAM and that actually has very

memory or RAM and that actually has very real world implications so for instance

real world implications so for instance here are some of the data types we've

here are some of the data types we've seen thus far and it turns out that each

seen thus far and it turns out that each of these in C has a specific number of

of these in C has a specific number of bits allocated to it now admittedly this

bits allocated to it now admittedly this can vary by System it's not so much the

can vary by System it's not so much the case nowaday days but for many years for

case nowaday days but for many years for decades computers were getting better

decades computers were getting better and better the earliest computers might

and better the earliest computers might have used fewer bits for some of these

have used fewer bits for some of these data types more modern computers might

data types more modern computers might use more bits so the numbers you're

use more bits so the numbers you're about to see are pretty much where we

about to see are pretty much where we are present day so when it comes to

are present day so when it comes to these data types A bu which is true or

these data types A bu which is true or false somewhat curiously uses a whole

false somewhat curiously uses a whole bite even though that's way Overkill

bite even though that's way Overkill because for a bull true or false you of

because for a bull true or false you of course only need one bit but it turns

course only need one bit but it turns out even though it's wasteful to use

out even though it's wasteful to use eight bits or one bite just to represent

eight bits or one bite just to represent true or false it's just easier for

true or false it's just easier for computers so a bull tends to be one bite

computers so a bull tends to be one bite an INT which we've been using a lot uses

an INT which we've been using a lot uses four bytes typically or 32 bits and if I

four bytes typically or 32 bits and if I do some quick math from week zero with

do some quick math from week zero with 32 bits you have four billion possible

32 bits you have four billion possible values roughly but if you want to

values roughly but if you want to represent positive and negative that

represent positive and negative that means you can represent roughly -2

means you can represent roughly -2 billion all the way up to positive2

billion all the way up to positive2 billion so that's the range typically

billion so that's the range typically within if that's to few numbers for you

within if that's to few numbers for you turns out there's things called Longs

turns out there's things called Longs and longs use 64 bits which allow you to

and longs use 64 bits which allow you to have like a quintilian number of

have like a quintilian number of possibilities which is a lot certainly a

possibilities which is a lot certainly a lot more than 4 billion so sometimes you

lot more than 4 billion so sometimes you might use a long but even that's finite

might use a long but even that's finite and so uh as we discussed at the end of

and so uh as we discussed at the end of last week bad things can happen if you

last week bad things can happen if you make certain assumptions as the data

make certain assumptions as the data because of things like Inger overflow or

because of things like Inger overflow or the like where things wrap around then

the like where things wrap around then there's a float which is a real number

there's a float which is a real number something with a decimal point by

something with a decimal point by convention it's four bytes or 32 bits

convention it's four bytes or 32 bits which gives you in short only a specific

which gives you in short only a specific amount of precision it doesn't

amount of precision it doesn't necessarily dictate how many numbers to

necessarily dictate how many numbers to the left or to the right in the

the left or to the right in the aggregate ultimately you have though uh

aggregate ultimately you have though uh three uh 4 billion possible permutations

three uh 4 billion possible permutations still if you need more Precision for

still if you need more Precision for scientific for medical for financial

scientific for medical for financial applications you might use eight bytes

applications you might use eight bytes AKA a double which just gives you more

AKA a double which just gives you more digits of precision they eventually get

digits of precision they eventually get imprecise per the example we looked at

imprecise per the example we looked at last week but it at least gets you

last week but it at least gets you further down the line as an aside in

further down the line as an aside in really really important applications in

really really important applications in finance and medicine and Military

finance and medicine and Military operations and the like where you really

operations and the like where you really can't have rounding errors long story

can't have rounding errors long story short humans have developed libraries in

short humans have developed libraries in C and other languages that use more even

C and other languages that use more even than 8 bytes so there are solutions to

than 8 bytes so there are solutions to these problems but they're always finite

these problems but they're always finite you have to pick uh an upper bound then

you have to pick uh an upper bound then there's Char which we saw briefly last

there's Char which we saw briefly last week when I asked the user for y or n

week when I asked the user for y or n for yes or no and then there's string

for yes or no and then there's string which I'm going to propose as a question

which I'm going to propose as a question mark because a string totally depends

mark because a string totally depends like high h i exclamation point would

like high h i exclamation point would seem to be three bytes d a ID would seem

seem to be three bytes d a ID would seem to be five so strings clearly are

to be five so strings clearly are variable based on what you or the human

variable based on what you or the human type in so we'll see what this means

type in so we'll see what this means though in just a bit this though is the

though in just a bit this though is the thing inside of your Mac your PC your

thing inside of your Mac your PC your phone might not look exactly like this

phone might not look exactly like this but this is a a memory module for a

but this is a a memory module for a modern computer and let's go ahead and

modern computer and let's go ahead and use this really as just representative

use this really as just representative of the finite amount of memory that any

of the finite amount of memory that any computer indeed has let's zoom in on one

computer indeed has let's zoom in on one of these little black chips on the uh uh

of these little black chips on the uh uh circuit board here zoom in and let me

circuit board here zoom in and let me propose that this rectangle really

propose that this rectangle really represents some number of bytes like

represents some number of bytes like tucked inside of this little black uh

tucked inside of this little black uh circuit on the board is maybe I don't

circuit on the board is maybe I don't know a gigabyte a billion bytes maybe

know a gigabyte a billion bytes maybe it's a 100 btes some number of bytes it

it's a 100 btes some number of bytes it totally depends on the computer and how

totally depends on the computer and how much you paid for the Stick of memory

much you paid for the Stick of memory but if there's a finite number of bytes

but if there's a finite number of bytes physically implemented somehow digitally

physically implemented somehow digitally inside of this Hardware well then it

inside of this Hardware well then it stands to reason that we could number

stands to reason that we could number those bytes we can just arbitrarily

those bytes we can just arbitrarily decide that the top left corner is bite

decide that the top left corner is bite number one or really bite number zero

number one or really bite number zero the one next to it is number one then

the one next to it is number one then number two number three dot dot dot

number two number three dot dot dot number two billion or whatever it is

number two billion or whatever it is however big this this memory is so if

however big this this memory is so if you are use a variable in a c program

you are use a variable in a c program that's only one bite like a Char it

that's only one bite like a Char it might literally be stored in that top

might literally be stored in that top left hand corner of the memory like in

left hand corner of the memory like in practice you don't care where physically

practice you don't care where physically it is but really the artist rendition

it is but really the artist rendition would be this a Char might use one of

would be this a Char might use one of those single btes somewhere in the

those single btes somewhere in the computer's memory if you use an INT

computer's memory if you use an INT which is four bytes it would give you

which is four bytes it would give you four bytes contiguous that is left to

four bytes contiguous that is left to right top to bottom but all 32 bits

right top to bottom but all 32 bits would be next to each other so the

would be next to each other so the computer knows that those indeed all

computer knows that those indeed all belong to the same int if you need a

belong to the same int if you need a long or a double for that matter then

long or a double for that matter then you might use a full eight bytes in this

you might use a full eight bytes in this case and you just keep using and using

case and you just keep using and using this memory kind of like a a canvas you

this memory kind of like a a canvas you know almost in Photoshop or a

know almost in Photoshop or a spreadsheet where you can just move uh

spreadsheet where you can just move uh pixels or you can move data around

pixels or you can move data around that's really what your computer's

that's really what your computer's memory is a canvas for storing uh

memory is a canvas for storing uh information in units of bytes or 8 Bits

information in units of bytes or 8 Bits now we don't need to keep looking at

now we don't need to keep looking at these circuit boards can abstract it

these circuit boards can abstract it away as we often do and let's go ahead

away as we often do and let's go ahead and zoom in on this grid just to

and zoom in on this grid just to consider some very specific variables so

consider some very specific variables so let me zoom in and now I see fewer but

let me zoom in and now I see fewer but larger uh boxes on the screen Each of

larger uh boxes on the screen Each of which again represents a bite and now

which again represents a bite and now let me propose that we play with some

let me propose that we play with some actual code so here in C albeit without

actual code so here in C albeit without a full program or three ins score one

a full program or three ins score one score two score three I have

score two score three I have coincidentally given myself two uh uh

coincidentally given myself two uh uh two scores at around 72 and 73 and then

two scores at around 72 and 73 and then a pretty low score at 33 of course

a pretty low score at 33 of course course last week or two weeks ago this

course last week or two weeks ago this would have been high but now we're

would have been high but now we're dealing with actual integers so these

dealing with actual integers so these are three so so scores on my quizzes or

are three so so scores on my quizzes or tests or the like so let me go to vs

tests or the like so let me go to vs code here and let's make a program

code here and let's make a program called scores. C so I'm going to write

called scores. C so I'm going to write code scores. C that's going to give me

code scores. C that's going to give me my new file and let me go ahead and

my new file and let me go ahead and Implement something like this include

Implement something like this include standard i.h int main void and then

standard i.h int main void and then inside of here let me do int score one

inside of here let me do int score one will be 72 uh int score two be 73 and

will be 72 uh int score two be 73 and int score three will be 33 and then let

int score three will be 33 and then let me just do something like write a

me just do something like write a program to average my three test scores

program to average my three test scores together something like that so let me

together something like that so let me do print F quote unquote my average is

do print F quote unquote my average is and I'm going to go ahead and do say

and I'm going to go ahead and do say percent I back sln and now let me plug

percent I back sln and now let me plug in the results and this is kind of grade

in the results and this is kind of grade school math now how do I compute the

school math now how do I compute the average of three values well just like

average of three values well just like in on paper I can do score one plus

in on paper I can do score one plus score two plus score three in

score two plus score three in parentheses because of order of

parentheses because of order of operations divided by three since

operations divided by three since there's three total scores all right so

there's three total scores all right so I think this checks out and indeed you

I think this checks out and indeed you can use parentheses and operators like

can use parentheses and operators like plus and your code like this in C let me

plus and your code like this in C let me go ahead now and do make scores no

go ahead now and do make scores no syntax error so that's good nothing

syntax error so that's good nothing missing there and now let me do dot

missing there and now let me do dot scores and see what my test average is

scores and see what my test average is all right you know it's not great but I

all right you know it's not great but I I I think I still passed and indeed my

I I think I still passed and indeed my average here is is 59 is it precisely 59

average here is is 59 is it precisely 59 though well let's see let's let's

though well let's see let's let's actually instead of using an INT how

actually instead of using an INT how about we go ahead and use something like

about we go ahead and use something like a floating point value here and let me

a floating point value here and let me go ahead and do this so let me recompile

go ahead and do this so let me recompile my code make scores huh all right I've

my code make scores huh all right I've got an issue let me zoom in on my

got an issue let me zoom in on my terminal window we've not seen this one

terminal window we've not seen this one necessarily before but error on line

necessarily before but error on line nine format specifies type double which

nine format specifies type double which is a lot of precision but the argument

is a lot of precision but the argument has Type in so what does this mean well

has Type in so what does this mean well it's showing me with these green

it's showing me with these green squigglies that something's bad between

squigglies that something's bad between the percent F and this thing over here

the percent F and this thing over here well on the left I'm implying a float or

well on the left I'm implying a float or a double for that matter on the right

a double for that matter on the right though what data type are score one

though what data type are score one score two score three all right so

score two score three all right so they're ins so Cent does not like this

they're ins so Cent does not like this the compiler just doesn't like that I'm

the compiler just doesn't like that I'm using ins on the right but I want floats

using ins on the right but I want floats on the left so there's going to be

on the left so there's going to be different ways of solving this one way

different ways of solving this one way would be to just ignore the problem like

would be to just ignore the problem like I originally did and just go back to

I originally did and just go back to percent I or as an aside percent D is

percent I or as an aside percent D is often an alternative to percent I for a

often an alternative to percent I for a decimal number but we use perc I cuz it

decimal number but we use perc I cuz it sounds like int so percent I is fine

sounds like int so percent I is fine here too but I don't want to just avoid

here too but I don't want to just avoid the problem I want to actually display a

the problem I want to actually display a floating point value so how can I fix

floating point value so how can I fix this well it turns out I can solve this

this well it turns out I can solve this in a few different ways the simplest is

in a few different ways the simplest is just to make sure that at least one

just to make sure that at least one number on the right is a floating point

number on the right is a floating point value like

value like 3.0 instead of just three now I think

3.0 instead of just three now I think clang will be happier let me do make

clang will be happier let me do make scores enter and indeed it's okay why as

scores enter and indeed it's okay why as soon as you have at least one more

soon as you have at least one more precise data data type on the right it

precise data data type on the right it just treats everything at that point as

just treats everything at that point as floating point value so that the math

floating point value so that the math works out so/ scores enter and now there

works out so/ scores enter and now there we go right you know uh some of us might

we go right you know uh some of us might really want that third of a point our

really want that third of a point our average was not 59 it's 59 and a third

average was not 59 it's 59 and a third as in this case here all right so we've

as in this case here all right so we've solved that there as an aside though

solved that there as an aside though there's one other technique to uh to

there's one other technique to uh to show here if you didn't want to change

show here if you didn't want to change it to 3.0 because that's a little weird

it to 3.0 because that's a little weird because I there were literally three

because I there were literally three scores it's not like that needs to have

scores it's not like that needs to have a decimal point you could also

a decimal point you could also explicitly convert the three to a float

explicitly convert the three to a float by saying in parentheses float this is

by saying in parentheses float this is what's called type casting and this will

what's called type casting and this will just convert the thing right after it to

just convert the thing right after it to that data type if it's possible so if I

that data type if it's possible so if I do this again make scores no errors now

do this again make scores no errors now do/ scores and I get in fact the same

do/ scores and I get in fact the same result there's a bit of a rounding issue

result there's a bit of a rounding issue here but we know the rounding relates to

here but we know the rounding relates to the imprecision from last week for now

the imprecision from last week for now let me just be happy with my 59.3

let me just be happy with my 59.3 something I'll take that for now but

something I'll take that for now but this is you know is close to um a good

this is you know is close to um a good enough correct answer for me now but how

enough correct answer for me now but how do I think about now what's going on

do I think about now what's going on inside of the computer's memory well

inside of the computer's memory well let's consider here's that same grid of

let's consider here's that same grid of memory each box represents a bite where

memory each box represents a bite where are score one score two and score three

are score one score two and score three in my memory well score one let me just

in my memory well score one let me just propose is at the top left but it's

propose is at the top left but it's taking up four boxes for four bytes

taking up four boxes for four bytes score two probably ends up right next to

score two probably ends up right next to it in memory though this isn't always

it in memory though this isn't always going to be the case but I've chosen

going to be the case but I've chosen simple examples 73 is next to it also

simple examples 73 is next to it also taking up four bytes and then lastly 33

taking up four bytes and then lastly 33 is in score three uh down there

is in score three uh down there underneath now if we don't if we really

underneath now if we don't if we really look at the computer's memory look at it

look at the computer's memory look at it with some kind of microscope or the like

with some kind of microscope or the like there's actually 32 bits 32 bits 32 bits

there's actually 32 bits 32 bits 32 bits in each of those four uh groups of four

in each of those four uh groups of four bytes representing those values but

bytes representing those values but again for today's purposes onwards we

again for today's purposes onwards we don't really need to think again and

don't really need to think again and again in binary it's just indeed these

again in binary it's just indeed these decimal numbers being stored there but I

decimal numbers being stored there but I claim now this isn't the best design

claim now this isn't the best design even if if you have never programmed

even if if you have never programmed before cs50 what you're looking at here

before cs50 what you're looking at here on the screen as an excerpt in what

on the screen as an excerpt in what sense is this perhaps bad design even

sense is this perhaps bad design even though it's a correct way of storing

though it's a correct way of storing three test

three test scores what's kind of bad here

yeah yeah always do exactly what you did extrapolate to four scores five scores

extrapolate to four scores five scores 50 scores this can't be that welld

50 scores this can't be that welld designed because now you're going to

designed because now you're going to have four lines of code five lines of

have four lines of code five lines of code 50 lines of code that are almost

code 50 lines of code that are almost identical except for this like arbitrary

identical except for this like arbitrary number that we're updating at the end of

number that we're updating at the end of the variable so indeed there's probably

the variable so indeed there's probably going to be a better way even though at

going to be a better way even though at least in C we haven't yet seen that

least in C we haven't yet seen that technique but the solution today onward

technique but the solution today onward is going to be something called an array

is going to be something called an array an array is a uh way of storing your

an array is a uh way of storing your data back to back to back in the

data back to back to back in the computer's memory in such a way that you

computer's memory in such a way that you can access each individual member easily

can access each individual member easily put another way with an array you can

put another way with an array you can instead do something like this instead

instead do something like this instead of saying int score one int score 2 in

of saying int score one int score 2 in score three giving each a value you can

score three giving each a value you can first tell the computer please give me a

first tell the computer please give me a variable called scores plural that you

variable called scores plural that you can call it anything you want of size

can call it anything you want of size three Each of which will be an integer

three Each of which will be an integer that is to say this is how you declare

that is to say this is how you declare an array in C that will have enough room

an array in C that will have enough room to store three integers put another way

to store three integers put another way this is the technical way of telling the

this is the technical way of telling the computer please give me uh 12 bytes in

computer please give me uh 12 bytes in total

total 3 * 4 each for an inch so give me 12

3 * 4 each for an inch so give me 12 bytes in total and what the computer

bytes in total and what the computer will do is guarantee that they're back

will do is guarantee that they're back to back to back in the computer's memory

to back to back in the computer's memory and that'll be useful in just a moment

and that'll be useful in just a moment so let me go ahead and do something

so let me go ahead and do something useful with this let me store three

useful with this let me store three actual scores here's how I could now

actual scores here's how I could now store those same numeric scores in this

store those same numeric scores in this array syntax is a little different but

array syntax is a little different but there's one variable called scores but

there's one variable called scores but if you want to go to its first location

if you want to go to its first location starting today you use square brackets

starting today you use square brackets and go to location zero first which

and go to location zero first which because things in C are zero indexed uh

because things in C are zero indexed uh so to speak you start counting at zero

so to speak you start counting at zero the first int is at bracket zero second

the first int is at bracket zero second int is at bracket 1 third int is at

int is at bracket 1 third int is at bracket two so it's not 1 two three it's

bracket two so it's not 1 two three it's literally 0 one2 and this is not

literally 0 one2 and this is not something you have control over you must

something you have control over you must start at zero so these lines now create

start at zero so these lines now create an array of size three and then insert

an array of size three and then insert one two three values into that array but

one two three values into that array but the upside now is that you only have one

the upside now is that you only have one name of the variable to remember it's

name of the variable to remember it's just called scores yes you need to go

just called scores yes you need to go into the array to get individual values

into the array to get individual values you need to index into it using those

you need to index into it using those square brackets but at least you don't

square brackets but at least you don't have this hackish approach of declaring

have this hackish approach of declaring a separate variable for each and every

a separate variable for each and every one of these values so let me go back to

one of these values so let me go back to scores do c here and let me propose that

scores do c here and let me propose that I do this let me just kind of use that

I do this let me just kind of use that same idea to do the following let me get

same idea to do the following let me get rid of these three separate integers let

rid of these three separate integers let me give myself an INT scores array of

me give myself an INT scores array of size three and then scores bracket Z

size three and then scores bracket Z will as before be 72 scores bracket 1

will as before be 72 scores bracket 1 will be 73 and scores bracket 2 will be

will be 73 and scores bracket 2 will be 33 and let me get rid of the Little Dot

33 and let me get rid of the Little Dot there all right so now if I go ahead and

there all right so now if I go ahead and run this again with make uh scores

run this again with make uh scores enter huh what did I do wrong here I

enter huh what did I do wrong here I think I got a little too ahead of myself

think I got a little too ahead of myself let me increase my terminal

let me increase my terminal window let's focus on line 10 here first

window let's focus on line 10 here first error use of Undeclared identifier score

error use of Undeclared identifier score one what did I do here that was dumb

yeah right so I didn't declare score one I've got old code right so I just kind

I've got old code right so I just kind of honestly got ahead of myself here not

of honestly got ahead of myself here not even intentionally so let me go ahead

even intentionally so let me go ahead and Shrink my terminal window again I

and Shrink my terminal window again I need to finish my thought here so let me

need to finish my thought here so let me clear my terminal and let me change this

clear my terminal and let me change this now to be scores brackets zero plus

now to be scores brackets zero plus scores bracket 1 plus scores bracket two

scores bracket 1 plus scores bracket two so it's a little more verbose because

so it's a little more verbose because I've got these these um square brackets

I've got these these um square brackets so to speak but I think now my code is

so to speak but I think now my code is consistent so let me make scores now now

consistent so let me make scores now now compiles do/ scores gives me indeed the

compiles do/ scores gives me indeed the same rough average with those same

same rough average with those same values all right so let me go ahead and

values all right so let me go ahead and maybe um tidy enhance this a little bit

maybe um tidy enhance this a little bit it's a little silly to have to write a

it's a little silly to have to write a special program just to check your

special program just to check your average of three test scores like 72 73

average of three test scores like 72 73 33 why don't I actually make the program

33 why don't I actually make the program Dynamic and ask the human for that

Dynamic and ask the human for that average uh for those scores so instead

average uh for those scores so instead let me do do this how about we get rid

let me do do this how about we get rid of the 72 and change this to get int and

of the 72 and change this to get int and I'll just prompt the user for a score

I'll just prompt the user for a score let me get rid of the 33 and change 73

let me get rid of the 33 and change 73 and get this to be get int score quote

and get this to be get int score quote unquote and then lastly get rid of the

unquote and then lastly get rid of the 33 and replace it with get int quote

33 and replace it with get int quote unquote score get int is a cs50 thing

unquote score get int is a cs50 thing for now so I need to include cs50.h as

for now so I need to include cs50.h as always but I think now it's sort of a

always but I think now it's sort of a better program because now I can compile

better program because now I can compile it once I can even share it with my

it once I can even share it with my friends and now any of us can average

friends and now any of us can average three scores on some classes test they

three scores on some classes test they don't need to know the code or rewrite

don't need to know the code or rewrite the code just to T type in their scores

the code just to T type in their scores so make scores worked scores now I can

so make scores worked scores now I can type anything I want maybe it's a 72 73

type anything I want maybe it's a 72 73 33 still get the same answer or maybe

33 still get the same answer or maybe I'm having a better semester 100 100

I'm having a better semester 100 100 maybe 99 and now we get still a pretty

maybe 99 and now we get still a pretty high score there but now it's Dynamic

high score there but now it's Dynamic now you don't need the source code you

now you don't need the source code you don't need to recompile the program it's

don't need to recompile the program it's just going to work work again and again

just going to work work again and again but this too let me propose that this

but this too let me propose that this code is correct if I want to get three

code is correct if I want to get three scores from the user but these

scores from the user but these highlighted lines now 6 through n are

highlighted lines now 6 through n are they welld designed would you

they welld designed would you say

say yeah yeah right this is we can use a

yeah yeah right this is we can use a loop is the spoiler here why I mean my

loop is the spoiler here why I mean my God it's like the same code again and

God it's like the same code again and again and again the only thing that's

again and again the only thing that's changing is the number and you it should

changing is the number and you it should have kind of had some code smell again

have kind of had some code smell again because if I keep typing the same thing

because if I keep typing the same thing again again and again like that's

again again and again like that's clearly an opportunity to better design

clearly an opportunity to better design something so let me do this let me go

something so let me do this let me go ahead and still create my ver my array

ahead and still create my ver my array of size three and but let me use our old

of size three and but let me use our old friend the for Loop for in I equals z i

friend the for Loop for in I equals z i Less Than 3 I ++ and then in here let me

Less Than 3 I ++ and then in here let me do scores bracket we haven't seen this

do scores bracket we haven't seen this before but any

before but any intuition scores bracket I because that

intuition scores bracket I because that will use whatever I is be it Z or one or

will use whatever I is be it Z or one or two in iteration and then I can get an

two in iteration and then I can get an INT asking the user for score without

INT asking the user for score without having to repeat myself again and again

having to repeat myself again and again so hopefully if I didn't make any typos

so hopefully if I didn't make any typos make scores all good do/ score 72 73 33

make scores all good do/ score 72 73 33 and we're back in business but the code

and we're back in business but the code is arguably now better designed because

is arguably now better designed because now I haven't actually uh I haven't

now I haven't actually uh I haven't actually hardcoded the scores and I

actually hardcoded the scores and I haven't actually copied and pasted any

haven't actually copied and pasted any of that code well if we consider now

of that code well if we consider now what's going on inside of the computer's

what's going on inside of the computer's memory it's pretty much the same in

memory it's pretty much the same in terms of the values but instead of the

terms of the values but instead of the variables being literally score one

variables being literally score one score two score three there's just one

score two score three there's just one variable it's an array called scores but

variable it's an array called scores but you can index into its three locations

you can index into its three locations by using scores bracket Z to get the

by using scores bracket Z to get the first scores bracket one to get the

first scores bracket one to get the second scores bracket two to get the

second scores bracket two to get the third but this is key the memory is

third but this is key the memory is contiguous it's only the screen is only

contiguous it's only the screen is only so large so it wraps around but

so large so it wraps around but physically digitally the memory is

physically digitally the memory is contiguous top to bottom left to right

contiguous top to bottom left to right and that's important why because the

and that's important why because the brackets indicate 0 1 2 that each of

brackets indicate 0 1 2 that each of these integers is just one integer away

these integers is just one integer away from the next it can't be randomly down

from the next it can't be randomly down here all of a sudden it's got to be back

here all of a sudden it's got to be back to back to back all right now equipped

to back to back all right now equipped with that Paradigm what more could we

with that Paradigm what more could we actually do here well it turns out it's

actually do here well it turns out it's worth knowing that it's possible in code

worth knowing that it's possible in code to even pass arrays around as arguments

to even pass arrays around as arguments and let me just whip this program up

and let me just whip this program up somewhat quickly just so you've seen it

somewhat quickly just so you've seen it before long but let me go ahead and do

before long but let me go ahead and do this let me propose that I create a

this let me propose that I create a function that does this averaging for me

function that does this averaging for me so I'm going to create a function called

so I'm going to create a function called average that returns a float uh and the

average that returns a float uh and the arguments this thing is going to take uh

arguments this thing is going to take uh let's see it's going to be the array so

let's see it's going to be the array so it turns out if you want to take in an

it turns out if you want to take in an array of numbers you can call it

array of numbers you can call it anything you want this is how you tell C

anything you want this is how you tell C that a function takes not an integer but

that a function takes not an integer but in Array of integers and you don't have

in Array of integers and you don't have to call it array I'm doing that just for

to call it array I'm doing that just for the sake of discussion it can be called

the sake of discussion it can be called X it can be numbers it can be anything

X it can be numbers it can be anything else I'm just calling an array to be

else I'm just calling an array to be super explicit as to what it is there

super explicit as to what it is there now how do I change my code down here

now how do I change my code down here what I think I'm going to do for the

what I think I'm going to do for the moment is just this I'm going to get rid

moment is just this I'm going to get rid of this code here where I manually

of this code here where I manually computed the average and let me just

computed the average and let me just call the average function here by

call the average function here by passing in the whole array of scores so

passing in the whole array of scores so this is just an example of abstraction

this is just an example of abstraction like now I have a fun function called

like now I have a fun function called average I don't care I don't have to

average I don't care I don't have to remember how it works once I implement

remember how it works once I implement it it just kind of tightens up my main

it it just kind of tightens up my main code a little bit but I do still have to

code a little bit but I do still have to implement this so later in my file let

implement this so later in my file let me repeat myself before the only time

me repeat myself before the only time it's okay and see to repeat yourself

it's okay and see to repeat yourself again and again by typing out again

again and again by typing out again average and then int array Open Bracket

average and then int array Open Bracket but now not a semicolon now I have to

but now not a semicolon now I have to implement this thing and I can implement

implement this thing and I can implement this in a bunch of different ways uh but

this in a bunch of different ways uh but I don't know huh in advance I can't just

I don't know huh in advance I can't just do this I can't just do array bracket 0

do this I can't just do array bracket 0 plus array bracket 1 plus array bracket

plus array bracket 1 plus array bracket 2 unless unless this program is only

2 unless unless this program is only ever going to work on three numbers so

ever going to work on three numbers so huh let me let me go ahead and do this

huh let me let me go ahead and do this let me first propose that there's a poor

let me first propose that there's a poor design here in my main function what

design here in my main function what value have I repeated

twice among the highlighted lines what jumps out at use twice L of the array

jumps out at use twice L of the array yeah the length of the array is just

yeah the length of the array is just three now it's not a huge deal that I

three now it's not a huge deal that I type the number three on line eight on

type the number three on line eight on line nine but this is exactly the kind

line nine but this is exactly the kind of like shortcut that's going to get you

of like shortcut that's going to get you in trouble eventually why because

in trouble eventually why because eventually you or someone else is going

eventually you or someone else is going to go in make the array bigger or

to go in make the array bigger or smaller and you're not going to realize

smaller and you're not going to realize that magically that same number is in

that magically that same number is in two places and indeed this is what a

two places and indeed this is what a programmer would often call a magic

programmer would often call a magic number a magic number is one that just

number a magic number is one that just kind of appears magically and you're on

kind of appears magically and you're on the honor System to change it here if

the honor System to change it here if you change it here and then you change

you change it here and then you change it over here like that's not going to

it over here like that's not going to end well if it the onus is on the

end well if it the onus is on the programmer to remember where they

programmer to remember where they hardcoded that is rote out three

hardcoded that is rote out three explicitly so anytime you reuse a value

explicitly so anytime you reuse a value like this you know what we should

like this you know what we should probably do what we did last week which

probably do what we did last week which was to declare a variable uh perhaps at

was to declare a variable uh perhaps at the very top of my program so it's super

the very top of my program so it's super obvious what it is called maybe n and

obvious what it is called maybe n and set that equal to three better yet what

set that equal to three better yet what did I do last week to make sure that I

did I do last week to make sure that I Can't Screw Up and accidentally change

Can't Screw Up and accidentally change that value yeah constant and the keyword

that value yeah constant and the keyword there was just const for short and now I

there was just const for short and now I have a global variable Global in the

have a global variable Global in the sense that I can access it anywhere that

sense that I can access it anywhere that is called n it's an INT and it's always

is called n it's an INT and it's always going to be three and now I can improve

going to be three and now I can improve my main function a little bit by just

my main function a little bit by just changing the threes to n so now if I if

changing the threes to n so now if I if a colleague realize oh wait a minute

a colleague realize oh wait a minute there's four tests this year you change

there's four tests this year you change n to four recompile the code and it just

n to four recompile the code and it just works everywhere else except in my

works everywhere else except in my average function let me change it back

average function let me change it back to three just for consistency this is

to three just for consistency this is not going to fly now to just uh sum up

not going to fly now to just uh sum up things like this for instance and then

things like this for instance and then return this divided three why will this

return this divided three why will this not work

not work now as I've defined

now as I've defined it

yeah okay I might be in returning an integer value when I intend to return a

integer value when I intend to return a float per this but I think I'm okay CU I

float per this but I think I'm okay CU I Ed that little trick where I made sure

Ed that little trick where I made sure that at least one of the numbers in my

that at least one of the numbers in my my arithmetic expression is in fact a

my arithmetic expression is in fact a floating point value and just by adding

floating point value and just by adding the point zero make sure that everything

the point zero make sure that everything gets treated as a float so I think

gets treated as a float so I think that's

that's okay sorry a little

ladder exactly so left hand's not talking to the right hand here and that

talking to the right hand here and that my current implementation of average is

my current implementation of average is still assuming that there's only going

still assuming that there's only going to be three tests or whatever but wait a

to be three tests or whatever but wait a minute I just went through the trouble

minute I just went through the trouble of modifying this to be n generically

of modifying this to be n generically and if I change this to four I'm not

and if I change this to four I'm not going to be happy perhaps with my

going to be happy perhaps with my average because now I'm going to ignore

average because now I'm going to ignore one of my test scores Al together so let

one of my test scores Al together so let me change this back to three and

me change this back to three and unfortunately if it's a variable now n

unfortunately if it's a variable now n and therefore I have literally a

and therefore I have literally a variable number of scores how do I take

variable number of scores how do I take the average of a variable number of

the average of a variable number of things I mean what's my building block

things I mean what's my building block there

yeah why don't I use a loop that goes through the array and adds things up as

through the array and adds things up as you go I mean kind of like grade school

you go I mean kind of like grade school as you take the average on your

as you take the average on your calculator or paper pencil you just keep

calculator or paper pencil you just keep adding the numbers together and then you

adding the numbers together and then you divide at the End by the total number of

divide at the End by the total number of things so how can I do this well let me

things so how can I do this well let me change my implementation of average to

change my implementation of average to First declare a a variable called sum or

First declare a a variable called sum or whatever set it equal to zero so this is

whatever set it equal to zero so this is like me on my piece of paper getting

like me on my piece of paper getting ready to count or my calculator of

ready to count or my calculator of course when you turn it on typically

course when you turn it on typically defaults to zero and now let me do four

defaults to zero and now let me do four in I equals z i is less than a VAR I no

in I equals z i is less than a VAR I no I didn't do that n i is less than n

I didn't do that n i is less than n i++ and now in here let me go ahead and

i++ and now in here let me go ahead and add to the current sum uh whatever is in

add to the current sum uh whatever is in the arrays location I and then down here

the arrays location I and then down here I think I can just return sum divided by

I think I can just return sum divided by 3 nope not 3.0 n perhaps here and

3 nope not 3.0 n perhaps here and actually I think I'm going to get let's

actually I think I'm going to get let's make sure it's a float let's use the the

make sure it's a float let's use the the type casting trick just to make sure I

type casting trick just to make sure I don't accidentally Short change someone

don't accidentally Short change someone and uh throw away everything after the

and uh throw away everything after the decimal point so it just escalated

decimal point so it just escalated quickly right like average just got a

quickly right like average just got a lot more involved it's just not just a

lot more involved it's just not just a single one line of code but now it's

single one line of code but now it's Dynamic I initialize a variable called

Dynamic I initialize a variable called sum to zero in this loop I go through

sum to zero in this loop I go through and just keep adding to sum which is

and just keep adding to sum which is initially zero whatever is in Array

initially zero whatever is in Array bracket I or specifically array bracket

bracket I or specifically array bracket zero array bracket one array bracket two

zero array bracket one array bracket two that gives me a total sum that I return

that gives me a total sum that I return divided by the total number of things

divided by the total number of things now this I can tighten slightly recall

now this I can tighten slightly recall that this is syntactic sugar for just

that this is syntactic sugar for just adding things I can't use plus plus

adding things I can't use plus plus because that only literally adds one but

because that only literally adds one but I can use here plus

I can use here plus equals questions on this implementation

equals questions on this implementation here really the only takeaway or the

here really the only takeaway or the most important takeaway is that this is

most important takeaway is that this is the Syntax for how you tell a function

the Syntax for how you tell a function that to it expects a whole array not a

that to it expects a whole array not a single variable like an INT or the like

single variable like an INT or the like you literally use square brackets but

you literally use square brackets but you don't specify the length inside

you don't specify the length inside there

top good question what do I have it defined as at the top this variable n it

defined as at the top this variable n it must be an integer if you're going to uh

must be an integer if you're going to uh use it inside of in arrays square

use it inside of in arrays square brackets here so this line 10 notice no

brackets here so this line 10 notice no longer says three it says n and so

longer says three it says n and so whatever n is three or four or something

whatever n is three or four or something else that's how many integers I will get

else that's how many integers I will get in that array and it must be by

in that array and it must be by definition of an array an integer that

definition of an array an integer that goes in those square brackets and here's

goes in those square brackets and here's a common source of confusion when you

a common source of confusion when you create the array that is declare it you

create the array that is declare it you use square brackets like this where you

use square brackets like this where you put the total number of elements you

put the total number of elements you want when you subsequently use the array

want when you subsequently use the array like I'm doing here you don't mention

like I'm doing here you don't mention int again just like you don't mention in

int again just like you don't mention in again and again once a variable exists

again and again once a variable exists you use the square bracket still but you

you use the square bracket still but you don't use n you use zero or one or two

don't use n you use zero or one or two or generically here I so when C was

or generically here I so when C was designed they sometimes use the same

designed they sometimes use the same Syntax for two different ideas or

Syntax for two different ideas or contexts

yeah good question do I have to include line six short answer yes because of the

line six short answer yes because of the reason we ran into last week C or clang

reason we ran into last week C or clang really reads your code top to bottom

really reads your code top to bottom left to right and so if the compiler

left to right and so if the compiler sees some mention of this function

sees some mention of this function average on line 16 but you haven't told

average on line 16 but you haven't told the compiler that average exists you're

the compiler that average exists you're going to get an error on the screen so

going to get an error on the screen so the conventional way to do that is you

the conventional way to do that is you just copy paste the one first line of

just copy paste the one first line of code from the function it's so-called

code from the function it's so-called prototype or Declaration

yeah really good question uh in a perfect segue is there a library you can

perfect segue is there a library you can use if you don't know the size of the

use if you don't know the size of the array no and so if any of you have

array no and so if any of you have programmed in uh in uh Java or python or

programmed in uh in uh Java or python or other Lang languages you can actually

other Lang languages you can actually just ask the array like how big is it in

just ask the array like how big is it in C you and I the programmers have to

C you and I the programmers have to remember it and so short answer no

remember it and so short answer no there's no function that will just

there's no function that will just automatically do this for us and in fact

automatically do this for us and in fact let me make a more subtle claim that

let me make a more subtle claim that it's fine to use Global variables like

it's fine to use Global variables like this if they're really for configuration

this if they're really for configuration options why it's just convenient to put

options why it's just convenient to put them at the very top of the file because

them at the very top of the file because everyone you your colleagues your Tas

everyone you your colleagues your Tas are going to see them at the top of the

are going to see them at the top of the code but you really shouldn't be using

code but you really shouldn't be using them everywhere throughout your code

them everywhere throughout your code it'd be better if the average function

it'd be better if the average function itself we're independent of that special

itself we're independent of that special variable so by that I mean this you know

variable so by that I mean this you know what I should really do if I really want

what I should really do if I really want to be welld designed I should pass in

to be welld designed I should pass in the length of the array to the average

the length of the array to the average function I should give the average

function I should give the average function a second argument I'll call it

function a second argument I'll call it length for instance but I could call it

length for instance but I could call it anything I want and so rather than

anything I want and so rather than putting n all the way down here at the

putting n all the way down here at the bottom of my file let me just

bottom of my file let me just dynamically say length instead and this

dynamically say length instead and this is a subtlety and no need need to get

is a subtlety and no need need to get too tripped up over this but this now is

too tripped up over this but this now is just an example of how the same function

just an example of how the same function can take not one but two arguments but

can take not one but two arguments but indeed in C you must remember yourself

indeed in C you must remember yourself what the length of an array is you can't

what the length of an array is you can't just ask the array via some syntax like

just ask the array via some syntax like you can those of you who've programmed

you can those of you who've programmed before in Java or python

yeah good question would it be better to write a function that computes the size

write a function that computes the size short answer can't do that in C as soon

short answer can't do that in C as soon as you pass an array into a function in

as you pass an array into a function in C you cannot figure out its size if it's

C you cannot figure out its size if it's a generic array like that of integers

a generic array like that of integers there are special cases that uh you can

there are special cases that uh you can do that but in general no it's just not

do that but in general no it's just not possible in see and if that's some

possible in see and if that's some frustration honestly this is why more

frustration honestly this is why more modern languages add that feature why

modern languages add that feature why because it was really annoying um as I'm

because it was really annoying um as I'm alluding here to not having that

alluding here to not having that information now just to make sure I

information now just to make sure I didn't screw up anywhere let me compile

didn't screw up anywhere let me compile this final version of

this final version of scores

scores suspense all good/ scores 72 73 33 and

suspense all good/ scores 72 73 33 and we're still back in business so this

we're still back in business so this version is more complicated and as

version is more complicated and as always we'll have this version on the

always we'll have this version on the course's website for reference but the

course's website for reference but the point really is that arrays not only can

point really is that arrays not only can be used as containers to store multiple

be used as containers to store multiple values three or more in this case um you

values three or more in this case um you can also even pass them around as

can also even pass them around as arguments as such all right now besides

arguments as such all right now besides that let's let's simplify for just a

that let's let's simplify for just a moment and consider now the world of

moment and consider now the world of chars if we've just got single bites

chars if we've just got single bites where uh where does this lead us and how

where uh where does this lead us and how does this get us ultimately to strings

does this get us ultimately to strings to solve problems like readability and

to solve problems like readability and cryptography and the like well here for

cryptography and the like well here for instance are three lines of code out of

instance are three lines of code out of context that simply store three chars

context that simply store three chars and you can already see where this is

and you can already see where this is going having three variables called C1

going having three variables called C1 C2 C3 is clearly going to end up being

C2 C3 is clearly going to end up being bad design because of all the silly

bad design because of all the silly redundancy here but notice I'm using

redundancy here but notice I'm using single quotes like last week because

single quotes like last week because these are single chars what does this

these are single chars what does this look like in the computer's memory well

look like in the computer's memory well it looks a little something like this if

it looks a little something like this if we clear out the old memory C1 C2 C3

we clear out the old memory C1 C2 C3 probably will end up here maybe not

probably will end up here maybe not literally in the top leftand Corner this

literally in the top leftand Corner this is just an artist rendition but C1 C2 C3

is just an artist rendition but C1 C2 C3 will probably end up like that now

will probably end up like that now what's really there it's really those

what's really there it's really those same three numbers 72 73 33 but how many

same three numbers 72 73 33 but how many bits does a bite

bits does a bite have just eight so if we were to look at

have just eight so if we were to look at the binary representation of these

the binary representation of these characters it would only be eight bits

characters it would only be eight bits each that's enough to store small

each that's enough to store small numbers like 72 73 33 we're not dealing

numbers like 72 73 33 we're not dealing with Unicode and emoji and the like but

with Unicode and emoji and the like but the point is the same you don't have to

the point is the same you don't have to use four bytes to store these numbers

use four bytes to store these numbers you can use a different data type like

you can use a different data type like chars and underneath the hood it's

chars and underneath the hood it's indeed going to use just single bytes

indeed going to use just single bytes for each but this is sort of like a this

for each but this is sort of like a this isn't really how we Implement strings

isn't really how we Implement strings right when you wanted to say hi last

right when you wanted to say hi last week or this we use double quotes and we

week or this we use double quotes and we wrote all of the things together and

wrote all of the things together and used one variable not three right when I

used one variable not three right when I typed in David I didn't have a variable

typed in David I didn't have a variable for d a v i d i had one variable called

for d a v i d i had one variable called name that stored the whole thing so in C

name that stored the whole thing so in C we keep talking about these things

we keep talking about these things called strings we'll see eventually that

called strings we'll see eventually that strings are not necessarily what they

strings are not necessarily what they seem to be but for now the key thing

seem to be but for now the key thing about strings is that they're variable

about strings is that they're variable length uh so to speak right they might

length uh so to speak right they might be three characters high or five

be three characters high or five characters David or anything uh smaller

characters David or anything uh smaller or larger so how do we go about

or larger so how do we go about implementing strings if all we have at

implementing strings if all we have at the end of the day is my memory well

the end of the day is my memory well here is an example of just creating uh

here is an example of just creating uh declaring and defining a string called s

declaring and defining a string called s s because it's just a simple string and

s because it's just a simple string and quote unquote high in double quotes what

quote unquote high in double quotes what does this look like in the computer's

does this look like in the computer's memory well let's clear it again and

memory well let's clear it again and here now because it's technically stored

here now because it's technically stored in one variable s here is how I might

in one variable s here is how I might draw it as an artist it's three bytes in

draw it as an artist it's three bytes in total H exclamation point but there's no

total H exclamation point but there's no C1 C2 C3 it's just the whole thing is uh

C1 C2 C3 it's just the whole thing is uh s but it turns out that a string fun

s but it turns out that a string fun fact is really just what underneath the

fact is really just what underneath the hood kind of leading up to this what is

hood kind of leading up to this what is a string if this is how it's laid out in

a string if this is how it's laid out in memory literally it's just an array of

memory literally it's just an array of characters and we didn't have to know

characters and we didn't have to know about arrays last week to use strings

about arrays last week to use strings this is where again the training wheels

this is where again the training wheels are starting to come off but a string is

are starting to come off but a string is just an array of characters Hi

just an array of characters Hi exclamation point for instance so

exclamation point for instance so technically an array a string called s

technically an array a string called s is really a variable called s that

is really a variable called s that allows you to get at the first character

allows you to get at the first character with s bracket zero if you want s

with s bracket zero if you want s bracket 1 s bracket2 you can literally

bracket 1 s bracket2 you can literally get individual characters just by

get individual characters just by treating S as though it's an array which

treating S as though it's an array which it really is underneath the hood in this

it really is underneath the hood in this case but there's a catch how do you know

case but there's a catch how do you know where strings end in the past when I

where strings end in the past when I drew some integers on the screen I know

drew some integers on the screen I know I claim they always take up four bytes

I claim they always take up four bytes if I had drawn a long it always takes up

if I had drawn a long it always takes up eight bytes if I drawn a a character it

eight bytes if I drawn a a character it always takes up one bite but how many

always takes up one bite but how many bites does a string take up yeah I mean

bites does a string take up yeah I mean that's kind of the right answer in this

that's kind of the right answer in this case three it would seem but if it's

case three it would seem but if it's David that's a good five characters but

David that's a good five characters but where do we put the number three where

where do we put the number three where do you put the number five right this is

do you put the number five right this is literally all that's into your computer

literally all that's into your computer this is all our building blocks in front

this is all our building blocks in front of us so how can we where does the three

of us so how can we where does the three go where does the five go well it turns

go where does the five go well it turns out you can solve this in a couple of

out you can solve this in a couple of different ways but the way humans

different ways but the way humans decided to implement strings years ago

decided to implement strings years ago is indeed an array but they added one

is indeed an array but they added one extra bite at the end of every such

extra bite at the end of every such string array just to make clear with a

string array just to make clear with a so-called Sentinel value that the string

so-called Sentinel value that the string ends here why so that if you have two

ends here why so that if you have two strings in the computer's memory like hi

strings in the computer's memory like hi and by you know where the barrier is

and by you know where the barrier is between like the exclamation point of

between like the exclamation point of one and the letter B in the next right

one and the letter B in the next right you need some kind of delimiter and so

you need some kind of delimiter and so what really is underneath the hood is

what really is underneath the hood is this when you store a string in memory

this when you store a string in memory when you type in a string as the user if

when you type in a string as the user if you type in three characters it's it's

you type in three characters it's it's going to use 3 + 1 = 4 bytes in total if

going to use 3 + 1 = 4 bytes in total if you type in David it's going to use 5 +

you type in David it's going to use 5 + 1al 6 bytes in total why because C

1al 6 bytes in total why because C automatically adds this special zero at

automatically adds this special zero at the end of the string I've drawn it with

the end of the string I've drawn it with back SL Zer because this is how you

back SL Zer because this is how you represent zero as a Char as a character

represent zero as a Char as a character but this is literally just zero as we'll

but this is literally just zero as we'll soon see so anytime there's a string in

soon see so anytime there's a string in memory it always takes up one more bite

memory it always takes up one more bite than you yourself as the programmer or

than you yourself as the programmer or human typed in in fact if we convert

human typed in in fact if we convert this again just for discussion sake to

this again just for discussion sake to those integers what's literally stored

those integers what's literally stored in the computer's memory is going to be

in the computer's memory is going to be 72 73 33 and now a zero and the computer

72 73 33 and now a zero and the computer because of c and how it was invented

because of c and how it was invented it's just smart enough to know that when

it's just smart enough to know that when you print out a string it prints out

you print out a string it prints out every character until it sees a zero and

every character until it sees a zero and then it just stops printing in

then it just stops printing in particular printf knows how this works

particular printf knows how this works and this is why print F knows when to

and this is why print F knows when to stop printing decimal numbers are not

stop printing decimal numbers are not that enlightening we'll generally write

that enlightening we'll generally write the characters like this and again back

the characters like this and again back back sl0 is just special symbology like

back sl0 is just special symbology like it's what the programmer types to make

it's what the programmer types to make clear that you're not saying hi zero

clear that you're not saying hi zero you're saying hi and then it's a special

you're saying hi and then it's a special zero specifically it is eight zero bits

zero specifically it is eight zero bits that indicate that it's the end of the

that indicate that it's the end of the string technically that back sl0 if you

string technically that back sl0 if you want to be fancy it's called null

want to be fancy it's called null NL and it turns out you've seen this

NL and it turns out you've seen this before that we didn't call it out here's

before that we didn't call it out here's that same asky chart from the past

that same asky chart from the past couple of weeks if I highlight this what

couple of weeks if I highlight this what is decimal number zero mapping to n

is decimal number zero mapping to n which is just programmer speak for the

which is just programmer speak for the special null character all zero bits

special null character all zero bits that means the string ends here this all

that means the string ends here this all happens automatically for you you do not

happens automatically for you you do not need to create these null characters or

need to create these null characters or these

these zeros any questions

zeros any questions then on this implementation thus

then on this implementation thus far any questions here no well let me do

far any questions here no well let me do this let me go back to vs code in a

this let me go back to vs code in a second and let's actually corroborate

second and let's actually corroborate this with some code let me go ahead and

this with some code let me go ahead and create a small program called high. C

create a small program called high. C and how about we do this let me include

and how about we do this let me include standard i.h let me include uh let me

standard i.h let me include uh let me type out int main void as always and now

type out int main void as always and now let me do something simple and kind of

let me do something simple and kind of bad but Char C1 equals quote unquote H

bad but Char C1 equals quote unquote H in single quotes Char C2 equals quote

in single quotes Char C2 equals quote unquote I in single quotes and lastly

unquote I in single quotes and lastly Char C3 equals exclamation point in

Char C3 equals exclamation point in single quotes and now let me just print

single quotes and now let me just print this out I can't use percent s cuz that

this out I can't use percent s cuz that is not a string that's literally three

is not a string that's literally three chars cuz that's the design decision I

chars cuz that's the design decision I made but I could do this percent C

made but I could do this percent C percent C percent C which we haven't

percent C percent C which we haven't seen before but percent s is string

seen before but percent s is string percent I is int percent C is indeed uh

percent I is int percent C is indeed uh Char so let me put a back sln at the end

Char so let me put a back sln at the end for cleanliness and now do C1 C2 C3 so

for cleanliness and now do C1 C2 C3 so this is like a Char based version of

this is like a Char based version of printing string so let me make high and

printing string so let me make high and then let me do/ high and it looks like I

then let me do/ high and it looks like I use print f with percent s but I did

use print f with percent s but I did things very manually by printing out

things very manually by printing out each individual character what's cool

each individual character what's cool now though is that once you know that

now though is that once you know that characters are just numbers and strings

characters are just numbers and strings are just characters you can kind of poke

are just characters you can kind of poke around let me change all three

around let me change all three placeholders to percent i instead and

placeholders to percent i instead and this is totally fine too let me rerun

this is totally fine too let me rerun this make

this make High um actually let me make one change

High um actually let me make one change just so we can see this let me add

just so we can see this let me add spaces just for Aesthetics sake let me

spaces just for Aesthetics sake let me do make high do SL High enter and voila

do make high do SL High enter and voila like now you can actually see the

like now you can actually see the numbers that I claimed back in week zero

numbers that I claimed back in week zero were in fact happening underneath the

were in fact happening underneath the hood well this is not how you would make

hood well this is not how you would make strings it'd be incredibly tedious to

strings it'd be incredibly tedious to have three variables for threel Words

have three variables for threel Words five variables for five letter words

five variables for five letter words we've been using of course strings since

we've been using of course strings since last week so let's do that instead uh

last week so let's do that instead uh string uh s equals quote unquote double

string uh s equals quote unquote double quotes high for this no because of these

quotes high for this no because of these training wheels I need to include the

training wheels I need to include the cs50 library but we'll come back to that

cs50 library but we'll come back to that in the coming week but for now I'm going

in the coming week but for now I'm going to go ahead and create a string s called

to go ahead and create a string s called quote unquote high and now I'm going to

quote unquote high and now I'm going to change this to be my familiar percent s

change this to be my familiar percent s and now just print out s itself this of

and now just print out s itself this of course is the same thing as last week

course is the same thing as last week high gives me the exact same thing but

high gives me the exact same thing but now we're dealing of course with strings

now we're dealing of course with strings but how can we see a little beyond that

but how can we see a little beyond that well how about this let's poke around

well how about this let's poke around further with today's Primitives even

further with today's Primitives even though s is a string I could technically

though s is a string I could technically print out its first character with

print out its first character with percent C by doing s bracket z i could

percent C by doing s bracket z i could technically print out its second

technically print out its second character with percent C by doing s

character with percent C by doing s bracket one I could print out its third

bracket one I could print out its third character with percent C and printing

character with percent C and printing out s bracket 2 so again this just

out s bracket 2 so again this just derives logically from my understanding

derives logically from my understanding now of that strings or arrays as you

now of that strings or arrays as you note let me do make let me do make high

note let me do make let me do make high SL high and no visual change but I'm

SL high and no visual change but I'm just kind of now tinkering around and in

just kind of now tinkering around and in fact if you're really curious let me do

fact if you're really curious let me do this let me change these back to I back

this let me change these back to I back to I I oops back to I and let me add a

to I I oops back to I and let me add a fourth one because if I'm really curious

fourth one because if I'm really curious now let's see it what's S braet 3 this

now let's see it what's S braet 3 this is the fourth bite and even though the

is the fourth bite and even though the string itself is hi I think we can

string itself is hi I think we can corroborate this whole null thing make

corroborate this whole null thing make high SL High enter and there it is you

high SL High enter and there it is you could have done this last week if you

could have done this last week if you really wanted to geek out on strings but

really wanted to geek out on strings but like for now it's just revealing what's

like for now it's just revealing what's going on underneath the hood questions

going on underneath the hood questions then on what these strings are

bracket uh uh why do you not need brackets good question why do I not need

brackets good question why do I not need brackets on line six uh to because uh s

brackets on line six uh to because uh s is a string we'll see in a couple of

is a string we'll see in a couple of weeks that s is essentially implemented

weeks that s is essentially implemented underneath the hood indeed as an array

underneath the hood indeed as an array but that happens automatically for you

but that happens automatically for you you can treat S as just a variable name

you can treat S as just a variable name without square brackets you will use

without square brackets you will use square brackets when you have arrays of

square brackets when you have arrays of ins or you manually create arrays of

ins or you manually create arrays of chars or doubles or Floats or anything

chars or doubles or Floats or anything else but strings are special why I mean

else but strings are special why I mean every program you write seems to use

every program you write seems to use strings text in some form we're humans

strings text in some form we're humans we like text not just numbers and and

we like text not just numbers and and such so this is just treated a little

such so this is just treated a little specially in C and many other languages

specially in C and many other languages as

as well other questions on this here no

well other questions on this here no let's add then one other string to the

let's add then one other string to the mix so instead of just saying hi why

mix so instead of just saying hi why don't we consider version of the program

don't we consider version of the program that says both high and by and I claim

that says both high and by and I claim now that that back sl0 that null

now that that back sl0 that null character is going to be ever more

character is going to be ever more important now if we've got two strings

important now if we've got two strings in memory so that c knows how to

in memory so that c knows how to distinguish one from the other so let me

distinguish one from the other so let me go ahead and just get rid of these two

go ahead and just get rid of these two lines for the moment let me recreate

lines for the moment let me recreate string s equals quote unquote double

string s equals quote unquote double quotes High Let me give myself another

quotes High Let me give myself another one and because I'm just playing around

one and because I'm just playing around I'll choose very short variable names

I'll choose very short variable names string T equals quote unquote by

string T equals quote unquote by exclamation point and then let me just

exclamation point and then let me just print them both out uh let me go ahead

print them both out uh let me go ahead and print out percent s back sln comma s

and print out percent s back sln comma s and then print F percent s uh back sln

and then print F percent s uh back sln and then T so very simple demonstration

and then T so very simple demonstration of just these two variables make high do

of just these two variables make high do SL high and of course it prints out two

SL high and of course it prints out two lines one after the other what's

lines one after the other what's actually going on underneath the hood

actually going on underneath the hood well let's go back to the computer's

well let's go back to the computer's memory High I think it's going to be I

memory High I think it's going to be I claim pretty much the same so s I'll

claim pretty much the same so s I'll claim is in the top left followed by the

claim is in the top left followed by the back sl0 and that's important now

back sl0 and that's important now because by probably is going to end up

because by probably is going to end up there and Visually it wraps just by

there and Visually it wraps just by nature of how I've drawn this grid of

nature of how I've drawn this grid of bytes but it's continuous B ye

bytes but it's continuous B ye exclamation point null AKA back0 this is

exclamation point null AKA back0 this is Now helpful to print F because now

Now helpful to print F because now printf knows where one begins and ends

printf knows where one begins and ends by way of that special null character

by way of that special null character but we can poke around now too what else

but we can poke around now too what else can I do here how about this how about I

can I do here how about this how about I go into my code here back to BS code and

go into my code here back to BS code and let me go ahead and say something like

let me go ahead and say something like well if I've got two of these uh strings

well if I've got two of these uh strings you know let's put them in an array

you know let's put them in an array let's kind of do this sort of arrays in

let's kind of do this sort of arrays in arrays sort of inception style here so

arrays sort of inception style here so string words bracket two so give me an

string words bracket two so give me an array of two strings is what I'm saying

array of two strings is what I'm saying here in code even though we've not done

here in code even though we've not done it with strings yet we only did it with

it with strings yet we only did it with ins and now let me do this the first

ins and now let me do this the first word AKA words bracket zero will equal

word AKA words bracket zero will equal as before high and now words bracket one

as before high and now words bracket one will equal quote unquote by bite so by

will equal quote unquote by bite so by and now I've done the exact same thing

and now I've done the exact same thing but again I'm just avoiding having s t

but again I'm just avoiding having s t QR and like all these different

QR and like all these different variables in my code I just now I'm

variables in my code I just now I'm treating them as one single array of

treating them as one single array of strings how do I change my code down

strings how do I change my code down here well if I want to print the first

here well if I want to print the first word I do words bracket zero and if I

word I do words bracket zero and if I want to print the second word I do words

want to print the second word I do words bracket one this is not a useful

bracket one this is not a useful exercise at the moment because I'm just

exercise at the moment because I'm just making my code more complicated but

making my code more complicated but again it allows us to post poke around

again it allows us to post poke around and see what's going on because there is

and see what's going on because there is that high and by but watch this if I

that high and by but watch this if I really want to be cool I can do this

really want to be cool I can do this let's print out percent C percent C

let's print out percent C percent C percent C back sln and then here percent

percent C back sln and then here percent C percent C percent C percent C so four

C percent C percent C percent C so four of those and now here's where things get

of those and now here's where things get interesting words is an array of strings

interesting words is an array of strings but again if I may what's a

but again if I may what's a string an array of characters so just

string an array of characters so just use the same logic if words is an array

use the same logic if words is an array of strings you get at the first string

of strings you get at the first string with words bracket zero how do you get

with words bracket zero how do you get at the first character in the first

at the first character in the first string bracket Zer words bracket Z

string bracket Zer words bracket Z bracket one and lastly words bracket Z

bracket one and lastly words bracket Z bracket two and now down here words

bracket two and now down here words bracket one but the first character is

bracket one but the first character is there words bracket one the second

there words bracket one the second character is here words bracket one the

character is here words bracket one the third character is here whoops third

third character is here whoops third character is here and words bracket one

character is here and words bracket one one the fourth character is here like

one the fourth character is here like this is not how people program this is

this is not how people program this is only for demonstration sake my God it's

only for demonstration sake my God it's so tedious and verbose already but if I

so tedious and verbose already but if I make high now do SL high now I'm like

make high now do SL high now I'm like manually Reinventing percent s if I

manually Reinventing percent s if I forgot it existed using percent C alone

forgot it existed using percent C alone but you can indeed manipulate arrays in

but you can indeed manipulate arrays in this way but because strings are arrays

this way but because strings are arrays of characters you can manipulate strings

of characters you can manipulate strings uh in this way too any question now on

uh in this way too any question now on this syntax

this syntax any questions here no no all right well

any questions here no no all right well let's go ahead and propose that we solve

let's go ahead and propose that we solve a couple of other problems we might not

a couple of other problems we might not have as before but first a quick visual

have as before but first a quick visual of what's been going on underneath the

of what's been going on underneath the hood here if here again is where we left

hood here if here again is where we left off on the screen high and by uh back to

off on the screen high and by uh back to back here is really how I just treated

back here is really how I just treated these things s bracket 0123 and then t0

these things s bracket 0123 and then t0 1 2 3 4 but really once I put them in an

1 2 3 4 but really once I put them in an array the picture becomes this words Z

array the picture becomes this words Z is the whole High words bracket one is

is the whole High words bracket one is the whole by but if I really get into

the whole by but if I really get into the weeds and start indexing into

the weeds and start indexing into individual characters in those strings

individual characters in those strings all I'm using is new syntax in order to

all I'm using is new syntax in order to represent these same values here

represent these same values here questions then on these

questions then on these representations before we Forge

representations before we Forge ahead no

yeah does the new line character say that one

more ah really good question does the new line character take up any space uh

new line character take up any space uh it does when so far as printf is

it does when so far as printf is concerned but I'm not storing the back

concerned but I'm not storing the back slash n in my strings printf is being

slash n in my strings printf is being manually handed that thing

manually handed that thing instead all right so let's go ahead then

instead all right so let's go ahead then and consider how we might solve some

and consider how we might solve some problems that have Arisen now with these

problems that have Arisen now with these strings as follows here suppose I let's

strings as follows here suppose I let's do this let me go back to VSS code here

do this let me go back to VSS code here and let me go ahead and open up a new

and let me go ahead and open up a new file called how about uh length uh. C

file called how about uh length uh. C and let's consider for a moment how I

and let's consider for a moment how I might actually figure out what the

might actually figure out what the length of a string is which is distinct

length of a string is which is distinct from the length of an array I claimed

from the length of an array I claimed earlier you cannot figure out

earlier you cannot figure out dynamically what the length of an array

dynamically what the length of an array is but I can figure out the length of a

is but I can figure out the length of a string specifically because of this

string specifically because of this implementation detail of that null

implementation detail of that null character so let me go ahead and do this

character so let me go ahead and do this let me include cs50.h in this second

let me include cs50.h in this second program here let me include standard i.h

program here let me include standard i.h as before and let me do this int main

as before and let me do this int main void and in the first thing I'll do is

void and in the first thing I'll do is just get a string from the user I'll ask

just get a string from the user I'll ask the user as always for their name so

the user as always for their name so I'll call get string and say what's your

I'll call get string and say what's your name question mark as always and then

name question mark as always and then down here if I want to figure out the

down here if I want to figure out the length of this string and print the name

length of this string and print the name the print the length out on the screen

the print the length out on the screen well I can kind of do this similar in

well I can kind of do this similar in spirit to the average where I'm

spirit to the average where I'm accumulating something let me go ahead

accumulating something let me go ahead and initialize n to zero let me give

and initialize n to zero let me give myself huh it's not not a for Loop

myself huh it's not not a for Loop because I don't have a I don't know in

because I don't have a I don't know in advance how long it is but what if I do

advance how long it is but what if I do this

this while the value at name bracket n does

while the value at name bracket n does not equal single quote back

not equal single quote back sl0 crazy syntax at the moment but it's

sl0 crazy syntax at the moment but it's just the culmination of these various

just the culmination of these various building blocks let me just finish the

building blocks let me just finish the thought here

thought here n++ and then down here let's just print

n++ and then down here let's just print out with print F and percent I that

out with print F and percent I that value of n

value of n so I claim this is going to show me the

so I claim this is going to show me the length of any string I type in whether

length of any string I type in whether it's high or by or David or anything

it's high or by or David or anything else I initialize a variable to zero and

else I initialize a variable to zero and that's good because that's where you

that's good because that's where you start counting in general while name

start counting in general while name bracket 0 does not equal back0 what is

bracket 0 does not equal back0 what is this saying well if name is the string

this saying well if name is the string the user typed in and name is just an

the user typed in and name is just an array as you noted then name bracket

array as you noted then name bracket zero is going to be the first character

zero is going to be the first character and I'm asking the question well does

and I'm asking the question well does the first character not equal back0 and

the first character not equal back0 and if I type in David D is it's not so I

if I type in David D is it's not so I keep going and I add one to n then I'm

keep going and I add one to n then I'm going to check name bracket one well if

going to check name bracket one well if I typed in David name bracket one is

I typed in David name bracket one is going to be a a does not equal back0 and

going to be a a does not equal back0 and so it's going to go again and again and

so it's going to go again and again and again but five steps in total later it's

again but five steps in total later it's going to get to the bite after David

going to get to the bite after David realize wait a minute that is a back

realize wait a minute that is a back slash n the Loop finishes and I print

slash n the Loop finishes and I print out the total length arrays in general

out the total length arrays in general do not have this null character however

do not have this null character however strings do again strings are special

strings do again strings are special versus all of the other data types we've

versus all of the other data types we've talked about thus far but how could I

talked about thus far but how could I for instance uh do this differently well

for instance uh do this differently well let's actually Factor this out as a

let's actually Factor this out as a function as I've I've commonly done but

function as I've I've commonly done but rather than implement it myself you know

rather than implement it myself you know what it turns out what's nice about

what it turns out what's nice about strings being so common there are many

strings being so common there are many other people who have solved these

other people who have solved these problems before and in fact there's a

problems before and in fact there's a whole string library in C it is used by

whole string library in C it is used by way of a header file called string.h and

way of a header file called string.h and what string H is is a library of string

what string H is is a library of string related functions in fact you can see in

related functions in fact you can see in cs50's manual pages for C uh the

cs50's manual pages for C uh the string.h functions at least those that

string.h functions at least those that we recommend as most useful and in

we recommend as most useful and in particular if you poke around there

particular if you poke around there you'll see that there's a function

you'll see that there's a function called Sterling means string length it

called Sterling means string length it was named very succinctly just because

was named very succinctly just because it's a little easier to type than string

it's a little easier to type than string length but Sterling tells you the length

length but Sterling tells you the length of a string so how might I use this in

of a string so how might I use this in my code here well it turns out I can

my code here well it turns out I can simplify this quite a bit let me get rid

simplify this quite a bit let me get rid of my Loop get rid of my uh my counting

of my Loop get rid of my uh my counting manually and do something like this int

manually and do something like this int n equals Sterling of the human's name

n equals Sterling of the human's name name and now I'll just use print f as

name and now I'll just use print f as before with percent I back sln and

before with percent I back sln and output the value of n but but there's a

output the value of n but but there's a bug at the moment what have I forgotten

bug at the moment what have I forgotten to

to do yeah I have to include the header

do yeah I have to include the header file at the top of the screen so let me

file at the top of the screen so let me at the top of the code so let me also

at the top of the code so let me also include string.h at the top of my file

include string.h at the top of my file so that c knows that in fact sterl

so that c knows that in fact sterl exists let me go ahead and make length

exists let me go ahead and make length as

as before length uh or actually really for

before length uh or actually really for the first time what's your name David

the first time what's your name David and hopefully I'm going to see in fact

and hopefully I'm going to see in fact five by contrast if I run it again and

five by contrast if I run it again and type in high exclamation point now I see

type in high exclamation point now I see three so Sterling is just one of the

three so Sterling is just one of the functions in that library and there's so

functions in that library and there's so many more in fact yet another library

many more in fact yet another library that might be useful moving forward is

that might be useful moving forward is this one C

this one C which relates to C data types and lots

which relates to C data types and lots of functions therein that can be useful

of functions therein that can be useful for instance if you review its

for instance if you review its documentation in the manual pages online

documentation in the manual pages online you'll see that there are functions via

you'll see that there are functions via which we can solve problems like this

which we can solve problems like this let me go ahead and propose here let me

let me go ahead and propose here let me see let's do um example here

see let's do um example here involving how about uh checking if

involving how about uh checking if something is uppercase or lowercase and

something is uppercase or lowercase and converting it uh to uppercase only let

converting it uh to uppercase only let me go back to vs code and code a program

me go back to vs code and code a program called uppercase Doc in this file I'm

called uppercase Doc in this file I'm going to start by including now as

going to start by including now as always

always cs50.h I'm going to include standard i.h

cs50.h I'm going to include standard i.h and I'm going to add one other to the

and I'm going to add one other to the mix which is

mix which is string.h now2 so I can access the length

string.h now2 so I can access the length of things as needed int main void comes

of things as needed int main void comes next and then within my main function

next and then within my main function I'm going to go ahead and declare a

I'm going to go ahead and declare a string called s I'm going to call get

string called s I'm going to call get string as before and I'm going to go

string as before and I'm going to go head and just ask the user for a string

head and just ask the user for a string called before I want to do it before and

called before I want to do it before and after whatever the user types in is

after whatever the user types in is before but I want to force everything to

before but I want to force everything to uppercase thereafter let me now in this

uppercase thereafter let me now in this Loop here do this let me print F quote

Loop here do this let me print F quote unquote after just so we can see this on

unquote after just so we can see this on the screen and let me do four in I gets

the screen and let me do four in I gets zero I is less than stir Lang of s I ++

zero I is less than stir Lang of s I ++ what am I about to do I'm about to

what am I about to do I'm about to iterate over every every character in

iterate over every every character in the string from left to right from zero

the string from left to right from zero on up two but not through the length of

on up two but not through the length of s and how do I check if something is

s and how do I check if something is lowercase so that I can actually force

lowercase so that I can actually force it to uppercase well it turns out I

it to uppercase well it turns out I could do this literally if the character

could do this literally if the character in s at location I is greater than or

in s at location I is greater than or equal to Capital a Ampersand Ampersand

equal to Capital a Ampersand Ampersand which means and instead of or which we

which means and instead of or which we saw in the past s braet i is less than

saw in the past s braet i is less than or equal to little Z that means

or equal to little Z that means logically in English that this is indeed

logically in English that this is indeed a lowercase how do I now convert it to

a lowercase how do I now convert it to uppercase this character well I could

uppercase this character well I could just literally print out the same

just literally print out the same character but that would not be the

character but that would not be the answer here because that's not changing

answer here because that's not changing the value but what could I do instead

the value but what could I do instead well let me actually pull up here real

well let me actually pull up here real fast the asky chart as before and let's

fast the asky chart as before and let's see if we can't glean some insight if I

see if we can't glean some insight if I pull up the same ask each chart and

pull up the same ask each chart and suppose the human has typed in a

suppose the human has typed in a lowercase a that's

lowercase a that's 97 what letter I want to convert it to

97 what letter I want to convert it to uppercase a so what number do I want to

uppercase a so what number do I want to convert the 97 to per week zero so 65 we

convert the 97 to per week zero so 65 we keep coming back to that one what if the

keep coming back to that one what if the user types in lowercase B I want to

user types in lowercase B I want to change the 98 value to 66 and so forth

change the 98 value to 66 and so forth and any quick math how far part of those

and any quick math how far part of those so it's always 32 like uppercase to

so it's always 32 like uppercase to lower case is always wonderfully good

lower case is always wonderfully good design 32 away one from the other so

design 32 away one from the other so what does this mean well I think we saw

what does this mean well I think we saw earlier that underneath the hood a Char

earlier that underneath the hood a Char is just a number you can certainly do

is just a number you can certainly do arithmetic on it and here again if you

arithmetic on it and here again if you understand these lower level Primitives

understand these lower level Primitives what if I do this whatever s bracket I

what if I do this whatever s bracket I is if I know on line 13 that it's up

is if I know on line 13 that it's up that it's lowercase do I want to add or

that it's lowercase do I want to add or subtract

subtract 32 so I want to subtract because I want

32 so I want to subtract because I want to go from like 97 to 65 or 98 to 66 so

to go from like 97 to 65 or 98 to 66 so indeed if you do some quick math that

indeed if you do some quick math that gives you 32 so it suffices to just

gives you 32 so it suffices to just treat chars as numbers subtract the 32

treat chars as numbers subtract the 32 and printing it with percent C I think

and printing it with percent C I think will just convert lowercase to uppercase

will just convert lowercase to uppercase if you now fast forward to like the real

if you now fast forward to like the real world Microsoft Word or Google Docs if

world Microsoft Word or Google Docs if you've ever chosen the menu option that

you've ever chosen the menu option that forces things to uppercase or lowercase

forces things to uppercase or lowercase on occasion literally that's what

on occasion literally that's what Microsoft and Google have done they

Microsoft and Google have done they iterate over every character in the

iterate over every character in the document check if it's lowercase and if

document check if it's lowercase and if so they subtract 32 from it and show you

so they subtract 32 from it and show you the new value what if though it is not a

the new value what if though it is not a lowercase letter I think I can keep it

lowercase letter I think I can keep it easy and just print out the current

easy and just print out the current letter unchanged if my goal is to Simply

letter unchanged if my goal is to Simply Force things to all uppercase and that

Force things to all uppercase and that letter then would be S bracket I so let

letter then would be S bracket I so let me go ahead now and make uppercase

me go ahead now and make uppercase hopefully no errors do/ uppercase and

hopefully no errors do/ uppercase and I'll now now type in David with an

I'll now now type in David with an uppercase D but lowercase everything

uppercase D but lowercase everything else but now the after version is David

else but now the after version is David an aesthetic bug notice here I forgot to

an aesthetic bug notice here I forgot to include just for prettiness sake about

include just for prettiness sake about back sln at the end no problem I'll add

back sln at the end no problem I'll add that let me fix my mistake make

that let me fix my mistake make uppercase SL uppercase enter daid enter

uppercase SL uppercase enter daid enter and voila and I I deliberately added

and voila and I I deliberately added another space after the after just so

another space after the after just so they would line up pretty even though

they would line up pretty even though before and after have different numbers

before and after have different numbers of letters questions then on this

of letters questions then on this implementation of forcing something to

implementation of forcing something to uppercase which in and of itself is not

uppercase which in and of itself is not all that enlightening but is

all that enlightening but is representative now of how you can

representative now of how you can leverage these low-level

leverage these low-level Primitives question

Primitives question now all right well this honestly is

now all right well this honestly is tedious my God like does Microsoft

tedious my God like does Microsoft Google everyone you have to literally

Google everyone you have to literally write out this code just to do something

write out this code just to do something simple well no that's again why we have

simple well no that's again why we have things like libraries and increasingly

things like libraries and increasingly now for problem sets projects and Beyond

now for problem sets projects and Beyond well you just use libraries more often

well you just use libraries more often off the shelf so as to solve problems

off the shelf so as to solve problems that surely other people have had before

that surely other people have had before you so how can I now use this uh Library

you so how can I now use this uh Library ctype.h well let me go back into my code

ctype.h well let me go back into my code let me include this among my header

let me include this among my header files here I tend just so I can skim

files here I tend just so I can skim things easily I tend to alphabetize my

things easily I tend to alphabetize my headers but that's not strictly

headers but that's not strictly necessary but it allows me at a glance

necessary but it allows me at a glance to realize did I or did I not include

to realize did I or did I not include something I need now let me go ahead and

something I need now let me go ahead and do this it turns out if you read the

do this it turns out if you read the documentation for the ctype library

documentation for the ctype library there is a function wonderfully called

there is a function wonderfully called if is lower that takes in a character as

if is lower that takes in a character as its argument essentially so s bracket I

its argument essentially so s bracket I and if that returns true a Boolean value

and if that returns true a Boolean value if you will well I'm going to force it

if you will well I'm going to force it to lowercase but I don't have to do this

to lowercase but I don't have to do this this math anymore turns out in the ctype

this math anymore turns out in the ctype library there's also a function called

library there's also a function called two upper that takes a character as

two upper that takes a character as input like s bracket I and it just does

input like s bracket I and it just does the math for you so that you can

the math for you so that you can abstract away the 32 thing and just know

abstract away the 32 thing and just know that someone else has solved that

that someone else has solved that problem for you otherwise I can leave my

problem for you otherwise I can leave my code unchanged down below because I'm

code unchanged down below because I'm not changing anything else so if I do

not changing anything else so if I do make uppercase now and then do SL

make uppercase now and then do SL uppercase daavid with just a capital D

uppercase daavid with just a capital D and now it still works but if you read

and now it still works but if you read the documentation further it turns out

the documentation further it turns out that two upper is smart if you pass in a

that two upper is smart if you pass in a character to two upper that's lowercase

character to two upper that's lowercase it obviously converts its uppercase by

it obviously converts its uppercase by doing that math but if you pass in a

doing that math but if you pass in a character to to Upper that's already

character to to Upper that's already uppercase the documentation you would

uppercase the documentation you would see tells you that it leaves it

see tells you that it leaves it unchanged so I can tighten all of this

unchanged so I can tighten all of this up I can get rid of the whole else I can

up I can get rid of the whole else I can get rid of the whole if and arguably now

get rid of the whole if and arguably now Implement a program that's just as

Implement a program that's just as correct but better designed why fewer

correct but better designed why fewer lines of code easier to read Le lower

lines of code easier to read Le lower probability of mistakes assuming the

probability of mistakes assuming the library is correct it just makes it

library is correct it just makes it easier and faster for me now to write

easier and faster for me now to write code so if I now do one last time make

code so if I now do one last time make uppercase enter uppercase and type in my

uppercase enter uppercase and type in my name still working but now notice we've

name still working but now notice we've whittel this down to far fewer lines of

whittel this down to far fewer lines of code albeit using now this additional

code albeit using now this additional Library questions then on how we did

Library questions then on how we did this no well even even though this code

this no well even even though this code I dare say is correct it's not

I dare say is correct it's not necessarily welld designed just yet in

necessarily welld designed just yet in fact there's one line of code one

fact there's one line of code one function call in this current

function call in this current implementation that's more inefficient

implementation that's more inefficient than it needs to be and allow me to draw

than it needs to be and allow me to draw our attention to this here line 10

our attention to this here line 10 wherein we're calling Sterling but we're

wherein we're calling Sterling but we're calling it inside of this for Loop

calling it inside of this for Loop specifically inside of the condition and

specifically inside of the condition and why might that not necessarily be the

why might that not necessarily be the best

best idea well is the length of the string s

idea well is the length of the string s changing ever I mean certainly not

changing ever I mean certainly not within the span of this Loop and so here

within the span of this Loop and so here we are within our for loop on line 10 11

we are within our for loop on line 10 11 12 and 13 asking on every iteration that

12 and 13 asking on every iteration that same question what's the length of s

same question what's the length of s what's the length of s what's the length

what's the length of s what's the length of s and in turn we're calling Sterling

of s and in turn we're calling Sterling every time even though we're getting

every time even though we're getting back the same answer so I dare say a

back the same answer so I dare say a better solution here would be to maybe

better solution here would be to maybe figure out the length of s earlier on in

figure out the length of s earlier on in my code and maybe declare a variable or

my code and maybe declare a variable or perhaps do something that's

perhaps do something that's syntactically a little more elegant and

syntactically a little more elegant and in fact a very common design in Loop

in fact a very common design in Loop like this would be to declare not just

like this would be to declare not just one variable like I but to actually

one variable like I but to actually declare a second variable called n for

declare a second variable called n for instance where n is just some number Set

instance where n is just some number Set n equal to the length of s but

n equal to the length of s but thereafter inside of this condition

thereafter inside of this condition instead of calling Sterling of s again

instead of calling Sterling of s again and again and again what might I now do

and again and again what might I now do I could instead just compare I against n

I could instead just compare I against n itself because n now will only be

itself because n now will only be calculated once when it's initialized

calculated once when it's initialized just as I is initialized to zero and

just as I is initialized to zero and thereafter we're going to be compar

thereafter we're going to be compar comparing I which is changing against n

comparing I which is changing against n which will not be so it's going to be

which will not be so it's going to be marginally more efficient by Design now

marginally more efficient by Design now with that said a good compiler could

with that said a good compiler could also recognize that there was this

also recognize that there was this optimization possibility and maybe do it

optimization possibility and maybe do it for us but for now best to get into the

for us but for now best to get into the Habit best to develop the muscle memory

Habit best to develop the muscle memory for making those better design decisions

for making those better design decisions yourselves questions then on how we did

yourselves questions then on how we did this no all right a few final building

this no all right a few final building blocks for the day so we started by

blocks for the day so we started by talking about those command line

talking about those command line arguments that clang uses whereby

arguments that clang uses whereby anything after the command that you type

anything after the command that you type at a prompt be it make or clang or even

at a prompt be it make or clang or even CD in Linux any word thereafter or

CD in Linux any word thereafter or something cryptic like- O is a

something cryptic like- O is a commandline argument it's an input to

commandline argument it's an input to the command it's different from a

the command it's different from a function argument because a function

function argument because a function argument of course is an input to a

argument of course is an input to a function but it's the same idea it's

function but it's the same idea it's just different syntax after the dollar

just different syntax after the dollar sign at the prompt well it turns out

sign at the prompt well it turns out that command line arguments are

that command line arguments are something you can now use in your own

something you can now use in your own programs by

programs by accessing uh words after the prompt and

accessing uh words after the prompt and let me propose that we invent we invent

let me propose that we invent we invent this as follows let me propose that we

this as follows let me propose that we switch back to vs code here and I'll

switch back to vs code here and I'll open a new file here called greet doc so

open a new file here called greet doc so in greet Doc is going to be a program

in greet Doc is going to be a program that very simply greets the user had we

that very simply greets the user had we written this last week we would have

written this last week we would have done this include

done this include cs50.h and then include uh standard

cs50.h and then include uh standard i.h and then int main void and then we

i.h and then int main void and then we might do something simple like string

might do something simple like string name equals get string quote unquote

name equals get string quote unquote what's your name question mark and then

what's your name question mark and then we would have printed out as always

we would have printed out as always hello comma percent s and then plugging

hello comma percent s and then plugging in that name so this is the same program

in that name so this is the same program we've implemented many times just to

we've implemented many times just to make sure it works although nope that's

make sure it works although nope that's not quite the same program semicolon in

not quite the same program semicolon in the wrong place this now is the same

the wrong place this now is the same program so make greet /g greet and I'll

program so make greet /g greet and I'll type in my own name hello David so we're

type in my own name hello David so we're back there now what's arguably a little

back there now what's arguably a little Annoying about this program if I type in

Annoying about this program if I type in something else like Carter enter you

something else like Carter enter you know I have to run the program wait for

know I have to run the program wait for the prompt type in my name hit enter and

the prompt type in my name hit enter and that's fine but imagine if every program

that's fine but imagine if every program worked like this like make suppose you

worked like this like make suppose you could only type make then you wait for a

could only type make then you wait for a prompt then you type the name of the

prompt then you type the name of the program you want to make then you hit

program you want to make then you hit enter or worse in Linux when you have to

enter or worse in Linux when you have to change directories as you might have for

change directories as you might have for problem set one what if you had to type

problem set one what if you had to type CD enter now type the name of the folder

CD enter now type the name of the folder you want to change into enter I mean it

you want to change into enter I mean it just slows life down and so it just gets

just slows life down and so it just gets annoying quickly so commandline

annoying quickly so commandline arguments just let you express your

arguments just let you express your whole thought all at once so how can I

whole thought all at once so how can I do this well if I want to express the

do this well if I want to express the notion of command line arguments in my

notion of command line arguments in my code I could do something like this I

code I could do something like this I could for the very first time go up and

could for the very first time go up and get rid of this void which as of today

get rid of this void which as of today means this program takes no command line

means this program takes no command line arguments and I can change it to exactly

arguments and I can change it to exactly this int ARG C string argv with brackets

this int ARG C string argv with brackets now it's cryptic admittedly and let me

now it's cryptic admittedly and let me zoom in but I think we can perhaps infer

zoom in but I think we can perhaps infer now what's going on if main Now does not

now what's going on if main Now does not have void as its input which means it

have void as its input which means it takes no arguments surely the spoiler

takes no arguments surely the spoiler here is that now main will take command

here is that now main will take command line argument somehow any guesses as to

line argument somehow any guesses as to what argv is or will be

what argv is or will be what might this

what might this represent it's an array of strings right

represent it's an array of strings right by way of the syntax

yeah exactly it will be all of the characters or really all of the words

characters or really all of the words that you type at the prompt argc as an

that you type at the prompt argc as an INT any

INT any guess

guess argument count is what it generally

argument count is what it generally stands for though technically you could

stands for though technically you could call these things any anything but this

call these things any anything but this is the convention because I claimed

is the convention because I claimed earlier that arrays don't keep track of

earlier that arrays don't keep track of their own length if you want to know how

their own length if you want to know how many words the human typed at the prompt

many words the human typed at the prompt after your program's name you have to be

after your program's name you have to be told not just the array of the words but

told not just the array of the words but the length of that array the strings you

the length of that array the strings you can figure out the length of using

can figure out the length of using Sterling but you can't figure out the

Sterling but you can't figure out the length of the array of strings the

length of the array of strings the collection of words that the human typed

collection of words that the human typed in so how can I now use this well let me

in so how can I now use this well let me go ahead and do this let me go ahead and

go ahead and do this let me go ahead and change this program now just to be print

change this program now just to be print F quote unquote

F quote unquote hello comma uh percent s back sln then

hello comma uh percent s back sln then Arvy bracket 1 so this is not the best

Arvy bracket 1 so this is not the best version of my code yet but it's my first

version of my code yet but it's my first make greet and now let me dog greet

make greet and now let me dog greet David all at once enter hello David now

David all at once enter hello David now let me run it again dogre Carter enter

let me run it again dogre Carter enter hello Carter you know it's a marginal

hello Carter you know it's a marginal Improvement but I don't have to wait for

Improvement but I don't have to wait for get string to prompt me to hit enter

get string to prompt me to hit enter it's just speeding things up you know is

it's just speeding things up you know is fast one less command to type in but I

fast one less command to type in but I deliberately did bracket one but where

deliberately did bracket one but where what's the beginning of arv it would be

what's the beginning of arv it would be bracket zero well what's that this is

bracket zero well what's that this is sometimes useful though for now it's not

sometimes useful though for now it's not suppose I recompile my code and run this

suppose I recompile my code and run this program now greet David anyone want to

program now greet David anyone want to guess what's in argv

Z say again greet enter hello. SLG greet so if

again greet enter hello. SLG greet so if you want sort of inception style your

you want sort of inception style your program to figure out what its own name

program to figure out what its own name is or at least how it was executed at

is or at least how it was executed at the command line at the terminal you can

the command line at the terminal you can look at ARG v0 in general probably not

look at ARG v0 in general probably not that useful probably better to start

that useful probably better to start looking at bracket one which was the

looking at bracket one which was the first word after the program name and if

first word after the program name and if there were more I could do this how

there were more I could do this how about RV bracket 2 let me add in a

about RV bracket 2 let me add in a second percent s let me recompile greet

second percent s let me recompile greet let me do greet David maen enter and

let me do greet David maen enter and that two now works taking in two words

that two now works taking in two words at the prompt if I really want to be

at the prompt if I really want to be smart at this now I could do something

smart at this now I could do something like this though how about if the count

like this though how about if the count of arguments AKA AR C equals equals 2

of arguments AKA AR C equals equals 2 then assume that the human typed in only

then assume that the human typed in only their first name and do print F uh hello

their first name and do print F uh hello comma percent s back sln and then uh

comma percent s back sln and then uh argv bracket 1 else if the human did not

argv bracket 1 else if the human did not provide exactly two arguments the name

provide exactly two arguments the name of the program and their own name let's

of the program and their own name let's just print out a default value L they

just print out a default value L they forgot their name or they typed in two

forgot their name or they typed in two names or three names let's just do uh

names or three names let's just do uh hello comma world as a default and we'll

hello comma world as a default and we'll just ignore what the human typed in if I

just ignore what the human typed in if I recompile this make greet I can dog

recompile this make greet I can dog greet and David again enter oops uh

greet and David again enter oops uh sorry what am I missing yeah so newbie

sorry what am I missing yeah so newbie mistake else all right make greet again

mistake else all right make greet again dog greet David enter there's my hello

dog greet David enter there's my hello David

David but if I omit my name I just get the

but if I omit my name I just get the generic like a default value and if I

generic like a default value and if I get a little curious and I type in both

get a little curious and I type in both names then I get ignored two why because

names then I get ignored two why because I just haven't built in support for arxy

I just haven't built in support for arxy of three I could do anything I want but

of three I could do anything I want but now we have access to these kinds of

now we have access to these kinds of building blocks all right what else

building blocks all right what else might I do here well it turns out there

might I do here well it turns out there might be some final features for us to

might be some final features for us to now execute um notice though that in C

now execute um notice though that in C despite what you might see in books or

despite what you might see in books or online tutorials nowadays the two

online tutorials nowadays the two official formats for defining a main

official formats for defining a main function are either this which we've

function are either this which we've been using now for Two Plus weeks or now

been using now for Two Plus weeks or now this whereby you change the void to int

this whereby you change the void to int argc and then for now string argv and

argc and then for now string argv and then empty brackets and we'll see that

then empty brackets and we'll see that this two is an a simplification some

this two is an a simplification some training wheels if you will but for now

training wheels if you will but for now those are the two forms even though you

those are the two forms even though you will see in online tutorials and even

will see in online tutorials and even books some people use Maine in different

books some people use Maine in different ways these are the two now to keep in

ways these are the two now to keep in mind and I'll note that these command

mind and I'll note that these command line arguments are kind of all over the

line arguments are kind of all over the place didn't probably expect to see this

place didn't probably expect to see this word on the screen here and what does it

word on the screen here and what does it mean well it turns out that for decades

mean well it turns out that for decades there's actually this program that comes

there's actually this program that comes with Linux systems in particular called

with Linux systems in particular called coway why probably because someone had

coway why probably because someone had too much free time once and decided to

too much free time once and decided to write a program that creates asy art out

write a program that creates asy art out of a cow saying something textually on

of a cow saying something textually on the screen but you use coway Just For

the screen but you use coway Just For Fun by way of command line arguments so

Fun by way of command line arguments so for instance let me propose that uh I go

for instance let me propose that uh I go back to vs code here not because I want

back to vs code here not because I want to write any code but I just want to use

to write any code but I just want to use my terminal window and let me uh

my terminal window and let me uh maximize my terminal window here and let

maximize my terminal window here and let me go ahead and type in something like

me go ahead and type in something like how about c space moo so cow is not a

how about c space moo so cow is not a program I wrote It's been around for

program I wrote It's been around for decades but we installed it in vs code

decades but we installed it in vs code for you in the cloud it takes at least

for you in the cloud it takes at least one command line argument what do you

one command line argument what do you want the cow to say I can say cow say

want the cow to say I can say cow say moo and hit enter and voila there's my

moo and hit enter and voila there's my asky art of a cow saying moo on the

asky art of a cow saying moo on the screen it can say multiple words so I

screen it can say multiple words so I can say hello world and enter and now it

can say hello world and enter and now it says hello world so this is just an

says hello world so this is just an example of a silly program that uses

example of a silly program that uses command line arguments but it takes

command line arguments but it takes others too just like clang use this

others too just like clang use this convention of hyphens to change the

convention of hyphens to change the output of the program Dash something is

output of the program Dash something is just a super common convention with

just a super common convention with commandline arguments when you want a

commandline arguments when you want a very tur notation for some option like

very tur notation for some option like output um in C I read the documentation

output um in C I read the documentation and it turns out there's a-f command

and it turns out there's a-f command line argument that allows you to change

line argument that allows you to change the uh the appearance of the cow if you

the uh the appearance of the cow if you will so if I do coway dasf duck and then

will so if I do coway dasf duck and then some other word like quack it's no

some other word like quack it's no longer a cow that command line argument

longer a cow that command line argument turns it into a tiny adorable duck

turns it into a tiny adorable duck instead and then lastly just for fun

instead and then lastly just for fun because I spent way too much time

because I spent way too much time playing with command line arguments uh

playing with command line arguments uh c-f dragon and then how about like raar

c-f dragon and then how about like raar enter you can even get this on the

enter you can even get this on the screen here so this too is just an

screen here so this too is just an example of what you can do with these

example of what you can do with these command line arguments now that we have

command line arguments now that we have this building block and there's one

this building block and there's one final thing we can now do with code

final thing we can now do with code there's one last feature today that

there's one last feature today that we'll introduce before we now connect

we'll introduce before we now connect all of these dots to readability and and

all of these dots to readability and and encryption by talking lastly about

encryption by talking lastly about something called exit status it turns

something called exit status it turns out that whenever your main function

out that whenever your main function exits it returns a secret integer that

exits it returns a secret integer that you can figure out as the programmer or

you can figure out as the programmer or an advanced user what it was and these

an advanced user what it was and these exit codes exit statuses are typically

exit codes exit statuses are typically used to indicate errors so for instance

used to indicate errors so for instance over over the past couple of years if

over over the past couple of years if you us zoom and you ever got some kind

you us zoom and you ever got some kind of error you might have seen a screen

of error you might have seen a screen like this it's usually not that helpful

like this it's usually not that helpful maybe tells you to click report problem

maybe tells you to click report problem or contact support but very often in our

or contact support but very often in our human world on Macs PCS and phones you

human world on Macs PCS and phones you see cryptic error codes like literally

see cryptic error codes like literally numbers that probably only Zoom knows or

numbers that probably only Zoom knows or Microsoft or Google or whatever company

Microsoft or Google or whatever company wrote the software you're using but that

wrote the software you're using but that number corresponds to a specific error

number corresponds to a specific error that some human somewhere knows might

that some human somewhere knows might very well happen these are used similar

very well happen these are used similar L although under a different name that

L although under a different name that we'll talk about later in the term uh on

we'll talk about later in the term uh on the web as well have you ever seen this

the web as well have you ever seen this maybe not character but number so 404

maybe not character but number so 404 means

means what so error yes but really not found

what so error yes but really not found so why I mean this is the most Arcane

so why I mean this is the most Arcane thing and we'll talk in a few weeks

thing and we'll talk in a few weeks about like what this and other numbers

about like what this and other numbers mean but numbers are all around us in

mean but numbers are all around us in technology and they very often mean

technology and they very often mean something to the technical people who

something to the technical people who wrote the software less so to humans

wrote the software less so to humans like you and me why so many of us

like you and me why so many of us recognize 404 is kind of weird that like

recognize 404 is kind of weird that like that's been around long enough that we

that's been around long enough that we all know it but it really is just a

all know it but it really is just a special number that represents an error

special number that represents an error of some sort so it turns out the last

of some sort so it turns out the last thing we'll reveal today about what

thing we'll reveal today about what we've been taking for granted for two

we've been taking for granted for two weeks is what the int is in Maine we've

weeks is what the int is in Maine we've seen just a moment ago that the thing in

seen just a moment ago that the thing in the parentheses which up until now has

the parentheses which up until now has been void which means no command line

been void which means no command line arguments now in Arc string arv brackets

arguments now in Arc string arv brackets just means yes command line arguments

just means yes command line arguments and we've seen how to access them so the

and we've seen how to access them so the last piece of the puzzle honestly of all

last piece of the puzzle honestly of all the cryptic syntax the past two weeks is

the cryptic syntax the past two weeks is just what int means int is always there

just what int means int is always there for Maine and it indicates that Maine

for Maine and it indicates that Maine will always return an integer even

will always return an integer even though you and I have never done so

though you and I have never done so explicitly usually Maine returns Zero by

explicitly usually Maine returns Zero by default but it would be weird if you saw

default but it would be weird if you saw an error message saying zero so zero is

an error message saying zero so zero is just hidden you would never see it on

just hidden you would never see it on the screen but it's happening

the screen but it's happening automatically by way of how C is

automatically by way of how C is designed so let me write one final

designed so let me write one final program here I'll call it for instance

program here I'll call it for instance status . C to show you these exit

status . C to show you these exit statuses code of status. c and then up

statuses code of status. c and then up here let me do something simple like

here let me do something simple like include

include cs50.h then include standard i.h and

cs50.h then include standard i.h and then int main uh let's do actually let's

then int main uh let's do actually let's use a command line argument in argc

use a command line argument in argc string argv so that's copy paste but now

string argv so that's copy paste but now let's do this if argc does not equal to

let's do this if argc does not equal to why don't we do something like this

why don't we do something like this let's not just um default to hello world

let's not just um default to hello world like last time let's yell at the user so

like last time let's yell at the user so let's say something like printf missing

let's say something like printf missing command line argument so that they know

command line argument so that they know they screwed up and they need to run the

they screwed up and they need to run the program again correctly else let's go

program again correctly else let's go ahead and say print out uh as before

ahead and say print out uh as before hello comma percent s and then plug in

hello comma percent s and then plug in argv bracket one so the human's name

argv bracket one so the human's name from The Prompt now at this point let me

from The Prompt now at this point let me go ahead and run

go ahead and run statusstatus and I'll will type nothing

statusstatus and I'll will type nothing first first I get yelled at this time

first first I get yelled at this time I'll type it again/ status David and it

I'll type it again/ status David and it works properly but now let me show you a

works properly but now let me show you a somewhat secret cryptic command you can

somewhat secret cryptic command you can type this at your prompt and it's just a

type this at your prompt and it's just a coincidence that there's another dollar

coincidence that there's another dollar sign Echo dollar sign question mark

sign Echo dollar sign question mark totally Arcane but it allows you to see

totally Arcane but it allows you to see what exit status your program has ended

what exit status your program has ended with so let me run this again the wrong

with so let me run this again the wrong way/ status okay I get the error message

way/ status okay I get the error message what was secretly returned I can't can't

what was secretly returned I can't can't see it there's obviously no error screen

see it there's obviously no error screen but by typing Echo dollar sign question

but by typing Echo dollar sign question mark I can see that oh my program

mark I can see that oh my program automatically by default return zero

automatically by default return zero however if I run it again correctly

however if I run it again correctly status David enter this is the correct

status David enter this is the correct version but if I run Echo question mark

version but if I run Echo question mark status again it's still entered with

status again it's still entered with zero and long story short this is just a

zero and long story short this is just a missed opportunity when something goes

missed opportunity when something goes wrong why don't I return a value other

wrong why don't I return a value other than zero zero by default means success

than zero zero by default means success and it's always there automatically but

and it's always there automatically but but you can control this I can go into

but you can control this I can go into my code here and return one else if

my code here and return one else if something works fine I can return zero

something works fine I can return zero by default and honestly if I omit the

by default and honestly if I omit the return zero again zero automatically is

return zero again zero automatically is returned so let me go ahead and though

returned so let me go ahead and though be explicit just so I know what's going

be explicit just so I know what's going on make status

on make status again/ status and let's do this

again/ status and let's do this correctly with David enter hello David

correctly with David enter hello David Echo question mark uh Echo dollar sign

Echo question mark uh Echo dollar sign question mark zero so all is well

question mark zero so all is well but now if I do/ status and nothing or

but now if I do/ status and nothing or multiple things but not just David enter

multiple things but not just David enter I get the error message but now if I do

I get the error message but now if I do Echo dollar sign question mark waa there

Echo dollar sign question mark waa there now is the one so what does this now

now is the one so what does this now mean this is in the graphical world we

mean this is in the graphical world we would just show something like this on

would just show something like this on the screen which is a little more

the screen which is a little more informative to the user but even in the

informative to the user but even in the Linux world we don't have a guey

Linux world we don't have a guey necessarily even for the programs we've

necessarily even for the programs we've written you can check these exit

written you can check these exit statuses and in fact more comfortable

statuses and in fact more comfortable more advanced programmers when they

more advanced programmers when they write code that calls programs be it

write code that calls programs be it coway or anything else you can encode

coway or anything else you can encode check what the exit status is of a

check what the exit status is of a program and then decide did my program

program and then decide did my program work or did it not and now let's connect

work or did it not and now let's connect the final dots uh before we adjourn for

the final dots uh before we adjourn for some fruit snacks uh cryptography namely

some fruit snacks uh cryptography namely one of the applications this week via

one of the applications this week via which you'll be able to send if you will

which you'll be able to send if you will secret messages and better yet decre

secret messages and better yet decre secret messages this will be in addition

secret messages this will be in addition to perhaps analyzing the readability of

to perhaps analyzing the readability of text using puristic like we identified

text using puristic like we identified at the start of class 2 so a

at the start of class 2 so a cryptography is just the art the science

cryptography is just the art the science of encrypting information scrambling

of encrypting information scrambling information so that if you have a secret

information so that if you have a secret message to send in so-called plain text

message to send in so-called plain text you can run it through some algorithm

you can run it through some algorithm and turn it into what's called Cipher

and turn it into what's called Cipher text thereby encrypting it and only

text thereby encrypting it and only someone who knows what algorithm you've

someone who knows what algorithm you've used and what input you've used to the

used and what input you've used to the algorithm theoretically can decrypt that

algorithm theoretically can decrypt that process and convert it back to the

process and convert it back to the original message so if we use our mental

original message so if we use our mental model from last week here is a problem

model from last week here is a problem here is an input and output the goal I

here is an input and output the goal I claim here is to take some plain text

claim here is to take some plain text like the message you want to send think

like the message you want to send think back to grade school if you ever passed

back to grade school if you ever passed a note to a friend or to your crush

a note to a friend or to your crush saying I love you it's a little awkward

saying I love you it's a little awkward if the teacher or someone else

if the teacher or someone else intercepts the paper and in English it

intercepts the paper and in English it just says I love you or whatever it is

just says I love you or whatever it is it'd be nice if you had at least

it'd be nice if you had at least encrypted it in some way but the other

encrypted it in some way but the other person needs to know what algorithm you

person needs to know what algorithm you used and what inputs you used to that

used and what inputs you used to that algorithm so that ultimately they can

algorithm so that ultimately they can decode the so-called Cipher text which

decode the so-called Cipher text which is the output so what go inside of the

is the output so what go inside of the box today well an algorithm as it

box today well an algorithm as it relates to cryptography is called a

relates to cryptography is called a cipher and a cipher is a fancy name for

cipher and a cipher is a fancy name for an algorithm that encrypts text from

an algorithm that encrypts text from plain text to Cipher text the catch is

plain text to Cipher text the catch is there needs to be not just the algorithm

there needs to be not just the algorithm there needs to be an input to it and so

there needs to be an input to it and so for instance you might draw the picture

for instance you might draw the picture like this for the first time today and

like this for the first time today and we've seen this in code you can give

we've seen this in code you can give multiple inputs or arguments to

multiple inputs or arguments to functions so in this black box can you

functions so in this black box can you imagine passing in the message you want

imagine passing in the message you want to send and then some Secret so for

to send and then some Secret so for instance suppose that the simplest thing

instance suppose that the simplest thing I could think of as a kid was instead of

I could think of as a kid was instead of sending the letter A why don't I write

sending the letter A why don't I write the letter b instead of the letter B why

the letter b instead of the letter B why don't I write the letter c so I can kind

don't I write the letter c so I can kind of shift the English alphabet by one

of shift the English alphabet by one space so a becomes b b becomes C dot dot

space so a becomes b b becomes C dot dot dot Z becomes a you can wrap around at

dot Z becomes a you can wrap around at the end and let's assume no punctuation

the end and let's assume no punctuation in this part of the story so that's a

in this part of the story so that's a very simple algorithm add a value to

very simple algorithm add a value to each letter and send the value as the

each letter and send the value as the cipher text and now the teacher the

cipher text and now the teacher the classmate they have to know that you use

classmate they have to know that you use not only this rotational algorithm also

not only this rotational algorithm also known as a Caesar Cipher they also need

known as a Caesar Cipher they also need to know what number you use did you add

to know what number you use did you add one to every letter two to every letter

one to every letter two to every letter 25 to every letter now if they're super

25 to every letter now if they're super smart and probably not the the young age

smart and probably not the the young age in this story they could also just try

in this story they could also just try all possibilities and that would be an

all possibilities and that would be an attack on the algorithm this is not a

attack on the algorithm this is not a sophisticated algorithm but it's enough

sophisticated algorithm but it's enough to send a message in class so if the two

to send a message in class so if the two inputs now are high as the plain text

inputs now are high as the plain text message and one as the so-called key the

message and one as the so-called key the secret number that only you and the

secret number that only you and the other person know you might uh be able

other person know you might uh be able to encrypt a message from one way to the

to encrypt a message from one way to the other and so in this case for instance

other and so in this case for instance high would become i j exclamation point

high would become i j exclamation point in this version of the algorithm we're

in this version of the algorithm we're not going to bother with numbers or

not going to bother with numbers or punctuation will only operate on a

punctuation will only operate on a through z be it uppercase or lowercase

through z be it uppercase or lowercase so now if you were to receive a a slip

so now if you were to receive a a slip of paper in class with i j on it you

of paper in class with i j on it you know you the recipient would know what

know you the recipient would know what it is so long as you know that the

it is so long as you know that the sender used one because you just reverse

sender used one because you just reverse the algorithm and you subtract one

the algorithm and you subtract one instead the teacher you know they

instead the teacher you know they probably don't know what this means and

probably don't know what this means and they're not going to spend time hacking

they're not going to spend time hacking the message so it just looks scrambled

the message so it just looks scrambled to them and that's what we get from

to them and that's what we get from encryption someone who intercepts it be

encryption someone who intercepts it be it in class or in the real world on the

it in class or in the real world on the Internet or anywhere else can't actually

Internet or anywhere else can't actually figure out ideally what it is you have

figure out ideally what it is you have sent the opposite of course is indeed

sent the opposite of course is indeed called decryption but the process is the

called decryption but the process is the same we now pass in negative one and so

same we now pass in negative one and so how about this why don't we end with a

how about this why don't we end with a demonstration here

demonstration here u y JT xbt

u y JT xbt dt50 there's a bit of a tell there if we

dt50 there's a bit of a tell there if we pass that in and do Nega -1 well how do

pass that in and do Nega -1 well how do we get out the plain text originally

we get out the plain text originally well if this is the cipher text and we

well if this is the cipher text and we subtract one from each letter I think U

subtract one from each letter I think U becomes t i becomes h j becomes i t

becomes t i becomes h j becomes i t becomes s x becomes w b becomes a t

becomes s x becomes w b becomes a t becomes s d becomes c t becomes s and

becomes s d becomes c t becomes s and this was indeed cs50 have a duck on your

this was indeed cs50 have a duck on your way out and some snacks in the

way out and some snacks in the [Applause]

[Applause] [Music]

lobby [Music]

all right all right this is cs50 and this is week three already wherein we'll

this is week three already wherein we'll take a look back actually at week zero

take a look back actually at week zero where we first began and in week zero

where we first began and in week zero recall that everything was very

recall that everything was very intuitive in a sense we talked not just

intuitive in a sense we talked not just about representation of information but

about representation of information but algorithms and we talked about tearing a

algorithms and we talked about tearing a phone book again and again and that

phone book again and again and that somehow got us to a better solution but

somehow got us to a better solution but today we'll try to start formalizing

today we'll try to start formalizing some of those ideas and capturing some

some of those ideas and capturing some of those same ideas not in pseudo code

of those same ideas not in pseudo code just yet but in uh actual code as well

just yet but in uh actual code as well but we'll also consider the the

but we'll also consider the the efficiency of those algorithms like just

efficiency of those algorithms like just how good how welld designed our

how good how welld designed our algorithms actually are and if you

algorithms actually are and if you recall when we did the phone book

recall when we did the phone book example where in I first had an

example where in I first had an algorithm searching one page at a time

algorithm searching one page at a time and then second when two pages at a time

and then second when two pages at a time and then third started tearing the thing

and then third started tearing the thing in half recall that we with a wave of

in half recall that we with a wave of the hand kind of analyzed it as follows

the hand kind of analyzed it as follows we proposed that if the x-axis here is

we proposed that if the x-axis here is the size of the problem like number of

the size of the problem like number of pages in a phone book and the y- axis is

pages in a phone book and the y- axis is the time required to solve the problem

the time required to solve the problem in seconds minutes page tears whatever

in seconds minutes page tears whatever your unit of measuring is recall that

your unit of measuring is recall that the first algorithm was this straight

the first algorithm was this straight line such that if you had n pages in the

line such that if you had n pages in the phone book it might have this slope of N

phone book it might have this slope of N and there's this one toone relationship

and there's this one toone relationship between pages and tears two pages at a

between pages and tears two pages at a time of course was twice as fast but

time of course was twice as fast but still really the same shape the yellow

still really the same shape the yellow line here indicating that yeah it's n /

line here indicating that yeah it's n / two maybe plus one if you have to double

two maybe plus one if you have to double back as we discussed but it's really

back as we discussed but it's really still fundamentally the same algorithm

still fundamentally the same algorithm one or two pages at a time but the third

one or two pages at a time but the third algorithm recall was this one here in

algorithm recall was this one here in green where we called it logarithmic in

green where we called it logarithmic in terms of how fast or how slow it was and

terms of how fast or how slow it was and indeed the implication of this algorithm

indeed the implication of this algorithm was that we could even double the size

was that we could even double the size of the phone book and no big deal one

of the phone book and no big deal one additional page tear and we take yet

additional page tear and we take yet another thousand page bite out of the

another thousand page bite out of the phone book so today we'll revisit some

phone book so today we'll revisit some of these ideas formalize them a bit but

of these ideas formalize them a bit but also translate some of them ultimately

also translate some of them ultimately to code and all of that now is possible

to code and all of that now is possible because we have this lower level

because we have this lower level understanding perhaps of like what's

understanding perhaps of like what's actually inside of your computer this of

actually inside of your computer this of course is your computer's Ram or memory

course is your computer's Ram or memory and recall that if we kind of start to

and recall that if we kind of start to abstract this away your compter

abstract this away your compter computer's memory is really just a grid

computer's memory is really just a grid of bites in fact we don't have to look

of bites in fact we don't have to look at the hardware anymore and we looked at

at the hardware anymore and we looked at a grid of bites like this whereby each

a grid of bites like this whereby each of these bites could be used to store a

of these bites could be used to store a Char an INT a long or even an entire

Char an INT a long or even an entire string at that but let's focus perhaps

string at that but let's focus perhaps just on a subset of this because last

just on a subset of this because last week of course we emphasized really

week of course we emphasized really arrays storing things in arrays and that

arrays storing things in arrays and that allowed us to start storing entire

allowed us to start storing entire strings sequences of characters and even

strings sequences of characters and even arrays of integers if we want to have

arrays of integers if we want to have multiple ones and not just multiple

multiple ones and not just multiple variables as well but the catch is that

variables as well but the catch is that if you look inside of an array in the

if you look inside of an array in the computer's memory and for instance

computer's memory and for instance suppose these integers here are stored

suppose these integers here are stored it's pretty easy for us humans to glance

it's pretty easy for us humans to glance at this and immediately find the number

at this and immediately find the number 50 you sort of have this bird's eye view

50 you sort of have this bird's eye view from where you're seated of everything

from where you're seated of everything on the screen and so it's pretty obvious

on the screen and so it's pretty obvious how you get to the number 50 but in the

how you get to the number 50 but in the world of computers of course it turns

world of computers of course it turns out that this is hardware and computers

out that this is hardware and computers for today's purposes can only do one

for today's purposes can only do one thing at a time they can't just take it

thing at a time they can't just take it all in and find instantly some number

all in and find instantly some number like 50 so perhaps a decent metaphor is

like 50 so perhaps a decent metaphor is to consider the array of memory inside

to consider the array of memory inside of your computer really is a sequence of

of your computer really is a sequence of of closed doors and if the computer

of closed doors and if the computer wants to find some value in an array it

wants to find some value in an array it has to do the digital equivalent of

has to do the digital equivalent of opening each of these doors one at a

opening each of these doors one at a time now how can code do that well of

time now how can code do that well of course we introduced indices or indexes

course we introduced indices or indexes last week whereby we by convention call

last week whereby we by convention call the first element of an array location

the first element of an array location zero the second location one the Third

zero the second location one the Third location two and so forth so-called zero

location two and so forth so-called zero indexed and this allowed us to now

indexed and this allowed us to now bridge this conceptual world of like

bridge this conceptual world of like what's going on in memory with actual

what's going on in memory with actual code because now we had this square

code because now we had this square bracket syntax via which we could go

bracket syntax via which we could go searching for something if we so choose

searching for something if we so choose and it turns out if I now uh paint these

and it turns out if I now uh paint these red instead of yellow it would seem that

red instead of yellow it would seem that we actually have a pretty good physical

we actually have a pretty good physical metaphor here standing in place for what

metaphor here standing in place for what would be a computer's uh array of memory

would be a computer's uh array of memory if for instance you're storing some

if for instance you're storing some seven numbers like that and so today we

seven numbers like that and so today we begin with a look of a specific type typ

begin with a look of a specific type typ of algorithm that is for searching like

of algorithm that is for searching like searching is all over the place all of

searching is all over the place all of us have probably gone to google.com or

us have probably gone to google.com or some equivalent like already multiple

some equivalent like already multiple times per day and getting back answers

times per day and getting back answers fast is what companies like Google are

fast is what companies like Google are really good at so how are they doing

really good at so how are they doing that how are they storing information in

that how are they storing information in computers uh memory well let's consider

computers uh memory well let's consider what this really is it's really just a

what this really is it's really just a problem as it was back in week zero the

problem as it was back in week zero the input though to the problem for now

input though to the problem for now might be this array of seven lockers so

might be this array of seven lockers so that's the input to the problem inside

that's the input to the problem inside of which is a number and maybe for

of which is a number and maybe for Simplicity now we just want a yes no a

Simplicity now we just want a yes no a true false answer a bull that is to say

true false answer a bull that is to say of whether or not some number like 50 is

of whether or not some number like 50 is in that array it's not quite as fancy as

in that array it's not quite as fancy as Google that doesn't just tell you yes we

Google that doesn't just tell you yes we have search results it actually gives

have search results it actually gives you the search results but for now we'll

you the search results but for now we'll keep it simple and just output as part

keep it simple and just output as part of this problem yes or no true or false

of this problem yes or no true or false we have found the number we're looking

we have found the number we're looking for given an input like that array but

for given an input like that array but it turns out inside of this black box

it turns out inside of this black box that we keep coming back to there's all

that we keep coming back to there's all sorts of possible algorithms and we talk

sorts of possible algorithms and we talk about this at a high level conceptually

about this at a high level conceptually in week zero with the phone book but

in week zero with the phone book but today let's consider it a little more

today let's consider it a little more concretely uh by way of a game that some

concretely uh by way of a game that some of you might have grown up with namely

of you might have grown up with namely Monopoly and so behind these doors it

Monopoly and so behind these doors it turns out we'll be hidden some

turns out we'll be hidden some denominations of Monopoly money but for

denominations of Monopoly money but for this we now have two volunteers if you'd

this we now have two volunteers if you'd like to greet the world hi I'm

like to greet the world hi I'm Jackson yay hi my name is Stephanie

Jackson yay hi my name is Stephanie Steph and you want to say a little

Steph and you want to say a little something about yourselves years house

something about yourselves years house first year living in Matthews

first year living in Matthews nice and I'm a first year in Canada nice

nice and I'm a first year in Canada nice well Welcome to our two

well Welcome to our two volunteers so why don't we do this would

volunteers so why don't we do this would one of you like to volunteer the other

one of you like to volunteer the other to go

to go first all go first okay all right so

first all go first okay all right so Stephanie's up first and behind one of

Stephanie's up first and behind one of these doors here we've hidden the

these doors here we've hidden the Monopoly money 50 and so we'd like you

Monopoly money 50 and so we'd like you to find the 50 we'll tell you nothing

to find the 50 we'll tell you nothing more about the lockers but we would like

more about the lockers but we would like you to execute a certain algorithm and

you to execute a certain algorithm and in fact I'm going to give you some

in fact I'm going to give you some pseudo code for this and I'm going to

pseudo code for this and I'm going to give you the name for it it's called

give you the name for it it's called linear search and as the name implies

linear search and as the name implies you're pretty much going to end up

you're pretty much going to end up walking in sort of a straight line but

walking in sort of a straight line but how are you going to do this well let me

how are you going to do this well let me propose that in a moment your first step

propose that in a moment your first step will be to think kind of like a loop for

will be to think kind of like a loop for each door from left to right what do we

each door from left to right what do we want you to do on each iteration well if

want you to do on each iteration well if 50 is behind that door then we want to

50 is behind that door then we want to go ahead and have you return true and

go ahead and have you return true and sort of hold up the 50 proudly if you

sort of hold up the 50 proudly if you will for the group otherwise if you get

will for the group otherwise if you get through that whole Loop and you haven't

through that whole Loop and you haven't found the number 50 you can just throw

found the number 50 you can just throw up your hands and disappointment false

up your hands and disappointment false you've not found the number 50 so to be

you've not found the number 50 so to be clear step one is going to be for each

clear step one is going to be for each uh door from left to right how would you

uh door from left to right how would you like to

begin yep oh and then yep there we go yep oh and if you'd like to at least

yep oh and if you'd like to at least tell oh good good acting here what have

tell oh good good acting here what have you found instead it's not 50 but 20 oh

you found instead it's not 50 but 20 oh okay so step one was a fail so let's

okay so step one was a fail so let's move on to step two inside of the Loop

move on to step two inside of the Loop what are you going to do next I'm going

what are you going to do next I'm going to move to the next door

okay almost okay almost sort of a 500 instead next Locker I would rather

no okay we're not telling the audience oh okay so keep going this is

audience oh okay so keep going this is step three

step three now

oh [Music]

[Music] man five okay few more lockers to

step this one kind of close all right and finally the last clearly you've been

and finally the last clearly you've been perhaps set up here let's go all right

perhaps set up here let's go all right so the number

50 and Stephanie if I may let me ask you a question here so on the screen this is

a question here so on the screen this is the pseudo code you just executed

the pseudo code you just executed suppose though I had done what many of

suppose though I had done what many of us have gotten into the habit of doing

us have gotten into the habit of doing when you have a if condition you often

when you have a if condition you often have an else Branch as well suppose that

have an else Branch as well suppose that I had done this now and I'm marking it

I had done this now and I'm marking it in red to be clear this is wrong but

in red to be clear this is wrong but what would have have been bad about this

what would have have been bad about this code using an if and an else might you

code using an if and an else might you say any

instincts um then you would end up like cancelling

um then you would end up like cancelling canceling the code before you found the

canceling the code before you found the 50 yeah exactly just be eternally sad

50 yeah exactly just be eternally sad indeed when Stephanie had opened the

indeed when Stephanie had opened the first Locker she had found 20 20 of

first Locker she had found 20 20 of course is not 50 she would have decreed

course is not 50 she would have decreed false but of course she hadn't checked

false but of course she hadn't checked all of the rest of the locker so that

all of the rest of the locker so that would seem to be a key detail that would

would seem to be a key detail that would this implementation of the SoDo coat we

this implementation of the SoDo coat we actually do go through as we did and

actually do go through as we did and only return false not even with an else

only return false not even with an else but just at the end of the loop such

but just at the end of the loop such that we only reach that line if we don't

that we only reach that line if we don't return truer uh earlier than that well

return truer uh earlier than that well let's go ahead and do this let me take

let's go ahead and do this let me take the mic from you if you'd like to take a

the mic from you if you'd like to take a seat next to Jackson Jackson in just a

seat next to Jackson Jackson in just a moment we'll have you come up Carter if

moment we'll have you come up Carter if you don't mind reorganizing the uh

you don't mind reorganizing the uh lockers for us but in the meantime let

lockers for us but in the meantime let me point out how we might now translate

me point out how we might now translate that same idea to code pretty high level

that same idea to code pretty high level pretty English oriented with that pseudo

pretty English oriented with that pseudo code but really now as of last week we

code but really now as of last week we have syntax via which Stephanie and soon

have syntax via which Stephanie and soon Jackson could treat this Locker the set

Jackson could treat this Locker the set of lockers as really indeed an array

of lockers as really indeed an array using bracket notation so we can now get

using bracket notation so we can now get a little closer in our pseudo code to

a little closer in our pseudo code to actual code and the way a computer

actual code and the way a computer scientist for instance would translate

scientist for instance would translate fairly high level English pseudo code

fairly high level English pseudo code like this to something that's a little

like this to something that's a little closer to C or any language that

closer to C or any language that supports arrays would be a little more

supports arrays would be a little more cryptically like this but you'll see

cryptically like this but you'll see more of this syntax in the coming days

more of this syntax in the coming days for I from 0 to n minus one this is

for I from 0 to n minus one this is still pseudo code but that's kind of

still pseudo code but that's kind of like the english-like way of expressing

like the english-like way of expressing what we've known come to know as a for

what we've known come to know as a for Loop if 50 is behind doors bracket I so

Loop if 50 is behind doors bracket I so I'm assuming for the sake of discussion

I'm assuming for the sake of discussion that doors now is the name of my

that doors now is the name of my variable this array of seven doors but

variable this array of seven doors but then the rest of the logic the rest of

then the rest of the logic the rest of the pseudo code really is the same way

the pseudo code really is the same way and so you'll find in time that

and so you'll find in time that programmers computer scientists more

programmers computer scientists more generally when you start expressing

generally when you start expressing ideas algorithms to someone else instead

ideas algorithms to someone else instead of maybe operating at this level here

of maybe operating at this level here you now have in your vocabulary really a

you now have in your vocabulary really a new new syntax that you can be a little

new new syntax that you can be a little more specific not getting so into the

more specific not getting so into the weeds of writing actual C code but at

weeds of writing actual C code but at least now doing something that's a

least now doing something that's a little closer to manipulating an array

little closer to manipulating an array like this so Jackson would you like to

like this so Jackson would you like to uh stand on up all right yes yes support

uh stand on up all right yes yes support for Jackson here too

for Jackson here too nice and here now I'm going to allow you

nice and here now I'm going to allow you an assumption that Stephanie did not

an assumption that Stephanie did not have Stephanie clearly was really doing

have Stephanie clearly was really doing her best searching from left to right

her best searching from left to right using linear searches we'll Now call it

using linear searches we'll Now call it but they were pretty much in random

but they were pretty much in random order right there was a 20 over there

order right there was a 20 over there there was a one over there and then a 50

there was a one over there and then a 50 so we deliberately jumbled things up and

so we deliberately jumbled things up and did not sort the numbers for her but

did not sort the numbers for her but Carter kindly has just come up to give

Carter kindly has just come up to give you a leg up Jackson by sorting the

you a leg up Jackson by sorting the numbers in advance and we'd like you

numbers in advance and we'd like you this time much like in week zero to do

this time much like in week zero to do something again and again but this time

something again and again but this time using what we'll Now call binary search

using what we'll Now call binary search it's exactly the same algorithm

it's exactly the same algorithm conceptually as we did in week zero but

conceptually as we did in week zero but if we translate to the context of this

if we translate to the context of this array we might now might say something

array we might now might say something like this the first step for Jackson

like this the first step for Jackson might be to ask ask the question if 50

might be to ask ask the question if 50 is behind the middle door where

is behind the middle door where presumably he's done some mental math to

presumably he's done some mental math to figure out what the middle is then he's

figure out what the middle is then he's going to just return true and hopefully

going to just return true and hopefully we'll get lucky and 50 will be right

we'll get lucky and 50 will be right there of course there's two other

there of course there's two other possibilities at least which would be

possibilities at least which would be what 50 is with respect to these

what 50 is with respect to these doors yeah so to the left or to the

doors yeah so to the left or to the right alternatively so if 50 is less

right alternatively so if 50 is less than the middle door then presumably

than the middle door then presumably Jackson's going to want to go left else

Jackson's going to want to go left else if 50 is greater than the middle door

if 50 is greater than the middle door he's going to want to go right much like

he's going to want to go right much like I did uh physically last week with the

I did uh physically last week with the phone book dividing and conquering left

phone book dividing and conquering left to right but there's actually a a fourth

to right but there's actually a a fourth case let's put it on the board first

case let's put it on the board first what else might happen here that Jackson

what else might happen here that Jackson should

should consider yeah oh it's not there it's not

consider yeah oh it's not there it's not there so let me actually go back and

there so let me actually go back and amend my my pseudo code here and just

amend my my pseudo code here and just say Jackson if we don't hand you any

say Jackson if we don't hand you any doors at all or eventually as he's

doors at all or eventually as he's dividing and conquering if he's left

dividing and conquering if he's left with no more doors we have to handle

with no more doors we have to handle that situation so that the def behavior

that situation so that the def behavior is defined find all right so with that

is defined find all right so with that said Jackson do you want to go ahead and

said Jackson do you want to go ahead and find us the number 50 and walk us

find us the number 50 and walk us through verbally what you're doing and

through verbally what you're doing and finding all right so it looks like this

finding all right so it looks like this one is the middle door so I'm going to

one is the middle door so I'm going to open

open it but it's 20 not 50 oh sad okay what's

it but it's 20 not 50 oh sad okay what's going through your head now so now I'm

going through your head now so now I'm looking because 50 is higher than 20 I

looking because 50 is higher than 20 I want to look to the right good um and

want to look to the right good um and look for the new middle door which would

look for the new middle door which would be here nice and it's 100 sad um but 50

be here nice and it's 100 sad um but 50 is less than 100 so now we know to look

is less than 100 so now we know to look left which would be here and Tada nice

left which would be here and Tada nice very well done this time around

very well done this time around too

too so thank you first to our volunteers

so thank you first to our volunteers here and in fact um since you're fan of

here and in fact um since you're fan of Monopoly as we're so informed we have

Monopoly as we're so informed we have the Cambridge edition of Monopoly with

the Cambridge edition of Monopoly with all your Harvard favorites here you go

all your Harvard favorites here you go thank you so thank you to our volunteers

thank you so thank you to our volunteers for finding us 50

so kind of was more popular than we expected so here we can translate this

expected so here we can translate this one more time into something a little

one more time into something a little closer to code and again still pseudo

closer to code and again still pseudo code but here now might be another

code but here now might be another formulation of exactly what Jackson just

formulation of exactly what Jackson just did just using the nomenclature now of

did just using the nomenclature now of arrays where you can be a little more

arrays where you can be a little more precise with your instructions and still

precise with your instructions and still leave it to someone else to translate

leave it to someone else to translate this finally to code but here we have

this finally to code but here we have same question at the beginning if no

same question at the beginning if no doors left return false if 50 is behind

doors left return false if 50 is behind doors bracket middle so I'm assuming

doors bracket middle so I'm assuming here because this is pseudo code that

here because this is pseudo code that somewhere I've done the mental math or

somewhere I've done the mental math or the actual math to figure out what the

the actual math to figure out what the index of middle is for instance if these

index of middle is for instance if these are seven doors in an array this would

are seven doors in an array this would be location zero 1 2 3 4 5 6 so somehow

be location zero 1 2 3 4 5 6 so somehow I've taken the total number of doors

I've taken the total number of doors seven divided by two to find the middle

seven divided by two to find the middle that's three and a half we have to deal

that's three and a half we have to deal with rounding but suffice it to say

with rounding but suffice it to say there's a well- defined formula for

there's a well- defined formula for finding the middle index given the total

finding the middle index given the total number of lockers divide by two and then

number of lockers divide by two and then round accordingly so that's presumably

round accordingly so that's presumably what Jackson did just by counting in his

what Jackson did just by counting in his head to find us door number three not

head to find us door number three not the third door the fourth door but door

the third door the fourth door but door bracket three so this is just saying if

bracket three so this is just saying if 50 is behind door is bracket middle

50 is behind door is bracket middle return true that was not the case he

return true that was not the case he found a $20 bill instead else if 50 is

found a $20 bill instead else if 50 is less than the uh doors bracket middle go

less than the uh doors bracket middle go ahead and now it gets interesting search

ahead and now it gets interesting search doors zero through doors middle minus

doors zero through doors middle minus one so it's getting a little more to the

one so it's getting a little more to the Weeds now but if middle is three this

Weeds now but if middle is three this one here well we want to now have

one here well we want to now have Jackson search if 50 had been uh if the

Jackson search if 50 had been uh if the number had been less we want to start at

number had been less we want to start at bracket zero and go up through this one

bracket zero and go up through this one when we deliberately subtract one CU

when we deliberately subtract one CU what's the point of looking in the same

what's the point of looking in the same Locker again we might as well do zero

Locker again we might as well do zero through middle minus one else if 50 is

through middle minus one else if 50 is greater than doors bracket middle which

greater than doors bracket middle which it was what did we then do Jackson

it was what did we then do Jackson intuitively searched for doors middle

intuitively searched for doors middle plus one through door

plus one through door n minus one and honestly it gets a

n minus one and honestly it gets a little Annoying having the pluses and

little Annoying having the pluses and the minuses here but just think of what

the minuses here but just think of what it means this is the middle door and

it means this is the middle door and Jackson then did proceed to search

Jackson then did proceed to search through doors middle plus one because

through doors middle plus one because there's no point in searching this one

there's no point in searching this one again and then the last element in any

again and then the last element in any array of size n where n is just our

array of size n where n is just our go-to number uh for the size is always

go-to number uh for the size is always going to be n minus one it's not going

going to be n minus one it's not going to be n it's going to be n minus one

to be n it's going to be n minus one because we always start counting at Rays

because we always start counting at Rays at zero so here then we have a

at zero so here then we have a translation into pseudo code that's a

translation into pseudo code that's a little closer to C of this exact same

little closer to C of this exact same idea and here we come full circle to

idea and here we come full circle to week zero like in week zero is pretty

week zero like in week zero is pretty intuitive to imagine dividing and

intuitive to imagine dividing and conquering a problem like this but if

conquering a problem like this but if you now think back to actual your iPhone

you now think back to actual your iPhone your Android phone or the like when

your Android phone or the like when you're doing autocomplete and searching

you're doing autocomplete and searching the list it's possible if you don't have

the list it's possible if you don't have many friends or family or colleagues in

many friends or family or colleagues in the phone you know what linear search

the phone you know what linear search just checking every name for the person

just checking every name for the person you're searching for might be perfectly

you're searching for might be perfectly fine but odds are your phones being

fine but odds are your phones being smarter than that especially if you

smarter than that especially if you start to have dozens hundreds thousands

start to have dozens hundreds thousands of people in your contacts over the

of people in your contacts over the years what would be better than linear

years what would be better than linear search well perhaps binary search but

search well perhaps binary search but but but there's an assumption a

but but there's an assumption a requirement which is what why was

requirement which is what why was Jackson ultimately able to find the 50

Jackson ultimately able to find the 50 in just like three steps instead of a

in just like three steps instead of a full seven like

full seven like Stephanie because the array was sorted

Stephanie because the array was sorted and so this is sort of a teaser for what

and so this is sort of a teaser for what we'll have to come back to later today

we'll have to come back to later today well you know how much effort did it

well you know how much effort did it take someone like Carter how much effort

take someone like Carter how much effort does it take your phone to sort all of

does it take your phone to sort all of those names and numbers in advance

those names and numbers in advance because maybe it's not actually worth

because maybe it's not actually worth the amount of time now someone like

the amount of time now someone like Google probably somehow keeps the

Google probably somehow keeps the database of web pages sorted you could

database of web pages sorted you could imagine it being super slow if when you

imagine it being super slow if when you type in cats or something else into

type in cats or something else into google.com if they searched linearly

google.com if they searched linearly over their entire data set ideally

over their entire data set ideally they're doing something a little smarter

they're doing something a little smarter than that so we'll formalize now exactly

than that so we'll formalize now exactly this kind of analysis and it's not going

this kind of analysis and it's not going to be so much mathy as it still will be

to be so much mathy as it still will be in intuitive but we'll introduce you to

in intuitive but we'll introduce you to some some jargon some terminology that

some some jargon some terminology that most any programmer or computer

most any programmer or computer scientist might use when analyzing their

scientist might use when analyzing their own algorithms let's formalize now what

own algorithms let's formalize now what this kind of analysis is so right right

this kind of analysis is so right right now I claim binary search better than

now I claim binary search better than linear search but how much better and

linear search but how much better and why exactly well it all comes back to

why exactly well it all comes back to this kind of graph so this recall is how

this kind of graph so this recall is how we analyzed the phone book back in week

we analyzed the phone book back in week zero and recall that indeed we had these

zero and recall that indeed we had these these formulas rough formulas that

these formulas rough formulas that describe the running time of those three

describe the running time of those three algorithms one page at a time two pages

algorithms one page at a time two pages at a time and then tearing the thing

at a time and then tearing the thing again and again in half and precisely if

again and again in half and precisely if you count it up the number of pages I

you count it up the number of pages I was touching or the number of pages I

was touching or the number of pages I was tearing it's fair to say that the

was tearing it's fair to say that the first algorithm in the worst case might

first algorithm in the worst case might have taken n total Pages it didn't

have taken n total Pages it didn't because I was searching for John Harvard

because I was searching for John Harvard at the time which is somewhat early in

at the time which is somewhat early in the alphabet but if I were searching

the alphabet but if I were searching with some for someone with the last name

with some for someone with the last name of Z I would have had to keep going and

of Z I would have had to keep going and going in the worst case through all n

going in the worst case through all n Pages not as bad for the second

Pages not as bad for the second algorithm and that's why we do n divided

algorithm and that's why we do n divided by two and even that's a bit of a white

by two and even that's a bit of a white lie right it's probably n / 2 + 1 in

lie right it's probably n / 2 + 1 in case I have to double back but again I'm

case I have to double back but again I'm sort of doing this more generally to

sort of doing this more generally to capture the essence of these things and

capture the essence of these things and then we really got into the weeds with

then we really got into the weeds with like log base 2 of n for that third and

like log base 2 of n for that third and final algorithm and at the time we

final algorithm and at the time we claimed anytime you're doing dividing

claimed anytime you're doing dividing something in half in half in half odds

something in half in half in half odds are there's going to be some kind of

are there's going to be some kind of logarithm involved and we'll see that

logarithm involved and we'll see that today but today we're going to actually

today but today we're going to actually start using computer science terminology

start using computer science terminology and we're going to sort of formalize

and we're going to sort of formalize this imprecision if you will we are not

this imprecision if you will we are not going to care generally about exactly

going to care generally about exactly how many steps some algorithm takes

how many steps some algorithm takes because that's not going to be that

because that's not going to be that enlightening especially if maybe you

enlightening especially if maybe you have a faster computer tomorrow than you

have a faster computer tomorrow than you did today it wouldn't really be fair to

did today it wouldn't really be fair to compare numbers too precisely we really

compare numbers too precisely we really kind of want to with a wave of the hand

kind of want to with a wave of the hand just get a sense of roughly how slow or

just get a sense of roughly how slow or how fast an algorithm is so the notation

how fast an algorithm is so the notation here is deliberate that is literally a

here is deliberate that is literally a capital O often italicized referred to

capital O often italicized referred to as Big O and so the first algorithm is

as Big O and so the first algorithm is in Big O of n the second algorithm is in

in Big O of n the second algorithm is in Big O of n / two the third algorithm is

Big O of n / two the third algorithm is in Big O of log base 2 of n but even

in Big O of log base 2 of n but even that is kind of unnecessary detail when

that is kind of unnecessary detail when using Big O notation you really don't

using Big O notation you really don't care about we'll see the smaller ordered

care about we'll see the smaller ordered terms right we're not going to care

terms right we're not going to care about the divided by two because you

about the divided by two because you know what the shape of these algorithms

know what the shape of these algorithms is almost the same and really the idea

is almost the same and really the idea the algorithm itself is sort of

the algorithm itself is sort of fundamentally the same okay and instead

fundamentally the same okay and instead of one page at a time I'm doing two but

of one page at a time I'm doing two but if you throw millions of pages billions

if you throw millions of pages billions of pages at me those algorithms are

of pages at me those algorithms are really going to kind of perform the same

really going to kind of perform the same as n gets really large goes off toward

as n gets really large goes off toward infinity and the same is true for

infinity and the same is true for logarithms even if you're a little rusty

logarithms even if you're a little rusty it turns out that whether you do the

it turns out that whether you do the math with log base 2 log base 3 log base

math with log base 2 log base 3 log base 10 you can just multiply one by the

10 you can just multiply one by the other to really get the same formula

other to really get the same formula this is only to say a computer scientist

this is only to say a computer scientist would generally say that the first two

would generally say that the first two algorithms are on the order of eps the

algorithms are on the order of eps the third algorithm is on the order of log

third algorithm is on the order of log end steps and we don't really care

end steps and we don't really care precisely what we mean beyond that and

precisely what we mean beyond that and this Big O notation as we'll see and

this Big O notation as we'll see and actually let me let me zoom out if you

actually let me let me zoom out if you can imagine suddenly making the x- axis

can imagine suddenly making the x- axis much longer so more pages on the screen

much longer so more pages on the screen at once it is indeed going to be the

at once it is indeed going to be the shapes of these curves that matter

shapes of these curves that matter because imagine in your mind's eye as

because imagine in your mind's eye as you zoom out zoom out zoom out zoom out

you zoom out zoom out zoom out zoom out and as n gets much much much bigger on

and as n gets much much much bigger on the xaxis the red and the yellow line

the xaxis the red and the yellow line are essentially going to look the same

are essentially going to look the same once n is sufficiently large but the

once n is sufficiently large but the green line is never going to look the

green line is never going to look the same it's going to be a fundamentally

same it's going to be a fundamentally different shape and so that's the

different shape and so that's the intuition of bigo to get a sense of

intuition of bigo to get a sense of these uh rates of performance like this

these uh rates of performance like this so here then is Big O here is perhaps a

so here then is Big O here is perhaps a cheat sheet of like the common formulas

cheat sheet of like the common formulas that a computer scientist certainly in

that a computer scientist certainly in an introductory context might use when

an introductory context might use when analyzing algorithms and let's consider

analyzing algorithms and let's consider for a moment which of our first two

for a moment which of our first two algorithms linear search and binary

algorithms linear search and binary search fall into these categories so

search fall into these categories so I've ordered them from sort of slowest

I've ordered them from sort of slowest to fastest so order of n s it's not

to fastest so order of n s it's not something we've actually seen yet but it

something we've actually seen yet but it tends to be slow because it's quadratic

tends to be slow because it's quadratic you're doing n * n that's got to add up

you're doing n * n that's got to add up to a lot of steps better today is going

to a lot of steps better today is going to be n Lin even better is going to be n

to be n Lin even better is going to be n even better than that is login and best

even better than that is login and best is soall order of one like one step or

is soall order of one like one step or maybe two steps maybe even a thousand

maybe two steps maybe even a thousand steps but a fixed finite number of steps

steps but a fixed finite number of steps that never changes no how matter how big

that never changes no how matter how big n is so given this chart just to be

n is so given this chart just to be clear linear search let's consider the

clear linear search let's consider the worst case in the worst case how many

worst case in the worst case how many steps did it take someone like Stephanie

steps did it take someone like Stephanie to find the uh solution to the problem

to find the uh solution to the problem assuming not seven doors but n doors

assuming not seven doors but n doors yeah so on the order of N and in this

yeah so on the order of N and in this case it's exactly n but you know what

case it's exactly n but you know what you know maybe it's arguably too n right

you know maybe it's arguably too n right because it took Stephanie a couple of

because it took Stephanie a couple of steps like she had to lift the latch she

steps like she had to lift the latch she had to open the door maybe it's three

had to open the door maybe it's three steps you had to show the money so now

steps you had to show the money so now it's 3n 2 N but there we don't really

it's 3n 2 N but there we don't really care about that level of precision we

care about that level of precision we really just care about the fundamental

really just care about the fundamental number of operations so we'll say yes on

number of operations so we'll say yes on the order of n so that might be an upper

the order of n so that might be an upper bound we'll call this for linear search

bound we'll call this for linear search and how about binary search in Jackson's

and how about binary search in Jackson's case or in general me and week zero if

case or in general me and week zero if there's end doors how many steps did it

there's end doors how many steps did it take Jackson or me using binary

take Jackson or me using binary search in this case it was literally

search in this case it was literally three but that's not a

three but that's not a formula yeah so it's on the order of log

formula yeah so it's on the order of log n and indeed if there's seven doors well

n and indeed if there's seven doors well that's almost eight if you just do a

that's almost eight if you just do a little bit of rounding and indeed if you

little bit of rounding and indeed if you take log base 2 of eight okay so that

take log base 2 of eight okay so that does actually give us three so the math

does actually give us three so the math actually checks out and if you're not

actually checks out and if you're not comy with logarithms no big deal just

comy with logarithms no big deal just think about it intuitively uh logarithm

think about it intuitively uh logarithm of base two is just dividing something

of base two is just dividing something again and again so on this chart when we

again and again so on this chart when we consider Big O which to be clear allows

consider Big O which to be clear allows you to describe the order of an

you to describe the order of an algorithm's running time like the

algorithm's running time like the magnitude of it but it also describes

magnitude of it but it also describes more specifically an upper bound so in

more specifically an upper bound so in the worst case for instance these are

the worst case for instance these are pretty good measures of how good uh or

pretty good measures of how good uh or rather of how bad linear search and

rather of how bad linear search and binary search might be why well suppose

binary search might be why well suppose you're searching a thousand page phone

you're searching a thousand page phone book and the person's name starts with z

book and the person's name starts with z the algorithm is still going to be on

the algorithm is still going to be on the order of n steps why because it

the order of n steps why because it might take you as many as all end steps

might take you as many as all end steps to find it now that's not necessarily

to find it now that's not necessarily going to be the case in practice if I

going to be the case in practice if I use Big O as an upper bound well it

use Big O as an upper bound well it would be nice if there's sort of a

would be nice if there's sort of a corresponding lower bound especially if

corresponding lower bound especially if you want to consider not just best uh

you want to consider not just best uh worst cases but maybe best cases so what

worst cases but maybe best cases so what might we use here well so this is a

might we use here well so this is a capital Greek omega symbol so Omega is

capital Greek omega symbol so Omega is the symbol that a computer scientist

the symbol that a computer scientist uses generally to describe a lower bound

uses generally to describe a lower bound on an algorithm often in the context of

on an algorithm often in the context of best case though not necessarily so a

best case though not necessarily so a lower bound means how few steps might an

lower bound means how few steps might an algorithm take and here too same

algorithm take and here too same formulas and we'll fill in these blanks

formulas and we'll fill in these blanks over time some algorithms might always

over time some algorithms might always take a minimum of n squ steps or on the

take a minimum of n squ steps or on the order of n steps some might only take n

order of n steps some might only take n logn or n or log n or one so something

logn or n or log n or one so something like uh linear search when Stephanie

like uh linear search when Stephanie started with linear search she didn't

started with linear search she didn't get lucky this time on stage but what if

get lucky this time on stage but what if she had and the first door she opened

she had and the first door she opened were 50 how much you then describe the

were 50 how much you then describe the lower bound on linear search in this

lower bound on linear search in this so-called best

so-called best case using this list of possible

case using this list of possible answers yeah yeah so Omega of one so in

answers yeah yeah so Omega of one so in the best case the lower bound on how

the best case the lower bound on how many steps it might take uh linear

many steps it might take uh linear search to find something might just be

search to find something might just be one step why because maybe Stephanie had

one step why because maybe Stephanie had gotten lucky and we had pre-filled these

gotten lucky and we had pre-filled these lockers with the numbers in some other

lockers with the numbers in some other order such that she might have open the

order such that she might have open the first locker and waila the number 50

first locker and waila the number 50 could have been there so a lower bound

could have been there so a lower bound arguably could indeed be Omega of one

arguably could indeed be Omega of one for linear search and how about now for

for linear search and how about now for Jackson he used binary search so he

Jackson he used binary search so he dived right into the middle of the

dived right into the middle of the problem but what would be a lower bound

problem but what would be a lower bound on binary search using this logic yeah

on binary search using this logic yeah yeah so again Omega of one why because

yeah so again Omega of one why because maybe he just gets lucky and indeed

maybe he just gets lucky and indeed right in the middle of the lockers could

right in the middle of the lockers could have been the number 50 it wasn't and so

have been the number 50 it wasn't and so more Germain in Jackson's uh actual

more Germain in Jackson's uh actual practice would have been the Big O

practice would have been the Big O discussion but Big O and Omega upper

discussion but Big O and Omega upper bound and lower bound just allow a

bound and lower bound just allow a computer scientist to kind of wrestle

computer scientist to kind of wrestle with what could happen maybe in the

with what could happen maybe in the worst case what can happen in the best

worst case what can happen in the best case and you can even get even more

case and you can even get even more precise like the average case or the

precise like the average case or the like and this is indeed what Engineers

like and this is indeed what Engineers might do at a whiteboard in a company in

might do at a whiteboard in a company in a university when designing an algorithm

a university when designing an algorithm and trying to make arguments as to like

and trying to make arguments as to like why their algorithm is better than

why their algorithm is better than someone else's by way of these kinds of

someone else's by way of these kinds of analyses and just so you've seen it it

analyses and just so you've seen it it turns out that if some algorithm happens

turns out that if some algorithm happens to have an identical upper bound and

to have an identical upper bound and lower bound you can actually use a

lower bound you can actually use a capital Greek Theta as well and this is

capital Greek Theta as well and this is the last of the the Greek symbols today

the last of the the Greek symbols today but a Greek Theta indicates a coinci

but a Greek Theta indicates a coinci idents of both upper bound and lower

idents of both upper bound and lower bound that is they are one and the same

bound that is they are one and the same that was not the case for our discussion

that was not the case for our discussion a second ago of linear search not the

a second ago of linear search not the case for binary search but you could use

case for binary search but you could use the same kinds of formulas if it turns

the same kinds of formulas if it turns out that your upper bound and lower

out that your upper bound and lower bound are the same so for instance if I

bound are the same so for instance if I were to count everyone like literally in

were to count everyone like literally in this room 1 2 3 4 five six and so forth

this room 1 2 3 4 five six and so forth you could actually say that counting in

you could actually say that counting in that way is in Theta of n right because

that way is in Theta of n right because I in the best case it's going to take me

I in the best case it's going to take me end points at the uh people in the

end points at the uh people in the audience in the worst case it's going to

audience in the worst case it's going to take me n it's always going to take me

take me n it's always going to take me end steps if I want to count everyone in

end steps if I want to count everyone in the room you can't really do better than

the room you can't really do better than that unless you skip people so that

that unless you skip people so that would be an example off the cuff of

would be an example off the cuff of something where Theta is instead germine

something where Theta is instead germine are any questions now on Big O on Omega

are any questions now on Big O on Omega or Theta which are now just more formal

or Theta which are now just more formal tools in the toolkit for talking about

tools in the toolkit for talking about the design of our

the design of our algorithms any

algorithms any questions

questions no seeing none yeah oh is this yes no

no seeing none yeah oh is this yes no okay so we're good so let's go ahead and

okay so we're good so let's go ahead and translate this perhaps to some some

translate this perhaps to some some actual code let me go over to vs code

actual code let me go over to vs code here and let's see if we can't now

here and let's see if we can't now translate some of these ideas to some

translate some of these ideas to some actual code not so much using new syntax

actual code not so much using new syntax yet we're going to still operate in this

yet we're going to still operate in this world of arrays like last week so let me

world of arrays like last week so let me go ahead and create a program called

go ahead and create a program called search. C by executing code space

search. C by executing code space search. c in my terminal and then up

search. c in my terminal and then up here let's go ahead and include our

here let's go ahead and include our usual so include cs50.h so I can get

usual so include cs50.h so I can get some input include standard i.h so I can

some input include standard i.h so I can print some output we'll do int main void

print some output we'll do int main void which the meaning of which we did start

which the meaning of which we did start to tease apart last week the fact that

to tease apart last week the fact that it's void again today just means no

it's void again today just means no command line arguments and let me go

command line arguments and let me go ahead and do this let me go ahead and

ahead and do this let me go ahead and declare just for discussion's sake a

declare just for discussion's sake a static array like an array that never

static array like an array that never changes and the Syntax for this is going

changes and the Syntax for this is going to be give me an array called numbers

to be give me an array called numbers using the square bracket not ation and

using the square bracket not ation and I'm going to immediately initialize it

I'm going to immediately initialize it to 20 500 10 5 100 1 and 50 reminiscent

to 20 500 10 5 100 1 and 50 reminiscent of those same denominations as before so

of those same denominations as before so this is a slightly new syntax that we've

this is a slightly new syntax that we've uh perhaps not seen and the curly braces

uh perhaps not seen and the curly braces here which are different from for loops

here which are different from for loops and while loops and functions just tell

and while loops and functions just tell the compiler please give me an array of

the compiler please give me an array of whatever size this is containing those

whatever size this is containing those numbers left to right I could

numbers left to right I could alternatively use last week's syntax of

alternatively use last week's syntax of saying something like this let's see one

saying something like this let's see one 2 3 4 5 six seven denominations I could

2 3 4 5 six seven denominations I could alternatively do this and then I could

alternatively do this and then I could say numbers bracket 0 uh numbers bracket

say numbers bracket 0 uh numbers bracket 0 equals 20 numbers bracket 1 equal 500

0 equals 20 numbers bracket 1 equal 500 and I could do this five more times

and I could do this five more times that's just a little tedious if you know

that's just a little tedious if you know the numbers in advance you don't have to

the numbers in advance you don't have to tell the compiler how many there are you

tell the compiler how many there are you can just let it figure it out that your

can just let it figure it out that your numbers are be will be 20 500 10 5 100 1

numbers are be will be 20 500 10 5 100 1 and 5050 so this is how you statically

and 5050 so this is how you statically Define an array all right let me just go

Define an array all right let me just go ahead and ask the user now for a number

ahead and ask the user now for a number we'll call it n by using get int and

we'll call it n by using get int and prompting them for a number so nothing

prompting them for a number so nothing new there and now let me go ahead and

new there and now let me go ahead and Implement linear search and the pseudo

Implement linear search and the pseudo code we had for this before used some

code we had for this before used some array like notation let me go ahead then

array like notation let me go ahead then and start similarly four in I and it's

and start similarly four in I and it's you almost always start counting at I by

you almost always start counting at I by convention so that's perhaps a good

convention so that's perhaps a good starting point I'm going to do this so

starting point I'm going to do this so long as I is less than seven not the

long as I is less than seven not the best design to hard code the seven but

best design to hard code the seven but this is just for demonstration sake for

this is just for demonstration sake for now because I know how many numbers I

now because I know how many numbers I put in there and then I'm going to do

put in there and then I'm going to do i++ so now I have the beginnings of a

i++ so now I have the beginnings of a loop that will just allow me to iterate

loop that will just allow me to iterate over the entire array and let me ask

over the entire array and let me ask this if the current number at location I

this if the current number at location I equals equals n which is the number the

equals equals n which is the number the human typed in then let's go ahead and

human typed in then let's go ahead and do something simple like print F quote

do something simple like print F quote unquote found back sln and then per our

unquote found back sln and then per our discussion last week to indicate that

discussion last week to indicate that this is successful I'm going to going to

this is successful I'm going to going to return zero if I found it and if I don't

return zero if I found it and if I don't find it I'm just going to go down here

find it I'm just going to go down here and by default say not found back sln

and by default say not found back sln and just for convention whoops just for

and just for convention whoops just for good measure per convention I'll return

good measure per convention I'll return one or really any value other than zero

one or really any value other than zero zero recall means success and any other

zero recall means success and any other integer tends to mean error of some sort

integer tends to mean error of some sort irrespective of the number I'm looking

irrespective of the number I'm looking for so just to revisit the only thing

for so just to revisit the only thing that's new here is the syntax for

that's new here is the syntax for creating an array of seven numbers these

creating an array of seven numbers these numbers and then after after that we

numbers and then after after that we have really highlighted here an

have really highlighted here an implementation of linear search I mean

implementation of linear search I mean this is the C version I dare say of what

this is the C version I dare say of what Stephanie did on the board whereas now

Stephanie did on the board whereas now the array is called numbers instead of

the array is called numbers instead of doors but I think it's pretty much the

doors but I think it's pretty much the same let me go ahead and open my

same let me go ahead and open my terminal window and run make

terminal window and run make search seems to compile okay search and

search seems to compile okay search and let's go ahead and search for a number

let's go ahead and search for a number we'll start with what we did before 50

we'll start with what we did before 50 and it's found let's go ahead and run it

and it's found let's go ahead and run it again/ search let's search for maybe 20

again/ search let's search for maybe 20 at the beginning that one too is found

at the beginning that one too is found let's run it one more time searching for

let's run it one more time searching for like 1,000 which is not uh in among the

like 1,000 which is not uh in among the uh denominations and that one indeed is

uh denominations and that one indeed is not found so we've taken an idea from

not found so we've taken an idea from week zero now formalized in week three

week zero now formalized in week three and just translated it now to code

and just translated it now to code questions on this implementation of

questions on this implementation of linear

linear search linear

search linear search nothing oh so successful so far

search nothing oh so successful so far today okay so let's see if we can't

today okay so let's see if we can't maybe make this a little more

maybe make this a little more interesting and see if we can't trip

interesting and see if we can't trip over a detail that's going to be

over a detail that's going to be important in C instead of doing numbers

important in C instead of doing numbers let me go ahead and do this we'll stay

let me go ahead and do this we'll stay on theme with Monopoly and I went down

on theme with Monopoly and I went down the rabbit hole of reading the

the rabbit hole of reading the wikkipedia article on Monopoly and the

wikkipedia article on Monopoly and the original uh pieces or tokens that came

original uh pieces or tokens that came with Monopoly and it turns out we can

with Monopoly and it turns out we can represent those with strings so I'm

represent those with strings so I'm going to create an array called strings

going to create an array called strings plural of whatever size I defined here

plural of whatever size I defined here and the very first first Monopoly pieces

and the very first first Monopoly pieces back in the day were a battleship that

back in the day were a battleship that you could play with a boot a cannon an

you could play with a boot a cannon an iron a

iron a thimble and a top hat some of which you

thimble and a top hat some of which you might know from the game nowadays turns

might know from the game nowadays turns out they've been changing these uh had

out they've been changing these uh had no idea over the years so here is now an

no idea over the years so here is now an array of strings let me go ahead and

array of strings let me go ahead and prompt the user now not for an integer

prompt the user now not for an integer anymore I want to Now search for one of

anymore I want to Now search for one of these strings still using linear search

these strings still using linear search so let me create a string s set it equal

so let me create a string s set it equal to string prompt the user for a string

to string prompt the user for a string to search for and then I think my code

to search for and then I think my code here is almost the same except for one

here is almost the same except for one detail I now have an array called

detail I now have an array called strings I now have a variable called s

strings I now have a variable called s but it turns out for reasons we'll

but it turns out for reasons we'll explore in more detail next week this

explore in more detail next week this line of code is not going to work and it

line of code is not going to work and it turns out the reason has to do with what

turns out the reason has to do with what we discussed last week of like what a

we discussed last week of like what a string really is and what is a string

string really is and what is a string again a string is an array and it turns

again a string is an array and it turns out though that equals equals is not

out though that equals equals is not going to generously compare all of the

going to generously compare all of the characters in an array for you just

characters in an array for you just because you use equal equals it turns

because you use equal equals it turns out it's not going to compare every

out it's not going to compare every letter and so thankfully there is in the

letter and so thankfully there is in the uh string library that we introduced

uh string library that we introduced last week a solution to this problem the

last week a solution to this problem the reason for the problem we'll explore in

reason for the problem we'll explore in more detail next week but for now just

more detail next week but for now just know that when you want to compare

know that when you want to compare strings in C especially if you've come

strings in C especially if you've come into the class knowing a bit of Java or

into the class knowing a bit of Java or python or some other language you cannot

python or some other language you cannot use equals equals even though you could

use equals equals even though you could in scratch you cannot in C so what I

in scratch you cannot in C so what I have to actually do here is this I have

have to actually do here is this I have to ask the question does the return

to ask the question does the return value of a function called stir compare

value of a function called stir compare or stir

or stir comp equal zero when passed in the

comp equal zero when passed in the current string and that user input so if

current string and that user input so if you read the documentation for this

you read the documentation for this function called stir compare you'll see

function called stir compare you'll see that it takes two strings as input first

that it takes two strings as input first one and second one it then someone

one and second one it then someone decades ago wrote the code that probably

decades ago wrote the code that probably uses a four Loop or a while loop to

uses a four Loop or a while loop to compare every character in each of those

compare every character in each of those strings and it turns out it returns zero

strings and it turns out it returns zero if they are in fact equal turns out too

if they are in fact equal turns out too it will return a positive number or a

it will return a positive number or a negative number in other situations any

negative number in other situations any intuition for why it might actually be

intuition for why it might actually be useful to have a function that allows

useful to have a function that allows you to check if two strings are equal

you to check if two strings are equal if they're not equal what else might be

if they're not equal what else might be interesting to know when comparing two

are okay possibly maybe you want to know just how similar they are um and that's

just how similar they are um and that's indeed an algorithm unto itself but stir

indeed an algorithm unto itself but stir compare is a little simpler than

that exactly if you're trying to like alphabetize a whole list of strings just

alphabetize a whole list of strings just like your phone probably is for your

like your phone probably is for your context or address book it turns out

context or address book it turns out that stir compare will actually return a

that stir compare will actually return a positive number or a negative number or

positive number or a negative number or a zero based on whether maybe it comes

a zero based on whether maybe it comes alphabetically first or later or or in

alphabetically first or later or or in fact equal so that can be a useful thing

fact equal so that can be a useful thing and that's just a teaser for a lower

and that's just a teaser for a lower level explanation that we'll see next

level explanation that we'll see next week so now let me cross my fingers and

week so now let me cross my fingers and see if I got this right let me go ahead

see if I got this right let me go ahead and do make

and do make search did compile okay albeit slowly

search did compile okay albeit slowly do/ search and let's search for

do/ search and let's search for something like the thimble and we see

something like the thimble and we see that that's indeed found otherwise let's

that that's indeed found otherwise let's search for something that I know isn't

search for something that I know isn't there like a race car which was there

there like a race car which was there when I grew up but huh segmentation

when I grew up but huh segmentation fault core dumped like and actually some

fault core dumped like and actually some of you have tripped over this error

of you have tripped over this error before anyone want to admit seeing this

before anyone want to admit seeing this so yeah not something we've talked about

so yeah not something we've talked about and um honestly not something I intended

and um honestly not something I intended just now but that too we'll see next

just now but that too we'll see next week any intuition for why my program

week any intuition for why my program just

just broke I didn't really change the logic

broke I didn't really change the logic it's still linear search let me hide the

it's still linear search let me hide the terminal so you can see all of the code

terminal so you can see all of the code at once the only thing I did was

at once the only thing I did was switched from integers to Strings and I

switched from integers to Strings and I switched to stir compare here but

switched to stir compare here but segmentation fault happened and the

segmentation fault happened and the teaser is that that somehow relates to

teaser is that that somehow relates to the computer's memory

yeah yeah and this is subtle but spot on so 1 2 3 4 5 six elements total in this

so 1 2 3 4 5 six elements total in this array versus the seven number

array versus the seven number of Monopoly denominations that we had

of Monopoly denominations that we had earlier and this is where see sort of

earlier and this is where see sort of case in point this came back to bite me

case in point this came back to bite me the fact that I hardcoded this value as

the fact that I hardcoded this value as to opposed to maybe separating it out as

to opposed to maybe separating it out as a constant or declaring it higher up

a constant or declaring it higher up kind of bit me here because now I'm

kind of bit me here because now I'm iterating over an array of size six but

iterating over an array of size six but clearly I'm going one step too far

clearly I'm going one step too far because I'm literally going to iterate

because I'm literally going to iterate seven times not six so it's as though

seven times not six so it's as though I'm looking at memory that's over here

I'm looking at memory that's over here and indeed next week we'll focus on

and indeed next week we'll focus on memory and that's just a bad thing so

memory and that's just a bad thing so odds are not even and seeing your code

odds are not even and seeing your code from this past week if any of you have

from this past week if any of you have had segmentation faults odds are you

had segmentation faults odds are you touched memory that you shouldn't have

touched memory that you shouldn't have you maybe looped too many times you

you maybe looped too many times you might have uh used a negative number to

might have uh used a negative number to get into your array in general you

get into your array in general you touched memory that you shouldn't have

touched memory that you shouldn't have and you touched a segment of memory that

and you touched a segment of memory that you shouldn't have the fix though at

you shouldn't have the fix though at least in my case is simple just don't do

least in my case is simple just don't do that so let me go ahead and recompile

that so let me go ahead and recompile this make uh

this make uh search do/ search and I'll search again

search do/ search and I'll search again for uh race car and

for uh race car and and now it does not crash but it does

and now it does not crash but it does tell me it's not found so subtle but

tell me it's not found so subtle but something you might yourself have

something you might yourself have tripped over already questions then on

tripped over already questions then on what I just did intentionally or

what I just did intentionally or otherwise yeah in

front return don't return Z return so what

what the a really good question so the

the a really good question so the program will still work even if I don't

program will still work even if I don't return zero or return one in fact let me

return zero or return one in fact let me go ahead head and do that and just hide

go ahead head and do that and just hide my terminal window for a second let's

my terminal window for a second let's get rid of the return here let's get rid

get rid of the return here let's get rid of the return here however Watch What

of the return here however Watch What Happens here uh let me go ahead and

Happens here uh let me go ahead and recompile this make search Let Me scroll

recompile this make search Let Me scroll up in my code here let me go ahead and

up in my code here let me go ahead and do do/ search and let me go ahead and

do do/ search and let me go ahead and search for the first thing in the list

search for the first thing in the list Battleship so I know that this should be

Battleship so I know that this should be found I hit

found I hit enter huh interesting so it's saying

enter huh interesting so it's saying found not found but do you see why

found not found but do you see why logically in this case

exactly so the loop is still running so there's a couple of solutions to this I

there's a couple of solutions to this I could for instance somehow break out of

could for instance somehow break out of the code here but that's going to still

the code here but that's going to still result in line 18 executing I could then

result in line 18 executing I could then instead just return here I don't

instead just return here I don't strictly need to return one down at the

strictly need to return one down at the bottom but I made this claim last week

bottom but I made this claim last week that it tends to be helpful as your

that it tends to be helpful as your programs get more sophisticated to at

programs get more sophisticated to at least signify just like a real world

least signify just like a real world programmer error codes when something

programmer error codes when something goes wrong so returning zero in main is

goes wrong so returning zero in main is the easiest way to signify my code is

the easiest way to signify my code is done I'm ready to exit successfully

done I'm ready to exit successfully that's it but down here I could

that's it but down here I could absolutely still return zero because

absolutely still return zero because it's not a huge deal it's not really an

it's not a huge deal it's not really an error that deserves annoying the user

error that deserves annoying the user with some kind of popup that something

with some kind of popup that something went wrong but return one is just a

went wrong but return one is just a lower level way of signaling H it didn't

lower level way of signaling H it didn't really find what I was looking for and

really find what I was looking for and remember from last week you can see this

remember from last week you can see this as follows if I recompile this again now

as follows if I recompile this again now that I've reverted those changes so make

that I've reverted those changes so make search and if I do uh /search and search

search and if I do uh /search and search for Battleship which is indeed found

for Battleship which is indeed found recall I can execute this magical

recall I can execute this magical command Echo dollar sign question mark

command Echo dollar sign question mark which you're not going to often execute

which you're not going to often execute but it shows you what main returned if I

but it shows you what main returned if I run search again and search for race car

run search again and search for race car which is not found I see not found but I

which is not found I see not found but I can also run this command again and see

can also run this command again and see that oh it returned one so now if you

that oh it returned one so now if you fast forward a few months a few years

fast forward a few months a few years when you're actually writing code in a

when you're actually writing code in a company or for larger projects you might

company or for larger projects you might want to be automating software you might

want to be automating software you might not want the human to necessarily be

not want the human to necessarily be running it manually you might want um

running it manually you might want um code to be automated by some nightly

code to be automated by some nightly process or something like that using

process or something like that using these exit codes can a program determine

these exit codes can a program determine yes or no that other code succeeded or

yes or no that other code succeeded or failed other questions on linear search

failed other questions on linear search in this

in this way no all right well let's translate

way no all right well let's translate this to one other feature of uh C here

this to one other feature of uh C here by incorporating these ideas now into

by incorporating these ideas now into one other program so I'm going to create

one other program so I'm going to create a phone book in C by doing code space

a phone book in C by doing code space phone

book.car a phone book for an actual name and getting back a number so I'm going

and getting back a number so I'm going to go ahead and quickly include some of

to go ahead and quickly include some of the same things cs50.h so we can get

the same things cs50.h so we can get input uh standard io. so we can print

input uh standard io. so we can print output and I'm going to preemptively

output and I'm going to preemptively include string.h in case we need that

include string.h in case we need that one as well uh int main void no need for

one as well uh int main void no need for uh command line arguments today and let

uh command line arguments today and let me me give myself now an array of names

me me give myself now an array of names for this phone book so string names

for this phone book so string names equals and then in curly braces how

equals and then in curly braces how about Carter will be one person in the

about Carter will be one person in the phone book and David myself will be the

phone book and David myself will be the other so we'll keep it short so we don't

other so we'll keep it short so we don't have to type too many names but this is

have to type too many names but this is a phone book with two people thus far

a phone book with two people thus far suppose now we want to also store

suppose now we want to also store Carter's phone number in mind so it's

Carter's phone number in mind so it's not just saying found or not found it's

not just saying found or not found it's literally looking up our phone numbers

literally looking up our phone numbers like a proper phone book well at the

like a proper phone book well at the moment there's really no way to do this

moment there's really no way to do this I could do something hackish like I

I could do something hackish like I could put a number like

could put a number like 617495 1000 after Carter I could maybe

617495 1000 after Carter I could maybe do something like 949 uh 468 2750 after

do something like 949 uh 468 2750 after me but now you're kind of doing the

me but now you're kind of doing the whole apples and oranges thing right

whole apples and oranges thing right like now it's not strings it's a string

like now it's not strings it's a string int string int all right so maybe I

int string int all right so maybe I could just make all of these strings but

could just make all of these strings but now it's just a conceptual mixing of

now it's just a conceptual mixing of apples and oranges like yes that's an

apples and oranges like yes that's an array of four strings but now you're on

array of four strings but now you're on the honor System to know that the first

the honor System to know that the first string is a name the second string is a

string is a name the second string is a number the third string is like you can

number the third string is like you can do it but it's a bit of a hack so to

do it but it's a bit of a hack so to speak so what might be cleaner than this

speak so what might be cleaner than this instead of combining our phone numbers

instead of combining our phone numbers into the same array as our names what

into the same array as our names what else might we do that's perhaps a little

else might we do that's perhaps a little better say a little

better say a little Lou a 2d array uh possibly something we

Lou a 2d array uh possibly something we could do I'm going to keep it even

could do I'm going to keep it even simpler now because we haven't used

simpler now because we haven't used those by name even though that is we saw

those by name even though that is we saw last week technically what argv is what

last week technically what argv is what else could I do if I want to store names

else could I do if I want to store names and numbers

and numbers yeahp yeah let me go with this

yeahp yeah let me go with this suggestion just it's a little simpler

suggestion just it's a little simpler rather than complicate things in

rather than complicate things in literally different dimensions let me go

literally different dimensions let me go ahead and do string well I could do int

ahead and do string well I could do int numbers but you know what so that we can

numbers but you know what so that we can support punctuation like dashes or even

support punctuation like dashes or even parentheses or country codes I'm going

parentheses or country codes I'm going to do this instead I'm going to do

to do this instead I'm going to do string numbers so that I can represent

string numbers so that I can represent Carter's number as quote unquote plus

Carter's number as quote unquote plus one for the US 617 495 1,000 complete

one for the US 617 495 1,000 complete with hyphens as is us convention and

with hyphens as is us convention and then for mine I'll go ahead and do + one

then for mine I'll go ahead and do + one 949 how about 468

949 how about 468 275 semi colon and now down below let's

275 semi colon and now down below let's actually enable the user to search this

actually enable the user to search this phone book just like in week zero we did

phone book just like in week zero we did string name equals get string and let's

string name equals get string and let's ask the user for a name presumably David

ask the user for a name presumably David or Carter or someone else and now let's

or Carter or someone else and now let's re-implement linear search so four in I

re-implement linear search so four in I gets zero I is less than two and do as I

gets zero I is less than two and do as I say not as I do I think we should beware

say not as I do I think we should beware this coding but we'll keep it simple for

this coding but we'll keep it simple for now i++ and then in this for loop I

now i++ and then in this for loop I think we have all of the ingredients to

think we have all of the ingredients to solve this so if the return value of

solve this so if the return value of stir compare of all of the names bracket

stir compare of all of the names bracket I comparing against the name that the

I comparing against the name that the human typed in if all of that equals

human typed in if all of that equals equals zero that is all of the

equals zero that is all of the characters in those two strings are

characters in those two strings are equal then I think we can go ahead and

equal then I think we can go ahead and say found just like last time but you

say found just like last time but you know what let's actually print Carter or

know what let's actually print Carter or my phone number so found percent s and

my phone number so found percent s and we'll plug in numbers bracket I and then

we'll plug in numbers bracket I and then just for consistency I'll return zero

just for consistency I'll return zero here and down here how about I'll say

here and down here how about I'll say something like uh print F not found just

something like uh print F not found just to be clear and then I'll return one as

to be clear and then I'll return one as well so just a recap here's all of the

well so just a recap here's all of the code it's almost the same as before

code it's almost the same as before except now it's useful I'm not just

except now it's useful I'm not just saying found or not found I found a

saying found or not found I found a number in Monopoly or I found a a piece

number in Monopoly or I found a a piece in Monopoly I'm looking up in one array

in Monopoly I'm looking up in one array of the strings and then I'm printing

of the strings and then I'm printing from the other array the answer so let

from the other array the answer so let me go ahead here and run the compiler

me go ahead here and run the compiler make phone book enter okay that's

make phone book enter okay that's promising no errors do/ phonebook now

promising no errors do/ phonebook now and let's search for for instance Carter

and let's search for for instance Carter enter all right so we found Carter's

enter all right so we found Carter's number all right let me do that again

number all right let me do that again phone book Let's search for David all

phone book Let's search for David all right we seem to have found David's

right we seem to have found David's number all right let's do it one last

number all right let's do it one last time phone book enter and now we'll

time phone book enter and now we'll search for like John Harvard enter not

search for like John Harvard enter not found

found all right so I dare say albeit with

all right so I dare say albeit with minimal testing this code is

minimal testing this code is correct would anyone now like to

correct would anyone now like to critique the design does something rub

critique the design does something rub you the wrong way perhaps about this

you the wrong way perhaps about this approach

here and as always think about how if the program maybe gets longer more

the program maybe gets longer more complicated how decisions like this

complicated how decisions like this might unfold

might unfold yeah okay so if I is less than two so

yeah okay so if I is less than two so technically I if I change the number of

technically I if I change the number of people in this phone book I'm going to

people in this phone book I'm going to have to update I and we've already seen

have to update I and we've already seen that I get myself into trouble so that's

that I get myself into trouble so that's bad design

goodes yeah so again I'm sort of trusting myself not to screw up if I add

trusting myself not to screw up if I add John or anyone else to the first array

John or anyone else to the first array but I forget to add their number to the

but I forget to add their number to the second array you know eventually things

second array you know eventually things are going to drift and be inconsistent

are going to drift and be inconsistent and then the just code will be incorrect

and then the just code will be incorrect at that point so so sort of a poor

at that point so so sort of a poor design setting me up for future failure

design setting me up for future failure if you will other thoughts

yeah yeah really good we're assuming the same order from left to right the names

same order from left to right the names go and from left to right the numbers go

go and from left to right the numbers go but that's kind of just the honor System

but that's kind of just the honor System like there's literally nothing in code

like there's literally nothing in code preventing me from reversing the order

preventing me from reversing the order for whatever reason or maybe sorting the

for whatever reason or maybe sorting the names like they're sorted now and maybe

names like they're sorted now and maybe that's deliberate but maybe it's not so

that's deliberate but maybe it's not so this honor System here too is just not

this honor System here too is just not good right I could put a comment in here

good right I could put a comment in here to remind myself to uh you know note to

to remind myself to uh you know note to self always update arrays the same way

self always update arrays the same way but like that's something's going to

but like that's something's going to happen eventually especially when we

happen eventually especially when we have not two but three but 30 300 names

have not two but three but 30 300 names and numbers it would be nice to keep all

and numbers it would be nice to keep all of the related data together and so in

of the related data together and so in fact the one new feature of C we'll

fact the one new feature of C we'll introduce today is one that actually

introduce today is one that actually allows us to implement our very own data

allows us to implement our very own data structures you can think of a raay as a

structures you can think of a raay as a very lightweight data structure and that

very lightweight data structure and that allows you to Cluster related data back

allows you to Cluster related data back to back to back to back and this is how

to back to back to back and this is how strings are implemented they are a data

strings are implemented they are a data structure effectively implemented with

structure effectively implemented with an array but with c and with other

an array but with c and with other languages it turns out you can invent

languages it turns out you can invent your own data types whether they're one

your own data types whether they're one dimensional two-dimensional even or

dimensional two-dimensional even or Beyond and with uh with C can you

Beyond and with uh with C can you specifically create your own types that

specifically create your own types that have their own name so for instance

have their own name so for instance wouldn't it have been nice if C came

wouldn't it have been nice if C came with not just Char and int and float and

with not just Char and int and float and uh uh long and and others wouldn't it be

uh uh long and and others wouldn't it be nice if C came with a data type called

nice if C came with a data type called person and ideally a person would have a

person and ideally a person would have a name and a number now that's a little

name and a number now that's a little naive and unrealistic like why would

naive and unrealistic like why would they define a person to have just those

they define a person to have just those two Fields certainly people could have

two Fields certainly people could have disagreed what a person is so they leave

disagreed what a person is so they leave it to us like the authors of C gave us

it to us like the authors of C gave us all of these Primitives inss and floats

all of these Primitives inss and floats and strings and so forth but it's up to

and strings and so forth but it's up to us now to use those in a more

us now to use those in a more interesting way so that we can create an

interesting way so that we can create an array of person variables if you will

array of person variables if you will inside of an array called people just to

inside of an array called people just to pluralize it here so how are we going to

pluralize it here so how are we going to do this well for now let's just

do this well for now let's just stipulate that a person in the world

stipulate that a person in the world will have a name and a number that we

will have a name and a number that we could argue all day long what else a

could argue all day long what else a person should have and that's fine you

person should have and that's fine you can invent your own person

can invent your own person eventually at the moment I'm using just

eventually at the moment I'm using just two variables to define a person's name

two variables to define a person's name and number but wouldn't it be nice to

and number but wouldn't it be nice to encapsulate that is combined these two

encapsulate that is combined these two data types into a new and improved data

data types into a new and improved data type called person and the Syntax for

type called person and the Syntax for that is going to be this so it's a bit

that is going to be this so it's a bit of a mouthful but you can perhaps infer

of a mouthful but you can perhaps infer what some of this is doing here so it

what some of this is doing here so it turns out C has a keyword called type

turns out C has a keyword called type def as the name kind of suggests this

def as the name kind of suggests this allows you to Define your own type

allows you to Define your own type struct is an indication that it's a

struct is an indication that it's a structure it's like a a structure that

structure it's like a a structure that has multiple values inside of it that

has multiple values inside of it that you are trying to Define and then at the

you are trying to Define and then at the very bottom here outside of the Curve

very bottom here outside of the Curve early braces is the name of the type

early braces is the name of the type that you want to create so you don't

that you want to create so you don't have discretion over using type def or

have discretion over using type def or struct in this particular case but you

struct in this particular case but you can name the thing whatever you want and

can name the thing whatever you want and you can put anything in the structure

you can put anything in the structure that you want as well and as soon as

that you want as well and as soon as this semicolon is executed at the bottom

this semicolon is executed at the bottom of the code every line thereafter can

of the code every line thereafter can now have access to a person data type

now have access to a person data type whether as a single variable or as an

whether as a single variable or as an entire array so if I want to build on

entire array so if I want to build on this then let me go ahead and do this

this then let me go ahead and do this let me go back to my C code here and I'm

let me go back to my C code here and I'm going to go ahead and uh change just a

going to go ahead and uh change just a couple of things let's go ahead and do

couple of things let's go ahead and do this I'm going to go ahead and first get

this I'm going to go ahead and first get rid of those two hardcoded arrays and

rid of those two hardcoded arrays and let me go ahead and at the top of my

let me go ahead and at the top of my file invent this type so type def struct

file invent this type so type def struct inside of it will be a string name and

inside of it will be a string name and then a string number and then the name

then a string number and then the name of this structure will be person and

of this structure will be person and best practice would have me Define at

best practice would have me Define at the very top of my file so that any of

the very top of my file so that any of my functions in fact could use it even

my functions in fact could use it even though I just have Main in this case now

though I just have Main in this case now if I want it I could do this like person

if I want it I could do this like person P1 and person P2 but we know from last

P1 and person P2 but we know from last week like that already is bad design if

week like that already is bad design if you want to have multiple instances of

you want to have multiple instances of the same type of variable we should

the same type of variable we should probably use what

instead and yeah an array so let me not even go down that road let me instead

even go down that road let me instead just do this person uh will be the type

just do this person uh will be the type of the array but I'm going to call it I

of the array but I'm going to call it I could call it persons but in English we

could call it persons but in English we typically say people so I'll call the

typically say people so I'll call the array people and I want two people to

array people and I want two people to exist in this array though I could

exist in this array though I could certainly change that number to be

certainly change that number to be anything I want how now do you put a

anything I want how now do you put a name inside of a person and then put the

name inside of a person and then put the number inside of that same person well

number inside of that same person well slightly new syntax today I'm going to

slightly new syntax today I'm going to go ahead and say this people bracket

go ahead and say this people bracket zero just gives me the first person in

zero just gives me the first person in the array that's not new but if you want

the array that's not new but if you want to go inside of that person in memory

to go inside of that person in memory you use a DOT and then you just specify

you use a DOT and then you just specify the name of the attribute therein so if

the name of the attribute therein so if I want to set the first person's name to

I want to set the first person's name to Carter I just use that so-called dot

Carter I just use that so-called dot notation and then if I want to set

notation and then if I want to set Carter's number using dot notation I

Carter's number using dot notation I would do this plus one

would do this plus one 617495 1000 and then if I want to do the

617495 1000 and then if I want to do the same for myself I would now do people

same for myself I would now do people bracket

bracket 1name equals quote unquote David and

1name equals quote unquote David and then people bracket one still number

then people bracket one still number equals quote unquote + 1

equals quote unquote + 1 949468

949468 2750 and now at the bottom of my file I

2750 and now at the bottom of my file I think my logic can pretty much stay the

think my logic can pretty much stay the same I can still on this line here

same I can still on this line here prompt the user for the name of the

prompt the user for the name of the person they want to look up for now even

person they want to look up for now even though I admit it's not the best design

though I admit it's not the best design I'm just doing this for demonstration

I'm just doing this for demonstration sake I'm going to leave the two there

sake I'm going to leave the two there because I know I have two people but

because I know I have two people but down here this is going to have to

down here this is going to have to change I don't want to compare names

change I don't want to compare names bracket I anymore what do I want to type

bracket I anymore what do I want to type here as the first first argument to stir

here as the first first argument to stir compare what do I want to do here

compare what do I want to do here yeah so people i. name yeah so I want to

yeah so people i. name yeah so I want to go into the people array at the I

go into the people array at the I location because that's what my Loop is

location because that's what my Loop is doing it's updating I again and again

doing it's updating I again and again and then look at name and that's good I

and then look at name and that's good I think now I need to change this too what

think now I need to change this too what do I want to print if the person is

do I want to print if the person is found someone

found someone else what do I want to print here if I

else what do I want to print here if I found the person's name

yeah say a little louder perfect so people bracket. number

louder perfect so people bracket. number if indeed I want to print the

if indeed I want to print the corresponding number to this person and

corresponding number to this person and then I think the rest of my code can

then I think the rest of my code can stay the same so let me go ahead and run

stay the same so let me go ahead and run make phonebook to recompile this version

make phonebook to recompile this version so far so good/ phonebook let's go ahead

so far so good/ phonebook let's go ahead and type in Carter's name found all

and type in Carter's name found all right let's go ahead and run it again

right let's go ahead and run it again David's name found let's go ahead and

David's name found let's go ahead and run it one more time type in John

run it one more time type in John Harvard for instance not found in this

Harvard for instance not found in this case so fundamentally the code isn't all

case so fundamentally the code isn't all that different linear search is still

that different linear search is still behaving the same way and I admit this

behaving the same way and I admit this is kind of ugly looking like we've kind

is kind of ugly looking like we've kind of made a two-line solution like five

of made a two-line solution like five lines of code now but if we fast forward

lines of code now but if we fast forward a week or two when we start saving

a week or two when we start saving information to files uh we'll introduce

information to files uh we'll introduce you to files like CSV value CSV files

you to files like CSV value CSV files comma separated values or spreadsheet

comma separated values or spreadsheet files which you've surely opened on your

files which you've surely opened on your Mac or PC at some point in the past

Mac or PC at some point in the past suffice it to say We'll soon learn

suffice it to say We'll soon learn Techni for storing information like

Techni for storing information like names and numbers in files and at that

names and numbers in files and at that point we're not going to do any of this

point we're not going to do any of this hackish sort of hardcoding of the number

hackish sort of hardcoding of the number two and manually typing my name and

two and manually typing my name and Carter's name and number into our

Carter's name and number into our program we'll read the information

program we'll read the information dynamically from a file and in a few

dynamically from a file and in a few weeks we'll read it dynamically from a

weeks we'll read it dynamically from a database instead but this is for now

database instead but this is for now just syntactically how we can create an

just syntactically how we can create an array of size two containing one person

array of size two containing one person each we can update the name and number

each we can update the name and number of the first person update the name and

of the first person update the name and the number of the second person and then

the number of the second person and then later search across those names and

later search across those names and print out the corresponding numbers and

print out the corresponding numbers and in this sense this is a better design

in this sense this is a better design why because my person data type

why because my person data type encapsulates now everything that it

encapsulates now everything that it means to be a person at least in this

means to be a person at least in this narrow world and if I want to add

narrow world and if I want to add something to the notion of a person for

something to the notion of a person for instance I could go up to my typed Def

instance I could go up to my typed Def and tomorrow add an address to every

and tomorrow add an address to every person and start reading that in as well

person and start reading that in as well and now it's not the honor System it's

and now it's not the honor System it's not a names array a numbers array and a

not a names array a numbers array and a addesses array and everything else you

addesses array and everything else you might imagine related to a person it's

might imagine related to a person it's all encapsulated which is a a term of

all encapsulated which is a a term of art inside of the same type reminiscent

art inside of the same type reminiscent if some of you have programmed before of

if some of you have programmed before of something called objectoriented

something called objectoriented programming but we're not there yet C is

programming but we're not there yet C is not that questions on this use of struct

not that questions on this use of struct or this new syntax the dot operator

or this new syntax the dot operator being really the juicy part

being really the juicy part here any questions

here any questions yeah

on what line number 16 so yes so syntactically we

number 16 so yes so syntactically we introduced the square brackets last week

introduced the square brackets last week so doing people bracket zero just means

so doing people bracket zero just means go to the first person in the array that

go to the first person in the array that was like when Stephanie literally opened

was like when Stephanie literally opened this door that's uh that's doors bracket

this door that's uh that's doors bracket zero but this is of course people

zero but this is of course people bracket zero instead today the dot is a

bracket zero instead today the dot is a new piece of syntax it means go inside

new piece of syntax it means go inside of that person in memory

of that person in memory and look at the name therein and set it

and look at the name therein and set it equal to Carter and do the same for

equal to Carter and do the same for number so that's all it's like open the

number so that's all it's like open the locker door go inside of it and check or

locker door go inside of it and check or set the name and the number

yeah attributes is fine uh good question in the struct can you set default values

in the struct can you set default values short answer no and this is where C

short answer no and this is where C becomes less featurable than more more

becomes less featurable than more more modern languages like Python and Java

modern languages like Python and Java and others where you can in fact do that

and others where you can in fact do that so when we transition to python in a few

so when we transition to python in a few weeks time we'll see how we can start

weeks time we'll see how we can start solving problems like that but for now

solving problems like that but for now it's up to you to initialize name and

it's up to you to initialize name and number to something

yeah really good question how can we adjust or critique the design of what

adjust or critique the design of what I'm doing

I'm doing this is one of the few situations where

this is one of the few situations where I would say hypocritically do as I say

I would say hypocritically do as I say not as I do I am using pretty ugly lines

not as I do I am using pretty ugly lines like this just to introduce the syntax

like this just to introduce the syntax but my claim pedagogically today is that

but my claim pedagogically today is that eventually when we start storing names

eventually when we start storing names and numbers or other things in files or

and numbers or other things in files or in databases you won't have this

in databases you won't have this redundancy you'll have one line of code

redundancy you'll have one line of code or two lines of code that read the

or two lines of code that read the information from the file or database

information from the file or database and then fill the entire array with that

and then fill the entire array with that data for now I'm just doing it manually

data for now I'm just doing it manually so is to keep our Focus only on the new

so is to keep our Focus only on the new syntax but that's it so forgive the bad

syntax but that's it so forgive the bad Design By Design today other questions

Design By Design today other questions on

on this all right that's been a lot already

this all right that's been a lot already why don't we go ahead and take our

why don't we go ahead and take our 10-minute break with snacks first we

10-minute break with snacks first we have some delightful brownies in the

have some delightful brownies in the lobby all right we are back and up until

lobby all right we are back and up until now it clearly seems to be a good thing

now it clearly seems to be a good thing if your data is sorted because you can

if your data is sorted because you can use binary search you know a little more

use binary search you know a little more some little something more about the

some little something more about the data uh but it turns out that sorting of

data uh but it turns out that sorting of itself is kind of a problem to solve too

itself is kind of a problem to solve too and you might think well if sorting is

and you might think well if sorting is going to be pretty fast we absolutely

going to be pretty fast we absolutely should do it before we start searching

should do it before we start searching because that'll just speed up all of our

because that'll just speed up all of our searches but if sorting is slow that

searches but if sorting is slow that kind of invites the question well should

kind of invites the question well should we bother sorting our data if we're only

we bother sorting our data if we're only going to search the data maybe once

going to search the data maybe once maybe twice and so here is going to be

maybe twice and so here is going to be potentially a trade-off so let's

potentially a trade-off so let's consider what it means really to sort

consider what it means really to sort data in our case it's just going to be

data in our case it's just going to be simple and use numbers but it might in

simple and use numbers but it might in the case of the Googles of the World be

the case of the Googles of the World be actual web pages or persons or the like

actual web pages or persons or the like so here is our typical picture for

so here is our typical picture for sorting input uh for solving any problem

sorting input uh for solving any problem input at left and output at right the

input at left and output at right the input to our sort problem is going to be

input to our sort problem is going to be uh some unsorted set of values and the

uh some unsorted set of values and the output ideally will be the same set of

output ideally will be the same set of values sorted and if we do this

values sorted and if we do this concretely let's suppose that we want to

concretely let's suppose that we want to go about sorting this list of numbers 7

go about sorting this list of numbers 7 2 5 4 1 6 03 so it's all of the numbers

2 5 4 1 6 03 so it's all of the numbers from 0 to 7 but they're somehow jumbled

from 0 to 7 but they're somehow jumbled up randomly that's going to be the input

up randomly that's going to be the input to the problem and the goal is now to

to the problem and the goal is now to sort those so that you indeed get out 0

sort those so that you indeed get out 0 1 2 3 4 5 67 instead so it turns out

1 2 3 4 5 67 instead so it turns out there's lots of different ways we can

there's lots of different ways we can actually sort numbers like these here um

actually sort numbers like these here um and in fact just to complement our

and in fact just to complement our search example earlier could we perhaps

search example earlier could we perhaps quickly get some eight volunteers to

quickly get some eight volunteers to come up if you're comfortable appearing

come up if you're comfortable appearing on the internet if you want to do one

on the internet if you want to do one okay uh two three four five 6 7 eight

okay uh two three four five 6 7 eight how about all right come on

down all right come on over here and I'll give

right come on over here and I'll give you each a number and if you want to

you each a number and if you want to start to organize yourselves in the same

start to organize yourselves in the same order you see the numbers on the

order you see the numbers on the board okay so look up on the overhead

board okay so look up on the overhead and organize yourselves from left to

and organize yourselves from left to right in that same

right in that same order and let's have the first of you

order and let's have the first of you perfect if you want to come uh right

perfect if you want to come uh right over here

over here how about right in line with this all

how about right in line with this all right and a few more

right and a few more numbers all right number

numbers all right number two six

two six and perfect just the right number all

and perfect just the right number all right no uhoh all right there we go

right no uhoh all right there we go number three all right so let's just do

number three all right so let's just do a quick check we have 7 2 5 4 1 603 very

a quick check we have 7 2 5 4 1 603 very good so far do you want to just scooch a

good so far do you want to just scooch a little this way just to make a little

little this way just to make a little more of room all right and let's

more of room all right and let's consider now who we have here on stage

consider now who we have here on stage you want to each say a quick hello to

you want to each say a quick hello to the audience hi my name is Ryan uh I'm a

the audience hi my name is Ryan uh I'm a first year from Penny

first year from Penny Packer hi my name is Cel I'm a first

Packer hi my name is Cel I'm a first year at Strauss um hi my name is Lucy

year at Strauss um hi my name is Lucy I'm a first year from

I'm a first year from greo hi my name is Shiloh I'm a first

greo hi my name is Shiloh I'm a first year in

year in Wigglesworth hi my name is Jack and I'm

Wigglesworth hi my name is Jack and I'm a first year in

a first year in St hi my name is Katherine I'm a first

St hi my name is Katherine I'm a first year in stra hi my name is Michael and

year in stra hi my name is Michael and my first year at Penny Packer hi my name

my first year at Penny Packer hi my name is Muhammad at my first here in massuse

is Muhammad at my first here in massuse nice welcome aboard all right so let's

nice welcome aboard all right so let's consider now how we might go about

consider now how we might go about sorting Our Kind volunteers here the

sorting Our Kind volunteers here the goal being to get them into order from

goal being to get them into order from uh smallest to largest so that

uh smallest to largest so that presumably then we can use something

presumably then we can use something smarter than just linear search we could

smarter than just linear search we could actually use binary search assuming that

actually use binary search assuming that they are already then sorted so let me

they are already then sorted so let me propose that we first consider an

propose that we first consider an algorithm that actually has a name

algorithm that actually has a name called selection sort and selection sort

called selection sort and selection sort is going to be one that literally has me

is going to be one that literally has me or really you as the programmer

or really you as the programmer selecting the smallest element again and

selecting the smallest element again and again and then putting them into the

again and then putting them into the appropriate place so let me go ahead and

appropriate place so let me go ahead and start this here uh starting with the

start this here uh starting with the number seven at the moment seven is the

number seven at the moment seven is the smallest number I found so I'm going to

smallest number I found so I'm going to make mental note of that with a mental

make mental note of that with a mental variable if you will I'm going to move

variable if you will I'm going to move on now oh number two is obviously

on now oh number two is obviously smaller so I'm just going to update my

smaller so I'm just going to update my mental reminder that two is now the

mental reminder that two is now the smallest effectively forgetting for now

smallest effectively forgetting for now number seven uh five not smaller four

number seven uh five not smaller four not smaller one smaller and I'm going to

not smaller one smaller and I'm going to make mental note of that six not smaller

make mental note of that six not smaller zero smaller I'll make mental note of

zero smaller I'll make mental note of that having forgotten now everything

that having forgotten now everything else and now number three is not smaller

else and now number three is not smaller so what's your name again Michael so

so what's your name again Michael so Michael is number zero he belongs of

Michael is number zero he belongs of course way down there but unfortunately

course way down there but unfortunately you are Ryan Ryan Ryan is in the way so

you are Ryan Ryan Ryan is in the way so what should we do how should we start to

what should we do how should we start to sort this

sort this list where should number zero

list where should number zero go yeah do you want to say the

L yeah so let's just go ahead and swap so if you want to go ahead and zero go

so if you want to go ahead and zero go on where seven is we need to make room

on where seven is we need to make room for number seven it would kind of be

for number seven it would kind of be cheating if maybe everyone kind of

cheating if maybe everyone kind of politely stepped over to the side why

politely stepped over to the side why because if we imagine all of our

because if we imagine all of our volunteers here to be in Array like

volunteers here to be in Array like that's a crazy amount of work to have

that's a crazy amount of work to have every element in the array shift to the

every element in the array shift to the left just to make room so we're going to

left just to make room so we're going to keep it simple and just evict whoever is

keep it simple and just evict whoever is there now maybe we get lucky and number

there now maybe we get lucky and number seven is actually closer to its

seven is actually closer to its destination maybe we get unlucky and it

destination maybe we get unlucky and it goes farther away but we've at least

goes farther away but we've at least solved one problem if we had n problems

solved one problem if we had n problems at first now we have n minus one because

at first now we have n minus one because number zero is indeed in the right place

number zero is indeed in the right place so if I continue to act this out let me

so if I continue to act this out let me go ahead and say two okay currently the

go ahead and say two okay currently the smallest five no four no one currently

smallest five no four no one currently the smallest I'll make mental note 6 7 3

the smallest I'll make mental note 6 7 3 and now let me pause one is obviously

and now let me pause one is obviously the now smallest element so did I need

the now smallest element so did I need to keep going well it turns out at least

to keep going well it turns out at least as I've defined selection sort I do need

as I've defined selection sort I do need to keep going because I only claim that

to keep going because I only claim that I'm using one variable in my mind to

I'm using one variable in my mind to remember the then smallest element I'm

remember the then smallest element I'm not smart enough like us humans to

not smart enough like us humans to remember oh wait a minute one is

remember oh wait a minute one is definitely the smallest now I don't have

definitely the smallest now I don't have that whole recollection so I just am

that whole recollection so I just am keeping track of the now smallest so

keeping track of the now smallest so number one what your name was Jack Jack

number one what your name was Jack Jack where should jack go probably there and

where should jack go probably there and what's your name itel itel okay so Jack

what's your name itel itel okay so Jack and itel if you want to swap places

and itel if you want to swap places we've now solved two of the end total

we've now solved two of the end total problems and now we'll do it a little

problems and now we'll do it a little faster if each of you want to sort of

faster if each of you want to sort of start to swap as I find the right person

start to swap as I find the right person so five smallest four is smaller two is

so five smallest four is smaller two is smaller got to keep checking okay two

smaller got to keep checking okay two was smaller all right now I'm going to

was smaller all right now I'm going to go back to the beginning all right four

go back to the beginning all right four is small five is not six is not seven is

is small five is not six is not seven is oh three is small where do you want to

oh three is small where do you want to go okay good I'm going to go back here

go okay good I'm going to go back here and I can be a little smart I don't have

and I can be a little smart I don't have to go all the way to the end because I

to go all the way to the end because I know these folks are already sorted so I

know these folks are already sorted so I can at least optimize slightly so now

can at least optimize slightly so now five is small six is small seven is four

five is small six is small seven is four four is smaller if you want to go in

four is smaller if you want to go in place

place there and now here things get

there and now here things get interesting I can optimize by not

interesting I can optimize by not looking at these folks anymore more cuz

looking at these folks anymore more cuz they're obviously problem solved but now

they're obviously problem solved but now five is small six is not seven is not

five is small six is not seven is not okay five you can stay where you are now

okay five you can stay where you are now a human in the room is obviously going

a human in the room is obviously going to question why I'm wasting any more

to question why I'm wasting any more time but with selection sort as I've

time but with selection sort as I've defined it thus far I still have to now

defined it thus far I still have to now check six is smallest not seven and now

check six is smallest not seven and now my final step okay they're all in place

my final step okay they're all in place so here too is this dichotomy between

so here too is this dichotomy between what we all have is this bird's eye view

what we all have is this bird's eye view of the whole problem where it's obvious

of the whole problem where it's obvious where everyone needs to go but a

where everyone needs to go but a computer implementing this with an array

computer implementing this with an array really has to be more methodical and

really has to be more methodical and we're actually saving a step here if we

we're actually saving a step here if we were really doing this none of these

were really doing this none of these numbers would be visible all eight of

numbers would be visible all eight of our volunteers would be inside of a

our volunteers would be inside of a locked door and only then could we see

locked door and only then could we see them one at a time but we're focusing

them one at a time but we're focusing now just on the Sorting aspect so let me

now just on the Sorting aspect so let me just before we do one other

just before we do one other demonstration here proposed that what I

demonstration here proposed that what I really just did here in pseudo code was

really just did here in pseudo code was something like this 4 I from 0er to n

something like this 4 I from 0er to n minus one keeping in mind that zero is

minus one keeping in mind that zero is always the left of the array n minus one

always the left of the array n minus one is always the right end of of the array

is always the right end of of the array for I from 0 to N minus1 I found the

for I from 0 to N minus1 I found the smallest number between numbers bracket

smallest number between numbers bracket I and numbers bracket n minus one and

I and numbers bracket n minus one and that's the very geeky way of expressing

that's the very geeky way of expressing this optimization I'm always starting

this optimization I'm always starting from numbers bracket I wherever I am and

from numbers bracket I wherever I am and then everything else to the right and

then everything else to the right and that's what was allowing me to ignore

that's what was allowing me to ignore the already sorted volunteers if though

the already sorted volunteers if though my last line says swap smallest number

my last line says swap smallest number with numbers I I think that implements

with numbers I I think that implements what our humans were doing by physically

what our humans were doing by physically walking to another spot all right so

walking to another spot all right so that then would what we'll call

that then would what we'll call selection sort let's go ahead and take a

selection sort let's go ahead and take a second approach here using an algorithm

second approach here using an algorithm that I'm going to call bubble sort but

that I'm going to call bubble sort but to do this we need you all to reset to

to do this we need you all to reset to your original locations we have a little

your original locations we have a little cheat sheet on the board if you'd like

cheat sheet on the board if you'd like to go back to this position here and let

to go back to this position here and let me take a fundamentally different

me take a fundamentally different approach because I'm not really liking

approach because I'm not really liking selection sort as is because it's kind

selection sort as is because it's kind of a lot of walking back and forth and a

of a lot of walking back and forth and a lot of walking suggests like a lot of

lot of walking suggests like a lot of lot of steps again and again so what

lot of steps again and again so what might I do instead well bubble sort is

might I do instead well bubble sort is going to have me focus a little more

going to have me focus a little more intuitively on just smaller problems and

intuitively on just smaller problems and let's see if this gets me somewhere else

let's see if this gets me somewhere else so if I just look at this list without

so if I just look at this list without looking at everyone else seven and two

looking at everyone else seven and two this is obviously a problem why because

this is obviously a problem why because you're out of order so let's just solve

you're out of order so let's just solve one tiny problem first so seven and two

one tiny problem first so seven and two why don't you swap I know two is in a

why don't you swap I know two is in a better place now because she's

better place now because she's definitely lower uh less than seven so I

definitely lower uh less than seven so I think I can now move on seven and five

think I can now move on seven and five problem so let's solve that seven and

problem so let's solve that seven and four problem let's solve that s and one

four problem let's solve that s and one let's solve that 7 and six let's solve

let's solve that 7 and six let's solve that 7 and zero solve that 7 and three

that 7 and zero solve that 7 and three solve that okay done sorted right all

solve that okay done sorted right all obviously not if you just glance at

obviously not if you just glance at these numbers here but we have

these numbers here but we have fundamentally taken a bite out of the

fundamentally taken a bite out of the problem seven is indeed in the right

problem seven is indeed in the right place so we maximally have n minus one

place so we maximally have n minus one other problems to solve so how do I do

other problems to solve so how do I do this I think I can just repeat the same

this I think I can just repeat the same logic let me go over here two and five

logic let me go over here two and five good five and four no five and one no

good five and four no five and one no five and six yes 6 and zero no six and

five and six yes 6 and zero no six and three no so so now we've solved two of

three no so so now we've solved two of the problems and what's nice about

the problems and what's nice about Bubble sword at least as this glance

Bubble sword at least as this glance it's nice and simple it's nice and local

it's nice and simple it's nice and local and you just keep incrementally solving

and you just keep incrementally solving more and more problems so let's go ahead

more and more problems so let's go ahead and do this again and I'll do it we can

and do this again and I'll do it we can do it faster two and four we know are

do it faster two and four we know are good four and one four and five five and

good four and one four and five five and zero five and three five and six six and

zero five and three five and six six and seven good so we go back two and one ah

seven good so we go back two and one ah now another problem solve two and four

now another problem solve two and four four and zero four and three

four and zero four and three four and five five and six six and seven

four and five five and six six and seven and so notice too as per its name the

and so notice too as per its name the largest elements have bubbled their way

largest elements have bubbled their way up to the top and that's what seems to

up to the top and that's what seems to be happening just as we're fixing some

be happening just as we're fixing some remaining problems so almost done one

remaining problems so almost done one and two two and zero two and three three

and two two and zero two and three three and four four and five five and six six

and four four and five five and six six and seven almost done obviously to us

and seven almost done obviously to us humans it looks done how do I know as

humans it looks done how do I know as the computer for sure what would be the

the computer for sure what would be the most Surefire way for me to now oh it's

most Surefire way for me to now oh it's not done sorry uh that's a bug

not done sorry uh that's a bug okay one and zero okay one and two two

okay one and zero okay one and two two and three three and four three four and

and three three and four three four and five five and six six and seven okay so

five five and six six and seven okay so now it's obviously sorted to the rest of

now it's obviously sorted to the rest of us on stage how could I confirm as much

us on stage how could I confirm as much as code right you're doing it with your

as code right you're doing it with your mind just glancing at this how would the

mind just glancing at this how would the computer the code know for sure that

computer the code know for sure that this list is now sorted

this list is now sorted yeah let's do one more time and look uh

yeah let's do one more time and look uh draw what

conclusion yeah let's do it one more time even though it's a little wasteful

time even though it's a little wasteful but logically if I go through the whole

but logically if I go through the whole list comparing pairs again again and

list comparing pairs again again and again and I don't do any work that time

again and I don't do any work that time now it's obviously logically safe to

now it's obviously logically safe to just stop because otherwise I'm wasting

just stop because otherwise I'm wasting my time doing the same thing again and

my time doing the same thing again and again if no one's actually moving so I'm

again if no one's actually moving so I'm afraid we don't have Monopoly games for

afraid we don't have Monopoly games for all of you but we do have eight stress

all of you but we do have eight stress balls and a round of applause if we

balls and a round of applause if we could for our

could for our volunteers if you want to put your

volunteers if you want to put your numbers on the Shelf

numbers on the Shelf there so if we consider for a moment

there so if we consider for a moment thank you thank you so

thank you thank you so much

much sure thank you thanks sure so if we

sure thank you thanks sure so if we consider now these two algorithms which

consider now these two algorithms which one is better any intuition for whether

one is better any intuition for whether selection sort the first is better or

selection sort the first is better or worse than bubble sort the

worse than bubble sort the second any

second any thoughts

yeah okay so bubbl swort seemed like less work especially since I was

less work especially since I was focusing on those localized problems

focusing on those localized problems other

other intuition selection sort versus bubble

intuition selection sort versus bubble sort well let me propose that we try to

sort well let me propose that we try to like quantize this so we can actually

like quantize this so we can actually analyze it in some way and this is not

analyze it in some way and this is not an exercise we'll do constantly for lots

an exercise we'll do constantly for lots of algorithms but these are pretty

of algorithms but these are pretty representative of algorithms so we can

representative of algorithms so we can wrap our minds around indeed the

wrap our minds around indeed the performance or the design of these

performance or the design of these things so here is my pseudo code for

things so here is my pseudo code for selection sort whereby as it's as per

selection sort whereby as it's as per its name I just iteratively select the

its name I just iteratively select the next smallest element El again and again

next smallest element El again and again so how can we go about analyzing

so how can we go about analyzing something like this well we could just

something like this well we could just do it on paper pencil and count up the

do it on paper pencil and count up the number of steps that seem to be implied

number of steps that seem to be implied logically by the code we could literally

logically by the code we could literally comp count like the number of steps I

comp count like the number of steps I was taking again and again left to right

was taking again and again left to right we could also just com uh count the

we could also just com uh count the number of comparisons I was making with

number of comparisons I was making with each of the persons involved and I was

each of the persons involved and I was doing it kind of quickly in selection

doing it kind of quickly in selection sort but every time I was looking at a

sort but every time I was looking at a person trying to decide do I want to

person trying to decide do I want to remember that number is small as that

remember that number is small as that number I was comparing two values with

number I was comparing two values with an equals equals or less than or greater

an equals equals or less than or greater than sign at least if we had done this

than sign at least if we had done this in code so that tends to be the norm

in code so that tends to be the norm when analyzing algorithms like these

when analyzing algorithms like these counting the number of comparisons

counting the number of comparisons because it's kind of a global uh it's

because it's kind of a global uh it's kind of a global unit of measure we can

kind of a global unit of measure we can use to compare different algorithms

use to compare different algorithms entirely so think too that in the

entirely so think too that in the general case when we have more than

general case when we have more than eight volunteers more than seven doors

eight volunteers more than seven doors we can generalize our our array in

we can generalize our our array in general as this is the first element at

general as this is the first element at bracket zero and the end of it is always

bracket zero and the end of it is always n minus one so arrays uh or doors in

n minus one so arrays uh or doors in this case or volunteers are always

this case or volunteers are always numerically indexed from zero on up to n

numerically indexed from zero on up to n minus one if there's n of them in total

minus one if there's n of them in total so how do we analyze the code of

so how do we analyze the code of selection sort well how many steps did

selection sort well how many steps did it take me to find the first smallest

it take me to find the first smallest element or more precisely how many

element or more precisely how many comparisons did I need to make when I

comparisons did I need to make when I walked left to right to find our first

walked left to right to find our first smallest person which ended up being

smallest person which ended up being zero how many comparisons did I do when

zero how many comparisons did I do when walking left to right if there were

walking left to right if there were eight people on

eight people on stage how many total comparisons did I

stage how many total comparisons did I do like if there's eight people I

do like if there's eight people I compared these folks then then like this

compared these folks then then like this person this person yeah yeah so seven

person this person yeah yeah so seven total right because if there's eight

total right because if there's eight people on stage you can only do seven

people on stage you can only do seven comparisons total because otherwise

comparisons total because otherwise you'd be comparing one number to itself

you'd be comparing one number to itself so it seems like in the general case if

so it seems like in the general case if you've got n numbers that you're trying

you've got n numbers that you're trying to sort finding the smallest element

to sort finding the smallest element first takes n minus one comparisons

first takes n minus one comparisons maybe n it's total steps left or right

maybe n it's total steps left or right but the number of comparisons which I

but the number of comparisons which I claim is just a useful unit of measure

claim is just a useful unit of measure is n minus one how about finding the

is n minus one how about finding the next smallest person how many steps did

next smallest person how many steps did it take me to find the next smallest

it take me to find the next smallest number which ended up being the number

one yeah yeah so just n minus two why

yeah yeah so just n minus two why because I'd already solved one problem

because I'd already solved one problem someone was already in the right

someone was already in the right position it would be silly to keep

position it would be silly to keep counting them again and again so I can

counting them again and again so I can Whittle down my number of comparisons

Whittle down my number of comparisons for the next past n minus 2 the third

for the next past n minus 2 the third past to find the third smallest number

past to find the third smallest number would be n minus 3 and then dot dot dot

would be n minus 3 and then dot dot dot presumably this story this formula ends

presumably this story this formula ends when you have just one final pair the

when you have just one final pair the people at the end to compare so if this

people at the end to compare so if this is looking a little reminiscent of some

is looking a little reminiscent of some kind of recurrence from high school or

kind of recurrence from high school or high school math or physics or the like

high school math or physics or the like let me just stipulate that if you

let me just stipulate that if you actually do out this math and generalize

actually do out this math and generalize it that is the same thing as n * nus1 /

it that is the same thing as n * nus1 / 2 and if you're Rusty on that no big

2 and if you're Rusty on that no big deal just kind of commit to memory that

deal just kind of commit to memory that anytime you add up this kind of series

anytime you add up this kind of series something plus something slightly

something plus something slightly smaller plus something slightly smaller

smaller plus something slightly smaller Each of which differs by one you're

Each of which differs by one you're going to get this formula n * n -1/ 2 if

going to get this formula n * n -1/ 2 if we of course uh uh multiply that out

we of course uh uh multiply that out that's really n^ 2 minus n all divided

that's really n^ 2 minus n all divided by two if we keep multiplying it out

by two if we keep multiplying it out that's n^2 / 2 minus n /2 and now we

that's n^2 / 2 minus n /2 and now we have kind of a vocabulary with which we

have kind of a vocabulary with which we can talk about the efficiency the design

can talk about the efficiency the design of this algorithm but honestly I don't

of this algorithm but honestly I don't really care about this level of

really care about this level of precision like n^2 / 2us n / 2 as n gets

precision like n^2 / 2us n / 2 as n gets really large which of these symbols

really large which of these symbols which of these terms is really going to

which of these terms is really going to dominate become the biggest influencer

dominate become the biggest influencer on the total value of

on the total value of steps right it's the square right like

steps right it's the square right like it's definitely not n divided by two

it's definitely not n divided by two that's shaving some time off but N

that's shaving some time off but N squared as n gets big is going to get

squared as n gets big is going to get really big if n is 100 then n squar is

really big if n is 100 then n squar is bigger if N is a million n squ is really

bigger if N is a million n squ is really bigger and so at the end of the day when

bigger and so at the end of the day when we really just talking about sort of a

we really just talking about sort of a wave of of the hand analysis and upper

wave of of the hand analysis and upper bound if you will let's just say that

bound if you will let's just say that selection sort as analyzed here it's on

selection sort as analyzed here it's on the order of n s steps it's not

the order of n s steps it's not precisely n squar steps but you know

precisely n squar steps but you know what n^2 divided two the intuition here

what n^2 divided two the intuition here might be that well it's half of that you

might be that well it's half of that you n squ is what really matters as n gets

n squ is what really matters as n gets really really large and that's when you

really really large and that's when you start thinking about and trying to solve

start thinking about and trying to solve the Google problems of the world when n

the Google problems of the world when n gets large that's when you have to be

gets large that's when you have to be smarter than just sort of naive

smarter than just sort of naive implementations of any algorithm so

implementations of any algorithm so where then does this algorithm fall into

where then does this algorithm fall into this categorization here well n^2 it

this categorization here well n^2 it turns out is on the order of n squ steps

turns out is on the order of n squ steps in the worst case whether it's sorted or

in the worst case whether it's sorted or not it turns out though lower bound if

not it turns out though lower bound if we consider this same code suppose the

we consider this same code suppose the best case scenario like our eight

best case scenario like our eight volunteers came up on stage and just

volunteers came up on stage and just because they already sorted themselves 0

because they already sorted themselves 0 through seven suppose they just happen

through seven suppose they just happen to be in that state how many steps would

to be in that state how many steps would selection store

selection store take to sort an already sorted list of

take to sort an already sorted list of volunteers any intuition

yeah would it still be n so for the first pass it would still

n so for the first pass it would still be seven for the first per uh pass

be seven for the first per uh pass across the humans because even though

across the humans because even though yeah I'm claiming zero is here I don't

yeah I'm claiming zero is here I don't know that zero is the smallest until I

know that zero is the smallest until I make my way all the way over there doing

make my way all the way over there doing all seven comparisons okay fine first

all seven comparisons okay fine first pass took seven more generally n minus

pass took seven more generally n minus one steps what if I look for the next

one steps what if I look for the next smallest element and the humans in this

smallest element and the humans in this story are already sorted 0 through 7

story are already sorted 0 through 7 well yes the number one's here and I see

well yes the number one's here and I see them first but I don't know they're the

them first but I don't know they're the smallest until I compare against

smallest until I compare against everyone else get to the end of the list

everyone else get to the end of the list oh well that was stupid I already had

oh well that was stupid I already had the smallest person in hand then and so

the smallest person in hand then and so this pseudo code this implementation of

this pseudo code this implementation of selection sort is sort of fixed like

selection sort is sort of fixed like this there's no special case that says

this there's no special case that says if already sorted quit early it's always

if already sorted quit early it's always going to take n squ steps and so in this

going to take n squ steps and so in this case if we borrow our uh jargon from

case if we borrow our uh jargon from earlier using Omega notation just to be

earlier using Omega notation just to be clear selection sword is also going to

clear selection sword is also going to be in this Incarnation on an Omega of n

be in this Incarnation on an Omega of n s because even in the best case where

s because even in the best case where the list is already sorted you're going

the list is already sorted you're going to waste a huge amount of time

to waste a huge amount of time essentially verifying as much or

essentially verifying as much or discovering as much even though we

discovering as much even though we humans of course could see it right away

humans of course could see it right away so selection sort would seem to take

so selection sort would seem to take both N squared steps in the uh worst

both N squared steps in the uh worst case n s steps in the best case and so

case n s steps in the best case and so you know what we can use our Theta

you know what we can use our Theta terminology for that here would be an

terminology for that here would be an algorithm just like counting earlier

algorithm just like counting earlier that always takes N squared steps no

that always takes N squared steps no matter whether the array is sorted or

matter whether the array is sorted or not from the get-go all right so

not from the get-go all right so hopefully we can do better and someone

hopefully we can do better and someone proposed earlier that bubble sort felt

proposed earlier that bubble sort felt like it was using fewer steps well let's

like it was using fewer steps well let's consider that next with bubble sort we

consider that next with bubble sort we had this pseudo code I claim whereby

had this pseudo code I claim whereby let's focus on the inside of the code

let's focus on the inside of the code first down here what was I doing for I

first down here what was I doing for I from 0 to n minus 2 that's curious we've

from 0 to n minus 2 that's curious we've never seen n minus 2 before but I asked

never seen n minus 2 before but I asked this question if numbers bracket I and

this question if numbers bracket I and numbers bracket I + 1 are out of order

numbers bracket I + 1 are out of order swap them so that was when I was

swap them so that was when I was pointing at our first two volunteers

pointing at our first two volunteers here I saw that they were out of order

here I saw that they were out of order so I swapped them how come I'm doing

so I swapped them how come I'm doing that again and again up to nus 2 though

that again and again up to nus 2 though instead of n

instead of n minus1 which we've always used up until

minus1 which we've always used up until now as our rightmost

now as our rightmost boundary any intuition for why I'm doing

boundary any intuition for why I'm doing this from 0 to n minus 2

yeah exactly because I'm looking at the E person per this pseudo code here and

E person per this pseudo code here and the E plus one person I better make sure

the E plus one person I better make sure I don't go Step Beyond the at boundaries

I don't go Step Beyond the at boundaries of my array so if you think of like my

of my array so if you think of like my left hand when my back was two here

left hand when my back was two here pointing at the current person at the

pointing at the current person at the first position my right hand for this if

first position my right hand for this if conditioner is essentially pointing at

conditioner is essentially pointing at the person next to them and you want to

the person next to them and you want to iterate with your left hand all through

iterate with your left hand all through these people but you don't want your

these people but you don't want your left hand to point at the last person

left hand to point at the last person you want it to point at the second to

you want it to point at the second to last person but we know that the last

last person but we know that the last person is always at n minus one so the

person is always at n minus one so the second to last person just

second to last person just mathematically is at n minus 2 so it's a

mathematically is at n minus 2 so it's a subtlety but this is like a Segal

subtlety but this is like a Segal waiting to happen if you implemented

waiting to happen if you implemented bubble sort using n minus one you will

bubble sort using n minus one you will my right hand would go beyond the

my right hand would go beyond the boundaries of the array so just bad all

boundaries of the array so just bad all right so why am I saying this end times

right so why am I saying this end times Well we did it very organically with

Well we did it very organically with humans but each time someone uh each

humans but each time someone uh each pass I did through the array someone

pass I did through the array someone bubbled their way up to the end number

bubbled their way up to the end number seven then number six then number five

seven then number six then number five so if on each pass through the array of

so if on each pass through the array of volunteers I was solving at Mo at least

volunteers I was solving at Mo at least one problem it seems like bubble sort

one problem it seems like bubble sort can just run n times total to solve all

can just run n times total to solve all n problems cuz the first pass will get

n problems cuz the first pass will get at least one one number into place

at least one one number into place second pass second number into place you

second pass second number into place you might get lucky and it would do more but

might get lucky and it would do more but worst case this feels like enough just

worst case this feels like enough just do this blindly end times and they'll

do this blindly end times and they'll all line up together well technically

all line up together well technically all right now we're getting into the

all right now we're getting into the weeds technically you can just repeat it

weeds technically you can just repeat it n minus one times because if you solve

n minus one times because if you solve all n minus one other problems and

all n minus one other problems and you're left with one like literally that

you're left with one like literally that person's where they need to be just

person's where they need to be just logically if you've already sorted

logically if you've already sorted everything else and you've got just the

everything else and you've got just the one left it's already bubbled up so how

one left it's already bubbled up so how do we analyze this well in bubble sore

do we analyze this well in bubble sore we might do something like this I'm

we might do something like this I'm essentially doing n minus one Things N

essentially doing n minus one Things N minus one times now let me back up to

minus one times now let me back up to the pseudo code because this one's a

the pseudo code because this one's a little less obvious this is where you

little less obvious this is where you can actually mathematically infer from

can actually mathematically infer from your Loop uh how many steps you're

your Loop uh how many steps you're taking so this first line literally says

taking so this first line literally says repeat the following n minus one times

repeat the following n minus one times so that's going to translate very

so that's going to translate very straightforwardly to our mathematical

straightforwardly to our mathematical formula do something n minus one times

formula do something n minus one times this Loop just because I'm using four

this Loop just because I'm using four Loop terminology it's framed a little

Loop terminology it's framed a little differently but but if you're iterating

differently but but if you're iterating from 0 to nus 2 you're iterating a total

from 0 to nus 2 you're iterating a total of nus1 times and again the arithmetic

of nus1 times and again the arithmetic is getting a little annoying but this

is getting a little annoying but this just means do the following n minus one

just means do the following n minus one times so do n minus one Things N minus

times so do n minus one Things N minus one times we can now uh run out the math

one times we can now uh run out the math as follows we have the formula n minus1

as follows we have the formula n minus1 * n minus1 we do our little foil method

* n minus1 we do our little foil method here N2 -1 * N - 1 * n + 1 we can

here N2 -1 * N - 1 * n + 1 we can combine like terms n^ 2 - 2 n plus one

combine like terms n^ 2 - 2 n plus one but at this point when n gets really

but at this point when n gets really large which term are we really going to

large which term are we really going to care about this is on the order

care about this is on the order of yeah n s so at least ASM totically

of yeah n s so at least ASM totically ASM totically means as n approaches

ASM totically means as n approaches Infinity gets really large turns out

Infinity gets really large turns out that the upper bounds on selection sort

that the upper bounds on selection sort and bubble sort are essentially the same

and bubble sort are essentially the same now if we really nitpicked and compared

now if we really nitpicked and compared the total number of comparisons they

the total number of comparisons they might differ slightly but as n gets

might differ slightly but as n gets large honestly you're barely going to

large honestly you're barely going to notice the difference it would seem

notice the difference it would seem between these two algorithms

between these two algorithms but what about um the lower bound if the

but what about um the lower bound if the upper bound on Bubble sword is also Big

upper bound on Bubble sword is also Big O of n what about the lower bound here

O of n what about the lower bound here well with this pseudo code what would

well with this pseudo code what would the lower bound be on bubble sort even

the lower bound be on bubble sort even in the best case when all of the

in the best case when all of the volunteers are

volunteers are sorted any

sorted any intuition in this pseudo code yeah in

intuition in this pseudo code yeah in the

middle good question isn't bubble sorts

good question isn't bubble sorts designed such that you wouldn't need to

designed such that you wouldn't need to compare numbers that have already uh

compare numbers that have already uh bubbled up that's what's happening here

bubbled up that's what's happening here in the middle implicitly I'm always

in the middle implicitly I'm always going from left to right but remember

going from left to right but remember that even when I screwed up at the end

that even when I screwed up at the end and the last two people were out of

and the last two people were out of order I do always need to restart at the

order I do always need to restart at the beginning because the big numbers are

beginning because the big numbers are going that way and the small numbers are

going that way and the small numbers are coming this

way so that is true there are some slight optimizations that I'm kind of

slight optimizations that I'm kind of glossing over here let me stipulate that

glossing over here let me stipulate that it would still end up being on the order

it would still end up being on the order of n squ but that would definitely shave

of n squ but that would definitely shave off some actual running time here but

off some actual running time here but what if the list is already sorted our

what if the list is already sorted our our pseudo code at the moment has no

our pseudo code at the moment has no allowance for if list is already sorted

allowance for if list is already sorted quit early so we're going to blindly do

quit early so we're going to blindly do n minus one Things N minus one times

n minus one Things N minus one times unless we modify our pseudo code as I

unless we modify our pseudo code as I did verbally earlier I propose this

did verbally earlier I propose this inside of that outer loop if you make a

inside of that outer loop if you make a pass across all of the volunteers and

pass across all of the volunteers and your mental counter has made no swaps

your mental counter has made no swaps you have to keep track with some kind of

you have to keep track with some kind of variable well then you might as well

variable well then you might as well stop because if you do a whole pass and

stop because if you do a whole pass and make no swaps why would you waste time

make no swaps why would you waste time doing it again expecting different

doing it again expecting different Behavior so to help visualize these

Behavior so to help visualize these whereby now bubble sort can be

whereby now bubble sort can be advantageous if the data is already

advantageous if the data is already sorted or mostly sorted why because it

sorted or mostly sorted why because it does have this short circuit detail at

does have this short circuit detail at least if we implement it like that how

least if we implement it like that how can we go about um visualizing these

can we go about um visualizing these things a little more clearly well let me

things a little more clearly well let me go ahead and do this let me pull up here

go ahead and do this let me pull up here a visualization of exactly these

a visualization of exactly these algorithms thanks to a third party tool

algorithms thanks to a third party tool here that's going to help us visualize

here that's going to help us visualize these sorting algorithms as follows

these sorting algorithms as follows small bars represent small numbers big

small bars represent small numbers big bars represent big numbers and so the

bars represent big numbers and so the idea now is when I hit a button here to

idea now is when I hit a button here to get all of the small bars this way all

get all of the small bars this way all of the big bars this way so just like

of the big bars this way so just like our volunteers but instead of holding

our volunteers but instead of holding lighted numbers its bars representing

lighted numbers its bars representing their magnitude so let's go ahead and

their magnitude so let's go ahead and start with for instance select

start with for instance select sort and you'll see in pink is being

sort and you'll see in pink is being highlighted the current number that is

highlighted the current number that is being selected and then pulled all the

being selected and then pulled all the way to the left so this is selection

way to the left so this is selection sort and again it's selecting the next

sort and again it's selecting the next smallest element but you can see here

smallest element but you can see here all the more visibly that just like my

all the more visibly that just like my human feet were taking a lot of steps so

human feet were taking a lot of steps so is this algorithm touching these

is this algorithm touching these elements again and again and again and

elements again and again and again and this is why the N squ is really a thing

this is why the N squ is really a thing there's got to be some inherent

there's got to be some inherent redundancy here like why do we keep

redundancy here like why do we keep looking at the same darn elements again

looking at the same darn elements again and again we do in terms of our pseudo

and again we do in terms of our pseudo code need to do so but it's this

code need to do so but it's this redundant comparisons that kind of

redundant comparisons that kind of explains why n s is indeed the case so

explains why n s is indeed the case so now it's done small bars here big bars

now it's done small bars here big bars there and I had to just keep talking

there and I had to just keep talking there to kill time because it's

there to kill time because it's relatively slow well let me randomize

relatively slow well let me randomize the array just so we start with a

the array just so we start with a different order and now let me click on

different order and now let me click on bubbl sore and you'll see similar idea

bubbl sore and you'll see similar idea but different algorithm so now the two

but different algorithm so now the two bars in pink are the two that are being

bars in pink are the two that are being compared and fixed potentially if

compared and fixed potentially if they're out of order and you can see

they're out of order and you can see already that the biggest bars are

already that the biggest bars are bubbling their way up to the top but now

bubbling their way up to the top but now you can also see like this redundancy

you can also see like this redundancy like we keep swooping through the list

like we keep swooping through the list again and again just like I kept walking

again and again just like I kept walking back and forth and this is n squar this

back and forth and this is n squar this is not that many bars what 10 20 there's

is not that many bars what 10 20 there's like 40 or something bars I'm guessing

like 40 or something bars I'm guessing that's pretty slow already just to sort

that's pretty slow already just to sort 40 numbers and I think it's going to get

40 numbers and I think it's going to get tedious if I keep talking over this so

tedious if I keep talking over this so let's just assume that this two is

let's just assume that this two is relatively slow had I gotten lucky and

relatively slow had I gotten lucky and the list were almost sorted already

the list were almost sorted already bubble sort would have been pretty fast

bubble sort would have been pretty fast but this was a truly random array so we

but this was a truly random array so we did not get lucky so indeed the worst

did not get lucky so indeed the worst case might be what's kicking in here so

case might be what's kicking in here so I don't I feel like it'll be

I don't I feel like it'll be anticlimactic like holding in a sneeze

anticlimactic like holding in a sneeze if I don't let you see the end of this

if I don't let you see the end of this so here we go nothing interesting is

so here we go nothing interesting is about to happen almost

about to happen almost done ah okay done all right so thank you

done ah okay done all right so thank you thank

thank you so still somewhat slow though how

you so still somewhat slow though how though can we perhaps do a little better

though can we perhaps do a little better fundamentally so we can do so if we

fundamentally so we can do so if we introduce yet another technique and this

introduce yet another technique and this one isn't so much a function of code as

one isn't so much a function of code as it is concept and it's something that

it is concept and it's something that you might have seen in the real world um

you might have seen in the real world um but perhaps not so obviously so so it

but perhaps not so obviously so so it turns out in programming recursion

turns out in programming recursion refers to the ability of a function to

refers to the ability of a function to call itself in the world of mathematics

call itself in the world of mathematics if you have a function f if F appears on

if you have a function f if F appears on both the left side and the right side of

both the left side and the right side of a formula that would be a recursive

a formula that would be a recursive function in the math world too whenever

function in the math world too whenever f is defined in terms of itself or in

f is defined in terms of itself or in our case in Compu in programming anytime

our case in Compu in programming anytime a function calls itself that function is

a function calls itself that function is said to be recursive and this is

said to be recursive and this is actually something we've seen already in

actually something we've seen already in class even though we didn't call it as

class even though we didn't call it as much so for instance consider this

much so for instance consider this pseudo code um from earlier whereby this

pseudo code um from earlier whereby this was the pseudo code for searching via AR

was the pseudo code for searching via AR search a whole bunch of doors if no

search a whole bunch of doors if no doors are left return false that was the

doors are left return false that was the the additional conditional we added but

the additional conditional we added but then if number behind middle door return

then if number behind middle door return true and here's the interesting part if

true and here's the interesting part if number is less than middle door search

number is less than middle door search the left half else if number is greater

the left half else if number is greater than middle door search the right half

than middle door search the right half this pseudo code earlier was itself

this pseudo code earlier was itself recursive why because here is an

recursive why because here is an algorithm for searching but what's the

algorithm for searching but what's the algorithm telling us well on this line

algorithm telling us well on this line and this line it's telling us to search

and this line it's telling us to search something else so even though it's not

something else so even though it's not explicitly defined in code as having a

explicitly defined in code as having a name if this is a search algorithm and

name if this is a search algorithm and yet the search algorithm is using a

yet the search algorithm is using a search algorithm this pseudo code is

search algorithm this pseudo code is recursive now that could quickly get you

recursive now that could quickly get you into trouble if a function just calls

into trouble if a function just calls itself again and again and again but why

itself again and again and again but why intuitively is it not problematic that

intuitively is it not problematic that this code this pseudo code calls

this code this pseudo code calls itself why will the algorithm still stop

itself why will the algorithm still stop yeah

exactly it has some exit condition like if no do is less and more more

if no do is less and more more importantly anytime you search the left

importantly anytime you search the left half you're searching a smaller version

half you're searching a smaller version of the problem anytime you search the

of the problem anytime you search the right half you're searching a smaller

right half you're searching a smaller version of the problem literally half

version of the problem literally half the size so this is why in the phone

the size so this is why in the phone book obviously I couldn't tear the phone

book obviously I couldn't tear the phone book in half uh infinitely many times

book in half uh infinitely many times because it was literally getting smaller

because it was literally getting smaller each time so recursion is this ability

each time so recursion is this ability to call yourself if you will it's but

to call yourself if you will it's but what's important is that you do it on a

what's important is that you do it on a small smaller smaller problem so that

small smaller smaller problem so that eventually you have no more problems to

eventually you have no more problems to solve or no more data no more doors at

solve or no more data no more doors at all so these two lines here would be the

all so these two lines here would be the recursive elements here but if we go

recursive elements here but if we go back to week zero we could have used

back to week zero we could have used recursion in some other way so this was

recursion in some other way so this was our Cudo code for the phone book back in

our Cudo code for the phone book back in week zero and recall that we described

week zero and recall that we described these yellow lines as really

these yellow lines as really representing a loop some kind of cycle

representing a loop some kind of cycle again and again but there was a missed

again and again but there was a missed opportunity here what if I had reinvent

opportunity here what if I had reinvent reimplemented this code to do this

reimplemented this code to do this instead

instead instead of saying open to middle of left

instead of saying open to middle of left half of book and then go back to line

half of book and then go back to line three like literally inducing a loop or

three like literally inducing a loop or open to Middle right half a book and go

open to Middle right half a book and go back to line three inducing another loop

back to line three inducing another loop why don't I just recognize that what I'm

why don't I just recognize that what I'm staring at now is a algorithm for

staring at now is a algorithm for searching a phone book and if you want

searching a phone book and if you want to search a smaller phone book like a

to search a smaller phone book like a through M or n through Z we'll just use

through M or n through Z we'll just use the same algorithm so I can replace

the same algorithm so I can replace these yellow lines with just this

these yellow lines with just this casually speaking search left half a

casually speaking search left half a book search right half a book this would

book search right half a book this would be implicitly and now I can shorten the

be implicitly and now I can shorten the whole thing a recursive implementation

whole thing a recursive implementation of the phone book pseudo code from week

of the phone book pseudo code from week zero and it's recursive because if this

zero and it's recursive because if this is a search algorithm and you're saying

is a search algorithm and you're saying go search something else that's fine

go search something else that's fine that's recursive but because you're

that's recursive but because you're searching half of the phone book it's

searching half of the phone book it's indeed going to get smaller and smaller

indeed going to get smaller and smaller even in the real world or the real

even in the real world or the real virtual world you can see recursive data

virtual world you can see recursive data structures in the wild or at least in

structures in the wild or at least in Super Mario Brothers like this let me

Super Mario Brothers like this let me get rid of all the distractions here and

get rid of all the distractions here and focus on this pyramid where you have one

focus on this pyramid where you have one block then two then three then four well

block then two then three then four well this itself is technically recursively

this itself is technically recursively defined in the sense that well what is a

defined in the sense that well what is a pyramid of height four well it's really

pyramid of height four well it's really what how would you describe a pyramid of

what how would you describe a pyramid of height four is actually the same thing

height four is actually the same thing as a pyramid

of of of height three plus one additional layer well what's a pyramid

additional layer well what's a pyramid of height three well it's technically a

of height three well it's technically a pyramid of height two plus one

pyramid of height two plus one additional layer and so even physical

additional layer and so even physical structures can be recursive if you can

structures can be recursive if you can Define them in terms of itself now at

Define them in terms of itself now at some point you have to say that if the

some point you have to say that if the pyramid is of height one there's just

pyramid is of height one there's just one block you can't forever say it's

one block you can't forever say it's defined in terms of a height negative 1

defined in terms of a height negative 1 negative2 you would never stop so you

negative2 you would never stop so you have to kind of have a special case

have to kind of have a special case there but let's go ahead and translate

there but let's go ahead and translate something like this in fact to code let

something like this in fact to code let me go back to uh vs code here and let me

me go back to uh vs code here and let me Implement a program called iteration

Implement a program called iteration that refers to a loop iterating and let

that refers to a loop iterating and let me Implement a very simple pyramid like

me Implement a very simple pyramid like that so let me go ahead and include the

that so let me go ahead and include the cs50 library I'll include our standard

cs50 library I'll include our standard i.h in main void no command line

i.h in main void no command line arguments today and let's go ahead and

arguments today and let's go ahead and do this let's declare a variable called

do this let's declare a variable called height ask the human for the height of

height ask the human for the height of this pyramid and then let's go ahead and

this pyramid and then let's go ahead and draw a pyramid of that height now of

draw a pyramid of that height now of course draw does not yet exist so I'm

course draw does not yet exist so I'm going to need to invent the draw a

going to need to invent the draw a function let me go ahead and Define a

function let me go ahead and Define a function that doesn't have a return

function that doesn't have a return value it's just going to have side

value it's just going to have side effects it's just going to print bricks

effects it's just going to print bricks on the screen call draw and it takes in

on the screen call draw and it takes in an integer n as its input and how am I

an integer n as its input and how am I going to implement this well again I

going to implement this well again I want to print one block then two then

want to print one block then two then three then four that's pretty

three then four that's pretty straightforward at least once you're

straightforward at least once you're comfortable with loops let me go back to

comfortable with loops let me go back to the code here let me go ahead and say 4

the code here let me go ahead and say 4 into I gets zero I is less than n i ++

into I gets zero I is less than n i ++ and that's going to iterate essentially

and that's going to iterate essentially row by row and on each row I want to

row by row and on each row I want to print out one then two then three then

print out one then two then three then four bricks but I'm iterating from Z to

four bricks but I'm iterating from Z to 1 to two to three so I think that's okay

1 to two to three so I think that's okay I can just say something like 4 in J

I can just say something like 4 in J gets zero J Let's Be Clever about this

gets zero J Let's Be Clever about this is less than I

is less than I j++ and now let me go ahead and inside

j++ and now let me go ahead and inside of this loop I think I can get away with

of this loop I think I can get away with just printing out a single hash sign but

just printing out a single hash sign but then outside of that Loop similar to

then outside of that Loop similar to last week I'm going to print my new line

last week I'm going to print my new line separately so a little non-obvious at

separately so a little non-obvious at first but this outer loop iterates row

first but this outer loop iterates row by row line by line if you will and then

by row line by line if you will and then the inner loop just Mak sure that when I

the inner loop just Mak sure that when I equals zero um let's see oh nope there's

equals zero um let's see oh nope there's a bug I need to make sure that it's j is

a bug I need to make sure that it's j is less than I + 1 so when I is zero on my

less than I + 1 so when I is zero on my first line of output I'm going to print

first line of output I'm going to print out one brick when I is one I'm going to

out one brick when I is one I'm going to print out two bricks and so forth so let

print out two bricks and so forth so let me go ahead and run make iteration

me go ahead and run make iteration all right and now seems to compile

all right and now seems to compile uhoh huh implicit Declaration of

uhoh huh implicit Declaration of function draw so I'm making week one

function draw so I'm making week one mistakes

mistakes again what say

again what say again yeah the the Prototype is missing

again yeah the the Prototype is missing I didn't declare it at the top that's an

I didn't declare it at the top that's an easy fix and the only time really it's

easy fix and the only time really it's okay and necessary to copy paste let me

okay and necessary to copy paste let me copy the functions declaration there end

copy the functions declaration there end it with a semicolon so that clang now

it with a semicolon so that clang now knows that draw will exist make

knows that draw will exist make iteration now it works thank you /

iteration now it works thank you / iteration we'll type in something like

iteration we'll type in something like four and there we have it our pyramid of

four and there we have it our pyramid of height 1 2 3 4 that looks pretty similar

height 1 2 3 4 that looks pretty similar to this albeit using hashes so that's

to this albeit using hashes so that's how we would have implemented this like

how we would have implemented this like two weeks ago in week one maybe last

two weeks ago in week one maybe last week but just using arrays but let me

week but just using arrays but let me propose that we could do something

propose that we could do something recursively instead let me close this

recursively instead let me close this version of the code and let me go back

version of the code and let me go back to VSS code and open up recursion doc

to VSS code and open up recursion doc just to demonstrate something

just to demonstrate something recursively and I'll do it incorrectly

recursively and I'll do it incorrectly deliberately the first time so let me

deliberately the first time so let me include cs50.h let me include standard

include cs50.h let me include standard i.h let me do uh int main void and let

i.h let me do uh int main void and let me just blindly draw a pyramid initially

me just blindly draw a pyramid initially of height one but now in my draw

of height one but now in my draw function let me reimplement it a little

function let me reimplement it a little differently so my draw function this

differently so my draw function this time is still going to take a number n

time is still going to take a number n but that's how many hashes it's going to

but that's how many hashes it's going to print so let's do four into I gets zero

print so let's do four into I gets zero I is less than n i

I is less than n i ++ then let's go ahead and print out a

++ then let's go ahead and print out a single hash mark here and then after

single hash mark here and then after that let's print out the end of the line

that let's print out the end of the line just as before but now this of course is

just as before but now this of course is only going to draw a single um row it's

only going to draw a single um row it's going to print out one hash or two

going to print out one hash or two hashes or three hashes but only on one

hashes or three hashes but only on one line let me now incorrectly but just

line let me now incorrectly but just kind of curiously say all right well if

kind of curiously say all right well if this draws a pyramid of height one let's

this draws a pyramid of height one let's just use ourself to draw a pyramid of

just use ourself to draw a pyramid of height n plus one so the first time I

height n plus one so the first time I call draw it will print out one hash

call draw it will print out one hash then the second time I call draw it will

then the second time I call draw it will print out two hashes then three then

print out two hashes then three then four so we're kind of laying These

four so we're kind of laying These Bricks down from top to bottom uh make

Bricks down from top to bottom uh make recursion uh oops I screwed up again so

recursion uh oops I screwed up again so let's copy the Prototype here let's put

let's copy the Prototype here let's put this down over here semicolon let's do

this down over here semicolon let's do this again uh make recursion all right

this again uh make recursion all right all good/ recursion and now let me

all good/ recursion and now let me increase the size of my terminal window

increase the size of my terminal window just so you can see more of the output

just so you can see more of the output and here we

and here we have okay bad but thank you so we have

have okay bad but thank you so we have an infinitely tall pyramid and it's just

an infinitely tall pyramid and it's just flying across the screen which is why it

flying across the screen which is why it looks kind of like a mess but I printed

looks kind of like a mess but I printed out a pyramid of height one and then two

out a pyramid of height one and then two and then three and then four and

and then three and then four and unfortunately what am I lacking any sort

unfortunately what am I lacking any sort of quick condition any kind of condition

of quick condition any kind of condition that says wait a minute when it's too

that says wait a minute when it's too tall stop all together so this is an

tall stop all together so this is an infinite Loop but it's not a loop it's a

infinite Loop but it's not a loop it's a recursive call and actually doing this

recursive call and actually doing this in en is very bad we'll see next week

in en is very bad we'll see next week that if you call a function too many

that if you call a function too many times you can actually trigger yet one

times you can actually trigger yet one yet another of those segmentation faults

yet another of those segmentation faults cuz you're using too much memory

cuz you're using too much memory essentially but for now I haven't

essentially but for now I haven't triggered that yet control C is your

triggered that yet control C is your friend to cancel and as an aside if

friend to cancel and as an aside if you're playing along at home or play

you're playing along at home or play with this code later I actually cheated

with this code later I actually cheated here we have a special clang

here we have a special clang configuration feature that prevents you

configuration feature that prevents you from calling a function like that and

from calling a function like that and creating a problem I overrode it just to

creating a problem I overrode it just to for demonstration sake but odds are at

for demonstration sake but odds are at home you wouldn't be able to compile

home you wouldn't be able to compile this code yourself but let me do a

this code yourself but let me do a proper version recursively of this code

proper version recursively of this code as follows let me go back into the code

as follows let me go back into the code here let me go ahead and not just

here let me go ahead and not just blindly start drawing one then two then

blindly start drawing one then two then three layers of bricks let me prompt the

three layers of bricks let me prompt the human as before for the height of the

human as before for the height of the pyramid they want using our get in

pyramid they want using our get in function and now let me call draw of

function and now let me call draw of height again so now I'm going back to

height again so now I'm going back to the looplike version but instead of

the looplike version but instead of using a loop now this is where recursion

using a loop now this is where recursion gets rather elegant if you will let me

gets rather elegant if you will let me go ahead and execute and code uh the

go ahead and execute and code uh the draw function as

draw function as follows per your definition if a pyramid

follows per your definition if a pyramid of height four is really just a pyramid

of height four is really just a pyramid of height three plus another row well

of height three plus another row well let's take that literally let me go back

let's take that literally let me go back to my code and if you want to draw a

to my code and if you want to draw a pyramid of height four well go right

pyramid of height four well go right ahead and draw a pyramid of height uh

ahead and draw a pyramid of height uh three first or more generally n minus

three first or more generally n minus one but what's the second step well once

one but what's the second step well once you've drawn a pyramid of height three

you've drawn a pyramid of height three draw an extra row so I at least have to

draw an extra row so I at least have to bite off that part of the problem myself

bite off that part of the problem myself so let me just do for in I gets zero I

so let me just do for in I gets zero I is less than n i++ and let me the

is less than n i++ and let me the programmer of this function print out my

programmer of this function print out my hashes and then at the very bottom print

hashes and then at the very bottom print out a new line so the cursor moves to

out a new line so the cursor moves to the next line but this is kind of

the next line but this is kind of elegant now I dare say in that draw is

elegant now I dare say in that draw is recursive because I'm literally

recursive because I'm literally translating from English to code this

translating from English to code this idea that a pyramid of height four is

idea that a pyramid of height four is really just a pyramid of height three so

really just a pyramid of height three so I do that first and I'm sort of trusting

I do that first and I'm sort of trusting that this will work then I just have to

that this will work then I just have to lay one more layer of bricks four of

lay one more layer of bricks four of them so if n is four this is just a

them so if n is four this is just a simple for Loop Allah week one that will

simple for Loop Allah week one that will print out an additional layer but this

print out an additional layer but this of course is going to be problematic

of course is going to be problematic eventually why it's not done yet this

eventually why it's not done yet this program how many times will draw call

program how many times will draw call itself in this

why yeah there's no there's no equivalent of quit like if you've

equivalent of quit like if you've printed enough already then quit well

printed enough already then quit well how do we capture that well I don't

how do we capture that well I don't think we want this to go negative right

think we want this to go negative right it would make no sense to draw a

it would make no sense to draw a negative height pyramid so I think we

negative height pyramid so I think we can just pluck off as the programmer a

can just pluck off as the programmer a an easy case an easy answer a so-called

an easy case an easy answer a so-called base case and I'm just going to do this

base case and I'm just going to do this at the top of my draw function let me

at the top of my draw function let me just say if n is less than or heck less

just say if n is less than or heck less than or equal to zero that's it go ahead

than or equal to zero that's it go ahead and just return there's nothing more to

and just return there's nothing more to do and that simple condition technically

do and that simple condition technically known as a base case will ensure that

known as a base case will ensure that the code doesn't run forever why well

the code doesn't run forever why well suppose that draw uh is called with an

suppose that draw uh is called with an argument of four four is of course not

argument of four four is of course not less than zero so we don't return but we

less than zero so we don't return but we do draw a pyramid of height three and

do draw a pyramid of height three and here's where things get a little

here's where things get a little mentally tricky you don't move on to

mentally tricky you don't move on to line 20 until draw has been called so

line 20 until draw has been called so when draw is called with an argument of

when draw is called with an argument of three it's as though you're executing

three it's as though you're executing from the top of this function again

from the top of this function again three is not less than zero so what do

three is not less than zero so what do you do you draw two okay how do you draw

you do you draw two okay how do you draw two well two is not less than zero so

two well two is not less than zero so you don't return so you draw one got to

you don't return so you draw one got to be careful here draw one and now we go

be careful here draw one and now we go ahead back to the beginning how do you

ahead back to the beginning how do you draw One well one is not less than zero

draw One well one is not less than zero so you don't return you draw height zero

so you don't return you draw height zero how do you draw height zero wait a

how do you draw height zero wait a minute 0o is less than or equal to zero

minute 0o is less than or equal to zero and you return and so it's kind of like

and you return and so it's kind of like this mental stack this to-do list you

this mental stack this to-do list you keep postponing executing these lower

keep postponing executing these lower lines of code because you keep

lines of code because you keep restarting restarting restarting the

restarting restarting restarting the draw function until finally one of those

draw function until finally one of those function calls says there's nothing to

function calls says there's nothing to do return and now the whole thing starts

do return and now the whole thing starts to unravel if you will and you pick back

to unravel if you will and you pick back up where you left off and this is

up where you left off and this is perhaps the best uh scenario we won't do

perhaps the best uh scenario we won't do it in class but if you'd like to wrestle

it in class but if you'd like to wrestle through this on your own using debug 50

through this on your own using debug 50 to keep stepping into step into step

to keep stepping into step into step into each of those lines logically

into each of those lines logically you'll see exactly what's actually

you'll see exactly what's actually happening so let me go to my terminal

happening so let me go to my terminal and do make recursion which is now this

and do make recursion which is now this correct version of the code do/

correct version of the code do/ recursion let's type in a height of four

recursion let's type in a height of four and voila now we have that same pyramid

and voila now we have that same pyramid not using iteration per se though

not using iteration per se though admittedly we're using iteration to

admittedly we're using iteration to print the additional layer we're now

print the additional layer we're now using draw recursively to print all of

using draw recursively to print all of the smaller pyramids that need come

the smaller pyramids that need come before

it no question is can you only use recursion with a void function no not at

recursion with a void function no not at all in fact it's very common to have a

all in fact it's very common to have a return value like an integer or

return value like an integer or something else so that uh you can

something else so that uh you can actually do something constructively

actually do something constructively with that actual value other questions

with that actual value other questions on

lad when is line 21 getting executed so if you continue to UNR let me uh scroll

if you continue to UNR let me uh scroll down a bit more so you can see the top

down a bit more so you can see the top of the code so line 21 will be executed

of the code so line 21 will be executed once line 19 is done executing itself

once line 19 is done executing itself now in the story I told we kept calling

now in the story I told we kept calling draw again again again but as soon as

draw again again again but as soon as one of those function calls where n

one of those function calls where n equals z returns immediately then we

equals z returns immediately then we don't keep drawing again and again so

don't keep drawing again and again so now if you kind of think of the process

now if you kind of think of the process as reversing then you continue to line

as reversing then you continue to line 21 then a line 21 again then line 21

21 then a line 21 again then line 21 again and as the sort of logic unravels

again and as the sort of logic unravels and next week we'll actually paint a

and next week we'll actually paint a picture of what's actually happening in

picture of what's actually happening in the computer's memory but for now it's

the computer's memory but for now it's just it's very similar to the pseudo

just it's very similar to the pseudo code for the phone book you're just

code for the phone book you're just searching again and again but you're

searching again and again but you're waiting until the very end to get back

waiting until the very end to get back the final result uh can Google Now whom

the final result uh can Google Now whom I keep mentioning by coincidence today

I keep mentioning by coincidence today is full of programmers of course um

is full of programmers of course um here's a fun exercise let me uh go back

here's a fun exercise let me uh go back to a browser I'm going to go ahead and

to a browser I'm going to go ahead and search for recursion because I want to

search for recursion because I want to learn a little something about recursion

learn a little something about recursion uh here is kind of an internet meme or

uh here is kind of an internet meme or joke if I zoom in here the engineers at

joke if I zoom in here the engineers at Google are kind of funny funny see

why ah there we go yes yes this is recursion and there's going to be so

recursion and there's going to be so many memes you'll come across now where

many memes you'll come across now where recursion like if you've ever pointed a

recursion like if you've ever pointed a camera at the TV that's showing the

camera at the TV that's showing the camera and you sort of see yourself for

camera and you sort of see yourself for the image again and again that's really

the image again and again that's really recursion and in that case it only stops

recursion and in that case it only stops once you hit the base case of a single

once you hit the base case of a single Pixel um but this is a very funny joke

Pixel um but this is a very funny joke in some circles uh when it comes to

in some circles uh when it comes to recursion uh and Google so how can we

recursion uh and Google so how can we actually use Google or rather how can we

actually use Google or rather how can we actually use recursion constructively

actually use recursion constructively well let me propose that we actually

well let me propose that we actually introduce a third and final algorithm

introduce a third and final algorithm for sorting that hopefully does better

for sorting that hopefully does better than the two sorts thus far we've done

than the two sorts thus far we've done selection sort and bubble sort bubble

selection sort and bubble sort bubble sort we liked a little better at least

sort we liked a little better at least in so far is in the best case where the

in so far is in the best case where the list is already sorted bubble sort is at

list is already sorted bubble sort is at least smarter and it will actually

least smarter and it will actually terminate early giving us a better lower

terminate early giving us a better lower Bound in terms of our Omega notation but

Bound in terms of our Omega notation but it turns out that recursion and this is

it turns out that recursion and this is not necessarily a feature of recursion

not necessarily a feature of recursion but something we can now leverage it

but something we can now leverage it turns out using recursion we can take a

turns out using recursion we can take a fundamentally different approach to

fundamentally different approach to sorting a whole bunch of numbers in such

sorting a whole bunch of numbers in such a way that we can do far fewer

a way that we can do far fewer comparisons and ideally speed up our

comparisons and ideally speed up our final results so here is the pseudo code

final results so here is the pseudo code for what we're about to see for

for what we're about to see for something called merge sort and it

something called merge sort and it really is this tur sort the left half of

really is this tur sort the left half of numbers sort the right half of numbers

numbers sort the right half of numbers merge the sorted halves right this is

merge the sorted halves right this is almost sort of non sensical because I uh

almost sort of non sensical because I uh if you're asked for an algorithm to sort

if you're asked for an algorithm to sort and you respond with well sort the left

and you respond with well sort the left half sort the right half like that's

half sort the right half like that's being sort of difficult because well I'm

being sort of difficult because well I'm asking you for a sorting algorithm

asking you for a sorting algorithm you're just telling me to sort the left

you're just telling me to sort the left half and the right half but implicit in

half and the right half but implicit in that last line merging is a pretty

that last line merging is a pretty powerful feature of this sort now we do

powerful feature of this sort now we do need another base case at the top so let

need another base case at the top so let me add this if we find ourselves with a

me add this if we find ourselves with a list an array of size one well that

list an array of size one well that array is obviously sorted if there's

array is obviously sorted if there's only one element in it there's no work

only one element in it there's no work to be done so that's going to be our

to be done so that's going to be our base case but allowing us now in just

base case but allowing us now in just these what uh four six lines of pseudo

these what uh four six lines of pseudo code to actually sort some elements but

code to actually sort some elements but let's focus first on just a subset of

let's focus first on just a subset of this let's consider for a moment what it

this let's consider for a moment what it means to merge sorted have so Carter has

means to merge sorted have so Carter has wonderfully come up to volunteer here

wonderfully come up to volunteer here just to help us reset these numbers

just to help us reset these numbers suppose that in the middle of the story

suppose that in the middle of the story we're about to tell we have two sorted

we're about to tell we have two sorted halves I've already sorted the left half

halves I've already sorted the left half of these numbers and indeed 2 4 57 is

of these numbers and indeed 2 4 57 is sorted from smallest to largest and the

sorted from smallest to largest and the right half appears to be already sorted

right half appears to be already sorted 0136 already sorted so in my pseudo code

0136 already sorted so in my pseudo code we're already done sorting the left half

we're already done sorting the left half and the right half somehow but we'll see

and the right half somehow but we'll see how in a moment well how do I go about

how in a moment well how do I go about merging these two halves well because

merging these two halves well because they're sorted already and you want to

they're sorted already and you want to merge them in order I think we can flip

merge them in order I think we can flip down we can hide all but the first

down we can hide all but the first numbers in each of these subl lists so

numbers in each of these subl lists so here we have a half that starts with two

here we have a half that starts with two and I don't really care what the other

and I don't really care what the other numbers are because they're clearly

numbers are because they're clearly larger than two I can focus only on Two

larger than two I can focus only on Two And Z too zero also we know that zero is

And Z too zero also we know that zero is the smallest there so let's just ignore

the smallest there so let's just ignore the numbers that Carter kindly flipped

the numbers that Carter kindly flipped down so how do I merge these two lists

down so how do I merge these two lists into a new sorted larger list well I

into a new sorted larger list well I compare the two on my left with the zero

compare the two on my left with the zero on my right obviously which comes first

on my right obviously which comes first the zero so let me put this down here

the zero so let me put this down here and Carter if you want to give us the

and Carter if you want to give us the next element now I have two sorted

next element now I have two sorted halves but I've already plucked one off

halves but I've already plucked one off so now I compare the two against the one

so now I compare the two against the one one obviously comes next so I'm going to

one obviously comes next so I'm going to take out the one and put it in place

take out the one and put it in place here now I'm going to compare the two

here now I'm going to compare the two halves again two and three which do I

halves again two and three which do I merge first obviously the two comes next

merge first obviously the two comes next and now notice each time I do this my

and now notice each time I do this my hands are theoretically making forward

hands are theoretically making forward progress I'm not doubling back like I

progress I'm not doubling back like I kept doing with selection sort or bubble

kept doing with selection sort or bubble sort back and forth back and forth my

sort back and forth back and forth my fingers are constantly advancing forward

fingers are constantly advancing forward and that's going to be a key detail so I

and that's going to be a key detail so I compare four and three three obviously I

compare four and three three obviously I compare three and uh I compare four and

compare three and uh I compare four and six four obviously I compare five five

six four obviously I compare five five and six five obviously and then I

and six five obviously and then I compare seven and six six of course and

compare seven and six six of course and then lastly we have just one element

then lastly we have just one element left and even though I'm kind of moving

left and even though I'm kind of moving awkwardly as a human my hands

awkwardly as a human my hands technically were only moving to the

technically were only moving to the right I was never looping back doing

right I was never looping back doing something again and again and that's

something again and again and that's perhaps the intuition and just enough

perhaps the intuition and just enough room for the seven so that then is how

room for the seven so that then is how you would merge two sorted halves we

you would merge two sorted halves we started with left half sorted right half

started with left half sorted right half sorted and merging is just like what you

sorted and merging is just like what you would do as a human and Carter just

would do as a human and Carter just flips the numbers down so our Focus was

flips the numbers down so our Focus was only on the smallest elements in each

only on the smallest elements in each any questions before we Forge ahead with

any questions before we Forge ahead with what it means then to be merged in this

what it means then to be merged in this way so now here is an original list we

way so now here is an original list we deliberately put it at the top because

deliberately put it at the top because there's one detail of Mer sort that's

there's one detail of Mer sort that's key mer sort is technically going to use

key mer sort is technically going to use a little more space and so whereas

a little more space and so whereas previously we just kept moving our

previously we just kept moving our humans around and swapping people and

humans around and swapping people and making sure they stayed ultimately in

making sure they stayed ultimately in the original positions with mer sort

the original positions with mer sort pretend ends that here's our original

pretend ends that here's our original array of uh memory I'm going to need at

array of uh memory I'm going to need at least one other array of memory and I'm

least one other array of memory and I'm going to cheat and I'm going to use even

going to cheat and I'm going to use even more memory but technically I could

more memory but technically I could actually go back and forth between one

actually go back and forth between one array and a secondary array but it isn't

array and a secondary array but it isn't going to it is going to take me more

going to it is going to take me more space so how do I go about implementing

space so how do I go about implementing merge sort on this Al on this code well

merge sort on this Al on this code well let's consider this here's a array of

let's consider this here's a array of size eight if only one number quit

size eight if only one number quit obviously not applicable so let's focus

obviously not applicable so let's focus on the juicy part there sort the left

on the juicy part there sort the left half of the numbers all right how do I

half of the numbers all right how do I sort the left half of the numbers I'm

sort the left half of the numbers I'm going to just nudge them over just to be

going to just nudge them over just to be clear which is the left half how here is

clear which is the left half how here is now a sub list of size four how do I

now a sub list of size four how do I sort the left

sort the left half well do I have an algorithm for

half well do I have an algorithm for sorting yeah what do I do here's a list

sorting yeah what do I do here's a list of size four how do I sort it what's

of size four how do I sort it what's step one sort the left half so I now

step one sort the left half so I now sort of conceptually in my mind take

sort of conceptually in my mind take this sublist of size four and I sort it

this sublist of size four and I sort it by first sorting the left half focusing

by first sorting the left half focusing now on the seven and two all right

now on the seven and two all right here's a list of size two how do I sort

here's a list of size two how do I sort a list of size

a list of size two sorry I think we just keep following

two sorry I think we just keep following our instruction sort the left half all

our instruction sort the left half all right here is a list of size one how do

right here is a list of size one how do I sort a list of size

I sort a list of size one I'm done like it's done so I leave

one I'm done like it's done so I leave this alone what was the next step in the

this alone what was the next step in the story I've just sorted the left half of

story I've just sorted the left half of the left half of the left half what

the left half of the left half what comes next I sort the right half of the

comes next I sort the right half of the left half of the left half and I'm done

left half of the left half and I'm done cuz it's just a list of size one what

cuz it's just a list of size one what comes after this merge so this is where

comes after this merge so this is where it gets a little trippy because you have

it gets a little trippy because you have to remember where we're pausing the

to remember where we're pausing the story to do things recursively again and

story to do things recursively again and again but if I've just sorted the left

again but if I've just sorted the left half and I've just sorted the right half

half and I've just sorted the right half now I merge them together this is a

now I merge them together this is a super short list so we don't need um uh

super short list so we don't need um uh Carter's help here as before but I think

Carter's help here as before but I think the first number I take here is the two

the first number I take here is the two and then the second number I take

and then the second number I take because it's the only option is the

because it's the only option is the seven but what's nice now is that notice

seven but what's nice now is that notice the left half of the left half is indeed

the left half of the left half is indeed sorted because I trivially sorted the

sorted because I trivially sorted the left half of it and the right half of it

left half of it and the right half of it but then merging is really where the

but then merging is really where the magic happens all right again if you rem

magic happens all right again if you rem if you rewind now in your mind if I've

if you rewind now in your mind if I've just sorted the left half of the left

just sorted the left half of the left half what happens

half what happens next sort the right half of the left

next sort the right half of the left half so again you kind of rewind in time

half so again you kind of rewind in time so how do I do this I've got a list of

so how do I do this I've got a list of size two I sort the left half the just

size two I sort the left half the just the five done sort the right half four

the five done sort the right half four done now the interesting part I merge

done now the interesting part I merge the left half and the right half of the

the left half and the right half of the right half of the left half so what do I

right half of the left half so what do I do four comes down here five comes down

do four comes down here five comes down here and

here and now notice what I have left half is

now notice what I have left half is sorted right half is sorted if you

sorted right half is sorted if you rewind in time where is my next step

rewind in time where is my next step three merge the two halves and so this

three merge the two halves and so this is what Carter helped me do before let's

is what Carter helped me do before let's focus only on the smallest elements just

focus only on the smallest elements just so there's less distraction I compare

so there's less distraction I compare the two and the four two comes first so

the two and the four two comes first so let's obviously put that here now I

let's obviously put that here now I compare the new beginning of this list

compare the new beginning of this list and the old beginning of this list four

and the old beginning of this list four obviously comes next and now I compare

obviously comes next and now I compare the seven against the five five

the seven against the five five obviously comes next and now lastly I'm

obviously comes next and now lastly I'm left with one number so now I'm down to

left with one number so now I'm down to the seven so even if you've kind of lost

the seven so even if you've kind of lost track of some of the nuances here if you

track of some of the nuances here if you just kind of take a step back we have

just kind of take a step back we have the original right half here still

the original right half here still untouched but the left half of the

untouched but the left half of the original input is now indeed sorted all

original input is now indeed sorted all by way of doing sorting left half right

by way of doing sorting left half right half left half right half but with those

half left half right half but with those merges in between all right so if we've

merges in between all right so if we've just sorted the left half we rewind all

just sorted the left half we rewind all the way to the beginning what do I now

the way to the beginning what do I now do all right so sort the right half so

do all right so sort the right half so sort the right half how do I sort a list

sort the right half how do I sort a list of size four well I first sort the left

of size four well I first sort the left half the one and the six how do I sort a

half the one and the six how do I sort a list of size two you sort the left half

list of size two you sort the left half just the number one obviously there's no

just the number one obviously there's no work to be done done sorting the left

work to be done done sorting the left half six done sorting the right half now

half six done sorting the right half now what do I do I merge the left half here

what do I do I merge the left half here with the right half here and that one's

with the right half here and that one's pretty straightforward now what do I do

pretty straightforward now what do I do I've just merged so now I sorted I've

I've just merged so now I sorted I've just sorted the left half of the right

just sorted the left half of the right half so now I sort the right half of the

half so now I sort the right half of the right half so I consider the zero done I

right half so I consider the zero done I consider the three done I now merge

consider the three done I now merge these two together zero of course comes

these two together zero of course comes first then comes the three and now I'm

first then comes the three and now I'm at the point of the story where I've

at the point of the story where I've sorted the left half of the right half

sorted the left half of the right half and the right half of the right half so

and the right half of the right half so step three is merge and I'll do it again

step three is merge and I'll do it again like we did with Carter all right one

like we did with Carter all right one and zero obviously the zero comes first

and zero obviously the zero comes first now compare the one and the three

now compare the one and the three obviously the one comes first compare

obviously the one comes first compare the six and the three obviously the

the six and the three obviously the three and then lastly the sixth so now

three and then lastly the sixth so now where are we we've taken the left half

where are we we've taken the left half of the whole thing and sorted the left

of the whole thing and sorted the left and sorted it we then took the right

and sorted it we then took the right half of the whole thing and sorted it so

half of the whole thing and sorted it so now we're at lastly step three for the

now we're at lastly step three for the last last time what do we do merge and

last last time what do we do merge and so just to be consistent let me push

so just to be consistent let me push these down and let's compare left hand

these down and let's compare left hand to right hand noticing that they only

to right hand noticing that they only make forward progress none of this back

make forward progress none of this back and forth comparisons two and zero of

and forth comparisons two and zero of course the zero so we'll put that in

course the zero so we'll put that in place two and one of course the one so

place two and one of course the one so we put that in place two and three we

we put that in place two and three we merge in of course the two in this case

merge in of course the two in this case four and three we now merge in the three

four and three we now merge in the three in this case four and six we now merge

in this case four and six we now merge of course the four in place and now we

of course the four in place and now we compare five and six we keep the

five bug okay well pretend that the five is

bug okay well pretend that the five is on uh oh this is why all right so now we

on uh oh this is why all right so now we compare the seven and the six sixth is

compare the seven and the six sixth is gone and lastly seven is the last one in

gone and lastly seven is the last one in place and even though I grant that of

place and even though I grant that of all the algorithms this is probably the

all the algorithms this is probably the hardest one to stay on top of especially

hardest one to stay on top of especially when I'm doing it as a voiceover realize

when I'm doing it as a voiceover realize that what we've just done is only those

that what we've just done is only those three steps recursively we started with

three steps recursively we started with a list of size eight we sorted the left

a list of size eight we sorted the left half we sorted the right half and then

half we sorted the right half and then we merged the two together but if you go

we merged the two together but if you go down each of those rabbit holes so to

down each of those rabbit holes so to speak sorting the left half involves

speak sorting the left half involves sorting the left half of the left half

sorting the left half of the left half and the right half of the left half and

and the right half of the left half and so forth but this germ of an idea of

so forth but this germ of an idea of really dividing and conquering the

really dividing and conquering the problem not such that you're having the

problem not such that you're having the problem and only dealing with one half

problem and only dealing with one half clearly we're sorting one half and the

clearly we're sorting one half and the other half and merging them together

other half and merging them together ultimately it does still lead us to the

ultimately it does still lead us to the same solution and if we visualize the

same solution and if we visualize the remnants of this now if I depict this as

remnants of this now if I depict this as follows where on the screen here you see

follows where on the screen here you see where the numbers originally started in

where the numbers originally started in the top row from left to right

the top row from left to right essentially even though this is in a

essentially even though this is in a different order I divided that list of

different order I divided that list of size eight ultimately into eight lists

size eight ultimately into eight lists of size one and that's where the base

of size one and that's where the base case kicked in and just said okay we're

case kicked in and just said okay we're done sorting that and after that

done sorting that and after that logically I then sorted I merged two

logically I then sorted I merged two lists of size one into many lists of

lists of size one into many lists of size two and those lists of size two

size two and those lists of size two into lists of size four and then finally

into lists of size four and then finally the list of size four into one big list

the list of size four into one big list sorted of size eight and so I put forth

sorted of size eight and so I put forth this picture with the little line

this picture with the little line indicators here because how many times

indicators here because how many times did I divide divide divide in half or

did I divide divide divide in half or really double double double so exponent

really double double double so exponent is the opposite oh spoiler uh how many

is the opposite oh spoiler uh how many times did I divide so three concretely

times did I divide so three concretely but if there's eight elements total and

but if there's eight elements total and there's N More generally it really is a

there's N More generally it really is a matter of dividing and conquering login

matter of dividing and conquering login times you start this and you can divide

times you start this and you can divide one two three times login times or

one two three times login times or conversely you can start here and

conversely you can start here and exponentially uh double double double

exponentially uh double double double three times which is log n but on every

three times which is log n but on every row every shelf literally I made a fuss

row every shelf literally I made a fuss about pointing my hands only from the

about pointing my hands only from the left to the right constantly advancing

left to the right constantly advancing them such that every time I did those

them such that every time I did those merges I touched every element once and

merges I touched every element once and only once there was none of this back

only once there was none of this back and forth back and forth on stage so if

and forth back and forth on stage so if I'm doing something log n times if I'm

I'm doing something log n times if I'm doing rather N Things log n times what

doing rather N Things log n times what would be our Big O formula perhaps N

would be our Big O formula perhaps N Things log n

Things log n times yeah so n log n the order of n log

times yeah so n log n the order of n log n is indeed how we would describe the

n is indeed how we would describe the running time of merge sort and so of all

running time of merge sort and so of all of the sorts thus far we've seen that

of the sorts thus far we've seen that merge sort here actually is n log n

merge sort here actually is n log n which is strictly better than n squ

which is strictly better than n squ which is where both merge uh both

which is where both merge uh both selection sort and bubble sort landed

selection sort and bubble sort landed but it's also slower than linear search

but it's also slower than linear search for instance but you would rather expect

for instance but you would rather expect that if you have to do a lot of work up

that if you have to do a lot of work up front sorting some elements versus just

front sorting some elements versus just searching them you're going to have to

searching them you're going to have to put in more effort and so the question

put in more effort and so the question of whether or not you should just search

of whether or not you should just search something blindly with linear search and

something blindly with linear search and not bother sorting it really boils down

not bother sorting it really boils down to can you afford to spend this amount

to can you afford to spend this amount of time and if you're the Googles of the

of time and if you're the Googles of the world odds are you don't want to be

world odds are you don't want to be searching their database linearly every

searching their database linearly every time why because you can sort it once

time why because you can sort it once and then benefit millions billions of

and then benefit millions billions of people subsequently using something like

people subsequently using something like binary search or frankly in practice

binary search or frankly in practice something even fancier and faster than

something even fancier and faster than binary search but there's always going

binary search but there's always going to be this tradeoff you can achieve

to be this tradeoff you can achieve binary search only if the elements are

binary search only if the elements are sorted how much does it cost you to sort

sorted how much does it cost you to sort them well maybe N squared if you use

them well maybe N squared if you use some of the earlier algorithms but it

some of the earlier algorithms but it turns out n log n is pretty fast as well

turns out n log n is pretty fast as well so at the end of the day

so at the end of the day these running times involve tradeoffs

these running times involve tradeoffs and indeed in merge sort 2 I should note

and indeed in merge sort 2 I should note that the lower bound on merge sort is

that the lower bound on merge sort is also going to be Omega of n log n as

also going to be Omega of n log n as such we can describe it in terms of our

such we can describe it in terms of our Theta notation saying that merge short

Theta notation saying that merge short is indeed in Theta of n log n so

is indeed in Theta of n log n so generally speaking probably better to

generally speaking probably better to use something like merge sort or some

use something like merge sort or some other algorithm that's in N log n in

other algorithm that's in N log n in practice most programmers are not

practice most programmers are not implementing these sorting algorithms

implementing these sorting algorithms themselves odds are they're using a

themselves odds are they're using a library off the shelf that themselves

library off the shelf that themselves have made the decision as to which of

have made the decision as to which of these algorithms to do but generally

these algorithms to do but generally speaking and we're seeing now this for

speaking and we're seeing now this for the first time if you want to improve

the first time if you want to improve time like use less time write faster

time like use less time write faster code you got to pay a price and that

code you got to pay a price and that might be your human time just takes you

might be your human time just takes you more time to code up something more

more time to code up something more sophisticated more difficult to

sophisticated more difficult to implement or you need to spend something

implement or you need to spend something like space and as these shelves suggest

like space and as these shelves suggest that too is one of the key details of

that too is one of the key details of merge sort you can't just have the

merge sort you can't just have the elements swapping in place you need at

elements swapping in place you need at least an auxiliary array so that when

least an auxiliary array so that when you do the merging you have a place to

you do the merging you have a place to put them and this is excessive this

put them and this is excessive this amount of memory I could have just gone

amount of memory I could have just gone back and forth between top shelf and

back and forth between top shelf and bottom shelf but it's a little more

bottom shelf but it's a little more interesting to go top down but you do

interesting to go top down but you do need more space back in the day decades

need more space back in the day decades ago space was really expensive and so

ago space was really expensive and so you know what it might have been better

you know what it might have been better to not use merge sort use bubble sort or

to not use merge sort use bubble sort or uh selection sort even or some other

uh selection sort even or some other algorithm alog together nowadays space

algorithm alog together nowadays space is relatively cheap and so these are

is relatively cheap and so these are more acceptable trade-offs but it

more acceptable trade-offs but it totally depends on the application the

totally depends on the application the very last thing we thought we'd do is

very last thing we thought we'd do is show you an actual compar comparison of

show you an actual compar comparison of some of these sorting algorithms it's

some of these sorting algorithms it's about 60 seconds long and it will

about 60 seconds long and it will compare for you uh selection sort bubble

compare for you uh selection sort bubble sort and merge sort in parallel

sort and merge sort in parallel simultaneously uh with some fun sorting

simultaneously uh with some fun sorting music showing you ultimately what it

music showing you ultimately what it really means to be an O of n s or better

really means to be an O of n s or better yet Big O of n

yet Big O of n Logan selection on the top bubble on the

Logan selection on the top bubble on the bottom merge in the

bottom merge in the [Music]

[Music] middle

middle [Music]

all right that's it for cs50 we'll see you next

[Music] time

time [Music]

[Music] all right this is cs50 and this is week

all right this is cs50 and this is week four and if you think back a few weeks

four and if you think back a few weeks ago already in week zero we started

ago already in week zero we started talking about like what images are and

talking about like what images are and we talked about representation of images

we talked about representation of images as this grid of pixels and each pixel

as this grid of pixels and each pixel has like some pattern of bits that

has like some pattern of bits that defines its color well it turns out

defines its color well it turns out today we'll take a deeper look

today we'll take a deeper look underneath the hood at how things like

underneath the hood at how things like images and so much more is actually

images and so much more is actually implemented using just these zeros and

implemented using just these zeros and ones and how now as a programmer you can

ones and how now as a programmer you can actually harness that for better for

actually harness that for better for worse to better understand and better

worse to better understand and better manipulate what's going on inside of a

manipulate what's going on inside of a computer's memory using a language like

computer's memory using a language like C in fact even this bowl of stress balls

C in fact even this bowl of stress balls that we keep happening is just a

that we keep happening is just a photograph of course but if you think

photograph of course but if you think back to week zero if you sort of enhance

back to week zero if you sort of enhance enhance enhance this image like they do

enhance enhance this image like they do in the movies it actually doesn't work

in the movies it actually doesn't work out the way you would think from

out the way you would think from Hollywood as I keep continue to zoom in

Hollywood as I keep continue to zoom in and zoom in and zoom in on a screen like

and zoom in and zoom in on a screen like this you'll see that yes it gets bigger

this you'll see that yes it gets bigger but if it gets too big what do you start

but if it gets too big what do you start to notice the so-called pixelation and

to notice the so-called pixelation and indeed you can see the individual dots

indeed you can see the individual dots so next time you watch some uh show or

so next time you watch some uh show or movie on uh TV that has this sort of

movie on uh TV that has this sort of notion of enhancing you know there's

notion of enhancing you know there's actually a finite limit there you can

actually a finite limit there you can only enhance so far as there's actually

only enhance so far as there's actually information there but once you zoom in

information there but once you zoom in to a certain level like this like that's

to a certain level like this like that's all that there you're not going to see

all that there you're not going to see the glint of the suspect in some crime

the glint of the suspect in some crime drama in their eye just because you've

drama in their eye just because you've enhanced the image there's only a finite

enhanced the image there's only a finite amount of information actually there but

amount of information actually there but we'll see today too that by

we'll see today too that by understanding what's going on inside of

understanding what's going on inside of a computer's memory we can start to

a computer's memory we can start to represent and even create and code more

represent and even create and code more interesting things so for instance here

interesting things so for instance here is a bit map if you will which is a term

is a bit map if you will which is a term of art a bit map is a type of image and

of art a bit map is a type of image and it's a map of bits in the sense that you

it's a map of bits in the sense that you have this coordinate system of up top

have this coordinate system of up top down left right at least in this artist

down left right at least in this artist representation here and suppose that

representation here and suppose that maybe we all defi decide as a as as the

maybe we all defi decide as a as as the world that one shall represent the color

world that one shall represent the color white and zero shall represent the color

white and zero shall represent the color black what might this map of bits this

black what might this map of bits this bit map actually be can you see through

bit map actually be can you see through it yeah it is indeed a smiley face so an

it yeah it is indeed a smiley face so an amazing eye if I actually turn all of

amazing eye if I actually turn all of the ones to White just to visualize this

the ones to White just to visualize this you'll see indeed this is what was

you'll see indeed this is what was embedded there but of course on our

embedded there but of course on our computer monitors and phones we have

computer monitors and phones we have this grid of square is this grid of

this grid of square is this grid of pixels so indeed if you were to actually

pixels so indeed if you were to actually see on your screen a smiley face like a

see on your screen a smiley face like a black and white one at that what's

black and white one at that what's probably going on underneath the hood is

probably going on underneath the hood is just some pattern of zeros and ones and

just some pattern of zeros and ones and maybe single bits one bit color if you

maybe single bits one bit color if you will where one here represents white and

will where one here represents white and zero represents black so if you kind of

zero represents black so if you kind of like this thing it turns out you can do

like this thing it turns out you can do pretty uh pretty beautiful pretty

pretty uh pretty beautiful pretty interesting pretty artistically inclined

interesting pretty artistically inclined things if you go to this URL at your

things if you go to this URL at your leisure cs50. lart it'll actually

leisure cs50. lart it'll actually redirect you to a Google spreadsheet

redirect you to a Google spreadsheet that we've made in advance and we've

that we've made in advance and we've kind of Shrunk the rows and columns to

kind of Shrunk the rows and columns to resemble a grid of pixels tiny little

resemble a grid of pixels tiny little squares all of which are white by

squares all of which are white by default not unlike this easel here that

default not unlike this easel here that we have a couple of volunteers working

we have a couple of volunteers working away at in fact would you guys like to

away at in fact would you guys like to come forward for a moment and say a

come forward for a moment and say a quick hello before we come back to you

quick hello before we come back to you uh hello my name is Daniel I'm from

uh hello my name is Daniel I'm from Chicago welcome to Daniel and hi

Chicago welcome to Daniel and hi everyone I'm Adam and I'm from chindon

everyone I'm Adam and I'm from chindon Tobago nice well welcome to you both

Tobago nice well welcome to you both thank you you'll see that in their hands

thank you you'll see that in their hands are actually a whole bunch of pixels uh

are actually a whole bunch of pixels uh Post-it notes that we've handed them in

Post-it notes that we've handed them in in advance so if you don't mind we'll

in advance so if you don't mind we'll come back to you in a couple of minutes

come back to you in a couple of minutes and see what they've created if you will

and see what they've created if you will on this grid of white paper much like

on this grid of white paper much like you could create on this Google

you could create on this Google spreadsheet in fact feel free to send us

spreadsheet in fact feel free to send us your Creations if so inclined uh via the

your Creations if so inclined uh via the URL you'll get at cs50. lart now let's

URL you'll get at cs50. lart now let's come back to week zero where we Define

come back to week zero where we Define some of the building blocks for images

some of the building blocks for images we talked about RGB which is just red

we talked about RGB which is just red green blue and it's just one of the

green blue and it's just one of the systems a popular system via which you

systems a popular system via which you can represent any color of the rainbow

can represent any color of the rainbow using some combination of red and green

using some combination of red and green and blue and if any of you are

and blue and if any of you are artistically inclined or have used

artistically inclined or have used Photoshop or similar programs you might

Photoshop or similar programs you might typically have some means of selecting a

typically have some means of selecting a color by some grid like this but down

color by some grid like this but down here notice there's explicit mentions of

here notice there's explicit mentions of the types of color systems in use R GB

the types of color systems in use R GB and in fact here you see 0000 0 and up

and in fact here you see 0000 0 and up here under new you see the color black

here under new you see the color black and that implies that if you have no red

and that implies that if you have no red no green no blue well that indeed would

no green no blue well that indeed would represent by convention the color black

represent by convention the color black by contrast if we play around with

by contrast if we play around with Photoshop or similar program if you have

Photoshop or similar program if you have a lot of red a lot of green and a lot of

a lot of red a lot of green and a lot of blue for instance 255 255 255 like

blue for instance 255 255 255 like really crank it up to the max value you

really crank it up to the max value you can represent with eight bits per week

can represent with eight bits per week zero well then it turns out you get the

zero well then it turns out you get the color white here and we can play with

color white here and we can play with these numbers endlessly for instance if

these numbers endlessly for instance if we use 255 of red but zero green and

we use 255 of red but zero green and zero blue not surprisingly the square at

zero blue not surprisingly the square at the top of the screen Becomes Of course

the top of the screen Becomes Of course red entirely because it's all red and no

red entirely because it's all red and no green no blue if we change it instead to

green no blue if we change it instead to 255 for green but zero for red and blue

255 for green but zero for red and blue of course we get green and then lastly

of course we get green and then lastly if we crank up the blue but leave red

if we crank up the blue but leave red and green as zero we of course get blue

and green as zero we of course get blue but all this while down here highlighted

but all this while down here highlighted is something that maybe some of you have

is something that maybe some of you have seen before like some combination of

seen before like some combination of numbers and letters if any of you have

numbers and letters if any of you have made personal web pages or use programs

made personal web pages or use programs like Photoshop you might have used these

like Photoshop you might have used these so-called color codes so indeed the

so-called color codes so indeed the world has this convention whereby using

world has this convention whereby using six digits or sometimes three you can

six digits or sometimes three you can represent a little more succinctly some

represent a little more succinctly some amount of red green blue and you'll see

amount of red green blue and you'll see here maybe by inference that if RGB is0

here maybe by inference that if RGB is0 0 255 respectively perhaps where we're

0 255 respectively perhaps where we're going with this is that

going with this is that 0000 FF is just an alternative way of

0000 FF is just an alternative way of expressing the exact same idea no red no

expressing the exact same idea no red no green and a lot of blue but why is that

green and a lot of blue but why is that and in fact we'll come full circle here

and in fact we'll come full circle here to introducing something that we could

to introducing something that we could have done in week zero but it doesn't

have done in week zero but it doesn't really solve a problem then but today as

really solve a problem then but today as we focus more on images and on memory

we focus more on images and on memory itself turns out understanding these

itself turns out understanding these patterns is pretty useful so back in

patterns is pretty useful so back in week zero we talked of course about

week zero we talked of course about binary and binary by implying two only

binary and binary by implying two only gives you two digits zero and one you

gives you two digits zero and one you and I as humans almost always use the

and I as humans almost always use the decimal system in normal conversation

decimal system in normal conversation deck meaning 10 so we have 0 through n

deck meaning 10 so we have 0 through n instead if a human like us wants to

instead if a human like us wants to count up as high as 10 or 11 or 12 we

count up as high as 10 or 11 or 12 we don't have a digit per se for 10 11 and

don't have a digit per se for 10 11 and 12 we start reusing digits so it's 1 0 1

12 we start reusing digits so it's 1 0 1 1 1 2 and so forth but in other systems

1 1 2 and so forth but in other systems not binary not decimal but systems

not binary not decimal but systems called heximal hex implying 16 there are

called heximal hex implying 16 there are actually more digits than these which

actually more digits than these which might come as a surprise um it's not

might come as a surprise um it's not pairs of digits like in decimal single

pairs of digits like in decimal single digits and frankly it doesn't really

digits and frankly it doesn't really matter what the digits are because at

matter what the digits are because at the end of the day these are just

the end of the day these are just symbols that you and I immediately

symbols that you and I immediately associate with some notion of math but

associate with some notion of math but just Strokes on the screen that

just Strokes on the screen that represent some have represent some

represent some have represent some actual value so it turns out that by

actual value so it turns out that by convention

convention when you want more than n 10 digits 0

when you want more than n 10 digits 0 through 9 you start using letters of the

through 9 you start using letters of the English alphabet A B C D E and F and you

English alphabet A B C D E and F and you can represent them in lower case it's

can represent them in lower case it's case insensitive so it doesn't really

case insensitive so it doesn't really matter you might see it in upper case or

matter you might see it in upper case or lower case but this is how you can count

lower case but this is how you can count Beyond N9 not using decimal but using

Beyond N9 not using decimal but using indeed something called hexadecimal if

indeed something called hexadecimal if we get really technical this is also

we get really technical this is also known as base 16 and it's the same idea

known as base 16 and it's the same idea as week zero where instead of using base

as week zero where instead of using base 2 for binary base 10 for decimal use 16

2 for binary base 10 for decimal use 16 as the base for heximal and so if we run

as the base for heximal and so if we run through just some simple examples here

through just some simple examples here in the world of heximal your columns are

in the world of heximal your columns are just powers of 16 16 to the 0er 16 to

just powers of 16 16 to the 0er 16 to the 1 16 to the 2 and so forth but in

the 1 16 to the 2 and so forth but in the world of hex we usually at least

the world of hex we usually at least thus far and today we'll see just pairs

thus far and today we'll see just pairs of digits like this so here for instance

of digits like this so here for instance is the ones column and the 16's column

is the ones column and the 16's column if we multiply that out so if you wanted

if we multiply that out so if you wanted to represent the number you and I know

to represent the number you and I know in in uh the real world as zero in

in in uh the real world as zero in heximal it would just be 0 0 if you want

heximal it would just be 0 0 if you want to represent the number one it would be

to represent the number one it would be 01 and from there we get 02 03 04 0 five

01 and from there we get 02 03 04 0 five 06 07 08 09 now things get potentially

06 07 08 09 now things get potentially interesting in decimal it would

interesting in decimal it would obviously become 10 but in heximal it

obviously become 10 but in heximal it just becomes 0 a and then 0 B which is

just becomes 0 a and then 0 B which is to say if I rewind after nine comes in

to say if I rewind after nine comes in heximal if I pronounce it in in decimal

heximal if I pronounce it in in decimal this is how you'd represent 10 this is

this is how you'd represent 10 this is how you'd represent 11 12 13 14 and then

how you'd represent 11 12 13 14 and then lastly in heximal the 16th value is f

lastly in heximal the 16th value is f which is just always going to represent

which is just always going to represent 15 so where to how do we connect this to

15 so where to how do we connect this to some of the past math well once you get

some of the past math well once you get to Zer F in heximal if f is the highest

to Zer F in heximal if f is the highest you can count just like in decimal nine

you can count just like in decimal nine is the highest you can count what comes

is the highest you can count what comes next if this is 15 I claim how do I

next if this is 15 I claim how do I represent 16 in heximal with what

represent 16 in heximal with what pattern of

pattern of symbols what pattern of symbols for

symbols what pattern of symbols for heximal yeah so one Z not 10 even though

heximal yeah so one Z not 10 even though you might read it like that as a typical

you might read it like that as a typical human but one zero because why well even

human but one zero because why well even if this is completely new to you the

if this is completely new to you the whole column system the places are

whole column system the places are exactly the same intuitively so you need

exactly the same intuitively so you need a one in the 16's place and a zero in

a one in the 16's place and a zero in the ones place and we won't count all

the ones place and we won't count all the way up to 255 but we count if we

the way up to 255 but we count if we count a little higher this would be 1 Z

count a little higher this would be 1 Z AKA 16 in decimal this would be 1 one

AKA 16 in decimal this would be 1 one AKA 17 in decimal and then 18 19 20 and

AKA 17 in decimal and then 18 19 20 and so forth dot dot dot and we can count

so forth dot dot dot and we can count all the way up to FF CU if f is the

all the way up to FF CU if f is the biggest digit in hexadecimal FF is

biggest digit in hexadecimal FF is indeed as high as we can count and if

indeed as high as we can count and if each F represents 15 well let's just do

each F represents 15 well let's just do the math like in week zero so 16 * F + 1

the math like in week zero so 16 * F + 1 * f is how all of us learned to do uh

* f is how all of us learned to do uh math in grade school even though not in

math in grade school even though not in heximal that's of course 16 * 15 + 1 *

heximal that's of course 16 * 15 + 1 * 15 multiply that out you get 240 plus 15

15 multiply that out you get 240 plus 15 and air go you can count as high as

and air go you can count as high as 255 using two heximal digits now this is

255 using two heximal digits now this is not the kind of thing where like this is

not the kind of thing where like this is going to be an interesting exercise

going to be an interesting exercise mentally to ever convert in your head

mentally to ever convert in your head generally you'll get used to the fact

generally you'll get used to the fact that after nine comes a and the biggest

that after nine comes a and the biggest digit is f and you'll just start to see

digit is f and you'll just start to see patterns like this in the worlds of

patterns like this in the worlds of Photoshop web pages in a few weeks and

Photoshop web pages in a few weeks and Beyond but like why is heximal useful

Beyond but like why is heximal useful like why are we complicating the world

like why are we complicating the world and adding on top of decimal something

and adding on top of decimal something else well it turns out that a single

else well it turns out that a single decimal digit like f the biggest one for

decimal digit like f the biggest one for instance is 15 and here let me just

instance is 15 and here let me just propose a bit of Mental Math how many

propose a bit of Mental Math how many bits do you need to represent the number

bits do you need to represent the number 15 in binary if you've got the ones

15 in binary if you've got the ones place two's Place fours and so forth how

place two's Place fours and so forth how many bits

many bits total so fewer than five to count this

total so fewer than five to count this highest 15 I think but

highest 15 I think but close someone

close someone else sing in hand yeah so four bits I

else sing in hand yeah so four bits I think suffice because if you want to

think suffice because if you want to count as high as F that is to say 15 I

count as high as F that is to say 15 I think if you have four bits you can do

think if you have four bits you can do that because if over here is the one's

that because if over here is the one's place from week zero for binary this is

place from week zero for binary this is the two's place this is The Four's place

the two's place this is The Four's place this is the e place do out some quick

this is the e place do out some quick math so 8 + 4 is 12 + 2 is 14 + 1 is 15

math so 8 + 4 is 12 + 2 is 14 + 1 is 15 so it turns out that by convenience

so it turns out that by convenience heximal digits can just be represented

heximal digits can just be represented consistently with four bits or fewer but

consistently with four bits or fewer but four and four of course is half of eight

four and four of course is half of eight and eight is like everywhere like eight

and eight is like everywhere like eight bits is a bite which is again just a

bits is a bite which is again just a convention we've seen and so the reason

convention we've seen and so the reason that you see hexad desmal in the world

that you see hexad desmal in the world of Photoshop and eventually web pages is

of Photoshop and eventually web pages is it actually just Maps really nicely to

it actually just Maps really nicely to expressing binary numbers more

expressing binary numbers more succinctly with a fixed number of digits

succinctly with a fixed number of digits so for instance anytime you see 11 one

so for instance anytime you see 11 one one one 1111 in the world as binary you

one one 1111 in the world as binary you know what that's a little tedious to

know what that's a little tedious to both say and write you can represent

both say and write you can represent more succinctly

more succinctly any uh group of four one bits more

any uh group of four one bits more succinctly in heximal as just F so 111

succinctly in heximal as just F so 111 1111 in binary more succinctly and more

1111 in binary more succinctly and more commonly now in the world of Photoshop

commonly now in the world of Photoshop memory images and the like is

memory images and the like is represented more succinctly as FF and

represented more succinctly as FF and that's why because it just Maps really

that's why because it just Maps really nicely to four bits and so we can be a

nicely to four bits and so we can be a little more succinct so any questions on

little more succinct so any questions on heximal which is just another way of

heximal which is just another way of representing information but using the

representing information but using the same grade school approach yeah good

same grade school approach yeah good question if you represent 15 with f it

question if you represent 15 with f it would use four bits so base systems are

would use four bits so base systems are really just a way for us humans on paper

really just a way for us humans on paper or on screens to represent information

or on screens to represent information if F represents the decimal number 15

if F represents the decimal number 15 the computer underneath the hood has to

the computer underneath the hood has to use four bits to represent it so one

use four bits to represent it so one heximal digit by convention always

heximal digit by convention always implies four bits underneath the hood so

implies four bits underneath the hood so therefore if you have two heximal digits

therefore if you have two heximal digits like 0 0 that means eight zero bits

like 0 0 that means eight zero bits underneath the hood like for red or for

underneath the hood like for red or for green if you see FF now we know that's

green if you see FF now we know that's four one bits and another four one Bits

four one bits and another four one Bits And if we do out the math that's 255

And if we do out the math that's 255 that's why in Photoshop

that's why in Photoshop 000000 FF means no red no green and

000000 FF means no red no green and 255 of blue and it's just way more

255 of blue and it's just way more succinct than writing out like what 8

succinct than writing out like what 8 plus 8 plus 8 24 zeros and ones and it's

plus 8 plus 8 24 zeros and ones and it's just cleaner than even using decimal

just cleaner than even using decimal when you're using units of eight which

when you're using units of eight which again computers just use everywhere so

again computers just use everywhere so it's just another system it's not one

it's just another system it's not one you need to dwell on very much but again

you need to dwell on very much but again it's fundamentally no different from

it's fundamentally no different from binary or decimal we're just using a

binary or decimal we're just using a slightly different base no all right

slightly different base no all right well we had this blank canvas here and I

well we had this blank canvas here and I think uh are you two perhaps ready to

think uh are you two perhaps ready to reveal for the world what you've created

reveal for the world what you've created do you want to go ahead and I'll I'll

do you want to go ahead and I'll I'll swivel it around for you all right here

swivel it around for you all right here we go big reveal and today's pixel

we go big reveal and today's pixel art a round of applause if we

art a round of applause if we could very nicely done well thank you

could very nicely done well thank you both if you want to come up after and

both if you want to come up after and tear this off and bring it home you're

tear this off and bring it home you're welcome to and keep the Post-it notes

welcome to and keep the Post-it notes too well thank you to our volunteers

too well thank you to our volunteers there let's now translate this to really

there let's now translate this to really more technical worlds where we're going

more technical worlds where we're going to see and consider it more often

to see and consider it more often because in fact sometimes when you've

because in fact sometimes when you've had error messages over the past few

had error messages over the past few weeks from clang the compiler you might

weeks from clang the compiler you might have even seen evidence of heximal we

have even seen evidence of heximal we didn't call it out it wasn't useful to

didn't call it out it wasn't useful to know at the time but it turns out a lot

know at the time but it turns out a lot of programs use and a lot of code uses

of programs use and a lot of code uses heximal for those reasons of more prec

heximal for those reasons of more prec more representation so for instance

more representation so for instance where else might we see it well here's

where else might we see it well here's that picture we keep pulling up of our

that picture we keep pulling up of our computer's memory and each of these

computer's memory and each of these squares in this grid represents a bite

squares in this grid represents a bite sort of top left to bottom right in the

sort of top left to bottom right in the computer's memory but again just an

computer's memory but again just an artist's representation a few weeks ago

artist's representation a few weeks ago I claimed that each of these bytes can

I claimed that each of these bytes can be numbered of course like this is bite

be numbered of course like this is bite zero at top left then bite one then bite

zero at top left then bite one then bite two then bite two billion if you have

two then bite two billion if you have two gigabytes of memory and so we could

two gigabytes of memory and so we could just number them like this 0 through 15

just number them like this 0 through 15 on up 16 17 18 and so forth but per the

on up 16 17 18 and so forth but per the reasons earlier it's just more common in

reasons earlier it's just more common in computer systems and in software to

computer systems and in software to actually use hexadecimal just to

actually use hexadecimal just to describe the locations of the addresses

describe the locations of the addresses of things in memory so instead a typical

of things in memory so instead a typical programmer or computer scientist would

programmer or computer scientist would call these first 16 bytes 0 through F

call these first 16 bytes 0 through F just because but that's because it's a

just because but that's because it's a predictable number of bits so if we keep

predictable number of bits so if we keep going beyond that you would get not 10

going beyond that you would get not 10 not 11 not 12 but in heximal 1 0 1 1 1 2

not 11 not 12 but in heximal 1 0 1 1 1 2 and so forth all the way down on the

and so forth all the way down on the screen to 1 F and if I you know shrunk

screen to 1 F and if I you know shrunk this down or had a bigger monitor we

this down or had a bigger monitor we would see eventually 255 bytes later

would see eventually 255 bytes later from the start 255 as well but there's a

from the start 255 as well but there's a potential problem here with using

potential problem here with using heximal in this way there's an

heximal in this way there's an ambiguity can anyone imagine like what

ambiguity can anyone imagine like what can go wrong if we use hex to just

can go wrong if we use hex to just simply describe locations in memory like

simply describe locations in memory like this

this yeah yeah yeah like one zero might also

yeah yeah yeah like one zero might also be 10 and you know maybe if you're you

be 10 and you know maybe if you're you know really thorough okay wait a minute

know really thorough okay wait a minute it can't be 10 cuz here's F over here so

it can't be 10 cuz here's F over here so it's obviously not decimal but why

it's obviously not decimal but why create potential confusion especially

create potential confusion especially when you're collaborating building

when you're collaborating building something with someone we want to avoid

something with someone we want to avoid that ambiguity and so the convention

that ambiguity and so the convention humans decided on years ago is that if

humans decided on years ago is that if you want to make clear that a number is

you want to make clear that a number is in hexadecimal just by convention you

in hexadecimal just by convention you prefix all of the digits with Zer X the

prefix all of the digits with Zer X the X is not like another character it's not

X is not like another character it's not like a 17th character it's just a human

like a 17th character it's just a human conven

conven of putting 0x to imply here comes

of putting 0x to imply here comes heximal and now it's unambiguous so now

heximal and now it's unambiguous so now we see 0x10 obviously is not 10 as we

we see 0x10 obviously is not 10 as we know it in decimal but rather it's the

know it in decimal but rather it's the number that comes after a single F so

number that comes after a single F so it's really the number in decimal 16 so

it's really the number in decimal 16 so 0x anytime you see it that's just a

0x anytime you see it that's just a visual cue that what is ahead is

visual cue that what is ahead is actually heximal so let's now start

actually heximal so let's now start playing around with this information so

playing around with this information so here's a super simple line of code from

here's a super simple line of code from like week one where I'm just declaring

like week one where I'm just declaring ing a variable n and I'm defining it to

ing a variable n and I'm defining it to be the value 50 and this is out of

be the value 50 and this is out of context we probably need a main function

context we probably need a main function and all of that but let's just rewind to

and all of that but let's just rewind to week one where we actually saw code like

week one where we actually saw code like this and do something useful with a line

this and do something useful with a line of code like this so let me go over here

of code like this so let me go over here to VSS code and in vs code I'll create a

to VSS code and in vs code I'll create a program called how about addresses since

program called how about addresses since the goal of this uh the goal here is to

the goal of this uh the goal here is to just play around ultimately with a

just play around ultimately with a variable like n and let me go ahead and

variable like n and let me go ahead and do this I'll include how about standard

do this I'll include how about standard i.h I'll do int main void so no command

i.h I'll do int main void so no command line Arguments for now in N gets 50 and

line Arguments for now in N gets 50 and now so that we can do something mildly

now so that we can do something mildly useful with it let's just go use print F

useful with it let's just go use print F and print out with percent I and then a

and print out with percent I and then a new line whatever that value of n is so

new line whatever that value of n is so this is not going to be interesting per

this is not going to be interesting per se it's just week one stuff where I'm

se it's just week one stuff where I'm defining a variable and printing it out

defining a variable and printing it out to the screen so let me go down to my

to the screen so let me go down to my terminal window and do make

terminal window and do make addresses no errors so that's good I'll

addresses no errors so that's good I'll do do/ addresses and of course I should

do do/ addresses and of course I should see the number 50 here now what's going

see the number 50 here now what's going on underneath the hood let's translate

on underneath the hood let's translate now code to really what's going on under

now code to really what's going on under under underneath the hood of the

under underneath the hood of the computer so if this is our grid of

computer so if this is our grid of memory I don't necessarily know as the

memory I don't necessarily know as the programmer and I definitely don't care

programmer and I definitely don't care as the programmer where exactly it's

as the programmer where exactly it's ending up in memory that's the whole

ending up in memory that's the whole point of using Code let the computer

point of using Code let the computer figure this out but at least

figure this out but at least conceptually I know that by declaring a

conceptually I know that by declaring a line of code like that the number 50

line of code like that the number 50 ends up somewhere in the computer's

ends up somewhere in the computer's memory and it's assigned the name n a

memory and it's assigned the name n a symbol n via which I the programmer can

symbol n via which I the programmer can refer to it and I very deliberately used

refer to it and I very deliberately used four of these squares for what

four of these squares for what reason what might be the reason for

reason what might be the reason for using four squares specifically yeah

using four squares specifically yeah yeah so an integer is four bytes at

yeah so an integer is four bytes at least most of the time on Modern systems

least most of the time on Modern systems an integer is four bytes on an older

an integer is four bytes on an older computer it might just use one or maybe

computer it might just use one or maybe even uh two bytes But Here by convention

even uh two bytes But Here by convention we're almost always going to see four

we're almost always going to see four bytes I don't know if it's going to end

bytes I don't know if it's going to end up here it might end up over here but

up here it might end up over here but for now who cares I just know that the

for now who cares I just know that the computer can store the the information

computer can store the the information in this way underneath the hood so let's

in this way underneath the hood so let's now introduce another feature of C that

now introduce another feature of C that we haven't had occasion to use just yet

we haven't had occasion to use just yet that's going to allow us to start poking

that's going to allow us to start poking around the computer's memory For Better

around the computer's memory For Better or For Worse and this is one of those

or For Worse and this is one of those situations where you're about to learn

situations where you're about to learn acquire a a skill a power that can

acquire a a skill a power that can actually come back to bite you because

actually come back to bite you because once you know how to start poking around

once you know how to start poking around a computer's memory you can do very

a computer's memory you can do very powerful things and next week we'll see

powerful things and next week we'll see what you can build in a computer's

what you can build in a computer's memory but you can also screw up pretty

memory but you can also screw up pretty easily and cause more of those

easily and cause more of those segmentation faults that a few of you

segmentation faults that a few of you have already suffered so with that said

have already suffered so with that said let's just stipulate that you know what

let's just stipulate that you know what I don't care necessarily where the 50 is

I don't care necessarily where the 50 is in memory but I know it exists at some

in memory but I know it exists at some address in memory and just so I have an

address in memory and just so I have an easy address to pronounce let's just

easy address to pronounce let's just suppose it lives at ox123 so that's the

suppose it lives at ox123 so that's the address in memory in heximal by

address in memory in heximal by convention and that just happens to be

convention and that just happens to be where it ends up when I write that line

where it ends up when I write that line of code but it turns out C has some

of code but it turns out C has some other operators we can use when we've

other operators we can use when we've seen the asterisk before the star and

seen the asterisk before the star and we've used it for multiplication but

we've used it for multiplication but today we're going to use it for

today we're going to use it for something more powerful and we're also

something more powerful and we're also going to introduce an Amper sand which

going to introduce an Amper sand which allows us to do something as well the

allows us to do something as well the Amper sand operator is going to allow us

Amper sand operator is going to allow us to get the address of a piece of data in

to get the address of a piece of data in memory like by literally putting

memory like by literally putting Ampersand before the name of a variable

Ampersand before the name of a variable C will tell us tell you what address

C will tell us tell you what address that variable lives at maybe it's ox123

that variable lives at maybe it's ox123 maybe it's Ox 456 who knows but that

maybe it's Ox 456 who knows but that will give you back the answer the Star

will give you back the answer the Star does the opposite it's sort of means go

does the opposite it's sort of means go there so using the star otherwise known

there so using the star otherwise known as the D reference operator I can

as the D reference operator I can actually go to a specific address if I

actually go to a specific address if I want and we'll see what this means in

want and we'll see what this means in code so how can I leverage this in some

code so how can I leverage this in some mildly interesting way to start poking

mildly interesting way to start poking around but eventually we'll use this

around but eventually we'll use this primitive to build more interesting

primitive to build more interesting things so let me go back to save vs code

things so let me go back to save vs code here and let me go ahead and do this

here and let me go ahead and do this I'll clear my terminal to start fresh

I'll clear my terminal to start fresh and I'll introduce another format code

and I'll introduce another format code for printf percent p and for now just

for printf percent p and for now just take on faith that this it is percent P

take on faith that this it is percent P because but percent p is going to allow

because but percent p is going to allow me to print the address of a variable if

me to print the address of a variable if I additionally tell C get the address of

I additionally tell C get the address of n so I'm changing percent I to percent p

n so I'm changing percent I to percent p and that's just something you have to do

and that's just something you have to do when printing addresses for now but I

when printing addresses for now but I need to change an ampersand in front of

need to change an ampersand in front of the variable name so I don't print in

the variable name so I don't print in the number 50 I print out something like

the number 50 I print out something like ox123 and it's not going to be as simple

ox123 and it's not going to be as simple as that we'll see on the screen though

as that we'll see on the screen though where it actually ended up in my code

where it actually ended up in my code spaces memory so here we go uh dot uh

spaces memory so here we go uh dot uh down in my terminal make addresses again

down in my terminal make addresses again to recompile and now dot SL addresses

to recompile and now dot SL addresses should reveal not the value of 50 but

should reveal not the value of 50 but the address of 50 and there it is it's

the address of 50 and there it is it's pretty long it's not quite as simple and

pretty long it's not quite as simple and pretty as ox123 but there's the ox

pretty as ox123 but there's the ox meaning here's a hexadecimal address and

meaning here's a hexadecimal address and it's 7f FC c784 a04 C suffice it to say

it's 7f FC c784 a04 C suffice it to say your codes space and even your Macs and

your codes space and even your Macs and PCs nowadays have a lot of memory that's

PCs nowadays have a lot of memory that's why in part this address is so big not

why in part this address is so big not as small as the thing on my slide so

as small as the thing on my slide so this at the moment isn't that useful yet

this at the moment isn't that useful yet but it introduces us to a concept that

but it introduces us to a concept that we'll Now call pointers and pointers are

we'll Now call pointers and pointers are admittedly one of the more challenging

admittedly one of the more challenging aspects of c and if in future life you

aspects of c and if in future life you tell friends that oh I took a class

tell friends that oh I took a class called cs50 and we learned C like you'll

called cs50 and we learned C like you'll probably get kind of a look at people

probably get kind of a look at people like why did you learn C or like oh c

like why did you learn C or like oh c was hard and it's largely because of

was hard and it's largely because of this topic which isn't to say that it's

this topic which isn't to say that it's that hard to wrap your mind around but

that hard to wrap your mind around but it's definitely very different and it's

it's definitely very different and it's not a feature that you can harness in

not a feature that you can harness in higher level languages that we'll see in

higher level languages that we'll see in class two like Python and Java and the

class two like Python and Java and the like C is about as close to the

like C is about as close to the computer's Hardware so to speak that you

computer's Hardware so to speak that you can get before things get actually scary

can get before things get actually scary the so-called Assembly Language we saw

the so-called Assembly Language we saw in week two when I had a link and

in week two when I had a link and compile and assemble and all of that

compile and assemble and all of that like that gets really lowlevel and you

like that gets really lowlevel and you really have to be an expert with the

really have to be an expert with the computer's CPU or brain to understand

computer's CPU or brain to understand that but with C you can actually poke

that but with C you can actually poke around the computer's memory and do

around the computer's memory and do powerful things with that but again with

powerful things with that but again with great power comes responsibility it's

great power comes responsibility it's very easy to break programs by misusing

very easy to break programs by misusing memory or just having a bug that touches

memory or just having a bug that touches memory in some way that you don't intend

memory in some way that you don't intend so pointers at the end of the day are

so pointers at the end of the day are pretty much what we just saw a pointer

pretty much what we just saw a pointer is really just a variable that contains

is really just a variable that contains the address of some value a pointer is a

the address of some value a pointer is a variable that contains the address of

variable that contains the address of some value or more simply it's fine to

some value or more simply it's fine to think of it as an address a pointer is

think of it as an address a pointer is an address of something in the

an address of something in the computer's memory now what might we do

computer's memory now what might we do to uh to uh actualize this well here's

to uh to uh actualize this well here's two lines of code it turns out by using

two lines of code it turns out by using our two new operators today I can

our two new operators today I can declare an INT call it n and assign it a

declare an INT call it n and assign it a value like 50 just like before if I want

value like 50 just like before if I want to store the address of n in a variable

to store the address of n in a variable and not just print it immediately via

and not just print it immediately via printf I can declare a variable for

printf I can declare a variable for instance called P but I could call it

instance called P but I could call it anything I want like any variable but

anything I want like any variable but because it's an address it's not int P

because it's an address it's not int P it has to be int star P so to speak and

it has to be int star P so to speak and the star here on the left hand side of

the star here on the left hand side of the equal sign is just a clue to C that

the equal sign is just a clue to C that means p is going to be a pointer that is

means p is going to be a pointer that is p is going to be the address of what the

p is going to be the address of what the address of an integer now technically

address of an integer now technically it's still an integer itself right

it's still an integer itself right because an address is just a number

because an address is just a number whether it's 1 2 3 or ox123 so this is

whether it's 1 2 3 or ox123 so this is really just a semantic difference so in

really just a semantic difference so in Star p just means that this variable

Star p just means that this variable doesn't contain any old number like 50

doesn't contain any old number like 50 it specifically contains a number that

it specifically contains a number that is the address of something

is the address of something else so how can I now use this well let

else so how can I now use this well let me go back to VSS code and let me

me go back to VSS code and let me propose that we add a line of code like

propose that we add a line of code like that so instead of just directly

that so instead of just directly printing out that value let's go ahead

printing out that value let's go ahead and Define a second variable called uh P

and Define a second variable called uh P that's of type in Star P set it equal to

that's of type in Star P set it equal to Ampersand n and then this time let's not

Ampersand n and then this time let's not just print out Amper sand n let's

just print out Amper sand n let's actually print out the value of P so the

actually print out the value of P so the only two new things here if I zoom in

only two new things here if I zoom in are I've used not only the Amper sand on

are I've used not only the Amper sand on the right to get the address of n I'm

the right to get the address of n I'm now using the star on the Left To Tell C

now using the star on the Left To Tell C that P is still a still a variable as

that P is still a still a variable as always but it's a pointer it is the

always but it's a pointer it is the address of some other value like this

address of some other value like this and I'm still going to print it with the

and I'm still going to print it with the same format code percent P so that

same format code percent P so that doesn't change so let me go ahead and

doesn't change so let me go ahead and zoom out and do make addresses and slash

zoom out and do make addresses and slash addresses and there it is exactly the

addresses and there it is exactly the same thing now in and of itself not that

same thing now in and of itself not that useful yet but the fact that you can now

useful yet but the fact that you can now access the addresses of things in memory

access the addresses of things in memory means that we'll be able to build things

means that we'll be able to build things and construct things and Link things

and construct things and Link things together by knowing where they live so

together by knowing where they live so to speak so any questions on this

to speak so any questions on this technique thus far

technique thus far [Music]

yeah a good question on line six must it be starp and Ampersand and in this case

be starp and Ampersand and in this case yes because what am I doing on the left

yes because what am I doing on the left and I'll get rid of the equal sign for

and I'll get rid of the equal sign for now this would give me a variable called

now this would give me a variable called P that's not an integer per se but

P that's not an integer per se but that's the address of an integer but

that's the address of an integer but without the equal sign I'm not storing

without the equal sign I'm not storing anything in that variable so by adding

anything in that variable so by adding the equal sign and then Ampersand N I am

the equal sign and then Ampersand N I am explicitly figuring out with Ampersand

explicitly figuring out with Ampersand what the address of n is which already

what the address of n is which already exists per line five and tucking it away

exists per line five and tucking it away in this new variable called

in this new variable called P other questions yeah every time you

P other questions yeah every time you run good question every time I run the

run good question every time I run the program it uses up a different piece of

program it uses up a different piece of memory short answer yes computers though

memory short answer yes computers though long story short also have something

long story short also have something called virtual memory so if you run it

called virtual memory so if you run it again and again you might actually see

again and again you might actually see the same addresses on the same Mac or PC

the same addresses on the same Mac or PC or cloud-based server but we'll see in a

or cloud-based server but we'll see in a bit where uh at a high level it's laid

bit where uh at a high level it's laid out but it will always exist at some

out but it will always exist at some address good question yeah some correct

address good question yeah some correct Ampersand n is the address of N and in

Ampersand n is the address of N and in Star p

Star p is a pointer called p and honestly in an

is a pointer called p and honestly in an Ideal World if C were made today and not

Ideal World if C were made today and not decades ago when humans were first

decades ago when humans were first creating languages you know ideally we

creating languages you know ideally we would just have a data type called

would just have a data type called pointer and then this would be a little

pointer and then this would be a little less complicated because it would

less complicated because it would literally be what it says you know the

literally be what it says you know the humans who invented SE didn't do this

humans who invented SE didn't do this but this is the idea so pointer is not a

but this is the idea so pointer is not a legitimate word in the code it is a term

legitimate word in the code it is a term of Art in English but this is really

of Art in English but this is really just the idea but the way you express

just the idea but the way you express pointer as a data type type is a little

pointer as a data type type is a little more cryptic as int star P here but

more cryptic as int star P here but notice in line seven when I print out p

notice in line seven when I print out p i don't use a star I don't use an

i don't use a star I don't use an ampersand y I literally just want to

ampersand y I literally just want to print the value of p and we've been

print the value of p and we've been doing that since week one if you want to

doing that since week one if you want to print a variable just describe the

print a variable just describe the variable by its name no special syntax

variable by its name no special syntax any other questions on this thus

any other questions on this thus far uh what's the advantage of using

far uh what's the advantage of using pointers with pointers we'll see today

pointers with pointers we'll see today some applications of them really the

some applications of them really the idea is going to come to fruition next

idea is going to come to fruition next week when we're going to create what are

week when we're going to create what are called uh data structures in memory

called uh data structures in memory where we can build not just uh for

where we can build not just uh for instance uh one-dimensional data

instance uh one-dimensional data structures like an array we'll see next

structures like an array we'll see next week we can actually create the

week we can actually create the equivalent of two-dimensional data

equivalent of two-dimensional data structures or even threedimensional data

structures or even threedimensional data structures by using these addresses and

structures by using these addresses and sort of linking things together and

sort of linking things together and we'll see the beginnings of that this

we'll see the beginnings of that this week but for now focus at least for now

week but for now focus at least for now on just really the syntax and what these

on just really the syntax and what these building blocks can do for us um does

building blocks can do for us um does the poin have to be does the p uh in

the poin have to be does the p uh in does the pop pointer have to be an point

does the pop pointer have to be an point to an integer short answer no and we'll

to an integer short answer no and we'll come back to this for now for the sake

come back to this for now for the sake of discussion we're only dealing with

of discussion we're only dealing with integers like the number 50 uh you

integers like the number 50 uh you mentioned strings or characters

mentioned strings or characters absolutely we're about to go there soon

absolutely we're about to go there soon so you can use the address of anything

so you can use the address of anything you want in the computer's memory so in

you want in the computer's memory so in fact let's translate this now to just

fact let's translate this now to just the same picture just to help you wrap

the same picture just to help you wrap your minds around what these two lines

your minds around what these two lines of code really fundamentally are doing

of code really fundamentally are doing so if I come back to my grid of memory

so if I come back to my grid of memory here let's plop the number 50 in the

here let's plop the number 50 in the variable n at the bottom right like it

variable n at the bottom right like it was before so this is that first line of

was before so this is that first line of code as before but with the new second

code as before but with the new second line of code as soon as I create P what

line of code as soon as I create P what do I do well first remember that n lives

do I do well first remember that n lives somewhere in the computer's memory

somewhere in the computer's memory usually I don't care precisely where it

usually I don't care precisely where it is but for the sake of discussion let's

is but for the sake of discussion let's suppose it's at ox123 which is easier to

suppose it's at ox123 which is easier to say than where it actually ended up and

say than where it actually ended up and now what is p well p is just another

now what is p well p is just another variable and variables live in memory

variable and variables live in memory too so let me just hypothesize that P

too so let me just hypothesize that P lives up here and it turns out that P

lives up here and it turns out that P once you assign it the value of

once you assign it the value of Ampersand N means that c will take a

Ampersand N means that c will take a look at the variable n realize oh it

look at the variable n realize oh it lives at ox123 and what goes in the

lives at ox123 and what goes in the value of p is literally

value of p is literally ox123 so again it's still an integer

ox123 so again it's still an integer which is confusing but it's technically

which is confusing but it's technically an integer being used as an address and

an integer being used as an address and now just a a prompt here notice that

now just a a prompt here notice that this pointer is pretty darn big it's

this pointer is pretty darn big it's like eight squares what's the

like eight squares what's the implication of that because I did that

implication of that because I did that deliberately

deliberately how big must a pointer apparently be in

how big must a pointer apparently be in most modern systems would you say okay

most modern systems would you say okay good computers today are very big you

good computers today are very big you have gigabytes of RAM in your computer

have gigabytes of RAM in your computer you therefore need big pointers to be

you therefore need big pointers to be able to point at memory that's

able to point at memory that's conceptually pretty far away so to be

conceptually pretty far away so to be clear how many bytes does a pointer

clear how many bytes does a pointer apparently take up well it seems to take

apparently take up well it seems to take up eight in total integers by convention

up eight in total integers by convention nowadays are usually four pointers

nowadays are usually four pointers though nowadays are typically eight in

though nowadays are typically eight in this case so I'm drawing it in a manner

this case so I'm drawing it in a manner consistent with the reality even though

consistent with the reality even though at the end of the day it's not really

at the end of the day it's not really that interesting what values are in here

that interesting what values are in here in fact let's emerge from these weeds I

in fact let's emerge from these weeds I don't really care what else is going on

don't really care what else is going on in my computer's memory at the moment

in my computer's memory at the moment because I've only got those two lines of

because I've only got those two lines of Juicy code defining n and defining P so

Juicy code defining n and defining P so let's hide all of the other squares and

let's hide all of the other squares and honestly I mean it when I say that

honestly I mean it when I say that programmers need to know that a variable

programmers need to know that a variable exists somewhere in memory and needs to

exists somewhere in memory and needs to be able to get that address using like

be able to get that address using like the Ampersand but you're never going to

the Ampersand but you're never going to print F like I did the actual address

print F like I did the actual address like it's not generally interesting

like it's not generally interesting unless you're debugging your code but

unless you're debugging your code but you're not going to like start typing

you're not going to like start typing out crazy Ox numbers in your code to

out crazy Ox numbers in your code to move things around you just need to know

move things around you just need to know that the computer can figure out where

that the computer can figure out where things are so frankly by that logic who

things are so frankly by that logic who cares that it's ox123 right tomorrow it

cares that it's ox123 right tomorrow it could be Ox 456 or something else so one

could be Ox 456 or something else so one of the ways to think of a pointer is

of the ways to think of a pointer is literally as a variable that points at

literally as a variable that points at something else and indeed in this case P

something else and indeed in this case P yeah technically it has an address and

yeah technically it has an address and yeah technically it's ox123 in the story

yeah technically it's ox123 in the story but honestly who cares I just need to

but honestly who cares I just need to know that you using p i can get to the

know that you using p i can get to the value n and so what are these addresses

value n and so what are these addresses and in fact if Carter wouldn't mind

and in fact if Carter wouldn't mind joining me up here for a moment what are

joining me up here for a moment what are these addresses well just like in our

these addresses well just like in our human world we have mailboxes even

human world we have mailboxes even though you might not check it very

though you might not check it very frequently nowadays but to get physical

frequently nowadays but to get physical mail every uh home every business has a

mail every uh home every business has a unique address the uh science and

unique address the uh science and engineering complex is 150 Western

engineering complex is 150 Western Avenue Austin Massachusetts 02134 USA

Avenue Austin Massachusetts 02134 USA and theoretically that uniquely

and theoretically that uniquely identifies that building in the world

identifies that building in the world well here we we have two mailboxes um

well here we we have two mailboxes um over here we have a value n that happens

over here we have a value n that happens to live I'll claim it address ox123 and

to live I'll claim it address ox123 and then over here I claim there's another

then over here I claim there's another address uh called by name p i don't

address uh called by name p i don't actually care where it is even though it

actually care where it is even though it definitely exists somewhere in the

definitely exists somewhere in the computer's memory but if this is p which

computer's memory but if this is p which is a variable and that's n another

is a variable and that's n another variable ideally this mailbox would be

variable ideally this mailbox would be twice as big because of the number of

twice as big because of the number of btes using but Home Depot only had

btes using but Home Depot only had identical size mailboxes but here is p

identical size mailboxes but here is p one variable there is in another

one variable there is in another variable if I open up this mailbox what

variable if I open up this mailbox what should I find inside of it based on our

should I find inside of it based on our story thus far like what value will I

story thus far like what value will I pull out dramatically in just a

pull out dramatically in just a moment yeah I think ox1 123 Now using

moment yeah I think ox1 123 Now using this you can kind of think of this as

this you can kind of think of this as like x marks the spot no pun intended

like x marks the spot no pun intended where I can now like walk around the

where I can now like walk around the computer's memory and find my way to

computer's memory and find my way to that location by sort of following the

that location by sort of following the treasure map or if I want it more

treasure map or if I want it more dramatically uh thanks to our little

dramatically uh thanks to our little Yale foam finger here here you can think

Yale foam finger here here you can think of it more abstractly as p is just

of it more abstractly as p is just pointing at n okay that's not going over

pointing at n okay that's not going over well so let's switch over to the Harvard

well so let's switch over to the Harvard one so p is

pointing so p is pointing at n and so it turns out we will be able to write code

turns out we will be able to write code now that will do the equivalent of me

now that will do the equivalent of me walking over to n but for now Carter if

walking over to n but for now Carter if you want to reveal what's in the mailbox

you want to reveal what's in the mailbox we should see indeed the number 50 so

we should see indeed the number 50 so that's really all that's oh I feel

that's really all that's oh I feel Carter's waiting for Applause so like

Carter's waiting for Applause so like really well nicely done

thank you so that's just like a physical metaphor of what's going on here in one

metaphor of what's going on here in one variable we have an address and that

variable we have an address and that variable by convention is called a

variable by convention is called a pointer in the other variable per week

pointer in the other variable per week one we just have a value like n and you

one we just have a value like n and you can yes follow the map and walk yourself

can yes follow the map and walk yourself to that particular address and we'll see

to that particular address and we'll see how to do that in code but what's really

how to do that in code but what's really interesting is this abstraction that

interesting is this abstraction that pointers literally or really I guess

pointers literally or really I guess figuratively point at some other value

figuratively point at some other value in memory all right questions then on

in memory all right questions then on pointers in this form pointers point to

pointers in this form pointers point to each other can pointers point to each

each other can pointers point to each other so yes there's things called

other so yes there's things called double pointers we're not going to see

double pointers we're not going to see them anytime soon but using star star

them anytime soon but using star star you can express an address of an address

you can express an address of an address um but we won't see that just yet other

um but we won't see that just yet other questions on

questions on pointers yeah in

front our array so to summarize our arrays then pointers so short answer

arrays then pointers so short answer there's a relationship and we'll come

there's a relationship and we'll come back to that in a little bit but arrays

back to that in a little bit but arrays are technically different from pointers

are technically different from pointers but we we're going to be able to blur

but we we're going to be able to blur the lines a little bit by using one like

the lines a little bit by using one like the other but let me come back to that

the other but let me come back to that in just a bit of time all right so if we

in just a bit of time all right so if we have now this mental model if you will

have now this mental model if you will of like what a pointer is in memory I

of like what a pointer is in memory I think we can start to peel back a layer

think we can start to peel back a layer of uh simplification that we've been

of uh simplification that we've been assuming for the past few weeks since

assuming for the past few weeks since week one so a string recall is a

week one so a string recall is a sequence of characters and so if you

sequence of characters and so if you want to create a string that says Hi in

want to create a string that says Hi in all caps and an next exclamation point

all caps and an next exclamation point we do string s equals quote unquote high

we do string s equals quote unquote high and we can hard code it like this or we

and we can hard code it like this or we could use get string but for now just

could use get string but for now just assume that I hardcoded it into my code

assume that I hardcoded it into my code to always say hi in all caps with an

to always say hi in all caps with an exclamation point well what does that

exclamation point well what does that look like in the computer's memory well

look like in the computer's memory well let's stop looking at the entire memory

let's stop looking at the entire memory let's just focus on really what's going

let's just focus on really what's going on once you create a string called s and

on once you create a string called s and store in it hi you know that a couple of

store in it hi you know that a couple of things are happening H and I and the

things are happening H and I and the exclamation point are ending up in the

exclamation point are ending up in the computer's memory we know from week two

computer's memory we know from week two that this thing the so-called null

that this thing the so-called null character n AKA back slz is also being

character n AKA back slz is also being added for you and it's somewhere in

added for you and it's somewhere in memory at the moment I don't really care

memory at the moment I don't really care where I drew it at the bottom right yes

where I drew it at the bottom right yes it has an address but for now it just

it has an address but for now it just ends up somewhere and in fact here's a

ends up somewhere and in fact here's a little visual cue as to how this happens

little visual cue as to how this happens in C anytime you use double quotes to

in C anytime you use double quotes to give you a string you can imagine that

give you a string you can imagine that the double quotes are like a a clue to

the double quotes are like a a clue to not only store Hi exclamation point but

not only store Hi exclamation point but also put the null character there for

also put the null character there for you and this is in contrast to what

you and this is in contrast to what chars if you want individual characters

chars if you want individual characters what syntax did we use

what syntax did we use instead so single quotes single quotes

instead so single quotes single quotes do not add magically a back SL zero they

do not add magically a back SL zero they literally just store one character so

literally just store one character so again strings have always been a little

again strings have always been a little special you get some extra an extra bite

special you get some extra an extra bite for free so that you know where the

for free so that you know where the string ends and functions like stir

string ends and functions like stir compare can then find their way there so

compare can then find their way there so in memory it might indeed look a little

in memory it might indeed look a little like this and if we assume that there's

like this and if we assume that there's going to be somewhere in memory these

going to be somewhere in memory these things are going to be somewhere in

things are going to be somewhere in memory we can address them per week two

memory we can address them per week two by way of the name of the variable so if

by way of the name of the variable so if s is the name of the variable S braet 0

s is the name of the variable S braet 0 is how you would refer to the first

is how you would refer to the first letter s bracket 1 s braet two and if

letter s bracket 1 s braet two and if you really want s bracket 3 would get

you really want s bracket 3 would get you at the uh null character at the very

you at the uh null character at the very end but what is s so technically in this

end but what is s so technically in this line of code here not only is the

line of code here not only is the computer giving you memory for h i

computer giving you memory for h i exclamation point back sl0 we turns out

exclamation point back sl0 we turns out that s itself must take up some amount

that s itself must take up some amount of space right because s is the variable

of space right because s is the variable and every time we' talked about

and every time we' talked about variables thus far I've given you a

variables thus far I've given you a rectangle on the screen in which to

rectangle on the screen in which to store its value so let's assume for the

store its value so let's assume for the sake of discussion that the H is at

sake of discussion that the H is at ox123 and I is at o x124 exclamation

ox123 and I is at o x124 exclamation points at o x125 and the null characters

points at o x125 and the null characters at ox1 126 well what then is s well s is

at ox1 126 well what then is s well s is just going to be some other variable and

just going to be some other variable and I'll draw it somewhat abstractly without

I'll draw it somewhat abstractly without all the other boxes up here and I'll

all the other boxes up here and I'll claim that the name of this variable is

claim that the name of this variable is s but it turns out what is s really how

s but it turns out what is s really how do strings Really Work Well s is a

do strings Really Work Well s is a variable and has been since week one but

variable and has been since week one but when you define it what the computer is

when you define it what the computer is doing for you automatically is when it

doing for you automatically is when it knows you want to store Hi exclamation

knows you want to store Hi exclamation point it puts that somewhere in memory

point it puts that somewhere in memory the computer then figures out for you

the computer then figures out for you what's the address of the very first

what's the address of the very first character and it stores that address and

character and it stores that address and only that address in the variable you

only that address in the variable you created on the left hand side of the

created on the left hand side of the equal sign and that's enough like to

equal sign and that's enough like to represent a string with three letters of

represent a string with three letters of the alph alphabet or punctuation you

the alph alphabet or punctuation you don't need three variables you just need

don't need three variables you just need one you just need to know the beginning

one you just need to know the beginning of the string why why is it sufficient

of the string why why is it sufficient for a variable to Only Store the first

for a variable to Only Store the first byes address and not all of the bytes

addresses exactly because of the design of strings per week two we always null

of strings per week two we always null terminate them so it suffices to only

terminate them so it suffices to only remember the first bites address because

remember the first bites address because from there you can sort of follow the

from there you can sort of follow the breadcrumbs bite after bite after bite

breadcrumbs bite after bite after bite and until you see the new line Sorry the

and until you see the new line Sorry the the null character you know that all of

the null character you know that all of those characters are apparently part of

those characters are apparently part of the same string so this is what's been

the same string so this is what's been going on in the me computer's memory all

going on in the me computer's memory all since week one and in fact if we

since week one and in fact if we abstract this away you can really think

abstract this away you can really think of s as being just this really a pointer

of s as being just this really a pointer to that chunk of memory so in fact what

to that chunk of memory so in fact what do we have here well in the left to

do we have here well in the left to recap on the code here on the left hand

recap on the code here on the left hand side string that's what ensures that

side string that's what ensures that we'll actually be able to store a string

we'll actually be able to store a string in a variable called s we're going to

in a variable called s we're going to have on the uh right hand side though

have on the uh right hand side though the actual value so let me switch back

the actual value so let me switch back to VSS code here and let me change my

to VSS code here and let me change my code to no longer involve integers alone

code to no longer involve integers alone so I'm going to add the uh cs50 Library

so I'm going to add the uh cs50 Library just so that I can use some shortcuts in

just so that I can use some shortcuts in there cs50.h and then in my main

there cs50.h and then in my main function I'm going to go ahead and do

function I'm going to go ahead and do this string s equals quote unquote high

this string s equals quote unquote high in all caps exclamation point and then

in all caps exclamation point and then I'm going to go ahead and print out

I'm going to go ahead and print out using percent S as always back sln the

using percent S as always back sln the value of s so this program at the moment

value of s so this program at the moment not interesting at all it's just week

not interesting at all it's just week one stuff again/ addresses indeed prints

one stuff again/ addresses indeed prints out high but it turns out that now that

out high but it turns out that now that I know this what's really been going on

I know this what's really been going on underneath the hood all this time well

underneath the hood all this time well here's that same line of code that

here's that same line of code that defines the variable called s and it

defines the variable called s and it turns out anyone want to guess what

turns out anyone want to guess what string is actually a synonym

string is actually a synonym for string it turns out is kind of a

for string it turns out is kind of a white lie we've been telling since week

white lie we've been telling since week one there is no such thing as string as

one there is no such thing as string as a keyword in in C it's technically a

a keyword in in C it's technically a cs50 thing

cs50 thing yeah it's a pointer to a character so

yeah it's a pointer to a character so really all this time we've kind of been

really all this time we've kind of been lying to you there is no string quote

lying to you there is no string quote unquote it's actually Char star and if I

unquote it's actually Char star and if I may it dramatically here go the training

may it dramatically here go the training wheels like okay that didn't land very

wheels like okay that didn't land very well so uh what have we been doing well

well so uh what have we been doing well it turns out that string is a much

it turns out that string is a much easier way conceptually to think about

easier way conceptually to think about what a string of characters is like my

what a string of characters is like my God if we had to start in week one by

God if we had to start in week one by having you type char

having you type char like yeah you might get past it but like

like yeah you might get past it but like this is just way too much ugly syntax

this is just way too much ugly syntax not intellectually interesting at all so

not intellectually interesting at all so we abstract it away what a Char star was

we abstract it away what a Char star was in the first week of C by telling you

in the first week of C by telling you it's actually called string now string

it's actually called string now string is a term of art like C programmers

is a term of art like C programmers programmers are in any language we'll

programmers are in any language we'll use the word string to mean a sequence

use the word string to mean a sequence of characters but in C it's not

of characters but in C it's not technically a word unto itself it's

technically a word unto itself it's rather a synonym that we ourselves

rather a synonym that we ourselves created in some form so in fact how did

created in some form so in fact how did we do this well think back to just last

we do this well think back to just last week last week I proposed that it'd be

week last week I proposed that it'd be really nice if we had a person data type

really nice if we had a person data type which the creators of C did not think of

which the creators of C did not think of decades ago but that's okay we can

decades ago but that's okay we can Define it ourselves what did we do here

Define it ourselves what did we do here well using syntax like this recall that

well using syntax like this recall that we defined a person to be what to be

we defined a person to be what to be this structure this structure using the

this structure this structure using the new keyword last week struct means that

new keyword last week struct means that a person is just a name and a number and

a person is just a name and a number and it could have been other things we just

it could have been other things we just kept it simple but how did I associate

kept it simple but how did I associate person with that structure well we

person with that structure well we claimed that it was this value here type

claimed that it was this value here type def which as you might expect defines a

def which as you might expect defines a data type so what did we do as cs50 back

data type so what did we do as cs50 back in week one without telling you well we

in week one without telling you well we could have done something like this like

could have done something like this like int itself is a little cryptic and maybe

int itself is a little cryptic and maybe we should have to keep things even

we should have to keep things even simpler said hey everyone turns out you

simpler said hey everyone turns out you can Define integers in C and if you

can Define integers in C and if you wanted to do this well if you want to

wanted to do this well if you want to create the keyword integer as a data

create the keyword integer as a data type you can just typ def it to int so

type you can just typ def it to int so typed def creates the word on the far

typed def creates the word on the far right integer and U creates a synonym

right integer and U creates a synonym for it in this case called int so what

for it in this case called int so what did we do in week one without telling

did we do in week one without telling you we have a line of code like this in

you we have a line of code like this in the cs-50 library that Associates quote

the cs-50 library that Associates quote unquote string with more cryptically

unquote string with more cryptically charar and this is why in week one

charar and this is why in week one onward anytime you use the cs50 library

onward anytime you use the cs50 library you can write the word string as though

you can write the word string as though it's a real C data type and that's just

it's a real C data type and that's just because we wanted to have this

because we wanted to have this abstraction these training wheels on for

abstraction these training wheels on for the first weeks so we don't have to get

the first weeks so we don't have to get weeds of all this crazy memory stuff we

weeds of all this crazy memory stuff we can sort of talk about strings at a

can sort of talk about strings at a higher level but that's all they are

higher level but that's all they are strings are the address of the first

strings are the address of the first character in that sequence of characters

character in that sequence of characters questions now on any of these details

questions now on any of these details yeah strings

yeah strings liar good question what about the

liar good question what about the strings Library which we have used um

strings Library which we have used um unrelated so it does not define the word

unrelated so it does not define the word string everything in there actually

string everything in there actually relates to char stars and so in fact if

relates to char stars and so in fact if you've used the CS5

you've used the CS5 uh manual uh which is just our

uh manual uh which is just our userfriendly version of the actual

userfriendly version of the actual manual pages for the official language C

manual pages for the official language C you'll see throughout that now if you

you'll see throughout that now if you start poking around or turning off less

start poking around or turning off less comfortable mode you'll actually see

comfortable mode you'll actually see that we've changed any mentions of

that we've changed any mentions of charar in the official documentation for

charar in the official documentation for these first weeks to just string to

these first weeks to just string to simplify it but underneath the hood C

simplify it but underneath the hood C does not know the word string per se as

does not know the word string per se as a keyword but it's absolutely a concept

a keyword but it's absolutely a concept that like every program in the world

that like every program in the world knows about and in fact in other

knows about and in fact in other languages in Python for instance there

languages in Python for instance there will actually be a proper string

will actually be a proper string although it's not going to be called

although it's not going to be called string it's going to be called stir St

string it's going to be called stir St Str for short questions on these strings

Str for short questions on these strings here well let me propose there's one

here well let me propose there's one other feature of this syntax that we can

other feature of this syntax that we can now leverage as follows let me propose

now leverage as follows let me propose that if we go back to the previous

that if we go back to the previous version of my code here wherein let me

version of my code here wherein let me switch back to vs code in just a moment

switch back to vs code in just a moment I'm going to rewind in vs code to the

I'm going to rewind in vs code to the integer version of my code from before

integer version of my code from before and most recently it looked looked like

and most recently it looked looked like this before when we were using integers

this before when we were using integers only and not in fact strings at all let

only and not in fact strings at all let me propose that there's this other

me propose that there's this other feature of C that we can use that

feature of C that we can use that actually allows us to go to an address

actually allows us to go to an address so at the moment let me just rewind and

so at the moment let me just rewind and do make addresses to remind you what

do make addresses to remind you what this program do did when it was using

this program do did when it was using integers alone and there's that address

integers alone and there's that address why because on line seven notice I'm

why because on line seven notice I'm printing out the value of P which is a

printing out the value of P which is a pointer so of course it's going to look

pointer so of course it's going to look like an address but let me zoom out now

like an address but let me zoom out now and make one change and instead of

and make one change and instead of printing out P how can I use today's

printing out P how can I use today's second new operator not the Ampersand

second new operator not the Ampersand but the star to actually go to that

but the star to actually go to that address well what I can actually do on

address well what I can actually do on this line of code is this if I want to

this line of code is this if I want to print out the actual integer 50 that's

print out the actual integer 50 that's in that variable or equivalently at that

in that variable or equivalently at that address I can go to P here and not print

address I can go to P here and not print P literally because that's just an

P literally because that's just an address I can now say star p and star p

address I can now say star p and star p means go there more technically

means go there more technically dreference p that is follow the treasure

dreference p that is follow the treasure map to the actual address and do what

map to the actual address and do what Carter did open the mailbox and print

Carter did open the mailbox and print whatever was in the mailbox which recall

whatever was in the mailbox which recall was the actual number 50 so let me try

was the actual number 50 so let me try this let me recompile the code so make

this let me recompile the code so make addresses okay let me clear my terminal

addresses okay let me clear my terminal window do/ addresses this time I

window do/ addresses this time I shouldn't see the ox anything I should

shouldn't see the ox anything I should see just the number 50 in this case and

see just the number 50 in this case and here to is kind of a unfortunate design

here to is kind of a unfortunate design decision certainly pedagogically I would

decision certainly pedagogically I would say in C if I zoom in on this code star

say in C if I zoom in on this code star is unfortunately being used in two

is unfortunately being used in two different ways in an Ideal World they

different ways in an Ideal World they would have used three different symbols

would have used three different symbols to make this more semantically clear but

to make this more semantically clear but this is what we're stuck with so in line

this is what we're stuck with so in line six when you declare a pointer that is a

six when you declare a pointer that is a variable that stores an address you put

variable that stores an address you put the type of variable that you want to

the type of variable that you want to point at then a star just because and

point at then a star just because and then the name of the variable and on the

then the name of the variable and on the right hand side you actually get the

right hand side you actually get the address of whatever using Ampersand but

address of whatever using Ampersand but when you want to go to an address you

when you want to go to an address you want to dreference a pointer you don't

want to dreference a pointer you don't use int again and we've never done that

use int again and we've never done that right once you declare a variable you

right once you declare a variable you never again mention the data type but in

never again mention the data type but in the world of pointers now if you want to

the world of pointers now if you want to not print out P but go to whatever

not print out P but go to whatever address p is storing you use star P here

address p is storing you use star P here so a good visual indicator would be when

so a good visual indicator would be when you declare a pointer that is make it

you declare a pointer that is make it exist in your program you have to

exist in your program you have to declare the data type with the star but

declare the data type with the star but when you use a pointer you just use the

when you use a pointer you just use the star in an Ideal World this would be a

star in an Ideal World this would be a completely different symbol but again

completely different symbol but again this is what we have questions now on

this is what we have questions now on that syntax

that syntax [Music]

[Music] yeah uh why can't we just do the Amper

yeah uh why can't we just do the Amper sand here are you saying it was still a

sand here are you saying it was still a little quiet so strictly speaking we do

little quiet so strictly speaking we do not need line six so this is really for

not need line six so this is really for pedagogical sake that I am um defining a

pedagogical sake that I am um defining a separate variable p and then printing it

separate variable p and then printing it out at this point though I'm just kind

out at this point though I'm just kind of you know going in circles if you will

of you know going in circles if you will because more simple would have been what

because more simple would have been what I would have done in week one which

I would have done in week one which would be get rid of P alt together get

would be get rid of P alt together get rid of P here and just print out n right

rid of P here and just print out n right but today we're just giving you this new

but today we're just giving you this new building block these this new syntax via

building block these this new syntax via which you can figure out the address of

which you can figure out the address of something and then reverse the process

something and then reverse the process later and actually go to it as

later and actually go to it as well other questions on what we've done

well other questions on what we've done here with these

here with these pointers all right well let's context

pointers all right well let's context switch back to the string now and see

switch back to the string now and see what more we can do with this here in

what more we can do with this here in the case of our strings here let me uh

the case of our strings here let me uh refine this to zoom out let me delete

refine this to zoom out let me delete the integer related code here let me do

the integer related code here let me do string s equals quote unquote high in

string s equals quote unquote high in all caps let me go ahead and for the

all caps let me go ahead and for the moment include cs50.h at the top so that

moment include cs50.h at the top so that indeed I can use the keyword s or string

indeed I can use the keyword s or string rather and let me go ahead now and do

rather and let me go ahead now and do something more than I did last time last

something more than I did last time last time I did print F of percent s back sln

time I did print F of percent s back sln and then I printed out s and again I'll

and then I printed out s and again I'll recompile this just for clarity make

recompile this just for clarity make addresses addresses that just prints out

addresses addresses that just prints out high so that's again week one stuff but

high so that's again week one stuff but now that we have this other bit of

now that we have this other bit of syntax we can do some interesting things

syntax we can do some interesting things too so for instance suppose I want to

too so for instance suppose I want to print out not s itself but what if I

print out not s itself but what if I want to print out the address of s like

want to print out the address of s like at what memory location is s well I can

at what memory location is s well I can change my percent s to percent P which

change my percent s to percent P which now we know P is for pointer so percent

now we know P is for pointer so percent p means print out the value of a pointer

p means print out the value of a pointer that is an address and here I can

that is an address and here I can actually print out s itself but why that

actually print out s itself but why that is we'll see in a moment let me do this

is we'll see in a moment let me do this here go the training wheels string does

here go the training wheels string does not technically EX exist but it does if

not technically EX exist but it does if I'm using the cs-50 library but if I get

I'm using the cs-50 library but if I get rid of the cs50 library as I'm

rid of the cs50 library as I'm metaphorically doing by taking off the

metaphorically doing by taking off the training wheels I can't use the word

training wheels I can't use the word string anymore and in fact let me make

string anymore and in fact let me make this mistake deliberately as you might

this mistake deliberately as you might have accidentally in past weeks here is

have accidentally in past weeks here is the error message I get if I forget the

the error message I get if I forget the cs50 library use of Undeclared

cs50 library use of Undeclared identifier string did you mean standard

identifier string did you mean standard in it's trying to be helpful but it's

in it's trying to be helpful but it's not because I didn't mean standard in so

not because I didn't mean standard in so indeed this is confirmation that c does

indeed this is confirmation that c does not know the word string exists at least

not know the word string exists at least as a keyword exists as a concept but not

as a keyword exists as a concept but not a keyword so I could fix this by adding

a keyword so I could fix this by adding back the cs50 library but that's kind of

back the cs50 library but that's kind of a step backwards educationally is

a step backwards educationally is instead of a step forward what could I

instead of a step forward what could I do instead to fix this now if the

do instead to fix this now if the training wheels are now off

yeah exactly replace string quote unquote with Char star instead so I'm

unquote with Char star instead so I'm going to go ahead and change this to

going to go ahead and change this to char technically you can put the literal

char technically you can put the literal star here the asterisk or you can put it

star here the asterisk or you can put it there or you can put it here by

there or you can put it here by convention is to do what I done from the

convention is to do what I done from the beginning put the star next to the name

beginning put the star next to the name of the variable as opposed to anywhere

of the variable as opposed to anywhere else uh let me go ahead now and or sorry

else uh let me go ahead now and or sorry I meant to add the spaces there you

I meant to add the spaces there you could do this too but this would be the

could do this too but this would be the most normal convention so now let's do

most normal convention so now let's do this make addresses compile is okay now

this make addresses compile is okay now do slash addresses what should I see

do slash addresses what should I see high or something else feel free to just

high or something else feel free to just call it

call it out so still high you say someone

out so still high you say someone else memory location a memory location

else memory location a memory location all right so could be one of the two

all right so could be one of the two options right either I'm going to see

options right either I'm going to see the string or I'm going to see a memory

the string or I'm going to see a memory address though I do in fact see a memory

address though I do in fact see a memory address and this one's quite different

address and this one's quite different from the integer one but does anyone now

from the integer one but does anyone now want to explain why you were correct why

want to explain why you were correct why am I seeing the address down here and

am I seeing the address down here and not hi it's subtle

yeah exactly because I left my percent P there which means hey print F show me a

there which means hey print F show me a pointer but this is where printf is

pointer but this is where printf is smart and has been smart since week zero

smart and has been smart since week zero humans who invented printf decades ago

humans who invented printf decades ago uh wrote code that notices that okay

uh wrote code that notices that okay percent s means to treat the following

percent s means to treat the following value not as just an address per se that

value not as just an address per se that gets printed literally but printed as

gets printed literally but printed as with the mailbox demo is sort of a

with the mailbox demo is sort of a treasure map that leads you to the

treasure map that leads you to the address of a character so simply by

address of a character so simply by changing one character percent P to

changing one character percent P to percent s and if I now do make addresses

percent s and if I now do make addresses again and slash addresses this now is

again and slash addresses this now is identical to week one but hopefully

identical to week one but hopefully makes sense because percent s is just a

makes sense because percent s is just a clue to print f that means go to this

clue to print f that means go to this address in s print out every character

address in s print out every character there and thereafter until you see what

there and thereafter until you see what the null character and then stop

the null character and then stop printing anything more and this is why

printing anything more and this is why hi has printed since week one today we

hi has printed since week one today we can see the address percent P but this

can see the address percent P but this combination of having access to

combination of having access to addresses and the null Terminator is all

addresses and the null Terminator is all the information printf needs to actually

the information printf needs to actually do something more useful by like

do something more useful by like printing the actual

printing the actual strings any questions now on this

strings any questions now on this approach to percent s yeah in

approach to percent s yeah in back oh so why is it traditionally being

back oh so why is it traditionally being used in this way honestly like the word

used in this way honestly like the word string has been around for decades it's

string has been around for decades it's not a keyword you should be able to type

not a keyword you should be able to type in C unless you're using a library like

in C unless you're using a library like cs50 um and so percent s just means

cs50 um and so percent s just means string so even though it doesn't exist

string so even though it doesn't exist as a keyword percent s connotes string

as a keyword percent s connotes string and humans decades ago like today just

and humans decades ago like today just kind of know what that means so they

kind of know what that means so they could have chosen any letter of the

could have chosen any letter of the alphabet but s sort of makes the most

alphabet but s sort of makes the most sense all right well let's in back other

sense all right well let's in back other question good question before let me

question good question before let me zoom in I did not use a star before the

zoom in I did not use a star before the S why well it's subtle here but printf

S why well it's subtle here but printf is was invented years ago to know given

is was invented years ago to know given an address like in the variable s printf

an address like in the variable s printf knows to go there so if we looked at the

knows to go there so if we looked at the source code that some human wrote years

source code that some human wrote years ago for C we would likely see the actual

ago for C we would likely see the actual uh asterisk that you're referring to

uh asterisk that you're referring to printf is taking on the responsibility

printf is taking on the responsibility for going to S if you were to do uh star

for going to S if you were to do uh star s here instead an asterisk and then s

s here instead an asterisk and then s that would now be literally a character

that would now be literally a character because if I say p uh star s that means

because if I say p uh star s that means go to the address in s and all you're

go to the address in s and all you're going to find there is a single

going to find there is a single character what printf wants to know is

character what printf wants to know is not what is the character there what is

not what is the character there what is the address of that character why

the address of that character why because printf needs to walk through the

because printf needs to walk through the rest of those characters looking for the

rest of those characters looking for the final null character and in fact let me

final null character and in fact let me see with a bit more syntax if we can

see with a bit more syntax if we can highlight this a bit more let me do this

highlight this a bit more let me do this in addition to printing s let's try out

in addition to printing s let's try out our syntax in another way let me print

our syntax in another way let me print out with percent s how about uh not s

out with percent s how about uh not s here but let's print out some addresses

here but let's print out some addresses percent s back sln close quote and then

percent s back sln close quote and then let's print out how about this the first

let's print out how about this the first character in the string s would be

character in the string s would be called s bracket Z but how do I get the

called s bracket Z but how do I get the address of the first character in s well

address of the first character in s well I could technically just use today's new

I could technically just use today's new primitive I can just add an ampon that

primitive I can just add an ampon that always gives me the address of some

always gives me the address of some value so when I end this thought and

value so when I end this thought and clear my terminal window and run make

clear my terminal window and run make addresses still compiles when I run

addresses still compiles when I run addresses in just a moment any guesses

addresses in just a moment any guesses as to what I will see line by line this

as to what I will see line by line this will print out two

will print out two things and you don't have to remember

things and you don't have to remember what the actual number was but at a high

what the actual number was but at a high level what will be printed now the same

level what will be printed now the same thing twice why well when I run this

thing twice why well when I run this what I'm printing here and let me zoom

what I'm printing here and let me zoom in at the bottom I ined see two really

in at the bottom I ined see two really long addresses but they're in fact the

long addresses but they're in fact the same why well that's because again if s

same why well that's because again if s is the address of a character as implied

is the address of a character as implied Now by either the cs50 word string or

Now by either the cs50 word string or the actual phrase Char star well then s

the actual phrase Char star well then s is just an address by contrast per week

is just an address by contrast per week two s bracket 0 is a Char always has

two s bracket 0 is a Char always has been a Char a specific Char but if you

been a Char a specific Char but if you want the address of that Char you just

want the address of that Char you just add the Ampersand well it turns out that

add the Ampersand well it turns out that strings per the definition we keep

strings per the definition we keep emphasizing is just the address of the

emphasizing is just the address of the first character in a string so of course

first character in a string so of course if you do this you're going to see the

if you do this you're going to see the exact same thing and if I do this a bit

exact same thing and if I do this a bit more generally you don't want to copy

more generally you don't want to copy paste but this is just for uh

paste but this is just for uh visualization sake let me print out all

visualization sake let me print out all the characters so another another

the characters so another another another and let me change this to print

another and let me change this to print out the address of bracket 1 bracket two

out the address of bracket 1 bracket two and bracket three so all four characters

and bracket three so all four characters Hi exclamation point and the null

Hi exclamation point and the null character notice I'm using percent P for

character notice I'm using percent P for all of them so if I now do make

all of them so if I now do make addresses and do/ addresses now notice

addresses and do/ addresses now notice and this is kind of cool the first two

and this is kind of cool the first two are indeed still the same but what's no

are indeed still the same but what's no noteworthy about the other values on the

screen yeah they're consecutive each of these is just one bite away even if

these is just one bite away even if you're not good at hex yet and there's a

you're not good at hex yet and there's a crazy number of digits here who cares

crazy number of digits here who cares they're all the same except for the last

they're all the same except for the last ones four four and then 5 six seven and

ones four four and then 5 six seven and this confirms what I've been claiming

this confirms what I've been claiming for weeks is that in an array all of the

for weeks is that in an array all of the characters are back to back to back

characters are back to back to back contiguous One Bites away so with just

contiguous One Bites away so with just this Ampersand with just this star like

this Ampersand with just this star like it's actually a pretty cool tool in the

it's actually a pretty cool tool in the toolkit to have because you can start to

toolkit to have because you can start to poke around what's actually going on

poke around what's actually going on inside of the computer's memory and in

inside of the computer's memory and in fact if we do this I can introduce one

fact if we do this I can introduce one other cool trick here if you will let me

other cool trick here if you will let me propose that we can actually now do

propose that we can actually now do arithmetic on pointers and you don't

arithmetic on pointers and you don't have to you'll see a simpler way to do

have to you'll see a simpler way to do this but now that you have perhaps this

this but now that you have perhaps this underlying understanding of where things

underlying understanding of where things are in memory and it's just addresses we

are in memory and it's just addresses we can actually do something kind of neat

can actually do something kind of neat we can do something like this uh let me

we can do something like this uh let me go back to how about uh the string

go back to how about uh the string version of this with high and let me do

version of this with high and let me do this instead let me um clean this up a

this instead let me um clean this up a bit get rid of some of these lines of

bit get rid of some of these lines of code and let me do this let me print out

code and let me do this let me print out percent C percent C percent C let me get

percent C percent C percent C let me get rid of all these Amper Sands we're going

rid of all these Amper Sands we're going to roll back to like week two stuff just

to roll back to like week two stuff just to be clear when I compile and run this

to be clear when I compile and run this version of the program and I'll zoom in

version of the program and I'll zoom in what should get printed on the screen

what should get printed on the screen this is just week two stuff now no

this is just week two stuff now no pointers per se yeah

pointers per se yeah mhm just Hi exclamation point one per

mhm just Hi exclamation point one per line because I have all of these back

line because I have all of these back slash ends so let me do that let me go

slash ends so let me do that let me go down here make addresses enter okay

down here make addresses enter okay pretty good/ addresses and indeed Hi

pretty good/ addresses and indeed Hi exclamation point But now if you're

exclamation point But now if you're getting a little more comfortable and

getting a little more comfortable and it's fine if you're not yet today but

it's fine if you're not yet today but over the coming week or weeks as you get

over the coming week or weeks as you get a little more comfortable with the

a little more comfortable with the equivalence of addresses with our

equivalence of addresses with our definition in the past of arrays and

definition in the past of arrays and strings and all of this you can start to

strings and all of this you can start to play around and I can do this instead if

play around and I can do this instead if I I want to print out the first

I I want to print out the first character in the string I could do like

character in the string I could do like week two s bracket zero like that will

week two s bracket zero like that will always work and you can keep using that

always work and you can keep using that that's not a cs50 thing it's just a

that's not a cs50 thing it's just a convenience in C but I could technically

convenience in C but I could technically print out not s because s is an address

print out not s because s is an address but what would be the syntax I could use

but what would be the syntax I could use to say print out the character at

to say print out the character at s any

s any Instinct how can I say go to the address

Instinct how can I say go to the address in

in s it's one of two possible answers

s it's one of two possible answers today so of our two new uh uh of our two

today so of our two new uh uh of our two new operators today we have the

new operators today we have the Ampersand and the star which one will

Ampersand and the star which one will lead us to what is that an address so

lead us to what is that an address so the star so in fact if I want to print

the star so in fact if I want to print out what is that address zero at the

out what is that address zero at the address S I can just do star s and if

address S I can just do star s and if you really want to get fancy how do you

you really want to get fancy how do you print out the second character that's

print out the second character that's immediately to the right of it so to

immediately to the right of it so to speak well you can go to with the D

speak well you can go to with the D reference operator and do you want to

reference operator and do you want to answer this

answer this one s+ one Argo pointer arithmetic like

one s+ one Argo pointer arithmetic like you can do math simple addition

you can do math simple addition subtraction whatever on pointers if you

subtraction whatever on pointers if you want and you can do this here too so

want and you can do this here too so star you want to pluck this one off too

star you want to pluck this one off too how do I print out the last character

how do I print out the last character the

the third s plus2 right because if you know

third s plus2 right because if you know and understand that like a string is

and understand that like a string is just a sequence of characters every

just a sequence of characters every character is just a bite and these bites

character is just a bite and these bites are back to back to back you can just go

are back to back to back you can just go wherever you want in the computer's

wherever you want in the computer's memory and here I can do make addresses

memory and here I can do make addresses again/ addresses and voila we now have

again/ addresses and voila we now have high exclamation point so we haven't

high exclamation point so we haven't printed out anything new but again just

printed out anything new but again just by using these two new operators the

by using these two new operators the erson and the star you can figure out

erson and the star you can figure out the address of something and you can go

the address of something and you can go to the address of something okay

to the address of something okay question in

question in back indeed it ends up being the exact

back indeed it ends up being the exact same and so I might have used this term

same and so I might have used this term before the Amper sand technique where

before the Amper sand technique where sorry the square bracket technique where

sorry the square bracket technique where you do s bracket 0o s bracket One S

you do s bracket 0o s bracket One S bracket 2 that's actually what we would

bracket 2 that's actually what we would really call syntactic Sugar like it

really call syntactic Sugar like it works and you can use it you should use

works and you can use it you should use it it's nice and simple

it it's nice and simple but the square bracket notation

but the square bracket notation underneath the hood is essentially being

underneath the hood is essentially being converted to this which this is not fun

converted to this which this is not fun right like this is when you want to show

right like this is when you want to show off to your friends like you know how to

off to your friends like you know how to do cool stuff in code but this is not as

do cool stuff in code but this is not as readable as just s bracket Z and one and

readable as just s bracket Z and one and two but that's all that's happening

two but that's all that's happening underneath the hood and so again this is

underneath the hood and so again this is why in cs50 we spend time on some of

why in cs50 we spend time on some of these lower level building blocks

these lower level building blocks because if you assume that indeed your

because if you assume that indeed your computer's memory is just this grid of

computer's memory is just this grid of btes and you have now the code ability

btes and you have now the code ability in code to get an address and go to an

in code to get an address and go to an address you can start doing any anything

address you can start doing any anything you want and you can poke around a

you want and you can poke around a computer's memory at any location and

computer's memory at any location and here in lies the danger like I'm kind of

here in lies the danger like I'm kind of on the honor System right now that if my

on the honor System right now that if my string is high exclamation point it's

string is high exclamation point it's kind of up to me to go to the first bite

kind of up to me to go to the first bite the second and the third but I could get

the second and the third but I could get kind of crazy now and if I want to see

kind of crazy now and if I want to see what's going on in the computer's memory

what's going on in the computer's memory I mean there's nothing stopping me from

I mean there's nothing stopping me from doing like s+ 50 and let's see what's

doing like s+ 50 and let's see what's there so make addresses do/ addresses Hi

there so make addresses do/ addresses Hi and then okay nothing it seems well how

and then okay nothing it seems well how about 5,000 bytes away let's poke around

about 5,000 bytes away let's poke around what's inside of the computer's memory

what's inside of the computer's memory so make addresses again uh make

so make addresses again uh make addresses addresses enter okay still

addresses addresses enter okay still nothing there let's try

nothing there let's try 50,000 all right do make addresses do SL

50,000 all right do make addresses do SL addresses okay there we see it so you've

addresses okay there we see it so you've probably done this some of you by

probably done this some of you by accident because you probably went too

accident because you probably went too far to the left or to the right in an

far to the left or to the right in an array touching memory that you shouldn't

array touching memory that you shouldn't suffice it to say I should not go

suffice it to say I should not go blindly touching 50,000 bytes away cuz

blindly touching 50,000 bytes away cuz who knows what's there and indeed in

who knows what's there and indeed in your computer computer when a program is

your computer computer when a program is running the computer uh segments it into

running the computer uh segments it into different segments of memory and if you

different segments of memory and if you get a little too greedy and you touch

get a little too greedy and you touch another segment of memory that

another segment of memory that technically was not allocated to you by

technically was not allocated to you by Mac OS or Windows or Linux or the

Mac OS or Windows or Linux or the operating system bad things happen and

operating system bad things happen and you get a segmentation fault and that

you get a segmentation fault and that means it's a bug in your code so you can

means it's a bug in your code so you can now do this and this means hackers too

now do this and this means hackers too can do things like this if they can

can do things like this if they can somehow inject code into your C program

somehow inject code into your C program maybe they can poke around the

maybe they can poke around the computer's memory and indeed this is

computer's memory and indeed this is kind of the technique whereby maybe a

kind of the technique whereby maybe a really sophisticated hacker can jump to

really sophisticated hacker can jump to this memory this memory this memory

this memory this memory this memory looking for something like your password

looking for something like your password or your financial information or

or your financial information or anything that's in the program but at

anything that's in the program but at some other address there's nothing

some other address there's nothing stopping an adversary at least right now

stopping an adversary at least right now from poking around if they can execute

from poking around if they can execute code on your computer from doing this

code on your computer from doing this kind of thing so there and again is the

kind of thing so there and again is the power of C but also the danger and

power of C but also the danger and you'll absolutely suffer more segals in

you'll absolutely suffer more segals in the coming days but ultimately the goal

the coming days but ultimately the goal is going to be to help you solve them

is going to be to help you solve them ultimately uh and fix things um but for

ultimately uh and fix things um but for now I think that was that was quite a

now I think that was that was quite a bit so let me propose that we go ahead

bit so let me propose that we go ahead and take our longer break here maybe 10

and take our longer break here maybe 10 minutes and have ourselves some whoopy

minutes and have ourselves some whoopy pies in the transcept we'll be back in

pies in the transcept we'll be back in 10 all right so we're back and to recap

10 all right so we're back and to recap where we left off you now have this new

where we left off you now have this new capability in code to do pointer

capability in code to do pointer arithmetic like treat addresses as

arithmetic like treat addresses as numbers which they really are in heximal

numbers which they really are in heximal or otherwise and like add them together

or otherwise and like add them together and kind of poke around a computer's

and kind of poke around a computer's memory and it was asked during break

memory and it was asked during break actually how we might further harness

actually how we might further harness this in the context of string so I

this in the context of string so I didn't change the code we wrote just

didn't change the code we wrote just before break recall that we last broke

before break recall that we last broke the program by checking out bytes 50,000

the program by checking out bytes 50,000 bytes away but let's not do that and

bytes away but let's not do that and let's actually try printing out not

let's actually try printing out not individual characters like I did per the

individual characters like I did per the percent C but why don't we try printing

percent C but why don't we try printing out strings and substrings if you will

out strings and substrings if you will so let me clear my terminal window let

so let me clear my terminal window let me change all of these percent C's to

me change all of these percent C's to percent s percent s percent s and then

percent s percent s percent s and then let me rewind to what we've been doing

let me rewind to what we've been doing since week one with strings which is

since week one with strings which is just print them out for instance with

just print them out for instance with that first line and the only difference

that first line and the only difference at the moment is that now I took off the

at the moment is that now I took off the training wheels I got rid of cs50.h

training wheels I got rid of cs50.h wherein string is typ Def to char star

wherein string is typ Def to char star for you got rid of that so now on line

for you got rid of that so now on line five I'm declaring S as being a Char

five I'm declaring S as being a Char star which just means the address of a

star which just means the address of a character and print f is smart enough to

character and print f is smart enough to know that the end of a string is

know that the end of a string is wherever that null character is but now

wherever that null character is but now that I can do pointer arithmetic notice

that I can do pointer arithmetic notice that I could do something like this if I

that I could do something like this if I want to print out s i just print out s

want to print out s i just print out s suppose I do s+ one here and s+2 here

suppose I do s+ one here and s+2 here again after changing percent C to

again after changing percent C to percent s any intuition around what this

percent s any intuition around what this code will now print on the screen line

code will now print on the screen line by

by line yeah

line yeah thoughts okay reasonable conjecture

thoughts okay reasonable conjecture maybe the memory address of H that of I

maybe the memory address of H that of I that of exclamation point but other

that of exclamation point but other [Music]

[Music] thoughts yeah I think it's going to do

thoughts yeah I think it's going to do the latter it's going to print high in

the latter it's going to print high in the usual way because honestly line five

the usual way because honestly line five is this rather line six is the same as

is this rather line six is the same as like week one stuff except we took off

like week one stuff except we took off the training wheel of string and we're

the training wheel of string and we're calling it Char star but I think line

calling it Char star but I think line seven is indeed going to print out I and

seven is indeed going to print out I and line eight is just going to print out

line eight is just going to print out because it'll be just the exclamation

because it'll be just the exclamation point prti will still be smart enough to

point prti will still be smart enough to know where each of those substrings a

know where each of those substrings a portions of the string End by the same

portions of the string End by the same logic as always but let me go ahead and

logic as always but let me go ahead and zoom out run make addresses enter

zoom out run make addresses enter compiles okay/ addresses and now indeed

compiles okay/ addresses and now indeed this is all a string is it's a sequence

this is all a string is it's a sequence of characters identified by its first

of characters identified by its first bite if you then start poking around and

bite if you then start poking around and tell printf to print at what's at the

tell printf to print at what's at the next bite or the next next bite it's

next bite or the next next bite it's going to do its same thing printing out

going to do its same thing printing out that character and everything after it

that character and everything after it up until that null character so again

up until that null character so again even though there's like a lot going on

even though there's like a lot going on we've introduced these two new operators

we've introduced these two new operators like there's nothing that's happening

like there's nothing that's happening today that hasn't been happening for

today that hasn't been happening for weeks but hopefully through this week uh

weeks but hopefully through this week uh this week's lecture this week's problem

this week's lecture this week's problem set and Beyond you'll start to realize

set and Beyond you'll start to realize that now you just have more tools via

that now you just have more tools via which to harness those lower level

which to harness those lower level implementation details so last week too

implementation details so last week too recall one other implementation detail I

recall one other implementation detail I claimed that you could not compare two

claimed that you could not compare two strings quite as easily as you could

strings quite as easily as you could compare to uh integers for instance and

compare to uh integers for instance and I told you to use a different function

I told you to use a different function instead that you probably used one or

instead that you probably used one or more times with the past problem set how

more times with the past problem set how are you supposed to compare strings

are you supposed to compare strings apparently yeah so string compare stir

apparently yeah so string compare stir comp that additional function that we

comp that additional function that we said H you just have to use it for now

said H you just have to use it for now but you might have a little intuition

but you might have a little intuition already as to like why we have to use

already as to like why we have to use Stir compare and we can't just use

Stir compare and we can't just use equals equals to compare strings like

equals equals to compare strings like any intuition for this already why was

any intuition for this already why was Stir compare necessary last week

Stir compare necessary last week equ perfect equals equals would compare

equ perfect equals equals would compare literally the two memory addresses

literally the two memory addresses instead of the actual strings character

instead of the actual strings character by character and unless the memory

by character and unless the memory addresses are literally the same so you

addresses are literally the same so you compare the that exact same memory

compare the that exact same memory address two different strings probably

address two different strings probably are not going to be considered equal

are not going to be considered equal even if to us humans they indeed look

even if to us humans they indeed look equal so let's see this let me go ahead

equal so let's see this let me go ahead and close addresses. C and actually

and close addresses. C and actually before I do one last mention one of the

before I do one last mention one of the powerful things about Pointer arithmetic

powerful things about Pointer arithmetic as an aside is that c and really the

as an aside is that c and really the compiler is smart enough to know how

compiler is smart enough to know how many bytes to keep adding and adding and

many bytes to keep adding and adding and by that I mean this right now we got

by that I mean this right now we got lucky because a string is a sequence of

lucky because a string is a sequence of characters and by definition every

characters and by definition every character is is a single bite you can

character is is a single bite you can poke around and do s+ one to get the

poke around and do s+ one to get the next bite S Plus 2 to get the uh the

next bite S Plus 2 to get the uh the third bite however if we weren't dealing

third bite however if we weren't dealing with strings suppose we were dealing

with strings suppose we were dealing with integers that were in an array back

with integers that were in an array back to back to back if you wanted to get at

to back to back if you wanted to get at the next integer you could still do plus

the next integer you could still do plus one or plus two to get at the next or

one or plus two to get at the next or the next next integer you would not

the next next integer you would not start to get into the weeds of doing

start to get into the weeds of doing plus four and then plus eight you don't

plus four and then plus eight you don't have to know or care how big the data

have to know or care how big the data types are in the computer C and the

types are in the computer C and the compiler will figure that out for you

compiler will figure that out for you based on the data type in question so

based on the data type in question so keep that in mind if ever doing this on

keep that in mind if ever doing this on a different data type uh than chars all

a different data type uh than chars all right so let me go ahead and open up a

right so let me go ahead and open up a file that I wrote in advance most of and

file that I wrote in advance most of and let me hide my terminal window and show

let me hide my terminal window and show you this so here is a program called

you this so here is a program called compare. C whose purpose in life is to

compare. C whose purpose in life is to compare two strings I'm back to using

compare two strings I'm back to using the cs50 library because at least for

the cs50 library because at least for now in probably a couple more weeks it

now in probably a couple more weeks it is so much easier to get input from the

is so much easier to get input from the user using cs50's function get in but

user using cs50's function get in but we'll conclude today by taking off those

we'll conclude today by taking off those training wheels as well so you can see

training wheels as well so you can see how you can actually get user input with

how you can actually get user input with nothing cs50 specific so line six and

nothing cs50 specific so line six and seven pretty boring week one stuff get

seven pretty boring week one stuff get an INT called I get an INT called J and

an INT called I get an INT called J and store them in two variables I and J

store them in two variables I and J respectively if I equals equals J print

respectively if I equals equals J print out the same else print out that they're

out the same else print out that they're different let me just stipulate for time

different let me just stipulate for time sake I'm pretty sure this code is

sake I'm pretty sure this code is correct this will get two integers from

correct this will get two integers from the human it will compare them and tell

the human it will compare them and tell me correctly if they're the same or

me correctly if they're the same or different and I'll prove as much by

different and I'll prove as much by running make compare

running make compare slash compare and I'll type in 50 for I

slash compare and I'll type in 50 for I 50 for J and they're the same and now

50 for J and they're the same and now I'll do how about 50 and say 13 and

I'll do how about 50 and say 13 and those are different so let me just

those are different so let me just stipulate this code is indeed correct

stipulate this code is indeed correct would have worked in week one also works

would have worked in week one also works now in week four but let me now change

now in week four but let me now change it to compare not two integers but as I

it to compare not two integers but as I hinted maybe two strings instead so let

hinted maybe two strings instead so let me go ahead and change this line of code

me go ahead and change this line of code to maybe be string s equals get string

to maybe be string s equals get string uh asking the user for s s uh then let's

uh asking the user for s s uh then let's change this second line here to be

change this second line here to be string T just to keep the variable name

string T just to keep the variable name short for now and T is a good uh Choice

short for now and T is a good uh Choice after s for something like this get

after s for something like this get string prompt the human for T and then

string prompt the human for T and then let's change our I and J here to do the

let's change our I and J here to do the wrong thing per the intuition earlier if

wrong thing per the intuition earlier if s equals equals T then print out the

s equals equals T then print out the same else print out that they're

same else print out that they're different now if I want I could take off

different now if I want I could take off at least some of the training wheels I

at least some of the training wheels I could change this to char star I could

could change this to char star I could change this to char star either is fine

change this to char star either is fine I still need the Cs library though

I still need the Cs library though because I'm using get string because

because I'm using get string because it's actually hard as we'll see today to

it's actually hard as we'll see today to get strings manually without using a

get strings manually without using a library but I'll keep it using string

library but I'll keep it using string just for now with the library all right

just for now with the library all right make compare

make compare again compare and now let me go ahead

again compare and now let me go ahead and type in for instance Hi exclamation

and type in for instance Hi exclamation point Enter and Hi exclamation point

point Enter and Hi exclamation point Enter and they're different all they're

Enter and they're different all they're obviously not visually but they are

obviously not visually but they are underneath the hood and you probably do

underneath the hood and you probably do have the intuition for this already

have the intuition for this already whereby what's going on underneath the

whereby what's going on underneath the hood is that we're comparing

hood is that we're comparing accidentally the two memory addresses so

accidentally the two memory addresses so in fact let's go there let's consider

in fact let's go there let's consider the memory and let me zoom out now so I

the memory and let me zoom out now so I can just have more btes to play with so

can just have more btes to play with so the squares are a little smaller than

the squares are a little smaller than before just so we can fit more in them

before just so we can fit more in them and let me propose that when I declare s

and let me propose that when I declare s on what was line six a moment ago it

on what was line six a moment ago it ends up somewhere in memory like the top

ends up somewhere in memory like the top leftand corner of my picture for

leftand corner of my picture for discussion sake and when I uh execute

discussion sake and when I uh execute that same line of code and get string is

that same line of code and get string is called and I type in high exclamation

called and I type in high exclamation point we know from week one that get

point we know from week one that get string puts it somewhere in the

string puts it somewhere in the computer's memory and I'll propose that

computer's memory and I'll propose that it's in like the bottom left hand corner

it's in like the bottom left hand corner of the screen here what happens after

of the screen here what happens after that well I know even though I don't

that well I know even though I don't generally care that Hi exclamation point

generally care that Hi exclamation point in the null character exist at some

in the null character exist at some address like ox123 12 4 125 126 for

address like ox123 12 4 125 126 for discussion sake and what's in s same as

discussion sake and what's in s same as before break 0x1 23 so that's all that's

before break 0x1 23 so that's all that's happening again on line six which is

happening again on line six which is pretty much the same as when we were

pretty much the same as when we were getting an S earlier but notice now with

getting an S earlier but notice now with line seven when I get a second variable

line seven when I get a second variable called T and I call get string again and

called T and I call get string again and by coincidence as the human I type the

by coincidence as the human I type the same thing well what happens here T gets

same thing well what happens here T gets its own chunk of memory maybe at the top

its own chunk of memory maybe at the top right uh that second version of high

right uh that second version of high gets somewhere else in memory you know

gets somewhere else in memory you know the computer could be smart and notice

the computer could be smart and notice it's the same but C doesn't generally do

it's the same but C doesn't generally do that for you it just plops it somewhere

that for you it just plops it somewhere else in memory and maybe it's at address

else in memory and maybe it's at address Ox 456 457 458 459 or wherever but you

Ox 456 457 458 459 or wherever but you can perhaps see where this is going

can perhaps see where this is going already T now of course contains the

already T now of course contains the address of that first bite and so in my

address of that first bite and so in my code on line nine when I compare s and t

code on line nine when I compare s and t for equality suffice it to say they are

for equality suffice it to say they are not equal because of the way the uh

not equal because of the way the uh strings are laid out in the computer's

strings are laid out in the computer's memory it's indeed looks the same the

memory it's indeed looks the same the same values are there but if we abstract

same values are there but if we abstract away further you can really see that s

away further you can really see that s and t are not the same themselves and so

and t are not the same themselves and so how did we fix this or really how did we

how did we fix this or really how did we avoid this last week without spilling

avoid this last week without spilling the beans and going down this Rabbit

the beans and going down this Rabbit Hole explaining like why you have to use

Hole explaining like why you have to use Stir compare well if I go back to my

Stir compare well if I go back to my code here let's do it now the right way

code here let's do it now the right way let me go ahead and include uh a line of

let me go ahead and include uh a line of code that says string compare of s comma

code that says string compare of s comma T both as inputs and then if you recall

T both as inputs and then if you recall what does stir compare return when two

what does stir compare return when two strings are equal there's three possible

strings are equal there's three possible return values so zero so one is for if

return values so zero so one is for if it comes alphabetically or atically

it comes alphabetically or atically first or second but for now I just want

first or second but for now I just want zero if I want to use S compare I do

zero if I want to use S compare I do need

need string.h so string.h does exist that's

string.h so string.h does exist that's not a cs50 thing there's no keyword

not a cs50 thing there's no keyword string as a data type that's a cs50

string as a data type that's a cs50 thing but string.h does exist so I think

thing but string.h does exist so I think now with that change on line 10 if I do

now with that change on line 10 if I do make compare and slash compare and then

make compare and slash compare and then run again uh type again High exclamation

run again uh type again High exclamation point High exclamation point I think now

point High exclamation point I think now they're the same and just as a uh second

they're the same and just as a uh second check high in L caps maybe high in

check high in L caps maybe high in lowercase those are in fact different

lowercase those are in fact different why well stir compare which was written

why well stir compare which was written by some other human decades ago is just

by some other human decades ago is just smart enough to know that it should go

smart enough to know that it should go to S and go to T start comparing them

to S and go to T start comparing them left to right stopping once it hits one

left to right stopping once it hits one or both null characters and return zero

or both null characters and return zero only if everything in s and and T are

only if everything in s and and T are exactly the same are any questions then

exactly the same are any questions then on this

on this here any questions on why we're using

here any questions on why we're using stir compare all right if no yeah oh in

stir compare all right if no yeah oh in the

middle yes so so why does why is it not the case with integers so it turns out

the case with integers so it turns out it's not the case with integers with

it's not the case with integers with floats with bulls with uh doubles with

floats with bulls with uh doubles with Longs like literally every other data

Longs like literally every other data type works correctly strings though are

type works correctly strings though are special they are useful enough in

special they are useful enough in programming and have been for decades

programming and have been for decades that the authors of printf and the

that the authors of printf and the authors of stir compare and bunches of

authors of stir compare and bunches of other functions Sterling for that matter

other functions Sterling for that matter just kind of treat strings special

just kind of treat strings special because they're just useful right we

because they're just useful right we humans interact using language be it

humans interact using language be it English or anything else and so it's

English or anything else and so it's just useful to have into the language C

just useful to have into the language C just sort of uh first class support for

just sort of uh first class support for this notion of strings of human text so

this notion of strings of human text so the short answer is just because like it

the short answer is just because like it just uh is necessar strings are

just uh is necessar strings are different they're implemented with this

different they're implemented with this address and the null character

address and the null character everything else though is just a value

everything else though is just a value but a string again is a white lie it's

but a string again is a white lie it's an address it's not a thing unto itself

an address it's not a thing unto itself good question yeah in front

oh really good question so in my code here in vs code what if I do this

here in vs code what if I do this instead of stir compare and instead of

instead of stir compare and instead of if s equals equals T what if I start

if s equals equals T what if I start playing around using star s and star T

playing around using star s and star T really interesting case to consider

really interesting case to consider let's go back to our sort of deductive

let's go back to our sort of deductive logic here so star the asteris operator

logic here so star the asteris operator today means go there so when I've typed

today means go there so when I've typed in high once and then high again both

in high once and then high again both uppercase for instance what is at the

uppercase for instance what is at the address s literally someone

address s literally someone else what is at the address s yeah uh so

else what is at the address s yeah uh so not quite at the address not so not what

not quite at the address not so not what is the address what is at the address

is the address what is at the address ox123 H and what is at the address Ox

ox123 H and what is at the address Ox 456 H also and so here you're kind of

456 H also and so here you're kind of cheating like you're comparing the first

cheating like you're comparing the first character of both strings but not every

character of both strings but not every other one now you could be really

other one now you could be really pedantic and here again this is like a

pedantic and here again this is like a good use of uh codee but you could do

good use of uh codee but you could do this if that and how about this

this if that and how about this craziness so star

craziness so star s+1 equals equals star t + one and and

s+1 equals equals star t + one and and you could do this for every character

you could do this for every character manually but that's why stir compare

manually but that's why stir compare exists it does all of this for you but

exists it does all of this for you but that's why and that's the intuition so I

that's why and that's the intuition so I would encourage you too anytime there's

would encourage you too anytime there's something kind of weird going on there's

something kind of weird going on there's I realize we might be um straining

I realize we might be um straining credibility now we haven't told you that

credibility now we haven't told you that many white lies and so most everything

many white lies and so most everything that we've seen thus far can explain

that we've seen thus far can explain pretty much all of the behavior up until

pretty much all of the behavior up until now from week one onward in C so let me

now from week one onward in C so let me revert this back to the right way if s

revert this back to the right way if s compare of S&T equals equals z this now

compare of S&T equals equals z this now is the right version of the code and now

is the right version of the code and now here is again where you can play so let

here is again where you can play so let me do this let me um clear my terminal

me do this let me um clear my terminal window just to tidy things up let me get

window just to tidy things up let me get rid of all of this comparison stuff and

rid of all of this comparison stuff and let's just see what's going on as you

let's just see what's going on as you are welcome to in your own code let's

are welcome to in your own code let's print out for instance as we might have

print out for instance as we might have in week one the value of s itself in a

in week one the value of s itself in a new line comma s and then let's just

new line comma s and then let's just print out T just to make sure it

print out T just to make sure it compiles and I'm not doing anything

compiles and I'm not doing anything wrong but this is not going to be that

wrong but this is not going to be that interesting and frankly I don't need

interesting and frankly I don't need string.h anymore because I'm not using

string.h anymore because I'm not using stir compare so make addresses addresses

stir compare so make addresses addresses there's my um oh sorry that's fun okay

there's my um oh sorry that's fun okay not percent T percent s here too ignore

not percent T percent s here too ignore that let's do this again make a oh and

that let's do this again make a oh and that's the wrong program okay dot SL

that's the wrong program okay dot SL let's do make

let's do make compare compare and let's type in high

compare compare and let's type in high again and high again and and now we just

again and high again and and now we just see the two strings I'm not comparing

see the two strings I'm not comparing but now we can kind of play around right

but now we can kind of play around right instead of printing out percent s which

instead of printing out percent s which prints the string how do I print the

prints the string how do I print the address in S I just need to make a

address in S I just need to make a slight change if I want to see not

slight change if I want to see not what's at s but I want to see s the

what's at s but I want to see s the address

yeah perfect so change percent s in both places here to percent P so now printf

places here to percent P so now printf will treat it literally as an dress it's

will treat it literally as an dress it's not going to do any fanciness this with

not going to do any fanciness this with a loop from left to right looking for

a loop from left to right looking for the null character it's just going to

the null character it's just going to print out S&T so let me clear my

print out S&T so let me clear my terminal run make compare whoops let's

terminal run make compare whoops let's do make comparecompare enter type in

do make comparecompare enter type in high type in high again and now you see

high type in high again and now you see oh so this is interesting it's not quite

oh so this is interesting it's not quite as straightforward as the other values

as straightforward as the other values which were slight one bite away they're

which were slight one bite away they're almost the same but this one ends in b0

almost the same but this one ends in b0 this one ends in F0 so they're indeed

this one ends in F0 so they're indeed separated by some number of bytes not

separated by some number of bytes not just one but a few because these strings

just one but a few because these strings are indeed longer all right so once

are indeed longer all right so once you've seen this here how can we now

you've seen this here how can we now maybe leverage this to solve other

maybe leverage this to solve other problems well let me propose that we do

problems well let me propose that we do this let me uh zoom out here let me

this let me uh zoom out here let me close compare and let me open up another

close compare and let me open up another program I wrote part of in advance

program I wrote part of in advance called

called copy.c so copy. C in theory makes a copy

copy.c so copy. C in theory makes a copy of a string how on line eight I'm using

of a string how on line eight I'm using the same thing as before get string

the same thing as before get string storing in a string or char star and

storing in a string or char star and asking the user for it then I'm not

asking the user for it then I'm not asking get string again I'm just making

asking get string again I'm just making a copy super simply with line 10 here

a copy super simply with line 10 here string T equals s now intuitively I

string T equals s now intuitively I think that's how I would copy a variable

think that's how I would copy a variable right that's how we've copied variables

right that's how we've copied variables every week thus far and see but

every week thus far and see but something's going to go wrong in line 12

something's going to go wrong in line 12 in English does someone want to explain

in English does someone want to explain what you think line 12 does don't worry

what you think line 12 does don't worry about finding any bugs or mistakes but

about finding any bugs or mistakes but what does line 12 seem to be doing using

what does line 12 seem to be doing using two upper which which is thanks to the

two upper which which is thanks to the ctype library which I've included the

ctype library which I've included the header file for

header file for yeah yeah right it's kind of like ugly

yeah yeah right it's kind of like ugly syntax but this would seem to be

syntax but this would seem to be capitalizing the first letter of T

capitalizing the first letter of T specifically and just changing it so we

specifically and just changing it so we have t bracket Zer here because we want

have t bracket Zer here because we want to save the change and we're passing to

to save the change and we're passing to two upper the first character here so

two upper the first character here so this is how we did uppercase in the past

this is how we did uppercase in the past and now I print out s and t respectively

and now I print out s and t respectively using percent s so this feels like it

using percent s so this feels like it should work I copied s and stored it in

should work I copied s and stored it in t on line 10 and then I change T and

t on line 10 and then I change T and only t on line 12 but you can perhaps if

only t on line 12 but you can perhaps if you're comfy thus far see where this is

you're comfy thus far see where this is going if I do make

going if I do make copy copy and let me type in lowercase

copy copy and let me type in lowercase Hi exclamation point this time just once

Hi exclamation point this time just once so I'm going to hit enter and watch what

so I'm going to hit enter and watch what we see for the value of s and

we see for the value of s and t huh the new value of SN T at the end

t huh the new value of SN T at the end of my program seems to be what it seems

of my program seems to be what it seems to be

to be the same high is capitalized both times

the same high is capitalized both times so what's the intuition then for this

so what's the intuition then for this why did this just

why did this just happen yeah and

happen yeah and back yeah I assigned S&T the same memory

back yeah I assigned S&T the same memory address so it did copy s into T but C

address so it did copy s into T but C takes this very literally what is s it's

takes this very literally what is s it's an address what is T it's a copy of that

an address what is T it's a copy of that address if you want to copy the whole

address if you want to copy the whole string like a normal human would expect

string like a normal human would expect hey you or someone has to do a lot more

hey you or someone has to do a lot more work you have to go to that address copy

work you have to go to that address copy this character this one this one this

this character this one this one this one and copy it to a new location in

one and copy it to a new location in memory that does not happen

memory that does not happen automatically here for you in C it does

automatically here for you in C it does in some other languages those of you

in some other languages those of you who've programmed in certain higher

who've programmed in certain higher level languages this just works as you

level languages this just works as you would hope and that's one of the

would hope and that's one of the benefits of python and other languages

benefits of python and other languages that we'll soon see but for now it

that we'll soon see but for now it literally takes at face value what this

literally takes at face value what this is copy the address into this address

is copy the address into this address and I'll make that more clear by getting

and I'll make that more clear by getting rid of the string keyword which again is

rid of the string keyword which again is just a type def this is technically an

just a type def this is technically an address here this is technically an

address here this is technically an address here so what's being being

address here so what's being being copied is the value of that address not

copied is the value of that address not all of the characters that might very

all of the characters that might very well follow it so I should make one note

well follow it so I should make one note to here I'm going to start getting more

to here I'm going to start getting more in the habit of trying to avoid

in the habit of trying to avoid segmentation faults because things could

segmentation faults because things could go wrong here for instance online 12

go wrong here for instance online 12 previously I was kind of blindly naively

previously I was kind of blindly naively dangerously assuming that there will be

dangerously assuming that there will be at least one character in s or t that

at least one character in s or t that might not be the case if the user Just

might not be the case if the user Just Hits enter there's no characters to

Hits enter there's no characters to uppercase and so this is reckless of me

uppercase and so this is reckless of me and could theoretically create a sigall

and could theoretically create a sigall so I should probably start to be smarter

so I should probably start to be smarter and say something like this if the

and say something like this if the length of T is greater than zero okay

length of T is greater than zero okay now it's safe to actually capitalize the

now it's safe to actually capitalize the first letter and that will decrease the

first letter and that will decrease the probability now of those segmentation

probability now of those segmentation faults by just not making any

faults by just not making any assumptions about what the human does

assumptions about what the human does almost always the your programs will

almost always the your programs will crash when you've made a mistake yes but

crash when you've made a mistake yes but the user gives you an input that you

the user gives you an input that you yourself did not

yourself did not expect so what does this all look like

expect so what does this all look like in memory well let's go back to the Big

in memory well let's go back to the Big Grid this time focusing on the copying

Grid this time focusing on the copying of values and let's do this here's S as

of values and let's do this here's S as in this new program just declared to be

in this new program just declared to be a Char star uh here is where my

a Char star uh here is where my lowercase High maybe ended up in the

lowercase High maybe ended up in the computer's memory that's probably at

computer's memory that's probably at ox123 12 4 125 whatever something like

ox123 12 4 125 whatever something like that and that's of course what ends up

that and that's of course what ends up in s as a value when I declare T I do

in s as a value when I declare T I do get a second variable called T just like

get a second variable called T just like before but when I copy s into T what

before but when I copy s into T what happens it's really just literally ox1 2

happens it's really just literally ox1 2 3 whatever the value of s is is now also

3 whatever the value of s is is now also the value of T and so if we abstract

the value of T and so if we abstract this away at a high level get rid of all

this away at a high level get rid of all of those extra squares this is what s

of those extra squares this is what s and t now are they're indeed copies but

and t now are they're indeed copies but copies of each other not copies of the

copies of each other not copies of the underlying characters and so if you

underlying characters and so if you follow those arrows um and try to print

follow those arrows um and try to print them both out after capitalizing one or

them both out after capitalizing one or the other you're going to unfortunately

the other you're going to unfortunately end up capitalizing not just one of them

end up capitalizing not just one of them s but both of them s and T because

s but both of them s and T because literally it's the same address any

literally it's the same address any questions then on this

questions then on this visualization

visualization yeah good question is this pass by

yeah good question is this pass by reference we haven't um we have not seen

reference we haven't um we have not seen in detail an example like that um right

in detail an example like that um right now you're copying by value but

now you're copying by value but references will come into play and

references will come into play and remind me in a bit if I haven't used

remind me in a bit if I haven't used that term yet but this is just copying

that term yet but this is just copying things by could have ended poorly value

things by could have ended poorly value other

other questions no all right so with this in

questions no all right so with this in mind like how do we actually copy things

mind like how do we actually copy things properly for this we actually need

properly for this we actually need another building block so today we give

another building block so today we give you two functions one of which is called

you two functions one of which is called Malo one of which is called free and

Malo one of which is called free and these are used all of the time by like

these are used all of the time by like every piece of software you and I use on

every piece of software you and I use on our Macs PCS and phones whether it's

our Macs PCS and phones whether it's written in C or some equivalent other

written in C or some equivalent other language Malo is for memory allocation

language Malo is for memory allocation it's a function that you can use to ask

it's a function that you can use to ask the operating system Mac OS Linux

the operating system Mac OS Linux Windows anything for some number of of

Windows anything for some number of of bytes one bite 100 bytes a gigabyte of

bytes one bite 100 bytes a gigabyte of memory you can ask malog for however

memory you can ask malog for however much memory you want in advance it will

much memory you want in advance it will return to you the address of the first

return to you the address of the first bite of memory that it found free for

bite of memory that it found free for you unlike a string it is not null

you unlike a string it is not null terminated and so the danger with malok

terminated and so the danger with malok is that it's on the honor System if you

is that it's on the honor System if you ask it for one bite or 10 bytes you the

ask it for one bite or 10 bytes you the programmer in like a variable have to

programmer in like a variable have to remember how many byes you requested one

remember how many byes you requested one or 10 or the like strings do that for

or 10 or the like strings do that for you not when we're getting now to this

you not when we're getting now to this low level Mal just going to give you

low level Mal just going to give you some memory and it's up to you to manage

some memory and it's up to you to manage it free does the opposite when you're

it free does the opposite when you're done with some chunk of memory you can

done with some chunk of memory you can free it by passing in that same address

free it by passing in that same address and just hand it back to Mac OS windows

and just hand it back to Mac OS windows or Linux and say I'm done with this you

or Linux and say I'm done with this you can let me use this for something else

can let me use this for something else later um as an aside if your computer

later um as an aside if your computer has ever like frozen or hung like the

has ever like frozen or hung like the whole thing maybe just spontaneously

whole thing maybe just spontaneously reboots yet another reason for a bug

reboots yet another reason for a bug like that might be if you write a

like that might be if you write a program with a bug that keeps Mal

program with a bug that keeps Mal loocking Mal loocking Mal loocking that

loocking Mal loocking Mal loocking that is asking for more and more and more

is asking for more and more and more memory but you make a mistake and you

memory but you make a mistake and you never free it well eventually the

never free it well eventually the computer is going to literally run out

computer is going to literally run out of memory and something's going to go

of memory and something's going to go wrong and that's often when computers

wrong and that's often when computers freeze like they're just out of memory

freeze like they're just out of memory it has the memory there but the program

it has the memory there but the program was trying to use too much of it

was trying to use too much of it endlessly so this too will be a mistake

endlessly so this too will be a mistake that some of us will surely make in the

that some of us will surely make in the coming weeks but hopefully you'll now

coming weeks but hopefully you'll now see the solution so let me go back to uh

see the solution so let me go back to uh vs code here and let me propose that we

vs code here and let me propose that we do the following I'll hide my terminal

do the following I'll hide my terminal window for a moment and I'm going to

window for a moment and I'm going to introduce another header file up here

introduce another header file up here and I promise there's not going to be

and I promise there's not going to be too many more of these but this one is

too many more of these but this one is called standard li. for standard library

called standard li. for standard library and in this file are the Declarations

and in this file are the Declarations the prototypes for malok and free and a

the prototypes for malok and free and a bunch of other stuff as well it lets me

bunch of other stuff as well it lets me now manage my own memory so let's focus

now manage my own memory so let's focus now on line 11 line 11 is where I went

now on line 11 line 11 is where I went wrong before because conceptually I want

wrong before because conceptually I want to copy the whole string but of course

to copy the whole string but of course I'm only copying modestly the individual

I'm only copying modestly the individual address so how do I copy the whole darn

address so how do I copy the whole darn thing well what I need to do is this

thing well what I need to do is this when when I declare T to be the address

when when I declare T to be the address of something in memory why don't I set T

of something in memory why don't I set T to be the address of a free chunk of

to be the address of a free chunk of memory so let me ask the operating

memory so let me ask the operating system give me this many bytes tell me

system give me this many bytes tell me what the address is and I'm going to

what the address is and I'm going to store that in t initially just so I know

store that in t initially just so I know where there's free space for me so how

where there's free space for me so how do I do that well quite simply I call

do I do that well quite simply I call malok and then I pass in the number of

malok and then I pass in the number of bytes that I need now for Hi exclamation

bytes that I need now for Hi exclamation point I think I need three although wait

point I think I need three although wait no I really need four because of the

no I really need four because of the null character but I don't think I

null character but I don't think I should be hardcoding numbers like this

should be hardcoding numbers like this cuz who knows what the human's going to

cuz who knows what the human's going to type in so I can actually use Sterling

type in so I can actually use Sterling of s and then plus one this will ask

of s and then plus one this will ask malok then for however many btes

malok then for however many btes corresponds to the number of characters

corresponds to the number of characters the human typed in plus one for again

the human typed in plus one for again the null character so it's just being

the null character so it's just being smart and defensive rather than choosing

smart and defensive rather than choosing a number myself but now all T is is a

a number myself but now all T is is a pointer if you will to some random chunk

pointer if you will to some random chunk of free space so there's nothing there

of free space so there's nothing there yet or there's you know bits there but

yet or there's you know bits there but who knows what value they are they're

who knows what value they are they're certainly not identical to what the

certainly not identical to what the human type did in I now have to do this

human type did in I now have to do this so how can I copy one string into the

so how can I copy one string into the other well let me do this instead of uh

other well let me do this instead of uh capitalizing something just yet let me

capitalizing something just yet let me do this how about four in I gets zero I

do this how about four in I gets zero I is less than the length of

is less than the length of s uh and then i++ so I'm going to

s uh and then i++ so I'm going to iterate for the whole length of the

iterate for the whole length of the string and in here I'm just going to do

string and in here I'm just going to do this the E character in t should be

this the E character in t should be identical to the E character in s so I'm

identical to the E character in s so I'm just literally copying from right to

just literally copying from right to left each and every character in s and I

left each and every character in s and I can trust that there's enough memory in

can trust that there's enough memory in t why cuz I asked for that many bytes

t why cuz I asked for that many bytes plus one now there's technically a bug

plus one now there's technically a bug here I actually should probably do this

here I actually should probably do this I should do plus one here or if you

I should do plus one here or if you prefer I should do less than or equal to

prefer I should do less than or equal to the Sterling but I think it's a little

the Sterling but I think it's a little clear to do the plus one why do I for

clear to do the plus one why do I for the first time want to go just beyond

the first time want to go just beyond the boundary of s and copy one more

the boundary of s and copy one more bite yeah yeah I need the null character

bite yeah yeah I need the null character like I could technically manually add it

like I could technically manually add it with some additional line of code but I

with some additional line of code but I might as well just copy it because back

might as well just copy it because back sl0 is back sl0 so this time and

sl0 is back sl0 so this time and probably only this time it's reasonable

probably only this time it's reasonable and correct to go just beyond the

and correct to go just beyond the boundary of your string so you copy the

boundary of your string so you copy the null terminating character so that the

null terminating character so that the computer also knows where T ends and now

computer also knows where T ends and now I think what I can do a little more

I think what I can do a little more safely is this let me go down here and

safely is this let me go down here and say t braet 0 equals 2 upper of t uh two

say t braet 0 equals 2 upper of t uh two upper of T bracket Z so same line of

upper of T bracket Z so same line of code as before if I actually want to be

code as before if I actually want to be really safe I should probably do this so

really safe I should probably do this so if the stir L of T is greater than zero

if the stir L of T is greater than zero so there's at least one bite there okay

so there's at least one bite there okay now it's safe to blindly capitalize the

now it's safe to blindly capitalize the first character and I think that now

first character and I think that now puts me in better shape so let me try

puts me in better shape so let me try this now let me uh open up my terminal

this now let me uh open up my terminal make uh copy do slash copy I'm going to

make uh copy do slash copy I'm going to type in Hi exclamation point in all

type in Hi exclamation point in all lowercase crossing my fingers this time

lowercase crossing my fingers this time and now if I zoom in it indeed

and now if I zoom in it indeed capitalized only T and not s in this

capitalized only T and not s in this case so pictorially let me switch over

case so pictorially let me switch over here here is as before the variable s

here here is as before the variable s pointing at high in all lowercase when I

pointing at high in all lowercase when I call malok though that gives me a chunk

call malok though that gives me a chunk of memory that I'm going to store the

of memory that I'm going to store the address in t of so if T is some other

address in t of so if T is some other variable as it is in my code and there's

variable as it is in my code and there's some other available chunk of memory I

some other available chunk of memory I don't know where it is but let's assume

don't know where it is but let's assume as always it's at Ox 456 4557 4558 N9 so

as always it's at Ox 456 4557 4558 N9 so four bytes total what is now happening

four bytes total what is now happening well T is defined as pointing to that

well T is defined as pointing to that because that's what Malo gives us the

because that's what Malo gives us the address of the first bite of the free

address of the first bite of the free memory and now with my for Loop I'm just

memory and now with my for Loop I'm just iterating over it copying the H then the

iterating over it copying the H then the I then the exclamation point and then

I then the exclamation point and then for good measure the back slash Zer

for good measure the back slash Zer instead questions then on this process

instead questions then on this process here a really good question um if I

here a really good question um if I omitted in my code the

omitted in my code the uh plus one and I didn't do less than or

uh plus one and I didn't do less than or equal to so that I'm copying the fourth

equal to so that I'm copying the fourth bite odds are in this program because

bite odds are in this program because it's so short you wouldn't notice that

it's so short you wouldn't notice that there's an actual error but what could

there's an actual error but what could happen is When I Call printf On T if

happen is When I Call printf On T if there's no null bite there it might

there's no null bite there it might print h i exclamation point some random

print h i exclamation point some random value some random value some random

value some random value some random value some random value until it gets

value some random value until it gets lucky and there happens to be a zero

lucky and there happens to be a zero bite a null bite by chance for instance

bite a null bite by chance for instance so if you don't include the back sl0

so if you don't include the back sl0 some way that's going to happen and I

some way that's going to happen and I say some way I could even do this I

say some way I could even do this I could technically just copy the length

could technically just copy the length of the string s and at the very bottom

of the string s and at the very bottom here I could do something like T bracket

here I could do something like T bracket I uh sorry T bracket um

I uh sorry T bracket um Sterling of T I could do this but this

Sterling of T I could do this but this is just not necessary like I could

is just not necessary like I could manually add it at the end of the string

manually add it at the end of the string but again i' claim that it's just

but again i' claim that it's just simpler to borrow that is copy the one

simpler to borrow that is copy the one that's already in s because it's the

that's already in s because it's the same thing at the end of the day good

same thing at the end of the day good question other questions on this copying

question other questions on this copying correctly

correctly [Music]

[Music] now all right is there any room for

now all right is there any room for improvement here well let me propose a

improvement here well let me propose a slight optimization this is kind of a

slight optimization this is kind of a throwback now to week one turns out that

throwback now to week one turns out that arguably my line 13 here wherein I have

arguably my line 13 here wherein I have this for Loop now that I'm doing things

this for Loop now that I'm doing things in Loops again and again and using a

in Loops again and again and using a function like Sterling this is correct

function like Sterling this is correct it will iterate from zero on up to the

it will iterate from zero on up to the length of I a length of uh s plus one

length of I a length of uh s plus one but it's kind of stupid of me to write

but it's kind of stupid of me to write this for Loop in this way why well

this for Loop in this way why well here's my initialization on the left

here's my initialization on the left here's my condition in the middle and in

here's my condition in the middle and in general calling a function inside of

general calling a function inside of your condition is probably not very good

your condition is probably not very good design like why why is it bad for me to

design like why why is it bad for me to be calling a function like Sterling in

be calling a function like Sterling in this condition in the middle of my for

this condition in the middle of my for Loop

yeah yeah you're just calling it again and again for no reason like the length

and again for no reason like the length of s never changes so like why are you

of s never changes so like why are you wasting everyone's Time by calling

wasting everyone's Time by calling Sterling of s again again again again

Sterling of s again again again again just to check this inequality whether I

just to check this inequality whether I is less than that value so it turns out

is less than that value so it turns out if you haven't discovered this already

if you haven't discovered this already there's a slight optimization we can do

there's a slight optimization we can do here that has nothing to do

here that has nothing to do fundamentally with strings or pointers

fundamentally with strings or pointers just with better design I can actually

just with better design I can actually Define two variables at once I could do

Define two variables at once I could do this let me remove this whole condition

this let me remove this whole condition and let me add a comma after I equals 0

and let me add a comma after I equals 0 set n or any variable equal to the stirl

set n or any variable equal to the stirl of s + 1 and then after the semicolon

of s + 1 and then after the semicolon just ask the question while I is less

just ask the question while I is less than n so it's almost the same but

than n so it's almost the same but notice now my condition in the very

notice now my condition in the very middle of this Loop is at least

middle of this Loop is at least comparing two static values n never

comparing two static values n never change sorry one static value n never

change sorry one static value n never changes all that changes is I but I'm

changes all that changes is I but I'm not foolishly calling sterling sterling

not foolishly calling sterling sterling sterling again and again why well how

sterling again and again why well how does Sterling work similar in spirit to

does Sterling work similar in spirit to PR print F Sterling given the name of a

PR print F Sterling given the name of a string looks at the first character and

string looks at the first character and then starts looking through the entire

then starts looking through the entire string looking for the null character

string looking for the null character and we saw this in week two counting up

and we saw this in week two counting up how many characters are there so it's

how many characters are there so it's just a waste of time again and

just a waste of time again and [Music]

again totally if you wanted to use n multiple times you could absolutely take

multiple times you could absolutely take it out of the for Loop put it right

it out of the for Loop put it right after s is defined and reuse n and again

after s is defined and reuse n and again and again absolutely but in General

and again absolutely but in General consider this when designing your for

consider this when designing your for Loops even though modern compilers like

Loops even though modern compilers like cang can actually fix this problem this

cang can actually fix this problem this inefficiency for you good practice would

inefficiency for you good practice would be don't call functions unnecessarily

be don't call functions unnecessarily especially if the answer is always going

especially if the answer is always going to be the same all right so what else

to be the same all right so what else should I perhaps refine here well how

should I perhaps refine here well how about I do one last thing and just

about I do one last thing and just comment on what exactly could go wrong

comment on what exactly could go wrong here well a couple of things well

here well a couple of things well actually this is just silly too like

actually this is just silly too like surely someone before me in the world

surely someone before me in the world has had to copy a string before surely

has had to copy a string before surely there's a function like called stir copy

there's a function like called stir copy maybe like stir compare like stir Ling

maybe like stir compare like stir Ling and indeed there is so let me propose

and indeed there is so let me propose that we actually get rid of this whole

that we actually get rid of this whole for Loop and we actually just call a

for Loop and we actually just call a function called stir copy no o just St

function called stir copy no o just St cpy and pass in the destination which is

cpy and pass in the destination which is T first and then the source that you

T first and then the source that you want to copy into the destination and

want to copy into the destination and that takes the place entirely of that

that takes the place entirely of that whole Loop so again I demonstrated the

whole Loop so again I demonstrated the loop first just to be very pedantic

loop first just to be very pedantic about it but that's wasting time you're

about it but that's wasting time you're wasting time writing lines of code you

wasting time writing lines of code you don't need to stir copy is what you can

don't need to stir copy is what you can use here instead and so this has now

use here instead and so this has now always existed and what more can I do

always existed and what more can I do well as one final point it turns out

well as one final point it turns out that there's actually things that can go

that there's actually things that can go wrong in this code even besides the

wrong in this code even besides the string being too short like if the human

string being too short like if the human Just Hits enter and there are no

Just Hits enter and there are no characters I don't want to blindly

characters I don't want to blindly capitalize the first character that

capitalize the first character that doesn't exist that's why I added that if

doesn't exist that's why I added that if condition but there's other things that

condition but there's other things that can go wrong and we introduced those to

can go wrong and we introduced those to you today it turns out that functions

you today it turns out that functions like get string and functions like Malo

like get string and functions like Malo return potentially a special value and

return potentially a special value and wonderfully confusingly it's also called

wonderfully confusingly it's also called null but with two L's all right so left

null but with two L's all right so left hand and right hand weren't talking so

hand and right hand weren't talking so well like decades ago NL is a back SL

well like decades ago NL is a back SL zero it's a single character as it

zero it's a single character as it always has been for a couple of weeks

always has been for a couple of weeks now

now n is technically a pointer it's an

n is technically a pointer it's an address but it's address zero it's like

address but it's address zero it's like the top left hand corner if you will of

the top left hand corner if you will of your computer's memory that just nothing

your computer's memory that just nothing is ever supposed to go in by convention

is ever supposed to go in by convention so null is a synonym for zero but it's

so null is a synonym for zero but it's specifically an address now why is this

specifically an address now why is this useful well suppose that in my code here

useful well suppose that in my code here something goes wrong with get string

something goes wrong with get string suppose you're being a little crazy and

suppose you're being a little crazy and you type in way too long of a string

you type in way too long of a string it's not just high but it's like an

it's not just high but it's like an entire essay of text and there's not

entire essay of text and there's not enough memory in the computer how does

enough memory in the computer how does get string signal to the programmer whoa

get string signal to the programmer whoa like that's way too big of a string I

like that's way too big of a string I can't fit it in memory well we never

can't fit it in memory well we never told you this but all of this time it

told you this but all of this time it turns out that that get string will

turns out that that get string will return this special value called null if

return this special value called null if something goes wrong so to be really

something goes wrong so to be really careful now you should do something like

careful now you should do something like this if s equals equals literally null

this if s equals equals literally null then you better um exit the program

then you better um exit the program entirely and return like one or two or

entirely and return like one or two or three to signify that something went

three to signify that something went wrong don't uh go any further similarly

wrong don't uh go any further similarly with malok it's possible if you ask for

with malok it's possible if you ask for way too much memory that could fail

way too much memory that could fail especially if you're asking now for

especially if you're asking now for double the memory after the human typed

double the memory after the human typed something in so if T equals equals null

something in so if T equals equals null then you know what but let's also return

then you know what but let's also return one or some other value to just get out

one or some other value to just get out before something crashes or freezes on

before something crashes or freezes on the human as well so honestly I tend not

the human as well so honestly I tend not to do this always in class because the

to do this always in class because the code just gets so bloated and

code just gets so bloated and complicated but you absolutely in

complicated but you absolutely in practice need to start doing this

practice need to start doing this otherwise you will be responsible for

otherwise you will be responsible for the freezes and the crashes and the

the freezes and the crashes and the reboots that users in the real world

reboots that users in the real world might actually encounter otherwise of

might actually encounter otherwise of course if we get to the bottom of this

course if we get to the bottom of this program now I should probably return

program now I should probably return zero explicitly or implicitly to just

zero explicitly or implicitly to just signify that everything is is

signify that everything is is successful but there's one other thing I

successful but there's one other thing I haven't done we introduced malok but

haven't done we introduced malok but what did I claim also

what did I claim also existed so free I'm also being a little

existed so free I'm also being a little reckless now here I am not practicing

reckless now here I am not practicing what I'm preaching I'm asking the

what I'm preaching I'm asking the computer for memory via get string I'm

computer for memory via get string I'm asking the computer for more memory my

asking the computer for more memory my via malok and I'm never technically

via malok and I'm never technically handing it back so really what I should

handing it back so really what I should be doing at the very bottom of my

be doing at the very bottom of my program to is freeing the memory I've

program to is freeing the memory I've asked for so henceforth it is a rule a

asked for so henceforth it is a rule a law if you will in see whenever you

law if you will in see whenever you allocate memory with malok or certain

allocate memory with malok or certain other functions as well you the

other functions as well you the programmer must free it when you're all

programmer must free it when you're all done with it now this is a bit of an

done with it now this is a bit of an overstatement because technically when

overstatement because technically when programs quit they'll free the memory

programs quit they'll free the memory automatically so you're not going to

automatically so you're not going to break someone's Mac or PC because you

break someone's Mac or PC because you necessarily have this bug but for

necessarily have this bug but for programs that are running all the time

programs that are running all the time like someone keeps a chrome their

like someone keeps a chrome their browser open Microsoft Word or the like

browser open Microsoft Word or the like bad things will happen if over time you

bad things will happen if over time you never never never call free and the

never never never call free and the program keeps running so always get into

program keeps running so always get into this habit here you you do not need to

this habit here you you do not need to free memory that comes from get string

free memory that comes from get string because the cs50 library automatically

because the cs50 library automatically frees it for you but you anytime you use

frees it for you but you anytime you use malok henceforth as you did or I did

malok henceforth as you did or I did here you must free that by just passing

here you must free that by just passing in the same address you got back

in the same address you got back questions now on malok and

questions now on malok and free questions

yeah oh really good question so free just so what does free do so free um

just so what does free do so free um just uh lets the computer know that you

just uh lets the computer know that you are done with that chunk of memory which

are done with that chunk of memory which means that if you have a another line of

means that if you have a another line of code elsewhere that same memory might be

code elsewhere that same memory might be reused and can be used again and again

reused and can be used again and again and that's going to be necessary

and that's going to be necessary certainly for any long running program

certainly for any long running program you can't ask for memory constantly

you can't ask for memory constantly you'll eventually run out so you need to

you'll eventually run out so you need to free it in this way other languages as

free it in this way other languages as an aside python you get another

an aside python you get another motivation in a couple of weeks for it

motivation in a couple of weeks for it is going to be Python and certain other

is going to be Python and certain other languages manage all this headache for

languages manage all this headache for you but in C the goal here is to really

you but in C the goal here is to really harness these capabilities ourselves all

harness these capabilities ourselves all right so it turns out like almost

right so it turns out like almost everyone in the room everyone in the

everyone in the room everyone in the room myself included you're going to

room myself included you're going to screw up when it comes to anything

screw up when it comes to anything memory related if you haven't already

memory related if you haven't already segals are in your future but hopefully

segals are in your future but hopefully there's tools via which you can detect

there's tools via which you can detect these things and fix them proactively

these things and fix them proactively and not just use print def uh or debug

and not just use print def uh or debug 50 or rubber duck we actually have

50 or rubber duck we actually have another tool we can equip you with now

another tool we can equip you with now that help you find some mistakes so let

that help you find some mistakes so let me do this let me close copy.c let me

me do this let me close copy.c let me open a program I wrote In Advance called

open a program I wrote In Advance called memory. C that doesn't do anything

memory. C that doesn't do anything really interesting but it's going to

really interesting but it's going to have two bugs in it notice that I've

have two bugs in it notice that I've included standard i.h as always I've

included standard i.h as always I've also included standard li. which is

also included standard li. which is necessary now for anything related to

necessary now for anything related to malok Andor free and the like line six

malok Andor free and the like line six it's a little weird what I've done here

it's a little weird what I've done here but this is like the manual way of

but this is like the manual way of asking for enough memory for an array in

asking for enough memory for an array in week two how do we ask for memory for an

week two how do we ask for memory for an array you very simply say int X3 and

array you very simply say int X3 and that gives you an array called X of size

that gives you an array called X of size three but if you do it manually Now

three but if you do it manually Now using Malo what you have to do is use

using Malo what you have to do is use syntax like this you call malok you ask

syntax like this you call malok you ask for three things times however big an

for three things times however big an INT is now we know it's four so you

INT is now we know it's four so you could literally write 12 here but this

could literally write 12 here but this is more generic so three times the size

is more generic so three times the size of an integer will give you 12

of an integer will give you 12 dynamically and what do Malo return the

dynamically and what do Malo return the address of the first bite you get back

address of the first bite you get back what do I want to put that well I want

what do I want to put that well I want to put it in a variable now the variable

to put it in a variable now the variable can't just be int X because that's a

can't just be int X because that's a number it's not an address per se if I

number it's not an address per se if I want to store this address in a variable

want to store this address in a variable I could call it X I could call it P but

I could call it X I could call it P but in Star X just means that X is now the

in Star X just means that X is now the address of a chunk of memory

address of a chunk of memory specifically a chunk of memory that's

specifically a chunk of memory that's big enough not for one but for three ins

big enough not for one but for three ins in total all right now I'm just sort of

in total all right now I'm just sort of naively putting our old friends 72 73

naively putting our old friends 72 73 and 33 at the first second and third

and 33 at the first second and third locations in memory but perhaps based on

locations in memory but perhaps based on week two or week four I'm clearly

week two or week four I'm clearly screwing up up here in a couple of ways

screwing up up here in a couple of ways someone want to identify at least one

someone want to identify at least one bug what did I do

bug what did I do wrong yeah like this is now you know

wrong yeah like this is now you know amateur uh stuff like I should be zero

amateur uh stuff like I should be zero indexing not one indexing so this has

indexing not one indexing so this has got to be 0 one2 ultimately and other

got to be 0 one2 ultimately and other bugs that are maybe more week four

bugs that are maybe more week four specific other bugs it's more subtle

specific other bugs it's more subtle yeah I'm not freeing the memory right so

yeah I'm not freeing the memory right so I'm not practicing what I'm preaching by

I'm not practicing what I'm preaching by freeing this memory now suppose these

freeing this memory now suppose these are non obvious and honestly after like

are non obvious and honestly after like an hour or two of this like this

an hour or two of this like this shouldn't be obvious yet it will be over

shouldn't be obvious yet it will be over time how could I find these bugs with uh

time how could I find these bugs with uh software as opposed to just staring at

software as opposed to just staring at the thing or asking someone for help

the thing or asking someone for help well let me propose this let me first go

well let me propose this let me first go ahead and run make memory to compile the

ahead and run make memory to compile the program and it seems to work look fine

program and it seems to work look fine there's no syntax errors at least

there's no syntax errors at least dotmemory notice seems to work fine too

dotmemory notice seems to work fine too now this program doesn't do anything

now this program doesn't do anything interesting there's no printf or

interesting there's no printf or anything like that but it didn't crash

anything like that but it didn't crash there's no segmentation fault but that

there's no segmentation fault but that doesn't mean there aren't bugs latent in

doesn't mean there aren't bugs latent in the software and this is true sadly of

the software and this is true sadly of all of today's software like Chrome and

all of today's software like Chrome and Microsoft Word and other programs surely

Microsoft Word and other programs surely have memory related bugs that people at

have memory related bugs that people at Google and Microsoft haven't yet found

Google and Microsoft haven't yet found but there are tools at least to find the

but there are tools at least to find the most obvious of those bugs and we're

most obvious of those bugs and we're going to introduce you now to a program

going to introduce you now to a program called valren so valren it's a fairly

called valren so valren it's a fairly fancy program but we'll use it for very

fancy program but we'll use it for very simple ways we'll look at your code and

simple ways we'll look at your code and find memory errors as it's executing and

find memory errors as it's executing and try to help you understand where they

try to help you understand where they are so let me go back to vs code here

are so let me go back to vs code here memory seems to be fine you know I feel

memory seems to be fine you know I feel like okay I'm going to submit this

like okay I'm going to submit this homework all is good no error messages

homework all is good no error messages that's no longer the case now you need

that's no longer the case now you need to poke a little more at your code to

to poke a little more at your code to see if maybe there's still some bug

see if maybe there's still some bug there so let me do this

there so let me do this valren and then space dotmemory so just

valren and then space dotmemory so just like debug 50 you run it on a program

like debug 50 you run it on a program you already compiled valren I'm going to

you already compiled valren I'm going to run it on a program I already compiled

run it on a program I already compiled let me uh zoom in on my terminal window

let me uh zoom in on my terminal window so we can see more at once and enter all

so we can see more at once and enter all right the output is crazy cryp IC for no

right the output is crazy cryp IC for no good reason there's lots of numbers and

good reason there's lots of numbers and equal signs it's a lot of clutter but

equal signs it's a lot of clutter but there is some juicy information here and

there is some juicy information here and let me start from the top down invalid

let me start from the top down invalid write of size four so write means to

write of size four so write means to change a value read means to like access

change a value read means to like access a value and this is again esoteric like

a value and this is again esoteric like a lot of our error messages are but it

a lot of our error messages are but it looks like uh after a block of size 12

looks like uh after a block of size 12 allock and then there's these weird hex

allock and then there's these weird hex notation there's some mention of malok

notation there's some mention of malok but honestly the juicy part here is

but honestly the juicy part here is memory. C line six that's probably my

memory. C line six that's probably my fault so let's look at line six per that

fault so let's look at line six per that output let me shrink the terminal window

output let me shrink the terminal window look at line six okay 12 is now Germain

look at line six okay 12 is now Germain right if you did the mental math of the

right if you did the mental math of the size of an INT times three 12 is somehow

size of an INT times three 12 is somehow involved here but line six is now uh

involved here but line six is now uh happening next year that's where the

happening next year that's where the memory came from what is this let me

memory came from what is this let me Zoom back in where is there invalid

Zoom back in where is there invalid right of size four like what's perhaps

right of size four like what's perhaps going wrong here invalid right of size

going wrong here invalid right of size four what does that mean it's like a

four what does that mean it's like a very technical way of explaining the bug

very technical way of explaining the bug is actually one line later on line seven

is actually one line later on line seven as we already identified

as we already identified yeah indeed and I I misspoke a moment

yeah indeed and I I misspoke a moment ago the bug actually arises here with

ago the bug actually arises here with line nine so after the allocation of

line nine so after the allocation of memory I'm somehow writing four bytes

memory I'm somehow writing four bytes incorrectly and unfortunately the onus

incorrectly and unfortunately the onus is kind of on you to sort of think

is kind of on you to sort of think through deductively like what could that

through deductively like what could that mean but I'm clearly touching four bytes

mean but I'm clearly touching four bytes of memory in these few lines of code

of memory in these few lines of code that I shouldn't be and hopefully here

that I shouldn't be and hopefully here is the light bulb already went off ear

is the light bulb already went off ear oh I'm not zero indexing okay that must

oh I'm not zero indexing okay that must mean that X bracket 3 as you note is

mean that X bracket 3 as you note is just too far past the chunk of memory so

just too far past the chunk of memory so I'm invalidly writing to four bytes that

I'm invalidly writing to four bytes that I shouldn't be so again it's not super

I shouldn't be so again it's not super obvious this is not super userfriendly

obvious this is not super userfriendly but at least it does give you a clue as

but at least it does give you a clue as to where that bug is so the fix there is

to where that bug is so the fix there is going to be quite simply to change the

going to be quite simply to change the one to a zero the two to a one and the

one to a zero the two to a one and the three to a two that'll fix that but

three to a two that'll fix that but there's still a second error and let me

there's still a second error and let me look at the cryptic output again Heap

look at the cryptic output again Heap summary some stuff there okay this does

summary some stuff there okay this does not sound good down here 12 bytes in one

not sound good down here 12 bytes in one blocks are definitely lost in Lost

blocks are definitely lost in Lost record one of one very Arcane output two

record one of one very Arcane output two but clearly related to line six again

but clearly related to line six again our allocation of memory now here too

our allocation of memory now here too it's not obvious what the solution is

it's not obvious what the solution is but memory is lost AKA this is a memory

but memory is lost AKA this is a memory leak and now the deductions kind of up

leak and now the deductions kind of up to you why what is leak oh wait I didn't

to you why what is leak oh wait I didn't call free and so the second solution

call free and so the second solution here is probably to free x at the very

here is probably to free x at the very end of the program and if you really

end of the program and if you really want to be you should probably check

want to be you should probably check like I proposed earlier if x is null

like I proposed earlier if x is null just get out now while you still can and

just get out now while you still can and don't even touch those other lines of

don't even touch those other lines of code but if you get to the bottom return

code but if you get to the bottom return zero but really the takeaways are I

zero but really the takeaways are I fixed my zero indexing of the array to

fixed my zero indexing of the array to avoid the invalid right of size four and

avoid the invalid right of size four and now I'm freeing the memory that I asked

now I'm freeing the memory that I asked for so there should be no leak lost all

for so there should be no leak lost all right let's try this again make

right let's try this again make memorymemory no visible errors yet but

memorymemory no visible errors yet but let me now increase my terminal window

let me now increase my terminal window again do valren of dotmemory crossing my

again do valren of dotmemory crossing my fingers and now all Heap blocks were

fingers and now all Heap blocks were freed no leaks are possible I don't see

freed no leaks are possible I don't see any invalid rights there's still a crazy

any invalid rights there's still a crazy amount of output but none of it is

amount of output but none of it is erroneous it's not bad now I fixed my

erroneous it's not bad now I fixed my memory bugs and so now my my ta my TF

memory bugs and so now my my ta my TF they're not going to find them either

they're not going to find them either because at least valr has proactively

because at least valr has proactively done that for me questions then on

done that for me questions then on valgren generally it's those two types

valgren generally it's those two types of Errors you might trip over there's

of Errors you might trip over there's not twoo much else in the way of Arcane

not twoo much else in the way of Arcane output questions then on

output questions then on this no all right well what else might

this no all right well what else might be going on so someone alluded to this

be going on so someone alluded to this earlier what happens when you for

earlier what happens when you for instance forget the null Terminator or

instance forget the null Terminator or you generally start poking around memory

you generally start poking around memory that you yourself didn't ask for or uh

that you yourself didn't ask for or uh looking at values you didn't put there

looking at values you didn't put there well let me go ahead and open this code

well let me go ahead and open this code of garbage. C in honor of Oscar the

of garbage. C in honor of Oscar the Grouch here of sorts and here is a

Grouch here of sorts and here is a simple program if I hide my terminal

simple program if I hide my terminal window that just does something kind of

window that just does something kind of arbitrary I first declare an array uh

arbitrary I first declare an array uh called scores uh but I made it crazy big

called scores uh but I made it crazy big like 1024 like that's a lot of uh

like 1024 like that's a lot of uh integers but so be it and then I iterate

integers but so be it and then I iterate over those integers and I print each of

over those integers and I print each of those scores out so I'm using week two

those scores out so I'm using week two syntax here but based on this program

syntax here but based on this program what have I clearly not done that I did

what have I clearly not done that I did do back in week two I've allocated the

do back in week two I've allocated the array I'm printing the array but but

array I'm printing the array but but Bute yeah I didn't initialize any values

Bute yeah I didn't initialize any values for that array back in week two we

for that array back in week two we didn't do 1024 we did like three and I

didn't do 1024 we did like three and I typed in like three test scores or

typed in like three test scores or something like that he here I'm

something like that he here I'm allocating memory even more than that

allocating memory even more than that just because I really want to be

just because I really want to be dramatic with what I'm demonstrating but

dramatic with what I'm demonstrating but I'm not initializing those values to

I'm not initializing those values to anything and so here it turns out in C

anything and so here it turns out in C generally if you do not initialize a

generally if you do not initialize a variable or you do not initialize an

variable or you do not initialize an array with explicit values there are

array with explicit values there are going to be garbage values there so to

going to be garbage values there so to speak remnants of that memory having

speak remnants of that memory having been used before probably by some other

been used before probably by some other function of yours some Library function

function of yours some Library function or something else while your program is

or something else while your program is running not a huge deal with a super

running not a huge deal with a super small program like this but for anything

small program like this but for anything sizable memory is going to be used and

sizable memory is going to be used and unused and used and unused that is

unused and used and unused that is macked and freed again and again there's

macked and freed again and again there's going to be lots of garbage values in

going to be lots of garbage values in the computer's memory by default so if I

the computer's memory by default so if I open my terminal window here uh let me

open my terminal window here uh let me do make garbage uh let me zoom in on my

do make garbage uh let me zoom in on my terminal so we can see the output when I

terminal so we can see the output when I runt garbage theoretically I should see

runt garbage theoretically I should see 1,24 integers but none of which have

1,24 integers but none of which have been initialized now I'm going to get

been initialized now I'm going to get lucky with some of them and it looks

lucky with some of them and it looks like wow okay a lot of them are

like wow okay a lot of them are initialized to zero and C does in some

initialized to zero and C does in some context initialize memory for you to

context initialize memory for you to zero at least at the beginning but not

zero at least at the beginning but not again and again typically but if I start

again and again typically but if I start scrolling backwards in time at this

scrolling backwards in time at this array of size

array of size 1024 where did these values come from so

1024 where did these values come from so just random positive and negative

just random positive and negative numbers interspersed among the zeros

numbers interspersed among the zeros well that's because I'm literally poking

well that's because I'm literally poking around a random 1,24 bytes of the

around a random 1,24 bytes of the computer's memory who knows what's there

computer's memory who knows what's there so the lesson here is that garbage

so the lesson here is that garbage values are indeed this like term of art

values are indeed this like term of art it means that a variable that you might

it means that a variable that you might have uh defined that you might have

have uh defined that you might have declared if you don't give it an

declared if you don't give it an explicit value who knows what's going to

explicit value who knows what's going to be there and the lesson here is just

be there and the lesson here is just don't do that always initialize

don't do that always initialize variables to something either yourself

variables to something either yourself or prompting the human for

or prompting the human for it questions about garbage

it questions about garbage values you'll see them sometimes if you

values you'll see them sometimes if you print things you shouldn't or touch

print things you shouldn't or touch arrays beyond their

arrays beyond their boundaries all right so maybe to make

boundaries all right so maybe to make this a little visual too it turns out

this a little visual too it turns out that a lot of things can go wrong

that a lot of things can go wrong unfortunately with poers and we've seen

unfortunately with poers and we've seen some of them and here's another program

some of them and here's another program that's a little contrived it's very

that's a little contrived it's very simple and it just is about manipulating

simple and it just is about manipulating values it doesn't do anything useful per

values it doesn't do anything useful per se except demonstrate some of today's

se except demonstrate some of today's Concepts so in main here let me propose

Concepts so in main here let me propose that we declare a pointer called X

that we declare a pointer called X that's going to store eventually the

that's going to store eventually the address of an integer apparently here's

address of an integer apparently here's another one called y that's going to

another one called y that's going to store the address of an integer as well

store the address of an integer as well here now I'm allocating enough memory to

here now I'm allocating enough memory to fit one integer now technically it's

fit one integer now technically it's four we know that but size of in just

four we know that but size of in just gives me that answer dynamically so it

gives me that answer dynamically so it will work on all systems

will work on all systems and I'm going to store the address that

and I'm going to store the address that Malo finds for me in X then I go to x

Malo finds for me in X then I go to x and put the number 42 there all right

and put the number 42 there all right why it's the sort of meaning of life the

why it's the sort of meaning of life the universe and everything here but star X

universe and everything here but star X again just means go to that address and

again just means go to that address and put a value there so why I don't know

put a value there so why I don't know but it's just uh correct at this point

but it's just uh correct at this point but what about this line here star y

but what about this line here star y equals 13 unlucky in this case what's

equals 13 unlucky in this case what's bad about this line here star y It's a

bad about this line here star y It's a combination now of today's Primitives

combination now of today's Primitives and that point here

and that point here yeah yeah we didn't ask the computer to

yeah yeah we didn't ask the computer to allocate any space so why was not

allocate any space so why was not initialized with an equal sign at any

initialized with an equal sign at any point to anything and so what is inside

point to anything and so what is inside y so to speak like a garbage value maybe

y so to speak like a garbage value maybe it's zero which isn't bad because at

it's zero which isn't bad because at least it's nice and simple but maybe

least it's nice and simple but maybe it's some crazy large uh positive number

it's some crazy large uh positive number some crazy large negative number either

some crazy large negative number either way odds are if I go to this address or

way odds are if I go to this address or that address randomly with star y bad

that address randomly with star y bad things are going to happen and so let me

things are going to happen and so let me go ahead and propose well let's not do

go ahead and propose well let's not do that let's let's actually do this

that let's let's actually do this instead assign y equal to X and we've

instead assign y equal to X and we've done that before and then I can go to Y

done that before and then I can go to Y now and change what was a 42 to a 13

now and change what was a 42 to a 13 again why this is just for educational

again why this is just for educational sake but for now this does not crash

sake but for now this does not crash because I only dreference y with star y

because I only dreference y with star y after actually giving it a value albe it

after actually giving it a value albe it a duplicate value similar to our copy

a duplicate value similar to our copy example earlier so our friends at

example earlier so our friends at Stanford have put together a wonderful

Stanford have put together a wonderful visual it's about 2 minutes long allow

visual it's about 2 minutes long allow me to dramatically dim the lights if we

me to dramatically dim the lights if we could and play with what happens with

could and play with what happens with memory when you do bad things like

memory when you do bad things like [Music]

[Music] this hey Binky wake up it's time for

this hey Binky wake up it's time for pointer fun what's that learn about

pointer fun what's that learn about pointers Oh goodie well to get started I

pointers Oh goodie well to get started I guess we're going to need a couple

guess we're going to need a couple pointers okay this code allocates two

pointers okay this code allocates two pointers which can point to integers

pointers which can point to integers okay well I see the two pointers but

okay well I see the two pointers but they don't seem to be pointing to

they don't seem to be pointing to anything that's right initially pointers

anything that's right initially pointers don't point to anything the things they

don't point to anything the things they point to are called Pointes and setting

point to are called Pointes and setting them up is a separate step oh right

them up is a separate step oh right right I knew that the Pointes are

right I knew that the Pointes are separate or so how do you allocate a

separate or so how do you allocate a point e okay well this code allocates a

point e okay well this code allocates a new integer point and this part sets X

new integer point and this part sets X to point to it hey that looks better so

to point to it hey that looks better so make it do something okay I'll

make it do something okay I'll dereference the pointer X to store the

dereference the pointer X to store the number 42 into its Point e for this

number 42 into its Point e for this trick I'll need my magic wand of

trick I'll need my magic wand of dereferencing your magic wand of D

dereferencing your magic wand of D referencing uh that that's great this is

referencing uh that that's great this is what the code looks like I'll just set

what the code looks like I'll just set up the number

up the number and hey look there it goes so doing a d

and hey look there it goes so doing a d reference on X follows the arrow to

reference on X follows the arrow to access its Point T in this case to store

access its Point T in this case to store 42 in there hey try using it to store

42 in there hey try using it to store the number 13 through the other pointer

the number 13 through the other pointer why okay I'll just go over here to Y and

why okay I'll just go over here to Y and get the number 13 set up and then take

get the number 13 set up and then take the wand of d referencing and

the wand of d referencing and just oh hey that didn't work say uh

just oh hey that didn't work say uh Binky I don't think de referencing Y is

Binky I don't think de referencing Y is a good idea cuz uh you know setting up

a good idea cuz uh you know setting up the point T is a separate step and uh I

the point T is a separate step and uh I don't think we ever did it good point

don't think we ever did it good point yeah we we allocated the pointer y but

yeah we we allocated the pointer y but we never set it to point to a point

we never set it to point to a point T very observant hey you're looking good

T very observant hey you're looking good there Binky can you fix it so that y

there Binky can you fix it so that y points to the same point as X sure I'll

points to the same point as X sure I'll use my magic wand of pointer assignment

use my magic wand of pointer assignment is that going to be a problem like

is that going to be a problem like before no this doesn't touch the Pointes

before no this doesn't touch the Pointes it just changes one pointer to point to

it just changes one pointer to point to the same thing is another oh I see now y

the same thing is another oh I see now y points to the same place as X so so wait

points to the same place as X so so wait now Y is fixed it has a point e so you

now Y is fixed it has a point e so you can try the wand of D referencing again

can try the wand of D referencing again to send the 13 over uh okay here it

to send the 13 over uh okay here it goes hey look at that now D referencing

goes hey look at that now D referencing works on why and because the pointers

works on why and because the pointers are sharing that one point e they both

are sharing that one point e they both see the 13 yeah sharing uh whatever so

see the 13 yeah sharing uh whatever so are we going to switch places now oh

are we going to switch places now oh look we're out of time but L thanks to

look we're out of time but L thanks to Professor Nick parlante of Stanford for

Professor Nick parlante of Stanford for spending a huge amount of time doing

spending a huge amount of time doing stop motion animation for that but

stop motion animation for that but hopefully now you have a sense of what

hopefully now you have a sense of what two can go wrong when you misuse memory

two can go wrong when you misuse memory in this way but at the end of the day we

in this way but at the end of the day we really only have these four new building

really only have these four new building blocks today like the star operator the

blocks today like the star operator the Ampersand operator malok and free and

Ampersand operator malok and free and really with that and the underlying

really with that and the underlying understanding of what your computer is

understanding of what your computer is doing underneath the hood we have this

doing underneath the hood we have this way now to really manipulate things in

way now to really manipulate things in memory For Better or For Worse and

memory For Better or For Worse and eventually we'll see how we can build

eventually we'll see how we can build things but we can also now use today's

things but we can also now use today's Primitives to better explain some things

Primitives to better explain some things that we've been asking you to take for

that we've been asking you to take for granted over the past several weeks so

granted over the past several weeks so for instance let me propose that we uh

for instance let me propose that we uh do one volunteer up here if we could

do one volunteer up here if we could could we get one volunteer who's you

could we get one volunteer who's you want to come straight up yep right in

want to come straight up yep right in the middle come on you'll have to take a

the middle come on you'll have to take a left right

there all right so we have two empty glasses

all right so we have two empty glasses here and two colors of liquid and we

here and two colors of liquid and we have let me give you the mic if you'd

have let me give you the mic if you'd like to say hello to the

like to say hello to the group hello um I'm Moen uh I'm in the

group hello um I'm Moen uh I'm in the and first year all right welcome well

and first year all right welcome well well welcome here I'm going to go ahead

well welcome here I'm going to go ahead and fill these two glasses with this

and fill these two glasses with this colored liquid um purple here on my

colored liquid um purple here on my right let's fill up a glass

here yeah don't drink uh and now we'll put some orange in here

put some orange in here and what we'd like you to do for the

and what we'd like you to do for the audience if you don't mind is swap the

audience if you don't mind is swap the two values we've got a purple value and

two values we've got a purple value and orange value and I'd like the purple

orange value and I'd like the purple liquid in this glass and the orange

liquid in this glass and the orange liquid in that glass

please can I have another glass oh okay good intuition but for the microphone uh

good intuition but for the microphone uh can I have another glass so you can and

can I have another glass so you can and just in fact I brought one here for you

just in fact I brought one here for you why are you asking for this though cuz

why are you asking for this though cuz if I just pour this into this and it'll

if I just pour this into this and it'll get mixed up right so obviously we need

get mixed up right so obviously we need like a temporary variable if you will so

like a temporary variable if you will so here is your temporary

here is your temporary variable and you want yeah there's

variable and you want yeah there's yeah all right so pouring the value of

yeah all right so pouring the value of the orange glass into this temporary

the orange glass into this temporary variable if you

variable if you will all right and now pouring the value

will all right and now pouring the value of the purple glass into the former

of the purple glass into the former orange

glass and now the temporary value goes back into

now the temporary value goes back into the original purple glass and now I

the original purple glass and now I think we give you a round of applause

think we give you a round of applause for having done that very well

for having done that very well okay thank you all right so so it should

okay thank you all right so so it should go without saying that in the real world

go without saying that in the real world like that's how you do this and in fact

like that's how you do this and in fact in code that's pretty much how you have

in code that's pretty much how you have to do this although ask us some time for

to do this although ask us some time for a super fancy way of doing it without a

a super fancy way of doing it without a temporary variable turns out that is

temporary variable turns out that is possible using bits but for now let's

possible using bits but for now let's suppose that indeed this demonstrates

suppose that indeed this demonstrates what is the reality in code if you want

what is the reality in code if you want to swap two values you need to have have

to swap two values you need to have have something like a temporary variable so

something like a temporary variable so for instance on the screen here is a uh

for instance on the screen here is a uh the beginning of a function called swap

the beginning of a function called swap whose purpose in life is to as you just

whose purpose in life is to as you just did swap two values call it a and b so

did swap two values call it a and b so orange and purple respectively are now

orange and purple respectively are now just a and b and integers to keep things

just a and b and integers to keep things simple well here is the corresponding

simple well here is the corresponding code if I may to what you just enacted

code if I may to what you just enacted as a human you declared a temporary

as a human you declared a temporary variable a called temp in this case

variable a called temp in this case which was like me handing you The Empty

which was like me handing you The Empty Glass and you stored the orange liquid

Glass and you stored the orange liquid in it akaa you then changed the value of

in it akaa you then changed the value of the formerly orange glass to be equal to

the formerly orange glass to be equal to the Purple by pouring one into the other

the Purple by pouring one into the other and then you did the opposite there now

and then you did the opposite there now at the end of this you still have a

at the end of this you still have a temporary variable that's now empty so

temporary variable that's now empty so it's temporary in literally that sense

it's temporary in literally that sense like you just don't need it anymore but

like you just don't need it anymore but it was necessary along the way so I dare

it was necessary along the way so I dare say this code is correct logically like

say this code is correct logically like this will swap two values A and B thanks

this will swap two values A and B thanks to the use of that temporary variable

to the use of that temporary variable unfortunately though if I actually do

unfortunately though if I actually do this in practice let me go over to VSS

this in practice let me go over to VSS code here and open a program I wrote in

code here and open a program I wrote in advance called swap. C which does this

advance called swap. C which does this as follows in here notice I have my

as follows in here notice I have my prototype for a swap function at the

prototype for a swap function at the very top and let me scroll down to the

very top and let me scroll down to the very bottom there is that exact same

very bottom there is that exact same code so I'm uh the same code for

code so I'm uh the same code for swapping two values A and B which I'm

swapping two values A and B which I'm claiming for now is correct now if I go

claiming for now is correct now if I go back up here what is main going to do

back up here what is main going to do for us main is really just meant to be a

for us main is really just meant to be a demonstration of the correctness of your

demonstration of the correctness of your algorithm so here I declare on line

algorithm so here I declare on line seven and eight two variables X and Y

seven and eight two variables X and Y being 1 and two arbitrarily respectively

being 1 and two arbitrarily respectively I then on line 10 just print out what

I then on line 10 just print out what the value of x is and Y is just so I can

the value of x is and Y is just so I can see it on the screen I then call the

see it on the screen I then call the swap function on line 11 and then I

swap function on line 11 and then I literally print the exact same thing

literally print the exact same thing again I print X and Y hopefully it'll

again I print X and Y hopefully it'll obviously be the opposite so I think

obviously be the opposite so I think logically swap is indeed correct let me

logically swap is indeed correct let me do make Swap and then do/ Swap and I

do make Swap and then do/ Swap and I should see X is one y is 2 and then

should see X is one y is 2 and then hopefully X is 2 Y is 1 enter but I

hopefully X is 2 Y is 1 enter but I don't and it did work in the sense that

don't and it did work in the sense that the code compiled the code ran so it's

the code compiled the code ran so it's not like some bug in that sense but

not like some bug in that sense but because I don't quite understand what's

because I don't quite understand what's going on underneath the hood at least as

going on underneath the hood at least as of right now or prior weeks this code

of right now or prior weeks this code here is indeed buggy in some way but

here is indeed buggy in some way but does anyone have an intuition perhaps

does anyone have an intuition perhaps based on today's discussion is to like

based on today's discussion is to like why this code while logically correct

why this code while logically correct clearly works in reality apparently does

clearly works in reality apparently does not work in

not work in C any intuition

yeah perfect and to summarize here's that term of art I prom

that term of art I prom when you call a function and pass in two

when you call a function and pass in two arguments Like A and B you're passing

arguments Like A and B you're passing those arguments by value so copies of

those arguments by value so copies of those values effectively and so when

those values effectively and so when swap is actually called here sorry when

swap is actually called here sorry when you pass in X and Y we call them A and B

you pass in X and Y we call them A and B but that's just a convention we could

but that's just a convention we could call the parameters anything we want

call the parameters anything we want what A and B are are indeed the values

what A and B are are indeed the values of X and Y respectively but copies of

of X and Y respectively but copies of the values so this code here is very

the values so this code here is very successfully in vs code 2 swapping the

successfully in vs code 2 swapping the values of a A and B but as you note

values of a A and B but as you note because I'm passing them in by value

because I'm passing them in by value literally one literally two and not by

literally one literally two and not by another term of art by reference AKA by

another term of art by reference AKA by their addresses swap has no capability

their addresses swap has no capability in C to go to those locations swap the

in C to go to those locations swap the actual locations just like we did

actual locations just like we did successfully in reality but I think we

successfully in reality but I think we really have the syntax already for

really have the syntax already for solving this if we consider that really

solving this if we consider that really this is just an issue of scope and we've

this is just an issue of scope and we've talked a bit about scope in the past

talked a bit about scope in the past whereby scope refers to the context in

whereby scope refers to the context in which a variable lives and generally

which a variable lives and generally I've claimed that a variable exists

I've claimed that a variable exists between the most recent curly braces and

between the most recent curly braces and that's pretty much true for the swap

that's pretty much true for the swap function because a and b i now claim

function because a and b i now claim again exist only in the context of these

again exist only in the context of these curly braces they have no effect on Main

curly braces they have no effect on Main up top which has different variables X

up top which has different variables X and Y but we can consider now what's

and Y but we can consider now what's really going on underneath the hood and

really going on underneath the hood and here's that same picture of memory as

here's that same picture of memory as we've seen in the past if we zoom in and

we've seen in the past if we zoom in and see on these little black chips this is

see on these little black chips this is a bunch of bites of memory if I create a

a bunch of bites of memory if I create a grid out of it just to kind of highlight

grid out of it just to kind of highlight that we can address each of these bites

that we can address each of these bites throw away the plastic circuit board and

throw away the plastic circuit board and focus only on those bites what's going

focus only on those bites what's going on underneath the hood when functions

on underneath the hood when functions are called in C which you've been doing

are called in C which you've been doing for weeks now well this rectangle of

for weeks now well this rectangle of memory if we kind of abstract it away

memory if we kind of abstract it away further is generally broken up into

further is generally broken up into different regions or segments like I

different regions or segments like I called them earlier and different things

called them earlier and different things get put in different parts of the

get put in different parts of the computer's memory and without getting

computer's memory and without getting too into the weeds when you double click

too into the weeds when you double click a program on your macro PC or when you

a program on your macro PC or when you do/ something on a Linux you are loading

do/ something on a Linux you are loading your machine code into the computer's

your machine code into the computer's memory from the computer's hard drive so

memory from the computer's hard drive so all the zeros and ones that compose

all the zeros and ones that compose Microsoft Word or Chrome or whatever are

Microsoft Word or Chrome or whatever are loaded into the computer's memory or RAM

loaded into the computer's memory or RAM and by convention it's put up top in the

and by convention it's put up top in the so-called machine code area and that's

so-called machine code area and that's how the CPU has access to them quickly

how the CPU has access to them quickly at that below that are what are be going

at that below that are what are be going to be our globals so Global variables

to be our globals so Global variables which we haven't used very much in C but

which we haven't used very much in C but you can declare them outside of main at

you can declare them outside of main at the very top of your files if you have

the very top of your files if you have globals they end up up there as well

globals they end up up there as well just FYI and then there's this big chunk

just FYI and then there's this big chunk of memory that we saw valgren mention

of memory that we saw valgren mention indirectly earlier called the Heap and

indirectly earlier called the Heap and it's kind of like Heap literally like

it's kind of like Heap literally like it's a heap of memory that you can use

it's a heap of memory that you can use as you see fit and the Heap is where

as you see fit and the Heap is where malok grabs memory from so initially

malok grabs memory from so initially there's nothing in the Heap it's just a

there's nothing in the Heap it's just a big chunk of free space anytime you call

big chunk of free space anytime you call malok Malo kind of carves out from the

malok Malo kind of carves out from the Heap area more and more bites and Malo

Heap area more and more bites and Malo keeps track of essentially which bites

keeps track of essentially which bites have already been allocated so initially

have already been allocated so initially it looks empty but different bites

it looks empty but different bites squares if you will keep getting

squares if you will keep getting requested again and again as a program

requested again and again as a program runs thanks to functions like malok and

runs thanks to functions like malok and it grows if you will conceptually down

it grows if you will conceptually down so the more and more memory you request

so the more and more memory you request from malok it starts up here but then

from malok it starts up here but then the next chunk you get is down here

the next chunk you get is down here conceptually the next chunk is down here

conceptually the next chunk is down here down here so it kind of fills the

down here so it kind of fills the available space in the computer's

available space in the computer's overall memory but there's this other

overall memory but there's this other chunk of memory called the stack and

chunk of memory called the stack and just like a stack of trays in like

just like a stack of trays in like annenburg or a cafeteria kind of grow

annenburg or a cafeteria kind of grow upward so does the stack of memory and

upward so does the stack of memory and it turns out the stack is where

it turns out the stack is where functions have variables and have

functions have variables and have arguments stored temporarily so whenever

arguments stored temporarily so whenever you call a function and it has variables

you call a function and it has variables inside of it or it has arguments there

inside of it or it has arguments there too this is the chunk of memory and the

too this is the chunk of memory and the computer's overall block of memory that

computer's overall block of memory that are used for functions but anytime you

are used for functions but anytime you call malok it's memory up here right at

call malok it's memory up here right at the end of the day they just had to pick

the end of the day they just had to pick a direction top bottom and technically

a direction top bottom and technically it's an artist rendition you could

it's an artist rendition you could Circle this thing around any orientation

Circle this thing around any orientation you want but you're just using a finite

you want but you're just using a finite amount of memory in this conventional

amount of memory in this conventional way Malo Starts Here functions start

way Malo Starts Here functions start here now you can kind of see where like

here now you can kind of see where like bad things can happen and indeed one of

bad things can happen and indeed one of the other reasons programs computers can

the other reasons programs computers can crash is if you ask for way too much

crash is if you ask for way too much memory from the Heap by calling malok

memory from the Heap by calling malok many many many times or if you call way

many many many times or if you call way too many functions or accidentally per

too many functions or accidentally per last week you recurse infinitely many

last week you recurse infinitely many times you might have a segmentation

times you might have a segmentation fault and that's because you're using

fault and that's because you're using too much stack memory so this is bound

too much stack memory so this is bound to be a problem eventually and the onus

to be a problem eventually and the onus is on the programmer to just minimize

is on the programmer to just minimize the probability of doing that and really

the probability of doing that and really avoid the possibility of doing that by

avoid the possibility of doing that by just checking return values like

just checking return values like checking if malok or get string return

checking if malok or get string return null because you can proactively with

null because you can proactively with conditionals make sure that these two

conditionals make sure that these two things do not Collide by just making

things do not Collide by just making sure that you get back non-null values

sure that you get back non-null values so let's consider the stack in the

so let's consider the stack in the context of Swap and what's really

context of Swap and what's really happening here and Carter if you

happening here and Carter if you wouldn't mind helping me animate the

wouldn't mind helping me animate the screen here when I call the main

screen here when I call the main function of any program it is allocated

function of any program it is allocated a slice of memory called a frame at the

a slice of memory called a frame at the bottom of this stack so if Carter you

bottom of this stack so if Carter you want to go ahead and uh Advance here

want to go ahead and uh Advance here here's like the First Slice of memory

here's like the First Slice of memory that will always be used by main whether

that will always be used by main whether it has commandline arguments or local

it has commandline arguments or local variables it just ends up here in memory

variables it just ends up here in memory suppose now per our swap. C program that

suppose now per our swap. C program that main calls swap well where does the

main calls swap well where does the memory for swap end up like right up

memory for swap end up like right up here so swap had two variable two

here so swap had two variable two arguments uh A and B and it also had a

arguments uh A and B and it also had a temporary variable so all of those end

temporary variable so all of those end up in here in memory and if you want to

up in here in memory and if you want to go ahead and Advance again Carter once

go ahead and Advance again Carter once swap is done executing whether it just

swap is done executing whether it just returns because there's no more lines of

returns because there's no more lines of code or you explicitly return this

code or you explicitly return this memory is just freed up automatically

memory is just freed up automatically you don't call free you don't undo malok

you don't call free you don't undo malok this just all happens automatically and

this just all happens automatically and has been since week one now technically

has been since week one now technically it's still there even though we've

it's still there even though we've removed it from the picture and there's

removed it from the picture and there's your first hint of garbage values right

your first hint of garbage values right like there's still zeros and ones there

like there's still zeros and ones there and they're left in the original the

and they're left in the original the previous configuration and so the reason

previous configuration and so the reason you get random values in the memory is

you get random values in the memory is because even though we hav't drawn swap

because even though we hav't drawn swap here there was stuff there a moment ago

here there was stuff there a moment ago it's going to be there the next time you

it's going to be there the next time you use that same memory now let's go ahead

use that same memory now let's go ahead and step through this a little more

and step through this a little more methodically main has two variables

methodically main has two variables called X and Y one and two so let's

called X and Y one and two so let's advance and represent X is one y is two

advance and represent X is one y is two taking up these two chunks of memory

taking up these two chunks of memory when we call swamp now swamp gets a new

when we call swamp now swamp gets a new slice of memory that then gives us three

slice of memory that then gives us three variables A and B technically the

variables A and B technically the arguments and temp so what happens well

arguments and temp so what happens well because functions automatically pass in

because functions automatically pass in values by value or rather pass in

values by value or rather pass in arguments by value X gets copied into a

arguments by value X gets copied into a y gets copied into B and then once we

y gets copied into B and then once we start executing the algorithm all the

start executing the algorithm all the watered glasses well what happens here

watered glasses well what happens here so if I execute the first line of code

so if I execute the first line of code temp equals a temp gets a copy of a what

temp equals a temp gets a copy of a what happens next a equals B so a takes on a

happens next a equals B so a takes on a copy of B and now we do the final swap

copy of B and now we do the final swap in the glasses b equals temp b gets a

in the glasses b equals temp b gets a copy of temp now we don't have to change

copy of temp now we don't have to change temp because it's essentially empty

temp because it's essentially empty although there's the garbage value one

although there's the garbage value one is always now going to be there until we

is always now going to be there until we reuse that memory the important thing

reuse that memory the important thing though is that a and b have been swapped

though is that a and b have been swapped but what obviously has not been swapped

but what obviously has not been swapped as is Manifest is when swap returns X

as is Manifest is when swap returns X and Y are untouched because copies

and Y are untouched because copies thereof were passed in so we need a

thereof were passed in so we need a solution to this problem and if we

solution to this problem and if we advance one more time if you don't mind

advance one more time if you don't mind let me step over here but then call you

let me step over here but then call you back in a second this code here is

back in a second this code here is logically correct this is what you did

logically correct this is what you did but this is now a detail of C you can't

but this is now a detail of C you can't just swap the things by value because

just swap the things by value because you're only changing it in the scope of

you're only changing it in the scope of the swap function function but I think

the swap function function but I think if we change it to this and add some

if we change it to this and add some annoying syntax we can solve the problem

annoying syntax we can solve the problem just like you can declare variables as

just like you can declare variables as storing addresses you can declare

storing addresses you can declare arguments to functions AKA parameters as

arguments to functions AKA parameters as taking addresses this new version of

taking addresses this new version of swap means that a shall be the address

swap means that a shall be the address of an integer B shall be the address of

of an integer B shall be the address of an integer and now it gets a little

an integer and now it gets a little cryptic here temp is the same because

cryptic here temp is the same because it's just an integer like it was in week

it's just an integer like it was in week one nothing special about temp but if

one nothing special about temp but if you want to get the value at a you do

you want to get the value at a you do star a and that goes to the address

star a and that goes to the address grabs the number one presumably if you

grabs the number one presumably if you want to change the value of a you go to

want to change the value of a you go to that address you follow the treasure map

that address you follow the treasure map to the other mailbox and you set it

to the other mailbox and you set it equal to whatever is at the value of B

equal to whatever is at the value of B you go to B as well last line you go to

you go to B as well last line you go to B now and change it to be whatever the

B now and change it to be whatever the temporary variable was which happened to

temporary variable was which happened to be the same as a so that's where the

be the same as a so that's where the final value gets swapped but here

final value gets swapped but here there's a lot more like crisscrossing

there's a lot more like crisscrossing metaphorically across the stage where

metaphorically across the stage where you're going to all of these different

you're going to all of these different addresses in the swap function to make

addresses in the swap function to make these changes so if we advance now to

these changes so if we advance now to the pictoral version of this here's the

the pictoral version of this here's the same story as before with Maine and X

same story as before with Maine and X and Y are 1 and two respectively when

and Y are 1 and two respectively when swap gets called now notice and I'll do

swap gets called now notice and I'll do it with arrows here a is effectively

it with arrows here a is effectively pointing to X B is effectively pointing

pointing to X B is effectively pointing to Y if we really get into the weeds

to Y if we really get into the weeds these are actually like addresses but

these are actually like addresses but who cares about the specifics it's

who cares about the specifics it's really just the concept here so now what

really just the concept here so now what happens in temp gets star a star a means

happens in temp gets star a star a means start at a and go there Follow the arrow

start at a and go there Follow the arrow if you will sort of shoots in ladder

if you will sort of shoots in ladder Style and then that's one so we put one

Style and then that's one so we put one and 10th all right star a equals star B

and 10th all right star a equals star B so let's do it from right to left star b

so let's do it from right to left star b means Follow the arrow it's two and then

means Follow the arrow it's two and then what do you do Follow the arrow it's now

what do you do Follow the arrow it's now two because you copy one to the other

two because you copy one to the other from right to left and then lastly star

from right to left and then lastly star b gets temp so start at B go to B and

b gets temp so start at B go to B and now store whatever the value is in temp

now store whatever the value is in temp so just by having this basic new syntax

so just by having this basic new syntax of like ampersands and stars and so

of like ampersands and stars and so forth we can actually now go to places

forth we can actually now go to places and circumvent what is otherwise a

and circumvent what is otherwise a feature of C that these variables are

feature of C that these variables are locally scoped but you can still access

locally scoped but you can still access things in other functions as well so

things in other functions as well so thank you so much for helping step

thank you so much for helping step through this so we now have a

through this so we now have a application of this that explains why

application of this that explains why now in this version of the C code this

now in this version of the C code this would actually now work so in fact let

would actually now work so in fact let me go back to my swap code here and let

me go back to my swap code here and let me change the function ever so slightly

me change the function ever so slightly in vs code so let me scroll down leaving

in vs code so let me scroll down leaving main the same and let me change swaps

main the same and let me change swaps prototype to taking in addresses let me

prototype to taking in addresses let me go to a here let me go to a here let me

go to a here let me go to a here let me go to B here and let me go to B here as

go to B here and let me go to B here as well but nothing else changes this

well but nothing else changes this change here in particular is enough of a

change here in particular is enough of a clue to see that means when you call

clue to see that means when you call Swap and pass into values I'm expecting

Swap and pass into values I'm expecting addresses now not integers but now that

addresses now not integers but now that I've made this change I do need to go up

I've made this change I do need to go up to Main and make one change does anyone

to Main and make one change does anyone have the intuition for what now need

have the intuition for what now need change in main so that I pass in X and Y

change in main so that I pass in X and Y by reference that is by address rather

by reference that is by address rather than than by value or copy oh yeah and

than than by value or copy oh yeah and [Music]

[Music] back so close so on the swap line it's

back so close so on the swap line it's not star that I want in front of the X

not star that I want in front of the X and the Y it's

instead what's the other one it's the Amper sand why because if I want to

Amper sand why because if I want to enable swap to go somewhere just like

enable swap to go somewhere just like Carter and I played this game with the

Carter and I played this game with the mailboxes I need to inform swap of the

mailboxes I need to inform swap of the address of X and the address of Y and

address of X and the address of Y and again per the beginning of today's class

again per the beginning of today's class Ampersand is the text via which we do

Ampersand is the text via which we do that so I add an ampersand here to get

that so I add an ampersand here to get the address of X Ampersand here to get

the address of X Ampersand here to get the address of Y and now this code lines

the address of Y and now this code lines up with the picture that Carter just

up with the picture that Carter just helped us walk through and so when I run

helped us walk through and so when I run make swap here I have a mistake oh what

make swap here I have a mistake oh what did I do wrong not intentional but I

did I do wrong not intentional but I guess worth pointing out I screwed up

guess worth pointing out I screwed up here it doesn't like Ampersand

here it doesn't like Ampersand X because of something on line three

X because of something on line three which is way early in the code what did

which is way early in the code what did I screw up yeah in the middle

yeah so this is why we you should not copy paste even though it's necessary

copy paste even though it's necessary for things like function prototypes if I

for things like function prototypes if I change swap at the bottom I need to

change swap at the bottom I need to change its prototype so let me add the

change its prototype so let me add the star there add the star there or just

star there add the star there or just recopy paste it at the top of the file

recopy paste it at the top of the file now let me do make swap again let me now

now let me do make swap again let me now do do/ Swap and I should now see X is

do do/ Swap and I should now see X is one y is 2 and hopefully X is 2 Y is 1

one y is 2 and hopefully X is 2 Y is 1 which I now do so the logic is the same

which I now do so the logic is the same the algorithm is the same all the weak

the algorithm is the same all the weak zero stuff is the same except now and

zero stuff is the same except now and four you just have a bit more

four you just have a bit more expressiveness via which you can tell

expressiveness via which you can tell the computer exactly what you want to

the computer exactly what you want to manipulate and how any questions then on

manipulate and how any questions then on this technique

this technique here no all right well when we fix this

here no all right well when we fix this there's still going to be problems and

there's still going to be problems and just so you've seen some terms of art

just so you've seen some terms of art here this is bad whenever you have two

here this is bad whenever you have two arrows pointing at one another certainly

arrows pointing at one another certainly if you might use and reuse more and more

if you might use and reuse more and more memory and it turns out there's some

memory and it turns out there's some terms of art that might suddenly now

terms of art that might suddenly now make sense especially if you've

make sense especially if you've programmed before bad things Can Happen

programmed before bad things Can Happen by this design but there's really only

by this design but there's really only this kind of design because it's a

this kind of design because it's a finite amount of memory so at some point

finite amount of memory so at some point bad things are going to happen no matter

bad things are going to happen no matter what if a computer runs out of memory so

what if a computer runs out of memory so it's not that this was a poor decision

it's not that this was a poor decision it's just sort of a necessary one given

it's just sort of a necessary one given finite amounts of memory in a computer

finite amounts of memory in a computer but a heap overflow so to speak is when

but a heap overflow so to speak is when you actually overflow the Heap and touch

you actually overflow the Heap and touch memory that you shouldn't up there stack

memory that you shouldn't up there stack Overflow is when you somehow overflow

Overflow is when you somehow overflow the stack and touch memory that you

the stack and touch memory that you shouldn't down there so with that said

shouldn't down there so with that said these are really just problems that can

these are really just problems that can happen and there specific incarnations

happen and there specific incarnations of what are generally called buffer

of what are generally called buffer overflows a buffer like in the YouTube

overflows a buffer like in the YouTube sense is just like a chunk of memory

sense is just like a chunk of memory that in the case of YouTube stores like

that in the case of YouTube stores like the next few seconds or minutes of video

the next few seconds or minutes of video but generally speaking a buffer is just

but generally speaking a buffer is just a chunk of memory that the computer is

a chunk of memory that the computer is using for some purpose be it the stack

using for some purpose be it the stack be it the Heap be it an array in the

be it the Heap be it an array in the computer and so buffer overflows are

computer and so buffer overflows are what happens when you just have logical

what happens when you just have logical bugs in your code but with these

bugs in your code but with these Primitives now in mind we wanted to

Primitives now in mind we wanted to conclude with a final Revelations and

conclude with a final Revelations and that's how some functions like these

that's how some functions like these here work the other thing in the cs50

here work the other thing in the cs50 library besides the type def for quote

library besides the type def for quote unquote string is of course all of these

unquote string is of course all of these functions and we give you these

functions and we give you these functions because honestly in C it is

functions because honestly in C it is hard it's annoying it's painful it's

hard it's annoying it's painful it's difficult to get user input correctly

difficult to get user input correctly it's very easy when you don't know how

it's very easy when you don't know how much how much the humans going to type

much how much the humans going to type to write buggy code when it comes to it

to write buggy code when it comes to it and indeed it's really hard to store it

and indeed it's really hard to store it correctly without accidentally having

correctly without accidentally having some kind of buffer overflow so for

some kind of buffer overflow so for instance let me show you a program here

instance let me show you a program here I'm going to go ahead and write this one

I'm going to go ahead and write this one from scratch so let me go ahead and open

from scratch so let me go ahead and open a file called get. C where I'm going to

a file called get. C where I'm going to go ahead and mimic the idea of getting

go ahead and mimic the idea of getting integers manually without the cs50

integers manually without the cs50 library so I'm going to include standard

library so I'm going to include standard io. only I'm going to Define main as not

io. only I'm going to Define main as not taking any command line arguments and

taking any command line arguments and then I'm going to do something like this

then I'm going to do something like this give me a variable x with no value yet

give me a variable x with no value yet and normally I would do something like

and normally I would do something like get int but let me take that away no

get int but let me take that away no more training wheels for get int either

more training wheels for get int either so let me just Define the int X let me

so let me just Define the int X let me then just print out something like uh a

then just print out something like uh a prompt and I'll just do x c just to make

prompt and I'll just do x c just to make it obvious to the human what we're

it obvious to the human what we're waiting for and now I'm going to use a

waiting for and now I'm going to use a builtin c function to get user input I'm

builtin c function to get user input I'm going to call a function called scan F

going to call a function called scan F which sort of scans the user's keyboard

which sort of scans the user's keyboard for input I'm going to scan it for an

for input I'm going to scan it for an integer so just like print F I'm going

integer so just like print F I'm going to use percent I because I expect an INT

to use percent I because I expect an INT and then I want to tell scanf where to

and then I want to tell scanf where to put the human's integer from the

put the human's integer from the keyboard it is not correct though to say

keyboard it is not correct though to say x because if I say x I run into the same

x because if I say x I run into the same swap problem scanf no function can

swap problem scanf no function can change the value of X unless I pass it

change the value of X unless I pass it not by value but

not by value but by reference so we're back to our

by reference so we're back to our Ampersand friend and now it has like a a

Ampersand friend and now it has like a a treasure map to the actual location of X

treasure map to the actual location of X and can therefore change it and so now

and can therefore change it and so now at the very end of this program let me

at the very end of this program let me do something simple like let's just go

do something simple like let's just go ahead and print out with print F uh the

ahead and print out with print F uh the value of x using percent I as always

value of x using percent I as always plugging in X not Ampersand X this is

plugging in X not Ampersand X this is now week one stuff I want to print the

now week one stuff I want to print the actual integer value of x so the only

actual integer value of x so the only change here is that instead of using get

change here is that instead of using get in I'm now using this new function that

in I'm now using this new function that as of today exists called scanf so let

as of today exists called scanf so let me go ahead and run get make get to

me go ahead and run get make get to create this program doget and let's go

create this program doget and let's go ahead and type in a value for x 50 enter

ahead and type in a value for x 50 enter and it just works so it turns out get in

and it just works so it turns out get in is pretty simple to implement however

is pretty simple to implement however notice what does not work if I type in

notice what does not work if I type in cat for instance cat gets converted to

cat for instance cat gets converted to zero and meanwhile get in recall will

zero and meanwhile get in recall will reprompt the user if a human not type an

reprompt the user if a human not type an actual integer you get automatically

actual integer you get automatically reprompt so that's one of the features

reprompt so that's one of the features we for cs50 added to get in just to make

we for cs50 added to get in just to make your programs more user friendly but

your programs more user friendly but otherwise get in is pretty

otherwise get in is pretty straightforward to re-implement using

straightforward to re-implement using scanf unfortunately that's not true for

scanf unfortunately that's not true for Strings because how do you know when you

Strings because how do you know when you write your code what word the human's

write your code what word the human's going to eventually type in how long

going to eventually type in how long they're greeting like high is if it's uh

they're greeting like high is if it's uh if their name is David or Carter or

if their name is David or Carter or anything else you just don't know in

anything else you just don't know in advance how much memory you need so how

advance how much memory you need so how might we do this in with string well let

might we do this in with string well let me go ahead and declare a string s

me go ahead and declare a string s although you know what there's no cs50

although you know what there's no cs50 Library so we do Char star s today

Library so we do Char star s today instead and that gives me not a string

instead and that gives me not a string per se but a pointer that will Point

per se but a pointer that will Point presumably to a string ideally I would

presumably to a string ideally I would use this get string but again we've

use this get string but again we've taken that training wheel away so now

taken that training wheel away so now that I have a pointer s suppose I prompt

that I have a pointer s suppose I prompt the human for a value for S just like

the human for a value for S just like before let me use scan F now and tell

before let me use scan F now and tell the user that I expect to read a string

the user that I expect to read a string percent s from the keyboard

percent s from the keyboard uh and store it in s now this is subtle

uh and store it in s now this is subtle I don't technically need an Amper sand

I don't technically need an Amper sand here even though I did for an INT and I

here even though I did for an INT and I would for a float and a double and a

would for a float and a double and a long and a bull and a Char why do I not

long and a bull and a Char why do I not need an Amper sand in this story to pass

need an Amper sand in this story to pass by

by reference because s is it's already an

reference because s is it's already an address again strings are just special

address again strings are just special strings now are always addresses so you

strings now are always addresses so you don't need to additionally add an Amper

don't need to additionally add an Amper sand here that's the only subtle

sand here that's the only subtle difference here but now if I go ahead

difference here but now if I go ahead and print out at the very end what the

and print out at the very end what the value of s is using percent S as before

value of s is using percent S as before this program looks like it's almost the

this program looks like it's almost the same as the int version but let's do

same as the int version but let's do make get and okay so this is not good

make get and okay so this is not good all right so it doesn't like an

all right so it doesn't like an initialized value so let me make it

initialized value so let me make it happy I said earlier to always

happy I said earlier to always initialize my variable so let's

initialize my variable so let's initialize it to null so that at least

initialize it to null so that at least something is there that's your good

something is there that's your good default value nowadays now if I do doget

default value nowadays now if I do doget now we're good and let me type in

now we're good and let me type in something like cat

something like cat okay cat is not X well let me try

okay cat is not X well let me try another word maybe it's just cat is

another word maybe it's just cat is wrong dog okay let me try David it just

wrong dog okay let me try David it just doesn't seem to be working moreover it's

doesn't seem to be working moreover it's printing it as a zero what logically

printing it as a zero what logically though is the bug here scanf worked a

though is the bug here scanf worked a moment ago for integers but it's not

moment ago for integers but it's not working for Strings and it seems to be

working for Strings and it seems to be forgetting C A it's forgetting dog it's

forgetting C A it's forgetting dog it's forgetting

forgetting David

David why what's Happening

why what's Happening Here think back to our yellow pictures

Here think back to our yellow pictures of

of memory when I

memory when I yeah it might be reading just the null

yeah it might be reading just the null itself because s is being initialized to

itself because s is being initialized to null and what step have I forgotten from

null and what step have I forgotten from just like a few minutes ago what did I

just like a few minutes ago what did I not actually request of the computer

not actually request of the computer like actual memory to store the C A the

like actual memory to store the C A the dog the d v d right there's nowhere have

dog the d v d right there's nowhere have I asked the computer for some amount of

I asked the computer for some amount of memory and so technically it might be

memory and so technically it might be reading it into some garbage location

reading it into some garbage location and that's really the problem here S is

and that's really the problem here S is initialized to null now and so in fact

initialized to null now and so in fact it is printing zero as null but I'm not

it is printing zero as null but I'm not seeing any of the other letters because

seeing any of the other letters because there was nowhere to put them C A do D

there was nowhere to put them C A do D ID because I didn't ask for three bytes

ID because I didn't ask for three bytes four bytes five bytes 100 bytes there's

four bytes five bytes 100 bytes there's no use of malok there's no use of an

no use of malok there's no use of an array there's no memory allocated for

array there's no memory allocated for anything other than the pointer itself

anything other than the pointer itself and this is where honestly life gets

and this is where honestly life gets hard with scanf like I could solve this

hard with scanf like I could solve this problem in a couple of ways let me go

problem in a couple of ways let me go ahead and do this instead of declaring s

ahead and do this instead of declaring s to be a pointer let me declare s to

to be a pointer let me declare s to actually be a an array of four chars and

actually be a an array of four chars and now let me go ahead and recompile the

now let me go ahead and recompile the code so make get doget and I'll type in

code so make get doget and I'll type in cat now that now works why well I'm

cat now that now works why well I'm allocating an explicit array of size

allocating an explicit array of size four enough for a one two three letters

four enough for a one two three letters plus a null character here's where to

plus a null character here's where to someone's question earlier it turns out

someone's question earlier it turns out then in some context you can treat

then in some context you can treat arrays as though they are Pointers

arrays as though they are Pointers themselves see we'll sort of do the

themselves see we'll sort of do the conversion for you but for now just

conversion for you but for now just assume that s is just an array of size

assume that s is just an array of size four and if you pass it into to scanf

four and if you pass it into to scanf that's like a treasure map that leads to

that's like a treasure map that leads to those four bytes so scanf can now

those four bytes so scanf can now successfully fill it with c a d o but

successfully fill it with c a d o but but let's try this again let's type in

but let's try this again let's type in David and here okay we got lucky but I

David and here okay we got lucky but I technically touched memory that I should

technically touched memory that I should not and in fact if I typed in a long

not and in fact if I typed in a long enough string and I don't think I could

enough string and I don't think I could do it very easily like by without typing

do it very easily like by without typing this thousands or hundreds of times

this thousands or hundreds of times still okay but you'll notice that it's

still okay but you'll notice that it's forgotten the rest of it now right so

forgotten the rest of it now right so somewhere we went beyond the boundary of

somewhere we went beyond the boundary of the array and we just don't have enough

the array and we just don't have enough storage space for that entire thing so

storage space for that entire thing so what do you do in your program if you

what do you do in your program if you don't know how long the person's name or

don't know how long the person's name or the the the animal name is going to be

the the the animal name is going to be what do you do 40 400 4,000 40,000 like

what do you do 40 400 4,000 40,000 like at some point you have to draw a line in

at some point you have to draw a line in the sand and that's why like getting

the sand and that's why like getting user input is so annoying in a language

user input is so annoying in a language like C and that's why get string exists

like C and that's why get string exists what we do if you're curious is we look

what we do if you're curious is we look at the user's input and we take baby

at the user's input and we take baby steps we look at it one character at a

steps we look at it one character at a time and every time we see another

time and every time we see another character we actually call malok again

character we actually call malok again and say no I need more than one bite I

and say no I need more than one bite I need two oh wait they typed in three

need two oh wait they typed in three letters I need three instead of two oh I

letters I need three instead of two oh I need four instead of two and we have

need four instead of two and we have this crazy loop essentially that keeps

this crazy loop essentially that keeps asking for more and more memory but by

asking for more and more memory but by taking baby steps and honestly if you

taking baby steps and honestly if you all had to do that in week one my God

all had to do that in week one my God like we couldn't even write hello world

like we couldn't even write hello world anymore and so that's why these training

anymore and so that's why these training wheels exist at least early on and

wheels exist at least early on and that's why in higher level languages

that's why in higher level languages like in uh python you don't have to do

like in uh python you don't have to do this at all it just works as you'd

this at all it just works as you'd expect so what more can we do well

expect so what more can we do well you'll see in problems set for this

you'll see in problems set for this coming week if I open up an example like

coming week if I open up an example like this phone book. C you'll see that you

this phone book. C you'll see that you can manipulate files now that you have a

can manipulate files now that you have a vocabulary for pointers it's going to be

vocabulary for pointers it's going to be new quickly but here we have an example

new quickly but here we have an example of how I have a program using some

of how I have a program using some familiar libraries here but as I claim

familiar libraries here but as I claim in my comment this saves names and

in my comment this saves names and numbers to a CSV file all of my examples

numbers to a CSV file all of my examples thus far I type in some words I type in

thus far I type in some words I type in some names and some phone numbers and

some names and some phone numbers and they disappear because we only store

they disappear because we only store them in memory but if you want to store

them in memory but if you want to store data in like a CSV file comma separated

data in like a CSV file comma separated values which is like a simple

values which is like a simple spreadsheet like Excel and apple numbers

spreadsheet like Excel and apple numbers and Google Sheets can open you can

and Google Sheets can open you can actually do this yourself so just as a

actually do this yourself so just as a teaser for this week here on line n I'm

teaser for this week here on line n I'm using a new data type not a cs50 thing

using a new data type not a cs50 thing this is a c thing called file but if you

this is a c thing called file but if you want to manipulate files you need to use

want to manipulate files you need to use addresses that is pointers so here is me

addresses that is pointers so here is me creating a variable called file that's

creating a variable called file that's going to point to an actual file on the

going to point to an actual file on the hard drive on the server or your Mac or

hard drive on the server or your Mac or PC fop is going to be a new function

PC fop is going to be a new function you'll use that will open open a file

you'll use that will open open a file and it will return effectively a pointer

and it will return effectively a pointer there to in memory the file name I want

there to in memory the file name I want to open is phonebook .csv and in this

to open is phonebook .csv and in this example it's going to be uh a pen mode

example it's going to be uh a pen mode it will keep allowing me to add more and

it will keep allowing me to add more and more names and numbers to this file

more names and numbers to this file here's some old get string stuff because

here's some old get string stuff because I'm not going to reinvent get string

I'm not going to reinvent get string with scan F but down here is a slightly

with scan F but down here is a slightly new function it's not print F but

new function it's not print F but fprintf and it turns out it's very easy

fprintf and it turns out it's very easy to print things not to the screen but to

to print things not to the screen but to a file with fprintf and it takes an

a file with fprintf and it takes an additional argument instead of starting

additional argument instead of starting with the quoted string you'll have to

with the quoted string you'll have to like say what file you want to write to

like say what file you want to write to and fr print F we'll figure out how to

and fr print F we'll figure out how to get the uh the bits into that file

get the uh the bits into that file passing in something like name comma

passing in something like name comma number so if I run this somewhat quickly

number so if I run this somewhat quickly here let me do this let me pre-create a

here let me do this let me pre-create a file called uh phonebook do CSV and in

file called uh phonebook do CSV and in phone book. CSV I'm going to create a

phone book. CSV I'm going to create a temporary row here name comma number

temporary row here name comma number just so that there's something in this

just so that there's something in this file and now let me go ahead and do this

file and now let me go ahead and do this and split my screen here if I have

and split my screen here if I have phonebook.com

phonebook.com and phone book. C on the left let me

and phone book. C on the left let me compile make phonebook which is the C

compile make phonebook which is the C version phonebook and now I'm prompted

version phonebook and now I'm prompted for a name and a number so I'll type in

for a name and a number so I'll type in David and then for instance + one

David and then for instance + one 949 uh what is it 468 275 o enter oh

949 uh what is it 468 275 o enter oh damn it bug uh pretend that didn't

damn it bug uh pretend that didn't happen I forgot to hit enter in the file

happen I forgot to hit enter in the file so let's do this again if I run the

so let's do this again if I run the program Again David and plus one 94 9

program Again David and plus one 94 9 468 2750 enter it's been saved now to

468 2750 enter it's been saved now to the file and if I close this file and I

the file and if I close this file and I reopen code of phonebook.com

or the like and I've actually created an actual CSV file uh if you're smiling

actual CSV file uh if you're smiling because I keep repeating my phone number

because I keep repeating my phone number out loud I would encourage you to call

out loud I would encourage you to call or text that number sometime it might

or text that number sometime it might very well well be a an Easter egg of

very well well be a an Easter egg of source but via these functions here do

source but via these functions here do we have now the ability to write files

we have now the ability to write files uh input and output and among the goals

uh input and output and among the goals then for this week as we'll see are to

then for this week as we'll see are to actually play with images in the spirit

actually play with images in the spirit of something like Instagram filters or

of something like Instagram filters or the like and we'll introduce you for

the like and we'll introduce you for instance to a file format called bmps

instance to a file format called bmps which to come full circle to the start

which to come full circle to the start of class are just maps of bits but more

of class are just maps of bits but more than just single bits for white and

than just single bits for white and black but rather colorful patterns as

black but rather colorful patterns as well and we'll give you images like this

well and we'll give you images like this of the week's Bridge here across the

of the week's Bridge here across the river at Harvard and you've run

river at Harvard and you've run after writing your own code in C and

after writing your own code in C and understanding how the data stored in the

understanding how the data stored in the computer's memory you'll be able to

computer's memory you'll be able to apply your own Instagram like filters to

apply your own Instagram like filters to make things uh grayscale instead or

make things uh grayscale instead or sepia in this case you can even flip the

sepia in this case you can even flip the bits around so that the thing is a

bits around so that the thing is a mirror image you can blur things further

mirror image you can blur things further or if you really are feeling more

or if you really are feeling more comfortable you can even write code that

comfortable you can even write code that finds the edges of the image and creates

finds the edges of the image and creates works of art like these so all that in

works of art like these so all that in more in problem set four we will see you

more in problem set four we will see you next

all right this is cs50 and this is week five which is going to be our last week

five which is going to be our last week in see uh but what that means is that

in see uh but what that means is that we'll have

we'll have okay

okay but with this week with last week and

but with this week with last week and really all of the weeks prior have you

really all of the weeks prior have you been hopefully acquiring if slowly and

been hopefully acquiring if slowly and with some challenge like some

with some challenge like some fundamental building blocks that are

fundamental building blocks that are still going to underly everything we

still going to underly everything we continue doing in the semester even as

continue doing in the semester even as we transition to so-called higher level

we transition to so-called higher level languages next week indeed we'll

languages next week indeed we'll introduce python a very popular language

introduce python a very popular language that does not have pointers does not

that does not have pointers does not have memory management at this very low

have memory management at this very low level but that's really just because

level but that's really just because someone else wrote the code that will do

someone else wrote the code that will do that for you and it's going to make our

that for you and it's going to make our lives easier because it means when you

lives easier because it means when you want to solve a problem concept up here

want to solve a problem concept up here to just get real work done or build

to just get real work done or build something amazing you don't have to

something amazing you don't have to really get into the same weeds as we

really get into the same weeds as we have been deliberately this week and now

have been deliberately this week and now last but the goal ultimately is that you

last but the goal ultimately is that you better understand and can better harness

better understand and can better harness than all that a computer can do even in

than all that a computer can do even in those higher level languages So today

those higher level languages So today we're going to focus particularly on

we're going to focus particularly on data structures how you might structure

data structures how you might structure your data in memory which really amounts

your data in memory which really amounts to building things digitally stitching

to building things digitally stitching together ideas and Concepts in memory

together ideas and Concepts in memory using this new building block from last

using this new building block from last week which of course are these pointers

week which of course are these pointers pointers allow you to store addresses in

pointers allow you to store addresses in memory like in variables but with those

memory like in variables but with those simple addresses we can sort of leave

simple addresses we can sort of leave these breadcrumbs we can point from here

these breadcrumbs we can point from here to there and we can conceptually stitch

to there and we can conceptually stitch pieces of data together but there's

pieces of data together but there's going to be different ways of doing that

going to be different ways of doing that and today we'll focus first on what's

and today we'll focus first on what's generally known as an abstract data type

generally known as an abstract data type and similar to a type in C more

and similar to a type in C more generally it does actually have some

generally it does actually have some properties in it but the underlying

properties in it but the underlying implementation details of an abstract

implementation details of an abstract data type are ultimately up to you that

data type are ultimately up to you that is to say an abstract data type can

is to say an abstract data type can represent one thing and can do something

represent one thing and can do something but how you implement it allows you some

but how you implement it allows you some discretion underneath the hood so for

discretion underneath the hood so for instance in the world of computer

instance in the world of computer science a q is actually a technical term

science a q is actually a technical term this is a type of data structure that we

this is a type of data structure that we could theoretically build in code in C

could theoretically build in code in C or really any other language but a q has

or really any other language but a q has properties just like cu's in the real

properties just like cu's in the real world for instance if you've ever lined

world for instance if you've ever lined up for something to get food in a uh and

up for something to get food in a uh and get food in a restaurant or go into a

get food in a restaurant or go into a store or wait for the airport to clear

store or wait for the airport to clear you well you've lined up in a queue Q

you well you've lined up in a queue Q being some sort of line but what's

being some sort of line but what's noteworthy about Q's are specific

noteworthy about Q's are specific properties they are first in first out

properties they are first in first out data structures either virtually or in

data structures either virtually or in the human world which is to say the

the human world which is to say the first person in the line should ideally

first person in the line should ideally be served first at the restaurant or the

be served first at the restaurant or the first person in the line should get

first person in the line should get through airport security first by

through airport security first by contrast if it weren't first in first

contrast if it weren't first in first out you can imagine how frustrated you

out you can imagine how frustrated you would be if you have this sort of

would be if you have this sort of inherent unfairness in fact if you've

inherent unfairness in fact if you've ever been in line at a store a

ever been in line at a store a supermarket or the like and all of a

supermarket or the like and all of a sudden they maybe open a new line and

sudden they maybe open a new line and now the person behind you gets to kind

now the person behind you gets to kind of cut ahead and go forward that's

of cut ahead and go forward that's because they've broken the concept of

because they've broken the concept of the cube so it has this inherent

the cube so it has this inherent potential for unfairness unless you

potential for unfairness unless you maintain this first in first out

maintain this first in first out property this would be true too for like

property this would be true too for like a to-do list just for productivity sake

a to-do list just for productivity sake if you're in the habit on paper or

if you're in the habit on paper or virtually making a to-do list ideally

virtually making a to-do list ideally you probably want to go through that

you probably want to go through that list top to bottom so that you actually

list top to bottom so that you actually get the first stuff you thought of done

get the first stuff you thought of done first rather than always focusing on

first rather than always focusing on your most recent thought now within the

your most recent thought now within the world of Q's there's generally two

world of Q's there's generally two operations two functions if you will

operations two functions if you will that any Q would have either in the real

that any Q would have either in the real world or in the virtual NQ is usually

world or in the virtual NQ is usually the technical term to mean adding

the technical term to mean adding something to a q but specifically it

something to a q but specifically it means adding it to the end of the que so

means adding it to the end of the que so that someone isn't cutting in line for

that someone isn't cutting in line for instance to go up front and then DQ is

instance to go up front and then DQ is just the opposite when it's time for the

just the opposite when it's time for the first person in line to be served the

first person in line to be served the time for the first person in line to go

time for the first person in line to go through security they are dced so to

through security they are dced so to speak so technical concept ultimately as

speak so technical concept ultimately as it's implemented in code but it's really

it's implemented in code but it's really just a real world concept and these are

just a real world concept and these are in contrast to another abstract data

in contrast to another abstract data type that we might call a stack and a

type that we might call a stack and a stack much like a stack of trays in the

stack much like a stack of trays in the cafeteria has sort of fundamentally

cafeteria has sort of fundamentally different properties you can still add

different properties you can still add and remove things from them but consider

and remove things from them but consider what happens whenever they clean all the

what happens whenever they clean all the trays in the cafeteria or the dining

trays in the cafeteria or the dining hall they put one of the trays down here

hall they put one of the trays down here and then the next one on top of it and

and then the next one on top of it and then the next one on top of it it it and

then the next one on top of it it it and so forth but of course which tray do you

so forth but of course which tray do you presume presumably take as a user of

presume presumably take as a user of that physical

that physical stack the top one presumably right like

stack the top one presumably right like you're not going to fuss down here and

you're not going to fuss down here and try to pull one out and so that would

try to pull one out and so that would seem to have an opposite property lifo

seem to have an opposite property lifo last in first out is what characterizes

last in first out is what characterizes something like a stack and that just

something like a stack and that just makes sense certainly in the physical

makes sense certainly in the physical world of stacking all of those cafeteria

world of stacking all of those cafeteria trays because it just makes more sense

trays because it just makes more sense to grab the most recently added one the

to grab the most recently added one the last added one first and at least in

last added one first and at least in that context the trays don't necessarily

that context the trays don't necessarily care what order they're using used in

care what order they're using used in but even then you could imagine that

but even then you could imagine that maybe there's some old dirty nasty tray

maybe there's some old dirty nasty tray at the very bottom that like never gets

at the very bottom that like never gets used because you're constantly

used because you're constantly replenishing the stack so there might

replenishing the stack so there might very well be side effects of these kinds

very well be side effects of these kinds of features um you might be familiar

of features um you might be familiar with using Gmail for instance or really

with using Gmail for instance or really any email program what you're looking at

any email program what you're looking at in your inbox is technically a stack at

in your inbox is technically a stack at least if you've left the defaults

least if you've left the defaults configured why you get a new email where

configured why you get a new email where does it end up not like five pages of

does it end up not like five pages of emails later presumably right at the top

emails later presumably right at the top and the next one's right at the top

and the next one's right at the top right at the the top right at the top

right at the the top right at the top and so if you're like me you're guilty

and so if you're like me you're guilty of eventually losing track of some

of eventually losing track of some emails why because you've pushed so many

emails why because you've pushed so many more emails onto the stack that you sort

more emails onto the stack that you sort of lose track of the things you got

of lose track of the things you got earliest last in first out though is

earliest last in first out though is maintained the most recent email you get

maintained the most recent email you get might very well be the first one you

might very well be the first one you reply to but that's not necessarily good

reply to but that's not necessarily good for responsiveness to everyone else out

for responsiveness to everyone else out there uh similarly if you store like all

there uh similarly if you store like all of your sweaters in a stack like this uh

of your sweaters in a stack like this uh the uh like a pile of black ones below

the uh like a pile of black ones below which is a red and then a blue stack

which is a red and then a blue stack might be perfectly fine for keeping

might be perfectly fine for keeping things organized it's sort of the sane

things organized it's sort of the sane way to do it if you just have a shelf in

way to do it if you just have a shelf in your dorm room or home but what's going

your dorm room or home but what's going to be a side effect of using a stack to

to be a side effect of using a stack to store your sweaters if they're these in

store your sweaters if they're these in this way as opposed to a queue

this way as opposed to a queue yeah it's harder to get the red and blue

yeah it's harder to get the red and blue one so presumably you're going to much

one so presumably you're going to much more often wear for instance if you will

more often wear for instance if you will black instead there now the operations

black instead there now the operations for adding things to a stack are similar

for adding things to a stack are similar in spirit but just different vocabulary

in spirit but just different vocabulary you push something on top of a stack um

you push something on top of a stack um even though it's more like in the tray

even though it's more like in the tray world you sort of place it or rest it

world you sort of place it or rest it but pushing means adding something to

but pushing means adding something to the top of the stack and popping means

the top of the stack and popping means some removing something also from the

some removing something also from the top of the stack so it's not a matter of

top of the stack so it's not a matter of enqing at the end and deqing at the

enqing at the end and deqing at the beginning with a stack everything's

beginning with a stack everything's happening on top you're pushing onto the

happening on top you're pushing onto the top and then popping off of the top now

top and then popping off of the top now when it comes to actual code how might

when it comes to actual code how might we Implement something like this well

we Implement something like this well let's just focus on really how you might

let's just focus on really how you might implement the data structure itself and

implement the data structure itself and we won't Implement any functions you

we won't Implement any functions you might implement the notion of a stack

might implement the notion of a stack like this we've seen type def before it

like this we've seen type def before it just means Define a new type of my own

just means Define a new type of my own struct means here comes a structure AKA

struct means here comes a structure AKA a data structure of one or more

a data structure of one or more variables within and let's suppose like

variables within and let's suppose like last time we've had we defined already

last time we've had we defined already like a person data type using a separate

like a person data type using a separate type Def and every person has like a

type Def and every person has like a name and a number or whatever let me

name and a number or whatever let me just stipulate that person exists

just stipulate that person exists already so here you might have to

already so here you might have to implement a stack an array called people

implement a stack an array called people of some capacity maybe it's an an array

of some capacity maybe it's an an array of size 10 or 100 or whatever this is a

of size 10 or 100 or whatever this is a constant here in this context capacity

constant here in this context capacity and every element in that array is a

and every element in that array is a person structure and I now have this to

person structure and I now have this to size now this almost seems like a

size now this almost seems like a synonym for capacity but maybe

synonym for capacity but maybe intuitively anyone want to propose why

intuitively anyone want to propose why I'm apparently maintaining separately

I'm apparently maintaining separately the capacity of the stack but also the

the capacity of the stack but also the size of the stack why these two

size of the stack why these two distinctions

distinctions here yeah

perfect the capacity is how high the stack could be like how much room is

stack could be like how much room is there for those sweatshirts in my closet

there for those sweatshirts in my closet for instance whereas size is just

for instance whereas size is just literally at this moment in time how

literally at this moment in time how many sweatshirts are in the stack it's

many sweatshirts are in the stack it's either capacity or fewer presumably in

either capacity or fewer presumably in total there so what is capacity well we

total there so what is capacity well we could implement this in you know perhaps

could implement this in you know perhaps a familiar way I might just Define a

a familiar way I might just Define a const somewhere else in my code of type

const somewhere else in my code of type int that just defines it to be capacity

int that just defines it to be capacity 50 but what perhaps is going to be the

50 but what perhaps is going to be the downside of implementing a stack in this

downside of implementing a stack in this way of how using an array inside here

way of how using an array inside here like what's a downside now of

like what's a downside now of implementing a stack using an array and

implementing a stack using an array and this size variable

this size variable within what's a caveat here

within what's a caveat here perhaps any hands

yeah okay so it's going to be harder to reach elements that aren't the last one

reach elements that aren't the last one that is the most recently added one so

that is the most recently added one so there could be some sweatshirts so to

there could be some sweatshirts so to speak way down below so we've seen that

speak way down below so we've seen that before too but at some point too a

before too but at some point too a limitation of this design is is what

limitation of this design is is what it's going to be finite right I can

it's going to be finite right I can maximally fit in this example 50

maximally fit in this example 50 sweatshirts or 50 emails or 50 cafeteria

sweatshirts or 50 emails or 50 cafeteria Trace which is fine but at least it's

Trace which is fine but at least it's indeed finite and at least in the

indeed finite and at least in the computer's memory it might be nice to

computer's memory it might be nice to use more and more and more maybe as more

use more and more and more maybe as more things are getting added to the computer

things are getting added to the computer so maybe I make this 500 or heck why

so maybe I make this 500 or heck why don't I make it 5,000 or 50,000 well

don't I make it 5,000 or 50,000 well what's the tradeoff there if I want to

what's the tradeoff there if I want to have enough room to grow seems like I

have enough room to grow seems like I should just crank up the value of

should just crank up the value of capacity endlessly but why might I not

capacity endlessly but why might I not want to change the 50 to 500 or 5,000 or

want to change the 50 to 500 or 5,000 or 50,000 what's the trade-off there

50,000 what's the trade-off there perhaps just intuitively

yeah okay you don't want to touch memory that you're not supposed to be touching

that you're not supposed to be touching and in this case it wouldn't be that

and in this case it wouldn't be that wouldn't be a risk per se unless you

wouldn't be a risk per se unless you indeed overflow the stack but there's a

indeed overflow the stack but there's a related issue in asking for that much

related issue in asking for that much memory what would another downside

be yeah okay exactly so if you've got a capacity of 5,000 but you're only using

capacity of 5,000 but you're only using one of those elements it's it's awkward

one of those elements it's it's awkward to say it non-technically which is just

to say it non-technically which is just to say very very wasteful right that's

to say very very wasteful right that's just bad design it's correct it will

just bad design it's correct it will work for up to 5,000 elements but my

work for up to 5,000 elements but my gosh you're wasting

gosh you're wasting 4,999 extra spots and that's not going

4,999 extra spots and that's not going to end well especially if you're using

to end well especially if you're using more data structures in memory like your

more data structures in memory like your Mac your PC your phone is surely going

Mac your PC your phone is surely going to run out of memory if you ask for that

to run out of memory if you ask for that much so it' be nice if there is a bit

much so it' be nice if there is a bit more dynamism there whether it's a stack

more dynamism there whether it's a stack or a CU both of which might be

or a CU both of which might be implemented a little similarly in spirit

implemented a little similarly in spirit but let's conclude this abstraction by

but let's conclude this abstraction by comparing thanks to a friend of ours

comparing thanks to a friend of ours Professor Shannon Duval of Elon

Professor Shannon Duval of Elon University who kindly put together this

University who kindly put together this graphical animation that's just under 2

graphical animation that's just under 2 minutes long that paints a picture of

minutes long that paints a picture of these two types of abstract data

these two types of abstract data structures and then we'll dive in

structures and then we'll dive in underneath to how we might Implement uh

underneath to how we might Implement uh problems like these if we could dim the

problems like these if we could dim the lights

lights [Music]

dramatically once upon a time there was a guy named Jack when it came to making

a guy named Jack when it came to making friends Jack did not have the Knack so

friends Jack did not have the Knack so Jack went to talk to the most popular

Jack went to talk to the most popular guy he knew he went up to Lou and asked

guy he knew he went up to Lou and asked what do I do Lou saw that his friend was

what do I do Lou saw that his friend was really distressed well Lou began just

really distressed well Lou began just look how you're dressed don't you have

look how you're dressed don't you have any clothes with a different look yeah

any clothes with a different look yeah yes said Jack I sure do come to my house

yes said Jack I sure do come to my house and I'll show them to you so they went

and I'll show them to you so they went off to Jacks and Jack showed Lou the box

off to Jacks and Jack showed Lou the box where he kept all his shirts and his

where he kept all his shirts and his pants and his socks L said I see you

pants and his socks L said I see you have all your clothes in a pile why

have all your clothes in a pile why don't you wear some others once in a

don't you wear some others once in a while Jack said well when I remove

while Jack said well when I remove clothes and socks I wash them and put

clothes and socks I wash them and put them away in the box then comes the next

them away in the box then comes the next morning and up I hop I go to the box and

morning and up I hop I go to the box and get my clothes off the top Lou quickly

get my clothes off the top Lou quickly realized the problem with Jack he kept

realized the problem with Jack he kept clothes CDs and books in a stack when he

clothes CDs and books in a stack when he reached for something to read or to wear

reached for something to read or to wear he chose the top book or underwear then

he chose the top book or underwear then when he was done he would put it right

when he was done he would put it right back back it would go on top of the

back back it would go on top of the stack I know the solution said a

stack I know the solution said a triumphant Lou you need to learn to

triumphant Lou you need to learn to start using a queue Lou took Jack's

start using a queue Lou took Jack's clothes and hung them in a closet and

clothes and hung them in a closet and when he had emptied the box he just

when he had emptied the box he just tossed it then he said now Jack at the

tossed it then he said now Jack at the end of the day put your clothes on the

end of the day put your clothes on the left when you put them away then

left when you put them away then tomorrow morning morning when you see

tomorrow morning morning when you see the sun shine get your clothes from the

the sun shine get your clothes from the right from the end of the line don't you

right from the end of the line don't you see said Lou it will be so nice you'll

see said Lou it will be so nice you'll wear everything once before you wear

wear everything once before you wear something twice and with everything in

something twice and with everything in cues in his closet and shelf Jack

cues in his closet and shelf Jack started to feel quite sure of himself

started to feel quite sure of himself all thanks to Lou and his wonderful

Q all right so sure so that paints a picture of these two abstract data

picture of these two abstract data structures but if we really were to dive

structures but if we really were to dive underneath the hood we could Implement

underneath the hood we could Implement them in a number of different ways but

them in a number of different ways but we really I think need some building

we really I think need some building blocks via which we could solve problems

blocks via which we could solve problems like those and we'll see today to some

like those and we'll see today to some others as well so let's rewind back to

others as well so let's rewind back to week two where we imple we introduced

week two where we imple we introduced you to your very first data structure

you to your very first data structure that is an array and an array of recall

that is an array and an array of recall was just a chunk of memory whereby

was just a chunk of memory whereby elements were stored by Design back to

elements were stored by Design back to back to back it's an array of contiguous

back to back it's an array of contiguous memory specifically so with an array we

memory specifically so with an array we could certainly store not just one thing

could certainly store not just one thing but two or three or even more and so for

but two or three or even more and so for instance if we treat uh my computer's

instance if we treat uh my computer's memory as this sort of abstraction here

memory as this sort of abstraction here and pictured here are three bytes or

and pictured here are three bytes or some uh uh multiplication thereof

some uh uh multiplication thereof suppose we're storing in the computer's

suppose we're storing in the computer's memory an array of size three storing

memory an array of size three storing the digits one two and three well

the digits one two and three well remember that if we zoom out per last

remember that if we zoom out per last week there's other stuff going on in

week there's other stuff going on in memory so even if we want to add another

memory so even if we want to add another number to this array that we didn't

number to this array that we didn't think of when we first started the

think of when we first started the program like the number four ideally we

program like the number four ideally we would put it right here next to it

would put it right here next to it otherwise it's no longer an array so by

otherwise it's no longer an array so by definition if we're using an array it's

definition if we're using an array it's got to end up right there after the

got to end up right there after the three but what else is going on inside

three but what else is going on inside of your computer's memory well assuming

of your computer's memory well assuming your program is of any length and you've

your program is of any length and you've got other variables other functions

got other variables other functions you've been running it for a while

you've been running it for a while there's a lot going on in your memory is

there's a lot going on in your memory is being used and reused so for instance

being used and reused so for instance somewhere in memory might be immediately

somewhere in memory might be immediately adjacent to this like hello comma world

adjacent to this like hello comma world back0 the null character just because

back0 the null character just because maybe you have another variable

maybe you have another variable somewhere in there that is storing that

somewhere in there that is storing that particular string alongside your

particular string alongside your existing array of size three and all of

existing array of size three and all of these Oscar the grouches here really

these Oscar the grouches here really just represent what we called last week

just represent what we called last week garbage values like there's obviously

garbage values like there's obviously bits there because they don't disappear

bits there because they don't disappear they're always going to be inside of the

they're always going to be inside of the computer somehow implemented but we

computer somehow implemented but we don't really know or care what they are

don't really know or care what they are they're the remnants of those bytes

they're the remnants of those bytes having been used for other older

having been used for other older variables previous function calls or the

variables previous function calls or the like but the problem clearly here is

like but the problem clearly here is that okay 1 two 3 is there but the H is

that okay 1 two 3 is there but the H is here and unless I want to start uh

here and unless I want to start uh taking a bite out of my string by

taking a bite out of my string by overriding the H with a four like we

overriding the H with a four like we just can't fit it right there and yet

just can't fit it right there and yet even though there's Oscars all over the

even though there's Oscars all over the place those are indeed garbage values

place those are indeed garbage values and therefore we could use that space

and therefore we could use that space because it's technically unused we just

because it's technically unused we just don't know or care what the values are

don't know or care what the values are so where could I put 1 2 3 4 well my

so where could I put 1 2 3 4 well my gosh like I have all this memory down

gosh like I have all this memory down here that's unused I could certainly

here that's unused I could certainly change those garbage values to be 1 2 3

change those garbage values to be 1 2 3 4 but to do that I might need to do a

4 but to do that I might need to do a bit of work here right it's not just a

bit of work here right it's not just a matter of just saying boom and it

matter of just saying boom and it happens now with c and with code I'd

happens now with c and with code I'd have to do this a little more

have to do this a little more methodically so let me abstract away

methodically so let me abstract away everything else that's a distraction let

everything else that's a distraction let me assume that there is indeed at least

me assume that there is indeed at least four bytes available for four numbers

four bytes available for four numbers just down here though we could have put

just down here though we could have put them in a bunch of different spots

them in a bunch of different spots what's involved now in moving the one

what's involved now in moving the one two three to this new chunk of memory so

two three to this new chunk of memory so we can add the four well I think

we can add the four well I think conceptually we're going to have to copy

conceptually we're going to have to copy the one from old to new copy the two

the one from old to new copy the two from old to new copy the three from old

from old to new copy the three from old to new and then ultimately we can get

to new and then ultimately we can get rid of the old memory those three

rid of the old memory those three original bites could now look like Oscar

original bites could now look like Oscar the Grouch and just be garbage values

the Grouch and just be garbage values for all intents and purposes but now I

for all intents and purposes but now I have room for a fourth bite wherein I

have room for a fourth bite wherein I can put the number four so this is nice

can put the number four so this is nice but what's a downside of this

but what's a downside of this approach what's a downside of solving

approach what's a downside of solving the problem in this way where the

the problem in this way where the problem at hand is just to grow the

problem at hand is just to grow the array so to speak to increase its size

array so to speak to increase its size to fit one or more

to fit one or more numbers seems pretty

numbers seems pretty straightforward but

yeah okay maybe it's out of order but I think that's okay because the order is

think that's okay because the order is just matters that it's relative so so

just matters that it's relative so so long it's it's still contiguous back to

long it's it's still contiguous back to back to back in a different chunk of

back to back in a different chunk of memory I think we're okay there it's not

memory I think we're okay there it's not like I changed it for 4 3 2 one but a

like I changed it for 4 3 2 one but a reasonable hunch

yeah yeah like I don't really plan ahead here like if I have to add another

here like if I have to add another number like five or anything else well I

number like five or anything else well I might have to jump through these hoops

might have to jump through these hoops again maybe I get lucky and maybe

again maybe I get lucky and maybe there's space there but not if I have

there's space there but not if I have other variables and other things going

other variables and other things going on that too might be used at some point

on that too might be used at some point other thoughts yeah slow efficiency slow

other thoughts yeah slow efficiency slow effic in slow efficiency why have

effic in slow efficiency why have to again yeah I mean it's just in it's

to again yeah I mean it's just in it's just inefficient it's sort of bad design

just inefficient it's sort of bad design arguably why because I had to copy all

arguably why because I had to copy all of my original work down here and as you

of my original work down here and as you note if I want to add a fifth number I'm

note if I want to add a fifth number I'm going to have to copy it again and again

going to have to copy it again and again and again and do things end times again

and again and do things end times again and again now maybe that's necessary

and again now maybe that's necessary we'll soon see for sure but it feels

we'll soon see for sure but it feels like this is not going to end well

like this is not going to end well especially if the array isn't of size

especially if the array isn't of size three or four but 300 400 your computer

three or four but 300 400 your computer ends up spending so much time just

ends up spending so much time just spinning its Wheels I mean honestly

spinning its Wheels I mean honestly better might be this like if this is my

better might be this like if this is my same array physically incarnated Now 1

same array physically incarnated Now 1 two 3 it's literally on the edge of the

two 3 it's literally on the edge of the shelf so there's no room for the number

shelf so there's no room for the number four you know maybe where we could take

four you know maybe where we could take this story is well let's just find room

this story is well let's just find room for the four like let's just put the

for the four like let's just put the four for instance over here replacing

four for instance over here replacing some available garbage value some spare

some available garbage value some spare bite over here but now wait a minute

bite over here but now wait a minute I've broken the definition of an array

I've broken the definition of an array right it's I can't have one two three

right it's I can't have one two three and then four over here so maybe there

and then four over here so maybe there maybe there could be a mechanism if I

maybe there could be a mechanism if I put this thing on again where when you

put this thing on again where when you get to the end of the existing elements

get to the end of the existing elements maybe I just somehow digitally point to

maybe I just somehow digitally point to the fourth array and maybe we can kind

the fourth array and maybe we can kind of stitch together all of these

of stitch together all of these different values in memory so that if

different values in memory so that if you follow the arrows so to speak we can

you follow the arrows so to speak we can reconstruct exactly what the order is

reconstruct exactly what the order is even without having to find or make room

even without having to find or make room here or pick up all of these numbers and

here or pick up all of these numbers and move all of them over there so that's

move all of them over there so that's perhaps the direction in which we'll go

perhaps the direction in which we'll go here so let's see how we might get to

here so let's see how we might get to that spot as follows let me go ahead and

that spot as follows let me go ahead and open up say VSS code here let me open up

open up say VSS code here let me open up a program called list. C in my terminal

a program called list. C in my terminal and let me go ahead and whip up a

and let me go ahead and whip up a relatively simple program that just

relatively simple program that just demonstrates what we did back in week

demonstrates what we did back in week two when we introduced arrays as follows

two when we introduced arrays as follows let me include uh standard io. so we can

let me include uh standard io. so we can print stuff out let me do int main void

print stuff out let me do int main void no command line Arguments for now let me

no command line Arguments for now let me give myself a an array uh called list of

give myself a an array uh called list of size three and I'll just hardcode it to

size three and I'll just hardcode it to keep it simple for lecture sake Each of

keep it simple for lecture sake Each of which is going to be an integer and now

which is going to be an integer and now just so we have some specifics to talk

just so we have some specifics to talk about let me put it list bracket zero

about let me put it list bracket zero the number one list bracket one the

the number one list bracket one the number two and list bracket two the

number two and list bracket two the number three so I'm just translating

number three so I'm just translating into codee what we just had uh

into codee what we just had uh pictorially on the screen and also

pictorially on the screen and also physically here with these numbers on

physically here with these numbers on the desk now let's just do something

the desk now let's just do something mildly useful for this how about we do

mildly useful for this how about we do four in I gets zero I is less than three

four in I gets zero I is less than three I ++ let's just print each of these

I ++ let's just print each of these numbers out just to make sure they're

numbers out just to make sure they're indeed in memory as I intended so

indeed in memory as I intended so percent I back sln comma I and then a

percent I back sln comma I and then a semicolon and I think that's it for now

semicolon and I think that's it for now so nothing interesting no problem solved

so nothing interesting no problem solved just yet just a proof of concept so that

just yet just a proof of concept so that now when I clear my terminal and run

now when I clear my terminal and run make

make list no apparent errors at the terminal

list no apparent errors at the terminal and so when I now do do/ list I should

and so when I now do do/ list I should see hopefully from left to right 1 two 3

see hopefully from left to right 1 two 3 but of course if I want to add a fourth

but of course if I want to add a fourth number now there's no mechanism for such

number now there's no mechanism for such certainly in the code that I just wrote

certainly in the code that I just wrote I could go back in here and change this

I could go back in here and change this to a four I could go down here and

to a four I could go down here and change lists bracket 3 equals 4 I could

change lists bracket 3 equals 4 I could just manually change the code recompile

just manually change the code recompile the code but of course that doesn't give

the code but of course that doesn't give me any additional run way for the fifth

me any additional run way for the fifth or sixth number so let me try to take a

or sixth number so let me try to take a different approach drawing some

different approach drawing some inspiration from last week if I want to

inspiration from last week if I want to allocate memory dynamically maybe

allocate memory dynamically maybe because I don't know when I wrote the

because I don't know when I wrote the program how many bytes I want we have

program how many bytes I want we have another function as of last week that

another function as of last week that does not require that you commit in

does not require that you commit in advance to a certain number of bytes via

advance to a certain number of bytes via what function can you just ask the

what function can you just ask the operating system for a chunk of memory

operating system for a chunk of memory okay so Malo to allocate memory now an

okay so Malo to allocate memory now an array is just a chunk of memory and even

array is just a chunk of memory and even though since week two 've been using

though since week two 've been using this syntactic sugar this convenience of

this syntactic sugar this convenience of just using square brackets and indexing

just using square brackets and indexing into it it's just making it easier to

into it it's just making it easier to manipulate a chunk of memory that's

manipulate a chunk of memory that's contiguous all together back to back to

contiguous all together back to back to back so today just like last week we can

back so today just like last week we can take those sort of training wheels off

take those sort of training wheels off and maybe be a little more deliberate in

and maybe be a little more deliberate in how we allocate memory let me go for

how we allocate memory let me go for instance and do this let me delete my

instance and do this let me delete my contents of my main function here go

contents of my main function here go back into Main and let me propose now

back into Main and let me propose now that I declare for instance how about my

that I declare for instance how about my list no longer as an array but as a

list no longer as an array but as a pointer so int star list and I'm going

pointer so int star list and I'm going to go ahead and initialize this to be

to go ahead and initialize this to be how about a chunk of three integers for

how about a chunk of three integers for now so I'm still going to hardcode it

now so I'm still going to hardcode it but I'm taking a step toward more

but I'm taking a step toward more dynamism for now so let me allocate

dynamism for now so let me allocate three times whatever the size is of an

three times whatever the size is of an INT but it's usually going to be four

INT but it's usually going to be four bytes as we know so this is really going

bytes as we know so this is really going to be 3 * 4al 12 but it's a little more

to be 3 * 4al 12 but it's a little more Dynamic and now what can I do down here

Dynamic and now what can I do down here well this is just a chunk of memory

well this is just a chunk of memory so I can do literally list bracket 0 uh

so I can do literally list bracket 0 uh equal 1 list bracket 1 = 2 list bracket

equal 1 list bracket 1 = 2 list bracket 2 equals 3 and voila achieve the exact

2 equals 3 and voila achieve the exact same effect because again an array is

same effect because again an array is just a chunk of contiguous memory but

just a chunk of contiguous memory but malok gives you any old chunk of

malok gives you any old chunk of contiguous memory so you can rather

contiguous memory so you can rather treat one like the other here now if you

treat one like the other here now if you really want to be cool you could do

really want to be cool you could do something like this instead you could

something like this instead you could dreference the address in list and go

dreference the address in list and go there you could go down here and

there you could go down here and dreference list + one and go there but

dreference list + one and go there but honestly no one really writes code like

honestly no one really writes code like this it's just too cryptic it's a little

this it's just too cryptic it's a little too far over the line at least for most

too far over the line at least for most people and so I think the syntactic

people and so I think the syntactic sugar as I keep describing it just the

sugar as I keep describing it just the more user friendly square bracket

more user friendly square bracket notation does the exact same thing

notation does the exact same thing figures out the pointer arithmetic and

figures out the pointer arithmetic and puts each of these integers in the right

puts each of these integers in the right chunks therein now just to be super

chunks therein now just to be super pedantic let me make sure if something

pedantic let me make sure if something went wrong so if list equals equals n

went wrong so if list equals equals n that means that something went wrong

that means that something went wrong like my computer is out of memory which

like my computer is out of memory which we should check for typically so let me

we should check for typically so let me just immediately return one signaling

just immediately return one signaling anything other than zero which means

anything other than zero which means success typically just to get out of

success typically just to get out of this program because something's wrong

this program because something's wrong but now let me propose that I've had a

but now let me propose that I've had a uh well let's do this for in I gets zero

uh well let's do this for in I gets zero I less than three i++ though a better

I less than three i++ though a better design would always be to use a con but

design would always be to use a con but I'm just doing this for demonstration

I'm just doing this for demonstration sake let's print out each of these ins

sake let's print out each of these ins too and just make sure I didn't mess

too and just make sure I didn't mess anything up and let me open my terminal

anything up and let me open my terminal window again let me do make list

window again let me do make list again okay huh implicitly declaring

again okay huh implicitly declaring Library function Malo with type void

Library function Malo with type void star something something implicitly de

star something something implicitly de declaring is the operative words there

declaring is the operative words there what did I mess up

what did I mess up yeah yeah I forgot the header file in

yeah yeah I forgot the header file in which malok is declared I remember now

which malok is declared I remember now okay that's in standard li. and it's

okay that's in standard li. and it's fine to look stuff like that up if you

fine to look stuff like that up if you forget so let me include standard libh

forget so let me include standard libh now let me clear my terminal run make

now let me clear my terminal run make list again okay now we're good /list and

list again okay now we're good /list and now what did I do

now what did I do [Music]

[Music] wrong oh okay not intended but teachable

wrong oh okay not intended but teachable moment what did I do wrong

moment what did I do wrong yeah yeah I'm printing the values of i

yeah yeah I'm printing the values of i instead of what is at location I in the

instead of what is at location I in the array so what I actually meant to do was

array so what I actually meant to do was print this out thank you you so now let

print this out thank you you so now let me recompile make

me recompile make listlist and now okay those are the

listlist and now okay those are the three values I was expecting not the

three values I was expecting not the indices thereof now let me propose that

indices thereof now let me propose that for the sake of discussion that I I

for the sake of discussion that I I regret having only allocated space for

regret having only allocated space for three integers and maybe I really should

three integers and maybe I really should have allocated enough space for four now

have allocated enough space for four now this is not how you would do this in

this is not how you would do this in practice because presumably if you have

practice because presumably if you have a change of thought just go back in and

a change of thought just go back in and correct the code but let me propose that

correct the code but let me propose that somewhere in here is a more complicated

somewhere in here is a more complicated program and time passes dot dot dot

program and time passes dot dot dot there's a lot of other interesting code

there's a lot of other interesting code there but at some point I might want to

there but at some point I might want to give myself more memory so how can I do

give myself more memory so how can I do this well let me just ask the operating

this well let me just ask the operating system now for four new bytes of memory

system now for four new bytes of memory so that we can at least in version one

so that we can at least in version one implement the ID on the board where I

implement the ID on the board where I just copied the three bytes into the new

just copied the three bytes into the new four bytes and then added a fourth value

four bytes and then added a fourth value so I'm going to use malok again and I'm

so I'm going to use malok again and I'm going to say here's a new pointer I'll

going to say here's a new pointer I'll call it temp TMP for short which is

call it temp TMP for short which is quite common when you just need it

quite common when you just need it briefly I'm going to then call malok

briefly I'm going to then call malok again I'm going to say give me four

again I'm going to say give me four integers using size of let me again make

integers using size of let me again make sure so if temp equals equals null

sure so if temp equals equals null something went wrong so let me just

something went wrong so let me just immediately return one and for good

immediately return one and for good measure before I return one one let me

measure before I return one one let me free the original list so that I don't

free the original list so that I don't leak memory so I'm not just immediately

leak memory so I'm not just immediately returning one I'm being a good citizen

returning one I'm being a good citizen and remembering well if this Malo call

and remembering well if this Malo call did succeed and indeed I got as far as

did succeed and indeed I got as far as line 18 but then line 18 failed I should

line 18 but then line 18 failed I should free the memory that I previously macked

free the memory that I previously macked so again that's the rule of thumb if you

so again that's the rule of thumb if you allocate it you should be the one to

allocate it you should be the one to free it even before you're about to quit

free it even before you're about to quit now once I've done that I think I need

now once I've done that I think I need to do what we did pictorially on the

to do what we did pictorially on the screen where I need to copy the One the

screen where I need to copy the One the two the three from the old array into

two the three from the old array into the new so how might I do this well let

the new so how might I do this well let me give myself a loop so for in I gets

me give myself a loop so for in I gets zero I less than three i++ because the

zero I less than three i++ because the size of the original is still the same

size of the original is still the same let me go ahead and treat the new chunk

let me go ahead and treat the new chunk of memory called 10 temp as an array

of memory called 10 temp as an array itself and so I can absolutely use these

itself and so I can absolutely use these square brackets just like before it's

square brackets just like before it's just a chunk of memory I'm treating it

just a chunk of memory I'm treating it like an array and let me add to that

like an array and let me add to that value whatever is at the original list

value whatever is at the original list at location I as well so this again is

at location I as well so this again is just this exercise of copying from right

just this exercise of copying from right uh from old to new step by step the one

uh from old to new step by step the one the two and the three but I still need

the two and the three but I still need one additional step if my goal at hand

one additional step if my goal at hand now is to have ultimately a fourth value

now is to have ultimately a fourth value here well I'm just going to hard coose

here well I'm just going to hard coose this for demonstration sake and I'm

this for demonstration sake and I'm going to go to the very last location of

going to go to the very last location of temp which is of size four which means

temp which is of size four which means the last element in temp is temp

the last element in temp is temp bracket three because it's zero indexed

bracket three because it's zero indexed but there's four total spaces there and

but there's four total spaces there and I'm just going to arbitrarily for the

I'm just going to arbitrarily for the sake of discussion put the number four

sake of discussion put the number four there and that is what happened when we

there and that is what happened when we proposed uh changing the final garbage

proposed uh changing the final garbage value there to that four but now I need

value there to that four but now I need to do what the slide did for us sort of

to do what the slide did for us sort of magically on the screen I should now do

magically on the screen I should now do a couple of final things I should free

a couple of final things I should free the original list which I've not done

the original list which I've not done yet CU I only called free earlier in

yet CU I only called free earlier in cases of error and that was just to be

cases of error and that was just to be safe I can now free the list and now if

safe I can now free the list and now if I want to inform the computer that I

I want to inform the computer that I want list quote unquote my variable

want list quote unquote my variable called list to point at not the old

called list to point at not the old chunk like it originally did but the new

chunk like it originally did but the new chunk I think I can just do this list

chunk I think I can just do this list equals

equals TMP and again that's just saying that if

TMP and again that's just saying that if list is a pointer which it was cuz look

list is a pointer which it was cuz look at the very top line here on line eight

at the very top line here on line eight on line six I declared list to be a

on line six I declared list to be a pointer uh to a chunk of memory temp

pointer uh to a chunk of memory temp meanwhile is a separate pointer to a

meanwhile is a separate pointer to a chunk of memory so down here this line

chunk of memory so down here this line 33 is just a matter of my saying okay

33 is just a matter of my saying okay now henceforth because I've already

now henceforth because I've already freed the old chunk of memory my list

freed the old chunk of memory my list variable should Point not at this chunk

variable should Point not at this chunk of three bytes but this chunk of four

of three bytes but this chunk of four bytes or really 12 in total now uh or

bytes or really 12 in total now uh or rather 16 now because we have four such

rather 16 now because we have four such bytes questions now on this code the

bytes questions now on this code the point of which was quite simply to

point of which was quite simply to demonstrate how we could Implement and

demonstrate how we could Implement and code this idea of Fairly correctly but

code this idea of Fairly correctly but inefficiently allocating a new array of

inefficiently allocating a new array of sufficient size and then populating it

sufficient size and then populating it with a new fourth

with a new fourth value questions on what we've just done

value questions on what we've just done here no

here no yeah

[Music] good question at this point in the story

good question at this point in the story with line 33 do I not have two different

with line 33 do I not have two different variables pointing at the same chunk of

variables pointing at the same chunk of memory short answer yes but here's where

memory short answer yes but here's where the semantics are perhaps compelling

the semantics are perhaps compelling list is the variable that I intend to

list is the variable that I intend to use longer term and keep around in

use longer term and keep around in memory and again assume that there's

memory and again assume that there's even more code going on here that we

even more code going on here that we just didn't write yet so it's useful to

just didn't write yet so it's useful to have that that variable temp was just

have that that variable temp was just kind of a necessary evil because up here

kind of a necessary evil because up here it would not have been correct to do

it would not have been correct to do this it would not have been correct to

this it would not have been correct to say list on line 18 equals the new chunk

say list on line 18 equals the new chunk of memory because this would have

of memory because this would have represented a memory leak if I pre uh

represented a memory leak if I pre uh prematurely change temp to point not at

prematurely change temp to point not at the old chunk but the new chunk at that

the old chunk but the new chunk at that point no one's pointing at the old chunk

point no one's pointing at the old chunk and so I've lost those three bites vren

and so I've lost those three bites vren for instance would yell at you for

for instance would yell at you for having lost as many btes in memory so in

having lost as many btes in memory so in this case here I do leave this as temp

this case here I do leave this as temp yes it's duplicative at this point but

yes it's duplicative at this point but it's it's not a huge deal if it was just

it's it's not a huge deal if it was just meant semantically to be a temporary

meant semantically to be a temporary value but down here at the risk of one

value but down here at the risk of one more line of code I still want to to be

more line of code I still want to to be a good citizen free list and maybe just

a good citizen free list and maybe just for good measure return zero explicitly

for good measure return zero explicitly but notice it's not doing it twice per

but notice it's not doing it twice per se on line 31 what am I freeing the

se on line 31 what am I freeing the original address of list the three

original address of list the three integer version then I change what list

integer version then I change what list points at so it's pointing at a

points at so it's pointing at a completely different chunk of memory

completely different chunk of memory this one of size four So eventually when

this one of size four So eventually when I'm all done using this memory for this

I'm all done using this memory for this demonstration I still need to free list

demonstration I still need to free list but at this point in the story line 40

but at this point in the story line 40 it's pointing at the new chunk of memory

it's pointing at the new chunk of memory which I similarly need to hand back to

which I similarly need to hand back to the operating system by free

yeah when would temp equal null so let me scroll back up slightly this is being

me scroll back up slightly this is being a good citizen and a good programmer

a good citizen and a good programmer whenever it comes to using malok malok

whenever it comes to using malok malok can return null if the computer's out of

can return null if the computer's out of memory so this is maybe a much bigger

memory so this is maybe a much bigger program you've got other things going on

program you've got other things going on in it and so you just don't have enough

in it and so you just don't have enough memory available to be handed malok

memory available to be handed malok needs to signal to you that there's some

needs to signal to you that there's some error and so it will by convention per

error and so it will by convention per the documentation per the manual pages

the documentation per the manual pages return null so this is just me being a

return null so this is just me being a good citizen otherwise here's another

good citizen otherwise here's another error that might cause your program to

error that might cause your program to crash with a segmentation fault if you

crash with a segmentation fault if you get back null but you assume that it's

get back null but you assume that it's good memory uh going to address zero AKA

good memory uh going to address zero AKA null will crash your program

null will crash your program intentionally

intentionally [Music]

yeah correct if I were to change my final line 40 here to be free temp this

final line 40 here to be free temp this would also work as well and here this is

would also work as well and here this is really a matter of design it's a very

really a matter of design it's a very nitpicky thing we could probably debate

nitpicky thing we could probably debate it but because at this point in the

it but because at this point in the story my main variable for remembering

story my main variable for remembering where the list is is called list this is

where the list is is called list this is sort of the more responsible way to do

sort of the more responsible way to do it freeing the list just so that my

it freeing the list just so that my colleagues my ta doesn't sort of wonder

colleagues my ta doesn't sort of wonder why are you freeing temporary ver memory

why are you freeing temporary ver memory that you already freed like it just is a

that you already freed like it just is a semantic thing at this point but good

semantic thing at this point but good Instinct it would also work correct

Instinct it would also work correct maybe just not good design all right so

maybe just not good design all right so it turns out that this gets annoying

it turns out that this gets annoying quickly as it did in the picture of

quickly as it did in the picture of doing all of this duplication and even

doing all of this duplication and even though technically it's necessary to

though technically it's necessary to copy those values if you need a newer

copy those values if you need a newer bigger chunk of memory

bigger chunk of memory there is at least a function in C that

there is at least a function in C that simplifies a lot of this for us and in

simplifies a lot of this for us and in fact let me go ahead and do this instead

fact let me go ahead and do this instead of using malok this second time on line

of using malok this second time on line 18 in addition to the first time I used

18 in addition to the first time I used it on line six I'm actually going to try

it on line six I'm actually going to try and introduce another function called

and introduce another function called realloc which as the name suggests tries

realloc which as the name suggests tries to reallocate memory for you and it

to reallocate memory for you and it works a little differently from malok

works a little differently from malok realloc expects two arguments the first

realloc expects two arguments the first one is what is the chunk of memory that

one is what is the chunk of memory that you want to try to grow or Shrink that

you want to try to grow or Shrink that is reallocate to be a different size and

is reallocate to be a different size and then you specify what size you would

then you specify what size you would want and indeed in this case I want four

want and indeed in this case I want four times size of int and that will now give

times size of int and that will now give me hopefully a new address of a chunk of

me hopefully a new address of a chunk of memory that's big enough to fit all four

memory that's big enough to fit all four numbers but what's wonderful about

numbers but what's wonderful about realloc is that it will handle all of

realloc is that it will handle all of the copying for me so in fact I'm going

the copying for me so in fact I'm going to go down here I'm going to get rid of

to go down here I'm going to get rid of all of this this extra for Loop and what

all of this this extra for Loop and what I'm simply going to do instead is this

I'm simply going to do instead is this once I can

once I can trust after lines 18 through 23 that

trust after lines 18 through 23 that rioc worked and it didn't return null

rioc worked and it didn't return null because I'm out of memory I can just say

because I'm out of memory I can just say okay just immediately remember that the

okay just immediately remember that the new list points at this new chunk of

new list points at this new chunk of memory instead and then I can still now

memory instead and then I can still now do this line but I can tweak the

do this line but I can tweak the semantics here and just say list bracket

semantics here and just say list bracket 3 the new Final a in uh um the new list

3 the new Final a in uh um the new list is for I don't need to free this here I

is for I don't need to free this here I don't need to do this all I need now at

don't need to do this all I need now at the bottom is the final for Loop to just

the bottom is the final for Loop to just print out these values so in short even

print out these values so in short even though that was somewhat quick using

though that was somewhat quick using realloc just moves the entire copying

realloc just moves the entire copying process that I implemented myself a

process that I implemented myself a moment ago using a for Loop it just

moment ago using a for Loop it just moves it to realloc and lets it deal

moves it to realloc and lets it deal with the copying for me it's no more

with the copying for me it's no more efficient but at least means uh I'm

efficient but at least means uh I'm writing less code which is more pleasant

writing less code which is more pleasant and hopefully the people who wrote aloc

and hopefully the people who wrote aloc or realloc are smarter than me and they

or realloc are smarter than me and they just will introduce bugs with lower

just will introduce bugs with lower probability

probability too all right that was a lot any

too all right that was a lot any [Music]

questions good question why do you still need to make list equal temp as I did on

need to make list equal temp as I did on line 24 so ideally I would do this

line 24 so ideally I would do this ideally I would just change this line 18

ideally I would just change this line 18 to be list that is to say call re or

to be list that is to say call re or actually even better ideally I would

actually even better ideally I would just say realloc this list to be of this

just say realloc this list to be of this new size but again things can go wrong

new size but again things can go wrong when allocating memory you need to check

when allocating memory you need to check a return value to see if it was

a return value to see if it was successful or not and so we need to use

successful or not and so we need to use a return value okay so let's not

a return value okay so let's not introduce temp let's just use list but

introduce temp let's just use list but here's where a memory leak might happen

here's where a memory leak might happen in the off chance realloc fails and

in the off chance realloc fails and doesn't have enough memory for your four

doesn't have enough memory for your four bytes therefore it returns by definition

bytes therefore it returns by definition null you can't overwrite the original

null you can't overwrite the original value of list with null

value of list with null to then check it why because now who

to then check it why because now who remembers where the original Three byes

remembers where the original Three byes were if you prematurely change the value

were if you prematurely change the value of list you've lost you've uh leaked

of list you've lost you've uh leaked memory in that sense and so that's why

memory in that sense and so that's why let me undo this change I declare a

let me undo this change I declare a temporary pointer for the sole purpose

temporary pointer for the sole purpose of making sure I can check the return

of making sure I can check the return value and then once it's good now I'll

value and then once it's good now I'll update the value of list so it's sort of

update the value of list so it's sort of doing a Switcheroo by making sure first

doing a Switcheroo by making sure first that you have a new value to swap with

that you have a new value to swap with the old other question

the old other question on this code

yeah indeed realloc automatically frees the previous memory for you and better

the previous memory for you and better yet it's even smarter than that if you

yet it's even smarter than that if you get lucky and they happens to be space

get lucky and they happens to be space right after your existing chunk of

right after your existing chunk of memory so one two three garbage value

memory so one two three garbage value instead of one two three hello world

instead of one two three hello world realloc won't even bother copying things

realloc won't even bother copying things from old to new it will just say okay

from old to new it will just say okay I'm going to now reserve for you more

I'm going to now reserve for you more bites than you originally asked for so

bites than you originally asked for so it doesn't have to waste time doing that

it doesn't have to waste time doing that copying and so in that sense this

copying and so in that sense this version is now not only still correct

version is now not only still correct it's even better designed because we're

it's even better designed because we're not wasting time with that for Loop we

not wasting time with that for Loop we might have to resort to it if there is

might have to resort to it if there is in fact hello world or something else in

in fact hello world or something else in the way but hopefully we'll get lucky

the way but hopefully we'll get lucky and save those steps other questions on

and save those steps other questions on this this manipulation of code

this this manipulation of code here yeah in the

middle what if you want to resize a

what if you want to resize a two-dimensional array so very similar in

two-dimensional array so very similar in Spirit uh whereby you can use the same

Spirit uh whereby you can use the same trickery let me wave my hand at that for

trickery let me wave my hand at that for now just because I think that's going to

now just because I think that's going to sort of significantly increase the

sort of significantly increase the complexity but very same Primitives

complexity but very same Primitives ultimately a two-dimensional array is

ultimately a two-dimensional array is essentially just a doubly long or

essentially just a doubly long or quadratically longer list of memory that

quadratically longer list of memory that using square bracket notation is doing

using square bracket notation is doing some of that mental math for you but

some of that mental math for you but it's fundamentally no different of

it's fundamentally no different of what's going on underneath the

what's going on underneath the hood all right so with that said and

hood all right so with that said and that code under our belt even though

that code under our belt even though that's not going to be something you'll

that's not going to be something you'll frequently need to code yourself let's

frequently need to code yourself let's propose now how we might go about

propose now how we might go about building some actual data structures

building some actual data structures ourselves the new ingredient here being

ourselves the new ingredient here being this reality that if you want to resize

this reality that if you want to resize a chunk of memory so is to make room for

a chunk of memory so is to make room for things we now have that ability memory

things we now have that ability memory addresses and pointers just give us the

addresses and pointers just give us the ability to like Point around at things

ability to like Point around at things and move things around in memory but now

and move things around in memory but now that we have malok and even realloc you

that we have malok and even realloc you can imagine maybe rewinding and you

can imagine maybe rewinding and you could Implement that stack that Q using

could Implement that stack that Q using not an array per se because you have to

not an array per se because you have to commit to an array size in advance but

commit to an array size in advance but if you implement your stack or your que

if you implement your stack or your que using a pointer and then malok and

using a pointer and then malok and realloc and maybe someone else writes

realloc and maybe someone else writes all that code for you perhaps now you

all that code for you perhaps now you can imagine that okay now the stack can

can imagine that okay now the stack can grow or Shrink by using realloc

grow or Shrink by using realloc accordingly you don't have to preact pre

accordingly you don't have to preact pre um preemptively say give me five bytes

um preemptively say give me five bytes or 50 or 500 or 5,000 you can say just

or 50 or 500 or 5,000 you can say just give me one initially and if I need more

give me one initially and if I need more I'll realloc realloc reloc and if you

I'll realloc realloc reloc and if you keep popping things off the stack you

keep popping things off the stack you can realloc in the other direction and

can realloc in the other direction and ask for fewer and fewer bytes and the

ask for fewer and fewer bytes and the operating system can take that memory

operating system can take that memory back as well so we now have this

back as well so we now have this building block let's see what we can do

building block let's see what we can do with it so we've had a few pieces of

with it so we've had a few pieces of syntax in recent weeks all of which

syntax in recent weeks all of which we're going to combine now in just a

we're going to combine now in just a slightly more clever way so struct is

slightly more clever way so struct is this keyword in C that lets us build our

this keyword in C that lets us build our own structure in memory like a

own structure in memory like a collection of two or three or more

collection of two or three or more variables like a person that we've seen

variables like a person that we've seen before the dot operator recall we've

before the dot operator recall we've used when you do have a struct like a a

used when you do have a struct like a a person and you want to go inside of it

person and you want to go inside of it so like uh person. name or person.

so like uh person. name or person. number we did this a few weeks ago now

number we did this a few weeks ago now but the dot Operator just allows you to

but the dot Operator just allows you to go inside of a structure and get the

go inside of a structure and get the individual variables within and then the

individual variables within and then the star operator unfortunately has a lot of

star operator unfortunately has a lot of uses now one was multiplication like my

uses now one was multiplication like my God that was easy back in the day now

God that was easy back in the day now it's used to declare pointers it's also

it's used to declare pointers it's also used to dreference pointers so to make

used to dreference pointers so to make one exist and then go to that address

one exist and then go to that address unfortunately it's the same symbol for

unfortunately it's the same symbol for all of those but it's all related but

all of those but it's all related but with these three symbols it turns out

with these three symbols it turns out you're going to get one last one today

you're going to get one last one today and my God it finally looks like the

and my God it finally looks like the concept it turns out there's a clever

concept it turns out there's a clever way anytime you want to use the dot and

way anytime you want to use the dot and the star together that is to go

the star together that is to go somewhere and go to an address and then

somewhere and go to an address and then look inside of a structure you can

look inside of a structure you can actually literally use an arrow symbol

actually literally use an arrow symbol on your keyboard it's not a single

on your keyboard it's not a single keystroke it's a hyphen and then an open

keystroke it's a hyphen and then an open angle bracket but at least it looks like

angle bracket but at least it looks like an arrow and we'll see indeed in code

an arrow and we'll see indeed in code today the things I was drawing

today the things I was drawing pictorially on the screen last time with

pictorially on the screen last time with yellow arrows you can actually now

yellow arrows you can actually now Express as well in code and so here we

Express as well in code and so here we have our next data structure called a

have our next data structure called a linked list and this is one of the most

linked list and this is one of the most useful powerful Concepts in C it's the

useful powerful Concepts in C it's the kind of thing that you can take for

kind of thing that you can take for granted in Java and Python and higher

granted in Java and Python and higher level languages but today we'll see how

level languages but today we'll see how we or others can actually build these

we or others can actually build these things just using these same Primitives

things just using these same Primitives so a linked list is going to allow us to

so a linked list is going to allow us to actually do what we you know used a foam

actually do what we you know used a foam finger for last week allow us to link

finger for last week allow us to link together for instance these three values

together for instance these three values maybe with that fourth value over there

maybe with that fourth value over there and then if there's a fifth you know

and then if there's a fifth you know maybe this other foam finger points even

maybe this other foam finger points even farther overway to that fifth value the

farther overway to that fifth value the key being that you can stitch together

key being that you can stitch together fancier data structures without having

fancier data structures without having to like pick all of these up and find

to like pick all of these up and find new space you just have to at least

new space you just have to at least connect the dots somehow we just need to

connect the dots somehow we just need to somehow point from one to the other and

somehow point from one to the other and that's going to make things much more

that's going to make things much more efficient it would seem so how do we get

efficient it would seem so how do we get there so here's my computer's memory as

there so here's my computer's memory as always suppose that I'm storing the

always suppose that I'm storing the value one somewhere in there and it's at

value one somewhere in there and it's at ox123 address whatever and I'm storing

ox123 address whatever and I'm storing the number two somewhere else in memory

the number two somewhere else in memory Ox 456 and number three at address Ox

Ox 456 and number three at address Ox 789 this is not an array by definition

789 this is not an array by definition why even though it's the only three

why even though it's the only three things on the screen what makes this not

things on the screen what makes this not an array it's not contiguous so this

an array it's not contiguous so this violates the definition of an array but

violates the definition of an array but you know especially since they're

you know especially since they're sequential it kind of looks to a human

sequential it kind of looks to a human like a list so it would be nice if there

like a list so it would be nice if there were a data type called list and there

were a data type called list and there isn't in C there will be in Python but

isn't in C there will be in Python but you know what if I could somehow like

you know what if I could somehow like Stitch together these three values so I

Stitch together these three values so I can get from one to the next to the next

can get from one to the next to the next then I think we could achieve the idea

then I think we could achieve the idea the concept of a list without this

the concept of a list without this really annoying constraint that they all

really annoying constraint that they all be contiguous as in an array so how do I

be contiguous as in an array so how do I do that well at the end of the day I

do that well at the end of the day I only have memory at my disposal right

only have memory at my disposal right there's no more training wheels to take

there's no more training wheels to take off here this is what we've got

off here this is what we've got underneath the hood of a computer so if

underneath the hood of a computer so if all I have is memory I think the

all I have is memory I think the solution to this problem of stitching

solution to this problem of stitching together those values in a list must be

together those values in a list must be to spend a bit more memory that's

to spend a bit more memory that's literally the only resource we have

literally the only resource we have right now so let me propose that if we

right now so let me propose that if we want to create a list conceptually out

want to create a list conceptually out of three values that are in random

of three values that are in random although pictorially pretty positions in

although pictorially pretty positions in memory let me just add a little bit more

memory let me just add a little bit more memory to the picture so in addition to

memory to the picture so in addition to storing the one I'm going to leave my

storing the one I'm going to leave my space myself some room a little scratch

space myself some room a little scratch pad if you will to use some other bits

pad if you will to use some other bits as well same for the two same for the

as well same for the two same for the three and you can perhaps see where this

three and you can perhaps see where this is going based on last week if I want to

is going based on last week if I want to somehow connect the one to the two any

somehow connect the one to the two any instincts as to what I should write in

instincts as to what I should write in this box here that would lead me

this box here that would lead me effectively from one to the two

effectively from one to the two what could go here

yeah we could store the address of Two And so specifically what would you have

And so specifically what would you have me write

here perfect ideally I would just put in this box another integer one that

this box another integer one that happens to be represented in heximal but

happens to be represented in heximal but that's just a base system it's just a

that's just a base system it's just a human thing for us to look at I'm going

human thing for us to look at I'm going to put the value Ox 456 here so let me

to put the value Ox 456 here so let me go ahead and reveal that Ox 456 goes

go ahead and reveal that Ox 456 goes there you can perhaps see further where

there you can perhaps see further where this is going well if I want to get from

this is going well if I want to get from the two to the three I think I need to

the two to the three I think I need to put below the two the address of the

put below the two the address of the three which gives me Ox 789 now if three

three which gives me Ox 789 now if three is the end of the list I don't want to

is the end of the list I don't want to let it be some garbage value because

let it be some garbage value because that would imply that it who knows where

that would imply that it who knows where it's pointing I need some definitive

it's pointing I need some definitive value and just what would your instincts

value and just what would your instincts be if I want to make clear with some

be if I want to make clear with some special Sentinel value that the buck

special Sentinel value that the buck stops here what do I put what my my

stops here what do I put what my my options be yeah so null not n per se but

options be yeah so null not n per se but n l which was the new keyword we

n l which was the new keyword we introduced last week which just

introduced last week which just represents an empty pointer if you will

represents an empty pointer if you will technically the address o x0 so

technically the address o x0 so literally the zero address and what

literally the zero address and what humans did years ago they just decided

humans did years ago they just decided you know what nothing should ever live

you know what nothing should ever live at address zero in memory we're just

at address zero in memory we're just going to reserve that one special bite

going to reserve that one special bite to be a special signal a sentinel value

to be a special signal a sentinel value such that if you ever see a zero address

such that if you ever see a zero address in a pointer it just means it's it's

in a pointer it just means it's it's invalid it does not exist now now we

invalid it does not exist now now we write that though a little more

write that though a little more pleasantly for the eyes as just n l in

pleasantly for the eyes as just n l in all caps and that's a key word in C as

all caps and that's a key word in C as well but of course last week I claimed

well but of course last week I claimed that who cares where things are in

that who cares where things are in memory and honestly like this quickly

memory and honestly like this quickly gets tedious even worrying about these

gets tedious even worrying about these values so let me abstract this away and

values so let me abstract this away and propose that if we want to remember

propose that if we want to remember where all of these numbers are in memory

where all of these numbers are in memory let's give oursel one final piece of

let's give oursel one final piece of memory that just allows us to start the

memory that just allows us to start the whole process let me allocate on the

whole process let me allocate on the left hand side here not room for a

left hand side here not room for a number like 1 two 3 just room for a

number like 1 two 3 just room for a pointer that henceforth I think I'll

pointer that henceforth I think I'll call list by convention and then store

call list by convention and then store in that one additional pointer a value

in that one additional pointer a value that just kickstarts the whole process

that just kickstarts the whole process this is the sort of treasure map if you

this is the sort of treasure map if you will that you get handed and this has

will that you get handed and this has the address of the very first actual

the address of the very first actual node in memory now technically we could

node in memory now technically we could just start with this but it turns out

just start with this but it turns out we'll see it's just a little cleaner to

we'll see it's just a little cleaner to use a simple single pointer that leads

use a simple single pointer that leads to the things you care about as opposed

to the things you care about as opposed to just starting with the first element

to just starting with the first element why well if you ever want to get rid of

why well if you ever want to get rid of this element it'd be nice if you could

this element it'd be nice if you could at least still hang on to an empty sheet

at least still hang on to an empty sheet of paper that indicates that the list is

of paper that indicates that the list is empty would be one argument for that so

empty would be one argument for that so again who cares about these addresses

again who cares about these addresses now now with the wave of the hand let's

now now with the wave of the hand let's just abstract it away and there are our

just abstract it away and there are our pointers each of those addresses in the

pointers each of those addresses in the rec uh the squares at the bottom are

rec uh the squares at the bottom are simply pointing to the next element in

simply pointing to the next element in the list the jargon to introduce here

the list the jargon to introduce here would be that now that we have these

would be that now that we have these integers 1 2 3 but they're in these like

integers 1 2 3 but they're in these like wrappers if you will these structures

wrappers if you will these structures that have metadata that is additional

that have metadata that is additional data that is related to but not the data

data that is related to but not the data you actually care about this is data

you actually care about this is data this is metadata this thing here

this is metadata this thing here rectangularly we'll call a node n o and

rectangularly we'll call a node n o and it's just a term of art that means it's

it's just a term of art that means it's like a container in code for storing

like a container in code for storing some

some values this then is a linked list and

values this then is a linked list and this then is the sort of graphical

this then is the sort of graphical incarnation of like one node pointing to

incarnation of like one node pointing to the other in this case case they happen

the other in this case case they happen to be by chance and by design of this

to be by chance and by design of this desk contiguous initially but there's no

desk contiguous initially but there's no requirement that they be such the one

requirement that they be such the one could be over there the two over there

could be over there the two over there the three over there I would just need

the three over there I would just need more foam fingers to point at one to the

more foam fingers to point at one to the next questions on this concept of a

next questions on this concept of a linked

linked list yeah and

[Music] back can you say that

again a good question do traditional arrays start with a pointer that's

arrays start with a pointer that's outside of the structure short answer no

outside of the structure short answer no arrays are special in C and certain

arrays are special in C and certain other languages and the name of an array

other languages and the name of an array is technically a symbol if you will that

is technically a symbol if you will that the computer the program knows maps to a

the computer the program knows maps to a specific location in memory it's just a

specific location in memory it's just a label a synonym for a memory address it

label a synonym for a memory address it does not take up space so to be clear

does not take up space so to be clear the name of an array does not take up

the name of an array does not take up space like that extra Square on the left

space like that extra Square on the left but you do need that extra Square on the

but you do need that extra Square on the left when implementing a link list so

left when implementing a link list so that you can determine if the list is of

that you can determine if the list is of size zero there's nothing being pointed

size zero there's nothing being pointed at or size three in this case we're sort

at or size three in this case we're sort of taking on more responsibility

of taking on more responsibility ourselves

yeah how do you point to the next element can you

element can you [Music]

elaborate ah good question if each of these elements is pointing to the next

these elements is pointing to the next how is three point to the others short

how is three point to the others short answer it doesn't at least in this

answer it doesn't at least in this design we have more technically what's

design we have more technically what's called a singly linked list and as the

called a singly linked list and as the arrows imply it only goes in One

arrows imply it only goes in One Direction so if you somehow find incode

Direction so if you somehow find incode maybe a for Loop maybe a while loop

maybe a for Loop maybe a while loop somehow you're sort of encode over here

somehow you're sort of encode over here you have no way in code to go backwards

you have no way in code to go backwards unless we changed this to a doubly link

unless we changed this to a doubly link list where I add another box that lets

list where I add another box that lets me have arrows in both directions or

me have arrows in both directions or maybe I just kind of make it uh uh

maybe I just kind of make it uh uh circular and I connect the three back to

circular and I connect the three back to the one which you can totally do but

the one which you can totally do but that tends to you know make life harder

that tends to you know make life harder because now you have to figure out when

because now you have to figure out when you're stuck in a loop in your data

you're stuck in a loop in your data structure but it's doable as well but as

structure but it's doable as well but as is it's a dead end by Design other

is it's a dead end by Design other questions on this design

here all right well how might we implement this structure in code well

implement this structure in code well let me just connect the dots to

let me just connect the dots to something like we've seen before here

something like we've seen before here like this is how a couple of weeks ago

like this is how a couple of weeks ago we introduced the notion of a a person

we introduced the notion of a a person and we claimed a person might have a

and we claimed a person might have a name and a number last week of course we

name and a number last week of course we took off some of these training wheels

took off some of these training wheels and a string is really technically a

and a string is really technically a Char star in both cases but really

Char star in both cases but really there's no conceptual difference beyond

there's no conceptual difference beyond that but let's use this same Paradigm to

that but let's use this same Paradigm to implement a node as I described it in

implement a node as I described it in that picture so let me get rid of the

that picture so let me get rid of the name and the number because that's

name and the number because that's related only to a person and let me

related only to a person and let me rename this structure for discussion

rename this structure for discussion sake to node that then invites the

sake to node that then invites the question well what needs to go inside of

question well what needs to go inside of a node well minimally an

a node well minimally an integer but this is now where we need to

integer but this is now where we need to think a little harder just conceptually

think a little harder just conceptually even if you have no idea how to type it

even if you have no idea how to type it at the keyboard what else needs to be

at the keyboard what else needs to be part of a node based on these

part of a node based on these rectangular pictures that we've

rectangular pictures that we've drawn what more do we need

drawn what more do we need yeah yeah we need a pointer to another

yeah yeah we need a pointer to another node so if I don't know how to implement

node so if I don't know how to implement this yet you know it could be something

this yet you know it could be something like you know pointer to another node

like you know pointer to another node how do I do that well you know what but

how do I do that well you know what but it turns out you would ideally say this

it turns out you would ideally say this if you know that the next node is itself

if you know that the next node is itself a node by definition well anytime we've

a node by definition well anytime we've needed a pointer we just use the data

needed a pointer we just use the data type and a star and I'm going to

type and a star and I'm going to arbitrarily but I think reasonably call

arbitrarily but I think reasonably call this second square at the bottom of

this second square at the bottom of those rectangles next as the name of my

those rectangles next as the name of my attribute here but node star just

attribute here but node star just connotes that the next variable is going

connotes that the next variable is going to be not a node per se but the address

to be not a node per se but the address of a node and that's exactly what we did

of a node and that's exactly what we did you had me put Ox 456 Ox 789 in that box

you had me put Ox 456 Ox 789 in that box which is the address of another node so

which is the address of another node so the way we would Express this in code

the way we would Express this in code would be node star next but we could

would be node star next but we could call the variable anything we want now

call the variable anything we want now this is a bit of a white lie but we'll

this is a bit of a white lie but we'll fix this right now this code won't

fix this right now this code won't actually compile C takes you pretty

actually compile C takes you pretty literally recall and if you use some

literally recall and if you use some term at the top of your file that you

term at the top of your file that you don't Define until later in your file

don't Define until later in your file you're going to see some error message

you're going to see some error message right we've seen this when I've messed

right we've seen this when I've messed up and forgot to include the function

up and forgot to include the function prototypes at the top of my code this is

prototypes at the top of my code this is related in spirit I seem here on my 1

related in spirit I seem here on my 1 two 3 fourth line of code I'm trying to

two 3 fourth line of code I'm trying to use this new term of art that I invented

use this new term of art that I invented here in my code called node even though

here in my code called node even though it's a CS term as well but nowhere Above

it's a CS term as well but nowhere Above This it would seem did I even Define

This it would seem did I even Define what a node is it's not a data type and

what a node is it's not a data type and C every computer scientist know what a

C every computer scientist know what a node is but it doesn't come for free

node is but it doesn't come for free with the language so I need to do

with the language so I need to do something else like I need this word

something else like I need this word here to come first so that I can use it

here to come first so that I can use it here and so we have this sort of Catch

here and so we have this sort of Catch 22 like how can a structure be

22 like how can a structure be self-referential that is point to

self-referential that is point to another version of itself if the word

another version of itself if the word doesn't yet exist so the solution to

doesn't yet exist so the solution to this in C which we didn't need for a

this in C which we didn't need for a person because there was no notion of

person because there was no notion of listing connecting as a list we need one

listing connecting as a list we need one more keyword here that we didn't need

more keyword here that we didn't need for a person and we reuse that keyword

for a person and we reuse that keyword here so kind of an annoying detail but

here so kind of an annoying detail but if we preemptively call this whole thing

if we preemptively call this whole thing struct node you can now refer to the

struct node you can now refer to the thing on the inside as a struct node

thing on the inside as a struct node star but then you can shorten the name

star but then you can shorten the name of the whole thing from struct node to

of the whole thing from struct node to just node sort of an annoying sequence

just node sort of an annoying sequence of steps but in short anytime you're

of steps but in short anytime you're building a node a linked list in memory

building a node a linked list in memory this is just the Paradigm you use type

this is just the Paradigm you use type def struct the name of the thing you

def struct the name of the thing you want to Define like node you use that

want to Define like node you use that name on the inside if you want to point

name on the inside if you want to point from one to another and then you can

from one to another and then you can shorten it down here to just be called

shorten it down here to just be called node questions then on this code

node questions then on this code here questions on what we just did well

here questions on what we just did well if I rewind just a moment to that final

if I rewind just a moment to that final picture what would be the upside to be

picture what would be the upside to be clear of having jumped through these

clear of having jumped through these hoops and added this complexity if you

hoops and added this complexity if you will what problem did we just solve by

will what problem did we just solve by linking together these three values to

linking together these three values to be clear

be clear [Music]

[Music] yeah making lists that

yeah making lists that are that are not contiguous if you will

are that are not contiguous if you will so making lists that are not contiguous

so making lists that are not contiguous in memory the upside of which is that if

in memory the upside of which is that if I want to add the number four to this

I want to add the number four to this list it looks like I could choose from

list it looks like I could choose from any chunks of available memory on the

any chunks of available memory on the screen I just need to sort of point from

screen I just need to sort of point from the end of the current list to wherever

the end of the current list to wherever that other one is in memory what I don't

that other one is in memory what I don't need to do to be clear is copy the One

need to do to be clear is copy the One the two or the three everything can just

the two or the three everything can just stay put which means TimeWise I can do

stay put which means TimeWise I can do this much more quickly it would seem

this much more quickly it would seem without copying things again and again

without copying things again and again and even without using realloc to let it

and even without using realloc to let it do all of the copying potentially for me

do all of the copying potentially for me all right but as we'll start seeing even

all right but as we'll start seeing even more in the coming weeks every time we

more in the coming weeks every time we benefit and solve some problem we pay a

benefit and solve some problem we pay a price there's a tradeoff what is a

price there's a tradeoff what is a downside as you might perceive now of

downside as you might perceive now of using a linked list instead of an array

using a linked list instead of an array yeah as much memory yeah I mean we use

yeah as much memory yeah I mean we use twice as much memory because now in

twice as much memory because now in addition to storing the integers one two

addition to storing the integers one two three I also need to store a pointer for

three I also need to store a pointer for each of those and honestly even this

each of those and honestly even this picture is a bit of uh simplification

picture is a bit of uh simplification technically in most systems today each

technically in most systems today each int would be four bytes technically

int would be four bytes technically today most pointers though would be

today most pointers though would be eight bytes I just didn't want to draw

eight bytes I just didn't want to draw this weird shape on the board where the

this weird shape on the board where the bottom square is even bigger than the

bottom square is even bigger than the top square but technically we're using

top square but technically we're using even more than twice as much space for

even more than twice as much space for these pointers so there's that trade-off

these pointers so there's that trade-off now thankfully decades after SE was

now thankfully decades after SE was invented memory is generally much

invented memory is generally much cheaper nowadays and so it's okay to

cheaper nowadays and so it's okay to sort of spend more of it if you need to

sort of spend more of it if you need to and it depends on what you want to

and it depends on what you want to optimize for but that's absolutely here

optimize for but that's absolutely here a downside what's another downside of

a downside what's another downside of having transitioned to in a uh link list

you can't index into it now I haven't even tried in code but when you have a

even tried in code but when you have a linked list you can no longer use square

linked list you can no longer use square bracket notation because why well square

bracket notation because why well square bracket notation just assumes the

bracket notation just assumes the contiguousness of memory location zero

contiguousness of memory location zero is here location one is literally one to

is here location one is literally one to the right location two is literally one

the right location two is literally one to the right one to the right these

to the right one to the right these things even though I've drawn it from

things even though I've drawn it from right to left to just keep things pretty

right to left to just keep things pretty there are gaps here and this is just my

there are gaps here and this is just my interpretation of this these gaps could

interpretation of this these gaps could be big they could be narrow they could

be big they could be narrow they could be down here up here they could be

be down here up here they could be anywhere so long as we're linking things

anywhere so long as we're linking things together in this list the computer can't

together in this list the computer can't just use bracket zero bracket 1 bracket

just use bracket zero bracket 1 bracket two anymore because it can't do simple

two anymore because it can't do simple arithmetic and jump to like the middle

arithmetic and jump to like the middle and now here's perhaps the worst price

and now here's perhaps the worst price we've paid if you don't have square

we've paid if you don't have square bracket notation or really you don't

bracket notation or really you don't have

have contiguousness what algorithm did we

contiguousness what algorithm did we just sacrifice for this

just sacrifice for this dynamism if you rewind even back to week

dynamism if you rewind even back to week zero and we gave it a name in week

zero and we gave it a name in week three what algorithm can we not use now

three what algorithm can we not use now if we can't assume that the memory is

if we can't assume that the memory is back to back to back to

back to back to back to back binary search why because binary

back binary search why because binary search just like the phone book back in

search just like the phone book back in the first week requires being able to

the first week requires being able to arithmetically jump right to the middle

arithmetically jump right to the middle right take the total length of it divide

right take the total length of it divide by two and boom you're right there in

by two and boom you're right there in the Middle with some simple arithmetic

the Middle with some simple arithmetic here they might be laid out again with

here they might be laid out again with these big or small gaps there's no

these big or small gaps there's no simple math I can do to just jump

simple math I can do to just jump immediately to the one in the middle and

immediately to the one in the middle and in fact again if this TV were bigger the

in fact again if this TV were bigger the two could technically be in memory be

two could technically be in memory be way down here or even way over here the

way down here or even way over here the foam finger could be pointing in any

foam finger could be pointing in any number of directions depending on where

number of directions depending on where malok put the thing there's just no way

malok put the thing there's just no way to do binary search and so it would seem

to do binary search and so it would seem that we've paid another price indeed in

that we've paid another price indeed in terms of it performance we're now

terms of it performance we're now talking about linear time again so

talking about linear time again so that's a regression now that's also a

that's a regression now that's also a lot things like feels like a good time

lot things like feels like a good time for some muffins and fruit out in the

for some muffins and fruit out in the lobby and when we come back we'll try to

lobby and when we come back we'll try to solve the problem we just created so see

solve the problem we just created so see you in 10 so we are back and let's see

you in 10 so we are back and let's see if we can't now take some of these

if we can't now take some of these higher level concepts of like stitching

higher level concepts of like stitching together these nodes in memory and

together these nodes in memory and translate it to some actual code but

translate it to some actual code but we'll do it step by step first before I

we'll do it step by step first before I actually start writing it in vs code so

actually start writing it in vs code so if Carter you wouldn't mind helping me

if Carter you wouldn't mind helping me step through with some visuals let me

step through with some visuals let me propose that line by line we solve some

propose that line by line we solve some of the problems that we've just created

of the problems that we've just created for ourselves in building this thing in

for ourselves in building this thing in memory so let's go ahead and first

memory so let's go ahead and first consider how we could build a length

consider how we could build a length list containing the numbers indeed one

list containing the numbers indeed one then two then three and let's translate

then two then three and let's translate each of those steps to code and then

each of those steps to code and then we'll put it all together into something

we'll put it all together into something that actually runs so how about first

that actually runs so how about first step here will just be this to declare a

step here will just be this to declare a pointer called list that's initially has

pointer called list that's initially has no value at least at this point in the

no value at least at this point in the story list is the name of the variable

story list is the name of the variable node star just means that this is

node star just means that this is essentially going to be our little

essentially going to be our little square over here that points to the

square over here that points to the beginning of the list of course it's

beginning of the list of course it's ideal If It ultimately has a value

ideal If It ultimately has a value because when we initially I'll call this

because when we initially I'll call this line of code it just gives us indeed

line of code it just gives us indeed that square over here on the left but

that square over here on the left but it's got a garbage value because there's

it's got a garbage value because there's no equal sign on the other side there so

no equal sign on the other side there so let's propose that we do one more step

let's propose that we do one more step here and actually initialize it to null

here and actually initialize it to null so that if only we know that it's not

so that if only we know that it's not garbage it at least has some known value

garbage it at least has some known value and null is a good way of signifying

and null is a good way of signifying that at this point in the story The List

that at this point in the story The List is empty indeed null indicates there's

is empty indeed null indicates there's no nodes in the list so that picture

no nodes in the list so that picture would now look like this whereby let's

would now look like this whereby let's just draw instead of writing null

just draw instead of writing null everywhere I'll just leave the squares

everywhere I'll just leave the squares blank when it's not a garbage value per

blank when it's not a garbage value per se it's literally Ox Z or null all right

se it's literally Ox Z or null all right so that's it for building a link list of

so that's it for building a link list of size zero like we're sort of done then

size zero like we're sort of done then but we want to now add a one and then a

but we want to now add a one and then a two then a three so next step here might

two then a three so next step here might be this if I want to allocate the first

be this if I want to allocate the first of my rectangles on our previous picture

of my rectangles on our previous picture I'm going to call malok and I'm going to

I'm going to call malok and I'm going to ask for enough memory to fit a whole

ask for enough memory to fit a whole node now technically I think that's

node now technically I think that's going to be like four bytes for the int

going to be like four bytes for the int and eight bytes for the pointer even

and eight bytes for the pointer even though I did not draw it to scale on the

though I did not draw it to scale on the board so that's technically going to be

board so that's technically going to be what 12 bytes but again size of node

what 12 bytes but again size of node just figures out how many bytes I

just figures out how many bytes I actually need dynamically that's going

actually need dynamically that's going to return to me the address of that

to return to me the address of that chunk of memory which apparently I'm

chunk of memory which apparently I'm going to store inside of a temporary

going to store inside of a temporary variable called n for short for node but

variable called n for short for node but let's see what this does pictorially so

let's see what this does pictorially so when this line of code is executed I

when this line of code is executed I first get on the left that variable n

first get on the left that variable n it's got a garbage value by default

it's got a garbage value by default because I haven't executed the whole

because I haven't executed the whole thing from right to left meanwhile on

thing from right to left meanwhile on the right hand side of the expression

the right hand side of the expression I've got now a node somewhere in memory

I've got now a node somewhere in memory it happened to be free here this is

it happened to be free here this is where malok put it for me but it does

where malok put it for me but it does have two garbage values initially but

have two garbage values initially but because it's a node per my type def

because it's a node per my type def earlier every node I proposed is going

earlier every node I proposed is going to have a number and a next pointer so

to have a number and a next pointer so we can see those labeled here but

we can see those labeled here but they've got two garbage values initially

they've got two garbage values initially but all I care about initially is that

but all I care about initially is that ultimately n is pointing at that chunk

ultimately n is pointing at that chunk of code so initially if we could back up

of code so initially if we could back up two

two steps we have two steps so we have initi

steps we have two steps so we have initi one step forward we have this line of

one step forward we have this line of code gives us this variable here which

code gives us this variable here which has garbage when this side of the

has garbage when this side of the expression is executed that allocates

expression is executed that allocates the memory and then when we copy from

the memory and then when we copy from right to left the address of that chunk

right to left the address of that chunk of memory that's what gives us

of memory that's what gives us conceptually this arrow and the garbage

conceptually this arrow and the garbage goes away because it's a valid pointer

goes away because it's a valid pointer now of course there's still two garbage

now of course there's still two garbage values there because we haven't set this

values there because we haven't set this node to store a number like the number

node to store a number like the number one so let's go ahead and execute one

one so let's go ahead and execute one other line of code like this which while

other line of code like this which while cryptic looking is just an application

cryptic looking is just an application of ideas we've seen in week four and

of ideas we've seen in week four and prior star N means to start at this

prior star N means to start at this variable and go there Follow the arrow

variable and go there Follow the arrow is what the star or the D reference

is what the star or the D reference operator does for us and then the dot

operator does for us and then the dot operator recall when we first introduce

operator recall when we first introduce structs like for a person struct allows

structs like for a person struct allows us to go at the number field or the next

us to go at the number field or the next field so if I do star n and then in

field so if I do star n and then in parentheses to make sure order of

parentheses to make sure order of operations is preserved do number and

operations is preserved do number and then assign it the actual number one

then assign it the actual number one which puts the one in the top of that

which puts the one in the top of that rectangle now admittedly this syntax is

rectangle now admittedly this syntax is not very user friendly it's annoying to

not very user friendly it's annoying to remember you have to the parentheses so

remember you have to the parentheses so there's another Syntax for this whenever

there's another Syntax for this whenever you're doing two things like this in

you're doing two things like this in code dereferencing a pointer that is

code dereferencing a pointer that is going to an address and then further

going to an address and then further using the dot notation to go inside of

using the dot notation to go inside of the structure you find that wonderfully

the structure you find that wonderfully C gives us this syntax whereby you can

C gives us this syntax whereby you can just change the star and the parentheses

just change the star and the parentheses and the dot to just be an arrow and

and the dot to just be an arrow and again it's not a single character on

again it's not a single character on your keyboard it's a hyphen and then an

your keyboard it's a hyphen and then an open angle bracket but I kind of like

open angle bracket but I kind of like the semantics of this because this code

the semantics of this because this code now pretty much matches the picture n

now pretty much matches the picture n arrow leads you to the value that you

arrow leads you to the value that you want to access or ultimately change in

want to access or ultimately change in this way there's one step though we've

this way there's one step though we've forgotten of course which is that we

forgotten of course which is that we can't leave this garbage value here

can't leave this garbage value here because the garbage value is some

because the garbage value is some unknown value that effectively is

unknown value that effectively is pointing who knows where and we don't

pointing who knows where and we don't want to accidentally misinterpret that

want to accidentally misinterpret that garbage value as being a valid address

garbage value as being a valid address and risk going there so of course what

and risk going there so of course what value should we put here instead our old

value should we put here instead our old friend null just to signify that this is

friend null just to signify that this is indeed the end of the list and we could

indeed the end of the list and we could do that with a line of code like this

do that with a line of code like this and again we'll canote as much by just

and again we'll canote as much by just leaving that empty box blank so now we

leaving that empty box blank so now we have a list of size one let's go ahead

have a list of size one let's go ahead and add the second number to it as with

and add the second number to it as with these lines here list equals n allows us

these lines here list equals n allows us to remember that indeed we have this

to remember that indeed we have this list here so if we can step one step

list here so if we can step one step forward here's what the picture now

forward here's what the picture now looks like and technically let's go one

looks like and technically let's go one step further here this is now really

step further here this is now really what's going on in memory once my list

what's going on in memory once my list of size exists my main variable called

of size exists my main variable called list is pointing at exactly that first

list is pointing at exactly that first node at this point in the story I don't

node at this point in the story I don't need to know or care about the temporary

need to know or care about the temporary variable that I called n even though it

variable that I called n even though it might very well still be there but

might very well still be there but indeed this now represents that link

indeed this now represents that link list let's now indeed add the number two

list let's now indeed add the number two so with the same line of code as before

so with the same line of code as before I'm going to allocate another node size

I'm going to allocate another node size of node ideally I would be checking for

of node ideally I would be checking for null here but we're doing the juicy

null here but we're doing the juicy Parts only on the slides let's now go

Parts only on the slides let's now go ahead and depict that so what happens

ahead and depict that so what happens with this this brings back our n pointer

with this this brings back our n pointer which might have been there the whole

which might have been there the whole time but we're doing this step by step

time but we're doing this step by step it's a garbage value though because we

it's a garbage value though because we haven't yet copied from right to left

haven't yet copied from right to left Malo of course gives us a second chunk

Malo of course gives us a second chunk of memory which maybe ends up there with

of memory which maybe ends up there with two garbage values by default I've

two garbage values by default I've omitted the labels now just because

omitted the labels now just because they're still going to be number and

they're still going to be number and next respectively once we copy from

next respectively once we copy from right to left the garbage value indeed

right to left the garbage value indeed becomes an arrow Oscar disappears

becomes an arrow Oscar disappears because it's now indeed a valid pointer

because it's now indeed a valid pointer pointing here now the values themselves

pointing here now the values themselves number and next are invalid garbage

number and next are invalid garbage values so here is where we can now start

values so here is where we can now start using our new syntax like the arrow

using our new syntax like the arrow notation or the star and the dot if you

notation or the star and the dot if you prefer and we can change the value of n

prefer and we can change the value of n Follow the arrow to number and that

Follow the arrow to number and that becomes two similarly we can do this

becomes two similarly we can do this again and set n arrow next so start at n

again and set n arrow next so start at n Follow the arrow access the next field

Follow the arrow access the next field and set that equal to null now we're not

and set that equal to null now we're not quite done yet because we haven't

quite done yet because we haven't actually linked things together so

actually linked things together so here's now where things get interesting

here's now where things get interesting how do I combine these two well let me

how do I combine these two well let me me propose this let me propose on our

me propose this let me propose on our next line here we actually update for

next line here we actually update for Now list equal to n that is to say

Now list equal to n that is to say whatever address this is whatever it's

whatever address this is whatever it's pointing at change list to be the same

pointing at change list to be the same address that is point at the same thing

address that is point at the same thing so if n is pointing here let's change

so if n is pointing here let's change list to point here and go ahead and do

list to point here and go ahead and do that Carter if you could I don't like

that Carter if you could I don't like this can you go one further step this is

this can you go one further step this is bad what is wrong about my sequence of

bad what is wrong about my sequence of operations here where I updated list to

operations here where I updated list to point my new

point my new node

node yeah yeah we lost the pointer to the

yeah yeah we lost the pointer to the other node so I don't even care about

other node so I don't even care about the ordering 21 or one two the bigger

the ordering 21 or one two the bigger problem now as the lack of arrows over

problem now as the lack of arrows over there suggests is that I have a memory

there suggests is that I have a memory leak I have orphaned my original node in

leak I have orphaned my original node in the sense that nothing is pointing at it

the sense that nothing is pointing at it anymore now absolutely I could fix this

anymore now absolutely I could fix this by adding some temporary variables I

by adding some temporary variables I could add it to the mix but at this

could add it to the mix but at this point in the story I have not done any

point in the story I have not done any such uh recollection thereof so let me

such uh recollection thereof so let me back this up and let's go forward in the

back this up and let's go forward in the slides this is where we left off a

slides this is where we left off a moment ago I think I need to take into

moment ago I think I need to take into account order of operations and I'm

account order of operations and I'm going to keep this simple I'm not going

going to keep this simple I'm not going to care about the order of the numbers

to care about the order of the numbers for now I'm fine with a list that is two

for now I'm fine with a list that is two and then one so with that said let me go

and then one so with that said let me go ahead and update I think this box here

ahead and update I think this box here to point at my original node so let's

to point at my original node so let's see how we can do this in code okay n

see how we can do this in code okay n arrow next so n arrow next should equal

arrow next so n arrow next should equal the current list and this is a little

the current list and this is a little weird again but recall what list is list

weird again but recall what list is list is this pointer here that just contains

is this pointer here that just contains the address of the original address of

the address of the original address of the list or equivalently it contains

the list or equivalently it contains this Arrow whatever it's pointing at so

this Arrow whatever it's pointing at so what this means in this line of code n

what this means in this line of code n bracket next means start at n Follow the

bracket next means start at n Follow the arrow access the next pointer and set it

arrow access the next pointer and set it equal to whatever list equals so if list

equal to whatever list equals so if list is pointing here then next should point

is pointing here then next should point there as well this I think is safe

there as well this I think is safe because now we have redundancy now we've

because now we have redundancy now we've got two pointers pointing at the

got two pointers pointing at the original list and now I think we can do

original list and now I think we can do another step whereby we update list to

another step whereby we update list to equal n same line of code before that

equal n same line of code before that got us into trouble but I'm doing it

got us into trouble but I'm doing it second now instead of first when I

second now instead of first when I execute list equals n this now sets list

execute list equals n this now sets list equal to the same thing that n equals

equal to the same thing that n equals and so now I have successfully inserted

and so now I have successfully inserted my new node containing two into the list

my new node containing two into the list and in fact if we advance one more we

and in fact if we advance one more we can just clear up the Clutter assume

can just clear up the Clutter assume that the temporary variable is gone from

that the temporary variable is gone from the story now we have a linked list

the story now we have a linked list where admittedly ordering is wrong it's

where admittedly ordering is wrong it's 21 instead of one two but at least it's

21 instead of one two but at least it's linked correctly and I didn't orphan or

linked correctly and I didn't orphan or leak any

leak any memory questions on this sequence of

memory questions on this sequence of steps

steps here yeah in

here yeah in [Music]

back yeah spot on so this would fall under that category of a stack if you

under that category of a stack if you will although I've not called it that by

will although I've not called it that by name because I just pushed the number

name because I just pushed the number two onto this data structure if you will

two onto this data structure if you will and indeed it ended up at the beginning

and indeed it ended up at the beginning of the list instead of the end and so

of the list instead of the end and so here's where we see a distinction

here's where we see a distinction between an abstract data structure which

between an abstract data structure which is where we began a stack is a thing

is where we began a stack is a thing like the pile of sweaters that just has

like the pile of sweaters that just has push and pop properties and lifo access

push and pop properties and lifo access like uh last in first out how do you

like uh last in first out how do you implement something like that in memory

implement something like that in memory well it would seem that you could

well it would seem that you could implement the notion of a stack here not

implement the notion of a stack here not for sweaters but for numbers using a

for sweaters but for numbers using a linked list so long as you implement

linked list so long as you implement insertion AKA pushing by prepending new

insertion AKA pushing by prepending new values to the list by prepending again

values to the list by prepending again and again and if Carter you don't mind

and again and if Carter you don't mind hitting the keyboard one more time if I

hitting the keyboard one more time if I wanted to add the number three now you

wanted to add the number three now you would could imagine prepending it to the

would could imagine prepending it to the list why well honestly especially as

list why well honestly especially as this list gets longer and longer I kind

this list gets longer and longer I kind of like the appeal of prepending these

of like the appeal of prepending these elements why because even if this list

elements why because even if this list gets crazy long and way way out here you

gets crazy long and way way out here you didn't notice me following all of the

didn't notice me following all of the arrows earlier to do the insert if I

arrows earlier to do the insert if I want to insert a fourth number a fifth

want to insert a fourth number a fifth number a sixth number all I have to do

number a sixth number all I have to do is like insert it here if you will point

is like insert it here if you will point it at the original uh start of the list

it at the original uh start of the list then update this pointer and done and I

then update this pointer and done and I would say that's like two steps give or

would say that's like two steps give or take it's not going to be end steps as

take it's not going to be end steps as it would be if I had to upend the new

it would be if I had to upend the new nodes to the end of the list now of

nodes to the end of the list now of course we've sacrificed ordering of

course we've sacrificed ordering of these numbers they're literally in the

these numbers they're literally in the opposite order or whatever order they

opposite order or whatever order they were inserted in but that might very

were inserted in but that might very well be okay depending on the goal at

well be okay depending on the goal at hand all right thank you to Carter for

hand all right thank you to Carter for stepping through this what if now we

stepping through this what if now we wanted to translate this oh sure thank

you it's all for you none for me in this example so here we have perhaps a way of

example so here we have perhaps a way of translating this now to some actual code

translating this now to some actual code and this will be the last of like the

and this will be the last of like the sort of intense code here just to give

sort of intense code here just to give you a sense of how we can translate this

you a sense of how we can translate this idea now to actual step so this is list.

idea now to actual step so this is list. C and VSS code here let me go ahead and

C and VSS code here let me go ahead and make a couple of changes up top let me

make a couple of changes up top let me go ahead and how about uh declaring a

go ahead and how about uh declaring a node using typ def uh struct node using

node using typ def uh struct node using our new framing as before I'm going to

our new framing as before I'm going to give every node a number as I proposed

give every node a number as I proposed and every node a pointer to the next

and every node a pointer to the next element which is going to be implemented

element which is going to be implemented just as before and I'm going to simplify

just as before and I'm going to simplify the whole name as just node so all of

the whole name as just node so all of that is is the exact same type depth

that is is the exact same type depth that we proposed earlier now let me go

that we proposed earlier now let me go ahead and get rid of all of this code

ahead and get rid of all of this code which we wrote earlier and recall that

which we wrote earlier and recall that this was the most recent version that

this was the most recent version that was not a linked list this was just in

was not a linked list this was just in Array that we allocated and then

Array that we allocated and then reallocated so this is sort of the old

reallocated so this is sort of the old way of doing things but it was

way of doing things but it was inefficient because we might have to

inefficient because we might have to lean on a for Loop or lean on realloc to

lean on a for Loop or lean on realloc to copy everything around we're now going

copy everything around we're now going to reimplement the notion of a list as

to reimplement the notion of a list as an actual linked list not as an array so

an actual linked list not as an array so my main function now might do something

my main function now might do something like this and I'm going to really just

like this and I'm going to really just copy the lines of code that we just

copy the lines of code that we just stepped through on the board so let me

stepped through on the board so let me give myself a uh special variable called

give myself a uh special variable called list that's going to be initialized to

list that's going to be initialized to null and this is just my pointer the

null and this is just my pointer the square on the left hand side of the

square on the left hand side of the screen that represents the start of the

screen that represents the start of the list and if it's null it means the list

list and if it's null it means the list is empty so done I'm done implementing a

is empty so done I'm done implementing a linked list of size zero well now how do

linked list of size zero well now how do I want to run this code well let me

I want to run this code well let me propose for the sake of discussion that

propose for the sake of discussion that this version of the program will take

this version of the program will take command line arguments so I want to be

command line arguments so I want to be able to do something like this I want to

able to do something like this I want to run this program ultimately and type in

run this program ultimately and type in three command line arguments like this 1

three command line arguments like this 1 2 3 and I want my program in a couple

2 3 and I want my program in a couple minutes to allocate one two three nodes

minutes to allocate one two three nodes and Stitch them together just like the

and Stitch them together just like the visualization on the board uh I could

visualization on the board uh I could use get int but it's just going to be

use get int but it's just going to be faster if we use command line arguments

faster if we use command line arguments so again I'm just borrowing some

so again I'm just borrowing some Concepts from week two but none of

Concepts from week two but none of that's possible yet until I change my

that's possible yet until I change my code here so let's do this in argc uh

code here so let's do this in argc uh string uh argv but you know what we know

string uh argv but you know what we know that strings are not actually a thing

that strings are not actually a thing anymore so I can change my command line

anymore so I can change my command line argument definition to be what it really

argument definition to be what it really is it's really charar but it's the exact

is it's really charar but it's the exact same thing as in week two just strings

same thing as in week two just strings are no more at least without the

are no more at least without the training wheels on anymore like last

training wheels on anymore like last week and now let me do this uh for in I

week and now let me do this uh for in I equal 1 uh I is less than ARG C i++ so

equal 1 uh I is less than ARG C i++ so what I'm doing with this Loop is I just

what I'm doing with this Loop is I just want to iterate over the command line

want to iterate over the command line argument so I have one number at a time

argument so I have one number at a time from The Prompt um what else do I want

from The Prompt um what else do I want to do here uh well let's go ahead and

to do here uh well let's go ahead and how about do this um let's get

how about do this um let's get a number so in number equals arv braet I

a number so in number equals arv braet I so a couple of notes Here one I'm

so a couple of notes Here one I'm starting my for loop at one instead of

starting my for loop at one instead of zero but I'm going up to RC RC is

zero but I'm going up to RC RC is argument count how many words are at the

argument count how many words are at the prompt why am I starting at one instead

prompt why am I starting at one instead of zero though given my

of zero though given my goal why am I starting at one

yeah yeah so the first value in RV is is actually the name of the program that's

actually the name of the program that's obviously not a number so I want the

obviously not a number so I want the second value so I'm going to start

second value so I'm going to start iterating over those command line

iterating over those command line arguments at I equals 1 so that's all I

arguments at I equals 1 so that's all I just want to get the actual numbers at

just want to get the actual numbers at the prompt um unfortunately argv bracket

the prompt um unfortunately argv bracket I is a string AKA Char star that is not

I is a string AKA Char star that is not an INT so this line of code won't work

an INT so this line of code won't work but can anyone think back to like week

but can anyone think back to like week two where we had a function for

two where we had a function for converting strings to

converting strings to integers anyone yeah so a to I is a

integers anyone yeah so a to I is a function that converts asky to an

function that converts asky to an integer assuming what you give it as an

integer assuming what you give it as an argument looks like a number like one or

argument looks like a number like one or two or three so let me fix this let me

two or three so let me fix this let me actually do the conversion if I were

actually do the conversion if I were really being careful I would error check

really being careful I would error check this make sure that there's no digits

this make sure that there's no digits just like you might have in problem set

just like you might have in problem set two but for today's purposes I'm just

two but for today's purposes I'm just going to assume the honor System that

going to assume the honor System that the user me is going to run the program

the user me is going to run the program correctly all right so now that I have a

correctly all right so now that I have a variable containing the number from the

variable containing the number from the command line let's just allocate a node

command line let's just allocate a node for it so let me do node star n just

for it so let me do node star n just like we did in the visualization and

like we did in the visualization and let's malok enough space for the size of

let's malok enough space for the size of one such node here I now need to just be

one such node here I now need to just be super safe so if n equals equals null

super safe so if n equals equals null like if I'm out of memory you know what

like if I'm out of memory you know what let me go ahead and just immediately

let me go ahead and just immediately return one here otherwise if that's not

return one here otherwise if that's not the case let me go ahead and update the

the case let me go ahead and update the number field of this new node which it

number field of this new node which it line 24 does exist because it did not

line 24 does exist because it did not return null so I did not exit early with

return null so I did not exit early with return and let me just store whatever

return and let me just store whatever number that human typed in first so the

number that human typed in first so the return value of a to I which per line 17

return value of a to I which per line 17 is in my variable called number and then

is in my variable called number and then let me go ahead and just prepend this to

let me go ahead and just prepend this to the list let me go ahead and say that um

the list let me go ahead and say that um this next field first has a known value

this next field first has a known value null just so that we get rid of that

null just so that we get rid of that second garbage value and let me go ahead

second garbage value and let me go ahead and now prepend it to the list so if I

and now prepend it to the list so if I want to prepend it that means this new

want to prepend it that means this new node must have a next field field that

node must have a next field field that points to the current beginning of the

points to the current beginning of the list and again the goal here is to preen

list and again the goal here is to preen Preen Preen so whatever the current list

Preen Preen so whatever the current list is let's change it so that this new node

is let's change it so that this new node points to that existing list and now

points to that existing list and now step two as before was to update the

step two as before was to update the actual list to point at this node so

actual list to point at this node so recall in red on the screen before I

recall in red on the screen before I screwed up originally and I only did

screwed up originally and I only did this line by moving the pointer too

this line by moving the pointer too early if you will but I fixed that once

early if you will but I fixed that once Carter helped me rewind and we got rid

Carter helped me rewind and we got rid of the red line which indicated error

of the red line which indicated error and I just do n arrow next to change the

and I just do n arrow next to change the next field of this new node to point to

next field of this new node to point to the existing list so I'm not orphaning

the existing list so I'm not orphaning anything all right at this point in the

anything all right at this point in the story I think my code is

story I think my code is correct not batting very well though

correct not batting very well though today but I think my code is correct but

today but I think my code is correct but the program doesn't do anything

the program doesn't do anything interesting so it would be nice to kind

interesting so it would be nice to kind of now iterate over this link list in

of now iterate over this link list in memory whatever its order is and print

memory whatever its order is and print things out well how do we do that well

things out well how do we do that well it turns out if you want to iterate over

it turns out if you want to iterate over a linked List the general Paradigm is to

a linked List the general Paradigm is to do something like this to define a

do something like this to define a temporary variable I could call it temp

temporary variable I could call it temp but another convention that you might as

but another convention that you might as well see is called pointer PTR for short

well see is called pointer PTR for short but you can call it anything you want

but you can call it anything you want and you can have a temporary variable

and you can have a temporary variable first point at the first node in the

first point at the first node in the list and then in some kind of loop like

list and then in some kind of loop like a while loop you point it at the second

a while loop you point it at the second node in the list and then you keep

node in the list and then you keep iterating you point it at the last node

iterating you point it at the last node in the list and then eventually you

in the list and then eventually you iterate too far effectively pointing at

iterate too far effectively pointing at null at which point your while loop can

null at which point your while loop can presumably terminate so how do I

presumably terminate so how do I Implement that idea of allocating a

Implement that idea of allocating a temporary pointer that just points at

temporary pointer that just points at each node in the list and lets me print

each node in the list and lets me print out ultimately each of those numbers

out ultimately each of those numbers well let's go back to my code here and

well let's go back to my code here and let me do this let me go ahead and

let me do this let me go ahead and declare this temporary pointer which is

declare this temporary pointer which is going to be a node star also why because

going to be a node star also why because it's the address of a node the first the

it's the address of a node the first the second the third and I'm going to set

second the third and I'm going to set that equal to whatever the beginning of

that equal to whatever the beginning of the list is so that is going to be

the list is so that is going to be equivalent to this version of the

equivalent to this version of the picture here where pointer is just

picture here where pointer is just temporarily pointing at the first node

temporarily pointing at the first node in the list it's not pointing at list

in the list it's not pointing at list per se it's pointing at the first node

per se it's pointing at the first node in the list which list is also pointing

in the list which list is also pointing at itself all right once I've done this

at itself all right once I've done this I think I can translate this to code

I think I can translate this to code that's a little new but it's

that's a little new but it's conceptually familiar perhaps now while

conceptually familiar perhaps now while that pointer does not equal null so

that pointer does not equal null so while I have a valid pointer like my

while I have a valid pointer like my finger or that arrow is pointing at an

finger or that arrow is pointing at an actual node in memory well let me go

actual node in memory well let me go ahead and print it out so let me print

ahead and print it out so let me print out with percent I back sln whatever is

out with percent I back sln whatever is in the current node at the number field

in the current node at the number field within and again this is going to have

within and again this is going to have the effect hopefully of first printing

the effect hopefully of first printing the three and I think I just need to Now

the three and I think I just need to Now update the pointer so that on the next

update the pointer so that on the next iteration it's pointing at the next

iteration it's pointing at the next value so if this is where the story is

value so if this is where the story is how do I update pointer to point at the

how do I update pointer to point at the second element of the list well I want

second element of the list well I want pointer to point at the two and I want

pointer to point at the two and I want pointer to eventually point at the three

pointer to eventually point at the three well how do I do that well the way in

well how do I do that well the way in code I can follow these arrows is as

code I can follow these arrows is as follows if I currently have pointer

follows if I currently have pointer pointing at this node but I want to

pointing at this node but I want to point it at the next node I can borrow

point it at the next node I can borrow this pointer here so whatever this

this pointer here so whatever this address is in the first node aka the

address is in the first node aka the next field I can copy that into pointer

next field I can copy that into pointer because then pointer will point at

because then pointer will point at whatever this is pointing at by just

whatever this is pointing at by just setting one equal to the other so once

setting one equal to the other so once I've done that the picture will

I've done that the picture will become this and how do I translate that

become this and how do I translate that to code it while new syntax is

to code it while new syntax is surprisingly straightforward all I need

surprisingly straightforward all I need do is say pointer after printing it

do is say pointer after printing it equals whatever pointer currently is but

equals whatever pointer currently is but grab its next field

grab its next field instead and this is a very common

instead and this is a very common Paradigm when iterating over a link list

Paradigm when iterating over a link list and you're using some temporary variable

and you're using some temporary variable like pointer you can simply set pointer

like pointer you can simply set pointer equal to pointer next and what that

equal to pointer next and what that means here is as follows if this is

means here is as follows if this is pointer pointing from here down to here

pointer pointing from here down to here pointer next is Follow the arrow grab

pointer next is Follow the arrow grab the next field so if you set pointer

the next field so if you set pointer equal to this thing that's the same

equal to this thing that's the same thing as pointing this at this same box

thing as pointing this at this same box and indeed if I advance to the next

and indeed if I advance to the next slide even though the arrows are

slide even though the arrows are technically pointing at different parts

technically pointing at different parts of the rectangles that's just for

of the rectangles that's just for graphic sake pointer is now pointing at

graphic sake pointer is now pointing at the second node and when I do this again

the second node and when I do this again on my next iteration it points at this

on my next iteration it points at this and then this last step notice when I

and then this last step notice when I keep doing pointer equals pointer next

keep doing pointer equals pointer next this will become eventually this value

this will become eventually this value but what's this value in this link list

but what's this value in this link list it's null technically so this Arrow will

it's null technically so this Arrow will eventually take on this value when I set

eventually take on this value when I set pointer equal to pointer next and at

pointer equal to pointer next and at that point PTR my temporary pointer is

that point PTR my temporary pointer is going to be null so it might as well

going to be null so it might as well look like this pictorially and what does

look like this pictorially and what does that mean for my Loop once pointer is

that mean for my Loop once pointer is null because you've walked off the end

null because you've walked off the end of the length list what's going to be

of the length list what's going to be true of this Loop here started in line

true of this Loop here started in line 32 any OB observations

32 any OB observations here what's going to be true what will

here what's going to be true what will happen now as soon as we hit the end of

happen now as soon as we hit the end of the list yeah

the list yeah sorry the loop is going to break out why

sorry the loop is going to break out why because line 32 which is constantly

because line 32 which is constantly asking well pointer does not equal null

asking well pointer does not equal null well if pointer finally equals null

well if pointer finally equals null three steps later the four the while

three steps later the four the while loop is now done and so what I can do at

loop is now done and so what I can do at the end of this program once I've

the end of this program once I've printed out those values well first

printed out those values well first let's go ahead and open my terminal

let's go ahead and open my terminal window let's make list okay a compile

window let's make list okay a compile do/ list and let me try the same values

do/ list and let me try the same values one and two and three that's going to

one and two and three that's going to again allocate one node two node three

again allocate one node two node three nodes by prepending prepending

nodes by prepending prepending prepending each of those values and it's

prepending each of those values and it's then going to iterate over them from

then going to iterate over them from left to right and so when I hit enter

left to right and so when I hit enter now what should I see on the screen if

now what should I see on the screen if my code is

correct what will I see feel fre to just call it

call it out 321 because I've prepended

out 321 because I've prepended presumably and here we go I indeed see

presumably and here we go I indeed see 321 so the list is is backwards but all

321 so the list is is backwards but all of the elements are there now

of the elements are there now technically if I ran valgrind on this

technically if I ran valgrind on this valgren would not be happy because I

valgren would not be happy because I have never freed any of my memory so I

have never freed any of my memory so I should probably now have a second Loop

should probably now have a second Loop here that does something like this let

here that does something like this let me again set pointer equal to list I

me again set pointer equal to list I don't need to redeclare it because I've

don't need to redeclare it because I've already created this thing on line 31 I

already created this thing on line 31 I just want to reset it to be the

just want to reset it to be the beginning of the list again and now I

beginning of the list again and now I can do the same kind of thing while PTR

can do the same kind of thing while PTR not equals null go ahead and do this

not equals null go ahead and do this well I don't want to just do free

well I don't want to just do free pointer and then do pointer gets pointer

pointer and then do pointer gets pointer next y my goal is to free all of my

next y my goal is to free all of my memory but I think this is going to get

memory but I think this is going to get me in trouble pointer equals list just

me in trouble pointer equals list just gives me a temporary pointer that points

gives me a temporary pointer that points at the three and then eventually the two

at the three and then eventually the two and then the one how well while pointer

and then the one how well while pointer not equal null I'm freeing the pointer

not equal null I'm freeing the pointer so this is like saying to Malo free that

so this is like saying to Malo free that node free that node free that node but

node free that node free that node but what's the problem with what I've just

what's the problem with what I've just done

done here this code is technically Bugg

here this code is technically Bugg [Music]

[Music] yeah exactly after you call free on

yeah exactly after you call free on pointer You Are by social contract with

pointer You Are by social contract with c not allowed to touch pointer anymore

c not allowed to touch pointer anymore it is invalid now it's still going to be

it is invalid now it's still going to be a number it's still going to be a

a number it's still going to be a pattern of bits but it's invalid and

pattern of bits but it's invalid and you'll very often get a segmentation

you'll very often get a segmentation fault if you tempt fate in that way so I

fault if you tempt fate in that way so I can't free the pointer and then use it

can't free the pointer and then use it literally the next line the solution

literally the next line the solution here kind of like our swapping of the

here kind of like our swapping of the liquids last time was to maybe just have

liquids last time was to maybe just have a temporary variable so I can do a

a temporary variable so I can do a Switcheroo and so a common way to solve

Switcheroo and so a common way to solve this problem to get the order of

this problem to get the order of operations right would be to do

operations right would be to do something like this give yourself a

something like this give yourself a temporary pointer like node star next

temporary pointer like node star next set it equal to the place you want to go

set it equal to the place you want to go next so one step ahead now you can free

next so one step ahead now you can free pointer and then you can update pointer

pointer and then you can update pointer to be that next value so essentially you

to be that next value so essentially you need need like two hands now you create

need need like two hands now you create on line 41 another pointer that if this

on line 41 another pointer that if this is pointing at the first Noe the three

is pointing at the first Noe the three your new pointer is pointing at the two

your new pointer is pointing at the two temporarily so now you can tell malok

temporarily so now you can tell malok via free release this memory but I

via free release this memory but I haven't forgotten where I want to go

haven't forgotten where I want to go next and so I can now continue on so a

next and so I can now continue on so a common Paradigm for just iterating over

common Paradigm for just iterating over these nodes and then freeing them a

these nodes and then freeing them a couple of observations strictly speaking

couple of observations strictly speaking I could have Consolidated this I don't

I could have Consolidated this I don't need two Loops to print the nodes and

need two Loops to print the nodes and then free the nodes I could do that all

then free the nodes I could do that all at once but let's assume that there's

at once but let's assume that there's other stuff of interest in my program

other stuff of interest in my program and I don't want to just immediately

and I don't want to just immediately free it there's one other bug that I

free it there's one other bug that I should probably address here there is

should probably address here there is still a potential memory leak up here

still a potential memory leak up here and this one is super subtle the valind

and this one is super subtle the valind would help you find it notice that in

would help you find it notice that in this Loop here when I'm calling malok

this Loop here when I'm calling malok this line of code is fine if the first

this line of code is fine if the first line of malok fails and returns null

line of malok fails and returns null because I immediately return and I'm

because I immediately return and I'm done but what if the second call but not

done but what if the second call but not the first or the third call but not the

the first or the third call but not the first or second fail this line of code

first or second fail this line of code has me returning immediately you really

has me returning immediately you really need to to do some garbage collection so

need to to do some garbage collection so to speak whereby you really need to go

to speak whereby you really need to go in and free any nodes that you did

in and free any nodes that you did allocate successfully earlier honestly

allocate successfully earlier honestly that's going to be a pin in the neck we

that's going to be a pin in the neck we won't do that here but probably what I'd

won't do that here but probably what I'd want to do is write a function called

want to do is write a function called free list or something like that and

free list or something like that and call that function to free any nodes I

call that function to free any nodes I had previously created so it's not quite

had previously created so it's not quite at the finish line but the building

at the finish line but the building blocks are indeed here questions on this

blocks are indeed here questions on this code and I think it's safe for me to

code and I think it's safe for me to promise that it won't escalate further

promise that it won't escalate further from

from that questions on

that questions on [Music]

[Music] this no well let me show you one

this no well let me show you one alternative that you might prefer and

alternative that you might prefer and I'm pretty sure this isn't an escalation

I'm pretty sure this isn't an escalation it's just an alternative formulation

it's just an alternative formulation another way you can iterate over nodes

another way you can iterate over nodes in a list could be this instead of a

in a list could be this instead of a while loop for instance let me actually

while loop for instance let me actually show you one other piece of syntax here

show you one other piece of syntax here you could technically use a for Loop you

you could technically use a for Loop you could give yourself a node pointer here

could give yourself a node pointer here that initialized is initialized to the

that initialized is initialized to the list you can then check in your for Loop

list you can then check in your for Loop that it's not equal to null and then you

that it's not equal to null and then you can do your update as usual like this

can do your update as usual like this either of these are equivalent even

either of these are equivalent even though this one I suspect looks scarier

though this one I suspect looks scarier it's doing the exact same thing in one

it's doing the exact same thing in one line instead of two but there's no

line instead of two but there's no reason we can't use four Loops instead

reason we can't use four Loops instead of while Loops to achieve the same idea

of while Loops to achieve the same idea but I'll leave these two as

but I'll leave these two as demonstrations of one approach or the

demonstrations of one approach or the other but that's just like in week one

other but that's just like in week one four Loops while Loops whatever looks

four Loops while Loops whatever looks simpler to you even though admittedly

simpler to you even though admittedly neither of these probably looks super

neither of these probably looks super clean all right so let's take the back

clean all right so let's take the back to things more conceptual here up until

to things more conceptual here up until now we've been inserting elements into

now we've been inserting elements into this link list by prepending them let's

this link list by prepending them let's consider what the running time then is

consider what the running time then is of these operations so if I've got a

of these operations so if I've got a link list of size three or size N More

link list of size three or size N More generally time has passed and I've added

generally time has passed and I've added a lot of things to it what's going to be

a lot of things to it what's going to be the running time for instance of

the running time for instance of searching a linked list for some value

searching a linked list for some value and I'll tell you already it's not login

and I'll tell you already it's not login because again binary search is off the

because again binary search is off the table as per before break so what might

table as per before break so what might the running time be of searching a

the running time be of searching a linked list for some value like two or

linked list for some value like two or three or 1 or

three or 1 or 50 what might the running time

50 what might the running time be o of I heard it over here o of n and

be o of I heard it over here o of n and y who was that oh in the middle here why

y who was that oh in the middle here why o of

[Music] n exactly you're going to have to go

n exactly you're going to have to go through every item in the list starting

through every item in the list starting from the left from the beginning which

from the left from the beginning which is how we've been drawing things and

is how we've been drawing things and connecting the dots and in the worst

connecting the dots and in the worst case the element might very well be at

case the element might very well be at the very end so it's going to be Big O

the very end so it's going to be Big O of n what about insertion how many steps

of n what about insertion how many steps in terms of Big O notation has it been

in terms of Big O notation has it been taking me to insert elements into the

taking me to insert elements into the link list using this prepended

design yeah so it's technically constant time Big O of one and again one is just

time Big O of one and again one is just representative of any constant it could

representative of any constant it could technically be two steps or three steps

technically be two steps or three steps or even 10 steps or 100 steps but if

or even 10 steps or 100 steps but if it's always finite and fixed then indeed

it's always finite and fixed then indeed you can say it's in Big O of one now why

you can say it's in Big O of one now why is that well over again no matter how

is that well over again no matter how long this list gets so long as there's

long this list gets so long as there's memory available for me I can just kind

memory available for me I can just kind of uh create a little splice at the

of uh create a little splice at the beginning of the list to put in the new

beginning of the list to put in the new node update the original list and I'm on

node update the original list and I'm on my way and it keeps getting longer even

my way and it keeps getting longer even though it might not be spread out in

though it might not be spread out in memory so big O of one is possible with

memory so big O of one is possible with these link lists if I indeed Preen

these link lists if I indeed Preen things of course if I Preen things

things of course if I Preen things everything's going to get out of order

everything's going to get out of order potentially and we're going to have

potentially and we're going to have maybe the stack property instead of a q

maybe the stack property instead of a q property so we might want to do things

property so we might want to do things slightly differently so instead of doing

slightly differently so instead of doing this

this whereby we kept prepending prepending

whereby we kept prepending prepending prepending suppose we append to the end

prepending suppose we append to the end of the list instead so if we now insert

of the list instead so if we now insert the one the two and the three as we

the one the two and the three as we might want to for a q to maintain that

might want to for a q to maintain that fairness property we might start with an

fairness property we might start with an empty list we might add the one we might

empty list we might add the one we might append the two append the three and so

append the two append the three and so it just is sort of laid out differently

it just is sort of laid out differently in memory and again if I can come to you

in memory and again if I can come to you in the middle what's the running time of

in the middle what's the running time of search again when the link list uses

search again when the link list uses this append

this append implementation yeah still Big O of n

implementation yeah still Big O of n because in the worst case you're going

because in the worst case you're going to have to go through the whole list

to have to go through the whole list just to find it and notice it doesn't

just to find it and notice it doesn't matter if you have an intuition now that

matter if you have an intuition now that the bigger numbers might very well be at

the bigger numbers might very well be at the end you have no way to jump to the

the end you have no way to jump to the end you have no way to jump to the

end you have no way to jump to the middle or do anything resembling binary

middle or do anything resembling binary search every search has to start from

search every search has to start from the left and follow the arrows again and

the left and follow the arrows again and again all right so I don't think we've

again all right so I don't think we've done any better there and in fact what

done any better there and in fact what is insertions running time now in Big O

is insertions running time now in Big O when we're appending to the list in this

when we're appending to the list in this way as we might to implement a Q instead

way as we might to implement a Q instead of a stack what's the running time of

of a stack what's the running time of inserting a new

inserting a new value Big O

value Big O of so not Big O of one in this case but

of so not Big O of one in this case but Big O of n because if I'm appending by

Big O of n because if I'm appending by definition I have to start here and

definition I have to start here and Traverse the whole thing looking for the

Traverse the whole thing looking for the end now this is a bit of an

end now this is a bit of an overstatement right you could obviously

overstatement right you could obviously optimize this slightly by maybe adding

optimize this slightly by maybe adding another variable that always points to

another variable that always points to the last element sort of a cheat sheet a

the last element sort of a cheat sheet a shortcut that gets you all the way to to

shortcut that gets you all the way to to the end that's totally fine it's not

the end that's totally fine it's not doesn't really fit the traditional

doesn't really fit the traditional definition of a singly linked list but

definition of a singly linked list but there's absolutely smart engineering

there's absolutely smart engineering solutions to these kinds of problems but

solutions to these kinds of problems but as designed it would indeed be Big O of

as designed it would indeed be Big O of end to insert to if you've got to go all

end to insert to if you've got to go all the way to the end and you're not using

the way to the end and you're not using a little extra memory to get yourself

a little extra memory to get yourself there quickly well what if we want to

there quickly well what if we want to take things one last step and not just a

take things one last step and not just a pen blindly because even though I

pen blindly because even though I inserted one two three if I inserted

inserted one two three if I inserted them in random order they would end up

them in random order they would end up in random order what if you want to

in random order what if you want to maintain a sorted list from smallest to

maintain a sorted list from smallest to largest well then you might want to

largest well then you might want to insert numbers like this starting from

insert numbers like this starting from an empty list we might have a two then

an empty list we might have a two then we might try inserting a one but we want

we might try inserting a one but we want to keep it sorted so now we're going to

to keep it sorted so now we're going to prepend in our code but then you might

prepend in our code but then you might want to insert a four so you would

want to insert a four so you would append the four because you're probably

append the four because you're probably going to look for the right spot to

going to look for the right spot to insert it then we're going to insert a

insert it then we're going to insert a three and this one's getting a little

three and this one's getting a little annoying because now you have to like

annoying because now you have to like iterate over the list look for the right

iterate over the list look for the right spot and then do a little smarter of a

spot and then do a little smarter of a splice but it's possible but you don't

splice but it's possible but you don't want to Orphan the four for instance and

want to Orphan the four for instance and then ultimately we get back to this

then ultimately we get back to this question what would the performance be

question what would the performance be of your linked list if you're trying to

of your linked list if you're trying to maintain sorted order well search I

maintain sorted order well search I think is going to be Big O of n for the

think is going to be Big O of n for the same reasons as before what about

same reasons as before what about insertion big go of what for inserting

insertion big go of what for inserting into a sorted linked

into a sorted linked list yeah in the worst

list yeah in the worst case yeah it's still Big O ofen so it's

case yeah it's still Big O ofen so it's no worse than but it's not really any

no worse than but it's not really any better than a pending but we gain the

better than a pending but we gain the additional property of maintaining a

additional property of maintaining a sorted list which might very well be

sorted list which might very well be useful if you're sorting your contacts

useful if you're sorting your contacts in your phone or something like that

in your phone or something like that where it just maintain makes sense to

where it just maintain makes sense to maintain sorted order now in the code

maintain sorted order now in the code for online today if you take a look at

for online today if you take a look at some of the final versions of code like

some of the final versions of code like list 6. C and list 5.c as we'll post on

list 6. C and list 5.c as we'll post on the website you can actually see code

the website you can actually see code that will solve all three of these

that will solve all three of these problems the Preen version that we wrote

problems the Preen version that we wrote live the appen version which we talk

live the appen version which we talk through as well as this sorted order one

through as well as this sorted order one but I think I'll avoid showing it live

but I think I'll avoid showing it live just because I do think that starts to

just because I do think that starts to escalate quickly but I think we have

escalate quickly but I think we have enough of a building block if we're

enough of a building block if we're comfortable with prepending to at least

comfortable with prepending to at least solve some real world problems with

solve some real world problems with these link

these link lists questions then on link list which

lists questions then on link list which we'll now leave behind on their own but

we'll now leave behind on their own but now use this technique to solve fancier

now use this technique to solve fancier problems but much less

problems but much less code questions on linked list all right

code questions on linked list all right so to C recap we've kind of taken a s

so to C recap we've kind of taken a s side step with link list like we have

side step with link list like we have this dynamism now where we can grow and

this dynamism now where we can grow and Shrink our chunks of memory without over

Shrink our chunks of memory without over allocating or accidentally

allocating or accidentally underallocation as in the world of an

underallocation as in the world of an array we don't have to worry about

array we don't have to worry about copying values endlessly because once

copying values endlessly because once you allocate the node it can just stay

you allocate the node it can just stay wherever it is in memory and you can

wherever it is in memory and you can just maintain uh you can just Stitch it

just maintain uh you can just Stitch it together somehow but unfortunately we've

together somehow but unfortunately we've sacrificed what we started the class

sacrificed what we started the class with in week zero which was like binary

with in week zero which was like binary search divide and conquer which was like

search divide and conquer which was like gave us that log and running time which

gave us that log and running time which was really compelling if you think back

was really compelling if you think back to the demonstrations and the the

to the demonstrations and the the visuals can we get the best of both

visuals can we get the best of both worlds can we get the sort of uh speed

worlds can we get the sort of uh speed of binary search something logarithmic

of binary search something logarithmic but the dynamism of something like a

but the dynamism of something like a link list well we can actually I think

link list well we can actually I think if we start to think not in a single

if we start to think not in a single Dimension just the x-axis if you will

Dimension just the x-axis if you will but two Dimensions such that our data

but two Dimensions such that our data structures can maybe now have width and

structures can maybe now have width and height if you will and so a tree is

height if you will and so a tree is perhaps the right term here much like a

perhaps the right term here much like a family tree if you have sort of your

family tree if you have sort of your elders up here in the tree and then the

elders up here in the tree and then the branches below them for their children

branches below them for their children and grandchildren and the like that's

and grandchildren and the like that's actually what a computer scientist means

actually what a computer scientist means when they talk about trees not a tree

when they talk about trees not a tree that grows up like this but really one

that grows up like this but really one that typically is depicted growing down

that typically is depicted growing down although this is just an artist's

although this is just an artist's depiction no matter what but there are

depiction no matter what but there are certain types of trees in the world

certain types of trees in the world called binary search trees that are

called binary search trees that are structured on paper and in visually like

structured on paper and in visually like a uh family tree but they have a special

a uh family tree but they have a special property that lends themselves to

property that lends themselves to exactly that feature binary search so

exactly that feature binary search so for instance here is an array back from

for instance here is an array back from week two and I've sorted a whole bunch

week two and I've sorted a whole bunch of numbers here in from 1 to seven we

of numbers here in from 1 to seven we know we can do binary search on this

know we can do binary search on this structure if it's implemented as an

structure if it's implemented as an array but what feature do arrays to be

array but what feature do arrays to be clear not have that link lists

clear not have that link lists do today's kind of a seesaw like what

do today's kind of a seesaw like what did we just gain by adding link list

did we just gain by adding link list that arrays do not

that arrays do not allow

allow yeah

yeah yeah you can insert more elements

yeah you can insert more elements without having to copy or moving

without having to copy or moving everything else around like right now in

everything else around like right now in this single Dimension if these values to

this single Dimension if these values to the left and or right are already used

the left and or right are already used then you have to move everything and

then you have to move everything and that's where we started today's story so

that's where we started today's story so arrays kind of paint you into a corner

arrays kind of paint you into a corner because you have to by definition decide

because you have to by definition decide in advance how big they are well

in advance how big they are well couldn't we have some kind of array that

couldn't we have some kind of array that can still grow but still is contiguous

can still grow but still is contiguous so we can do binary search in some way

so we can do binary search in some way well yes if we sort of rethink how we

well yes if we sort of rethink how we Implement binary search Let Me propose

Implement binary search Let Me propose that this I've chosen these seven

that this I've chosen these seven elements in the array much like the

elements in the array much like the lockers from uh week two to be ordered

lockers from uh week two to be ordered from smallest to largest I've

from smallest to largest I've highlighted now in yellow the middle

highlighted now in yellow the middle elements here and if we were telling the

elements here and if we were telling the story of week two going left or going

story of week two going left or going right let me highlight in red the middle

right let me highlight in red the middle elements of the left half and the right

elements of the left half and the right half and then let me further highlight

half and then let me further highlight in green the other elements in between

in green the other elements in between those and there's a pattern here as you

those and there's a pattern here as you might notice whereby there's one yellow

might notice whereby there's one yellow in the middle and then there's the two

in the middle and then there's the two red and the four green there's kind of

red and the four green there's kind of an implicit structure there if you will

an implicit structure there if you will and what if I do start to think in two

and what if I do start to think in two dimensions and instead of laying out an

dimensions and instead of laying out an array of lockers like this on the x-axis

array of lockers like this on the x-axis only what if I kind of like Slide the

only what if I kind of like Slide the four up and pull the uh the the one the

four up and pull the uh the the one the three the five down and kind of draw

three the five down and kind of draw this in two Dimensions instead well let

this in two Dimensions instead well let me do that as by separating these things

me do that as by separating these things like this such that now let me propose

like this such that now let me propose that each of these squares maybe it

that each of these squares maybe it doesn't have to be contiguous it can be

doesn't have to be contiguous it can be anywhere in the computer's memory but I

anywhere in the computer's memory but I can't have these crazy gaps among them

can't have these crazy gaps among them how could I perhaps keep these things

how could I perhaps keep these things connected conceptually what should I add

connected conceptually what should I add to the picture if you

will yeah branches say again so branches metaphorically here and more technically

metaphorically here and more technically in the language of C maybe just some

in the language of C maybe just some arrows some pointers so I won't bother

arrows some pointers so I won't bother drawing things as rectangles constantly

drawing things as rectangles constantly let me propose that we're now just

let me propose that we're now just abstracting away what a node is but let

abstracting away what a node is but let me claim that each of these squares now

me claim that each of these squares now is a node and a node might have a number

is a node and a node might have a number but it might also have a pointer heck

but it might also have a pointer heck maybe even two or more pointers and let

maybe even two or more pointers and let me draw those now I don't care about

me draw those now I don't care about addresses like ox1 2 3 4 5 6 7 8 n

addresses like ox1 2 3 4 5 6 7 8 n anymore let's just draw our pointers

anymore let's just draw our pointers with arrows but now let me propose that

with arrows but now let me propose that we could very well think about this as a

we could very well think about this as a tree storing what was previous

tree storing what was previous previously array data but now each of

previously array data but now each of these nodes can be anywhere in memory

these nodes can be anywhere in memory and moreover even though I've kind of

and moreover even though I've kind of painted myself into a corner visually on

painted myself into a corner visually on the screen so long as there's more

the screen so long as there's more memory in the computer I could put the

memory in the computer I could put the number zero over here I could put the

number zero over here I could put the number eight over here and I'm if I'm

number eight over here and I'm if I'm smart I could probably if I want to

smart I could probably if I want to insert other numbers like 2.5 or 1.5 or

insert other numbers like 2.5 or 1.5 or values in between you know I bet we

values in between you know I bet we could kind of make room by swiveling

could kind of make room by swiveling things around and just kind of hanging

things around and just kind of hanging things off of these branches slightly

things off of these branches slightly differently and so what does this gain

differently and so what does this gain me well if I instead start to model my

me well if I instead start to model my data not single dimensionally but in two

data not single dimensionally but in two dimensions and I connect those nodes

dimensions and I connect those nodes with these pointers what can I now do I

with these pointers what can I now do I think I just gave myself back binary

think I just gave myself back binary search why suppose I'm searching for the

search why suppose I'm searching for the number five how do I find it Well just

number five how do I find it Well just like in a family tree where you might

like in a family tree where you might visually start reading from top to

visually start reading from top to bottom I'm always going to start from

bottom I'm always going to start from the so-called root of a binary search

the so-called root of a binary search tree this is just like the list pointer

tree this is just like the list pointer that kicks off the whole link list

that kicks off the whole link list process

process this is the so-called route all right

this is the so-called route all right here I am at the number four I want to

here I am at the number four I want to find the number five what decision can I

find the number five what decision can I make when I see that I'm currently at

make when I see that I'm currently at the number four just like the phone book

the number four just like the phone book from week zero where is five

from week zero where is five not it's not to the left and if I were

not it's not to the left and if I were had you know built a little mobile here

had you know built a little mobile here or something we could very dramatically

or something we could very dramatically snip off this Branch this is this is

snip off this Branch this is this is like very lowbudget animation these

like very lowbudget animation these nodes could like fall to the ground and

nodes could like fall to the ground and we're left with half of essentially a

we're left with half of essentially a tree but what do I now know it's

tree but what do I now know it's obviously the five to the right so let

obviously the five to the right so let me go to the right six is obviously not

me go to the right six is obviously not the one I'm looking for but what do I

the one I'm looking for but what do I now know about the five well five is

now know about the five well five is less than the sixth so I can sort of

less than the sixth so I can sort of snip this off here because I know it's

snip this off here because I know it's not going to be down there and I can

not going to be down there and I can follow the remaining Arrow here and

follow the remaining Arrow here and voila I just found it and now without

voila I just found it and now without getting into the weeds of the math I've

getting into the weeds of the math I've got here what Seven Elements that's

got here what Seven Elements that's roughly eight if I round up and if I do

roughly eight if I round up and if I do some log base two I actually 1 two three

some log base two I actually 1 two three is the key detail here the height of

is the key detail here the height of this tree

this tree is three because I took a list of size

is three because I took a list of size seven and IED it and IED it in order to

seven and IED it and IED it in order to let it dangle in these two Dimensions

let it dangle in these two Dimensions plus or minus one for rounding sake so

plus or minus one for rounding sake so what do I get back I now have binary

what do I get back I now have binary search but it's not like H the middle of

search but it's not like H the middle of the middle of the middle I now follow

the middle of the middle I now follow these arrows in one of two directions so

these arrows in one of two directions so each of these nodes now has an INT and

each of these nodes now has an INT and maybe a left pointer and a right pointer

maybe a left pointer and a right pointer but you can call them anything you want

but you can call them anything you want and so I've gotten back binary search

and so I've gotten back binary search and dynamism because if you want to add

and dynamism because if you want to add zero or eight or 9 or 10 we can just

zero or eight or 9 or 10 we can just dangle them at the bottom of the binary

dangle them at the bottom of the binary search tree so what would this look like

search tree so what would this look like in code but we won't actually implement

in code but we won't actually implement it line by line well here was previously

it line by line well here was previously our definition of a node for a link list

our definition of a node for a link list which was onedimensional if you will

which was onedimensional if you will even though it might bounce up and down

even though it might bounce up and down on the screen it was still just a line

on the screen it was still just a line if you will well let me get rid of the

if you will well let me get rid of the single pointer in the linked list let me

single pointer in the linked list let me make a little bit of room here in this

make a little bit of room here in this type death and let me propose that we

type death and let me propose that we just add two pointers each of which is a

just add two pointers each of which is a struct node star one will be called left

struct node star one will be called left by convention one will be called right

by convention one will be called right by convention and so long as someone not

by convention and so long as someone not me not today not in class writes the

me not today not in class writes the code that stitches together this data

code that stitches together this data structure too handling both the left

structure too handling both the left child and the right child so to speak I

child and the right child so to speak I think we can indeed stitch together that

think we can indeed stitch together that two-dimensional structure and moreover

two-dimensional structure and moreover once you have this in memory you can

once you have this in memory you can translate pretty elegantly to code

translate pretty elegantly to code binary search itself using a principle

binary search itself using a principle we talked about recently too here is for

we talked about recently too here is for instance a function that I'll write by

instance a function that I'll write by just clicking through steps called

just clicking through steps called search whose purpose in life is to

search whose purpose in life is to return a Boolean true or false the

return a Boolean true or false the number I'm looking for is in the tree

number I'm looking for is in the tree this search function therefore takes two

this search function therefore takes two arguments the number I'm looking for

arguments the number I'm looking for called number and then a pointer to the

called number and then a pointer to the tree the so-called root of the tree now

tree the so-called root of the tree now how can I Implement binary search in

how can I Implement binary search in code will recall our brief discussion of

code will recall our brief discussion of recursion it turns out recursion is a

recursion it turns out recursion is a beautiful technique and honestly more

beautiful technique and honestly more obvious technique when you have two

obvious technique when you have two dimensional structures which finally

dimensional structures which finally after five plus weeks we now do here's

after five plus weeks we now do here's maybe my first line of code here if the

maybe my first line of code here if the tree is null then obviously return false

tree is null then obviously return false you've handed me an empty tree there's

you've handed me an empty tree there's nothing going on obviously the number

nothing going on obviously the number you're looking for is not going to be

you're looking for is not going to be here so that's my like safe base case to

here so that's my like safe base case to make sure I don't screw up and recurse

make sure I don't screw up and recurse infinitely well what else might be the

infinitely well what else might be the case well if the number I'm looking for

case well if the number I'm looking for is less than the tree's own number and

is less than the tree's own number and now recall that trees a node star so

now recall that trees a node star so even though I'm calling it a tree it's

even though I'm calling it a tree it's really the current node that's been

really the current node that's been passed in so if the number I'm looking

passed in so if the number I'm looking for is less than the current nodes

for is less than the current nodes number then I must know that the number

number then I must know that the number I'm looking for is to the left so to

I'm looking for is to the left so to speak so how can I solve that well this

speak so how can I solve that well this is where the magic of recursion just

is where the magic of recursion just return whatever the answer is to calling

return whatever the answer is to calling search again but on a sub tree if you

search again but on a sub tree if you will this is the sort of equivalent of

will this is the sort of equivalent of snipping off half of the tree pass in

snipping off half of the tree pass in the left sub tree if you will with the

the left sub tree if you will with the same number else if the number you're

same number else if the number you're looking for isn't less than the current

looking for isn't less than the current nodes number but greater than snip off

nodes number but greater than snip off the other subtree instead and just

the other subtree instead and just return whatever search says it finds in

return whatever search says it finds in the right subtree here and then there's

the right subtree here and then there's a fourth and final case what else could

a fourth and final case what else could be true

be true logically

logically [Music]

[Music] yeah perfect if the number you're

yeah perfect if the number you're looking for equals equals the number in

looking for equals equals the number in this node then I'm just going to return

this node then I'm just going to return true and you might recall from our

true and you might recall from our recurring discussions of design I don't

recurring discussions of design I don't strictly need to ask that explicitly

strictly need to ask that explicitly either there's no node it's to the left

either there's no node it's to the left it's to the right or you found it so I

it's to the right or you found it so I can just Whittle that down as usual to

can just Whittle that down as usual to an else and this now returns my true so

an else and this now returns my true so here too this is where recursion once

here too this is where recursion once you get comfy with it sort of gets

you get comfy with it sort of gets pretty elegant and cool in the sense

pretty elegant and cool in the sense that wow even though there's a lot of

that wow even though there's a lot of lines here I mean there's only a few

lines here I mean there's only a few interesting lines a lot of it's like

interesting lines a lot of it's like Curly braces at that which strictly

Curly braces at that which strictly speaking I could get rid of and so

speaking I could get rid of and so recurs lends itself to Elegance when it

recurs lends itself to Elegance when it comes to traversing these

comes to traversing these two-dimensional data structures as well

two-dimensional data structures as well so that is in code how you might

so that is in code how you might Implement something like

Implement something like search questions then on these

search questions then on these trees we have dynamism we can insert

trees we have dynamism we can insert more nodes to them they're faster

more nodes to them they're faster because we get B search back but but but

because we get B search back but but but there's got to be a price paid any

there's got to be a price paid any downsides or question or

downsides or question or downside okay let me come back to that

downside okay let me come back to that in just one sec downside though what

in just one sec downside though what price of we paid for this dynamism and

price of we paid for this dynamism and for this binary

for this binary searchability even though I've

searchability even though I've abstracted it away in the

abstracted it away in the picture say

picture say again we're using a lot of memory right

again we're using a lot of memory right I'm kind of misleading you now because

I'm kind of misleading you now because I'm just drawing these little squares

I'm just drawing these little squares with the simple numbers but there's

with the simple numbers but there's actually three things in there a four

actually three things in there a four byte integer an 8 by left pointer a 8 by

byte integer an 8 by left pointer a 8 by right pointer so we're already up to 16

right pointer so we're already up to 16 20 bytes now to store individual ins

20 bytes now to store individual ins that's probably okay though if memory is

that's probably okay though if memory is relatively cheap and voluminous as it

relatively cheap and voluminous as it nowadays is but these are the kinds of

nowadays is but these are the kinds of trade-offs and here too you see a hint

trade-offs and here too you see a hint of why some people still do like and use

of why some people still do like and use C and in fact it's so omnipresent

C and in fact it's so omnipresent because when you have C you can really

because when you have C you can really fine-tune how much memory is being used

fine-tune how much memory is being used for Better or For Worse under the hood

for Better or For Worse under the hood as we transition soon to python these

as we transition soon to python these decisions get made for you and you have

decisions get made for you and you have much much less control about how many me

much much less control about how many me how much memory is being used by your

how much memory is being used by your program because someone else made those

program because someone else made those designed decisions for you

designed decisions for you question

question is it bad if we don't know the parent

is it bad if we don't know the parent node uh not necessarily there's no

node uh not necessarily there's no reason why you need to have pointers in

reason why you need to have pointers in both directions however that can lend

both directions however that can lend itself to efficiency by spending more

itself to efficiency by spending more space and having arrows go up too you

space and having arrows go up too you can actually save more time when

can actually save more time when searching the tree in other context this

searching the tree in other context this though would be the canonical way the

though would be the canonical way the typical way to implement it um but

typical way to implement it um but absolutely just like a doubly link list

absolutely just like a doubly link list that could help you solve other problems

that could help you solve other problems too all right so turns out I'm kind of

too all right so turns out I'm kind of overselling binary search trees there

overselling binary search trees there are perversions of them so to speak

are perversions of them so to speak whereby they won't actually behave as

whereby they won't actually behave as advertised for instance here's a a good

advertised for instance here's a a good situation suppose you've got an empty

situation suppose you've got an empty tree initially and you insert the number

tree initially and you insert the number two well it's got to go somewhere so it

two well it's got to go somewhere so it might as well become the root of this

might as well become the root of this binary search tree and let's assume that

binary search tree and let's assume that someone wrote the code to do this now

someone wrote the code to do this now you want to insert the number one and

you want to insert the number one and you want to maintain the searchability

you want to maintain the searchability of this tree well it's important to note

of this tree well it's important to note that binary search tree is different

that binary search tree is different from tree if you just got a tree in

from tree if you just got a tree in memory there is no social contract with

memory there is no social contract with where the numbers need to go they can be

where the numbers need to go they can be completely random all over the place

completely random all over the place binary search tree means that you can do

binary search tree means that you can do binary search means that any node here

binary search means that any node here is going to be greater than every node

is going to be greater than every node here and less than every node here and

here and less than every node here and that's a definition it's a recursive

that's a definition it's a recursive structural definition that must be true

structural definition that must be true to be a binary search tree or BST so if

to be a binary search tree or BST so if we maintain that property ourselves let

we maintain that property ourselves let me insert two let me insert one one

me insert two let me insert one one belongs there by that definition let me

belongs there by that definition let me insert three three belongs there by that

insert three three belongs there by that definition but I kind of got lucky in

definition but I kind of got lucky in that I in the story inserted two and

that I in the story inserted two and then one and then three let me propose a

then one and then three let me propose a sort of perversion of the algorithm

sort of perversion of the algorithm whereby we just get unlucky let me

whereby we just get unlucky let me propose that we insert in uh insert one

propose that we insert in uh insert one first and then we insert two well where

first and then we insert two well where does two go well logically it goes to

does two go well logically it goes to the right because it's larger all right

the right because it's larger all right now the user inserts three where does it

now the user inserts three where does it go okay it goes there logically and how

go okay it goes there logically and how does this story un fold the user inserts

does this story un fold the user inserts four five six it's wonderfully sorted in

four five six it's wonderfully sorted in advance by luck but this is a perversion

advance by luck but this is a perversion of the structure in what

of the structure in what sense it's still technically a binary

sense it's still technically a binary search tree but what does it look more

search tree but what does it look more like it really is devolving if you will

like it really is devolving if you will into a linked list and so if you the

into a linked list and so if you the programmer don't Implement a binary

programmer don't Implement a binary search tree with some kind of repairs

search tree with some kind of repairs going on such that as soon as something

going on such that as soon as something gets whoa a little too long in stringy I

gets whoa a little too long in stringy I think I can fix this it's going to be an

think I can fix this it's going to be an annoying line number of lines of code

annoying line number of lines of code which we're not going to write here or

which we're not going to write here or or in a pet but we could kind of pivot

or in a pet but we could kind of pivot this thing right and we could just

this thing right and we could just rejigger things so that the two becomes

rejigger things so that the two becomes the new root the one becomes the left

the new root the one becomes the left child the three becomes the right child

child the three becomes the right child but that's what like two three plus

but that's what like two three plus lines of code it's possible it's doable

lines of code it's possible it's doable but it's it's extra work it's extra code

but it's it's extra work it's extra code so unless you write that code though and

so unless you write that code though and maintain balance of these trees just

maintain balance of these trees just because it's a binary search tree does

because it's a binary search tree does not mean its height is going to be log

not mean its height is going to be log base 2 of n the height could be log it

base 2 of n the height could be log it could be n in which case you don't get

could be n in which case you don't get those properties so when it comes to

those properties so when it comes to looking up in a balanced binary search

looking up in a balanced binary search tree yes it's log in but if it's

tree yes it's log in but if it's unbalanced if you don't add that

unbalanced if you don't add that additional logic and those repairs so to

additional logic and those repairs so to speak you could it could devolve into

speak you could it could devolve into Big O of N and this is a whole category

Big O of N and this is a whole category of algorithms and fanciness that you

of algorithms and fanciness that you would explore in a higher level course

would explore in a higher level course on algorithms and data structures

on algorithms and data structures there's lots of way to do that sort of

there's lots of way to do that sort of fixing that I'm alluding to in the

fixing that I'm alluding to in the picture there on the screen

picture there on the screen screen all right a few other data

screen all right a few other data structures if you will toward an end of

structures if you will toward an end of a sort of computer science Holy Grail so

a sort of computer science Holy Grail so log in is repeatedly a really good place

to end up we started in week zero when we got log in we lost it this earlier

we got log in we lost it this earlier today by introducing link list but we

today by introducing link list but we just got it back albeit at the price of

just got it back albeit at the price of spending more space but the Holy Grail

spending more space but the Holy Grail so to speak when it comes to algorithms

so to speak when it comes to algorithms would not be Big O of n certainly

would not be Big O of n certainly definitely not n squar like our mer like

definitely not n squar like our mer like our bubble sorts and selection sorts and

our bubble sorts and selection sorts and not even Big O of Logan what's better

not even Big O of Logan what's better than all of

than all of those big O of one constant time right

those big O of one constant time right that's the Holy Grail because if we

that's the Holy Grail because if we could store huge amounts of data but

could store huge amounts of data but find it instantly in one step or two

find it instantly in one step or two steps or heck even 10 or 20 steps but

steps or heck even 10 or 20 steps but independent of the size of the data

independent of the size of the data structure that's pretty powerful I mean

structure that's pretty powerful I mean that's the secret sauce of the Googles

that's the secret sauce of the Googles and the twitters of the world trying to

and the twitters of the world trying to get back results really really fast well

get back results really really fast well it turns out another abstract data type

it turns out another abstract data type or abstract data structure might be

or abstract data structure might be something called a dictionary just like

something called a dictionary just like the maram Webster Oxford English

the maram Webster Oxford English dictionaries that you might know which

dictionaries that you might know which associate say words with definitions

associate say words with definitions well you can think of a dictionary

well you can think of a dictionary really abstractly as this like two

really abstractly as this like two columns maybe on a spreadsheet of sorts

columns maybe on a spreadsheet of sorts where the left column represents

where the left column represents something and the right column

something and the right column represents something else like the word

represents something else like the word is on the left and its definition is on

is on the left and its definition is on the right and that's almost literally

the right and that's almost literally what a dictionary is on paper you've got

what a dictionary is on paper you've got all the words and all the definitions

all the words and all the definitions right next to it but more generally in

right next to it but more generally in Computing a dictionary really just has

Computing a dictionary really just has not words and definitions per se but key

not words and definitions per se but key value pairs this is a term of Art and

value pairs this is a term of Art and we're going to see this again and again

we're going to see this again and again especially as we transition to web

especially as we transition to web programming keys and values key is what

programming keys and values key is what you use to look for something the value

you use to look for something the value is what you find ultimately via that key

is what you find ultimately via that key so that's the generic term there we've

so that's the generic term there we've seen key value pairs really in the past

seen key value pairs really in the past in week zero we talked about your

in week zero we talked about your contacts in your iPhone or Android phone

contacts in your iPhone or Android phone uh being an app that has a whole bunch

uh being an app that has a whole bunch of contexts presumably alphabetized by

of contexts presumably alphabetized by first name or last name or the like well

first name or last name or the like well one one of those contact cards

one one of those contact cards ultimately has someone's number for

ultimately has someone's number for instance like John Harvard in this case

instance like John Harvard in this case so in that type of application the keys

so in that type of application the keys is the name like John Harvard that you

is the name like John Harvard that you use to find information and the value is

use to find information and the value is the number that you find there or if

the number that you find there or if there's more information like where he

there's more information like where he lives and uh email address and the like

lives and uh email address and the like the whole contact card could be the

the whole contact card could be the value thereof the key is what you use to

value thereof the key is what you use to look up John Harvard now back in week

look up John Harvard now back in week zero oh and rather the corresponding

zero oh and rather the corresponding table then if we draw this in two

table then if we draw this in two columns wouldn't be word and definition

columns wouldn't be word and definition or key value generically it would be

or key value generically it would be name and number for instance so we're

name and number for instance so we're just slapping some new terminology on

just slapping some new terminology on this old contact problem well this is

this old contact problem well this is the picture we drew way back in week

the picture we drew way back in week zero whereby I claimed that log of n was

zero whereby I claimed that log of n was really really good and indeed it was and

really really good and indeed it was and has been since but the Holy Grail would

has been since but the Holy Grail would indeed be something more like this in

indeed be something more like this in this dashed Green Line constant time and

this dashed Green Line constant time and maybe not literally one step but a fixed

maybe not literally one step but a fixed number of steps that even as the problem

number of steps that even as the problem gets huge and you go way way out on the

gets huge and you go way way out on the right of the X AIS the problem does not

right of the X AIS the problem does not depend on the side the uh the uh the

depend on the side the uh the uh the time to solve the problem does not

time to solve the problem does not depend at all on the size of the problem

depend at all on the size of the problem itself you can have a thousand contacts

itself you can have a thousand contacts or 100,000 contacts constant time means

or 100,000 contacts constant time means it takes the same number of steps no

it takes the same number of steps no matter what well how can we get to that

matter what well how can we get to that point well there's a couple of final

point well there's a couple of final building blocks today and there's one

building blocks today and there's one called hashing and this is something

called hashing and this is something that will recur a few times but for now

that will recur a few times but for now hashing is all about taking as input

hashing is all about taking as input some value

some value and outputting a simpler version thereof

and outputting a simpler version thereof so for instance here's a a gratuitously

so for instance here's a a gratuitously large deck of cards which are all the

large deck of cards which are all the more visible as a result and in a deck

more visible as a result and in a deck of cards typically you've got like what

of cards typically you've got like what 52 cards plus maybe the Jokers and

52 cards plus maybe the Jokers and whatnot and each of those cards has a

whatnot and each of those cards has a number of sorts and a suit on it and

number of sorts and a suit on it and here are literally four buckets on the

here are literally four buckets on the stage and how might I go about sorting

stage and how might I go about sorting these cards not just by number but also

these cards not just by number but also by suit well you could certainly like

by suit well you could certainly like spread them all out and sort of make a

spread them all out and sort of make a mess of things and just kind of reason

mess of things and just kind of reason your way through it and get everything

your way through it and get everything in order according to suit and corded by

in order according to suit and corded by number but most of us even if you don't

number but most of us even if you don't have four buckets at home probably are

have four buckets at home probably are going to do something a little more

going to do something a little more intuitive feels like an optimization

intuitive feels like an optimization where if I find like the nine of Hearts

where if I find like the nine of Hearts I'm going to put that into the hearts

I'm going to put that into the hearts bucket the King of Spades I'm going to

bucket the King of Spades I'm going to put that into the Spades bucket the jack

put that into the Spades bucket the jack of diamonds over here and I'll do this

of diamonds over here and I'll do this with the Queen of Diamonds and uh the

with the Queen of Diamonds and uh the Ace of clubs here and the three here and

Ace of clubs here and the three here and the 10 here and even though it's still

the 10 here and even though it's still going to be 52 steps why am I and maybe

going to be 52 steps why am I and maybe at home like why would you perhaps do

at home like why would you perhaps do this step

this step first what's the value of bucketization

reduces the probability of errors or the like and what I'm doing here to give it

like and what I'm doing here to give it a technical term is that I'm hashing the

a technical term is that I'm hashing the values I'm taking as input a card like

values I'm taking as input a card like this and I'm reducing it more simply

this and I'm reducing it more simply from a larger domain to a much smaller

from a larger domain to a much smaller range if you will so here's a domain of

range if you will so here's a domain of like 52 possibilities I want to map that

like 52 possibilities I want to map that to a range of four possible outcomes the

to a range of four possible outcomes the Diamonds the clubs the carts or the

Diamonds the clubs the carts or the Spades here and by doing that I'm just

Spades here and by doing that I'm just shrinking the size of the problem so

shrinking the size of the problem so hashing does that it's like literally an

hashing does that it's like literally an F ofx type Arrangement whereby you pass

F ofx type Arrangement whereby you pass something in and you get back a simpler

something in and you get back a simpler known value well a hash function more

known value well a hash function more technically is the algorithm or even the

technically is the algorithm or even the math or even the code that implements

math or even the code that implements that idea converting something bigger to

that idea converting something bigger to something smaller to this indeed finite

something smaller to this indeed finite range of values and it turns out that

range of values and it turns out that hash tables are a wonderful application

hash tables are a wonderful application of arrays and length lists to try to

of arrays and length lists to try to Leverage The Best of Both Worlds the

Leverage The Best of Both Worlds the goal being theoretically to achieve that

goal being theoretically to achieve that Holy Grail of constant time and that's

Holy Grail of constant time and that's going to be a bit of an overstatement

going to be a bit of an overstatement because you're not always going to

because you're not always going to achieve it exactly but at least we can

achieve it exactly but at least we can get a little closer there too so with

get a little closer there too so with hash tables you have something that

hash tables you have something that looks like this this is just an array

looks like this this is just an array this is an artist rendition of drawing

this is an artist rendition of drawing it vertically instead of horizontally

it vertically instead of horizontally but that's just a a detail graphically

but that's just a a detail graphically and this array for instance maybe uh is

and this array for instance maybe uh is of size 26 and where am I going with

of size 26 and where am I going with this well how does Apple how does Google

this well how does Apple how does Google Store your contexts alphabetically in

Store your contexts alphabetically in your phone and search for things quickly

your phone and search for things quickly well they might they probably

well they might they probably alphabetize at least in English

alphabetize at least in English according to a through z or if we

according to a through z or if we convert that to numbers it's like what

convert that to numbers it's like what 65 through whatever or really 0 through

65 through whatever or really 0 through 25 suffices if we're using an array of

25 suffices if we're using an array of size 26 we start counting at zero and we

size 26 we start counting at zero and we count up to 25 but let's abstract that

count up to 25 but let's abstract that away as just letters of the alphabet so

away as just letters of the alphabet so maybe what Google and apple are doing in

maybe what Google and apple are doing in your phone is storing all of the A's up

your phone is storing all of the A's up there all of the Z's down there and

there all of the Z's down there and everything else in between and so this

everything else in between and so this works pretty well if you start adding

works pretty well if you start adding your friends and your family so for

your friends and your family so for instance and I'll get rid of the letter

instance and I'll get rid of the letter so it's to not distract uh alvus might

so it's to not distract uh alvus might go in that first spot because a you

go in that first spot because a you subtract the 65 maps to zero so we put

subtract the 65 maps to zero so we put him in the first bucket the a bucket uh

him in the first bucket the a bucket uh maybe Zacharias ends up all the way at

maybe Zacharias ends up all the way at the end there and then in the middle

the end there and then in the middle might here be Hermione and if we do this

might here be Hermione and if we do this dot dot dot you keep adding all of your

dot dot dot you keep adding all of your classmates you might get a uh contact

classmates you might get a uh contact database that has all of this data here

database that has all of this data here in now each of these nodes they're drawn

in now each of these nodes they're drawn differently because this is just another

differently because this is just another artist rendition these rectangles these

artist rendition these rectangles these long rectangles represent a contact card

long rectangles represent a contact card like John Harvard's that's got the name

like John Harvard's that's got the name maybe email definitely phone number and

maybe email definitely phone number and things like that so this seems great why

things like that so this seems great why how can I find Albus well I go to the a

how can I find Albus well I go to the a bucket how do I find Zacharias I go to

bucket how do I find Zacharias I go to the Z bucket how do I find Hermione I go

the Z bucket how do I find Hermione I go to the H bucket but but but I've done

to the H bucket but but but I've done this very deliberately what problem will

this very deliberately what problem will arise eventually assuming you have

arise eventually assuming you have enough classmates

yeah there'll be too many people too many contacts for all of the available

many contacts for all of the available spaces in the array there's still some

spaces in the array there's still some room here but I'm pretty sure if I think

room here but I'm pretty sure if I think back to this particular class uh we've

back to this particular class uh we've got not hermy but also Harry who's also

got not hermy but also Harry who's also an H Hagrid who's also an H so where do

an H Hagrid who's also an H so where do I put them I could just put them

I put them I could just put them arbitrarily in any of the open spots but

arbitrarily in any of the open spots but then you lose the immediacy of jumping

then you lose the immediacy of jumping right to the H right to the a right to

right to the H right to the a right to the Z but now that we have link lists we

the Z but now that we have link lists we can kind of combine these ideas right

can kind of combine these ideas right use an array to get to the first letter

use an array to get to the first letter of the name you care about and then if

of the name you care about and then if you have a collision so to speak whereby

you have a collision so to speak whereby someone's already there you don't do

someone's already there you don't do something stupid like put Harry down

something stupid like put Harry down here just because it's available or

here just because it's available or maybe Hagrid down here just because it's

maybe Hagrid down here just because it's available because then you're losing the

available because then you're losing the immediacy of the lookup why don't you

immediacy of the lookup why don't you just kind of stitch them together in a

just kind of stitch them together in a linked list now what does this mean this

linked list now what does this mean this means for most of the characters here

means for most of the characters here you have constant time lookup you look

you have constant time lookup you look up alvas boom you're done Zacharias boom

up alvas boom you're done Zacharias boom you're done okay Harry Hermione Hagrid

you're done okay Harry Hermione Hagrid it might be one two or three steps so

it might be one two or three steps so that's actually devolving into something

that's actually devolving into something linear but here we make a distinction

linear but here we make a distinction today between theoretical running times

today between theoretical running times which we keep talking about and honestly

which we keep talking about and honestly a clock on the wall running times that

a clock on the wall running times that actual humans care about this is way

actual humans care about this is way faster than a linked list because you

faster than a linked list because you don't have to search every name it's

don't have to search every name it's even faster than a an array because you

even faster than a an array because you don't need to do binary search

don't need to do binary search you can literally for most of the names

you can literally for most of the names find them in constant time one step and

find them in constant time one step and again it's not theoretically constant

again it's not theoretically constant because these if you only befriend

because these if you only befriend people who have H names it's going to be

people who have H names it's going to be a crazy long link list anyway so again

a crazy long link list anyway so again it really kind of depends on what the

it really kind of depends on what the nature of the data is here but this is

nature of the data is here but this is pretty close to constant time and in

pretty close to constant time and in fact how could we get even closer how

fact how could we get even closer how could we reduce the probability of

could we reduce the probability of collisions for the H's or any other

collisions for the H's or any other letters how could we avoid putting too

letters how could we avoid putting too many H names together

L okay yeah so we could add another dimension if you will but let's not add

dimension if you will but let's not add a third dimension per se but let's

a third dimension per se but let's indeed look at not just the first letter

indeed look at not just the first letter of everyone's name but the first and the

of everyone's name but the first and the second and in fact let's see if that

second and in fact let's see if that gets us a little uh further along so let

gets us a little uh further along so let me go ahead and propose if you go

me go ahead and propose if you go through the whole Harry Potter Universe

through the whole Harry Potter Universe there's actually a lot of collisions if

there's actually a lot of collisions if we keep going and so we've got the L's

we keep going and so we've got the L's here the RS the S's and so forth well

here the RS the S's and so forth well let's clean this up here Hermione

let's clean this up here Hermione originally went to the H location but

originally went to the H location but let's decrease the probability of

let's decrease the probability of collisions there and everywhere else

collisions there and everywhere else instead of putting hermion Harry and

instead of putting hermion Harry and Hagrid Al together let's go ahead and do

Hagrid Al together let's go ahead and do this instead instead of labeling these

this instead instead of labeling these buckets A through Z let's just give

buckets A through Z let's just give ourselves more buckets so in fact this

ourselves more buckets so in fact this might be H well instead of H maybe this

might be H well instead of H maybe this should be ha and then this should be HB

should be ha and then this should be HB HC HD h e HF now some of those are a

HC HD h e HF now some of those are a little nonsensical cuz I can't think of

little nonsensical cuz I can't think of names that match most of those but it's

names that match most of those but it's deterministic at least we know the

deterministic at least we know the bucket will be there which is important

bucket will be there which is important even if it's empty and now we can put

even if it's empty and now we can put Hermione here we can put Harry here but

Hermione here we can put Harry here but uhoh we didn't do this perfectly well

uhoh we didn't do this perfectly well Hagrid still Collide so let me come back

Hagrid still Collide so let me come back to you how can we reduce the probability

to you how can we reduce the probability of Harry and Hagrid

of Harry and Hagrid colliding yeah so we could look at the

colliding yeah so we could look at the third letter okay so let me try that

third letter okay so let me try that instead of ha let's look at haa ha ha C

instead of ha let's look at haa ha ha C dot dot dot haq dot dot dot h h e q h e

dot dot dot haq dot dot dot h h e q h e r hes and so forth and now I think those

r hes and so forth and now I think those names and probably all the others we saw

names and probably all the others we saw are now much more cleanly distributed

are now much more cleanly distributed there's much lower probability of

there's much lower probability of collisions unless two people have like

collisions unless two people have like almost the same names or one is like a

almost the same names or one is like a prefix of the other but but but even

prefix of the other but but but even though we're now closer than ever to

though we're now closer than ever to constant time because the odds that we

constant time because the odds that we hit a collision and have to devolve to a

hit a collision and have to devolve to a link list or much lower what's the

link list or much lower what's the downside that's not completely obvious

downside that's not completely obvious from how I've depicted this on

from how I've depicted this on screen what's the price I'm paying here

screen what's the price I'm paying here yeah this is a huge amount of memory the

yeah this is a huge amount of memory the number of cells here in the array is now

number of cells here in the array is now what 26 * 26 * 26 for the first the

what 26 * 26 * 26 for the first the second and the third possible characters

second and the third possible characters all combinatorically combined here

all combinatorically combined here that's a lot I didn't even draw them I

that's a lot I didn't even draw them I have the dot dot dot to evoke that

have the dot dot dot to evoke that instead that's a huge amount of memory

instead that's a huge amount of memory this is a very sparse data set now and

this is a very sparse data set now and odds are you're going to waste so much

odds are you're going to waste so much memory even for the names like Hae ha

memory even for the names like Hae ha like HQ like I can't even think of names

like HQ like I can't even think of names so many of those buckets are going to be

so many of those buckets are going to be empty not to mention the AAA and the zzz

empty not to mention the AAA and the zzz and everything else in between so it's a

and everything else in between so it's a trade-off and it might be too expensive

trade-off and it might be too expensive a trade-off and so you might have to

a trade-off and so you might have to tolerate something like the collisions

tolerate something like the collisions we had earlier whereby even though they

we had earlier whereby even though they might very well happen at least you uh

might very well happen at least you uh are decreasing the probability by

are decreasing the probability by perhaps having more buckets like this

perhaps having more buckets like this and in fact if I rewind now to where we

and in fact if I rewind now to where we might have gone with this here here's

might have gone with this here here's how we might represent these nodes in

how we might represent these nodes in the tree previously in the past we've

the tree previously in the past we've had a person who had a string name and a

had a person who had a string name and a string number AKA now Char star and so

string number AKA now Char star and so here now might be how in this hash table

here now might be how in this hash table we represent someone's name and number

we represent someone's name and number as well as a pointer to the next element

as well as a pointer to the next element in the list we let me rewind just to the

in the list we let me rewind just to the picture here we keep drawing different

picture here we keep drawing different shapes because again these are

shapes because again these are abstractions who really cares if they're

abstractions who really cares if they're to scale now we've got enough room for

to scale now we've got enough room for the person's name not pictured on the

the person's name not pictured on the screen is herm's number that's somewhere

screen is herm's number that's somewhere in this rectangle but yes pictured here

in this rectangle but yes pictured here in this little square is a pointer to

in this little square is a pointer to the next node in the list so by storing

the next node in the list so by storing name and number maybe her address maybe

name and number maybe her address maybe her mailing address whatever in addition

her mailing address whatever in addition to a pointer allows each of these nodes

to a pointer allows each of these nodes to be connectable just like the nodes in

to be connectable just like the nodes in a linked list but where they're starting

a linked list but where they're starting is in an array so the array gets us like

is in an array so the array gets us like 126 of or gets us um Narrows the problem

126 of or gets us um Narrows the problem from size 26 to one gets us to the link

from size 26 to one gets us to the link list in question hopefully it's a single

list in question hopefully it's a single person or perhaps it has more than that

person or perhaps it has more than that meanwhile what is the hash table itself

meanwhile what is the hash table itself the hash table the whole thing is

the hash table the whole thing is literally just an array I've hardcoded

literally just an array I've hardcoded the simplest version of size 26 but what

the simplest version of size 26 but what do each of those boxes in the vertical

do each of those boxes in the vertical array represent just a pointer to

array represent just a pointer to potentially a node a node in a length

potentially a node a node in a length list and if there's no one there if

list and if there's no one there if there's no one in location y or X or the

there's no one in location y or X or the like in that Universe well it's just a

like in that Universe well it's just a null pointer signifying there's no uh

null pointer signifying there's no uh one there but if there is it's going to

one there but if there is it's going to be a pointer to a valid node from which

be a pointer to a valid node from which we can get to any of the others as well

we can get to any of the others as well and that so-called hash function just

and that so-called hash function just like the one I did with the greeting

like the one I did with the greeting cards well it's just an a black box if

cards well it's just an a black box if you will but implemented somewhere in

you will but implemented somewhere in code like in C and so if you pass in

code like in C and so if you pass in Albus what is the hash value of Albus

Albus what is the hash value of Albus well in the first version of the story

well in the first version of the story with 26 buckets it should be a zero if

with 26 buckets it should be a zero if you pass in Zacharias it should be 25

you pass in Zacharias it should be 25 and so just as my cards were being

and so just as my cards were being hashed to one of 1 2 3 four values now

hashed to one of 1 2 3 four values now these names are being hashed to one of

these names are being hashed to one of 26 possibilities or 26 time 26 or 26 to

26 possibilities or 26 time 26 or 26 to the 3 power if you have more and more

the 3 power if you have more and more granularity than

granularity than that questions on this implementation

that questions on this implementation now of this idea of a hash

table if uh if you say that again with the

[Music] null oh a good question so if there's so

null oh a good question so if there's so much spareness there's all of these

much spareness there's all of these empty cells in the array couldn't you

empty cells in the array couldn't you just go in and free them or delete them

just go in and free them or delete them or just kind of shrink the array and not

or just kind of shrink the array and not have a a AA and aab and a a only have

have a a AA and aab and a a only have the prefixes two or three characters

the prefixes two or three characters that you need you absolutely could do

that you need you absolutely could do that but now what you you lose is the

that but now what you you lose is the arithmetic benefit of being able to map

arithmetic benefit of being able to map each letter to a number if you start uh

each letter to a number if you start uh freeing up unused space you don't know

freeing up unused space you don't know that Zacharias is necessarily at

that Zacharias is necessarily at location 25 Albus is still going to be

location 25 Albus is still going to be at location zero but if you've deleted

at location zero but if you've deleted some of the elements in the middle

some of the elements in the middle Zacharias could be at 24 if you've

Zacharias could be at 24 if you've deleted one 23 if you've deleted another

deleted one 23 if you've deleted another and so you don't have that arithmetic

and so you don't have that arithmetic immediacy that you need in order to

immediacy that you need in order to index into the array with constant time

index into the array with constant time and the same is going to be true if it's

and the same is going to be true if it's uh two letters or three letters you need

uh two letters or three letters you need to be able to trust that you can do some

to be able to trust that you can do some quick math and jump to the right index

quick math and jump to the right index in constant time and that's again the

in constant time and that's again the appeal of these arrays so when it comes

appeal of these arrays so when it comes to the running time of a hash table

to the running time of a hash table inserting values into it searching for

inserting values into it searching for values therein at the end of the day it

values therein at the end of the day it technically is Big O of n because in the

technically is Big O of n because in the craziest case you might have a huge

craziest case you might have a huge fancy hash table but everyone in the

fancy hash table but everyone in the universe has a name starting with h and

universe has a name starting with h and then it just evolves into a really long

then it just evolves into a really long link list just like a binary search tree

link list just like a binary search tree could do the same but if you choose a

could do the same but if you choose a smarter hash function maybe you mitigate

smarter hash function maybe you mitigate that and you don't rely only on the

that and you don't rely only on the first letter but on the second or the

first letter but on the second or the third as well or some other combination

third as well or some other combination of that input and make your hash

of that input and make your hash function smarter odds are if you get a

function smarter odds are if you get a good hash function you want it get it to

good hash function you want it get it to be more of like order of n divided by K

be more of like order of n divided by K where K means constant mathematically

where K means constant mathematically and so K is the number of buckets so

and so K is the number of buckets so ideally you want like a uniform

ideally you want like a uniform distribution you want like this many

distribution you want like this many people here this many people here you

people here this many people here you don't want there to be some or no people

don't want there to be some or no people you want a uniform statistical

you want a uniform statistical distribution and maybe you get that from

distribution and maybe you get that from Human names maybe you don't but that's

Human names maybe you don't but that's kind of the challenge of a hash function

kind of the challenge of a hash function of course Big O of n over K is not a

of course Big O of n over K is not a thing because we always throw away

thing because we always throw away constants like K so it's still Big O of

constants like K so it's still Big O of n but again the distinction today is

n but again the distinction today is that okay yes academically you learned

that okay yes academically you learned in cs50 that sure it's Big O ofin but my

in cs50 that sure it's Big O ofin but my God it's 26 times faster if you do the

God it's 26 times faster if you do the hash function well and you spread

hash function well and you spread everyone out over the hash table and

everyone out over the hash table and that's the appeal of these kinds of

that's the appeal of these kinds of structures

structures and we've got one more for you if I may

and we've got one more for you if I may something now known as a try so it turns

something now known as a try so it turns out that a try is even cooler if you

out that a try is even cooler if you like this kind of thing in that it does

like this kind of thing in that it does not devolve into Big O of n it is truly

not devolve into Big O of n it is truly constant time but there's going to be a

constant time but there's going to be a price there's going to be a gotcha a try

price there's going to be a gotcha a try is sort of a fancier tree and it's short

is sort of a fancier tree and it's short for retrieval but pronounced try for

for retrieval but pronounced try for weird historical reasons but a try is a

weird historical reasons but a try is a tree each of whose nodes is an array

tree each of whose nodes is an array right so this is all like crazy mashups

right so this is all like crazy mashups now people started inventing data

now people started inventing data structures just by combining different

structures just by combining different ones unfortunately a lot of the good

ones unfortunately a lot of the good ideas are taken but you just have

ideas are taken but you just have benefits from certain aspects of those

benefits from certain aspects of those data structures and combining them

data structures and combining them ideally gives you the best of both

ideally gives you the best of both worlds so to speak so here might be the

worlds so to speak so here might be the root of a try it's literally a big node

root of a try it's literally a big node a big rectangle but it's actually an

a big rectangle but it's actually an array so there's like 26 locations in

array so there's like 26 locations in this picture here and here's how you use

this picture here and here's how you use a try for instance to store names just

a try for instance to store names just like the hash table it you treat each of

like the hash table it you treat each of the elements of that array in that node

the elements of that array in that node as like a letter of the alphabet so a

as like a letter of the alphabet so a through z or 0 through 25 and if you

through z or 0 through 25 and if you want to store someone's name in here you

want to store someone's name in here you do so as follows if you want to store

do so as follows if you want to store like uh an H you index into the H

like uh an H you index into the H location and if you want to store the

location and if you want to store the second letter of someone's name like an

second letter of someone's name like an a well you add another node below it and

a well you add another node below it and such one is connected to the other and

such one is connected to the other and you then identify the a in that array

you then identify the a in that array and then you go on and maybe put a g if

and then you go on and maybe put a g if the goal is to store spoiler now Hagrid

the goal is to store spoiler now Hagrid in this data structure and then the r

in this data structure and then the r and the I and then the D but when you

and the I and then the D but when you get to the D the end of the name you

get to the D the end of the name you have to somehow flag that this is the

have to somehow flag that this is the end of a name that we've embedded into

end of a name that we've embedded into this data structure so whereas all of

this data structure so whereas all of these are called out in white just to

these are called out in white just to make obvious what we're connecting to

make obvious what we're connecting to what green has to be like a bull that's

what green has to be like a bull that's true that just indicates like the buck

true that just indicates like the buck stops here like d is the last letter in

stops here like d is the last letter in someone's actual name and what's kind of

someone's actual name and what's kind of cool now about a try is that we can

cool now about a try is that we can repeat this for other names as well so

repeat this for other names as well so for instance here is where we might put

for instance here is where we might put Harry as well and notice they share a

Harry as well and notice they share a common prefix ha for Hagrid ha for Harry

common prefix ha for Hagrid ha for Harry so we're reusing some of these nodes

so we're reusing some of these nodes some of these arrays we can even slip

some of these arrays we can even slip Hermione in here too borrowing only the

Hermione in here too borrowing only the H but she gets the H then the E then r r

H but she gets the H then the E then r r m i o n e and so forth and we Mark at

m i o n e and so forth and we Mark at the end of her name too that she's in

the end of her name too that she's in there now what's the takeaway here well

there now what's the takeaway here well what is the running time of a try how

what is the running time of a try how many steps does it take to find someone

many steps does it take to find someone in this data structure and let me zoom

in this data structure and let me zoom out so that it sort of suddenly becomes

out so that it sort of suddenly becomes a massive data structure with even more

a massive data structure with even more in it uh maybe it looks sorry no I'll

in it uh maybe it looks sorry no I'll keep it on this one maybe it looks a

keep it on this one maybe it looks a little something like this with just

little something like this with just these three names but how many steps

these three names but how many steps does it take to find Hagrid or Harry or

does it take to find Hagrid or Harry or Hermione no matter how many names are in

Hermione no matter how many names are in this data structure there's three at the

this data structure there's three at the moment but it takes what h a g r i d so

moment but it takes what h a g r i d so six steps to find Hagrid uh h a r r y

six steps to find Hagrid uh h a r r y five steps to find Harry h e r m i o n e

five steps to find Harry h e r m i o n e eight steps to find Hermione but notice

eight steps to find Hermione but notice that those steps are only dependent on

that those steps are only dependent on what the lengths of the human's names

what the lengths of the human's names and let's assume that no one's going to

and let's assume that no one's going to have a infinitely long name it's going

have a infinitely long name it's going to max out at what like eight no maybe

to max out at what like eight no maybe eight 18 maybe 20 30 there's actually

eight 18 maybe 20 30 there's actually some pretty long human names out there

some pretty long human names out there but it's going to be finite you know

but it's going to be finite you know it's abounded whereas most contexts n

it's abounded whereas most contexts n could grow forever so what's compelling

could grow forever so what's compelling here is if you assume that the longest

here is if you assume that the longest name is I don't know 50 for the sake of

name is I don't know 50 for the sake of a theme here then you know that finding

a theme here then you know that finding anyone in this data structure will take

anyone in this data structure will take you no more than 50 steps 50 is thus a

you no more than 50 steps 50 is thus a constant which means you have big O of

constant which means you have big O of one running time it doesn't matter if

one running time it doesn't matter if there's a million people in this phone

there's a million people in this phone book or a billion people in this phone

book or a billion people in this phone book that's going to definitely add more

book that's going to definitely add more nodes to it but it's still going to take

nodes to it but it's still going to take you h a r uh sorry h a g r i d six steps

you h a r uh sorry h a g r i d six steps to find Hagrid h a r r y five steps to

to find Hagrid h a r r y five steps to find Harry even if there's a billion

find Harry even if there's a billion other people in that data structure so

other people in that data structure so now we actually do seem to have constant

now we actually do seem to have constant time if you assume that there's going to

time if you assume that there's going to be a bound on the length of the name why

be a bound on the length of the name why don't we use tries for everything then

don't we use tries for everything then what's the price we're paying for this

what's the price we're paying for this data structure even though we've

data structure even though we've represented just three characters here

represented just three characters here yeah it's a lot of memory yeah and you

yeah it's a lot of memory yeah and you can see it even with these three names

can see it even with these three names most of the squares on the screen are

most of the squares on the screen are empty like bites and bits that are there

empty like bites and bits that are there and are allocated and they need to be

and are allocated and they need to be there because you need to be able to do

there because you need to be able to do that arithmetic thing of this being zero

that arithmetic thing of this being zero this being 25 so you can jump from boom

this being 25 so you can jump from boom boom boom boom based on each of the

boom boom boom based on each of the letters but it's a hugely sparse data

letters but it's a hugely sparse data structure which means it takes up a

structure which means it takes up a crazy amount of memory now maybe that's

crazy amount of memory now maybe that's tolerable especially for short names but

tolerable especially for short names but that's going to be the trade-off as well

that's going to be the trade-off as well and this is such a tension in Computing

and this is such a tension in Computing almost any time you want to improve time

almost any time you want to improve time you want to speed up the efficiency the

you want to speed up the efficiency the speed of your algorithm you're going to

speed of your algorithm you're going to spend space if by contrast you want to

spend space if by contrast you want to decrease the amount of space you might

decrease the amount of space you might very well have to increase the running

very well have to increase the running time it is indeed this seesaw back and

time it is indeed this seesaw back and forth and you your colleagues your

forth and you your colleagues your company need to decide what resource is

company need to decide what resource is the most precious heck it might be much

the most precious heck it might be much harder to code one of these data

harder to code one of these data structures than another you're a human

structures than another you're a human your time is valuable do you really want

your time is valuable do you really want to spend hours implementing a try when

to spend hours implementing a try when you know hey in 30 minutes I can bang

you know hey in 30 minutes I can bang out an array nowadays or a linked list

out an array nowadays or a linked list even there too development time is going

even there too development time is going to be yet another resource and why

to be yet another resource and why sometimes there's good code or bad code

sometimes there's good code or bad code it depends on what you're prioritizing

it depends on what you're prioritizing so what do each of these nodes look like

so what do each of these nodes look like in a try well we can keep calling it a

in a try well we can keep calling it a node this is a very generic term for

node this is a very generic term for just a container in these data

just a container in these data structures in this story though let me

structures in this story though let me claim that everyone has a number like a

claim that everyone has a number like a phone number a string akhr star every

phone number a string akhr star every every node has 26 children or

every node has 26 children or technically an array of size 26 that can

technically an array of size 26 that can point to more of these nodes notice that

point to more of these nodes notice that I don't need to store the name of

I don't need to store the name of someone in a try because it's implicit

someone in a try because it's implicit in the path that you take to find them

in the path that you take to find them so that's a minor optimization but it

so that's a minor optimization but it saves us some space but this would be

saves us some space but this would be just a different data structure we could

just a different data structure we could use to actually solve this problem as

use to actually solve this problem as well albeit at a very expensive cost and

well albeit at a very expensive cost and what do we need our variable to be that

what do we need our variable to be that stores the try just like four we just

stores the try just like four we just need a single pointer that hangs on to

need a single pointer that hangs on to the root of this structure that's null

the root of this structure that's null if it's empty or non-null if it's

if it's empty or non-null if it's actually pointing at

actually pointing at something any questions then on

something any questions then on tries and if it's feeling like a lot the

tries and if it's feeling like a lot the fire the the fire hydrant it is we

fire the the fire hydrant it is we started with arrays then link list then

started with arrays then link list then tries but questions on how we've just

tries but questions on how we've just assembled from these basic building

assembled from these basic building blocks

blocks [Music]

[Music] yeah a good question why is this not uh

yeah a good question why is this not uh size 26 uh it's just like with the try

size 26 uh it's just like with the try just like with the link list before it

just like with the link list before it just tends to be en code convenient to

just tends to be en code convenient to have a separate additional pointer

have a separate additional pointer that's small that just points to the

that's small that just points to the beginning of the data structure because

beginning of the data structure because that way it can be null thereby clearly

that way it can be null thereby clearly indicating there are no nodes the whole

indicating there are no nodes the whole structure is empty if you allocated one

structure is empty if you allocated one of those nodes you absolutely could but

of those nodes you absolutely could but then you'd be just wasting space even if

then you'd be just wasting space even if it's empty and it creates an ambiguity

it's empty and it creates an ambiguity so just having a single pointer linked

so just having a single pointer linked to the beginnings of all of these things

to the beginnings of all of these things is a good thing other questions now on

is a good thing other questions now on tries or trees or hash tables or

tries or trees or hash tables or arrays so what problems might arise well

arrays so what problems might arise well here's a counter example what names are

here's a counter example what names are manifest in this try

out what do you see Danielle and Danielle so presumably

see Danielle and Danielle so presumably if these are two uh names here one of

if these are two uh names here one of which is a prefix of another notice that

which is a prefix of another notice that the data structure still works and I

the data structure still works and I chose you know a friend's name and then

chose you know a friend's name and then appended a couple of more characters to

appended a couple of more characters to it that's also a name because we have

it that's also a name because we have here d a n i e l and the green

here d a n i e l and the green technically is implemented as a bull or

technically is implemented as a bull or something like that that indicates a

something like that that indicates a word stops here but we don't want to

word stops here but we don't want to preclude storing Danielle as well who's

preclude storing Danielle as well who's a super string if you will of Daniel and

a super string if you will of Daniel and so that's okay too so long as the

so that's okay too so long as the structure allows for the pointers to

structure allows for the pointers to keep going so even that works out okay

keep going so even that works out okay whereas it might not have otherwise and

whereas it might not have otherwise and in terms of the running time just to be

in terms of the running time just to be clear at the end of the day tries do

clear at the end of the day tries do give you actual constant time for

give you actual constant time for insertions lookups deletions and the

insertions lookups deletions and the like because it's dependent only on the

like because it's dependent only on the length of the input the key if you will

length of the input the key if you will and not on how many other people are in

and not on how many other people are in your phone or a dress book and now

your phone or a dress book and now thought we'd conclude with a visual if

thought we'd conclude with a visual if you've gotten out into the square anyone

you've gotten out into the square anyone recognize this okay okay sweet green a

recognize this okay okay sweet green a local salad place what are we looking at

local salad place what are we looking at here and what's its connection to today

here and what's its connection to today um you're about to become all the

um you're about to become all the geekier in the real world cuz you will

geekier in the real world cuz you will start to see data structures

start to see data structures everywhere what is this or how does this

everywhere what is this or how does this work maybe in salad form who's been to

work maybe in salad form who's been to Sweet

Sweet green okay either of you so how does

green okay either of you so how does this

this [Music]

work okay good so if you order a salad for someone named L when it's ready they

for someone named L when it's ready they put it in the l section here and so this

put it in the l section here and so this is kind of a set of key value pairs

is kind of a set of key value pairs right if L is the first letter of

right if L is the first letter of someone's name the value hopefully is

someone's name the value hopefully is the salad and so what you kind of have

the salad and so what you kind of have here is a dictionary key value pairs

here is a dictionary key value pairs where it's not words and definitions

where it's not words and definitions it's names and salads and you can think

it's names and salads and you can think of this too as kind of a hash table why

of this too as kind of a hash table why even though it actually doesn't fit on

even though it actually doesn't fit on one long shelf cu the store is only so

one long shelf cu the store is only so big this is really an array and

big this is really an array and apparently a is missing or maybe it's

apparently a is missing or maybe it's around around the corner but this array

around around the corner but this array just happens to wrap onto multiple lines

just happens to wrap onto multiple lines but it's still conceptually a single

but it's still conceptually a single Dimension but suppose two people have

Dimension but suppose two people have the name L what do they do

typically yeah so maybe they they well if they run put that much effort into it

if they run put that much effort into it they might look at the second letter and

they might look at the second letter and then the third letter odds are this is

then the third letter odds are this is not that interesting a problem to solve

not that interesting a problem to solve optimally in that way but they probably

optimally in that way but they probably do start stacking the salads on top of

do start stacking the salads on top of each other maybe scooching it over just

each other maybe scooching it over just a little bit and so what do you have

a little bit and so what do you have there well now you start to view the

there well now you start to view the lens through like cs50 glasses like okay

lens through like cs50 glasses like okay you have an array and then you have like

you have an array and then you have like these link lists that are sort of

these link lists that are sort of growing here but even then you run into

growing here but even then you run into a problem why because it's not really a

a problem why because it's not really a link list because at some point you're

link list because at some point you're going to hit the boundary here so it's

going to hit the boundary here so it's kind of like an array of arrays because

kind of like an array of arrays because you can only fit what like three or four

you can only fit what like three or four salads here and so long story short we

salads here and so long story short we started today deliberately talking about

started today deliberately talking about real world things like stacks and cues

real world things like stacks and cues and even though it did escalate quickly

and even though it did escalate quickly into binary search trees and hash tables

into binary search trees and hash tables and tries even those things are

and tries even those things are everywhere even though they don't call

everywhere even though they don't call them as such these are just solutions to

them as such these are just solutions to problems and now with this final week of

problems and now with this final week of SE under your belt you have all the more

SE under your belt you have all the more of a technical toolkit via which to

of a technical toolkit via which to implement these things and code next

implement these things and code next week we'll be able to trust that someone

week we'll be able to trust that someone else solved all these problems we'll

else solved all these problems we'll introduce Python and lines of code like

introduce Python and lines of code like this will finally become lines of code

this will finally become lines of code like that so that's the promise ahead

like that so that's the promise ahead and we'll see you next

and we'll see you next [Applause]

[Applause] [Music]

all right this is cs50 and this is week six wherein we finally transitioned from

six wherein we finally transitioned from scratch to see to now Python and indeed

scratch to see to now Python and indeed this is going to be somewhat of a unique

this is going to be somewhat of a unique experience in that just like a few weeks

experience in that just like a few weeks past perhaps for the first time and now

past perhaps for the first time and now uh today you're going to learn a new

uh today you're going to learn a new language but the goal isn't just to

language but the goal isn't just to throw another fire hose of content and

throw another fire hose of content and syntax and whatnot at you but rather to

syntax and whatnot at you but rather to really equip you all to actually teach

really equip you all to actually teach yourself new languages in the future and

yourself new languages in the future and so indeed what we'll do today what we'll

so indeed what we'll do today what we'll do this coming week is sort of prepare

do this coming week is sort of prepare you to stand on your own and once python

you to stand on your own and once python is p a and the world has moved on to

is p a and the world has moved on to some other language in some number of

some other language in some number of years you'll be well equipped to figure

years you'll be well equipped to figure out how to wrap your mind around some

out how to wrap your mind around some new syntax some new language and solve

new syntax some new language and solve problems as well now you recall in week

problems as well now you recall in week zero this is where we started just

zero this is where we started just saying hello to the world and that

saying hello to the world and that quickly escalated just a week later in C

quickly escalated just a week later in C to see be something much much more

to see be something much much more cryptic and if you've still sort of

cryptic and if you've still sort of struggled with some of the syntax find

struggled with some of the syntax find yourself checking your notes or your

yourself checking your notes or your previous code like that's totally normal

previous code like that's totally normal and that's one of the reasons why there

and that's one of the reasons why there are languages besides C out there among

are languages besides C out there among them this language called python humans

them this language called python humans over the decades have realized gee that

over the decades have realized gee that wasn't necessarily the best designed

wasn't necessarily the best designed decision or humans have realized wow you

decision or humans have realized wow you know what now that computers have gotten

know what now that computers have gotten faster with more memory and more faster

faster with more memory and more faster CPUs we can actually do more with our

CPUs we can actually do more with our programming languages so just as human

programming languages so just as human languages evolve so do actual

languages evolve so do actual programming languages and even within a

programming languages and even within a programming language there's typically

programming language there's typically different versions We for instance have

different versions We for instance have been using version C11 of C which was

been using version C11 of C which was updated in 2011 um but python itself

updated in 2011 um but python itself continues to evolve and it's now up to

continues to evolve and it's now up to version three plus and so there too

version three plus and so there too these things will evolve in the coming

these things will evolve in the coming days thankfully what you're about to see

days thankfully what you're about to see is hello world for the third time but

is hello world for the third time but it's going to be literally this none of

it's going to be literally this none of the crazy syntax above or below fewer

the crazy syntax above or below fewer semicolons if any fewer curly braces and

semicolons if any fewer curly braces and really a lot of the distractions get out

really a lot of the distractions get out of the way so to get there let's

of the way so to get there let's consider exactly how we've been

consider exactly how we've been programming up until now so you write a

programming up until now so you write a program in C and you've got hopefully no

program in C and you've got hopefully no syntax error so you're ready to build it

syntax error so you're ready to build it that is compil it and so you've run make

that is compil it and so you've run make and then you've run the program like/

and then you've run the program like/ hello or if you think back to week two

hello or if you think back to week two where we took a peak underneath the hood

where we took a peak underneath the hood of what make is doing it's really

of what make is doing it's really running the actual compiler something

running the actual compiler something called clang maybe with some command

called clang maybe with some command line arguments creating a program called

line arguments creating a program called hello and then you could do do/ hello so

hello and then you could do do/ hello so today you're going to start doing

today you're going to start doing something similar in spirit but fewer

something similar in spirit but fewer steps no longer will you have to compile

steps no longer will you have to compile your code and then run it and then maybe

your code and then run it and then maybe fix or change it and then compile your

fix or change it and then compile your code and run it and then repeat repeat

code and run it and then repeat repeat the process of running your code is

the process of running your code is going to be distilled into just a single

going to be distilled into just a single step and the way to think of this for

step and the way to think of this for now is that where a c is frequently used

now is that where a c is frequently used as indeed a compiled language whereby

as indeed a compiled language whereby you convert it first to zeros and ones

you convert it first to zeros and ones Python's going to let you speed things

Python's going to let you speed things up whereby you the human programmer

up whereby you the human programmer don't have to compile it you're just

don't have to compile it you're just going to run what's called an

going to run what's called an interpreter which by Design is named the

interpreter which by Design is named the exact same thing as the language itself

exact same thing as the language itself and by running this program installed in

and by running this program installed in VSS code or eventually on your own Max

VSS code or eventually on your own Max or PCS this is just going to tell your

or PCS this is just going to tell your computer to interpret this code and

computer to interpret this code and figure out how to get down to that lower

figure out how to get down to that lower level of zeros and ones but you don't

level of zeros and ones but you don't have to compile the code yourself

have to compile the code yourself anymore so with that said let's consider

anymore so with that said let's consider what the code is going to look like side

what the code is going to look like side by side in fact let's look back at some

by side in fact let's look back at some scratch blocks just like we did with C

scratch blocks just like we did with C in week one and do some side by sides

in week one and do some side by sides because even though some of the syntax

because even though some of the syntax this week and Beyond's going to be

this week and Beyond's going to be different like the ideas are truly going

different like the ideas are truly going to be the same there's not all that much

to be the same there's not all that much intellectually new just yet so whereas

intellectually new just yet so whereas in week zero we might have said hello to

in week zero we might have said hello to the world with this purple puzzle piece

the world with this purple puzzle piece today of course uh or rather in week one

today of course uh or rather in week one it looked like this in C but today

it looked like this in C but today moving forward it's going to quite

moving forward it's going to quite simply look like this instead and if we

simply look like this instead and if we go back and forth for just a moment here

go back and forth for just a moment here again is the version in C noticing the

again is the version in C noticing the very seike characteristics and just at a

very seike characteristics and just at a glance here in Python I claim it's now

glance here in Python I claim it's now this what do you apparently need not

this what do you apparently need not worry about

worry about anymore what's gone so semicolon is gone

anymore what's gone so semicolon is gone and indeed you don't need those to

and indeed you don't need those to finish most of your thoughts anymore

finish most of your thoughts anymore anything else so the back sln is absent

anything else so the back sln is absent and that's kind of curious because we're

and that's kind of curious because we're still going to get a new line but we'll

still going to get a new line but we'll see that it's become the default and

see that it's become the default and this one's a little more subtle but now

this one's a little more subtle but now the function is called print instead of

the function is called print instead of print F so it's a little more familiar

print F so it's a little more familiar in that sense all right so when it comes

in that sense all right so when it comes to using libraries that is code that

to using libraries that is code that other people have written in the past

other people have written in the past we've done things like hash include

we've done things like hash include cs50.h to use cs50's own header file or

cs50.h to use cs50's own header file or standard IO or standard lib or string or

standard IO or standard lib or string or any number of other header files you

any number of other header files you have all used well moving forward we're

have all used well moving forward we're going to give you for this first week a

going to give you for this first week a similar cs-50 Library just very

similar cs-50 Library just very short-term uh training wheels that will

short-term uh training wheels that will quickly take off because in reality it's

quickly take off because in reality it's a lot easier to do things in python as

a lot easier to do things in python as we'll see but the Syntax for this now is

we'll see but the Syntax for this now is going to be to import the cs50 library

going to be to import the cs50 library in this way and when we have now this

in this way and when we have now this ability we can actually start writing

ability we can actually start writing some code right away in fact let me

some code right away in fact let me switch over to vs code here and just as

switch over to vs code here and just as in the past I'll create a new file but

in the past I'll create a new file but instead of creating something called C

instead of creating something called C I'm going to go ahead and create my

I'm going to go ahead and create my first program called hello.py using Code

first program called hello.py using Code space hello.py that of course gives me

space hello.py that of course gives me this new tab and let me actually quite

this new tab and let me actually quite simply do what I proposed print quote

simply do what I proposed print quote unquote hello world without the back

unquote hello world without the back slash without the semicolon without the

slash without the semicolon without the F in print and now let me go down to my

F in print and now let me go down to my terminal window and I don't have to

terminal window and I don't have to compile it I don't have to do dot slash

compile it I don't have to do dot slash i instead run a program called python

i instead run a program called python whose purpose in life is now to

whose purpose in life is now to interpret my code top to bottom left to

interpret my code top to bottom left to right and if I run python of hello.py

right and if I run python of hello.py crossing my fingers as always voila now

crossing my fingers as always voila now I have printed out hello world so we

I have printed out hello world so we seem to have gotten the new line for

seem to have gotten the new line for free in this sense where it's

free in this sense where it's automatically happening the dollar sign

automatically happening the dollar sign isn't weirdly on the same line like it

isn't weirdly on the same line like it want was in week one but that's just a a

want was in week one but that's just a a minor detail here if we switch back to

minor detail here if we switch back to now some other capabilities well indeed

now some other capabilities well indeed with the cs50 library you can also not

with the cs50 library you can also not just import the library itself but

just import the library itself but specific functions and you'll see that

specific functions and you'll see that temporarily we're going to give you a

temporarily we're going to give you a helper function called get string just

helper function called get string just like in C that just makes it work

like in C that just makes it work exactly the same way as in C and we'll

exactly the same way as in C and we'll see a couple of other functions that

see a couple of other functions that will just make life easier initially but

will just make life easier initially but quickly will we take those training

quickly will we take those training wheels off so that nothing is indeed

wheels off so that nothing is indeed cs50 specific all right well how about

cs50 specific all right well how about functions more generally in Python let's

functions more generally in Python let's do a whirlwind tour if you will much

do a whirlwind tour if you will much like we did in that first week of C

like we did in that first week of C comparing one to the other so back in

comparing one to the other so back in our world of scratch one of the first

our world of scratch one of the first programs we wrote was this one here

programs we wrote was this one here whereby we asked the human their name we

whereby we asked the human their name we then used the return value that was sort

then used the return value that was sort of automatically stored in this answer

of automatically stored in this answer variable as a second argument to join so

variable as a second argument to join so that we could say hello David or hello

that we could say hello David or hello Carter so this was back in week one uh

Carter so this was back in week one uh week zero in week one we converted it to

week zero in week one we converted it to this and here's a perfect example of

this and here's a perfect example of things like escalating quickly and again

things like escalating quickly and again this is why we start in scratch there's

this is why we start in scratch there's just so much distraction here to achieve

just so much distraction here to achieve the same idea but even today we're going

the same idea but even today we're going to chip away at some of that syntax so

to chip away at some of that syntax so in C we had to declare the argument as a

in C we had to declare the argument as a we had to declare the variable as a

we had to declare the variable as a string here we of course had the

string here we of course had the semicolon and more well in Python the

semicolon and more well in Python the comparable code now is going to look

comparable code now is going to look more simply like this so semicolon is

more simply like this so semicolon is again gone on both lines for that matter

again gone on both lines for that matter so that's good what else appears to have

so that's good what else appears to have changed or disappeared yeah type of

changed or disappeared yeah type of variable yeah so I didn't have to

variable yeah so I didn't have to specifically say that answer is now a

specifically say that answer is now a string and indeed python is is

string and indeed python is is dynamically typed and in fact it will

dynamically typed and in fact it will infer from Context exactly what it is

infer from Context exactly what it is you are storing in that variable other

you are storing in that variable other details that seem a little bit

different little bit different what else jumps out at you here I'll go back this

jumps out at you here I'll go back this was the C version and maybe Focus now on

was the C version and maybe Focus now on the second line because we've rather

the second line because we've rather exhausted the first here's now the

exhausted the first here's now the python version what's different here

yeah yeah there's no percent s anymore there's no second argument at the moment

there's no second argument at the moment per se to print now it is still a little

per se to print now it is still a little weird it's as though I've like deployed

weird it's as though I've like deployed some addition here arithmetically but

some addition here arithmetically but that's not the case some of you have

that's not the case some of you have programmed before and plus some of you

programmed before and plus some of you might know means what in this

might know means what in this context so to combine or more

context so to combine or more technically anyone know the buzz word

technically anyone know the buzz word yeah

yeah to concatenate so to concatenate is like

to concatenate so to concatenate is like the fancy way of what scratch calls

the fancy way of what scratch calls joining which is to take one string on

joining which is to take one string on the left one string on the right and to

the left one string on the right and to join them together to glue them together

join them together to glue them together if you will so this is not addition it

if you will so this is not addition it would be if it were numbers involved

would be if it were numbers involved instead but because we've got a string

instead but because we've got a string hello comma and another string

hello comma and another string implicitly in this variable based on

implicitly in this variable based on what the human typed in in response to

what the human typed in in response to this get string function that's going to

this get string function that's going to concatenate hello comma space and then

concatenate hello comma space and then David or Carter or whatever the human

David or Carter or whatever the human has typed in but turns out there's going

has typed in but turns out there's going to be different ways to do this in

to be different ways to do this in Python and we'll show you a few

Python and we'll show you a few different ones and here too try not to

different ones and here too try not to get too hung up on or frustrated by like

get too hung up on or frustrated by like all of the different ways you can solve

all of the different ways you can solve problems odds are you're going to be

problems odds are you're going to be picking up tips and techniques for years

picking up tips and techniques for years to come if you continue programming so

to come if you continue programming so let's just give you a few of the

let's just give you a few of the possible ways so here's a second way you

possible ways so here's a second way you could print out hello comma David or

could print out hello comma David or hello comma Carter but what has changed

hello comma Carter but what has changed in the previous version I used

in the previous version I used concatenation explicitly and the space

concatenation explicitly and the space here is important grammatically just so

here is important grammatically just so we get that in the final phrase

we get that in the final phrase now I'm proposing to get rid of that

now I'm proposing to get rid of that space to add a comma outside of the

space to add a comma outside of the double quotes as well but if you think

double quotes as well but if you think back to C this probably just means that

back to C this probably just means that print similar in spirit to print F can

print similar in spirit to print F can take not just one argument but even two

take not just one argument but even two and in fact because of this comma in the

and in fact because of this comma in the middle that's outside of the double

middle that's outside of the double quotes it's hello comma and then it will

quotes it's hello comma and then it will be automatically concatenated with even

be automatically concatenated with even without using the plus to whatever the

without using the plus to whatever the value of answer is and by default just

value of answer is and by default just for grammatical prettiness the print fun

for grammatical prettiness the print fun function always gives you a space for

function always gives you a space for free in between each of the multiple

free in between each of the multiple arguments you pass in we'll see how you

arguments you pass in we'll see how you can override that down the line but for

can override that down the line but for now that's just another way to do it now

now that's just another way to do it now perhaps the better if slightly cryptic

perhaps the better if slightly cryptic way to do this or just the increasingly

way to do this or just the increasingly common way is probably this third

common way is probably this third version which looks a little weird too

version which looks a little weird too and probably the weirdness jumps out

and probably the weirdness jumps out we've automatic we've suddenly

we've automatic we've suddenly introduced these like Curly braces which

introduced these like Curly braces which I promised were mostly gone and they are

I promised were mostly gone and they are but inside of this string here I've done

but inside of this string here I've done a curly brace which might mean what just

a curly brace which might mean what just intuitively and here is sort of an

intuitively and here is sort of an example of how you learn a new language

example of how you learn a new language just kind of infer from Context how

just kind of infer from Context how python probably

python probably works what might this mean yeah able to

works what might this mean yeah able to tell that this is not one of the actual

tell that this is not one of the actual stat

stat inside yeah so this is an indication

inside yeah so this is an indication because the curly braces because this is

because the curly braces because this is the way python was designed that we want

the way python was designed that we want to plug in the value of answer not

to plug in the value of answer not literally a ANS w r and the fancy word

literally a ANS w r and the fancy word here is that the answer variable will be

here is that the answer variable will be interpolated that is substituted with

interpolated that is substituted with its actual value but but but and this is

its actual value but but but and this is actually weird looking and this was

actually weird looking and this was introduced a few years ago to python

introduced a few years ago to python what else did I have to change to make

what else did I have to change to make these curly braces work

these curly braces work apparently

apparently yeah yeah there's this weird F and so

yeah yeah there's this weird F and so it's sort of like part of print F but

it's sort of like part of print F but now it's inside the curly it's inside

now it's inside the curly it's inside the parenthesis there this is just the

the parenthesis there this is just the way python designed this so a few years

way python designed this so a few years ago when they introduced what are called

ago when they introduced what are called format strings or F strings you

format strings or F strings you literally prefix your quoted string with

literally prefix your quoted string with the uh letter F and then you can use

the uh letter F and then you can use trickery like this like putting curly

trickery like this like putting curly braces so that the value will be

braces so that the value will be substituted automatically if you forget

substituted automatically if you forget the F you're going to literally see

the F you're going to literally see hello comma curly brace answer close

hello comma curly brace answer close curly brace if you add the F it's indeed

curly brace if you add the F it's indeed interpolated the value is plugged in all

interpolated the value is plugged in all right questions on how we can just say

right questions on how we can just say hello to the world via python in this

hello to the world via python in this case

case yeah if you do this without the without

yeah if you do this without the without the F if you omit the F you will

the F if you omit the F you will literally see c h e l l o comma curly

literally see c h e l l o comma curly brace a NS w r close curly brace so in

brace a NS w r close curly brace so in fact let's do this let me go back to vs

fact let's do this let me go back to vs code here quickly I've still got my file

code here quickly I've still got my file called hello.py open and let me go ahead

called hello.py open and let me go ahead and change this ever so slightly so I'm

and change this ever so slightly so I'm going to go ahead and uh let's say from

going to go ahead and uh let's say from cs50 import get string and that's just

cs50 import get string and that's just the new syntax I propose using to import

the new syntax I propose using to import a function from someone else's Library

a function from someone else's Library I'm going to now go ahead and ask the

I'm going to now go ahead and ask the question uh let's go ahead and use get

question uh let's go ahead and use get string storing the result and answer so

string storing the result and answer so get string quote unquote what's your

get string quote unquote what's your name question mark and then on this line

name question mark and then on this line I'm going to deliberately make a mistake

I'm going to deliberately make a mistake here exactly to your question let me

here exactly to your question let me just say hello comma answer and just

just say hello comma answer and just this now even though answer is a

this now even though answer is a variable Python's not going to be so

variable Python's not going to be so presumptuous as to just plug in the

presumptuous as to just plug in the value of a variable called answer what

value of a variable called answer what it's going to do of course is if I type

it's going to do of course is if I type in my name whoops I typed too fast let

in my name whoops I typed too fast let me go ahead and rerun that again if I

me go ahead and rerun that again if I run python of hello.py type in my name

run python of hello.py type in my name and hit enter I get hello comma answer

and hit enter I get hello comma answer well let me do one better let me apply

well let me do one better let me apply these curly braces as before let me

these curly braces as before let me rerun python of hello.py What's Your

rerun python of hello.py What's Your Name daav ID and here's again the answer

Name daav ID and here's again the answer to your question now we get literally

to your question now we get literally the curly braces so the fix here

the curly braces so the fix here ultimately is just going to be to add

ultimately is just going to be to add the F there rerun my program again with

the F there rerun my program again with daav ID and now hello comma David so

daav ID and now hello comma David so this is admittedly a little more cryptic

this is admittedly a little more cryptic than the ones with the Plus the comma

than the ones with the Plus the comma but this is just increasingly common why

but this is just increasingly common why because you can read it left to right

because you can read it left to right it's nice and convenient it's less

it's nice and convenient it's less cryptic than the percent s's so it's

cryptic than the percent s's so it's sort of a new and improved version if

sort of a new and improved version if you will of printf in C based on Decades

you will of printf in C based on Decades of experience of programmers doing

of experience of programmers doing things like this questions on printing

things like this questions on printing in this

in this way we're now on our way to programming

way we're now on our way to programming in

in Python anything all right well what more

Python anything all right well what more can we do with this language here well

can we do with this language here well let me propose that we consider that we

let me propose that we consider that we have for

have for instance a few other features that we

instance a few other features that we can add to the mix as well

can add to the mix as well namely let's say some data types as well

namely let's say some data types as well so let me flip over here to um back to

so let me flip over here to um back to the slides and there's different data

the slides and there's different data types in python as we'll soon see but

types in python as we'll soon see but they're not as explicit as we already

they're not as explicit as we already saw by using a string from get string

saw by using a string from get string you don't have to explicitly State what

you don't have to explicitly State what it is but you solve recall and see all

it is but you solve recall and see all of these various data types and then in

of these various data types and then in python kind of nicely enough this list

python kind of nicely enough this list is about to get shorter and so here is

is about to get shorter and so here is our list in C here is an abbreviated

our list in C here is an abbreviated list in Python so we're still going to

list in Python so we're still going to have strings but they're going to be

have strings but they're going to be more succinctly called stirs now St Str

more succinctly called stirs now St Str we're still going to have in for

we're still going to have in for integers we're still going to have

integers we're still going to have floats for floating Point values we're

floats for floating Point values we're even going to have bulls for true and

even going to have bulls for true and false but what's missing now from the

false but what's missing now from the list is long and floats and why is that

list is long and floats and why is that or rather long and double well recall

or rather long and double well recall that in C those used more bits well in

that in C those used more bits well in Python the smaller data types previously

Python the smaller data types previously int and Float themselves just use more

int and Float themselves just use more bits for you and so you don't need to

bits for you and so you don't need to distinguish between small and large you

distinguish between small and large you just use one data type and the language

just use one data type and the language gives you a bigger range than before it

gives you a bigger range than before it turns out though there's going to be

turns out though there's going to be some other features as well of python

some other features as well of python these data types one of which will be

these data types one of which will be called range another of which will be

called range another of which will be list So Gone will be arrays we'll

list So Gone will be arrays we'll actually use something literally called

actually use something literally called a list tle sort of like XY pairs for

a list tle sort of like XY pairs for coordinates and things like that uh dict

coordinates and things like that uh dict for dictionaries so we have built-in

for dictionaries so we have built-in capabilities for storing keys and values

capabilities for storing keys and values we'll see and even a set sort of

we'll see and even a set sort of mathematically a set is like a

mathematically a set is like a collection of values but it

collection of values but it automatically gets rid of duplicates for

automatically gets rid of duplicates for you so all of these things we could

you so all of these things we could absolutely Implement in C if we wanted

absolutely Implement in C if we wanted and indeed in problem set five you've

and indeed in problem set five you've been implementing your very own spell

been implementing your very own spell checker using some form of hashtable

checker using some form of hashtable well it turns out that in Python you can

well it turns out that in Python you can solve those same problems but perhaps a

solve those same problems but perhaps a little more readily in fact let me go

little more readily in fact let me go back over here to vs code and let me

back over here to vs code and let me propose that I do the following let me

propose that I do the following let me go ahead and create a file called

go ahead and create a file called dictionary. let me propose that I try to

dictionary. let me propose that I try to implement say problem set five our spell

implement say problem set five our spell checker in Python instead of c and

checker in Python instead of c and Achieve ultimately the same kind of

Achieve ultimately the same kind of behavior uh whereby I'll be able to

behavior uh whereby I'll be able to spell check a whole bunch of words so

spell check a whole bunch of words so this is jumping the gun a little bit

this is jumping the gun a little bit because you're about to see syntax will

because you're about to see syntax will revisit over the course of today but for

revisit over the course of today but for now I've got a new file called

now I've got a new file called dictionary. py and let me begin to

dictionary. py and let me begin to create uh some placehold ERS for

create uh some placehold ERS for functions we'll see in just a bit that

functions we'll see in just a bit that in Python you can define a function

in Python you can define a function called check and that check function can

called check and that check function can take a word as it's input and I'll come

take a word as it's input and I'll come back to this in just a moment in Python

back to this in just a moment in Python I can define a second function like load

I can define a second function like load which itself will take a whole

which itself will take a whole dictionary just like in problem set five

dictionary just like in problem set five and I'll go ahead and come back to the

and I'll go ahead and come back to the implementation of this meanwhile we

implementation of this meanwhile we might similarly Implement a function

might similarly Implement a function called size which takes no arguments but

called size which takes no arguments but ultimately is going to return the size

ultimately is going to return the size of my dictionary of words and then last

of my dictionary of words and then last lastly for consistency with problem set

lastly for consistency with problem set five we might Define an unload function

five we might Define an unload function whose purpose in life is to free any

whose purpose in life is to free any memory that you've been using just to

memory that you've been using just to give it back to the computer now odds

give it back to the computer now odds are whether you're still working on

are whether you're still working on speller or half finished speller like

speller or half finished speller like you wrote a decent amount of lines of

you wrote a decent amount of lines of code and indeed it's been by design a

code and indeed it's been by design a challenge but one of the reasons for

challenge but one of the reasons for these higher level languages like python

these higher level languages like python is that you can stand on the shoulders

is that you can stand on the shoulders of programmers before you and solve very

of programmers before you and solve very common problems much more quickly so

common problems much more quickly so that you can f us on building your new

that you can f us on building your new app or your web application or your own

app or your web application or your own project to solve problems of interest to

project to solve problems of interest to you so at the risk of crushing some

you so at the risk of crushing some Spirits let me propose that in Python if

Spirits let me propose that in Python if you want a dictionary for something like

you want a dictionary for something like a spell checker well that's fine go

a spell checker well that's fine go ahead and give yourself a variable like

ahead and give yourself a variable like words to store all of those words and

words to store all of those words and just assign it equal to a dictionary or

just assign it equal to a dictionary or dict for short in Python that will give

dict for short in Python that will give you a hashtable now it turns out in

you a hashtable now it turns out in speller recall you don't need to worry

speller recall you don't need to worry about words and definition conditions

about words and definition conditions it's just about spellchecking the words

it's just about spellchecking the words so strictly speaking we don't need keys

so strictly speaking we don't need keys and values we just need keys so I'm

and values we just need keys so I'm going to save myself a few more key

going to save myself a few more key strokes by just saying that technically

strokes by just saying that technically in Python using a set suffices again a

in Python using a set suffices again a set is just a collection of values with

set is just a collection of values with no duplicates but they don't necessarily

no duplicates but they don't necessarily have uh keys and values it's just one or

have uh keys and values it's just one or the other but now that I have on line

the other but now that I have on line one I claim the equivalent in Python of

one I claim the equivalent in Python of a hash table I can actually do something

a hash table I can actually do something like this here's how I might implement

like this here's how I might implement the check function in Python if the word

the check function in Python if the word passed into this function is in my

passed into this function is in my variable called words well return true

variable called words well return true else go ahead and return false done wait

else go ahead and return false done wait you're thinking if anything at all maybe

you're thinking if anything at all maybe we want to handle lowercase instead of

we want to handle lowercase instead of just uppercase and lowercase well you

just uppercase and lowercase well you know what in Python if you want to force

know what in Python if you want to force a whole word to lowercase you don't have

a whole word to lowercase you don't have to iterate over it with a loop you don't

to iterate over it with a loop you don't have to use any of that ctype functions

have to use any of that ctype functions or anything just say word. lower and

or anything just say word. lower and that will convert the whole thing to

that will convert the whole thing to lowercase for parody with the dictionary

lowercase for parody with the dictionary all right how about something like the

all right how about something like the load function in Python well in Python

load function in Python well in Python you can open files just like in C for

you can open files just like in C for instance in Python I might do open the

instance in Python I might do open the dictionary argument in read mode just

dictionary argument in read mode just like fopen in Python I might do

like fopen in Python I might do something like this for each line in

something like this for each line in that file let me go ahead and add to my

that file let me go ahead and add to my words variable that line and then let me

words variable that line and then let me go ahead and close that file and I think

go ahead and close that file and I think I'm done I'm just going to go ahead and

I'm done I'm just going to go ahead and return true just because I think think

return true just because I think think I'm already done now here too I could

I'm already done now here too I could nitpick a little bit technically if I'm

nitpick a little bit technically if I'm reading in every line from the file

reading in every line from the file every line in the dictionary ends with

every line in the dictionary ends with technically a back sln but there's an

technically a back sln but there's an easy way to get rid of that uh just like

easy way to get rid of that uh just like you might in see with an alternative

you might in see with an alternative syntax what I'm actually going to do is

syntax what I'm actually going to do is this let me grab from the current line

this let me grab from the current line the current word by stripping off with

the current word by stripping off with reverse strip R strip a function will

reverse strip R strip a function will again see that just gets rid of the

again see that just gets rid of the trailing new line the back sln at the

trailing new line the back sln at the end of that line and what I really want

end of that line and what I really want to do then is add this word to that

to do then is add this word to that dictionary meanwhile if I want to figure

dictionary meanwhile if I want to figure out what the size is of my dictionary

out what the size is of my dictionary well and see you're probably writing

well and see you're probably writing code to iterate over all of those lines

code to iterate over all of those lines and you're just going to uh count them

and you're just going to uh count them up using a variable not so in Python you

up using a variable not so in Python you can just return the length of those

can just return the length of those words and better still in Python you

words and better still in Python you don't have to manage your own memory no

don't have to manage your own memory no more Malo no more free no more manual

more Malo no more free no more manual thinking about memory the language just

thinking about memory the language just deals with all of that for you so you

deals with all of that for you so you know what it suffices for me to just

know what it suffices for me to just return true and claim that unloading is

return true and claim that unloading is done for me and that's it again whether

done for me and that's it again whether you're in the middle of or already

you're in the middle of or already finished this might perhaps suggest some

finished this might perhaps suggest some frustration but also Enlightenment in

frustration but also Enlightenment in this in that this is why higher level

this in that this is why higher level languages exist you can build on top of

languages exist you can build on top of the same principles the same ideas with

the same principles the same ideas with which you've been dealing struggling

which you've been dealing struggling even this past week but you can now

even this past week but you can now express yourself all the more succinctly

express yourself all the more succinctly like this one line implements a hash

like this one line implements a hash table for you and all of this now now

table for you and all of this now now just uses that hash table in a simpler

just uses that hash table in a simpler way any questions now on this keeping in

way any questions now on this keeping in mind that the point nonetheless of

mind that the point nonetheless of speller and P said 5 is to understand

speller and P said 5 is to understand what's really going on underneath the

what's really going on underneath the hood and better still to notice this

hood and better still to notice this this might seem all rather amazing but

this might seem all rather amazing but let me go ahead and do this I've

let me go ahead and do this I've actually got a couple of versions of

actually got a couple of versions of speller written here and I've got a

speller written here and I've got a version written in C that I won't show

version written in C that I won't show the source code for but I'm going to go

the source code for but I'm going to go ahead and make that version of speller

ahead and make that version of speller in C and I'm going to go ahead here and

in C and I'm going to go ahead here and let's say split my window here for just

let's say split my window here for just a moment and I'm going to go into a

a moment and I'm going to go into a python version of spell really that I

python version of spell really that I just wrote and on the left hand side

just wrote and on the left hand side here let me go ahead and run speller the

here let me go ahead and run speller the version I compiled in C using a big text

version I compiled in C using a big text like uh the Sherlock Holmes text which

like uh the Sherlock Holmes text which is a whole lot of words in it and on the

is a whole lot of words in it and on the right hand side let me run python of

right hand side let me run python of spell. Pi which is a separate file I

spell. Pi which is a separate file I wrote in advance just like we give you

wrote in advance just like we give you speller.c and I'll similarly run this on

speller.c and I'll similarly run this on the Sherlock Holmes text and I'm going

the Sherlock Holmes text and I'm going to do my best to hit enter on the left

to do my best to hit enter on the left and the right of my screen at the same

and the right of my screen at the same time but we should see hopefully the

time but we should see hopefully the same list of misspelled words and the

same list of misspelled words and the timings thereof so here we go on the

timings thereof so here we go on the right here we go on the

right here we go on the left all right sort of a race to see

left all right sort of a race to see which one wins here C is on the left

which one wins here C is on the left python is on the right okay

python is on the right okay interesting hopefully Python's close

interesting hopefully Python's close behind note that some of the is internet

behind note that some of the is internet delay and so it might not necessarily be

delay and so it might not necessarily be a crazy number of seconds but the system

a crazy number of seconds but the system is indeed using if we measure at a low

is indeed using if we measure at a low level how much time the CPU spent

level how much time the CPU spent executing my code C took a total of 1.64

executing my code C took a total of 1.64 seconds that was pretty fast even though

seconds that was pretty fast even though it took a moment more for all of the

it took a moment more for all of the btes to come over the Internet the

btes to come over the Internet the python version though took what 2.44

python version though took what 2.44 seconds so what might an inference be I

seconds so what might an inference be I mean one maybe I'm just better at

mean one maybe I'm just better at programming in c than I am in Python

programming in c than I am in Python which is probably not true

which is probably not true but what else might you infer from this

example should we maybe give up on python stick with

python stick with C no so where what might be going on

C no so where what might be going on here like why is the python version that

here like why is the python version that I claim is correct and I think the

I claim is correct and I think the numbers all line up just not the

numbers all line up just not the times where's the trade-off here well

times where's the trade-off here well here again is sort of this design

here again is sort of this design tradeoff

yeah yeah exactly in order to save the human programmer time there's a lot more

human programmer time there's a lot more features built into python more

features built into python more functions more automatic management of

functions more automatic management of memory and so forth and you have to pay

memory and so forth and you have to pay a price like someone else's code is

a price like someone else's code is doing all of that work for you but if

doing all of that work for you but if they've written some number of lines of

they've written some number of lines of code those are just more lines of code

code those are just more lines of code that need to be executed for you whereas

that need to be executed for you whereas here the computer is at the risk of

here the computer is at the risk of oversimplifying only running my lines of

oversimplifying only running my lines of code so there's just less overhead and

code so there's just less overhead and so this is a Perpetual trade-off

so this is a Perpetual trade-off typically when using a more userfriendly

typically when using a more userfriendly and more modern language one of the

and more modern language one of the prices you might pay is performance now

prices you might pay is performance now there's a lot of smart computer

there's a lot of smart computer scientists in the world though trying to

scientists in the world though trying to push back on those same tradeoffs and so

push back on those same tradeoffs and so these interpreters like the command I

these interpreters like the command I wrote python technically can especially

wrote python technically can especially if you run a program again and again

if you run a program again and again they can actually sort of secretly

they can actually sort of secretly behind the scenes compile your code for

behind the scenes compile your code for you down to zeros and ones and then the

you down to zeros and ones and then the second the third the fourth time you run

second the third the fourth time you run that program it might very well be

that program it might very well be faster so this is a bit of a head fake

faster so this is a bit of a head fake here in that I'm running them once and

here in that I'm running them once and only once but we could get benefit over

only once but we could get benefit over time if we kept running the python

time if we kept running the python version again and again and perhaps

version again and again and perhaps fine-tune the performance but in general

fine-tune the performance but in general there's going to be this trade-off now

there's going to be this trade-off now would you rather spend the 60 seconds I

would you rather spend the 60 seconds I wrote implementing a spell checker or

wrote implementing a spell checker or the 6 hours 16 hours you might be or

the 6 hours 16 hours you might be or have spent implementing the same and C

have spent implementing the same and C you know probably not for productivity

you know probably not for productivity sake this is why we have these

sake this is why we have these additional languages just for fun let me

additional languages just for fun let me flip over to another screen here and

flip over to another screen here and open up a version of python that's

open up a version of python that's actually on on my in just a second on my

actually on on my in just a second on my own uh Mac instead of the cloud so that

own uh Mac instead of the cloud so that I can actually do something with

I can actually do something with Graphics so here I just have a black and

Graphics so here I just have a black and white terminal window on my very own Mac

white terminal window on my very own Mac and I've pre-installed python just like

and I've pre-installed python just like we've done so for VSS code in the cloud

we've done so for VSS code in the cloud for you uh notice that I've got this uh

for you uh notice that I've got this uh photo of uh perhaps one of your favorite

photo of uh perhaps one of your favorite TV shows here with the cast of The

TV shows here with the cast of The Office notice all of the faces in this

Office notice all of the faces in this image here and let me propose that we

image here and let me propose that we try to find one face in the crowd sort

try to find one face in the crowd sort of CSI style whereby we want to find

of CSI style whereby we want to find perhaps the stranton Strangler so to

perhaps the stranton Strangler so to speak and so here is an example of this

speak and so here is an example of this this guy's face now how do we go about

this guy's face now how do we go about finding this specific face in the crowd

finding this specific face in the crowd well our human eyes obviously can pluck

well our human eyes obviously can pluck him out especially if you're familiar

him out especially if you're familiar with the show but let me go ahead and do

with the show but let me go ahead and do this instead let me go ahead and propose

this instead let me go ahead and propose that we run code that I already wrote in

that we run code that I already wrote in advance here this is a Python program

advance here this is a Python program with more lines of code that we won't

with more lines of code that we won't dwell on for today but it's meant to

dwell on for today but it's meant to motivate what we can do from a a pillow

motivate what we can do from a a pillow uh Library implying a python image

uh Library implying a python image Library I want to import some type of

Library I want to import some type of information called some type of some

information called some type of some feature called image so that I can

feature called image so that I can manipulate images not unlike our own

manipulate images not unlike our own problem set 4 and this is kind of

problem set 4 and this is kind of powerful you in Python you can just

powerful you in Python you can just import face recognition as a library

import face recognition as a library that someone else wrote from there I'm

that someone else wrote from there I'm going to create a variable called image

going to create a variable called image I'm going to use this face recognitions

I'm going to use this face recognitions libraries load image file function it's

libraries load image file function it's a little verbose but it's similar in

a little verbose but it's similar in Spirit to F open and I'm going to open

Spirit to F open and I'm going to open office. jpeg I'm going to then declare a

office. jpeg I'm going to then declare a second variable called face locations

second variable called face locations plural because what I'm expecting to get

plural because what I'm expecting to get back per the documentation for this

back per the documentation for this library is a list of all of the faces

library is a list of all of the faces locations that are detected all right

locations that are detected all right then I'm going to iterate over each of

then I'm going to iterate over each of those uh faces using a for Loop that

those uh faces using a for Loop that we'll see in more detail I'm going to

we'll see in more detail I'm going to then infer what the top right bottom and

then infer what the top right bottom and left Corners are of that face and then

left Corners are of that face and then what I'm going to do here is show that

what I'm going to do here is show that face alone if I've detected the face in

face alone if I've detected the face in question so let me go ahead here and run

question so let me go ahead here and run detect. and we'll see not just the one

detect. and we'll see not just the one face we're looking for but if I run

face we're looking for but if I run python of detect. piy it's going to do

python of detect. piy it's going to do all of the analysis I'll see a big

all of the analysis I'll see a big opening here now of all of the faces

opening here now of all of the faces that were detected in this here program

that were detected in this here program okay some better than others I guess if

okay some better than others I guess if you zoom in on catching someone typical

you zoom in on catching someone typical Angela if you now want to Now find that

Angela if you now want to Now find that one face I think we need to train the

one face I think we need to train the software a bit more so let me actually

software a bit more so let me actually open up a second program called

open up a second program called recognize that's got more going on but

recognize that's got more going on but let me with a wave of a hand point out

let me with a wave of a hand point out that I'm now loading not only the

that I'm now loading not only the office. JPEG but also toby. JPEG to sort

office. JPEG but also toby. JPEG to sort of train the algorithm to find that

of train the algorithm to find that specific face and so now if I run this

specific face and so now if I run this second version recognize. with python of

second version recognize. with python of recognize. py hold my breath for just a

recognize. py hold my breath for just a moment it's an analyzing presumably all

moment it's an analyzing presumably all of the faces you see the same original

of the faces you see the same original photo but do you see one such face

photo but do you see one such face highlighted here this adversion of the

highlighted here this adversion of the code found Toby highlighted him with

code found Toby highlighted him with this green and voila we have face

this green and voila we have face recognition so for better for worse this

recognition so for better for worse this is what's happening increasingly

is what's happening increasingly societally nowadays and honestly even

societally nowadays and honestly even though I didn't write the code live

though I didn't write the code live because it's a good dozen or more lines

because it's a good dozen or more lines of code it's not terribly many and

of code it's not terribly many and literally all the authorities all we

literally all the authorities all we have to do is import face recognition

have to do is import face recognition and voila you have access like these

and voila you have access like these techn IES are here already but let's

techn IES are here already but let's consider for just a moment how did we

consider for just a moment how did we find Toby like how might that Library

find Toby like how might that Library even though we're not going to look at

even though we're not going to look at its implementation details how does it

its implementation details how does it find Toby and distinguish him from all

find Toby and distinguish him from all of these other faces in the crowd what

of these other faces in the crowd what might it be doing

might it be doing intuitively think back even to pet 4

intuitively think back even to pet 4 like what you yourselves have access to

like what you yourselves have access to data wise

yeah you know pixels in one area area and a lot

and a lot of

of that it's a lot of a lot of

simar yeah exactly and to summarize for for camera here we have trained the

for camera here we have trained the software if you will by giving it a

software if you will by giving it a photo of Toby's face by so by looking

photo of Toby's face by so by looking for the same or really similar pixels

for the same or really similar pixels especially if it's a slightly different

especially if it's a slightly different image of Toby we can perhaps identify

image of Toby we can perhaps identify him in the crowd and what really is a

him in the crowd and what really is a human face well at the end of the day

human face well at the end of the day the computer only knows it as a pattern

the computer only knows it as a pattern of bits or really at a higher level a

of bits or really at a higher level a pattern of pixels so maybe a human face

pattern of pixels so maybe a human face is perhaps best defined in general as

is perhaps best defined in general as like two eyes and a nose and a mouth

like two eyes and a nose and a mouth that even though all of us look similar

that even though all of us look similar structurally odds are the measurement

structurally odds are the measurement between the eyes and the nose and the

between the eyes and the nose and the width of the mouth the skin tone and all

width of the mouth the skin tone and all of these other physical characteristics

of these other physical characteristics are patterns that software could perhaps

are patterns that software could perhaps detect and then look sort of

detect and then look sort of statistically through the image looking

statistically through the image looking for the closest possible match to these

for the closest possible match to these various measurement shapes colors and

various measurement shapes colors and sizes and the like and indeed that might

sizes and the like and indeed that might might be the intuition but what's

might be the intuition but what's powerful here again is just how easy and

powerful here again is just how easy and readily available this technology now is

readily available this technology now is all right so with that said let's

all right so with that said let's propose to consider what more we can do

propose to consider what more we can do with python itself get back to the

with python itself get back to the fundamentals so that you yourselves can

fundamentals so that you yourselves can start to implement something along those

start to implement something along those same lines so besides having access to

same lines so besides having access to things like a get string function um the

things like a get string function um the cs50 library provides a few other things

cs50 library provides a few other things as well namely in C we had these but in

as well namely in C we had these but in Python we're going to have fewer in

Python we're going to have fewer in Python our library short term is going

Python our library short term is going to give you not only get string but also

to give you not only get string but also get in and get float why it's actually

get in and get float why it's actually just kind of annoying as we'll student

just kind of annoying as we'll student see to get back an integer or a float

see to get back an integer or a float from a user and just make sure that it's

from a user and just make sure that it's an INT and a float and not like a word

an INT and a float and not like a word like cat or dog or some string that's

like cat or dog or some string that's not actually a number well we can import

not actually a number well we can import not just the specific function get

not just the specific function get string but we can actually import all of

string but we can actually import all of these functions one at a time like this

these functions one at a time like this as we'll soon see or you can even in

as we'll soon see or you can even in Python import specific functions from a

Python import specific functions from a file one of you asked a while back when

file one of you asked a while back when you import when you include something

you import when you include something like cs50.h or standard i.h you're

like cs50.h or standard i.h you're actually getting all of the code in that

actually getting all of the code in that file which potentially can add bulk to

file which potentially can add bulk to your own program or time in this case

your own program or time in this case when you import specific functions from

when you import specific functions from python you can be a little more narrowly

python you can be a little more narrowly uh precise as to what it is you want to

uh precise as to what it is you want to have access to all right so with that

have access to all right so with that said let's go ahead and see what

said let's go ahead and see what conditionals look like in Python so in

conditionals look like in Python so in the left hand side again here we'll see

the left hand side again here we'll see scrp

scrp and here for instance was just kind of a

and here for instance was just kind of a contrived example asking if x is less

contrived example asking if x is less than y then say x is less than y in C it

than y then say x is less than y in C it looked like this in Python now it's

looked like this in Python now it's going to look like this instead and

going to look like this instead and here's before in C and here's after and

here's before in C and here's after and just to call out a few of the obvious

just to call out a few of the obvious differences what has changed in Python

differences what has changed in Python for conditionals it would

seem sort of what's the difference yeah yeah so there's no more curly

yeah yeah so there's no more curly braces and indeed you don't use those

braces and indeed you don't use those what appears to be taking their place if

what appears to be taking their place if you might

you might infer what seems to have taken their

infer what seems to have taken their place what do you think so the colon at

place what do you think so the colon at the start of this line here but also

the start of this line here but also even more important now is this

even more important now is this indentation below it so some of you and

indentation below it so some of you and we know this from Office hours have a

we know this from Office hours have a habit of like uh indenting everything on

habit of like uh indenting everything on the left right and it's just kind of

the left right and it's just kind of this crazy mess to look at frustrating

this crazy mess to look at frustrating for you surely but C and clang is pretty

for you surely but C and clang is pretty tolerant when it comes to things like

tolerant when it comes to things like Whit space in a program python uh-uh

Whit space in a program python uh-uh they realized years ago that let's help

they realized years ago that let's help humans help themselves and just require

humans help themselves and just require standard indentation so four spaces

standard indentation so four spaces would be the norm here but because it's

would be the norm here but because it's indented below that colon that indeed

indented below that colon that indeed indicates that this now is part of that

indicates that this now is part of that condition something else has gone

condition something else has gone missing versus C in this conditional

missing versus C in this conditional what else is a little

what else is a little simplified yeah so no more parentheses

simplified yeah so no more parentheses you can still use them especially when

you can still use them especially when you need to logically to do order of

you need to logically to do order of operations like in math but in this case

operations like in math but in this case if you just want to ask a simple

if you just want to ask a simple question like if x less than y you can

question like if x less than y you can just do it like that how about when you

just do it like that how about when you have an if else well this is almost the

have an if else well this is almost the same here with these same changes in C

same here with these same changes in C this looked like this and it's starting

this looked like this and it's starting to get a bit bulky at least if we use

to get a bit bulky at least if we use our curly braces in this way in Python

our curly braces in this way in Python we can tighten things up further even

we can tighten things up further even though strictly speaking in C you don't

though strictly speaking in C you don't always need the curly braces but here

always need the curly braces but here gone are the parentheses again gone are

gone are the parentheses again gone are the curly braces indentation is

the curly braces indentation is consistent and we've just added another

consistent and we've just added another keyword else with a colon but no more

keyword else with a colon but no more semicolons as well how about something

semicolons as well how about something larger like this and if else if else

larger like this and if else if else this one's a little curious but in C it

this one's a little curious but in C it looked like this if else if else in

looked like this if else if else in Python it now looks like this and

Python it now looks like this and there's perhaps one curiosity here that

there's perhaps one curiosity here that honestly all these years later I still

honestly all these years later I still can't remember how to spell it half the

can't remember how to spell it half the time what's weird about

time what's weird about this what do you spot as

this what do you spot as different uh yeah over

here yeah instead of else if it's l if why apparently El space if was just too

why apparently El space if was just too many keystrokes for humans to type so

many keystrokes for humans to type so they condensed it into this way probably

they condensed it into this way probably means it's a little more distinguishable

means it's a little more distinguishable too for the computer between the if and

too for the computer between the if and the else to but just something to

the else to but just something to remember now it's indeed L if and not

remember now it's indeed L if and not else if all right so what about

else if all right so what about variables in Python I've used a couple

variables in Python I've used a couple of them already but let's let's um

of them already but let's let's um distill exactly how you define uh and

distill exactly how you define uh and declare these things as well so in

declare these things as well so in scratch if we wanted to create a

scratch if we wanted to create a variable called counter and set it equal

variable called counter and set it equal initially to zero we would do something

initially to zero we would do something like this specify that it's an INT use

like this specify that it's an INT use the assignment op operator and the

the assignment op operator and the thought with a semicolon in Python it's

thought with a semicolon in Python it's just simpler you name the variable you

just simpler you name the variable you use the assignment operator as before

use the assignment operator as before you set it equal to some value and

you set it equal to some value and that's it you don't mention the type you

that's it you don't mention the type you don't mention the semicolon or anything

don't mention the semicolon or anything more what if you want to change a

more what if you want to change a variable like the counter by one that is

variable like the counter by one that is incremented by one you have a few

incremented by one you have a few different ways here in C we saw syntax

different ways here in C we saw syntax like this where you can say counter

like this where you can say counter equals counter plus one which again

equals counter plus one which again feels like illogical how can counter

feels like illogical how can counter equal counter plus one but again we read

equal counter plus one but again we read this code really right to left updating

this code really right to left updating its value by one um in Python it's

its value by one um in Python it's almost the same you just get rid of the

almost the same you just get rid of the semicolon so that logic is there but

semicolon so that logic is there but recall in C we could do something

recall in C we could do something slightly different that we can also do

slightly different that we can also do in Python in Python you can also more

in Python in Python you can also more succinctly do this plus equals and then

succinctly do this plus equals and then whatever number you want to add or you

whatever number you want to add or you can even change it to subtract if you

can even change it to subtract if you prefer sadly gone is something you've

prefer sadly gone is something you've probably typed a whole lot what was the

probably typed a whole lot what was the other way you can add

other way you can add one Plus+ is no more sadly in Python

one Plus+ is no more sadly in Python just too many ways to do the same thing

just too many ways to do the same thing so they got rid of it in favor of just

so they got rid of it in favor of just this syntax here so keep that in mind as

this syntax here so keep that in mind as well what about loops when you want to

well what about loops when you want to do something in Python again and again

do something in Python again and again well in scratch in week zero here's how

well in scratch in week zero here's how we meowed three times specifically in C

we meowed three times specifically in C we had a couple of ways of doing this

we had a couple of ways of doing this this was like the more mechanical

this was like the more mechanical approach where you create a variable

approach where you create a variable called I you set it equal to zero you

called I you set it equal to zero you then do while I is less than three the

then do while I is less than three the following and then you yourself

following and then you yourself increment I again and again mechanical

increment I again and again mechanical in the sense that like you have to

in the sense that like you have to implement all of these gears and make

implement all of these gears and make them turn yourself but this was a

them turn yourself but this was a correct way to do that in Python we can

correct way to do that in Python we can still achieve the same idea but we don't

still achieve the same idea but we don't need the int keyword we don't need any

need the int keyword we don't need any of the semicolons we don't need the

of the semicolons we don't need the parentheses we don't need the curly

parentheses we don't need the curly braces we can't use the Plus+ so maybe

braces we can't use the Plus+ so maybe that's a minor step backwards if you're

that's a minor step backwards if you're a fan but otherwise the code the logic

a fan but otherwise the code the logic is exactly the same but there's other

is exactly the same but there's other ways to achieve this same idea recall

ways to achieve this same idea recall that in C we could also do this you

that in C we could also do this you could use a for Loop which it does

could use a for Loop which it does exactly the same thing both are correct

exactly the same thing both are correct both are arguably welld designed it's

both are arguably welld designed it's kind of to each their own when it comes

kind of to each their own when it comes to choosing between these in Python

to choosing between these in Python though we're going to have to think

though we're going to have to think through how to do this so you don't do

through how to do this so you don't do the same for loop as in C the closest I

the same for loop as in C the closest I could come up with is this where you say

could come up with is this where you say four I or whatever variable you want to

four I or whatever variable you want to do the counting in literally the

do the counting in literally the preposition and then you use square

preposition and then you use square brackets here and we've used square

brackets here and we've used square brackets before in the context of like

brackets before in the context of like arrays and things like that and the 012

arrays and things like that and the 012 looks like an array in some sense even

looks like an array in some sense even though we've also seen arrays with curly

though we've also seen arrays with curly braces but these square brackets for now

braces but these square brackets for now denote a list python does not have

denote a list python does not have arrays an array is that contiguous chunk

arrays an array is that contiguous chunk of memory back to back to back that you

of memory back to back to back that you have to resize somehow by moving things

have to resize somehow by moving things around in memory as per two weeks ago in

around in memory as per two weeks ago in Python though you can just create a list

Python though you can just create a list like this using square brackets and

like this using square brackets and better still as we'll see you can add or

better still as we'll see you can add or even remove things from that list down

even remove things from that list down the road um this though is not going to

the road um this though is not going to be very welld designed this will work

be very welld designed this will work this will iterate in Python three

this will iterate in Python three times but what might rub you the wrong

times but what might rub you the wrong way about this design even if you've

way about this design even if you've never seen python before how does this

never seen python before how does this example not end well

yeah yeah like if you're making a large list you have to type out each one of

list you have to type out each one of these numbers like comma three comma

these numbers like comma three comma four comma 5 comma dot dot do 50 comma

four comma 5 comma dot dot do 50 comma dot dot dot 500 like surely that's not

dot dot dot 500 like surely that's not the best solution to have all of these

the best solution to have all of these numbers on the code on the screen

numbers on the code on the screen wrapping endlessly on the screen so in

wrapping endlessly on the screen so in Python another way to do this would be

Python another way to do this would be to use a function called range which

to use a function called range which technically is a data type unto itself

technically is a data type unto itself and this returns to you as many values

and this returns to you as many values as you ask for it range takes some other

as you ask for it range takes some other arguments as well but the simplest use

arguments as well but the simplest use case here is if you want back the

case here is if you want back the numbers 0 1 and two a total of three

numbers 0 1 and two a total of three values you say hey python please give me

values you say hey python please give me a range of three values and by default

a range of three values and by default they start at zero on up but this is

they start at zero on up but this is more efficient than it would be to

more efficient than it would be to hardcode the entire list at once and the

hardcode the entire list at once and the best metaphor I could up with is

best metaphor I could up with is something like this like here for

something like this like here for instance is a deck of cards this is sort

instance is a deck of cards this is sort of normal human size and there's

of normal human size and there's presumably 52 cards here so writing out

presumably 52 cards here so writing out 0 through 51 on code would be a little

0 through 51 on code would be a little ridiculous for the reasons you know it

ridiculous for the reasons you know it would just be very unwieldy and ugly and

would just be very unwieldy and ugly and wrapping and all of that it would be the

wrapping and all of that it would be the phys it would be the virtual equivalent

phys it would be the virtual equivalent of me like handing you all of these

of me like handing you all of these cards at once to just deal with and

cards at once to just deal with and right you know they're not that big but

right you know they're not that big but like it's a lot of cards to hold on to

like it's a lot of cards to hold on to it requires a lot of memory or physical

it requires a lot of memory or physical storage if you will what range does

storage if you will what range does metaphorically is if you ask me for

metaphorically is if you ask me for three cards I hand you them one at a

three cards I hand you them one at a time like this so that at any point in

time like this so that at any point in time you only have one number in the

time you only have one number in the computer's memory until you're handed

computer's memory until you're handed the next the alternative the previous

the next the alternative the previous version would be to hand me all three

version would be to hand me all three cards at once or all 52 cards at once

cards at once or all 52 cards at once but in this case range is just way more

but in this case range is just way more efficient you can do range of a thousand

efficient you can do range of a thousand that's not going to give you a list of a

that's not going to give you a list of a thousand values all at once it's going

thousand values all at once it's going to give you a thousand values one at a

to give you a thousand values one at a time reducing memory sign signicantly in

time reducing memory sign signicantly in the computer itself all right so besides

the computer itself all right so besides this what about doing something forever

this what about doing something forever in scratch well we could do this

in scratch well we could do this literally with a forever block which

literally with a forever block which didn't quite exist in C in C we had had

didn't quite exist in C in C we had had to hack it together by saying while true

to hack it together by saying while true because true is by definition TR always

because true is by definition TR always true so this just in uh deliberately

true so this just in uh deliberately induces an infinite Loop for us in

induces an infinite Loop for us in Python the logic's going to be almost

Python the logic's going to be almost the same and infinite Loops in Python

the same and infinite Loops in Python tend to actually be even more common

tend to actually be even more common because you can always break out of them

because you can always break out of them as you couldn't see in Python it looks

as you couldn't see in Python it looks like this and this is slightly more

like this and this is slightly more subtle but gone are the curly braces

subtle but gone are the curly braces gone or the parentheses but ever so

gone or the parentheses but ever so slight difference

slight difference toal capital T for true and it's going

toal capital T for true and it's going to be a capital f for false stupid

to be a capital f for false stupid little differences eventually you're

little differences eventually you're going to mistype one or the other but

going to mistype one or the other but these are the kinds of things to keep an

these are the kinds of things to keep an eye out and to start recognizing in your

eye out and to start recognizing in your mind's eye when you read code questions

mind's eye when you read code questions now on any of these building

now on any of these building blocks

blocks yeah

I in the for Loop was I re uh it was set to zero on the first iteration then one

to zero on the first iteration then one on the next then two on the third and

on the next then two on the third and the same thing for range it just doesn't

the same thing for range it just doesn't use up as much memory all at once other

use up as much memory all at once other questions now on any of these building

questions now on any of these building blocks of

blocks of python no all right well let's go ahead

python no all right well let's go ahead and build something a little more than

and build something a little more than hello let me propose that over here we

hello let me propose that over here we Implement maybe the the simplest of

Implement maybe the the simplest of calculators here so let me go back to vs

calculators here so let me go back to vs code here open my terminal uh window and

code here open my terminal uh window and open up say a file called

open up say a file called calculator. and in calculator. we'll

calculator. and in calculator. we'll have an opportunity to explore some of

have an opportunity to explore some of these building blocks but we'll allow

these building blocks but we'll allow things to escalate pretty quickly to

things to escalate pretty quickly to more interesting examples so that we can

more interesting examples so that we can do the same thing ultimately as well and

do the same thing ultimately as well and in fact let me go ahead and do this

in fact let me go ahead and do this moreover I've brought some code with me

moreover I've brought some code with me in advance uh for instance something

in advance uh for instance something called calculator 0. C from the first

called calculator 0. C from the first week of C

week of C and let me go ahead and split my window

and let me go ahead and split my window here in fact so that I can now do

here in fact so that I can now do something like uh this let me move this

something like uh this let me move this over here here calculator. Pi so now I

over here here calculator. Pi so now I have on the left of my screen

have on the left of my screen calculator. C or calculator z.c because

calculator. C or calculator z.c because that's the first version I made and

that's the first version I made and calculator. Pi on the right let me go

calculator. Pi on the right let me go ahead and Implement really the same idea

ahead and Implement really the same idea here so on the right hand side the

here so on the right hand side the analog of including cs50.h would be from

analog of including cs50.h would be from cs50 import get int if I want to indeed

cs50 import get int if I want to indeed use this function now I'm going to go

use this function now I'm going to go ahead and give myself a variable X

ahead and give myself a variable X without defining its type I'm going to

without defining its type I'm going to use this get int function and I'm going

use this get int function and I'm going to prompt the user for X just like in C

to prompt the user for X just like in C I'm then going to go ahead and prompt

I'm then going to go ahead and prompt the user for another int like y here

the user for another int like y here just like in C and at the very end I'm

just like in C and at the very end I'm going to go ahead and do print X Plus Y

going to go ahead and do print X Plus Y and that's it now granted I have some

and that's it now granted I have some comments in my C version of the code

comments in my C version of the code just to remind you of what each line is

just to remind you of what each line is doing but I've still distilled this into

doing but I've still distilled this into like six lines or really four if I get

like six lines or really four if I get rid of the blank line so it's already

rid of the blank line so it's already perhaps a bit tighter here here but

perhaps a bit tighter here here but there's also it's tighter because

there's also it's tighter because something really important historically

something really important historically is missing what did I seem to Omit

is missing what did I seem to Omit altogether that we haven't really

altogether that we haven't really highlighted yet

highlighted yet yeah yeah the main function is gone and

yeah yeah the main function is gone and in fact maybe you took for granted that

in fact maybe you took for granted that it just worked a moment ago when I wrote

it just worked a moment ago when I wrote hello but I didn't have a main function

hello but I didn't have a main function in hello either and this too is a

in hello either and this too is a feature of python and a lot of other

feature of python and a lot of other languages as well instead of having to

languages as well instead of having to adhere to these long-standing Traditions

adhere to these long-standing Traditions if you just want to write code and get

if you just want to write code and get something done fine just write code and

something done fine just write code and get something done without necessarily

get something done without necessarily all of the same boiler plate so whatever

all of the same boiler plate so whatever is in your python file left indented if

is in your python file left indented if you will by default is just going to be

you will by default is just going to be the code that The Interpreter runs top

the code that The Interpreter runs top to bottom left to right well let me go

to bottom left to right well let me go ahead now and run code like this let me

ahead now and run code like this let me go ahead and open that back up my

go ahead and open that back up my terminal window run python of

terminal window run python of calculator. piy and I'll do X is one y

calculator. piy and I'll do X is one y is 2 and as you might expect it gives me

is 2 and as you might expect it gives me three slight aesthetic bug I put my

three slight aesthetic bug I put my space in the wrong place here so that's

space in the wrong place here so that's a new mistake let me fix that

a new mistake let me fix that aesthetically let me rerun python of

aesthetically let me rerun python of calculator. Pi type in one type in two

calculator. Pi type in one type in two and voila there is now my same version

and voila there is now my same version again but let me propose now that we get

again but let me propose now that we get rid of this training wheel we don't want

rid of this training wheel we don't want to keep taking one step forward and then

to keep taking one step forward and then two steps back by adding these training

two steps back by adding these training wheels so let me instead do this in my

wheels so let me instead do this in my version of calculator. suppose that we

version of calculator. suppose that we take away already the training wheel

take away already the training wheel that is the cs50 library here and let me

that is the cs50 library here and let me instead then use just Python's built-in

instead then use just Python's built-in function called input which literally

function called input which literally does just that it gets input from the

does just that it gets input from the user and it stores it as before in X and

user and it stores it as before in X and Y so this is not cs50 specific this is

Y so this is not cs50 specific this is real world Python Programming well let

real world Python Programming well let me go ahead and run again python of

me go ahead and run again python of calculator. Pi and of course if x is one

calculator. Pi and of course if x is one and Y is 2 X + Y should of course still

and Y is 2 X + Y should of course still be three

be three hm it's apparently 12 according to

hm it's apparently 12 according to python until cs50's Library gets

python until cs50's Library gets involved but does anyone want to

involved but does anyone want to infer what's just went wrong

yeah exactly the input function by Design always returns a string of text

Design always returns a string of text after all that's what the human typed in

after all that's what the human typed in and even though yes I type the number

and even though yes I type the number keys on the keyboard it's still coming

keys on the keyboard it's still coming back is all text now maybe we should use

back is all text now maybe we should use like a get in function well that doesn't

like a get in function well that doesn't exist in Python all you can do is get

exist in Python all you can do is get textual input a string from the user but

textual input a string from the user but we can convert one to the other and so a

we can convert one to the other and so a fix for this so that we don't accident

fix for this so that we don't accident Al concatenate that is join X+ y

Al concatenate that is join X+ y together would be to do something like

together would be to do something like this let me go back to my python code

this let me go back to my python code here and whereas in C we could

here and whereas in C we could previously do type casting we could

previously do type casting we could convert one type to another that

convert one type to another that generally wasn't the case when you were

generally wasn't the case when you were doing something complex like a string to

doing something complex like a string to an INT you could do a Char to an INT and

an INT you could do a Char to an INT and vice versa but for a string recall there

vice versa but for a string recall there was a special function in the ctype

was a special function in the ctype library called a to I like ask e to

library called a to I like ask e to integer that's uh that closest analog

integer that's uh that closest analog here and in fact the way to do this in

here and in fact the way to do this in Python would be to use a function called

Python would be to use a function called int which indeed is the name of the data

int which indeed is the name of the data type 2 even though I have not yet had to

type 2 even though I have not yet had to type it and I can convert the output of

type it and I can convert the output of the input function automatically from a

the input function automatically from a string immediately to an INT and now if

string immediately to an INT and now if I go back to my terminal window rerun

I go back to my terminal window rerun python of calculator. piy with one and

python of calculator. piy with one and two for X and Y now I'm back in business

two for X and Y now I'm back in business so that then is for instance what the

so that then is for instance what the cs50 library does if temporarily this

cs50 library does if temporarily this week is it just deals with the

week is it just deals with the conversion for you and in fact bad

conversion for you and in fact bad things could happen if I type the wrong

things could happen if I type the wrong thing like dog or cat instead of a

thing like dog or cat instead of a number but we'll cross that bridge in

number but we'll cross that bridge in just a moment as well all right what if

just a moment as well all right what if we do something slightly different now

we do something slightly different now with our calculator instead of just

with our calculator instead of just addition let me go ahead and do how

addition let me go ahead and do how about uh div instead of addition let's

about uh div instead of addition let's do division instead so Z equals x / y

do division instead so Z equals x / y thereby giving me a third variable Z let

thereby giving me a third variable Z let me go ahead and run python of

me go ahead and run python of calculator. piy again I'll type in one

calculator. piy again I'll type in one I'll type in three this time and what

I'll type in three this time and what prog what problem do you think we're

prog what problem do you think we're about to

about to see or is it gone what happened when I

see or is it gone what happened when I did this in C albeit with some slightly

did this in C albeit with some slightly more cryptic syntax when I divided one

more cryptic syntax when I divided one number like one one divided by

number like one one divided by three anyone recall

three anyone recall yeah yeah so it would round down to the

yeah yeah so it would round down to the nearest integer whereby you experience

nearest integer whereby you experience trunc so if you take an integer like one

trunc so if you take an integer like one you divide it by another integer like

you divide it by another integer like three that technically should be

three that technically should be 0.33333 infinitely long but in uh C

0.33333 infinitely long but in uh C recall you truncate the value if you

recall you truncate the value if you divide an INT by an INT you get back an

divide an INT by an INT you get back an INT which means you get only the integer

INT which means you get only the integer part which was the zero now python

part which was the zero now python actually handles this for us and avoids

actually handles this for us and avoids the truncation but it leaves us still

the truncation but it leaves us still with one other problem here which is

with one other problem here which is going to be for instance not necessarily

going to be for instance not necessarily visible at a GL

visible at a GL this looks correct this has solved the

this looks correct this has solved the problem in C so truncation does not

problem in C so truncation does not happen the integers are automatically

happen the integers are automatically converted to a float a floating point

converted to a float a floating point value but what other problem did we trip

value but what other problem did we trip over back

over back in week uh

one what else got a little dicey when dealing with simple

dealing with simple arithmetic anyone recall well the syntax

arithmetic anyone recall well the syntax in Python's a little different but let

in Python's a little different but let me go ahead and do this it turns out in

me go ahead and do this it turns out in Python if you want to see more

Python if you want to see more significant digits than what I'm seeing

significant digits than what I'm seeing here by the default which is a dozen or

here by the default which is a dozen or so let me go ahead and print out Z as

so let me go ahead and print out Z as follows let me first print out a format

follows let me first print out a format string because I want to format Z in an

string because I want to format Z in an interesting way and notice this would

interesting way and notice this would have no effect on the difference this is

have no effect on the difference this is just a format string that for no

just a format string that for no compelling reason at the moment is

compelling reason at the moment is interpolating z in those curly braces

interpolating z in those curly braces using an F string or format string if I

using an F string or format string if I run this again with one and three we'll

run this again with one and three we'll see indeed the exact same thing but when

see indeed the exact same thing but when you use an F string you indeed have the

you use an F string you indeed have the ability to format that string more

ability to format that string more precisely just like with percent F in

precisely just like with percent F in Python you could start to fine-tune how

Python you could start to fine-tune how many significant digits you see in uh p

many significant digits you see in uh p in C rather in Python you can do the

in C rather in Python you can do the same but the syntax is a little

same but the syntax is a little different if you want the computer to

different if you want the computer to interpolate Z and show you 50

interpolate Z and show you 50 significant digits that is 50 numbers

significant digits that is 50 numbers after the decimal point syntax is

after the decimal point syntax is similar to C but it's a little different

similar to C but it's a little different you literally put a colon after the

you literally put a colon after the variable's name 50 means show me the

variable's name 50 means show me the decimal point and then 50 digits to the

decimal point and then 50 digits to the right and the F just indicates please

right and the F just indicates please treat this as a floating point value so

treat this as a floating point value so now if I rerun python of calculator. Pi

now if I rerun python of calculator. Pi divide 1 by 3 unfortunately python has

divide 1 by 3 unfortunately python has not solved all of the world's problems

not solved all of the world's problems for us this again was an example of

for us this again was an example of floating point in Precision so that

floating point in Precision so that problem is still latent so just because

problem is still latent so just because the world has advanced doesn't

the world has advanced doesn't necessarily mean that all of our

necessarily mean that all of our problems from C have gone away there are

problems from C have gone away there are solutions using third-party libraries

solutions using third-party libraries for scientific calculations and the like

for scientific calculations and the like but out of the box floating point in

but out of the box floating point in Precision is still an issue meanwhile

Precision is still an issue meanwhile there was one other problem in C that we

there was one other problem in C that we ran into involving numbers and that was

ran into involving numbers and that was this integer overflow recall that an

this integer overflow recall that an integer in C only took up what like 32

integer in C only took up what like 32 bits typically which meant you could

bits typically which meant you could count as high as four billion or maybe

count as high as four billion or maybe if you're doing positive and negatives

if you're doing positive and negatives as high as two billion after which weird

as high as two billion after which weird things would happen the number would go

things would happen the number would go to zero or negative or just it would

to zero or negative or just it would overflow or wrap back around well

overflow or wrap back around well wonderfully in Python they did it at

wonderfully in Python they did it at least address this whereby you can count

least address this whereby you can count as high as you want and python will just

as high as you want and python will just use more and more and more and more bits

use more and more and more and more bits and bites to store really big numbers so

and bites to store really big numbers so integer overflow is not a thing with

integer overflow is not a thing with that said python is limited to how many

that said python is limited to how many digits it will show you on the screen at

digits it will show you on the screen at once as a string but mathematically your

once as a string but mathematically your math will be correct now so we've taken

math will be correct now so we've taken a couple steps forward One Step sideways

a couple steps forward One Step sideways but indeed we've solved some of our

but indeed we've solved some of our problems here all right questions now

problems here all right questions now now on any of these examples thus

now on any of these examples thus far

far question all right well how about uh how

question all right well how about uh how about another problem that we

about another problem that we encountered in C let's revisit it here

encountered in C let's revisit it here in python as well so let me go ahead and

in python as well so let me go ahead and on the left hand side here let me open

on the left hand side here let me open up a file called say compare let's see

up a file called say compare let's see uh how about a file called compare 3. C

uh how about a file called compare 3. C on the left and let me go ahead and

on the left and let me go ahead and create a new file on the right called

create a new file on the right called compare. Pi because recall that bad

compare. Pi because recall that bad things happened when we needed to

things happened when we needed to compare two values in C so on the left

compare two values in C so on the left here is a reminder of what we once did

here is a reminder of what we once did in C whereby if we want to compare

in C whereby if we want to compare values we can get an INT in C stored in

values we can get an INT in C stored in x a get int in C stored in y we then

x a get int in C stored in y we then have our familiar conditional logic here

have our familiar conditional logic here just printing out if x is less than y or

just printing out if x is less than y or not well we can certainly do the same

not well we can certainly do the same thing ultimately in Python by using some

thing ultimately in Python by using some fairly familiar syntax and let's just

fairly familiar syntax and let's just demonstrate this one quickly let me go

demonstrate this one quickly let me go over here too I'll do from cs50 import

over here too I'll do from cs50 import uh get int even though I could do this

uh get int even though I could do this instead with the input function itself x

instead with the input function itself x equals get int and I'll prompt the user

equals get int and I'll prompt the user for that y equals get int and I'll

for that y equals get int and I'll prompt the user for that after that

prompt the user for that after that recall that I can say without

recall that I can say without parentheses if x is less than y then

parentheses if x is less than y then print out without the F uh X is less

print out without the F uh X is less than y then I can go ahead and say else

than y then I can go ahead and say else if x is greater than y i can print out

if x is greater than y i can print out uh quote unquote X is greater than y if

uh quote unquote X is greater than y if you'd like to interject now what did I

you'd like to interject now what did I screw

screw up anyone yeah L if right so L if L if x

up anyone yeah L if right so L if L if x is greater than y else this part's the

is greater than y else this part's the same print X is equal to Y so there's

same print X is equal to Y so there's not all that much new there's no New

not all that much new there's no New Logic going on here but at least

Logic going on here but at least syntactically it's a little cleaner

syntactically it's a little cleaner indeed this program is only 11 lines

indeed this program is only 11 lines long albeit without any comments let me

long albeit without any comments let me go ahead and run python of compare. Pi

go ahead and run python of compare. Pi let's see is 1 less than two indeed

let's see is 1 less than two indeed let's run it again is 2 less than 1 no

let's run it again is 2 less than 1 no it's greater than and let's lastly type

it's greater than and let's lastly type in one and one twice X is equal to Y so

in one and one twice X is equal to Y so we've got a pretty side by side onetoone

we've got a pretty side by side onetoone conversion here let's do something a

conversion here let's do something a little more interesting then and see how

little more interesting then and see how about I open instead something where we

about I open instead something where we actually compared for a purpose so if I

actually compared for a purpose so if I open up from last uh from earlier in the

open up from last uh from earlier in the course how about uh agree do C which

course how about uh agree do C which prompt the user to agree to something or

prompt the user to agree to something or not and let me code up a new version

not and let me code up a new version here called agree. pi and I'll do this

here called agree. pi and I'll do this on the right hand side with agree. pi

on the right hand side with agree. pi but on gree do c on the left notice that

but on gree do c on the left notice that this is how we did this sort of yes no

this is how we did this sort of yes no thing in C we compared c a character

thing in C we compared c a character equal to single quotes y or equal to

equal to single quotes y or equal to single quotes little Y and then the same

single quotes little Y and then the same thing for n now in Python this one's

thing for n now in Python this one's actually going to be a little bit

actually going to be a little bit different here let me go ahead and in

different here let me go ahead and in the python version of this let me do

the python version of this let me do something like this uh we'll use get

something like this uh we'll use get string uh actually no we'll just use

string uh actually no we'll just use input in this case so let's do uh s

input in this case so let's do uh s equals input and we'll ask the user the

equals input and we'll ask the user the same thing do you agree question mark

same thing do you agree question mark then let's go ahead and say if s equals

then let's go ahead and say if s equals equals how about uh

equals how about uh y huh how do I do this well a few things

y huh how do I do this well a few things turns out I'm going to do this s equals

turns out I'm going to do this s equals equals little y then I'm going to go

equals little y then I'm going to go ahead and print out a agreed and L if s

ahead and print out a agreed and L if s equals equals capital N or S equals

equals equals capital N or S equals equals lowercase n I'm going to go ahead

equals lowercase n I'm going to go ahead and print out not agreed and I claimed

and print out not agreed and I claimed for the moment that this is identical

for the moment that this is identical now to the program on the right the

now to the program on the right the program on the left in C but what's

program on the left in C but what's different so we're still doing the same

different so we're still doing the same kind of logic these equal equals for

kind of logic these equal equals for comparing for equality but notice that

comparing for equality but notice that nicely enough python got rid of the two

nicely enough python got rid of the two vertical bars and it's just literally

vertical bars and it's just literally the word or if you recall seeing

the word or if you recall seeing Ampersand Ampersand to express a logical

Ampersand Ampersand to express a logical and in C you can just write literally

and in C you can just write literally the word and and so here's a hint of why

the word and and so here's a hint of why python tends to be pretty popular people

python tends to be pretty popular people just like that it's a little closer to

just like that it's a little closer to English there's a little less of the

English there's a little less of the cryptic syntax here now this is correct

cryptic syntax here now this is correct as this code will now work but I've also

as this code will now work but I've also used double quotes instead of single

used double quotes instead of single quotes and I also omitted a few minutes

quotes and I also omitted a few minutes ago for my list of data types in Python

ago for my list of data types in Python the word Char in Python there are no

the word Char in Python there are no chars there are no individual characters

chars there are no individual characters if you want to manipulate individual

if you want to manipulate individual character you use a string that is to

character you use a string that is to say a stir of size one now in Python you

say a stir of size one now in Python you can use single quotes or double quotes

can use single quotes or double quotes I'm deliberately using double quotes

I'm deliberately using double quotes everywhere just for consistency with how

everywhere just for consistency with how we treat strings in C it's pretty common

we treat strings in C it's pretty common though to use single quotes instead if

though to use single quotes instead if only because on most keyboards you don't

only because on most keyboards you don't have to hold the shift key anymore I

have to hold the shift key anymore I mean humans have really started to

mean humans have really started to optimize just how quickly they want to

optimize just how quickly they want to be able to code so using a single quote

be able to code so using a single quote tends to be pretty popular in Python and

tends to be pretty popular in Python and other languages as well they are

other languages as well they are fundamentally the same uh single or

fundamentally the same uh single or double unlike in C where they have

double unlike in C where they have meaning so this is correct I claim and

meaning so this is correct I claim and in fact let me run this real quick I'll

in fact let me run this real quick I'll open up my terminal window here let me

open up my terminal window here let me get rid of the version and see run

get rid of the version and see run python of agree. piy and I'll type in y

python of agree. piy and I'll type in y okay I'll run it again and type in

okay I'll run it again and type in little Y and I'll stipulate it's going

little Y and I'll stipulate it's going to work for no as well but this isn't

to work for no as well but this isn't necessarily the only way we can do this

necessarily the only way we can do this there are other ways to implement the

there are other ways to implement the same idea and in fact I can go about

same idea and in fact I can go about doing this this instead let me go back

doing this this instead let me go back up to my code here and we saw a hint of

up to my code here and we saw a hint of this earlier we know that lists exist in

this earlier we know that lists exist in Python and you can create them just by

Python and you can create them just by using square brackets so what if I

using square brackets so what if I simplify the code a little bit and just

simplify the code a little bit and just say if s is in the following list of

say if s is in the following list of values capital y or lowercase y it's not

values capital y or lowercase y it's not all that different logically but it's a

all that different logically but it's a little tighter it's a little more

little tighter it's a little more compact so L ifs is in capital N or

compact so L ifs is in capital N or lowercase n i can express that same idea

lowercase n i can express that same idea too so here again it's just getting a

too so here again it's just getting a little more pleasant to write code

little more pleasant to write code there's less like hitting of the

there's less like hitting of the keyboard you can express yourself a

keyboard you can express yourself a little more succinctly and using the

little more succinctly and using the keyword in Python will figure out how to

keyword in Python will figure out how to search the entire list for whatever the

search the entire list for whatever the value of s is and if it finds it it will

value of s is and if it finds it it will return true automatically else it will

return true automatically else it will return false so if I run agree. Pi again

return false so if I run agree. Pi again and type in capital y or lowercase y

and type in capital y or lowercase y that still now works well I can typen

that still now works well I can typen tighten this up further if I want to add

tighten this up further if I want to add more features well what if I want to

more features well what if I want to support not just why Big Y and little y

support not just why Big Y and little y but how about yes or yes or in case the

but how about yes or yes or in case the user's yelling or you know someone who

user's yelling or you know someone who doesn't really isn't good with caps lock

doesn't really isn't good with caps lock types in yes wait a minute but it could

types in yes wait a minute but it could be weird like do we want to support this

be weird like do we want to support this or this I mean this this just gets

or this I mean this this just gets really tedious quickly combinatorically

really tedious quickly combinatorically if you consider all of these possible

if you consider all of these possible permutations what would be smarter than

permutations what would be smarter than doing something like this if you want to

doing something like this if you want to just be able to tolerate yes in any form

just be able to tolerate yes in any form of capitalization like logically what

of capitalization like logically what would be nice maybe whatever theut is

would be nice maybe whatever theut is you justf over to all lower and then

you justf over to all lower and then exactly super common Paradigm why don't

exactly super common Paradigm why don't we just force the users's input to all

we just force the users's input to all lowercase or all uppercase doesn't

lowercase or all uppercase doesn't matter so long as we're self-consistent

matter so long as we're self-consistent and just compare against all uppercase

and just compare against all uppercase or all lowercase and that will get rid

or all lowercase and that will get rid of all of the possible permutations

of all of the possible permutations otherwise now in C we might have done

otherwise now in C we might have done something like this we might have

something like this we might have simplified this whole list and just said

simplified this whole list and just said let's say uh we'll do how about

let's say uh we'll do how about lowercase so y or yes and we'll just

lowercase so y or yes and we'll just leave it at that but we need to force

leave it at that but we need to force now s to lowercase well in C we would

now s to lowercase well in C we would have used the ctype library we would

have used the ctype library we would have done like two lower and called that

have done like two lower and called that function passing it in although not

function passing it in although not really cuz in C type those operate on

really cuz in C type those operate on individual characters or chars not whole

individual characters or chars not whole strings we have actually didn't see a

strings we have actually didn't see a function that could convert the whole

function that could convert the whole string in C to lowercase but in Python

string in C to lowercase but in Python we're going to benefit from some other

we're going to benefit from some other feature as well it turns out that python

feature as well it turns out that python supports what's called objectoriented

supports what's called objectoriented programming and we're only going to

programming and we're only going to scratch the surface of this in cs50 but

scratch the surface of this in cs50 but if you take a higher level course in

if you take a higher level course in programming or CS you explore this as a

programming or CS you explore this as a different Paradigm up until now in C

different Paradigm up until now in C we've been focusing on what's called

we've been focusing on what's called really procedural programming you write

really procedural programming you write procedures you write functions top to

procedures you write functions top to bottom uh left to right and when you

bottom uh left to right and when you want to change some value we were in the

want to change some value we were in the habit of using a procedure that is a

habit of using a procedure that is a function you would pass something like a

function you would pass something like a variable into a function like to Upper

variable into a function like to Upper or to lower and it would do its thing

or to lower and it would do its thing and hand you back a value well it turns

and hand you back a value well it turns out that it would be nicer programming

out that it would be nicer programming wise if some data types just had

wise if some data types just had built-in functionality like why do we

built-in functionality like why do we have our variables over here and all of

have our variables over here and all of our helper functions like two upper and

our helper functions like two upper and two lower over here such that we

two lower over here such that we constantly have to pass one into the

constantly have to pass one into the other it would be nice to sort of bake

other it would be nice to sort of bake into our data types buil-in

into our data types buil-in functionality so that you can change

functionality so that you can change variables using their own default be uh

variables using their own default be uh buil-in functionality and so

buil-in functionality and so object-oriented programming otherwise

object-oriented programming otherwise known as oop is a technique whereby

known as oop is a technique whereby certain types of values like a string

certain types of values like a string AKA stir not only have properties inside

AKA stir not only have properties inside of them attributes just like a struct in

of them attributes just like a struct in C your data can also have functions

C your data can also have functions built into them as well so whereas in C

built into them as well so whereas in C which is not objectoriented you have

which is not objectoriented you have strs and strs can only store data like a

strs and strs can only store data like a name and a number when implementing a

name and a number when implementing a person in Python you can for instance

person in Python you can for instance have not just a structure otherwise

have not just a structure otherwise known as a class storing a name and a

known as a class storing a name and a number you can have a function like call

number you can have a function like call that person or email that person or

that person or email that person or actual verbs or actions associated with

actual verbs or actions associated with that piece of data now in the context of

that piece of data now in the context of strings it turns out that strings come

strings it turns out that strings come with a lot of useful functionality and

with a lot of useful functionality and in fact this URL here which is in docs.

in fact this URL here which is in docs. python.org which is the official

python.org which is the official documentation for python you'll see a

documentation for python you'll see a whole list of methods that is functions

whole list of methods that is functions that come with strings that you can

that come with strings that you can actually use to modify their values and

actually use to modify their values and what I mean by this is the following if

what I mean by this is the following if we go through the documentation poke

we go through the documentation poke around it turns out that strings come

around it turns out that strings come with a function called Lower and if you

with a function called Lower and if you want to use that function you just have

want to use that function you just have to use slightly different syntax than in

to use slightly different syntax than in C you do not do to lower and you do not

C you do not do to lower and you do not say as I just did lower because this

say as I just did lower because this function is built into s itself and just

function is built into s itself and just like in C when you want to go inside of

like in C when you want to go inside of a variable like a structure and access a

a variable like a structure and access a piece of data inside of it like name or

piece of data inside of it like name or number when you also have functions

number when you also have functions built into data types AKA methods a

built into data types AKA methods a method is just a function that is built

method is just a function that is built into a piece of data you can do s do

into a piece of data you can do s do lower open pen closed pen in this case

lower open pen closed pen in this case and I can do this down here as well if

and I can do this down here as well if if s do lower in quote unquote uh n or

if s do lower in quote unquote uh n or no the whole thing I can force this

no the whole thing I can force this whole thing to lowercase so the only

whole thing to lowercase so the only difference here now is in

difference here now is in object-oriented programming instead of

object-oriented programming instead of constantly passing a value into a

constantly passing a value into a function you just access a function

function you just access a function that's inside of the value it just works

that's inside of the value it just works because of how the language itself is

because of how the language itself is defined and the only way you know that

defined and the only way you know that these functions exist is the

these functions exist is the documentation a Class A book a website

documentation a Class A book a website or the like questions now on this

technique all right I claim this is correct now even though you've never

correct now even though you've never programmed most of you in Python before

programmed most of you in Python before not super well-designed there's an

not super well-designed there's an subtle inefficiency now on lines three

subtle inefficiency now on lines three and five together what's dumb about how

and five together what's dumb about how I've used lower might you

I've used lower might you think

yeah yeah if you're going to use the same function twice and ask the same

same function twice and ask the same question expecting the same answer why

question expecting the same answer why are you calling the function itself

are you calling the function itself twice maybe we should just store the

twice maybe we should just store the result in a variable so we could do this

result in a variable so we could do this in a couple of different ways we for

in a couple of different ways we for instance could go up here and create

instance could go up here and create another variable called T and set that

another variable called T and set that equal to s. lower and then we could just

equal to s. lower and then we could just change this to be T here but honestly I

change this to be T here but honestly I don't think we technically need another

don't think we technically need another variable Al together here I could just

variable Al together here I could just do something like this let's change the

do something like this let's change the value of s to be the lowercase version

value of s to be the lowercase version thereof and so now I can quite simply

thereof and so now I can quite simply refer to S again and again like this

refer to S again and again like this this reusing that same value now to be

this reusing that same value now to be sure I have now just lost the user's

sure I have now just lost the user's original input and if I care about that

original input and if I care about that if they typed in all caps I have no idea

if they typed in all caps I have no idea anymore so maybe I do want to use a

anymore so maybe I do want to use a separate variable altogether but a

separate variable altogether but a takeaway here too is that strings in

takeaway here too is that strings in Python are technically what we'll call

Python are technically what we'll call immutable that is they cannot be changed

immutable that is they cannot be changed this was not true in C once we gave you

this was not true in C once we gave you arrays in week two or memory in week

arrays in week two or memory in week four you could go to town on a string

four you could go to town on a string and change any of the characters you

and change any of the characters you want upper casing lower casing changing

want upper casing lower casing changing it shortening it and so forth but in

it shortening it and so forth but in this case uh this returns a copy of s

this case uh this returns a copy of s forced to lowercase it doesn't change

forced to lowercase it doesn't change the original string that is the memory

the original string that is the memory the bytes in the computer's memory when

the bytes in the computer's memory when you assign it back to S you're

you assign it back to S you're essentially forgetting about the old

essentially forgetting about the old version of s but because python does

version of s but because python does memory management for you there's no

memory management for you there's no maloc there's no free python

maloc there's no free python automatically frees up the original

automatically frees up the original bytes like yees and hands them back to

bytes like yees and hands them back to the operating system for you all right

the operating system for you all right questions now on this

questions now on this technique questions on

technique questions on this in general I'll call out the python

this in general I'll call out the python documentation will start to be your

documentation will start to be your friend because in class we'll only

friend because in class we'll only scratch the surface with some of these

scratch the surface with some of these things but in docs. python.org for

things but in docs. python.org for instance there's a whole reference of

instance there's a whole reference of all of the built-in functions that come

all of the built-in functions that come with the language as well as for

with the language as well as for instance those with the string all right

instance those with the string all right well let me go ahead and before we take

well let me go ahead and before we take a break let's go ahead and create

a break let's go ahead and create something a little familiar to based on

something a little familiar to based on our week here in C let me propose that

our week here in C let me propose that we revisit those examples in involving

we revisit those examples in involving some meow so for instance when we had

some meow so for instance when we had our cat meow back in the first week and

our cat meow back in the first week and then second in C we did something that

then second in C we did something that was a little stupid at first whereby we

was a little stupid at first whereby we created a file as I'll do here this time

created a file as I'll do here this time called meow. p and if I want a cat to

called meow. p and if I want a cat to meow three times I could run it once

meow three times I could run it once like this little copy paste and now

like this little copy paste and now python of meow. py and I'm done now

python of meow. py and I'm done now we've visited this example like two

we've visited this example like two times at least now in scratch it and see

times at least now in scratch it and see it's correct I'll stipulate but what's

it's correct I'll stipulate but what's obviously poorly

obviously poorly designed what's the fault here

designed what's the fault here yeah it should just be a loop right like

yeah it should just be a loop right like why type it three times literally

why type it three times literally copying and pasting is almost always a

copying and pasting is almost always a bad thing except in C when you have the

bad thing except in C when you have the function prototypes that you need to

function prototypes that you need to borrow but in this case this is just

borrow but in this case this is just inefficient so what could we do better

inefficient so what could we do better here in Python well in Python we could

here in Python well in Python we could probably change this in a few different

probably change this in a few different ways we could borrow some of the syntax

ways we could borrow some of the syntax we proposed in slide form earlier like

we proposed in slide form earlier like give me a variable called I set it to

give me a variable called I set it to zero no semicolon while I is less than

zero no semicolon while I is less than three if I want to do this three times I

three if I want to do this three times I can go ahead and print out meow and then

can go ahead and print out meow and then I can do i+ equals one and I think this

I can do i+ equals one and I think this would do the trick python of meow. and

would do the trick python of meow. and we're back in business already well if I

we're back in business already well if I wanted to change this to a for Loop well

wanted to change this to a for Loop well in Python it would be a little tighter

in Python it would be a little tighter but this would not be the best approach

but this would not be the best approach so for I in uh

so for I in uh 012 I could just do print meow like this

012 I could just do print meow like this and that too would get the job done but

and that too would get the job done but to my to our discussion earlier this

to my to our discussion earlier this would get stupid pretty quickly if you

would get stupid pretty quickly if you had to keep enumerating all of these

had to keep enumerating all of these values like what did we introduce

values like what did we introduce instead the the range function exactly

instead the the range function exactly so that hands me back way more

so that hands me back way more efficiently just the values I want

efficiently just the values I want indeed one at a time so even this if I

indeed one at a time so even this if I run it a third a third or fourth time

run it a third a third or fourth time we've got the same result but now let's

we've got the same result but now let's transition to where we went with this

transition to where we went with this back in the day how can we start to

back in the day how can we start to modularize this like just like it would

modularize this like just like it would be nice I claimed if MIT had given us a

be nice I claimed if MIT had given us a meow function wouldn't it be nice if

meow function wouldn't it be nice if like python had given us a meow function

like python had given us a meow function maybe less compelling in Python but how

maybe less compelling in Python but how can I build my own function well I did

can I build my own function well I did this briefly with the spell checker

this briefly with the spell checker earlier but let me go ahead and propose

earlier but let me go ahead and propose that we could Implement now our own

that we could Implement now our own version of this in python as follows let

version of this in python as follows let me go ahead and start fresh here and use

me go ahead and start fresh here and use the keyword defa so this did not exist

the keyword defa so this did not exist in C you had the return value the

in C you had the return value the function name the arguments in Python

function name the arguments in Python you literally say defa to define a

you literally say defa to define a function you give it a name like meow

function you give it a name like meow and now now I'm going to go ahead and in

and now now I'm going to go ahead and in this function just print out meow and

this function just print out meow and this lets me change it to anything else

this lets me change it to anything else I want in the future but for now it's an

I want in the future but for now it's an abstraction and in fact I can uh move it

abstraction and in fact I can uh move it out of sight out of mind just going to

out of sight out of mind just going to hit enter a bunch of times to pretend

hit enter a bunch of times to pretend like now it exists but I don't care how

like now it exists but I don't care how it is implemented and up here now I can

it is implemented and up here now I can do something like this 4 I in range of

do something like this 4 I in range of three let me go ahead and not print meow

three let me go ahead and not print meow anymore let me just call meow and

anymore let me just call meow and tightening up my code further but I

tightening up my code further but I think let's see python of meow. py this

think let's see python of meow. py this is I think going to be the first time it

is I think going to be the first time it does not work

does not work correctly okay so here we have sadly our

correctly okay so here we have sadly our first python error and let's see the

first python error and let's see the syntax is going to be different from C

syntax is going to be different from C or clangs output traceback is like the

or clangs output traceback is like the term of art here this is like a trace

term of art here this is like a trace back of all of the lines of code that

back of all of the lines of code that were just executed or really functions

were just executed or really functions you called the file name is

you called the file name is uninteresting this is like my codes

uninteresting this is like my codes space specifically but the file name is

space specifically but the file name is important here meow. py uh line two is

important here meow. py uh line two is is the issue okay I didn't get very far

is the issue okay I didn't get very far before I screwed up and then there's a

before I screwed up and then there's a name error and you'll see in Python

name error and you'll see in Python there's typically these capitalized uh

there's typically these capitalized uh keywords that hint at what the issue is

keywords that hint at what the issue is it's something related to names of

it's something related to names of variables meow is not defined all right

variables meow is not defined all right you're programming python for the first

you're programming python for the first time you've screwed up you're following

time you've screwed up you're following some online tutorial you're seeing this

some online tutorial you're seeing this reason through it like why might meow

reason through it like why might meow not be defined what can weer infer about

not be defined what can weer infer about python how to troubleshoot

python how to troubleshoot logically is it me ised after maybe is

logically is it me ised after maybe is it because meow is defined after you

it because meow is defined after you know as smart as python seems to be Visa

know as smart as python seems to be Visa VC they have some similar design

VC they have some similar design characteristics so let's try that so let

characteristics so let's try that so let me scroll all the way back down to where

me scroll all the way back down to where I move this earlier let me uh get rid of

I move this earlier let me uh get rid of it way down there I'll copy it to my

it way down there I'll copy it to my clipboard and let me just kind of hack

clipboard and let me just kind of hack something together let me just put it up

something together let me just put it up here and let's see if this works so now

here and let's see if this works so now let me clear my terminal run python of

let me clear my terminal run python of meow. okay we're back in business so

meow. okay we're back in business so that was actually really good intuition

that was actually really good intuition good debugging technique to sort of

good debugging technique to sort of reason through it now this is kind of

reason through it now this is kind of contradicting what I claimed back in

contradicting what I claimed back in week one which was that you know the

week one which was that you know the main part of your program ideally should

main part of your program ideally should just be at the top of the file like

just be at the top of the file like don't make me look for it it's not a

don't make me look for it it's not a huge deal with like a four-line program

huge deal with like a four-line program but if you've got 40 lines 400 lines you

but if you've got 40 lines 400 lines you don't want like the juice juicy part of

don't want like the juice juicy part of your program to be way down here and all

your program to be way down here and all of these functions way up here so it

of these functions way up here so it would be nice maybe if we actually have

would be nice maybe if we actually have a main function and so it actually turns

a main function and so it actually turns out to be a convention in Python to

out to be a convention in Python to define a main function it's not a

define a main function it's not a special function that's automatically

special function that's automatically called like in C but humans realized you

called like in C but humans realized you know what that was a pretty useful

know what that was a pretty useful feature let me Define a function called

feature let me Define a function called main let me indent these lines

main let me indent these lines underneath it let me practice what I'm

underneath it let me practice what I'm preaching which is put the main code at

preaching which is put the main code at the top of the file and wonderfully in

the top of the file and wonderfully in Python now you do not need prototypes

Python now you do not need prototypes there's none of that hackish copying and

there's none of that hackish copying and pasting of the return type the name and

pasting of the return type the name and the arguments to a function like we

the arguments to a function like we needed in C this is now

needed in C this is now okay instead except for one Minor Detail

okay instead except for one Minor Detail let me go ahead and run python of

let me go ahead and run python of meow. hopefully now I've solve this

meow. hopefully now I've solve this problem by having a main function but

problem by having a main function but now nothing has happened all right even

now nothing has happened all right even if you've never programmed in Python

if you've never programmed in Python before What might explain this

before What might explain this behavior and how do I

behavior and how do I fix again when you're off in the real

fix again when you're off in the real world learning some new language all you

world learning some new language all you have is deductive logic to debug yeah I

have is deductive logic to debug yeah I remember

right so the solution to be clear in C was that we had to put the Prototype up

was that we had to put the Prototype up here otherwise we'd get an error message

here otherwise we'd get an error message in this case I'm actually not getting an

in this case I'm actually not getting an error message and indeed I'll claim that

error message and indeed I'll claim that you don't need the prototypes in Python

you don't need the prototypes in Python just not necessary because that was

just not necessary because that was annoying if nothing

annoying if nothing else but what else might explain yeah

else but what else might explain yeah I'm

I'm back yeah maybe you have to call Main

back yeah maybe you have to call Main itself if main is not some some special

itself if main is not some some special status in Python maybe just because it

status in Python maybe just because it exists isn't enough and indeed if you

exists isn't enough and indeed if you want to call Maine the new convention is

want to call Maine the new convention is actually going to be as the very last

actually going to be as the very last line of your program typically to

line of your program typically to literally call Main it's a little stupid

literally call Main it's a little stupid stupid looking but you know they made a

stupid looking but you know they made a design decision and this is how now we

design decision and this is how now we work around it python of meowy now we're

work around it python of meowy now we're back in business but now logically why

back in business but now logically why does this work the way it does well in

does this work the way it does well in this case top to bottom line one is

this case top to bottom line one is telling python to define a fun fun

telling python to define a fun fun called Main and then Define it as

called Main and then Define it as follows lines two and three but it's not

follows lines two and three but it's not calling main yet line six is telling

calling main yet line six is telling python how to define a function called

python how to define a function called meow but it's not calling these lines

meow but it's not calling these lines yet now line 10 you're telling python

yet now line 10 you're telling python call Main and at that point python has

call Main and at that point python has been trained if you will to know what

been trained if you will to know what main is on line one to know what meow is

main is on line one to know what meow is on line six and so it's now perfectly

on line six and so it's now perfectly okay for Maine to be above meow because

okay for Maine to be above meow because you never called them yet you defined

you never called them yet you defined defined and then you called and that's

defined and then you called and that's the logic behind this any questions now

the logic behind this any questions now on the structure of this technique

on the structure of this technique here now let's do one more then recall

here now let's do one more then recall that the last thing we did in scratch

that the last thing we did in scratch and in Python uh scratch and in C was to

and in Python uh scratch and in C was to actually

actually parameterize uh these same function so

parameterize uh these same function so suppose that you don't want Maine to be

suppose that you don't want Maine to be responsible for the loop here you

responsible for the loop here you instead want to very simply do something

instead want to very simply do something like meow three times and be done with

like meow three times and be done with it well in Python it's going to be

it well in Python it's going to be similar in spirit toy but again we don't

similar in spirit toy but again we don't need to keep mentioning data types if

need to keep mentioning data types if you want me now to take some argument

you want me now to take some argument like a number n you can just specify n

like a number n you can just specify n as the name of that argument or you

as the name of that argument or you could call it anything else of course

could call it anything else of course that you want you don't have to specify

that you want you don't have to specify int or anything else in your code now

int or anything else in your code now inside of meow you can do something like

inside of meow you can do something like for I in let's say I definitely now

for I in let's say I definitely now can't do this because like that would be

can't do this because like that would be weird to start the list and end it with

weird to start the list and end it with n so if I can come back over here what's

n so if I can come back over here what's the solution how can I do something n

the solution how can I do something n times yeah using range so range is nice

times yeah using range so range is nice cuz I can pass in now this variable n

cuz I can pass in now this variable n and now I can meow whoops now I can

and now I can meow whoops now I can print out quote unquote meow so it's

print out quote unquote meow so it's almost the same as in scratch almost the

almost the same as in scratch almost the same as in C but it's a little simpler

same as in C but it's a little simpler and if now I run meow. P I'll have the

and if now I run meow. P I'll have the ability now to do this here as well all

ability now to do this here as well all right questions on any of this right now

right questions on any of this right now we're sort of like taking the stroll

we're sort of like taking the stroll through week one we're going to

through week one we're going to momentarily escalate things to look not

momentarily escalate things to look not only at some of these basic but also

only at some of these basic but also other features like we saw with face

other features like we saw with face recognition with the speller or the like

recognition with the speller or the like um because of how many of us are here we

um because of how many of us are here we have a huge amount of candy out in the

have a huge amount of candy out in the lobby so why don't we go ahead and take

lobby so why don't we go ahead and take a 10-minute break and we come back we'll

a 10-minute break and we come back we'll do even fancier more powerful things

do even fancier more powerful things with python in 10 all right so we are

with python in 10 all right so we are back among our goals now are to

back among our goals now are to introduce a few more building blocks so

introduce a few more building blocks so that we can solve more interesting

that we can solve more interesting problems at the end much like those that

problems at the end much like those that we began with you recall from a few

we began with you recall from a few weeks ago we played with this sort of

weeks ago we played with this sort of two-dimensional Super Mario World and we

two-dimensional Super Mario World and we tried to print a vertical column of like

tried to print a vertical column of like three or more bricks well let me propose

three or more bricks well let me propose that we use this as an opportunity to

that we use this as an opportunity to now Tinker with some of Python's more uh

now Tinker with some of Python's more uh useful more userfriendly functionality

useful more userfriendly functionality as well so let me code a file called

as well so let me code a file called mario. py and let's just print out like

mario. py and let's just print out like that the equivalent of that vertical

that the equivalent of that vertical column so it's of height three each one

column so it's of height three each one is a hash so let's do for I in range of

is a hash so let's do for I in range of three initially and let's just print out

three initially and let's just print out a single hash and I think now python of

a single hash and I think now python of mario. py voila we're in business

mario. py voila we're in business printing out just that same pyramid

printing out just that same pyramid there or just that same column there

there or just that same column there what if though we want to print a column

what if though we want to print a column of like some variable height where the

of like some variable height where the user tells us how tall they want it to

user tells us how tall they want it to be well let me go up here for instance

be well let me go up here for instance and instead how about we'll use um let's

and instead how about we'll use um let's do this how about uh from cs50

do this how about uh from cs50 import how about the get in function as

import how about the get in function as before so it will deal with making sure

before so it will deal with making sure the user gives us an integer and now in

the user gives us an integer and now in the past whenever we wanted to get a

the past whenever we wanted to get a number from a user we've actually

number from a user we've actually followed a certain Paradigm in fact if I

followed a certain Paradigm in fact if I open up here for instance uh how about

open up here for instance uh how about Mario in how about Mario 1. C from a

Mario in how about Mario 1. C from a while back you might recall that we had

while back you might recall that we had code like this and we specifically use

code like this and we specifically use the do while loop and see whenever we

the do while loop and see whenever we want to like get something from the user

want to like get something from the user maybe again and again and again until

maybe again and again and again until they cooperate at which point we finally

they cooperate at which point we finally break out of the loop so it turns out

break out of the loop so it turns out python does have while Loops does have

python does have while Loops does have four Loops does not have do while loops

four Loops does not have do while loops and yet pretty much anytime you've

and yet pretty much anytime you've gotten user input you've probably used

gotten user input you've probably used this Paradigm so it turns out that the

this Paradigm so it turns out that the python equivalent of this is to do

python equivalent of this is to do similar in spirit but using only a while

similar in spirit but using only a while loop and a common Paradigm in python as

loop and a common Paradigm in python as I alluded earlier is to actually

I alluded earlier is to actually deliberately induce an infinite Loop

deliberately induce an infinite Loop while true capital T and then do what

while true capital T and then do what you want to do like get an INT from the

you want to do like get an INT from the user and prompt them for the height for

user and prompt them for the height for instance in question and then if you're

instance in question and then if you're sure that the user has given you what

sure that the user has given you what you want like n is greater than zero

you want like n is greater than zero which is what I want in this case cuz I

which is what I want in this case cuz I want a positive integer otherwise

want a positive integer otherwise there's nothing to print you literally

there's nothing to print you literally just break out of the loop and so we

just break out of the loop and so we could actually use this technique in C

could actually use this technique in C it's just not really done in C you could

it's just not really done in C you could absolutely in C have done a while true

absolutely in C have done a while true loop with the parentheses lowercase true

loop with the parentheses lowercase true you could break out of it and so forth

you could break out of it and so forth but in Python this is like the python

but in Python this is like the python way and this is actually a term of art

way and this is actually a term of art this way in Python is pythonic like this

this way in Python is pythonic like this is the way everyone does it quote

is the way everyone does it quote unquote doesn't mean you have to but

unquote doesn't mean you have to but that's sort of the way like the cool

that's sort of the way like the cool python programmers would Implement an

python programmers would Implement an idea like this trying to do something

idea like this trying to do something again and again and again until the user

again and again and again until the user actually cooperates but all we've done

actually cooperates but all we've done is take away the do while loop but still

is take away the do while loop but still logically we can implement the same idea

logically we can implement the same idea now below this let me go ahead and just

now below this let me go ahead and just print out for I in range of n this time

print out for I in range of n this time because I want it to be variable and not

because I want it to be variable and not three I can go ahead and print out the

three I can go ahead and print out the hash let me go ahead and get rid of the

hash let me go ahead and get rid of the C version here open my terminal window

C version here open my terminal window and I'll run again python of mario. py

and I'll run again python of mario. py I'll type in three and I get back those

I'll type in three and I get back those three hashes but if I instead type in

three hashes but if I instead type in four I now get four hashes instead so

four I now get four hashes instead so the takeaway here is quite simply that

the takeaway here is quite simply that this would be the way for instance to

this would be the way for instance to actually get back a value in Python that

actually get back a value in Python that is consistent with some parameter like

is consistent with some parameter like greater than zero how about this let's

greater than zero how about this let's actually uh practice what we preached a

actually uh practice what we preached a moment ago with our meowing examples and

moment ago with our meowing examples and kind of factoring all this out let me go

kind of factoring all this out let me go ahead and Define a main function as

ahead and Define a main function as before let me go ahead and assume for

before let me go ahead and assume for the moment that a get height function

the moment that a get height function exists which is not a thing in Python

exists which is not a thing in Python I'm going to invent it in just a moment

I'm going to invent it in just a moment and now I'm going to go ahead and do

and now I'm going to go ahead and do something like this for I in the range

something like this for I in the range of that height well let's go ahead and

of that height well let's go ahead and print out those hashes so I'm assuming

print out those hashes so I'm assuming that get height exists let me go ahead

that get height exists let me go ahead and Implement that abstraction so Define

and Implement that abstraction so Define a function now called get height it's

a function now called get height it's not going to take any arguments in this

not going to take any arguments in this design while true I can go ahead and do

design while true I can go ahead and do the same thing as before assign a

the same thing as before assign a variable n the return value of get int

variable n the return value of get int prompting the user for that height and

prompting the user for that height and then if n is greater than zero I can go

then if n is greater than zero I can go ahead and break but if I break here I

ahead and break but if I break here I logically just like can see end up

logically just like can see end up executing below the loop in question but

executing below the loop in question but there's nothing there but if I want get

there's nothing there but if I want get height to return the height what should

height to return the height what should I type here on line 14

logically what do I want to return to be clear yeah so I actually want to return

clear yeah so I actually want to return n and here's another curiosity of python

n and here's another curiosity of python Visa BC there doesn't seem to be an

Visa BC there doesn't seem to be an issue of scope anymore right in C it was

issue of scope anymore right in C it was super important to not only declare your

super important to not only declare your variables with the data types you also

variables with the data types you also had to be mindful of like where they

had to be mindful of like where they exist inside of those curly braces in

exist inside of those curly braces in Python it turns out you can be a little

Python it turns out you can be a little looser with things for better for worse

looser with things for better for worse and so on line 11 if I create a variable

and so on line 11 if I create a variable called n it exists on line 11 12 and

called n it exists on line 11 12 and even 13

even 13 outside of the while loop so to be clear

outside of the while loop so to be clear in C with a while loop we would have

in C with a while loop we would have ordinarily had not the colon we would

ordinarily had not the colon we would have had the curly brace like here and

have had the curly brace like here and over here and a week ago I would have

over here and a week ago I would have claimed that in C N does not exist

claimed that in C N does not exist outside of the while loop by nature of

outside of the while loop by nature of those curly braces even though the curly

those curly braces even though the curly braces are gone python actually allows

braces are gone python actually allows you to use a variable anytime after you

you to use a variable anytime after you have assigned it a value so slightly

have assigned it a value so slightly more powerful as such how however I can

more powerful as such how however I can tighten this up a little bit logically

tighten this up a little bit logically and this is true in C I don't really

and this is true in C I don't really need to break out of the loop by using

need to break out of the loop by using break recall that or know that I can

break recall that or know that I can actually once I'm ready to go I can just

actually once I'm ready to go I can just return the value I care about even

return the value I care about even inside of the loop and that will have

inside of the loop and that will have the side effect of breaking me out of

the side effect of breaking me out of the loop and also breaking me out of and

the loop and also breaking me out of and returning from the entire function so

returning from the entire function so nothing too new here in terms of C

nothing too new here in terms of C versus python except for this issue of

versus python except for this issue of scope and I indeed returned n at the

scope and I indeed returned n at the bottom there just to make clear that n

bottom there just to make clear that n would still exist so either of those are

would still exist so either of those are correct now I just have a Python program

correct now I just have a Python program that I think is going to allow me to

that I think is going to allow me to implement this same Mario idea so let's

implement this same Mario idea so let's run python of mario. and okay so nothing

run python of mario. and okay so nothing happened uh python of mario.

happened uh python of mario. py what did I do wrong yeah I have to

py what did I do wrong yeah I have to call Main so at the bottom of my code I

call Main so at the bottom of my code I have to call Main here and this is a

have to call Main here and this is a stylistic detail that's been subtle um

stylistic detail that's been subtle um generally speaking when when you are

generally speaking when when you are writing in Python um there's not a cs50

writing in Python um there's not a cs50 style guide per se there's actually a

style guide per se there's actually a python style guide that most people

python style guide that most people adhere to um it's and in this case

adhere to um it's and in this case double blank lines between functions is

double blank lines between functions is the norm I'm doing that deliberately

the norm I'm doing that deliberately although uh it might otherwise not be

although uh it might otherwise not be obvious but now that I've called main on

obvious but now that I've called main on line 16 let's run mario. once more aha

line 16 let's run mario. once more aha now we get there now we see it type in

now we get there now we see it type in three and I'm back in business printing

three and I'm back in business printing out the values there

out the values there yeah

sure why do I need the if condition at all why can't I just return n here as by

all why can't I just return n here as by doing return n or if I really want to be

doing return n or if I really want to be succinct I could technically just do

succinct I could technically just do this the only reason I added the if

this the only reason I added the if condition is because if the user types

condition is because if the user types in negative 1 -2 I wanted to prompt them

in negative 1 -2 I wanted to prompt them again and again that's all but that

again and again that's all but that would be totally acceptable too if you

would be totally acceptable too if you were okay with that result instead well

were okay with that result instead well let me do one other thing here to point

let me do one other thing here to point out why we are using get in so

out why we are using get in so frequently this new training wheel all

frequently this new training wheel all be it temporarily so let me go back to

be it temporarily so let me go back to the way it was a moment ago and let me

the way it was a moment ago and let me propose now to take away get int I

propose now to take away get int I claimed earlier that if you're not using

claimed earlier that if you're not using get int you can just use the input

get int you can just use the input function itself from

function itself from python but that always returns a string

python but that always returns a string or a stir and so recall that you have to

or a stir and so recall that you have to pass the output of the input function to

pass the output of the input function to an INT either on the same line or if you

an INT either on the same line or if you prefer on another line instead but it

prefer on another line instead but it turns out what I didn't do was show you

turns out what I didn't do was show you what happens if you uh don't cooperate

what happens if you uh don't cooperate with the user uh with the program so if

with the user uh with the program so if I run python of mario. now works great

I run python of mario. now works great even without the get int function and I

even without the get int function and I can do it with four still works great

can do it with four still works great but let me clear my terminal and be

but let me clear my terminal and be difficult now as the user and type in C

difficult now as the user and type in C for the height instead enter now we see

for the height instead enter now we see one of those tracebacks again this one

one of those tracebacks again this one is different this isn't a name error but

is different this isn't a name error but apparently a value error and if I kind

apparently a value error and if I kind of ignore the stuff I don't understand I

of ignore the stuff I don't understand I can see invalid literal for INT with

can see invalid literal for INT with base 10 cat that's a super cryptic way

base 10 cat that's a super cryptic way of saying that c a is not a number in

of saying that c a is not a number in decimal notation and so I would seem to

decimal notation and so I would seem to have to somehow handle this case and if

have to somehow handle this case and if you want to be more Curious you'll see

you want to be more Curious you'll see that this is indeed a trace back and um

that this is indeed a trace back and um C tends to do this too or the debugger

C tends to do this too or the debugger would do this for you too you can see

would do this for you too you can see all of the functions that have been

all of the functions that have been called to get you to this point so

called to get you to this point so apparently my problem is initially in

apparently my problem is initially in line 14 but line 14 if I keep scrolling

line 14 but line 14 if I keep scrolling is uninteresting it's main but line 14

is uninteresting it's main but line 14 leads me to execute line two which is

leads me to execute line two which is indeed in main that leads me to execute

indeed in main that leads me to execute line nine which is in get height and

line nine which is in get height and okay here's the issue so the closest

okay here's the issue so the closest line number to the error message is the

line number to the error message is the one that probably reveals the most line

one that probably reveals the most line nine is where my issue is so I can't

nine is where my issue is so I can't just blindly ask the user for input and

just blindly ask the user for input and then convert it to an INT if they're not

then convert it to an INT if they're not going to give me an INT now how do we

going to give me an INT now how do we deal with this well back in problem set

deal with this well back in problem set two you might recall validating that the

two you might recall validating that the user typed in a number and using a for

user typed in a number and using a for Loop and the like well it turns out

Loop and the like well it turns out there's a better way to do this in

there's a better way to do this in Python and the are kind of there if you

Python and the are kind of there if you want to try to convert something for a

want to try to convert something for a number to a number that might not

number to a number that might not actually be a number turns out Python

actually be a number turns out Python and certain other languages literally

and certain other languages literally have a keyword called try and if only

have a keyword called try and if only this existed for the past few weeks I

this existed for the past few weeks I know but like you can try to do the

know but like you can try to do the following with your code what do I want

following with your code what do I want to try to do well I want to try to

to try to do well I want to try to execute those few lines except if

execute those few lines except if there's an error so I can say except if

there's an error so I can say except if there's a value error specifically the

there's a value error specifically the one I screwed up and created a moment

one I screwed up and created a moment ago and if there is a value error I can

ago and if there is a value error I can print out an informative message to the

print out an informative message to the user like not an integer or anything

user like not an integer or anything else and what's happening here now is

else and what's happening here now is literally this operative word try the

literally this operative word try the pro python is going to try to get input

pro python is going to try to get input and try to convert it to an in and it's

and try to convert it to an in and it's going to try to check if it's greater

going to try to check if it's greater than zero and then try to return it all

than zero and then try to return it all why all of three of those lines are

why all of three of those lines are inside of indented underneath the tri

inside of indented underneath the tri block except if something goes wrong

block except if something goes wrong specifically a value error happens then

specifically a value error happens then it prints this but it doesn't return

it prints this but it doesn't return anything and because I'm in a loop that

anything and because I'm in a loop that means it's going to do it again and

means it's going to do it again and again and again until the human actually

again and again until the human actually cooperates and gives me an actual number

cooperates and gives me an actual number and so this too is what the world would

and so this too is what the world would call pythonic in Python you don't

call pythonic in Python you don't necessarily rigorously try to validate

necessarily rigorously try to validate the users's input make sure they haven't

the users's input make sure they haven't screwed up you honestly take a more

screwed up you honestly take a more laxidasical approach and just try to do

laxidasical approach and just try to do something but catch an error if it

something but catch an error if it happens so catch is also a term of art

happens so catch is also a term of art even though it's not a keyword here

even though it's not a keyword here except if something happens you handle

except if something happens you handle it so you try and you handle it it's

it so you try and you handle it it's sort of best effort programming if you

sort of best effort programming if you will but this is baked into the mindset

will but this is baked into the mindset of the Python uh programming community

of the Python uh programming community so now if I do python of mario. py and I

so now if I do python of mario. py and I cooperate works great as before try and

cooperate works great as before try and succeed three Works four works if though

succeed three Works four works if though I try and fail by typing in cat it

I try and fail by typing in cat it doesn't crash per se it doesn't show me

doesn't crash per se it doesn't show me an error it shows me something more user

an error it shows me something more user friendly like not an integer and then I

friendly like not an integer and then I can try again with dog not an integer I

can try again with dog not an integer I can try again with five and now it works

can try again with five and now it works so we won't generally have you write

so we won't generally have you write much in the way of these try except

much in the way of these try except blocks only because they get a little

blocks only because they get a little sophisticated quickly but that is to

sophisticated quickly but that is to reveal what the get int function is

reveal what the get int function is doing this is why we give you the

doing this is why we give you the training wheels so that when you want to

training wheels so that when you want to get an INT you don't have to jump

get an INT you don't have to jump through all these annoying Hoops to do

through all these annoying Hoops to do so but that's all the library is really

so but that's all the library is really doing for you is just try and accept you

doing for you is just try and accept you won't be left with any training wheels

won't be left with any training wheels ultimately questions now on getting

ultimately questions now on getting inputs and trying in this way

yeah Tri block it say that oh you could you put the condition outside of the tri

you put the condition outside of the tri block short answer yes and in fact I

block short answer yes and in fact I struggled with this last night when

struggled with this last night when tweaking this example to show the

tweaking this example to show the simplest version I will disclaim that

simplest version I will disclaim that really I should only be trying literally

really I should only be trying literally to do the The Fragile part and then down

to do the The Fragile part and then down here I should be really doing what

here I should be really doing what you're proposing which is do the

you're proposing which is do the condition out here the problem is though

condition out here the problem is though that logically this gets messy quickly

that logically this gets messy quickly right because except if there's a value

right because except if there's a value error I want to print out not an integer

error I want to print out not an integer I can't compare n against zero then

I can't compare n against zero then because n doesn't exist because there

because n doesn't exist because there was an error so it turns out and I'll

was an error so it turns out and I'll show you this this is now the advanced

show you this this is now the advanced version of python there's actually an

version of python there's actually an else keyword you can use in Python that

else keyword you can use in Python that does not accompany if or L if it

does not accompany if or L if it accompanies try and accept which I think

accompanies try and accept which I think is weirdly confusing a different word

is weirdly confusing a different word would have been better but if you really

would have been better but if you really prefer I could have done this instead

prefer I could have done this instead dead and this is one of these design

dead and this is one of these design things where like reasonable people will

things where like reasonable people will disagree generally speaking you should

disagree generally speaking you should only try to do the one line that might

only try to do the one line that might very well fail but honestly this looks

very well fail but honestly this looks kind of stupid now it's just

kind of stupid now it's just unnecessarily complicated and so my own

unnecessarily complicated and so my own preference was actually the original

preference was actually the original which was yeah I'm trying a few extra

which was yeah I'm trying a few extra lines that really aren't going to fail

lines that really aren't going to fail mathematically but it's just tighter

mathematically but it's just tighter it's cleaner this way and here's again

it's cleaner this way and here's again the sort of like you know arguments

the sort of like you know arguments you'll start to make yourself as you get

you'll start to make yourself as you get more comfortable with programming you'll

more comfortable with programming you'll have an opinion you'll disagree with

have an opinion you'll disagree with someone and so long as you can back your

someone and so long as you can back your argument up pretty

argument up pretty probably all right so how about we now

probably all right so how about we now take away some piece of magic that's

take away some piece of magic that's been here for a while let me go ahead

been here for a while let me go ahead and uh Delete all of this here and let

and uh Delete all of this here and let me propose that we revisit uh not that

me propose that we revisit uh not that vertical column and the exceptions that

vertical column and the exceptions that might result from getting input but

might result from getting input but these like horizontal question marks

these like horizontal question marks that we saw a while ago so I want all of

that we saw a while ago so I want all of those question marks on the same line

those question marks on the same line and yet I worry we're about to see a

and yet I worry we're about to see a challenge here because print up until

challenge here because print up until now has been putting new lines

now has been putting new lines everywhere automatically even without

everywhere automatically even without those backslash NS well let me propose

those backslash NS well let me propose that we do this for I in the range of

that we do this for I in the range of four if I want four question marks let

four if I want four question marks let me just print four question marks

me just print four question marks unfortunately I don't think this is

unfortunately I don't think this is correct yet let me run python of mario.

correct yet let me run python of mario. and of course this gives me a column

and of course this gives me a column instead of the row of question marks

instead of the row of question marks that I want so how do we do this well it

that I want so how do we do this well it turns out if you read the documentation

turns out if you read the documentation for the print function it turns out that

for the print function it turns out that print not surprisingly perhaps takes a

print not surprisingly perhaps takes a lot of different arguments as well and

lot of different arguments as well and in fact if you go to the documentation

in fact if you go to the documentation for it you'll see that it takes not just

for it you'll see that it takes not just positional arguments that is from left

positional arguments that is from left to right separated by commas turns out

to right separated by commas turns out python has supports a fancier feature

python has supports a fancier feature with arguments where you can pass the

with arguments where you can pass the names of arguments to functions too so

names of arguments to functions too so what do I mean by this if I go back to

what do I mean by this if I go back to vs code here and I've read the

vs code here and I've read the documentation it turns out that yes as

documentation it turns out that yes as before you can pass multiple arguments

before you can pass multiple arguments to python like this like hello comma

to python like this like hello comma David comma me that will just

David comma me that will just automatically concatenate all three of

automatically concatenate all three of those positional arguments together

those positional arguments together they're positional in the sense that

they're positional in the sense that they literally flow from left to right

they literally flow from left to right separated by commas but if you don't

separated by commas but if you don't want to just pass in values like that

want to just pass in values like that you want to actually print out as I did

you want to actually print out as I did before a question mark but you want to

before a question mark but you want to override the default behavior of print

override the default behavior of print by changing the line ending you can

by changing the line ending you can actually do this you can use the name of

actually do this you can use the name of an argument that you know exists from

an argument that you know exists from the documentation set it equal to some

the documentation set it equal to some alternative value and in fact even

alternative value and in fact even though this looks cryptic this is how I

though this looks cryptic this is how I would override the end of each line to

would override the end of each line to be quote unquote that is nothing because

be quote unquote that is nothing because if you read the documentation the

if you read the documentation the default value for this end argument does

default value for this end argument does someone want to guess is is back sln so

someone want to guess is is back sln so if you read the documentation you'll see

if you read the documentation you'll see that back sln is the implied default for

that back sln is the implied default for this end argument and so if you want to

this end argument and so if you want to change it you just say end equals

change it you just say end equals something else and so here I can change

something else and so here I can change change it to nothing and now rerun

change it to nothing and now rerun python of mario. and now they're all on

python of mario. and now they're all on the same line now looks a little stupid

the same line now looks a little stupid cuz I made that sort of week one mistake

cuz I made that sort of week one mistake where I still need to move the cursor to

where I still need to move the cursor to the next line that's just a different

the next line that's just a different problem I'm just going to go over here

problem I'm just going to go over here and print nothing I don't even need to

and print nothing I don't even need to print back sln because if print

print back sln because if print automatically gives you a backslash n

automatically gives you a backslash n just call print with nothing and you'll

just call print with nothing and you'll get that for free so let me rerun python

get that for free so let me rerun python of mario. Pi and now it looks a little

of mario. Pi and now it looks a little prettier at the prompt and to be super

prettier at the prompt and to be super clear as to what's going on suppose I

clear as to what's going on suppose I want to sort of make an exclamation here

want to sort of make an exclamation here I could change the back sln default to

I could change the back sln default to like an exclamation point Just for kicks

like an exclamation point Just for kicks and if I run python of mario. py again

and if I run python of mario. py again now I get this sort of you know

now I get this sort of you know exclamation with question marks and

exclamation with question marks and exclamation points as well so that's all

exclamation points as well so that's all that's going on here and this is what's

that's going on here and this is what's called a named argument it literally has

called a named argument it literally has a name that you can specify when calling

a name that you can specify when calling it in and it's different from positional

it in and it's different from positional in that you're literally using the

in that you're literally using the name let me propose something else

name let me propose something else though and this is why people kind of

though and this is why people kind of like python there's just kind of cool

like python there's just kind of cool ways to do things that's kind of a you

ways to do things that's kind of a you know ver it's a three line verbose way

know ver it's a three line verbose way of printing out four question marks you

of printing out four question marks you know I could certainly take the you know

know I could certainly take the you know shortcut and just do this but that's not

shortcut and just do this but that's not really that interesting for anyone

really that interesting for anyone especially if I want to do it a variable

especially if I want to do it a variable number of times but python does let you

number of times but python does let you do this if you want to uh multiply a

do this if you want to uh multiply a character some number of times not only

character some number of times not only can you use Plus for concatenation you

can you use Plus for concatenation you can use star or an asterisk for

can use star or an asterisk for multiplication if you will that is

multiplication if you will that is concatenation again and again and again

concatenation again and again and again so if I just print out quote unquote

so if I just print out quote unquote question mark Time 4 that's actually

question mark Time 4 that's actually going to be the tightest way the most

going to be the tightest way the most distinct way I can print four question

distinct way I can print four question marks instead and if I don't use four I

marks instead and if I don't use four I use n where I get n from the user bang

use n where I get n from the user bang like now I've gotten rid of the four

like now I've gotten rid of the four Loop entirely and I'm using the the star

Loop entirely and I'm using the the star operator to manipulate it instead and to

operator to manipulate it instead and to be super clear here in so far as python

be super clear here in so far as python does not have milock or free or memory

does not have milock or free or memory management that you have to do guess

management that you have to do guess what python also doesn't

have anything on your minds the past couple of

couple of weeks doesn't have pointers yes so

weeks doesn't have pointers yes so python does not have pointers which just

python does not have pointers which just means that all of that happens for you

means that all of that happens for you automatically underneath the hood Again

automatically underneath the hood Again by way of code that someone else wrote

by way of code that someone else wrote how about one more throwback with Mario

how about one more throwback with Mario we've talked about in week one this sort

we've talked about in week one this sort of two-dimensional structure where it's

of two-dimensional structure where it's like I claim like 3x3 a grid of bricks

like I claim like 3x3 a grid of bricks if you will well how can we do this in

if you will well how can we do this in Python we can do this in a couple of

Python we can do this in a couple of ways now let me go back to my mario. py

ways now let me go back to my mario. py and let me do something like for I in

and let me do something like for I in range of we'll just do three even though

range of we'll just do three even though I know now I could use get int or I

I know now I could use get int or I could use input and int and if I want to

could use input and int and if I want to do something two-dimensionally just like

do something two-dimensionally just like in C you can Nest your for Loop so maybe

in C you can Nest your for Loop so maybe I could do 4J in range of three and then

I could do 4J in range of three and then in here I could print out a uh hash

in here I could print out a uh hash symbol and then let's see if that gives

symbol and then let's see if that gives me nine total so if I've got a nested

me nine total so if I've got a nested Loop like this python of mario.

Loop like this python of mario. hopefully gives me a grid no it gave me

hopefully gives me a grid no it gave me a column of

a column of nine why

nine why logically even though I've got my row

logically even though I've got my row and my

and my columns

columns yeah yeah the line ending so in my row I

yeah yeah the line ending so in my row I can't let print just keep adding new

can't let print just keep adding new line adding new lines so I just have to

line adding new lines so I just have to override this here and let me not screw

override this here and let me not screw up like before let me print one at the

up like before let me print one at the end of the whole row just to move the

end of the whole row just to move the cursor down and I think now together now

cursor down and I think now together now we've got our 3x3 of course we could

we've got our 3x3 of course we could tighten this up further like if I don't

tighten this up further like if I don't like the nested loop I probably could go

like the nested loop I probably could go in here and just print out for instance

in here and just print out for instance a uh a brick times three or I could

a uh a brick times three or I could change the three to a variable if I've

change the three to a variable if I've gotten it from the user so I can tighten

gotten it from the user so I can tighten this up further so again just different

this up further so again just different ways to solve the same problem and again

ways to solve the same problem and again sort of evidence of why a lot of people

sort of evidence of why a lot of people like python there's just some more

like python there's just some more pleasant ways to solve problems without

pleasant ways to solve problems without getting into the weeds constantly of

getting into the weeds constantly of doing things like like with um uh for

doing things like like with um uh for loops and wild Loops endlessly all right

loops and wild Loops endlessly all right well how about some other building

well how about some other building blocks lists are going to be so

blocks lists are going to be so incredibly useful in Python just as

incredibly useful in Python just as arrays were in C but arrays are annoying

arrays were in C but arrays are annoying because you have to manage the memory

because you have to manage the memory yourself you have to know in advance how

yourself you have to know in advance how big they are or you have to use pointers

big they are or you have to use pointers and malok or realloc to resize them like

and malok or realloc to resize them like oh my God like the past two weeks have

oh my God like the past two weeks have been painful in that sense but python

been painful in that sense but python does this all for free for you in fact

does this all for free for you in fact there's a whole bunch of functions that

there's a whole bunch of functions that come with python that involve lists and

come with python that involve lists and they'll ow us ultimately um to do things

they'll ow us ultimately um to do things again and again and again uh with uh

again and again and again uh with uh within the same data structure and for

within the same data structure and for instance we'll be able to get the length

instance we'll be able to get the length of a list you don't have to remember it

of a list you don't have to remember it yourself in a variable you can just ask

yourself in a variable you can just ask python how many elements are in this

python how many elements are in this list and with this I think we can solve

list and with this I think we can solve some some old problems too so let me go

some some old problems too so let me go back here to vs code let me close Mario

back here to vs code let me close Mario and give us a new program called scores.

and give us a new program called scores. piy and rather than show the C and the

piy and rather than show the C and the python now let's just focus on Python

python now let's just focus on Python and in scores. C way back when we just

and in scores. C way back when we just averaged like three test scores or

averaged like three test scores or something like that 72 73 and 33 a few

something like that 72 73 and 33 a few weeks ago so if I want to create a list

weeks ago so if I want to create a list in this python version of 72 73 33 I

in this python version of 72 73 33 I just use my square bracket notation C

just use my square bracket notation C let you use curly braces if you know the

let you use curly braces if you know the values in advance but Python's just this

values in advance but Python's just this and now if I want to compute the average

and now if I want to compute the average in pi in C recall I did something with a

in pi in C recall I did something with a loop I added all the values together I

loop I added all the values together I then divided by the total number values

then divided by the total number values just like you would in grade school and

just like you would in grade school and that gave me the average well python

that gave me the average well python comes with a lot of super handy

comes with a lot of super handy functions not just length but others as

functions not just length but others as well and so in fact if you want to

well and so in fact if you want to compute the average you can take the sum

compute the average you can take the sum of all of those scores and divide it by

of all of those scores and divide it by the length of all of those scores so

the length of all of those scores so python comes with length comes with sum

python comes with length comes with sum you can just pass in a whole list of any

you can just pass in a whole list of any size and let it deal with that problem

size and let it deal with that problem for you so if I want to now print out

for you so if I want to now print out this average I can print out average

this average I can print out average colon and then I'll plug in my average

colon and then I'll plug in my average uh string for variable for interpolation

uh string for variable for interpolation let me make this an F string so that it

let me make this an F string so that it gets formatted and let me just run

gets formatted and let me just run python of scores. Pi and there's my

python of scores. Pi and there's my average it's sort of rounding weird

average it's sort of rounding weird because we're still vulnerable to some

because we're still vulnerable to some floating point in Precision but at least

floating point in Precision but at least I didn't need loops and I didn't have to

I didn't need loops and I didn't have to write all this darn code just to do

write all this darn code just to do something that you know Excel and Google

something that you know Excel and Google spreadsheets can just do like that well

spreadsheets can just do like that well python is closer to those kinds of tools

python is closer to those kinds of tools but more powerful and that you can

but more powerful and that you can manipulate the data yourself how about

manipulate the data yourself how about though if I want to um get a bunch of

though if I want to um get a bunch of scores manually from the user and then

scores manually from the user and then sum them together well let's combine a

sum them together well let's combine a few ideas here how about this first let

few ideas here how about this first let me go ahead and uh import um the cs50 LI

me go ahead and uh import um the cs50 LI get in function from the cs50 library

get in function from the cs50 library just so we don't have to deal with try

just so we don't have to deal with try and accept or all of that and let me go

and accept or all of that and let me go ahead and give myself an empty list and

ahead and give myself an empty list and this is powerful in py in C there's

this is powerful in py in C there's really there's no point to an empty

really there's no point to an empty array because if you create an empty

array because if you create an empty array with square bracket notation like

array with square bracket notation like it's not useful for anything but in

it's not useful for anything but in Python you can create it empty because

Python you can create it empty because python will grow and shrink the list for

python will grow and shrink the list for you automatically as you add things to

you automatically as you add things to it so if I want to get three scores from

it so if I want to get three scores from the user I could do something like this

the user I could do something like this for I in range of three and then I can

for I in range of three and then I can grab a variable called score or anything

grab a variable called score or anything I could call get int prompt the human

I could call get int prompt the human for the score that they want to type in

for the score that they want to type in and then once they do I can do this

and then once they do I can do this thinking back to our objectoriented

thinking back to our objectoriented programming capability now I could do

programming capability now I could do scores dot a pen

scores dot a pen and I Canen that score to it and you

and I Canen that score to it and you would only know this from having read

would only know this from having read the documentation heard it in class in a

the documentation heard it in class in a book or whatnot but it turns out that

book or whatnot but it turns out that just like strings have functions like

just like strings have functions like lower built into them lists have

lower built into them lists have functions like append built into them

functions like append built into them that just literally appends to the end

that just literally appends to the end of the list for you and python will grow

of the list for you and python will grow or Shrink it as needed no more Malo or C

or Shrink it as needed no more Malo or C or Realo or the like so this just

or Realo or the like so this just appends to the scores array the scores

appends to the scores array the scores list that score and then again and again

list that score and then again and again and again so the array starts at

and again so the array starts at sorry the list starts at size zero then

sorry the list starts at size zero then grows to one then two then three without

grows to one then two then three without you having to do anything else and so

you having to do anything else and so now down here I can compute an average

now down here I can compute an average with the sum of those scores divided by

with the sum of those scores divided by the length of the total number of scores

the length of the total number of scores and to be clear length is the total

and to be clear length is the total number of elements in the list doesn't

number of elements in the list doesn't matter how big the values themselves are

matter how big the values themselves are now I can go ahead and print out an FST

now I can go ahead and print out an FST string uh with something like average

string uh with something like average colon average and curly braces and and

colon average and curly braces and and if I run python of scores. piy I'll type

if I run python of scores. piy I'll type in just for the sake of discussion the

in just for the sake of discussion the three values I still get the same answer

three values I still get the same answer but that would have been painful to do

but that would have been painful to do in see unless you committ it in advance

in see unless you committ it in advance to a fixed size array which we already

to a fixed size array which we already decided weeks ago was annoying or uh you

decided weeks ago was annoying or uh you uh grew it dynamically using malok or

uh grew it dynamically using malok or realloc or the like all right what else

realloc or the like all right what else can I do well there's some nice things

can I do well there's some nice things you might as well know exist um instead

you might as well know exist um instead of scores. aen you can do slight

of scores. aen you can do slight fanciness like this like if you want to

fanciness like this like if you want to append something to a list you can

append something to a list you can actually do plus equals and then put

actually do plus equals and then put that thing in a a temporary list of its

that thing in a a temporary list of its own and just use what is essentially

own and just use what is essentially concatenation but not concatenation of

concatenation but not concatenation of strings but concatination of lists so

strings but concatination of lists so this new line six appends to the scores

this new line six appends to the scores list this tiny little list I'm

list this tiny little list I'm temporarily creating with just the

temporarily creating with just the current new score so just another piece

current new score so just another piece of syntax that's worth seeing that

of syntax that's worth seeing that allows you to do something like that as

allows you to do something like that as well all right well how about we go back

well all right well how about we go back to strings for a moment and all these

to strings for a moment and all these examples as always are on the course's

examples as always are on the course's website afterward suppose we want to do

website afterward suppose we want to do something like converting characters to

something like converting characters to uppercase well to be clear I could do

uppercase well to be clear I could do something like this let me create a

something like this let me create a program called uppercase dop let me

program called uppercase dop let me prompt the user for a before string as

prompt the user for a before string as by using the input function or get

by using the input function or get string which is almost the same and I'll

string which is almost the same and I'll prompt the user for a string

prompt the user for a string beforeand then let me go ahead and print

beforeand then let me go ahead and print out uh how about the keyword after

out uh how about the keyword after and then end the new line with nothing

and then end the new line with nothing just so that I can see before on one

just so that I can see before on one line and after on the next line and then

line and after on the next line and then let me do this and here's where python

let me do this and here's where python gets pleasant too with loops for C in

gets pleasant too with loops for C in before print c. uper n equals quote

before print c. uper n equals quote unquote and then I'll print this here

unquote and then I'll print this here all right that was fast but let's try to

all right that was fast but let's try to infer what's going on so line one just

infer what's going on so line one just gets input from the user stores it in a

gets input from the user stores it in a variable called before line two

variable called before line two literally just prints after but doesn't

literally just prints after but doesn't move the new line to uh the cursor to

move the new line to uh the cursor to the next line what it then does is this

the next line what it then does is this and in C this was a little more annoying

and in C this was a little more annoying you needed a for loop with I you needed

you needed a for loop with I you needed array in uh notation with the square

array in uh notation with the square brackets but python if you say four

brackets but python if you say four variable in string so for c for

variable in string so for c for character in string Python's going to

character in string Python's going to automatically assign C to the first word

automatically assign C to the first word letter that the user types in then on

letter that the user types in then on the next iteration the second letter the

the next iteration the second letter the third letter and the fourth so you don't

third letter and the fourth so you don't need any square bracket notation you

need any square bracket notation you just you see and python will do it for

just you see and python will do it for you and just hand you back one at a time

you and just hand you back one at a time each of the letters that the user has

each of the letters that the user has typed in so if I go back over here and I

typed in so if I go back over here and I run for instance python of uppercase

run for instance python of uppercase dopy and I'll type in how about uh David

dopy and I'll type in how about uh David in all lowercase and hit enter you'll

in all lowercase and hit enter you'll now see that it's all uppercase instead

now see that it's all uppercase instead by iterating over it indeed one

by iterating over it indeed one character at a time but we already know

character at a time but we already know thanks to object-oriented programming

thanks to object-oriented programming strings themselves have the

strings themselves have the functionality built in to not just

functionality built in to not just uppercase Single Character

uppercase Single Character but the whole string so honestly this

but the whole string so honestly this was a bit of a silly exercise I don't

was a bit of a silly exercise I don't need to use a loop anymore like in see

need to use a loop anymore like in see and so some of the habits you've only

and so some of the habits you've only just developed in recent weeks it's time

just developed in recent weeks it's time to start breaking them when they're not

to start breaking them when they're not necessary I can create a variable called

necessary I can create a variable called after set it equal to before do uper

after set it equal to before do uper which indeed exists just like lower

which indeed exists just like lower exists and then what I can go ahead and

exists and then what I can go ahead and print out is for instance uh let's get

print out is for instance uh let's get rid of this print line here and do it at

rid of this print line here and do it at the end after and print the value of

the end after and print the value of that variable so now if I rerun

that variable so now if I rerun uppercase Pi type in David and all

uppercase Pi type in David and all lowercase I can just uppercase the whole

lowercase I can just uppercase the whole thing all at once because again in C in

thing all at once because again in C in Python you don't have to operate on

Python you don't have to operate on characters individually questions on any

characters individually questions on any of these tricks up until

of these tricks up until now now all right how about a few other

now now all right how about a few other techniques that we saw and C that we'll

techniques that we saw and C that we'll bring back now in Python so it turns out

bring back now in Python so it turns out in Python there are other librar you can

in Python there are other librar you can use two that unlock even more

use two that unlock even more functionality so in C if you wanted

functionality so in C if you wanted command line arguments you just change

command line arguments you just change the proo the signature for main to be

the proo the signature for main to be void instead of void to be int argc

void instead of void to be int argc comma string argv Open brackets for an

comma string argv Open brackets for an array or Char star eventually well it

array or Char star eventually well it turns out in Python that if you want to

turns out in Python that if you want to access command line arguments it's a

access command line arguments it's a little simpler but they're tucked away

little simpler but they're tucked away in a library otherwise known as a module

in a library otherwise known as a module called CIS the CIS or system module now

called CIS the CIS or system module now this is similar in spirit to the cs50

this is similar in spirit to the cs50 library and that it's got a bunch of

library and that it's got a bunch of functionality built in but this one

functionality built in but this one comes with python itself so if I want to

comes with python itself so if I want to create a program like greet py in VSS

create a program like greet py in VSS code here let me go ahead and do this

code here let me go ahead and do this from the CIS Library let's import argv

from the CIS Library let's import argv and that's just a thing that exists it's

and that's just a thing that exists it's not built into main because there is no

not built into main because there is no main per se anymore so it's tucked away

main per se anymore so it's tucked away in that library and now I can do

in that library and now I can do something like this if the length of

something like this if the length of argv equals equals 2 well let's go ahead

argv equals equals 2 well let's go ahead and print out something friendly like

and print out something friendly like hello comma AR V bracket 1 and then

hello comma AR V bracket 1 and then close quotes else if the length of RV is

close quotes else if the length of RV is not equal to two let's just go ahead and

not equal to two let's just go ahead and print out hello world now at a glance

print out hello world now at a glance this might look a little cryptic but

this might look a little cryptic but it's identical to what we did a few

it's identical to what we did a few weeks ago when I run this python of

weeks ago when I run this python of greet with no arguments it just says

greet with no arguments it just says hello world but if I instead add a

hello world but if I instead add a command line argument like my first name

command line argument like my first name and hit enter now the length of arv is

and hit enter now the length of arv is no longer one it's going to be two and

no longer one it's going to be two and so it prints out hello David instead so

so it prints out hello David instead so the takeaway here is that whereas in C

the takeaway here is that whereas in C argv technically contained the name of

argv technically contained the name of your program like hello or dog greet and

your program like hello or dog greet and then everything the human typed Python's

then everything the human typed Python's a little different in that because we're

a little different in that because we're using The Interpreter in this way

using The Interpreter in this way Technically when you run python of greet

Technically when you run python of greet py the length of arv is only one it

py the length of arv is only one it contains only greet so the name of the

contains only greet so the name of the file it does not unnecessarily contain

file it does not unnecessarily contain python itself because what's the point

python itself because what's the point of that being there omnes it does

of that being there omnes it does contain the number of words that the

contain the number of words that the human typed after python itself so argv

human typed after python itself so argv is length one here argv is length two

is length one here argv is length two here and that's why when it did equal to

here and that's why when it did equal to I saw hello David instead of the default

I saw hello David instead of the default hello world so same ability to access

hello world so same ability to access command line arguments add these kinds

command line arguments add these kinds of inputs to your functions but you have

of inputs to your functions but you have to unlock it by way of using Arvy uh

to unlock it by way of using Arvy uh instead in this way if you want to see

instead in this way if you want to see all of the words you could do something

all of the words you could do something like this uh just as if we combine ideas

like this uh just as if we combine ideas here for I in range of how about length

here for I in range of how about length of

of arv then I can do this print argv

arv then I can do this print argv bracket I all right a little cryptic but

bracket I all right a little cryptic but line three is just a for Loop iterating

line three is just a for Loop iterating over the range of length of argv so if

over the range of length of argv so if the human types in two words the length

the human types in two words the length of argv will be two so this is just a

of argv will be two so this is just a way of saying iterate over all of the

way of saying iterate over all of the words in arv printing them one at a time

words in arv printing them one at a time so python of greet dopy enter just

so python of greet dopy enter just prints out the name of the program

prints out the name of the program python of greet dopy with David prints

python of greet dopy with David prints out greet dopy and then David I can keep

out greet dopy and then David I can keep running it though with more words and

running it though with more words and they'll each get printed one at a time

they'll each get printed one at a time but what's nice too about Python and

but what's nice too about Python and this is the point of this exercise

this is the point of this exercise honestly this looks pretty cryptic this

honestly this looks pretty cryptic this is not very pleasant to look at if you

is not very pleasant to look at if you just want to iterate over every word in

just want to iterate over every word in a list which argv is watch what I can do

a list which argv is watch what I can do I can do for ARG or any variable name in

I can do for ARG or any variable name in ARG V let me just now print out that

ARG V let me just now print out that argument I could keep calling it I but I

argument I could keep calling it I but I seems weird when it's not a number so

seems weird when it's not a number so I'm changing to AR as a word instead if

I'm changing to AR as a word instead if I now do python of greet py it does this

I now do python of greet py it does this if I do python of greet David it does

if I do python of greet David it does that again David ma it does that again

that again David ma it does that again so this is again why Python's just very

so this is again why Python's just very appealing you want to do something this

appealing you want to do something this many times iterate over a list just say

many times iterate over a list just say it and it reads a little more like

it and it reads a little more like English and there's even other fanciness

English and there's even other fanciness too if I may it's a little stupid that I

too if I may it's a little stupid that I keep seeing the name of the program

keep seeing the name of the program greet dopy so I it'd be nice if I could

greet dopy so I it'd be nice if I could remove that python also supports what

remove that python also supports what are called slices of arrays sorry slices

are called slices of arrays sorry slices of lists even though I get the

of lists even though I get the terminology confused if Arvy is a list

terminology confused if Arvy is a list then it's going to print out everything

then it's going to print out everything in it but if I want a slice of it that

in it but if I want a slice of it that starts at location one all the way to

starts at location one all the way to the end you can use this funky syntax in

the end you can use this funky syntax in between the square brackets which we've

between the square brackets which we've not seen yet that's going to start at

not seen yet that's going to start at item one and go all the way to the end

item one and go all the way to the end and so this is a nice clever way of

and so this is a nice clever way of slicing off if you will the very first

slicing off if you will the very first element because now when I run greet doy

element because now when I run greet doy David men I should only see David and

David men I should only see David and men if I only want one element I could

men if I only want one element I could do one to two if I want all of them I

do one to two if I want all of them I could do zero onward I could give myself

could do zero onward I could give myself just two of one of them in this way so

just two of one of them in this way so you can play with the start value and

you can play with the start value and the end value in this way to sort of

the end value in this way to sort of slice and dice these lists in different

slice and dice these lists in different ways that would have been a pain in see

ways that would have been a pain in see just because we didn't really have the

just because we didn't really have the built-in support for manipulating arrays

built-in support for manipulating arrays as cleanly as this all right just so

as cleanly as this all right just so you've seen it too though this one is

you've seen it too though this one is less uh exciting to see live if I go

less uh exciting to see live if I go ahead and create a quick program here it

ahead and create a quick program here it turns out there's something else in the

turns out there's something else in the CIS Library the ability to exit programs

CIS Library the ability to exit programs either exiting with status code one or

either exiting with status code one or zero as we've been doing anytime

zero as we've been doing anytime something goes right or wrong so for

something goes right or wrong so for instance Let Me Whip up a quick program

instance Let Me Whip up a quick program that just says if the length of cy.

that just says if the length of cy. argv uh does not equal two then let's y

argv uh does not equal two then let's y at the user and say you're missing a

at the user and say you're missing a command line argument otherwise command

command line argument otherwise command line argument and let's then return cy.

line argument and let's then return cy. exit one else let's go ahead and

exit one else let's go ahead and logically just say print a formatted

logically just say print a formatted string that says hello as before cy.

string that says hello as before cy. arv1 now things look different all of a

arv1 now things look different all of a sudden but I'm doing something

sudden but I'm doing something deliberately first let's see what this

deliberately first let's see what this does so on line one I'm importing not

does so on line one I'm importing not argv specifically I'm importing the

argv specifically I'm importing the whole CIS library and we'll see why in

whole CIS library and we'll see why in second well it turns out that this Arvy

second well it turns out that this Arvy the CIS library has not only the Arvy

the CIS library has not only the Arvy list it also has a function called exit

list it also has a function called exit which I'd like to be able to use as well

which I'd like to be able to use as well so it turns out that if you import a

so it turns out that if you import a whole library in this way that's fine

whole library in this way that's fine but you have to refer to the things

but you have to refer to the things inside of it by using that same

inside of it by using that same library's name and a DOT to sort of

library's name and a DOT to sort of namespace it so to speak so here I'm

namespace it so to speak so here I'm just saying if the user types in does

just saying if the user types in does not type in two words yell at them with

not type in two words yell at them with missing command line argument and then

missing command line argument and then exit with one just like in C when you do

exit with one just like in C when you do exit one just means something went wrong

exit one just means something went wrong otherwise print out hello to this and

otherwise print out hello to this and this is starting to look cryptic but

this is starting to look cryptic but it's just a combination of ideas the

it's just a combination of ideas the curly braces means interpolate this

curly braces means interpolate this value plug it in here cy. Arvy is just

value plug it in here cy. Arvy is just the verbose way of saying go into the

the verbose way of saying go into the CIS library and get the argv variable

CIS library and get the argv variable therein and bracket one of course just

therein and bracket one of course just like arrays and C is just the second

like arrays and C is just the second element at the prompt so when I run this

element at the prompt so when I run this version now python of exit. py with no

version now python of exit. py with no arguments I get yelled at in this way if

arguments I get yelled at in this way if however I type in two arguments total

however I type in two arguments total the name of the file and my own name now

the name of the file and my own name now I get greeted with hello David and it's

I get greeted with hello David and it's the same idea before this was a very

the same idea before this was a very low-level technique but same thing here

low-level technique but same thing here if you do Echo dollar sign question mark

if you do Echo dollar sign question mark enter you'll see the exit code of your

enter you'll see the exit code of your program so if I do this incorrectly

program so if I do this incorrectly again let me rerun it without my name

again let me rerun it without my name enter I get yelled at but if I do Echo

enter I get yelled at but if I do Echo dollar sign question mark there's the

dollar sign question mark there's the secret one that's returned again just to

secret one that's returned again just to show You parody with C in this case

show You parody with C in this case questions now on any of these techniques

here all right how about something that's a little more powerful too we

that's a little more powerful too we spent so much time in week zero and one

spent so much time in week zero and one doing searching and then eventually

doing searching and then eventually sorting in week three well it turns out

sorting in week three well it turns out python can help with some of this too

python can help with some of this too let me go ahead and create a program

let me go ahead and create a program called names. py that's just going to be

called names. py that's just going to be an opportunity to maybe search over a

an opportunity to maybe search over a whole bunch of names let me go ahead and

whole bunch of names let me go ahead and import CIS and then just so I have

import CIS and then just so I have access to exit and let me go ahead and

access to exit and let me go ahead and create a variable called names that's

create a variable called names that's going to be a list with a whole bunch of

going to be a list with a whole bunch of names uh how about Here Charlie and Fred

names uh how about Here Charlie and Fred and George and Jenny and Percy and

and George and Jenny and Percy and lastly Ron so a whole bunch of names

lastly Ron so a whole bunch of names here and you know it'd be a little

here and you know it'd be a little Annoying to implement code that iterates

Annoying to implement code that iterates over that from left to right and see

over that from left to right and see searching for one of those names in fact

searching for one of those names in fact what name well let's go ahead and ask

what name well let's go ahead and ask the user to input the name that they

the user to input the name that they want to search for so that we can tell

want to search for so that we can tell them if the name is there or not and we

them if the name is there or not and we could do this similar to C in Python

could do this similar to C in Python doing something like this so for n in

doing something like this so for n in names where n is just a variable to

names where n is just a variable to iterate over each name if how about the

iterate over each name if how about the name I'm looking for equals the current

name I'm looking for equals the current name in the list AKA n let's print out

name in the list AKA n let's print out something friendly like found and then

something friendly like found and then let's do cy. Exit 0 to indicate that we

let's do cy. Exit 0 to indicate that we found whoever that is otherwise if we

found whoever that is otherwise if we get all the way to the bottom here

get all the way to the bottom here outside of this Loop let's just print

outside of this Loop let's just print not found because if we haven't exited

not found because if we haven't exited yet and then let's just exit with one

yet and then let's just exit with one just to be clear I can continue

just to be clear I can continue importing all of CIS or I could do from

importing all of CIS or I could do from CIS import exit and then I could get rid

CIS import exit and then I could get rid of CIS dot everywhere else but you know

of CIS dot everywhere else but you know sometimes it's helpful to know exactly

sometimes it's helpful to know exactly where functions came from so this two is

where functions came from so this two is just a matter of style in this case all

just a matter of style in this case all right so let's go ahead and run this

right so let's go ahead and run this python of names. piy and let's look for

python of names. piy and let's look for like Ron all the way at the end you know

like Ron all the way at the end you know all right he's found and let's search

all right he's found and let's search for someone outside of the family here

for someone outside of the family here like herion not found okay so it seems

like herion not found okay so it seems to be working in this way but I've

to be working in this way but I've essentially implemented what

essentially implemented what algorithm what algorithm would this seem

algorithm what algorithm would this seem to be per lines seven and eight and nine

to be per lines seven and eight and nine and

and 10 yeah so it's just linear search it's

10 yeah so it's just linear search it's a loop even though the syntax is a

a loop even though the syntax is a little more succinct today and it's just

little more succinct today and it's just iterating over the whole thing well

iterating over the whole thing well honestly we've seen an even more tur way

honestly we've seen an even more tur way to do this in Python and this again is

to do this in Python and this again is what makes it a more pleasant language

what makes it a more pleasant language sometimes why don't I just do this

sometimes why don't I just do this instead in of iterating one at a time

instead in of iterating one at a time why don't I just say this let me go

why don't I just say this let me go ahead and change my condition to just be

ahead and change my condition to just be How about if the name we're looking for

How about if the name we're looking for is in the names list we're done we found

is in the names list we're done we found it use the in preposition that we've

it use the in preposition that we've seen a couple of times now that itself

seen a couple of times now that itself asks the question is something in

asks the question is something in something else and python will take care

something else and python will take care of linear search for us and it's going

of linear search for us and it's going to work exactly the same if I do python

to work exactly the same if I do python of names. search for run it's still

of names. search for run it's still going to find him and it's still going

going to find him and it's still going to do it linearly in this case but I

to do it linearly in this case but I don't have to write all of the lower

don't have to write all of the lower level code myself in this

level code myself in this case questions now on any of this the

case questions now on any of this the code's just getting shorter and

code's just getting shorter and shorter now what about uh let's see what

shorter now what about uh let's see what else might we have here how about this

else might we have here how about this it turns out let's go ahead and

it turns out let's go ahead and Implement that phone book that we

Implement that phone book that we started metaphorically with in the

started metaphorically with in the beginning of the course let's code up a

beginning of the course let's code up a program called phonebook dopy and in

program called phonebook dopy and in this case let's go ahead and let's

this case let's go ahead and let's create a dictionary this time recall

create a dictionary this time recall that a dictionary is a little something

that a dictionary is a little something that implements something like this like

that implements something like this like a two column table that's got keys and

a two column table that's got keys and values words and definitions names and

values words and definitions names and numbers and let's focus on the last of

numbers and let's focus on the last of those names and numbers in this case

those names and numbers in this case well I claimed earlier that python has

well I claimed earlier that python has built-in support for dictionaries dict

built-in support for dictionaries dict objects that you can create with one

objects that you can create with one line I didn't need it for speller

line I didn't need it for speller because a set is sufficient when you

because a set is sufficient when you only want one of the keys or the values

only want one of the keys or the values not both but now I want some names and

not both but now I want some names and numbers so it turns out in pyth python

numbers so it turns out in pyth python you can create an empty dictionary by

you can create an empty dictionary by saying dict open parenthesis close and

saying dict open parenthesis close and that just gives you essentially a chart

that just gives you essentially a chart that looks like this with nothing in it

that looks like this with nothing in it or there's more succinct syntax you can

or there's more succinct syntax you can alternatively do uh this with two curly

alternatively do uh this with two curly braces instead and in fact I've been

braces instead and in fact I've been using a shortcut all this time when I

using a shortcut all this time when I had a list earlier where my

had a list earlier where my variable uh was called scores and I did

variable uh was called scores and I did this that was actually the shorthand

this that was actually the shorthand version of this hey python give me an

version of this hey python give me an empty list so there's different Syntax

empty list so there's different Syntax for achieving the same goal in this case

for achieving the same goal in this case if I want a dictionary for people I can

if I want a dictionary for people I can either do this or more commonly just two

either do this or more commonly just two curly braces like that all right well

curly braces like that all right well what do I want to put in this well let

what do I want to put in this well let me actually put some things in this and

me actually put some things in this and I'm going to just move my Clos curly

I'm going to just move my Clos curly brace to a new line if I want to

brace to a new line if I want to implement this idea of keys and values

implement this idea of keys and values the way you do this in Python is key

the way you do this in Python is key colon value comma key colon value so

colon value comma key colon value so you'd implement it more in code so for

you'd implement it more in code so for instance if I want Carter to be the

instance if I want Carter to be the first key in my phone book and I want

first key in my phone book and I want his number to be+ one 617495 1000 I can

his number to be+ one 617495 1000 I can put that as the corresponding value the

put that as the corresponding value the colon is in between both are strings or

colon is in between both are strings or stirs so I've quoted both deliberately

stirs so I've quoted both deliberately if I want to add myself I can put a

if I want to add myself I can put a comma and then just to keep things

comma and then just to keep things pretty I'm moving the cursor to the next

pretty I'm moving the cursor to the next line but that's not strictly required

line but that's not strictly required aesthetically it's just good style and

aesthetically it's just good style and here I might do+ 1 949 468 uh

here I might do+ 1 949 468 uh 2750 and now I have a dictionary that

2750 and now I have a dictionary that essentially has two rows here David uh

essentially has two rows here David uh Carter and his number and David and his

Carter and his number and David and his number as well and if I kept adding to

number as well and if I kept adding to this this call this chart would just get

this this call this chart would just get longer and longer suppose I want to

longer and longer suppose I want to search for one of our numbers well let's

search for one of our numbers well let's prompt the user for the name for whose

prompt the user for the name for whose number you want to search by getting

number you want to search by getting string or you know what we don't need

string or you know what we don't need the cs50 library let's just use input

the cs50 library let's just use input and prompt the user for a name and now

and prompt the user for a name and now we can use this super syntax and just

we can use this super syntax and just say if name in people print the format

say if name in people print the format added string number colon and here we

added string number colon and here we can do this people bracket name okay so

can do this people bracket name okay so this is getting kind of cool kind of

this is getting kind of cool kind of quickly kind of confusingly so let me

quickly kind of confusingly so let me run this python of phone

run this python of phone book. let's type in Carter and indeed I

book. let's type in Carter and indeed I see his number let's run it again with

see his number let's run it again with David and I see my number here so what's

David and I see my number here so what's going on well it turns out that a

going on well it turns out that a dictionary is very similar in spirit to

dictionary is very similar in spirit to a list it's actually very similar in

a list it's actually very similar in spirit to an array in C but instead of

spirit to an array in C but instead of being limited to keys that are numbers

being limited to keys that are numbers like bracket 0o bracket 1 bracket two

like bracket 0o bracket 1 bracket two you can actually use words and that's

you can actually use words and that's all I'm doing here on line eight if I

all I'm doing here on line eight if I want to check for the name Carter which

want to check for the name Carter which is currently in this variable called

is currently in this variable called name I can index into my people

name I can index into my people dictionary using not a number but using

dictionary using not a number but using literally a string the name Carter or

literally a string the name Carter or David or anything else to make this

David or anything else to make this clear too notice that I'm at the moment

clear too notice that I'm at the moment you using this format string which is

you using this format string which is adding some undue complexity but I could

adding some undue complexity but I could clarify this perhaps further as this I

clarify this perhaps further as this I could give myself another variable

could give myself another variable called number set it equal to the people

called number set it equal to the people dictionary indexing into it using the

dictionary indexing into it using the current name and now I can shorten this

current name and now I can shorten this to make it clear that all I'm doing is

to make it clear that all I'm doing is printing the value of that and in fact I

printing the value of that and in fact I can do this even more cryptically if I

can do this even more cryptically if I this would be weird to do but if I only

this would be weird to do but if I only ever want to show David's phone number

ever want to show David's phone number and never Carter's I can literally quote

and never Carter's I can literally quote unquote index into the people dictionary

unquote index into the people dictionary because now when I run this even if I

because now when I run this even if I type Carter I'm going to get back my

type Carter I'm going to get back my number instead but that's all that's

number instead but that's all that's happening if I undo that because that's

happening if I undo that because that's now a bug but I index into it using the

now a bug but I index into it using the value of name dictionaries are just so

value of name dictionaries are just so wonderfully convenient because now you

wonderfully convenient because now you can associate anything with anything

can associate anything with anything else but not using numbers but entire

else but not using numbers but entire keywords instead so here's how if in

keywords instead so here's how if in spell we gave you not just words but

spell we gave you not just words but hundreds of thousands of definitions as

hundreds of thousands of definitions as well you could essentially store them as

well you could essentially store them as this and then when the human wants to

this and then when the human wants to look up a definition in a proper

look up a definition in a proper dictionary not just for spellchecking

dictionary not just for spellchecking you could index into the dictionary

you could index into the dictionary using square brackets and get back the

using square brackets and get back the definition in English as well

definition in English as well questions on this

yeah location a really good question so how

location a really good question so how to summarize how is python finding that

to summarize how is python finding that name within that dictionary this is

name within that dictionary this is where honestly speller in pet 5 is what

where honestly speller in pet 5 is what Python's all about so you have struggled

Python's all about so you have struggled are struggling with implementing your

are struggling with implementing your own spell checker and implementing your

own spell checker and implementing your own hash table and recall that per last

own hash table and recall that per last week the goal of a hash table is to

week the goal of a hash table is to ideally get constant time access not

ideally get constant time access not something linear which is slow and even

something linear which is slow and even better than something uh uh logarithmic

better than something uh uh logarithmic like log base 2 of n so Python and the

like log base 2 of n so Python and the really smart people who invented it they

really smart people who invented it they have written the code that does its best

have written the code that does its best to give you constant time searches of

to give you constant time searches of dictionaries and they're not always

dictionaries and they're not always going to succeed just as you and your

going to succeed just as you and your own problem set probably going to have

own problem set probably going to have some collisions once in a while and

some collisions once in a while and start to have chains of length lists of

start to have chains of length lists of words but this is where again you defer

words but this is where again you defer to someone else someone smarter than you

to someone else someone smarter than you someone with more time than you to solve

someone with more time than you to solve these problems for you and if you read

these problems for you and if you read Python's documentation you'll see that

Python's documentation you'll see that it doesn't guarantee constant time but

it doesn't guarantee constant time but it's going to ideally optimize the data

it's going to ideally optimize the data structure for you to get as fast as

structure for you to get as fast as possible and of all of the data

possible and of all of the data structures um like a dictionary a

structures um like a dictionary a hashtable is really like the Swiss army

hashtable is really like the Swiss army knife of computing because it just lets

knife of computing because it just lets you associate something with something

you associate something with something else and even though we keep focusing on

else and even though we keep focusing on names and numbers that's a really

names and numbers that's a really powerful thing because it's more

powerful thing because it's more powerful than lists and arrays which are

powerful than lists and arrays which are only numbers and something else now you

only numbers and something else now you can have any sorts of relationships

can have any sorts of relationships instead all right let me show a few

instead all right let me show a few other examples before we culminate with

other examples before we culminate with some more powerful techniques in pythons

some more powerful techniques in pythons thanks to libraries how about this

thanks to libraries how about this problem we encountered in week four

problem we encountered in week four which was this let me code up a program

which was this let me code up a program called again compare. Pi here but this

called again compare. Pi here but this time compare to Strings and not numbers

time compare to Strings and not numbers so let me for instance do uh get one

so let me for instance do uh get one string from the user called s just for

string from the user called s just for the sake of discussion let me get

the sake of discussion let me get another string from the user uh called T

another string from the user uh called T so that we can actually do some

so that we can actually do some comparison here and if s equals equals T

comparison here and if s equals equals T let's go ahead and print out that

let's go ahead and print out that they're the same else let's go ahead and

they're the same else let's go ahead and print out that they're different so this

print out that they're different so this is very similar to what we did in week

is very similar to what we did in week four but in week four recall we did this

four but in week four recall we did this spef specifically because we had

spef specifically because we had encountered a problem for instance if I

encountered a problem for instance if I run whoops

run whoops uh if I

run what's going on uh input T come

on oh the okay wow okay long day all right if I run the proper command python

right if I run the proper command python of compare. py then let's go ahead and

of compare. py then let's go ahead and type in something like cat in all

type in something like cat in all lowercase cats in all lowercase and

lowercase cats in all lowercase and they're the same uh if though I do this

they're the same uh if though I do this again with dog and dog they're the same

again with dog and dog they're the same and of course cat and dog they're

and of course cat and dog they're different but does anyone recall from

different but does anyone recall from two weeks ago when I typed in my name

two weeks ago when I typed in my name twice both identically capitalized what

twice both identically capitalized what did it say that they were in

did it say that they were in fact different and why was that like why

fact different and why was that like why were two strings in C different even

were two strings in C different even though I typed literally the same

though I typed literally the same thing two different places in memory so

thing two different places in memory so each string might look the same

each string might look the same aesthetically but of course was stored

aesthetically but of course was stored elsewhere in memory and yet python

elsewhere in memory and yet python appears to be using the equality

appears to be using the equality operator equals equals like you and I

operator equals equals like you and I would expect as humans actually

would expect as humans actually comparing for us Char by Char in each of

comparing for us Char by Char in each of those strings for actual equality so

those strings for actual equality so this is a feature of python in that it's

this is a feature of python in that it's just easier to do and why well this

just easier to do and why well this derives from the reality that in Python

derives from the reality that in Python there are no pointers anymore there's no

there are no pointers anymore there's no underlying memory management it's not up

underlying memory management it's not up to you now to worry about those lower

to you now to worry about those lower level details the language itself takes

level details the language itself takes care of that for you and so similarly if

care of that for you and so similarly if I do this and don't ask the user for two

I do this and don't ask the user for two strings but just one and then I do

strings but just one and then I do something like this how about give

something like this how about give myself a second variable T set it equal

myself a second variable T set it equal to s.

to s. capitalize which note is not the same as

capitalize which note is not the same as upper capitalize by Design per Python's

upper capitalize by Design per Python's documentation will only capitalize the

documentation will only capitalize the first letter for you I can now print out

first letter for you I can now print out say two F strings here what the value of

say two F strings here what the value of s is and then let me print out with

s is and then let me print out with another F string what the value of t is

another F string what the value of t is and recall that in C this was a problem

and recall that in C this was a problem because if you capitalize s and store it

because if you capitalize s and store it in t we accidentally capitalized both s

in t we accidentally capitalized both s and t but in this case in Python when I

and t but in this case in Python when I actually run this and type in cat in all

actually run this and type in cat in all lowercase the original s is unchanged

lowercase the original s is unchanged because when I use capitalize on line

because when I use capitalize on line three this is indeed capitalizing s but

three this is indeed capitalizing s but it's returning a copy of the result it

it's returning a copy of the result it cannot change S itself because again for

cannot change S itself because again for that technical term s is IM mutable

that technical term s is IM mutable strings once they exist cannot be

strings once they exist cannot be changed themselves but you can return

changed themselves but you can return copies and modified mutated copies of

copies and modified mutated copies of those same strings so in short all of

those same strings so in short all of those headaches we encountered in week

those headaches we encountered in week four are now solved really in the way

four are now solved really in the way you might expect and here's another one

you might expect and here's another one that we dwell on in week four with the

that we dwell on in week four with the colored uh liquid in glasses let me code

colored uh liquid in glasses let me code up a program called swap. pi and in

up a program called swap. pi and in swap. pi let me set x equal to 1 y equal

swap. pi let me set x equal to 1 y equal to 2 and then let me just print out an F

to 2 and then let me just print out an F string here so how about X X is this

string here so how about X X is this comma Y is that and then let me do that

comma Y is that and then let me do that twice just for the sake of demonstration

twice just for the sake of demonstration and in here recall that we had to create

and in here recall that we had to create a swap function but then we had to pass

a swap function but then we had to pass it in by reference with the Amper sand

it in by reference with the Amper sand and like oh my God like that was kind of

and like oh my God like that was kind of peak complexity and see well if you want

peak complexity and see well if you want to swap X and Y in Python you could do X

to swap X and Y in Python you could do X comma yal y comma X and now python of

comma yal y comma X and now python of swap whoops python of

swap whoops python of swap. and there we go all of that's

swap. and there we go all of that's handled for you sort of like a shell

handled for you sort of like a shell game without even a temporary variable

game without even a temporary variable in mind so what more can we do here how

in mind so what more can we do here how about a few final building blocks and

about a few final building blocks and these related now to files from that

these related now to files from that week four suppose that I want to save

week four suppose that I want to save some names and numbers in like a a CSV

some names and numbers in like a a CSV file comma separated values which is

file comma separated values which is like a very lightweight spreadsheet well

like a very lightweight spreadsheet well first let me create a uh phonebook CSV

first let me create a uh phonebook CSV file that just has name comma number as

file that just has name comma number as like the first row there but after after

like the first row there but after after that I'm going to go ahead now and code

that I'm going to go ahead now and code up a phonebook dopy program that

up a phonebook dopy program that actually allows me to add things to this

actually allows me to add things to this phone book so let me split my screen

phone book so let me split my screen here so that we can see the old and the

here so that we can see the old and the new and down here in my code for phone

new and down here in my code for phone book. in this new and improved version

book. in this new and improved version I'm going to actually import a whole

I'm going to actually import a whole other Library this one called CSV and

other Library this one called CSV and here too especially for people in data

here too especially for people in data science and the like really like being

science and the like really like being in a manipulate files and data that

in a manipulate files and data that might very well be stored in

might very well be stored in spreadsheets or csvs comma separated

spreadsheets or csvs comma separated values which we we saw briefly in week

values which we we saw briefly in week four in phonebook then it suffices to

four in phonebook then it suffices to just import CSV after reading the

just import CSV after reading the documentation therefore because this is

documentation therefore because this is going to give me functionality in code

going to give me functionality in code related to CSV files so here's how I

related to CSV files so here's how I might open a file in Python I literally

might open a file in Python I literally call open it's not fop now it's just

call open it's not fop now it's just open and I open this file called

open and I open this file called phonebook.com

phonebook.com going to open it in a pend mode not

going to open it in a pend mode not right where it would change the whole

right where it would change the whole thing I want to append new line at a

thing I want to append new line at a time uh after this I want to get maybe

time uh after this I want to get maybe how about a name from the user so let's

how about a name from the user so let's prompt the user for some input for their

prompt the user for some input for their name and then let's prompt the user for

name and then let's prompt the user for a number as well using input prompting

a number as well using input prompting for number all right and now this is a

for number all right and now this is a little cryptic and you'd only know this

little cryptic and you'd only know this from the documentation but if you want

from the documentation but if you want to write line rows to a CSV file that

to write line rows to a CSV file that you can then view in Excel or the like

you can then view in Excel or the like you can do this give me a variable

you can do this give me a variable called writer but I could call it

called writer but I could call it anything I want let me use a CSV do

anything I want let me use a CSV do writer function that comes with this CSV

writer function that comes with this CSV Library passing in the file this is like

Library passing in the file this is like saying hey python treat this open file

saying hey python treat this open file as a CSV file so that things are

as a CSV file so that things are separated with commas and nicely

separated with commas and nicely formatted in rows and columns now I'm

formatted in rows and columns now I'm going to do this use that writer to

going to do this use that writer to write a row well what do I want to write

write a row well what do I want to write I want to write a short list namely the

I want to write a short list namely the current name and the current number to

current name and the current number to that file but I don't want to use FR

that file but I don't want to use FR printf and percent s and all of that

printf and percent s and all of that stuff that we might have had in the past

stuff that we might have had in the past and now I just want to close the file

and now I just want to close the file let me reopen my ter teral let me run

let me reopen my ter teral let me run python of phone book. and let me type in

python of phone book. and let me type in how about David and then +1

how about David and then +1 949468 2750 and hold crossing my fingers

949468 2750 and hold crossing my fingers watching the actual CSV at top left my

watching the actual CSV at top left my code has just added me to the file and

code has just added me to the file and if I were to run it again for instance

if I were to run it again for instance with Carter in plus1 617495 1000

with Carter in plus1 617495 1000 crossing my fingers again we've updated

crossing my fingers again we've updated the file and it turns out there's code

the file and it turns out there's code now via which I can even read that file

now via which I can even read that file but I can first tighten this up just so

but I can first tighten this up just so you've seen it it turns out into python

you've seen it it turns out into python it's so common to open files and close

it's so common to open files and close them you know humans make mistakes and

them you know humans make mistakes and they often forget to close files which

they often forget to close files which might then end up using more memory than

might then end up using more memory than you intend so you can alternatively do

you intend so you can alternatively do this in Python so that you don't have to

this in Python so that you don't have to worry about closing files you can use

worry about closing files you can use this keyword instead you can say with

this keyword instead you can say with the opening of this file as a variable

the opening of this file as a variable called file do all of the following

called file do all of the following underneath so I'm indenting most of my

underneath so I'm indenting most of my code I'm using this new python specific

code I'm using this new python specific keyword called wi and this is just a

keyword called wi and this is just a matter of saying with the following

matter of saying with the following opening of the file do those next four

opening of the file do those next four lines of code and then automatically

lines of code and then automatically close it for me at the end of the

close it for me at the end of the indentation it's a minor optimization

indentation it's a minor optimization but this again is sort of the pythonic

but this again is sort of the pythonic way to do things instead how else might

way to do things instead how else might I do this too well it turns out that the

I do this too well it turns out that the code I've written here on line n

code I've written here on line n especially is a little fragile right if

especially is a little fragile right if any human opens the spreadsheet the CSV

any human opens the spreadsheet the CSV file in Excel Google spreadsheets Apple

file in Excel Google spreadsheets Apple numbers and maybe like moves the columns

numbers and maybe like moves the columns around just because maybe they're

around just because maybe they're fussing they SA it and they don't

fussing they SA it and they don't realize they've now changed my

realize they've now changed my assumptions I don't want to necessarily

assumptions I don't want to necessarily write name and number always in that

write name and number always in that order CU what if someone screws up and

order CU what if someone screws up and flips those two columns by literally

flips those two columns by literally dragging and dropping so it turns out

dragging and dropping so it turns out that instead of using a list here we can

that instead of using a list here we can use another feature of this Library as

use another feature of this Library as follows instead of using a writer

follows instead of using a writer there's something called a dictionary

there's something called a dictionary writer or dict writer that takes the

writer or dict writer that takes the same argument as input the file that's

same argument as input the file that's opened but now the one difference here

opened but now the one difference here is that you need to tell this dictionary

is that you need to tell this dictionary writer

writer that your field names are name and

that your field names are name and number and let me close the CSV here

number and let me close the CSV here name and number are the names of the

name and number are the names of the fields The Columns in this CSV file and

fields The Columns in this CSV file and when it comes time to write a new row

when it comes time to write a new row the syntax here is going to be a little

the syntax here is going to be a little uglier but it's just a dictionary the

uglier but it's just a dictionary the name I want to write to the dictionary

name I want to write to the dictionary is going to be whatever name the human

is going to be whatever name the human typed in the number that I want to write

typed in the number that I want to write to the the CSV file is going to be

to the the CSV file is going to be whatever the number the human typed in

whatever the number the human typed in but what's different next about this

but what's different next about this code is by simply using a dictionary

code is by simply using a dictionary writer here instead of the generic

writer here instead of the generic writer now the columns can be in this

writer now the columns can be in this order or this order or any order and the

order or this order or any order and the dictionary writer is going to figure out

dictionary writer is going to figure out based on the first line of text in that

based on the first line of text in that CSV where to put name where to put

CSV where to put name where to put number so if you flip them no big deal

number so if you flip them no big deal it's going to notice oh wait the columns

it's going to notice oh wait the columns changed and it's going to insert the

changed and it's going to insert the columns correctly so just again another

columns correctly so just again another more powerful feature that lets you

more powerful feature that lets you focus

focus on lets you focus on real work as

on lets you focus on real work as opposed to actually uh getting tied up

opposed to actually uh getting tied up in the weeds of writing code like this

in the weeds of writing code like this otherwise questions on this one as well

otherwise questions on this one as well but what we will do now is come full

but what we will do now is come full circle to some of the more uh

circle to some of the more uh sophisticated examples with which we

sophisticated examples with which we began and I'm going to go back over to

began and I'm going to go back over to my own Mac laptop here where I've got my

my own Mac laptop here where I've got my own terminal window up and running and I

own terminal window up and running and I was just going to introduce a couple of

was just going to introduce a couple of final libraries that really speak to

final libraries that really speak to just how powerful python can can be and

just how powerful python can can be and how quickly you can get up and running

how quickly you can get up and running to be fair can't necessarily do all of

to be fair can't necessarily do all of these things in the cloud like in code

these things in the cloud like in code spaces because you need access to your

spaces because you need access to your own speakers or microphone or the like

own speakers or microphone or the like so that's why I'm doing it on my own Mac

so that's why I'm doing it on my own Mac here but let me go ahead and open up a

here but let me go ahead and open up a program called

program called speech. and I'm not using VSS code here

speech. and I'm not using VSS code here I'm using a program called VI that's

I'm using a program called VI that's entirely terminal window based but it's

entirely terminal window based but it's going to allow me for instance to import

going to allow me for instance to import the python text to speech version three

the python text to speech version three Library I'm going to give myself a

Library I'm going to give myself a variable called engine that's going to

variable called engine that's going to be set equal to the python text to

be set equal to the python text to speech 3 libraries init method which is

speech 3 libraries init method which is just going to initialize this library

just going to initialize this library that relates to text to speech I'm going

that relates to text to speech I'm going to then use the engines say function to

to then use the engines say function to say something like how about hello comma

say something like how about hello comma world and then as my last line I'm going

world and then as my last line I'm going to say engine. runand wait capitalized

to say engine. runand wait capitalized as such to tell my program now to run

as such to tell my program now to run that speech and wait until it's done all

that speech and wait until it's done all right I'm going to save this file I'm

right I'm going to save this file I'm going to run python of speech. py and

going to run python of speech. py and I'm going to cross my fingers as always

I'm going to cross my fingers as always and hello world all right so now I have

and hello world all right so now I have a program that's actually synthesizing

a program that's actually synthesizing speech using a library like this how can

speech using a library like this how can I now modify this to be a little more

I now modify this to be a little more interesting well how about this let me

interesting well how about this let me go ahead and prompt the user for their

go ahead and prompt the user for their name like we've done several times here

name like we've done several times here using Python's built-in name function

using Python's built-in name function and now let me go ahead and use a format

and now let me go ahead and use a format string in conjunction with this Library

string in conjunction with this Library interpolating the value of name there

interpolating the value of name there and at least if my name is somewhat

and at least if my name is somewhat phonetically pronounceable let's go

phonetically pronounceable let's go ahead and run python ofp

ahead and run python ofp speech. type in my name and hello David

speech. type in my name and hello David okay it's a sort of weird choice of

okay it's a sort of weird choice of inflection but we're starting to

inflection but we're starting to synthesize voice not unlike Siri or

synthesize voice not unlike Siri or Google assistant or Alexa or the like

Google assistant or Alexa or the like now we can maybe do something a little

now we can maybe do something a little more advanced too in addition to

more advanced too in addition to synthesizing speech in this way we could

synthesizing speech in this way we could synthesize for instance uh an actual

synthesize for instance uh an actual graphic let me go ahead now and do

graphic let me go ahead now and do something like this let me create a

something like this let me create a program called

program called qr. I'm going to go ahead and import a

qr. I'm going to go ahead and import a library called OS which gives you access

library called OS which gives you access to operating system related

to operating system related functionality in Python I'm going to

functionality in Python I'm going to import a library I've pre-installed

import a library I've pre-installed called QR code which is a

called QR code which is a two-dimensional barcode that you might

two-dimensional barcode that you might have seen in the real world I'm going to

have seen in the real world I'm going to go ahead and create an image variable

go ahead and create an image variable using this QR code library's make

using this QR code library's make function which per its documentation

function which per its documentation takes a URL like one of cs50's own

takes a URL like one of cs50's own videos so we'll do this with YouTube uh

videos so we'll do this with YouTube uh /xv f

zj5 p g g0 so hopefully that's the right

p g g0 so hopefully that's the right lecture and now we've got image. saave

lecture and now we've got image. saave which is going to allow me to create a

which is going to allow me to create a file called qr. ping think back now on

file called qr. ping think back now on problem set 4 and how painful it was to

problem set 4 and how painful it was to save files we'll just use the save

save files we'll just use the save function now in Python and save this as

function now in Python and save this as a ping file portable Network graphic and

a ping file portable Network graphic and then lastly let's just go ahead and open

then lastly let's just go ahead and open uh the with the command open qr. ping on

uh the with the command open qr. ping on my Mac so that hopefully this just

my Mac so that hopefully this just automatically opens all right I'm going

automatically opens all right I'm going to go ahead and just double check my

to go ahead and just double check my syntax here so that I haven't made any

syntax here so that I haven't made any mistakes I'm going to go ahead and run

mistakes I'm going to go ahead and run python of qr. enter that opens up this

python of qr. enter that opens up this let me go ahead and zoom in if you've

let me go ahead and zoom in if you've got a phone handy and you'd like to scan

got a phone handy and you'd like to scan this code

this code here whether in person or

here whether in person or online I apologize you won't appreciate

online I apologize you won't appreciate it amazing okay and lastly let me go

it amazing okay and lastly let me go back into our speech example here uh

back into our speech example here uh create a final ending here on our final

create a final ending here on our final moments and how about we just say

moments and how about we just say something like this was cs50 like this

something like this was cs50 like this let's go ahead here fix my

let's go ahead here fix my capitalization just for tidiness get rid

capitalization just for tidiness get rid of the name and now with our final

of the name and now with our final flourish and your introduction to python

flourish and your introduction to python equipped here we go this was cs50 all

equipped here we go this was cs50 all right we'll see you next

right we'll see you next [Applause]

all right this is cs50 and this is already week seven and this is the week

already week seven and this is the week where we'll continue where we left off

where we'll continue where we left off with python introducing you to a bit

with python introducing you to a bit more syntax and capabilities of the

more syntax and capabilities of the language so you can solve like

language so you can solve like interesting problems but a lot of those

interesting problems but a lot of those problems increasingly are now going to

problems increasingly are now going to involve data in some form after all if

involve data in some form after all if you think of most any website or mobile

you think of most any website or mobile app uh or uh process nowadays that

app uh or uh process nowadays that involves solving problems it almost

involves solving problems it almost always involves some amount of data and

always involves some amount of data and often data at scale lots and lots of

often data at scale lots and lots of data and so what we're going to see

data and so what we're going to see first today is that yes you can use

first today is that yes you can use Python to solve all the problems past

Python to solve all the problems past that we've seen and also some data

that we've seen and also some data specific ones but sometimes it's just

specific ones but sometimes it's just going to be annoying it's going to be a

going to be annoying it's going to be a little painful it's just going to be

little painful it's just going to be more work than you might like to just

more work than you might like to just get to some answer and so today we'll

get to some answer and so today we'll too introduce you to a new language

too introduce you to a new language called SQL structured query language and

called SQL structured query language and this is a language that wur is actually

this is a language that wur is actually much smaller relatively speaking than C

much smaller relatively speaking than C and python it sort of does less but it

and python it sort of does less but it does it really well and it's a language

does it really well and it's a language for querying databases storing data in

for querying databases storing data in it updating it inserting it deleting it

it updating it inserting it deleting it and so much more and it's the kind of

and so much more and it's the kind of Technology that's used nowadays in

Technology that's used nowadays in indeed web apps and mobile apps data

indeed web apps and mobile apps data science analytics and and so much more

science analytics and and so much more it's really good at storing lots and

it's really good at storing lots and lots of data now this is yet another

lots of data now this is yet another language and believe it or not next week

language and believe it or not next week we'll introduce you to three more

we'll introduce you to three more languages HTML and CSS which are not

languages HTML and CSS which are not technically programming languages

technically programming languages they're all about Aesthetics and markup

they're all about Aesthetics and markup of information but also JavaScript which

of information but also JavaScript which is in fact a programming language but

is in fact a programming language but the goals here in cs50 really are going

the goals here in cs50 really are going to be to empower you to program more

to be to empower you to program more generally and indeed when you're out

generally and indeed when you're out there in the real world some years from

there in the real world some years from now invariably there's going to be some

now invariably there's going to be some new other popular language out there and

new other popular language out there and hopefully in this week and next week and

hopefully in this week and next week and Beyond among the goals is not just to

Beyond among the goals is not just to teach you these languages specifically

teach you these languages specifically but again like how to teach yourself the

but again like how to teach yourself the future languages that we've not even

future languages that we've not even heard about just yet so with that said

heard about just yet so with that said let's begin with a survey of sorts if

let's begin with a survey of sorts if you go to this URL on your phone or

you go to this URL on your phone or laptop cs50. l/f favorites a very simple

laptop cs50. l/f favorites a very simple Google form awaits you that's just going

Google form awaits you that's just going to ask you a couple of multiple choice

to ask you a couple of multiple choice questions so go to cs50.

questions so go to cs50. l/f favorites and that should lead you

l/f favorites and that should lead you to a Google form that looks a little

to a Google form that looks a little something like this asking you first as

something like this asking you first as of now in week seven what is your

of now in week seven what is your favorite language among those options

favorite language among those options here and then further down one more

here and then further down one more question if you think back on problem

question if you think back on problem sets 0 through six what was if any your

sets 0 through six what was if any your favorite uh problem set problem be it in

favorite uh problem set problem be it in scratch or C or python so answer those

scratch or C or python so answer those two

two questions and in a moment I'll flip over

questions and in a moment I'll flip over to my screen here where you'll see and

to my screen here where you'll see and anyone who's used Google forms knows the

anyone who's used Google forms knows the spreadsheet that's collecting now this

spreadsheet that's collecting now this data um Microsoft Office 365 can do the

data um Microsoft Office 365 can do the same if you use one of those forms and

same if you use one of those forms and what you see here now is a spreadsheet

what you see here now is a spreadsheet in Google Sheets enumerating all of the

in Google Sheets enumerating all of the audience's questions language is in

audience's questions language is in column B problem is in column C and each

column B problem is in column C and each row represents one student who has

row represents one student who has responded uh a few of you were super

responded uh a few of you were super eager for class today at 8:33 a.m.

eager for class today at 8:33 a.m. eastern time 10:32 11:10 okay so now

eastern time 10:32 11:10 okay so now we're getting into the actual class time

we're getting into the actual class time here and if I scroll down we'll probably

here and if I scroll down we'll probably see few dozen couple hundred Answers by

see few dozen couple hundred Answers by now and yeah so we're getting a whole

now and yeah so we're getting a whole lot of answers here and I'm seeing some

lot of answers here and I'm seeing some patterns emerge but but it's not

patterns emerge but but it's not necessarily obvious to the human eyes

necessarily obvious to the human eyes what those patterns are now of course

what those patterns are now of course you can use Google spreadsheets you can

you can use Google spreadsheets you can like highlight the data and you can

like highlight the data and you can create charts magically out of it but

create charts magically out of it but you can only do what Google lets you do

you can only do what Google lets you do with the data and same thing for

with the data and same thing for Microsoft Excel or apple numbers but

Microsoft Excel or apple numbers but wouldn't it be nice to just be able to

wouldn't it be nice to just be able to manipulate the raw data relatively

manipulate the raw data relatively simple though it is to just answer

simple though it is to just answer questions about the data Maybe long-term

questions about the data Maybe long-term create your own charts customize it just

create your own charts customize it just the way you want rather than be holding

the way you want rather than be holding to like software that's off the shelf

to like software that's off the shelf like this well how could we go about

like this well how could we go about doing this well let me propose that we

doing this well let me propose that we treat this data set now as what we're

treat this data set now as what we're going to call for now a flat file

going to call for now a flat file database we'll see today that there's

database we'll see today that there's fancier databases but the simplest

fancier databases but the simplest database in the world is really just

database in the world is really just like a CSV file and we saw that a couple

like a CSV file and we saw that a couple of weeks ago in C we wrote a bit of C

of weeks ago in C we wrote a bit of C code that used F printf to write data to

code that used F printf to write data to a file using commas as the separator we

a file using commas as the separator we didn't really do much more with csvs at

didn't really do much more with csvs at the time though because it's really

the time though because it's really annoying painful timec consuming not fun

annoying painful timec consuming not fun to use see for something like that

to use see for something like that because of malok and memory and all that

because of malok and memory and all that stuff but with python it's going to be

stuff but with python it's going to be much easier and so anytime you have

much easier and so anytime you have access to some data set where you can

access to some data set where you can just like download it to your own Mac or

just like download it to your own Mac or PC or your Cloud environment it's sort

PC or your Cloud environment it's sort of a candidate for now writing code to

of a candidate for now writing code to do something with the data Maybe analyze

do something with the data Maybe analyze it right away if it's been human inputed

it right away if it's been human inputed manually maybe you have to clean it up

manually maybe you have to clean it up by doing a lot of find and replace but

by doing a lot of find and replace but not with your keyboard but rather with

not with your keyboard but rather with code and so let me go ahead and do this

code and so let me go ahead and do this let me go back to my uh Google sheet

let me go back to my uh Google sheet here that has all of the data that's

here that has all of the data that's come in now and let me go ahead and

come in now and let me go ahead and download this via uh the file menu

download this via uh the file menu here and let's see download and you can

here and let's see download and you can see a whole bunch of options of most

see a whole bunch of options of most formats might be familiar but today

formats might be familiar but today we'll focus just on this one comma

we'll focus just on this one comma separated values or CSV that's going to

separated values or CSV that's going to go ahead and download it on my Mac here

go ahead and download it on my Mac here into my own downloads folder and now I'm

into my own downloads folder and now I'm going to go ahead and do this let me go

going to go ahead and do this let me go ahead and pull up VSS code in the cloud

ahead and pull up VSS code in the cloud here and if you've never done this

here and if you've never done this before there's a couple of ways to do it

before there's a couple of ways to do it but the simplest way to upload a file to

but the simplest way to upload a file to your codes space so to speak is just a

your codes space so to speak is just a sort of drag and drop that's going to

sort of drag and drop that's going to magically upload it to the server there

magically upload it to the server there and we'll see that one it has a very

and we'll see that one it has a very long file name which I'm I'm actually

long file name which I'm I'm actually going to clean this up because that's

going to clean this up because that's going to be very tedious to type in my

going to be very tedious to type in my code so I could either rightclick of of

code so I could either rightclick of of course up here but I'm going to use my

course up here but I'm going to use my Linux command so let's move this file

Linux command so let's move this file called cs50 2022 something or other and

called cs50 2022 something or other and let's just name it more simply

let's just name it more simply favorites. CSV so all lowercase no

favorites. CSV so all lowercase no spaces sort of good Basics and let me go

spaces sort of good Basics and let me go ahead now and open up this file with

ahead now and open up this file with code favorites. CSV I'll close my file

code favorites. CSV I'll close my file explorer and we'll see exactly the same

explorer and we'll see exactly the same data as before but not quite as pretty

data as before but not quite as pretty as Google Sheets makes it be rather we

as Google Sheets makes it be rather we see here that I still have three columns

see here that I still have three columns timestamp language problem and then all

timestamp language problem and then all of the values down below including the

of the values down below including the timestamps and the answers therefore but

timestamps and the answers therefore but it doesn't have proper columns it just

it doesn't have proper columns it just has commas separating them now we could

has commas separating them now we could very easily write python code just like

very easily write python code just like we wrote code to manipulate files like

we wrote code to manipulate files like this either to write or read but in

this either to write or read but in instead let's do something that's a

instead let's do something that's a little more pleasant which is indeed in

little more pleasant which is indeed in the form of python so python actually

the form of python so python actually comes with Native support for csvs it

comes with Native support for csvs it has indeed a package called CSV that

has indeed a package called CSV that just lets you read and write and do a

just lets you read and write and do a whole bunch of useful stuff when it

whole bunch of useful stuff when it comes to CSV files so let's go ahead and

comes to CSV files so let's go ahead and do something with this file let me go

do something with this file let me go back here to VSS code I'm going to close

back here to VSS code I'm going to close favorites. CSV for now but just remember

favorites. CSV for now but just remember in your mind that timestamp was the

in your mind that timestamp was the first column language was the second

first column language was the second column and problem was the third and

column and problem was the third and notice because we're using commas they

notice because we're using commas they don't again line up perfectly but that's

don't again line up perfectly but that's not a problem there are two commas in

not a problem there are two commas in every line presumably and I'm going to

every line presumably and I'm going to go ahead and now create a file called

go ahead and now create a file called how about uh favorites. piy so that I

how about uh favorites. piy so that I can start writing some code to

can start writing some code to manipulate this data and let's do

manipulate this data and let's do something simple let's just write a

something simple let's just write a simple program in Python that opens this

simple program in Python that opens this file reads it and prints something out

file reads it and prints something out just as like a safety check that I know

just as like a safety check that I know what I'm doing even though it's not

what I'm doing even though it's not going to be useful so in Python if you

going to be useful so in Python if you want CSV support you import CSV and that

want CSV support you import CSV and that gives you access to all the magical

gives you access to all the magical capabilities thereof let me now go ahead

capabilities thereof let me now go ahead and use this technique to open a file in

and use this technique to open a file in Python which is similar in C but with

Python which is similar in C but with python I'm going to do this the keyword

python I'm going to do this the keyword with I'm going to open a file called

with I'm going to open a file called favorites. CSV which was the shorter

favorites. CSV which was the shorter name I gave it this is optional but just

name I gave it this is optional but just for explicitness I'm going to open it in

for explicitness I'm going to open it in read mode explicitly just like f open

read mode explicitly just like f open took a second argument as well and I'm

took a second argument as well and I'm going to name this file once open quite

going to name this file once open quite simply file though I could call it

simply file though I could call it anything I want and now it's just an

anything I want and now it's just an open file so far as python knows at this

open file so far as python knows at this moment it's just text or better yet it's

moment it's just text or better yet it's just zeros and ones if you want this

just zeros and ones if you want this python package called CSV to actually do

python package called CSV to actually do something useful with it you have to

something useful with it you have to load this file now into the library and

load this file now into the library and the simplest way to do this is to give

the simplest way to do this is to give myself like a variable called reader

myself like a variable called reader because I want to read this file though

because I want to read this file though this too I could call anything else I'm

this too I could call anything else I'm going to then set that equal to the

going to then set that equal to the return value of a function called CSV do

return value of a function called CSV do reader and I pass to that per the

reader and I pass to that per the documentation the open file so step one

documentation the open file so step one I open the file and this just gives me

I open the file and this just gives me me access to the bytes there in step two

me access to the bytes there in step two now with csvreader tells the python

now with csvreader tells the python package called CSV to do something

package called CSV to do something useful with it and start analyzing the

useful with it and start analyzing the commas and uh allow me to uh parse it

commas and uh allow me to uh parse it further so let's go ahead and do this

further so let's go ahead and do this let me go ahead now and within this Loop

let me go ahead now and within this Loop let's say this with sorry within this

let's say this with sorry within this open file let's do this for every row if

open file let's do this for every row if you will or line in the file AKA reader

you will or line in the file AKA reader let's go ahead and print out now just

let's go ahead and print out now just how about Row Bracket one now what's

how about Row Bracket one now what's going on here well it turns out if you

going on here well it turns out if you read the documentation for the CSV

read the documentation for the CSV reader function what it hands you back

reader function what it hands you back is essentially this special object so to

is essentially this special object so to speak that allows you to treat it as

speak that allows you to treat it as though it's just a really big list of

though it's just a really big list of lines from the file AKA reader so by

lines from the file AKA reader so by saying for Row in reader this is a way

saying for Row in reader this is a way more succinct way of saying uh give me

more succinct way of saying uh give me the first line in the file plus plus

the first line in the file plus plus give me the second line in the file Plus

give me the second line in the file Plus plus and so forth that we would have

plus and so forth that we would have done what much more mechanically in C

done what much more mechanically in C this is just much more pythonic and

this is just much more pythonic and English friendly if you will so in every

English friendly if you will so in every iteration of this Loop row is going to

iteration of this Loop row is going to contain all of the data from the current

contain all of the data from the current row but better yet what the reader

row but better yet what the reader function does for me is it hands me each

function does for me is it hands me each row not just as a big string or Stir of

row not just as a big string or Stir of text in Python it gives me what

text in Python it gives me what apparently based on the syntax on line

apparently based on the syntax on line six any Instinct yeah it's giving me

six any Instinct yeah it's giving me back indeed a list and I'm I presume the

back indeed a list and I'm I presume the visual clue for you was the fact that

visual clue for you was the fact that we're using square brackets here and

we're using square brackets here and indeed Row Bracket one is going to be

indeed Row Bracket one is going to be not the first but the second element in

not the first but the second element in that list and so just take a guess when

that list and so just take a guess when I run this code in a moment What's going

I run this code in a moment What's going to get printed the timestamp the

to get printed the timestamp the language or the

problem the yeah oh the language the language because it's the second column

language because it's the second column that is in the file delimited by those

that is in the file delimited by those those commas so let me go ahead and do

those commas so let me go ahead and do this let me clear my terminal down here

this let me clear my terminal down here here let me run python of favorites.i

here let me run python of favorites.i and enter and there's everything it was

and enter and there's everything it was super fast but there's a really long

super fast but there's a really long list here and in fact if I increase the

list here and in fact if I increase the size of my terminal and start scrolling

size of my terminal and start scrolling up you'll just see all of the raw data

up you'll just see all of the raw data now this isn't that useful yet I could

now this isn't that useful yet I could have just glanced at the CSV but clearly

have just glanced at the CSV but clearly now I have the ability to open the file

now I have the ability to open the file parse it so to speak that is break it up

parse it so to speak that is break it up into its constituent parts and do

into its constituent parts and do something with specific Parts therein

something with specific Parts therein all right so if I want to do this a

all right so if I want to do this a little more pleasantly though let me at

little more pleasantly though let me at least least make this semantically a

least least make this semantically a little cleaner and you know what just

little cleaner and you know what just for clarity let me just give myself a

for clarity let me just give myself a variable it's not strictly necessary but

variable it's not strictly necessary but I know that this is the favorite uh for

I know that this is the favorite uh for instance uh language so let's just call

instance uh language so let's just call it favorite set it equal to Row Bracket

it favorite set it equal to Row Bracket 1 and now just to be more explicit in my

1 and now just to be more explicit in my code even though again we don't need the

code even though again we don't need the variable per se this codee's of course

variable per se this codee's of course going to do the same thing it's just

going to do the same thing it's just using an additional variable called

using an additional variable called favorite if I go down here scroll up run

favorite if I go down here scroll up run the program again I get back the exact

the program again I get back the exact same data but this is a stepping stone

same data but this is a stepping stone to something that's even more powerful

to something that's even more powerful about python support for CSV files is

about python support for CSV files is that you don't have to just treat the

that you don't have to just treat the return value as a list with zero and one

return value as a list with zero and one and two so just thinking intuitively

and two so just thinking intuitively here why is this maybe not the best

here why is this maybe not the best design to hand you the programmer back

design to hand you the programmer back the data in a list that's numerically

the data in a list that's numerically indexed with

indexed with 012 it clearly works but critique this

012 it clearly works but critique this what could go wrong what's a little

what could go wrong what's a little poorly designed yeah you have to always

poorly designed yeah you have to always remember what

remember what are

yeah exactly so if yeah so it's up to you to repeat it's up to you to remember

you to repeat it's up to you to remember like what column the data is actually in

like what column the data is actually in and you know God forbid you're you're

and you know God forbid you're you're collaborating with someone else on the

collaborating with someone else on the spreadsheet you know you've if you've

spreadsheet you know you've if you've used Google spreadsheets you can move

used Google spreadsheets you can move columns around maybe just because you

columns around maybe just because you want to visually reorganize things and

want to visually reorganize things and if you do this and then someone else

if you do this and then someone else downloads that same data all of their

downloads that same data all of their code is going to break so that's just

code is going to break so that's just really bad design it's fragile just

really bad design it's fragile just because it's you're sort of on the honor

because it's you're sort of on the honor System that one means the data that you

System that one means the data that you want so wouldn't it be nice if we could

want so wouldn't it be nice if we could be a little more explicit well recall

be a little more explicit well recall that the very first line in this file is

that the very first line in this file is actually this and I paused the output

actually this and I paused the output this time so that we can see more

this time so that we can see more optionally I just reran favorites St pi

optionally I just reran favorites St pi and notice one of these things is not

and notice one of these things is not like the other every output was either

like the other every output was either scratch or C or python except for this

scratch or C or python except for this first one why am I seeing the word

first one why am I seeing the word language

here where did language come from you didn't have the ability to manual input

didn't have the ability to manual input oh no where did it come from

oh no where did it come from yeah yeah the header the very first row

yeah yeah the header the very first row in the file which by human convention

in the file which by human convention generally just defines what the columns

generally just defines what the columns represent so that there's some human

represent so that there's some human useful information there now that's not

useful information there now that's not really intended to be part of my output

really intended to be part of my output at the moment so there is a way to skip

at the moment so there is a way to skip this if you want to skip the first row

this if you want to skip the first row you can actually do something like this

you can actually do something like this you can say next row and that will just

you can say next row and that will just ignore that row so that I'm starting

ignore that row so that I'm starting really with the every row thereafter but

really with the every row thereafter but there's a better way to handle this than

there's a better way to handle this than that that will get rid of the row in the

that that will get rid of the row in the output but let me go ahead and use a

output but let me go ahead and use a different feature of the CSV package

different feature of the CSV package that's just going to make this a little

that's just going to make this a little cleaner Al together so let me clear my

cleaner Al together so let me clear my terminal window here let me undo this

terminal window here let me undo this next thing that I just added and instead

next thing that I just added and instead of using a reader let me go ahead and

of using a reader let me go ahead and use a dictionary reader abbreviated dict

use a dictionary reader abbreviated dict reader that's going to now return me the

reader that's going to now return me the equivalent of all of the rows one at a

equivalent of all of the rows one at a time so I can still call it Reader just

time so I can still call it Reader just as before but as the name implies what

as before but as the name implies what this reader is going to return is not a

this reader is going to return is not a list after list after list but a

list after list after list but a dictionary a dictionary a dictionary and

dictionary a dictionary a dictionary and remember a dictionary is just a

remember a dictionary is just a collection of key value pairs so what

collection of key value pairs so what does that mean what are the keys what

does that mean what are the keys what are the values well now that I'm using a

are the values well now that I'm using a dictionary reader I can actually do this

dictionary reader I can actually do this instead of sort of on the honor System

instead of sort of on the honor System remembering that I want column one I can

remembering that I want column one I can treat row now not as a list but as a

treat row now not as a list but as a dictionary and that means I can go in

dictionary and that means I can go in here and say quote unquote language and

here and say quote unquote language and we saw that back in week six python

we saw that back in week six python allows you to index into dictionaries

allows you to index into dictionaries using square bracket notation in strings

using square bracket notation in strings or stirs on the inside just like lists

or stirs on the inside just like lists allow for numbers but this now I think

allow for numbers but this now I think is going to be a little more robust if I

is going to be a little more robust if I run this again python of favorites. py

run this again python of favorites. py all of that worked out fine and let me

all of that worked out fine and let me pause the output to by using this

pause the output to by using this program called more now I don't even see

program called more now I don't even see the header so now whoever works it uh

the header so now whoever works it uh with python wrote the code for this

with python wrote the code for this package to just analyze that first line

package to just analyze that first line of code use the header as you just

of code use the header as you just called it as the keys and then every

called it as the keys and then every time you iterate through this Loop it

time you iterate through this Loop it updates the values the values the values

updates the values the values the values but the keys stay the same any questions

but the keys stay the same any questions then on this

then on this technique spiced to say this would be

technique spiced to say this would be painful in C

Associated exactly so the keys are always going to be quote unquote

always going to be quote unquote timestamp uh Pro language and problem

timestamp uh Pro language and problem but on each iteration of this Loop here

but on each iteration of this Loop here uh the row is going to contain a

uh the row is going to contain a different row of values different row

different row of values different row values different row values so you're

values different row values so you're going to get back one dictionary for

going to get back one dictionary for every student who submitted the Google

every student who submitted the Google form if you will while iterating through

form if you will while iterating through it there all right so once we have this

it there all right so once we have this ability here why don't we go ahead and

ability here why don't we go ahead and transition

transition into how about not just using that

into how about not just using that dictionary reader which makes the code a

dictionary reader which makes the code a little more robust because now if you

little more robust because now if you move the columns around no big deal it

move the columns around no big deal it doesn't matter if the numeric indices

doesn't matter if the numeric indices change you can still use those keywords

change you can still use those keywords instead well let's actually analyze the

instead well let's actually analyze the data now I'm just spitting it out which

data now I'm just spitting it out which is not solving any problems for anyone

is not solving any problems for anyone so let's go ahead and and count the

so let's go ahead and and count the popularity of scratch C and Python and

popularity of scratch C and Python and see what everyone's uh been thinking

see what everyone's uh been thinking here all right so how might I do this

here all right so how might I do this well let me go ahead and do this up here

well let me go ahead and do this up here before I start iterating let me give

before I start iterating let me give myself let's say three variables and to

myself let's say three variables and to keep things simple I'll say one variable

keep things simple I'll say one variable called scratch set it equal to zero for

called scratch set it equal to zero for zero students so far C is going to equal

zero students so far C is going to equal zero and python is going to equal zero

zero and python is going to equal zero there's a slightly prettier way of doing

there's a slightly prettier way of doing this just because this is like three

this just because this is like three lines of code to do something very

lines of code to do something very simple you could alternatively in Python

simple you could alternatively in Python but not C do scratch comma C comma

but not C do scratch comma C comma python equals 0 comma 0 so kind of

python equals 0 comma 0 so kind of slightly more elegant just to fit it all

slightly more elegant just to fit it all into one line but now let's just do

into one line but now let's just do something more interesting on line seven

something more interesting on line seven I'm still going to figure out what the

I'm still going to figure out what the current favorite language is and now I'm

current favorite language is and now I'm just going to do some conditional checks

just going to do some conditional checks How about if that favorite equals equals

How about if that favorite equals equals quote unquote scratch let's go ahead and

quote unquote scratch let's go ahead and increment Scratch by one we can't do

increment Scratch by one we can't do plus plus in Python but we can do plus

plus plus in Python but we can do plus equals 1 how about L if favorite equals

equals 1 how about L if favorite equals equals c then let's do C+ equal 1 L and

equals c then let's do C+ equal 1 L and not we could do else this is actually a

not we could do else this is actually a good design question should I do else

good design question should I do else should I do L if any instincts here

yeah yeah really good instincts just in case someone goes and adds another

case someone goes and adds another language to the form next week because

language to the form next week because we're obviously going to introduce

we're obviously going to introduce another language today you don't want

another language today you don't want your code to now artificially inflate

your code to now artificially inflate the scores for python just because

the scores for python just because you're conflating multiple languages

you're conflating multiple languages together so the more defensive sort of

together so the more defensive sort of better way to write this code I agree

better way to write this code I agree would be L if favorite equals equals

would be L if favorite equals equals python then let's go ahead and increment

python then let's go ahead and increment python plus equals 1 and if there's a

python plus equals 1 and if there's a new language next week we're obviously

new language next week we're obviously going to have to update the code but at

going to have to update the code but at least we're not miscounting we're just

least we're not miscounting we're just missing the new language so I think

missing the new language so I think that's slightly more robust all right

that's slightly more robust all right now at the very bottom of this program

now at the very bottom of this program and outside of the loop when I'm all

and outside of the loop when I'm all done counting let me go ahead and print

done counting let me go ahead and print out using some f strings how about the

out using some f strings how about the total number of people who uh whose

total number of people who uh whose favorite is scratch so this is just uh

favorite is scratch so this is just uh week six F string syntax let me go ahead

week six F string syntax let me go ahead and print out another F string for C and

and print out another F string for C and I'm of course putting the variables in

I'm of course putting the variables in curly braces all lowercase but the

curly braces all lowercase but the English words I'm doing capitalization

English words I'm doing capitalization for Let's do an final one with f uh

for Let's do an final one with f uh python colon and then in curly braces

python colon and then in curly braces python close quote and I think I'm done

python close quote and I think I'm done so let me just hide my terminal for a

so let me just hide my terminal for a second here's the total program same

second here's the total program same stuff as before open favorites. CSV open

stuff as before open favorites. CSV open it further with the dictionary reader to

it further with the dictionary reader to do that processing for us initialize

do that processing for us initialize three variables to zero just so we have

three variables to zero just so we have something to count with and then iterate

something to count with and then iterate over the file row by row and this is

over the file row by row and this is just some sort of week one style

just some sort of week one style conditional logic albe it in Python

conditional logic albe it in Python counting things all right so how can we

counting things all right so how can we now execute this let me go back to my

now execute this let me go back to my terminal python of favorites.i and here

terminal python of favorites.i and here we go uh as of today everyone who's

we go uh as of today everyone who's reporting in live via the Google form

reporting in live via the Google form their favorite languages are

their favorite languages are interesting that's pretty interesting

interesting that's pretty interesting too after just one week of python no

too after just one week of python no less so but scratch is a healthy

less so but scratch is a healthy Contender there a lot of C so a pretty

Contender there a lot of C so a pretty good mix here so is this going to be the

good mix here so is this going to be the best way to write this program long term

best way to write this program long term well as you noted if there's a new

well as you noted if there's a new language next week this week we're going

language next week this week we're going to have to constantly update this and

to have to constantly update this and here's where you should let your mind

here's where you should let your mind wander to like the future like if we

wander to like the future like if we have a fourth language fifth language

have a fourth language fifth language 6th sth eth which aspect here might kind

6th sth eth which aspect here might kind of have some code smell to it like this

of have some code smell to it like this probably isn't the best design to set us

probably isn't the best design to set us up for the

up for the future what might be better than this

future what might be better than this yeah add language line yeah we have to

yeah add language line yeah we have to keep adding a language to line five and

keep adding a language to line five and okay not a big deal we could add like

okay not a big deal we could add like SQL today and maybe JavaScript next week

SQL today and maybe JavaScript next week but you know anytime a a line of code a

but you know anytime a a line of code a line of logic is just going to kind of

line of logic is just going to kind of grow out of control and we've had this

grow out of control and we've had this chat a couple of times with different

chat a couple of times with different syntax there's probably a better way

syntax there's probably a better way than that so let's do that instead of

than that so let's do that instead of using these individual variables we

using these individual variables we could make maybe use a list but a list

could make maybe use a list but a list would be a little confusing because like

would be a little confusing because like what does braet zero mean what is

what does braet zero mean what is bracket 1 bracket two but a dictionary

bracket 1 bracket two but a dictionary recall is like this Swiss army knife of

recall is like this Swiss army knife of data structures whereby you can

data structures whereby you can associate anything with anything else

associate anything with anything else keys with values so I dare say a cleaner

keys with values so I dare say a cleaner way to solve this problem that sets us

way to solve this problem that sets us up for Less work or confusion later

up for Less work or confusion later would be to create like a new variable

would be to create like a new variable called counts if that's what we're doing

called counts if that's what we're doing counting things up and just set it equal

counting things up and just set it equal to an empty dictionary and you can

to an empty dictionary and you can literally say adct with the open

literally say adct with the open parenthesis closed parenthesis nothing

parenthesis closed parenthesis nothing or the more pythonic just use open and

or the more pythonic just use open and Clos curly braces with nothing inside

Clos curly braces with nothing inside that gives me an empty dictionary just

that gives me an empty dictionary just like square brackets gives me a list now

like square brackets gives me a list now my logic down here has to change a

my logic down here has to change a little bit but what's nice is I don't

little bit but what's nice is I don't need one conditional for every language

need one conditional for every language because again if we have a fourth a

because again if we have a fourth a fifth a sixth that chunk of code is also

fifth a sixth that chunk of code is also going to grow a bit out of control too

going to grow a bit out of control too so I can get rid of this here and what I

so I can get rid of this here and what I think I'm going to do is say this

think I'm going to do is say this whatever the current favorite is from

whatever the current favorite is from the current Row in the file why don't we

the current Row in the file why don't we go into our counts variable at that key

go into our counts variable at that key and again favorite is a variable it's

and again favorite is a variable it's not quote unquote favorite it's going to

not quote unquote favorite it's going to be scratch or C or Python and then why

be scratch or C or Python and then why don't we go ahead and just increment

don't we go ahead and just increment whatever the value of that count is at

whatever the value of that count is at that key now this is technically bugy

that key now this is technically bugy we're really close but there is a

we're really close but there is a bug does anyone want to conjecture what

bug does anyone want to conjecture what the bug is

yeah a good question that answers my question in uh nonetheless so no like

question in uh nonetheless so no like the magic you describe will not happen

the magic you describe will not happen and to repeat the the hypothesis will

and to repeat the the hypothesis will this automatically create a key for

this automatically create a key for every uh language that we try plugging

every uh language that we try plugging into those square brackets short answer

into those square brackets short answer no odds are this is going to create a

no odds are this is going to create a key error one of those traceback error

key error one of those traceback error messages that you've probably seen by

messages that you've probably seen by now either in class or in problem sets

now either in class or in problem sets whereby if scratch hasn't appeared in

whereby if scratch hasn't appeared in the dictionary before or C or python

the dictionary before or C or python like then the dictionary has no clue

like then the dictionary has no clue what you're talking about so I think we

what you're talking about so I think we actually still need some conditional

actually still need some conditional logic but not that's going to grow

logic but not that's going to grow longer and longer with each language

longer and longer with each language what I think we probably want to do is

what I think we probably want to do is this if the current favorite is in the

this if the current favorite is in the counts dictionary and this is the

counts dictionary and this is the pythonic way of just saying is this key

pythonic way of just saying is this key in this dictionary then go ahead and

in this dictionary then go ahead and safely do count favorite plus equals 1

safely do count favorite plus equals 1 else to your conjecture now else what do

else to your conjecture now else what do I want to do counts

I want to do counts favorite

favorite equals yeah one so initialize a brand

equals yeah one so initialize a brand new key to a brand new value of one

new key to a brand new value of one because I'm obviously just seen this

because I'm obviously just seen this language otherwise increment again and

language otherwise increment again and again and now down here I just need to

again and now down here I just need to tweak my syntax a little bit I don't

tweak my syntax a little bit I don't need to print out all of these things

need to print out all of these things one at a time manually I can actually

one at a time manually I can actually get away I think with another loop at

get away I think with another loop at the very bottom here so how about I do

the very bottom here so how about I do this for each favorite in those counts

this for each favorite in those counts and this is again the pythonic way to

and this is again the pythonic way to iterate over all of the keys in a

iterate over all of the keys in a dictionary go ahead and print out using

dictionary go ahead and print out using an F string whatever the current

an F string whatever the current favorite is scratch or C or Python and

favorite is scratch or C or Python and then a colon and then figure out what

then a colon and then figure out what its count is and you can do that by

its count is and you can do that by going into the counts dictionary looking

going into the counts dictionary looking at the favorite key and get back its

at the favorite key and get back its value so close my curly braces I close

value so close my curly braces I close my quotes and even though this looks

my quotes and even though this looks ugly at the moment now this is much more

ugly at the moment now this is much more dynamic because if we go and add SQL to

dynamic because if we go and add SQL to the CSV file tomorrow where we add

the CSV file tomorrow where we add JavaScript next week this will just work

JavaScript next week this will just work it will keep working now automatically

it will keep working now automatically all I change is the Google form not my

all I change is the Google form not my actual code all right let's try python

actual code all right let's try python of favorites.i cross my fingers as

of favorites.i cross my fingers as always and there now is the data as of

always and there now is the data as of now questions on this code here

yeah really good good question what if you wanted to print it in a particular

you wanted to print it in a particular order uh well I could give you a couple

order uh well I could give you a couple of solutions like if you want to print

of solutions like if you want to print it out in it's already coincidentally in

it out in it's already coincidentally in alphabetical order so you got that for

alphabetical order so you got that for free although that's just by chance here

free although that's just by chance here but there is a way to do this and let me

but there is a way to do this and let me propose that we go down here to my Loop

propose that we go down here to my Loop and I explicitly use a function you

and I explicitly use a function you might not have seen in Python yet but

might not have seen in Python yet but it's literally called sorted which is

it's literally called sorted which is going to take either a list or in this

going to take either a list or in this case a dictionary and by default sort it

case a dictionary and by default sort it by key alphabetically now if my

by key alphabetically now if my intuition is correct this is not going

intuition is correct this is not going to change the output because it's

to change the output because it's already alphabetical but if you read the

already alphabetical but if you read the documentation for the sorted function it

documentation for the sorted function it takes multiple parameters potentially

takes multiple parameters potentially some of which are named parameters and

some of which are named parameters and so you can actually do this if you want

so you can actually do this if you want to sort the counts but you want to

to sort the counts but you want to reverse the order for whatever reason

reverse the order for whatever reason here so that it's reverse alphabetical

here so that it's reverse alphabetical order now let me go ahead and rerun this

order now let me go ahead and rerun this and I'll keep the previous output on the

and I'll keep the previous output on the screen enter and now it's backwards uh

screen enter and now it's backwards uh alphabetically if you will other

alphabetically if you will other questions on this here

questions on this here here no how about then how about then we

here no how about then how about then we transition

transition to changing sorting by value and let me

to changing sorting by value and let me this is going to escalate a little

this is going to escalate a little quickly briefly but then we'll we'll

quickly briefly but then we'll we'll tone it down again notice that right now

tone it down again notice that right now this is indeed sorting by key what if

this is indeed sorting by key what if especially if I have lots of data it'd

especially if I have lots of data it'd be nice to make like a top 10 list or in

be nice to make like a top 10 list or in this case a top three list and actually

this case a top three list and actually see in order of the counts the values uh

see in order of the counts the values uh what these popular ones are so it's not

what these popular ones are so it's not C python scratch it should ideally be

C python scratch it should ideally be python then C then scratch because of

python then C then scratch because of the values and the magnitude thereof so

the values and the magnitude thereof so how can I do this well it turns out

how can I do this well it turns out there's another key another uh parameter

there's another key another uh parameter that you can pass to the sorted function

that you can pass to the sorted function that is typically implemented as a

that is typically implemented as a function itself and so I'm going to go

function itself and so I'm going to go ahead and do this I'm going to

ahead and do this I'm going to temporarily Define a function called get

temporarily Define a function called get value just to make my life easier and

value just to make my life easier and this get value function is going to take

this get value function is going to take I'll say a language par parameter and

I'll say a language par parameter and then all I'm going to do is return

then all I'm going to do is return whatever the count is of that language

whatever the count is of that language so out of context this is just a super

so out of context this is just a super simple function that you hand it a

simple function that you hand it a language like scratch or C or python

language like scratch or C or python it's just going to tell you what the

it's just going to tell you what the count is thereof in that uh in that

count is thereof in that uh in that dictionary called counts but what I can

dictionary called counts but what I can do now down here in my newly introduced

do now down here in my newly introduced call to sorted is I can tell it what to

call to sorted is I can tell it what to use as its key instead of using

use as its key instead of using literally the key scratch C python I can

literally the key scratch C python I can sort of override that behavior and say

sort of override that behavior and say you know what to figure out what to sort

you know what to figure out what to sort by go ahead and call this function

by go ahead and call this function called get value notice that I have not

called get value notice that I have not put parentheses after get value because

put parentheses after get value because I don't want to call get value right

I don't want to call get value right then and there I want to pass the get

then and there I want to pass the get value function as itself in argument to

value function as itself in argument to the sorted function so that the sorted

the sorted function so that the sorted function written years ago by the people

function written years ago by the people at python can call my version of get

at python can call my version of get value again and again and again when

value again and again and again when they try to sort sort this actual data

they try to sort sort this actual data so now if I add that and I leave reverse

so now if I add that and I leave reverse equals true let's see what happens

equals true let's see what happens python of favorites. py enter and now I

python of favorites. py enter and now I get my top 10 or in this case top three

get my top 10 or in this case top three list and if I had more sophisticated

list and if I had more sophisticated data with like more columns Al together

data with like more columns Al together that I actually care about I could even

that I actually care about I could even sort this more uh powerfully as well but

sort this more uh powerfully as well but let me clean this up a little bit just

let me clean this up a little bit just so you've seen it even though we won't

so you've seen it even though we won't use these that often in cs50 until the

use these that often in cs50 until the end of the class will they come up again

end of the class will they come up again technically this is a little bit H this

technically this is a little bit H this isn't necessarily the best design to

isn't necessarily the best design to spend all this time implementing a

spend all this time implementing a function and then only use it in one

function and then only use it in one place in general we've argued that H you

place in general we've argued that H you don't necessarily need a variable if

don't necessarily need a variable if you're only going to use it in one place

you're only going to use it in one place you don't really need a function if

you don't really need a function if you're only going to use it in one place

you're only going to use it in one place and here we kind of have a good

and here we kind of have a good candidate for that and so it turns out

candidate for that and so it turns out in Python if you don't want to bother

in Python if you don't want to bother creating a function just to use it once

creating a function just to use it once you can create what's called an

you can create what's called an anonymous function AKA a Lambda function

anonymous function AKA a Lambda function like the lamb Lambda symbol familiar and

like the lamb Lambda symbol familiar and a Lambda function the syntax is a little

a Lambda function the syntax is a little strange looking but you say this you

strange looking but you say this you literally say Lambda you literally then

literally say Lambda you literally then say the name of the argument that you

say the name of the argument that you want this Anonymous function with no

want this Anonymous function with no name to take then you have a colon and

name to take then you have a colon and then quite simply you write what you

then quite simply you write what you want the return value of this function

want the return value of this function to be you don't even say return

to be you don't even say return literally these Lambda functions are

literally these Lambda functions are meant to be used super turly so that you

meant to be used super turly so that you can in one line Express something like

can in one line Express something like this and I admit this looks more cryptic

this and I admit this looks more cryptic I think than the previous version but as

I think than the previous version but as you get more comfortable with python or

you get more comfortable with python or other languages that support this

other languages that support this feature it allows you to not bother with

feature it allows you to not bother with lines of code like that and just tighten

lines of code like that and just tighten up your code a little bit so this line

up your code a little bit so this line here lamba language colon counts

here lamba language colon counts language is the oneline version of this

language is the oneline version of this and you don't even need to bother

and you don't even need to bother picking a name for it Lambda tells

picking a name for it Lambda tells python I didn't waste any time thinking

python I didn't waste any time thinking of a name for this

of a name for this function so questions then on this

function so questions then on this technique technique of using python to

technique technique of using python to analyze data like

analyze data like this any

this any questions we're almost done with python

questions we're almost done with python questions no okay so why don't we make

questions no okay so why don't we make things a little more interesting because

things a little more interesting because we had a much juicier data set with the

we had a much juicier data set with the problems that we've assigned over the

problems that we've assigned over the past several weeks why don't we go ahead

past several weeks why don't we go ahead and quite simply you know I think we

and quite simply you know I think we wrote pretty darn good code here so I

wrote pretty darn good code here so I think we can pretty much just change a

think we can pretty much just change a bit of it to say let's see if I don't

bit of it to say let's see if I don't want language I want problem and if I

want language I want problem and if I want to sort by not language but problem

want to sort by not language but problem I think that's it I think if I didn't

I think that's it I think if I didn't Overlook something here just by changing

Overlook something here just by changing what column I'm reading the data from

what column I'm reading the data from and then just to be consistent renaming

and then just to be consistent renaming my variables just so I know what I'm

my variables just so I know what I'm looking at what will this program now do

looking at what will this program now do after those minor

after those minor changes what will I see when I run

changes what will I see when I run this

this what would be the first thing I see when

what would be the first thing I see when I run

this tough crowd today yes problem yeah the problem this the top problem so the

the problem this the top problem so the most popular problem which I'm a little

most popular problem which I'm a little worried it might be hello or just

worried it might be hello or just scratch but let's go ahead and see so

scratch but let's go ahead and see so let me go ahead and open my terminal

let me go ahead and open my terminal window I'll even maximize my terminal

window I'll even maximize my terminal window so we can see a lot let me go

window so we can see a lot let me go ahead and run python of favorites. py

ahead and run python of favorites. py I'm going to go ahead now and cross my

I'm going to go ahead now and cross my fingers that I didn't mess up and hit

fingers that I didn't mess up and hit enter and okay great we peaked early so

enter and okay great we peaked early so scratch was the most popular program

scratch was the most popular program according to the data at the time I

according to the data at the time I downloaded it I'm sure other votes have

downloaded it I'm sure other votes have come in since filter uh in week four was

come in since filter uh in week four was tied then with tan as well Mario is a

tied then with tan as well Mario is a close third there and so forth so this

close third there and so forth so this is helpful for us on staff that uh not

is helpful for us on staff that uh not so much love down here at the bottom of

so much love down here at the bottom of the list so it was a bunch of code to

the list so it was a bunch of code to write but now that we've written it in

write but now that we've written it in this very versatile Dynamic way it's

this very versatile Dynamic way it's pretty good for just like crunching data

pretty good for just like crunching data and doing some analytics but it's still

and doing some analytics but it's still a decent number of lines to have had to

a decent number of lines to have had to write manually and this is where

write manually and this is where sometimes it isn't necessarily the right

sometimes it isn't necessarily the right tool for the job but rather the job that

tool for the job but rather the job that uh but rather a candidate for using some

uh but rather a candidate for using some other language alog together especially

other language alog together especially when it's not just a one-time program

when it's not just a one-time program that you run and you want to see the

that you run and you want to see the answer what if you want to take input

answer what if you want to take input from the user and ask uh answer

from the user and ask uh answer questions dynamically like a mobile app

questions dynamically like a mobile app would like a website would like

would like a website would like Microsoft Excel or apple numbers or

Microsoft Excel or apple numbers or Google Sheets would for for you well

Google Sheets would for for you well let's make one final change for now to

let's make one final change for now to this version of the program and actually

this version of the program and actually take in some user input so besides just

take in some user input so besides just loading all of the data into memory

loading all of the data into memory let's go ahead and down below here not

let's go ahead and down below here not just print out the top 10 list if you

just print out the top 10 list if you will but prompt the user for their

will but prompt the user for their favorite so I'm going to use Python's

favorite so I'm going to use Python's input function and I'm just going to

input function and I'm just going to prompt them with favorite quote unquote

prompt them with favorite quote unquote like tell me what your favorite problem

like tell me what your favorite problem what problem uh rather uh you interested

what problem uh rather uh you interested in and now let me go ahead and say if

in and now let me go ahead and say if that favorite is in the counts variable

that favorite is in the counts variable so you didn't type in something random

so you didn't type in something random that we didn't actually assign as a

that we didn't actually assign as a problem then let me go ahead and print

problem then let me go ahead and print with a format string whatever that

with a format string whatever that favorite is of yours and show you the

favorite is of yours and show you the actual popularity thereof by indexing

actual popularity thereof by indexing into counts using that favorite as the

into counts using that favorite as the key and printing this so now it's a

key and printing this so now it's a dynamic program it doesn't dump all of

dynamic program it doesn't dump all of the data and all of the summations

the data and all of the summations rather it's going to allow me to see

rather it's going to allow me to see what my choice of favorite is and I'm

what my choice of favorite is and I'm going to go ahead and say uh let's see

going to go ahead and say uh let's see I'm a fan of Mario here so enter and

I'm a fan of Mario here so enter and indeed we see the same value we saw a

indeed we see the same value we saw a moment ago but just for Mario but the

moment ago but just for Mario but the point now is that one all of this is

point now is that one all of this is possible two it's way easier and more

possible two it's way easier and more pleasant than this would have been in C

pleasant than this would have been in C this is still only 15 lines of code and

this is still only 15 lines of code and in C again there's the memory management

in C again there's the memory management there's the iterating over the strings

there's the iterating over the strings trying to find the commas there's just a

trying to find the commas there's just a lot more work but honestly even when you

lot more work but honestly even when you just want to answer a question like this

just want to answer a question like this in Excel in Apple Numbers Google sheets

in Excel in Apple Numbers Google sheets you know generally you can just

you know generally you can just highlight things you can click a button

highlight things you can click a button and boom you get your answer for

and boom you get your answer for summation or Max or Min or any of those

summation or Max or Min or any of those sort of Basics wouldn't it be nice if we

sort of Basics wouldn't it be nice if we weren't taking a step backwards as

weren't taking a step backwards as programmers and being sort of more

programmers and being sort of more powerful and yet we now have to do more

powerful and yet we now have to do more of the work so maybe sometimes Python's

of the work so maybe sometimes Python's not or any language is not the best tool

not or any language is not the best tool for the job and that's going to now

for the job and that's going to now allow us to introduce more generally

allow us to introduce more generally something called a relational database

something called a relational database graduating from Mere flat file databases

graduating from Mere flat file databases like text files or binary files in which

like text files or binary files in which all of your data is stored to something

all of your data is stored to something more proper but first

questions really good question to reiterate if I were to is this case

reiterate if I were to is this case sensitive so if I were to type in Mario

sensitive so if I were to type in Mario in all lowercase and hit enter I

in all lowercase and hit enter I actually get no such response now that

actually get no such response now that might be acceptable because the problem

might be acceptable because the problem technically what is a capital M but

technically what is a capital M but that's a little uh ridiculous to be that

that's a little uh ridiculous to be that pedantic about the input so how could we

pedantic about the input so how could we solve

solve this any tips for how we can make this a

this any tips for how we can make this a little more robust

yeah yeah so we could use a few different functions one of which is

different functions one of which is called title which will change it to

called title which will change it to title case where it capitalizes like in

title case where it capitalizes like in most English sentences the first letter

most English sentences the first letter of that sentence we could use capitalize

of that sentence we could use capitalize we could use upper we could use lower

we could use upper we could use lower but indeed we could just decide how we

but indeed we could just decide how we want to standardize the capitalization

want to standardize the capitalization either uppercase lowercase or some

either uppercase lowercase or some combination thereof and just make sure

combination thereof and just make sure that you change the counts themselves

that you change the counts themselves make sure that you do the same to

make sure that you do the same to favorite and make sure that maybe you

favorite and make sure that maybe you keep a backup of the data if you want to

keep a backup of the data if you want to show the original version that came from

show the original version that came from the CSV without presuming to just

the CSV without presuming to just capitalize everything for the user but

capitalize everything for the user but indeed that would be the most common

indeed that would be the most common scenario you just make things case

scenario you just make things case insensitive when doing those matches

insensitive when doing those matches other questions now on python before we

other questions now on python before we leave it behind for the coming

leave it behind for the coming week all right well then let's introduce

week all right well then let's introduce these relational databases so relational

these relational databases so relational database is what like every is a super

database is what like every is a super popular way of storing lots of data like

popular way of storing lots of data like this is what the Twitter's of the world

this is what the Twitter's of the world the Googles of the world the metas of

the Googles of the world the metas of the world use to store some of their

the world use to store some of their data at scale there are alternatives to

data at scale there are alternatives to relational databases um indeed today

relational databases um indeed today we'll talk about a language called SQL

we'll talk about a language called SQL there's so a movement if you will or an

there's so a movement if you will or an alternative generally called No SQL

alternative generally called No SQL which is just the opposite you don't use

which is just the opposite you don't use SQL there are things called

SQL there are things called objectoriented databases and the like

objectoriented databases and the like but if you've ever heard of MySQL or

but if you've ever heard of MySQL or postgress SQL or uh Microsoft SQL server

postgress SQL or uh Microsoft SQL server or Oracle or Maria DB or a bunch of

or Oracle or Maria DB or a bunch of other products both free and Commercial

other products both free and Commercial this is what they're talking about

this is what they're talking about databases that are designed to store

databases that are designed to store lots of data and what's nice about

lots of data and what's nice about relational databases is that they're

relational databases is that they're really similar to the spreadsheets with

really similar to the spreadsheets with which you were presumably familiar long

which you were presumably familiar long before today's class so a relational

before today's class so a relational database is going to store as you'll see

database is going to store as you'll see all of the data in rows and columns now

all of the data in rows and columns now the terminology will thereafter be a

the terminology will thereafter be a little different instead of having

little different instead of having sheets you're going to have tables but

sheets you're going to have tables but those tables are still going to have

those tables are still going to have rows and columns and you're going to

rows and columns and you're going to have even more control over the

have even more control over the performance of your data when you start

performance of your data when you start to access it using this structured query

to access it using this structured query language or SQL this is a language you

language or SQL this is a language you can use for web apps mobile apps uh a

can use for web apps mobile apps uh a lot of analysts would sit down and their

lot of analysts would sit down and their Mac or PC and actually ask questions of

Mac or PC and actually ask questions of data to get back the answer and

data to get back the answer and wonderfully even though there will be

wonderfully even though there will be some new syntax today SQL really just

some new syntax today SQL really just does four basic things crud is the sort

does four basic things crud is the sort of crude acronym here crud is a way of

of crude acronym here crud is a way of remembering that a relational database

remembering that a relational database supports ultimately creating data

supports ultimately creating data reading data updating data and deleting

reading data updating data and deleting data so even if you're feeling like wow

data so even if you're feeling like wow this is a lot of new syntax which it

this is a lot of new syntax which it isn't relative to our past languages the

isn't relative to our past languages the only things you're doing really are

only things you're doing really are creating dat data reading data updating

creating dat data reading data updating and deleting the same now a little

and deleting the same now a little confusingly in SQL the corresponding

confusingly in SQL the corresponding functions or commands that exist that

functions or commands that exist that map to crud are actually this so it's

map to crud are actually this so it's still create but there's another one

still create but there's another one called insert uh it's not read which is

called insert uh it's not read which is more of the computer scientist way of

more of the computer scientist way of saying it but select which is a little

saying it but select which is a little more explicit like select data you care

more explicit like select data you care about update is still update delete is

about update is still update delete is still delete but there's another command

still delete but there's another command called drop which lets you drop that is

called drop which lets you drop that is delete entire tables as well so you can

delete entire tables as well so you can create tables using syntax that's

create tables using syntax that's generally going to look like this you'll

generally going to look like this you'll say create table you'll give the name of

say create table you'll give the name of the table uh which you can call almost

the table uh which you can call almost anything you want but generally all

anything you want but generally all lowercase no spaces is best then in

lowercase no spaces is best then in parentheses you can specify a comma

parentheses you can specify a comma separated list of the columns that you

separated list of the columns that you might want in this table so this is the

might want in this table so this is the code equivalent in the SQL language of

code equivalent in the SQL language of like manually opening Google Sheets or

like manually opening Google Sheets or Excel or numbers and like clicking in

Excel or numbers and like clicking in the top left cell and like typing

the top left cell and like typing timestamp and then in the next typing

timestamp and then in the next typing language and then in the third typing

language and then in the third typing problem this is the way to sort of

problem this is the way to sort of Define what your headers are if you will

Define what your headers are if you will in a spreadsheet but now it's called a

in a spreadsheet but now it's called a table now we won't use this command

table now we won't use this command manually first let's do something a

manually first let's do something a little simpler we're going to start off

little simpler we're going to start off by just importing this data ourselves

by just importing this data ourselves and I'm going to go ahead and do this

and I'm going to go ahead and do this let me go back to vs code here I'm going

let me go back to vs code here I'm going to leave behind favorites. py for now

to leave behind favorites. py for now because now we're going to transition to

because now we're going to transition to this other language called SQL and to do

this other language called SQL and to do this I am going to create a new database

this I am going to create a new database file and I'm going to do so using a

file and I'm going to do so using a command called SQL light 3 which is just

command called SQL light 3 which is just the third version thereof and I'm going

the third version thereof and I'm going to give the database a name of

to give the database a name of favorites. DB there's different

favorites. DB there's different conventions but this is one of the most

conventions but this is one of the most common when I hit enter this is going to

common when I hit enter this is going to create for me a new empty database just

create for me a new empty database just like opening an Untitled spreadsheet in

like opening an Untitled spreadsheet in Excel Google Sheets or Apple Numbers I'm

Excel Google Sheets or Apple Numbers I'm being prompted do I want to create

being prompted do I want to create favorites. DB I'll hit y for yes okay

favorites. DB I'll hit y for yes okay we're up and running now you're going to

we're up and running now you're going to notice a different prompt I'm not in my

notice a different prompt I'm not in my Linux prompt per se which is always the

Linux prompt per se which is always the dollar sign I'm now inside of the

dollar sign I'm now inside of the program called SQL light and we're going

program called SQL light and we're going to use SQL light SQL light 3 as just an

to use SQL light SQL light 3 as just an interactive way for now of playing with

interactive way for now of playing with SQL code at the end of today we'll show

SQL code at the end of today we'll show you how you can use SQL in Python code

you how you can use SQL in Python code so that you still write python code to

so that you still write python code to do whatever you want but you can talk to

do whatever you want but you can talk to databases using Python and this is

databases using Python and this is exactly how web apps mobile apps work

exactly how web apps mobile apps work for instance on iOS uh and an i phone an

for instance on iOS uh and an i phone an iPad or the like if you want to store

iPad or the like if you want to store data it's very often stored in a SQL

data it's very often stored in a SQL database as we're about to do um but you

database as we're about to do um but you might use a language called Swift or

might use a language called Swift or objective c and same exists in the world

objective c and same exists in the world of Android using Java or cotlin or

of Android using Java or cotlin or something else to query the database so

something else to query the database so we're going to see SQL in isolation for

we're going to see SQL in isolation for now like an analyst might just use at

now like an analyst might just use at their Mac or PC but we're going to tie

their Mac or PC but we're going to tie it together by Day end so at this

it together by Day end so at this terminal uh SQL light let me go ahead

terminal uh SQL light let me go ahead and execute uh this command first I'm

and execute uh this command first I'm going to first put SQL light into CSV

going to first put SQL light into CSV mode because I'm going to cut some

mode because I'm going to cut some Corners initially and I'm just going to

Corners initially and I'm just going to automatically import all of the data

automatically import all of the data that was submitted via that Google form

that was submitted via that Google form which I exported as a CSV and uploaded

which I exported as a CSV and uploaded to my code space and I'm just going to

to my code space and I'm just going to automatically say turn this CSV file

automatically say turn this CSV file into a SQL database for me just so I

into a SQL database for me just so I don't have to figure out what those

don't have to figure out what those create table commands are so to do this

create table commands are so to do this I'm going to say mode CSV so that SQL

I'm going to say mode CSV so that SQL light knows that this is the command uh

light knows that this is the command uh knows that this is a CSV file it's

knows that this is a CSV file it's literally mode so the dot comes before

literally mode so the dot comes before the keyword there and now I'm going to

the keyword there and now I'm going to say do import and then the name of the

say do import and then the name of the file I want to import which is

file I want to import which is favorites. CSV and now the name of the

favorites. CSV and now the name of the table that I want to create with that

table that I want to create with that data and just for consistency I'm going

data and just for consistency I'm going to call it favorites I could change

to call it favorites I could change these things to be anything I want but

these things to be anything I want but I'm going to do that and voila nothing

I'm going to do that and voila nothing seems to have happened but just like in

seems to have happened but just like in C and in Python in Linux when nothing

C and in Python in Linux when nothing seems to happen that's usually a good

seems to happen that's usually a good thing it means I didn't mess up so if I

thing it means I didn't mess up so if I want to see what just happened there's

want to see what just happened there's this other command and these commands

this other command and these commands that start with dots these are SQL light

that start with dots these are SQL light specific which is indeed a lightweight

specific which is indeed a lightweight version of SQL they're not SQL per se so

version of SQL they're not SQL per se so if you're using Oracle or something else

if you're using Oracle or something else like that you're not going to use these

like that you're not going to use these exact commands you'll see the ones we

exact commands you'll see the ones we use in just a moment and here's the

use in just a moment and here's the first when I type schema the schema of a

first when I type schema the schema of a database is the design of the database

database is the design of the database what are the tables what are the columns

what are the tables what are the columns and all of that so when I type schema

and all of that so when I type schema this actually in this case shows me that

this actually in this case shows me that create table command that was

create table command that was automatically run for me by just doing

automatically run for me by just doing this import line once I get more

this import line once I get more comfortable with SQL I could literally

comfortable with SQL I could literally type this out myself or use some program

type this out myself or use some program to generate that as well but what it's

to generate that as well but what it's creating for me is this create table if

creating for me is this create table if it doesn't exist even though it's more

it doesn't exist even though it's more tur than that I want to create a table

tur than that I want to create a table called favorites and then the columns

called favorites and then the columns for that table are going to be timestamp

for that table are going to be timestamp which is going to be text comma language

which is going to be text comma language which is also going to be text comma

which is also going to be text comma problem which is also going to be text

problem which is also going to be text that was just inferred very trivially by

that was just inferred very trivially by the do import command to just figure out

the do import command to just figure out that yes just give me a three column

that yes just give me a three column database table based on the Google

database table based on the Google form okay questions on this these are

form okay questions on this these are commands you run once to get up and

commands you run once to get up and running you don't run these commands

running you don't run these commands frequently but we have them on the slide

frequently but we have them on the slide just for reference all right so now

just for reference all right so now let's do something a little more

let's do something a little more interesting I'm going to clear my SQL

interesting I'm going to clear my SQL light terminal here but I'm still in SQL

light terminal here but I'm still in SQL light I'm going to now use some of my

light I'm going to now use some of my first SQL commands which recall were uh

first SQL commands which recall were uh were among them uh select so crud c r UD

were among them uh select so crud c r UD D the r was select this is maybe the

D the r was select this is maybe the most common the most useful the most

most common the most useful the most powerful thing to use with a SQL

powerful thing to use with a SQL database selecting data to answer

database selecting data to answer questions akin to the ones we were

questions akin to the ones we were trying to answer with python this is the

trying to answer with python this is the general syntax anytime you want to

general syntax anytime you want to select data from a SQL database you

select data from a SQL database you literally say select you then specify

literally say select you then specify the column or columns that you want to

the column or columns that you want to select data from you literally write the

select data from you literally write the word from and then you specify the name

word from and then you specify the name of the table you want to get that data

of the table you want to get that data from semicolon in this case everything

from semicolon in this case everything that's in capitals here is a sequel

that's in capitals here is a sequel keyword strictly speaking you don't have

keyword strictly speaking you don't have to capitalize things but we would

to capitalize things but we would encourage you to do so stylistically and

encourage you to do so stylistically and especially as you're learning and even

especially as you're learning and even as you're writing it it just helps to

as you're writing it it just helps to distinguish SQL from like words you

distinguish SQL from like words you chose like the names of the columns and

chose like the names of the columns and the data there're in so uh do adopt

the data there're in so uh do adopt early on this convention so let me go

early on this convention so let me go back now to my code space here I'm

back now to my code space here I'm running my terminal window with SQL

running my terminal window with SQL light 3 inside of it suppose that I just

light 3 inside of it suppose that I just want to get all of the data from the

want to get all of the data from the favorites table which was automatically

favorites table which was automatically imported let's do this select I want

imported let's do this select I want everything well I can do timestamp comma

everything well I can do timestamp comma language comma problem but you know what

language comma problem but you know what here's a uh convenience already if you

here's a uh convenience already if you want everything there's what's called a

want everything there's what's called a wild card character in SQL which is just

wild card character in SQL which is just a star in asterisk which means give me

a star in asterisk which means give me every column without my knowing even

every column without my knowing even what they're called let me go ahead now

what they're called let me go ahead now and say from favorite it's semicolon and

and say from favorite it's semicolon and this is the SQL way of opening the

this is the SQL way of opening the database iterating over every row

database iterating over every row they're in printing out every row

they're in printing out every row therein done so those three steps which

therein done so those three steps which was like nine lines of python code give

was like nine lines of python code give or take earlier is now one line of SQL I

or take earlier is now one line of SQL I hit enter there is all of the data so I

hit enter there is all of the data so I see now all of the data just outputed as

see now all of the data just outputed as a CSV here but it's not the CSV file

a CSV here but it's not the CSV file it's now actually the table and in fact

it's now actually the table and in fact just for good measure let me do this CU

just for good measure let me do this CU you'll see the behavior a little

you'll see the behavior a little different the next time we open the file

different the next time we open the file I've just exited out of SQL light 3 I'm

I've just exited out of SQL light 3 I'm going to rerun it but I'm not going to

going to rerun it but I'm not going to reimport the data or do anything like

reimport the data or do anything like that because my file now exists in fact

that because my file now exists in fact let me take one step back if I type LS

let me take one step back if I type LS at my Linux prompt there's my favorites.

at my Linux prompt there's my favorites. py from before there's my favorites. CSV

py from before there's my favorites. CSV from before and here's a third file that

from before and here's a third file that I did create a moment ago when I first

I did create a moment ago when I first ran SQL light 3 so the data is

ran SQL light 3 so the data is persistent it's not using Ram or memory

persistent it's not using Ram or memory anything I do now is save there so let's

anything I do now is save there so let's go ahead and rerun SQL light 3 with the

go ahead and rerun SQL light 3 with the same file but I'm not going to I don't

same file but I'm not going to I don't have to reort everything because the

have to reort everything because the file already exists let me now do that

file already exists let me now do that same thing again select star from

same thing again select star from favorites to get all of the data and

favorites to get all of the data and what you'll see now is the same data but

what you'll see now is the same data but it's a little prettier now because I

it's a little prettier now because I reran it I effectively disabled CSV mode

reran it I effectively disabled CSV mode this time and what I'm now seeing is the

this time and what I'm now seeing is the entire contents of this database table

entire contents of this database table called favorit now there's nothing new

called favorit now there's nothing new here but you're just seeing now like an

here but you're just seeing now like an asky or Unicode version of all of the

asky or Unicode version of all of the same data from that database well

same data from that database well suppose I want to get a subset of the

suppose I want to get a subset of the data well let me clear my screen and

data well let me clear my screen and just like in Linux I can hit contrl L

just like in Linux I can hit contrl L just to clean things up aesthetically

just to clean things up aesthetically suppose I want to get just the languages

suppose I want to get just the languages so I could do select language from

so I could do select language from favorites and this will now select not

favorites and this will now select not all three columns AKA star this will

all three columns AKA star this will only select the language column and all

only select the language column and all of the data they're in if I hit enter

of the data they're in if I hit enter voila now I just see those there no time

voila now I just see those there no time Stamps no problems it's just a slice of

Stamps no problems it's just a slice of the table if you will all right not that

the table if you will all right not that interesting still still because it's

interesting still still because it's just a big column of data but now things

just a big column of data but now things get more interesting it turns out in SQL

get more interesting it turns out in SQL that there are functions that come with

that there are functions that come with this language just like C just like

this language just like C just like python in SQL some of the more useful

python in SQL some of the more useful ones some of the simpler ones are these

ones some of the simpler ones are these here average count distinct lower Max

here average count distinct lower Max Min upper which pretty much uh do what

Min upper which pretty much uh do what they say and count is a particularly

they say and count is a particularly useful one let's start with that you

useful one let's start with that you know it's a reasonable question to be

know it's a reasonable question to be asked uh how many people submitted the

asked uh how many people submitted the Google form by the time I actually

Google form by the time I actually downloaded the CSV well why don't we go

downloaded the CSV well why don't we go ahead and do this let me go back to VSS

ahead and do this let me go back to VSS code here in my terminal window let me

code here in my terminal window let me select not star but the count of star so

select not star but the count of star so give me the count of the rows that are

give me the count of the rows that are being returned from the database called

being returned from the database called uh the database table called favorites

uh the database table called favorites now when I hit enter I'm not going to

now when I hit enter I'm not going to get all the data I'm just going to get

get all the data I'm just going to get simply a number 430 rows came back so

simply a number 430 rows came back so that's pretty good I now know how much

that's pretty good I now know how much data is in there well what languages

data is in there well what languages were in there well I could do select

were in there well I could do select language from favorit just as before but

language from favorit just as before but that's not that useful especially if I'm

that's not that useful especially if I'm inheriting the data like I'm the analyst

inheriting the data like I'm the analyst who's been handed a data set by my boss

who's been handed a data set by my boss and they want me to like crunch some

and they want me to like crunch some numbers okay I could like load this into

numbers okay I could like load this into Excel I could sort it but you can use

Excel I could sort it but you can use SQL now to answer pretty basic questions

SQL now to answer pretty basic questions too if you want to select the distinct

too if you want to select the distinct languages in the data set because you

languages in the data set because you didn't you weren't privy to the Google

didn't you weren't privy to the Google form let me go ahead and select only the

form let me go ahead and select only the distinct languages from the favorites

distinct languages from the favorites table and now I hit enter and I get back

table and now I hit enter and I get back a much more succinct answer just the

a much more succinct answer just the three languages in question not really

three languages in question not really that useful since I created the Google

that useful since I created the Google form but certainly if you're inheriting

form but certainly if you're inheriting data from someone else you've just

data from someone else you've just downloaded a data set at least now I'm

downloaded a data set at least now I'm arguably wrapping my mind around what's

arguably wrapping my mind around what's going on now this is not necessary for

going on now this is not necessary for such a small data set but I can combine

such a small data set but I can combine these things select the count of the

these things select the count of the distinct languages in this data set

distinct languages in this data set called favorites and now I should get

called favorites and now I should get back what

back what answer so hopefully indeed an answer

answer so hopefully indeed an answer called three and what you're getting

called three and what you're getting back notice aesthetically too is like a

back notice aesthetically too is like a mini temporary table when I asked for uh

mini temporary table when I asked for uh just the distinct languages what SQL

just the distinct languages what SQL hands me back is this temporary table in

hands me back is this temporary table in memory that has one column called

memory that has one column called language and then two row uh three rows

language and then two row uh three rows now this is not saved anywhere it's just

now this is not saved anywhere it's just executed ephemerally like this but

executed ephemerally like this but that's why it's depicted in this way

that's why it's depicted in this way what you're getting is subsets of your

what you're getting is subsets of your data smaller tables containing some of

data smaller tables containing some of your data and same thing down here this

your data and same thing down here this is like a crazy long

is like a crazy long uh column name you can rename it if you

uh column name you can rename it if you really want uh but uh that's all we're

really want uh but uh that's all we're seeing there and in fact if that's a

seeing there and in fact if that's a little ugly we can actually Alias These

little ugly we can actually Alias These Things N is a common uh name for a

Things N is a common uh name for a variable a number in any programming

variable a number in any programming language so I can actually alas this to

language so I can actually alas this to be a column called n hit enter and now

be a column called n hit enter and now I'm getting a tiny tiny table whose

I'm getting a tiny tiny table whose column is called n that just has the one

column is called n that just has the one value

value there all right questions on

there all right questions on these application of these functions

here no questions yeah say a little louder as oh as as

yeah say a little louder as oh as as literally in English so name this column

literally in English so name this column rename this column as this technically

rename this column as this technically it creates an alias for the column so

it creates an alias for the column so that's all

yeah exactly distinct will operate on whatever you handed in parentheses and

whatever you handed in parentheses and get rid of all of the duplicates giving

get rid of all of the duplicates giving you back just the unique

you back just the unique correct other questions here

yeah good question when you define an alias like n which I just did does it

alias like n which I just did does it become like a variable you can reuse

become like a variable you can reuse short answer no in this case but you can

short answer no in this case but you can reuse it within your same query even

reuse it within your same query even though these these queries are getting a

though these these queries are getting a little longer admittedly statements that

little longer admittedly statements that they are uh you can actually reuse n in

they are uh you can actually reuse n in even longer queries so later in your

even longer queries so later in your query and we'll see a few that are start

query and we'll see a few that are start to going to start to grow in length so

to going to start to grow in length so it's a a nice way of nicknaming things

it's a a nice way of nicknaming things just to be a little more tur in your

just to be a little more tur in your query so we can transition to some of

query so we can transition to some of these more sophisticated queries because

these more sophisticated queries because it turns out there's some other uh

it turns out there's some other uh techniques we can introduce as well here

techniques we can introduce as well here are some other keywords in SQL and again

are some other keywords in SQL and again even though like this is another list of

even though like this is another list of things there's only four things

things there's only four things fundamentally we're doing creating

fundamentally we're doing creating reading updating and deleting data these

reading updating and deleting data these are just allowing us to like fine-tune

are just allowing us to like fine-tune how we do it exactly so where is going

how we do it exactly so where is going to allow us to filter data as we'll do

to allow us to filter data as we'll do in just a moment like select data where

in just a moment like select data where this conditional is true uh like is

this conditional is true uh like is going to be an alternative to an equal

going to be an alternative to an equal sign so instead of looking for exactly

sign so instead of looking for exactly scratch or exactly python or exactly C

scratch or exactly python or exactly C you can look for something like dot dot

you can look for something like dot dot dot and it can be a little bit of a

dot and it can be a little bit of a fuzzier match if you will uh with other

fuzzier match if you will uh with other characters as well order by is going to

characters as well order by is going to deal with sorting limit is going to just

deal with sorting limit is going to just let me limit the total number of rows

let me limit the total number of rows that come back to one or 10 or finite if

that come back to one or 10 or finite if I don't want to see all 400 plus rows

I don't want to see all 400 plus rows all at once because I'm just trying to

all at once because I'm just trying to wrap my mind around it and group by is

wrap my mind around it and group by is best shown by example so let's play with

best shown by example so let's play with just a couple of these as well let me go

just a couple of these as well let me go back to vs code here I'll clear my

back to vs code here I'll clear my screen I'm still in the same SQL light

screen I'm still in the same SQL light instance and let's count how many of you

instance and let's count how many of you likeed C without writing python code as

likeed C without writing python code as before so let me go ahead and select the

before so let me go ahead and select the count of the rows from favorites where

count of the rows from favorites where the language in each row equals c and

the language in each row equals c and the convention in SQL light is to use

the convention in SQL light is to use single quotes anytime you're surrounding

single quotes anytime you're surrounding a string that's meant to represent a

a string that's meant to represent a literal piece of text uh as opposed to C

literal piece of text uh as opposed to C which was double quotes or python which

which was double quotes or python which was either so this is selecting the

was either so this is selecting the count of rows from favorites table where

count of rows from favorites table where the language in question is C enter and

the language in question is C enter and this gives me 98 notice though if I omit

this gives me 98 notice though if I omit that predicate like we did before you'll

that predicate like we did before you'll get back the total number of rows that

get back the total number of rows that were in the table so where is what's

were in the table so where is what's called a predicate that just allows me

called a predicate that just allows me to filter things just like an if

to filter things just like an if condition or the like in a language that

condition or the like in a language that we've seen before you can be a little

we've seen before you can be a little more specific like how many people

more specific like how many people really liked C and the Mario program uh

really liked C and the Mario program uh problem specifically well let's do this

problem specifically well let's do this uh let's go ahead and do select the

uh let's go ahead and do select the number of rows from the favorites table

number of rows from the favorites table where the language is C and so it's uh

where the language is C and so it's uh still literally the word ands and or

still literally the word ands and or just like in Python but not like in C uh

just like in Python but not like in C uh and

and equals Mario so let's see if there's any

equals Mario so let's see if there's any fans of both C and the Mario problem and

fans of both C and the Mario problem and three of us really like those two things

three of us really like those two things together in this case all right what

together in this case all right what else can we do well more compelling

else can we do well more compelling might be to see kind of like in Python

might be to see kind of like in Python for each language what was the

for each language what was the popularity thereof and at the moment we

popularity thereof and at the moment we don't really have a way of doing that

don't really have a way of doing that except in Python where we had the loop

except in Python where we had the loop and we had those variables with the

and we had those variables with the dictionary that did all that counting

dictionary that did all that counting for us you know totally doable but

for us you know totally doable but tedious especially if your job is to

tedious especially if your job is to analyze data my God like even writing 15

analyze data my God like even writing 15 lines of code to answer simple questions

lines of code to answer simple questions is kind of ridiculous SQL can do better

is kind of ridiculous SQL can do better for us so let me go ahead and do this

for us so let me go ahead and do this let me go ahead and select every

let me go ahead and select every language and the count thereof from the

language and the count thereof from the favorites table but this time Group by

favorites table but this time Group by language so this was another one of the

language so this was another one of the keywords that we can use in this

keywords that we can use in this abbreviated list of extra features of

abbreviated list of extra features of SQL and this one's a little takes a

SQL and this one's a little takes a moment to wrap your mind around but this

moment to wrap your mind around but this is going to give me a two column

is going to give me a two column temporary table where the First Column

temporary table where the First Column is a language and the second column is

is a language and the second column is the count thereof from this data set and

the count thereof from this data set and group by language just means that only

group by language just means that only show me scratch once only show me c once

show me scratch once only show me c once only show me python once that is group

only show me python once that is group all of the identical values together but

all of the identical values together but keep track of how many of them there are

keep track of how many of them there are and so now if I go over to SQL light and

and so now if I go over to SQL light and I hit enter now I have in SQL version

I hit enter now I have in SQL version the exact same output that I had from

the exact same output that I had from python that took me what 15 plus lines

python that took me what 15 plus lines before now we're down to just one

before now we're down to just one because SQL structured query language is

because SQL structured query language is all about constructing queries like this

all about constructing queries like this to answer questions and get back answers

to answer questions and get back answers quickly if we want to clean this up a

quickly if we want to clean this up a little bit you asked earlier about

little bit you asked earlier about sorting order well we can do that too

sorting order well we can do that too there's another uh key phrase we can use

there's another uh key phrase we can use here we can order by the count of those

here we can order by the count of those rows and then run that query here so now

rows and then run that query here so now unfortunately they're from smallest to

unfortunately they're from smallest to biggest but we can reverse that it turns

biggest but we can reverse that it turns out and my query is starting to wrap

out and my query is starting to wrap here I'll I'll zoom out for a moment if

here I'll I'll zoom out for a moment if you want to order by count the default

you want to order by count the default is in ascending order abbreviated ASC if

is in ascending order abbreviated ASC if you want to reverse the sort in SQL

you want to reverse the sort in SQL instead of using reverse equals true

instead of using reverse equals true like we did in Python you say DC for

like we did in Python you say DC for descending order and now we get almost

descending order and now we get almost the same output but flipped in Reverse

the same output but flipped in Reverse so it's just a lot faster to answer

so it's just a lot faster to answer questions once of course you get some

questions once of course you get some muscle memory and some comfort with it

muscle memory and some comfort with it well what else can I do you know what if

well what else can I do you know what if I just care about the most popular

I just care about the most popular language I don't care about the second

language I don't care about the second place or the third place languages or

place or the third place languages or anything else well let me add one more

anything else well let me add one more Clause here limit the answer to one and

Clause here limit the answer to one and no matter how many rows should come back

no matter how many rows should come back now I just get the number one language

now I just get the number one language as of the data set we collected with 27

as of the data set we collected with 27 uh 270 votes for

uh 270 votes for it

it questions on

questions on this any questions

this any questions here no well what if uh you know we're

here no well what if uh you know we're starting to introduce SQL and it was

starting to introduce SQL and it was kind of too late to make it into the

kind of too late to make it into the this the Google form so it turns out

this the Google form so it turns out there Syntax for this too you can create

there Syntax for this too you can create data of course not just the tables but

data of course not just the tables but the data they're in and here's like the

the data they're in and here's like the typical Syntax for inserting data into a

typical Syntax for inserting data into a SQL database you literally say insert

SQL database you literally say insert into the name of the table and then in

into the name of the table and then in parentheses you specify one or more

parentheses you specify one or more columns for which you have values that

columns for which you have values that you want to insert this is to say you

you want to insert this is to say you don't have to give values for every

don't have to give values for every column in the given row if you only have

column in the given row if you only have answers to some of those questions you

answers to some of those questions you can enumerate them here like this but

can enumerate them here like this but the values you insert are going to be

the values you insert are going to be these so you literally say after the

these so you literally say after the closed parenthesis values and then in a

closed parenthesis values and then in a second set of parentheses with the same

second set of parentheses with the same length comma separated list you specify

length comma separated list you specify what values do you want to insert so

what values do you want to insert so it's a little verbose and frankly longer

it's a little verbose and frankly longer term you're going to use like python

term you're going to use like python code to automatically do these kinds of

code to automatically do these kinds of insertions but let's go ahead and try

insertions but let's go ahead and try this right now if I do select

this right now if I do select distinct uh language from favorites

distinct uh language from favorites again we see this just these three

again we see this just these three candidates but we've now taught you a

candidates but we've now taught you a bit of sequel so let's do insert into

bit of sequel so let's do insert into favorites the column called

favorites the column called language uh and you know what let's I'm

language uh and you know what let's I'm going to give a problem here the values

going to give a problem here the values for which and let me Zoom back out are

for which and let me Zoom back out are going to be quote unquote SQL and quote

going to be quote unquote SQL and quote unquote 50v you'll see soon see what

unquote 50v you'll see soon see what that's all about semicolon nothing seems

that's all about semicolon nothing seems to happen but that's usually a good

to happen but that's usually a good thing and now if I scroll back up in my

thing and now if I scroll back up in my my queries in SQL light three you can

my queries in SQL light three you can scroll back and forth in time and uh to

scroll back and forth in time and uh to avoid retyping things now I should see

avoid retyping things now I should see indeed four candidate languages here now

indeed four candidate languages here now suppose that you were never really a fan

suppose that you were never really a fan of c and maybe you uh programmed a

of c and maybe you uh programmed a little bit in high school or in the real

little bit in high school or in the real world and you liked C++ well there's a

world and you liked C++ well there's a whole lot of answers for C so select

whole lot of answers for C so select star from favorites where language

star from favorites where language equals quote unquote C so here's

equals quote unquote C so here's everyone who submitted the answer for C

everyone who submitted the answer for C let's presume that no they didn't really

let's presume that no they didn't really want C they wanted C++ which is not a

want C they wanted C++ which is not a language we teach in the class but I

language we teach in the class but I could also now do this you can use the

could also now do this you can use the update command to set a column or

update command to set a column or columns to different values where some

columns to different values where some condition is met so if I do update table

condition is met so if I do update table name set column name equal to some value

name set column name equal to some value filtering it perhaps by where some

filtering it perhaps by where some condition is true so suppose I've

condition is true so suppose I've changed my mind or you know what let's

changed my mind or you know what let's go ahead and do

go ahead and do update uh favorites set language equal

update uh favorites set language equal to maybe C++ where language equals c now

to maybe C++ where language equals c now this is destructive so you generally

this is destructive so you generally don't want to do this unless you have a

don't want to do this unless you have a backup of your data too overriding what

backup of your data too overriding what people's answers are this seems to have

people's answers are this seems to have been successful because no error

been successful because no error messages and if I rerun the previous

messages and if I rerun the previous select that gives me all of the

select that gives me all of the favorites where language equals c now

favorites where language equals c now indeed I get none but if I search for

indeed I get none but if I search for C++ now I get a lot and if I get rid of

C++ now I get a lot and if I get rid of that wear Clause altogether and just

that wear Clause altogether and just look at the contents of my database now

look at the contents of my database now you see that indeed C++ is comingled

you see that indeed C++ is comingled with all the other data this is not what

with all the other data this is not what you all intended of course so I can undo

you all intended of course so I can undo this let me go ahead and undo what I

this let me go ahead and undo what I just did let me set my favorite language

just did let me set my favorite language to C where language equals C plus plus

to C where language equals C plus plus but the predicate is important this I'm

but the predicate is important this I'm not going to do what if I accidentally

not going to do what if I accidentally omitted this predicate the wear Clause

omitted this predicate the wear Clause how would that screw things up might you

think uh yeah I'm back it would set Every Rose language to

back it would set Every Rose language to indeed see and this is dangerous and if

indeed see and this is dangerous and if you start googling around for like

you start googling around for like sequel mistakes or the like people in

sequel mistakes or the like people in the real world have accidentally run

the real world have accidentally run commands like this and without naming

commands like this and without naming names a former member of our teaching

names a former member of our teaching staff at one point accidentally re ran a

staff at one point accidentally re ran a command like this and changed every

command like this and changed every student's name in our database to Bobby

student's name in our database to Bobby I think it was the same name for every

I think it was the same name for every row because they simply forgot a

row because they simply forgot a predicate so here to two like there's

predicate so here to two like there's dangers in code and you should adopt The

dangers in code and you should adopt The Habit quite quickly of always one

Habit quite quickly of always one backing up your data like with CP for

backing up your data like with CP for instance in Linux or any other technique

instance in Linux or any other technique or just making sure before you hit enter

or just making sure before you hit enter that yes this is indeed the query I want

that yes this is indeed the query I want to execute and generally speaking in the

to execute and generally speaking in the real world there should be process

real world there should be process controls in place like the intern should

controls in place like the intern should not have access to the datab the

not have access to the datab the production database the live database

production database the live database and the like but you have a lot of power

and the like but you have a lot of power now with these queries so just be all

now with these queries so just be all the more careful cuz very easily can you

the more careful cuz very easily can you do bad things so let me undo this where

do bad things so let me undo this where language equals quote unquote C++ and

language equals quote unquote C++ and I'll Zoom back out enter and now I think

I'll Zoom back out enter and now I think we're back in business C is among the

we're back in business C is among the answers

yeah is essentially doing what at the end replace it's essentially find and

end replace it's essentially find and replace yes in like lay person's terms

replace yes in like lay person's terms this is find and replace implemented

this is find and replace implemented with SQL and in fact the authors of

with SQL and in fact the authors of Microsoft Word or Google Docs might very

Microsoft Word or Google Docs might very well be using language like this sequel

well be using language like this sequel when you go to the nice graphical

when you go to the nice graphical userfriendly find and replace box this

userfriendly find and replace box this may very well be what they're doing

may very well be what they're doing underneath the hood or of course they

underneath the hood or of course they could be using some other language

could be using some other language altogether there's one last uh syntax

altogether there's one last uh syntax that's worth knowing delete which for

that's worth knowing delete which for better for worse is even more

better for worse is even more destructive whereby it allows you to

destructive whereby it allows you to delete rows from tables it's distinct

delete rows from tables it's distinct from drop which lets you delete tables

from drop which lets you delete tables themselves this focuses on rows so

themselves this focuses on rows so suppose that you really really didn't

suppose that you really really didn't like let's say uh tedin was a little

like let's say uh tedin was a little challenging if you adopt if you tackle

challenging if you adopt if you tackle that more comfortable problem so if you

that more comfortable problem so if you really don't want to even think about t

really don't want to even think about t in anymore so why don't we do uh delete

in anymore so why don't we do uh delete from favorites where problem equals and

from favorites where problem equals and I won't execute it for real tiamin this

I won't execute it for real tiamin this would have the effect of deleting every

would have the effect of deleting every row including the language therein and

row including the language therein and the time stamp where the student

the time stamp where the student answered tiamin worse than this would be

answered tiamin worse than this would be this why might this be bad

okay chuckling because like there's no predicate there's no filter which means

predicate there's no filter which means literally this would delete all of the

literally this would delete all of the data so again with great power here

data so again with great power here comes great responsibility now this has

comes great responsibility now this has just been a data set of what 430 rows by

just been a data set of what 430 rows by us dynamically created there's of course

us dynamically created there's of course some really juicy data sets in the real

some really juicy data sets in the real world and one website you might have

world and one website you might have heard or an app you might have used is

heard or an app you might have used is IMDb the internet movie database which

IMDb the internet movie database which wonderfully makes some of their data

wonderfully makes some of their data available for download as CSV files or

available for download as CSV files or technically tsv files tab separated

technically tsv files tab separated values but what we did in advance of

values but what we did in advance of class was download some of that data for

class was download some of that data for both TV shows in the real world and

both TV shows in the real world and movies in the real world and what's

movies in the real world and what's wonderful about this data set is it's

wonderful about this data set is it's not just dozens or hundreds or even

not just dozens or hundreds or even thousands of lines there are millions of

thousands of lines there are millions of rows of Juicy data TV shows and movies

rows of Juicy data TV shows and movies with which most folks are probably

with which most folks are probably familiar at least with the subset and

familiar at least with the subset and we'll see in just a little bit that this

we'll see in just a little bit that this data comes in the form of now six

data comes in the form of now six different tables that we've given you

different tables that we've given you and the tables and question for today

and the tables and question for today are going to be the people in the TV

are going to be the people in the TV business the Stars they in the shows

business the Stars they in the shows that people are producing and the like

that people are producing and the like this is a picture we'll revisit to let

this is a picture we'll revisit to let enable you to wrap your minds around

enable you to wrap your minds around what the actual data is this feels like

what the actual data is this feels like a good opportunity though for a snack in

a good opportunity though for a snack in fact in just a moment we have a whole

fact in just a moment we have a whole lot of Rice Krispie treats out in the

lot of Rice Krispie treats out in the lobby but if folks could perhaps

lobby but if folks could perhaps acknowledge uh this mini wedding cake

acknowledge uh this mini wedding cake here cs50 own Carter zeni is getting

here cs50 own Carter zeni is getting married this week so congratulations to

married this week so congratulations to Carter

Carter as

as well

well congrats all right there's

only okay there's only one piece of cake in that box but a lot of Rice Krispy

in that box but a lot of Rice Krispy Treats in the transip let's take 10

Treats in the transip let's take 10 minutes and we'll be back with Internet

minutes and we'll be back with Internet Movie Database in 10 all right we are

Movie Database in 10 all right we are back so if you've never been like you

back so if you've never been like you can actually go to I imdb.com right now

can actually go to I imdb.com right now and play around or download the mobile

and play around or download the mobile app and it's just big database of a lot

app and it's just big database of a lot of TV show and movies and actors and the

of TV show and movies and actors and the like but what indeed is nice is you can

like but what indeed is nice is you can download some of that data and that's in

download some of that data and that's in what I've done in advance and what we've

what I've done in advance and what we've done is we wrote some python code to

done is we wrote some python code to convert some of the uh flat file

convert some of the uh flat file databases that they let you download and

databases that they let you download and we converted it into a SQL database with

we converted it into a SQL database with six tables so not just one but six that

six tables so not just one but six that ultimately are these here and let me

ultimately are these here and let me just help you wrap your minds around

just help you wrap your minds around what this picture is which is a entity

what this picture is which is a entity relationship diagram which is just to

relationship diagram which is just to say each of these boxes on the screen

say each of these boxes on the screen represents a table and each of the

represents a table and each of the arrows or edges represents some kind of

arrows or edges represents some kind of relationship across the tables because

relationship across the tables because up until now the only data we had were

up until now the only data we had were those three columns in the favorites

those three columns in the favorites table but what's gets really useful

table but what's gets really useful about SQL databases just like a Google

about SQL databases just like a Google spreadsheet or an Excel file is you can

spreadsheet or an Excel file is you can have multiple sheets or in a database

have multiple sheets or in a database multiple tables and so what we're about

multiple tables and so what we're about to see is that in this IMDb database for

to see is that in this IMDb database for TV shows there's going to be a dedicated

TV shows there's going to be a dedicated table for all the people in the TV

table for all the people in the TV business there's going to be a dedicated

business there's going to be a dedicated table for all of the TV shows that are

table for all of the TV shows that are in their database as of right now

in their database as of right now there's going to be a dedicated table

there's going to be a dedicated table for writers in that industry for the

for writers in that industry for the ratings of uh shows for the genres to

ratings of uh shows for the genres to which shows belong comedy and the like

which shows belong comedy and the like and then lastly there's going to be this

and then lastly there's going to be this table which somehow

table which somehow Associates people with the TV shows that

Associates people with the TV shows that they star in and vice versa and so let's

they star in and vice versa and so let's consider first what this looks like in

consider first what this looks like in code and we'll see that it's going to

code and we'll see that it's going to overwhelm intentionally at first but I'm

overwhelm intentionally at first but I'm going to do this I'm going to go back to

going to do this I'm going to go back to my terminal window and during the break

my terminal window and during the break I downloaded from the course's website a

I downloaded from the course's website a file called shows. DB which we made in

file called shows. DB which we made in advance for you and if I type LS I'll

advance for you and if I type LS I'll see all of my favorites files from

see all of my favorites files from before the CSV the DB and the python

before the CSV the DB and the python file but now there's shows. DB so I'm

file but now there's shows. DB so I'm going to go ahead in my full screen

going to go ahead in my full screen terminal window here I'm not using

terminal window here I'm not using actual tabs or code files now I'm going

actual tabs or code files now I'm going to run SQL light3 on the file called

to run SQL light3 on the file called shows. DB and I'm just going to see this

shows. DB and I'm just going to see this version information here let me clear my

version information here let me clear my screen and run the one command I ran

screen and run the one command I ran earlier to show us the schema of the

earlier to show us the schema of the favorites database now we'll see the

favorites database now we'll see the schema for the shows database and

schema for the shows database and there's a lot going on here but let me

there's a lot going on here but let me scroll back up to the very top the

scroll back up to the very top the beginning and we see this here so when I

beginning and we see this here so when I run. schema we see a dump really of all

run. schema we see a dump really of all of the SQL create table commands that

of the SQL create table commands that were run in order to create this

were run in order to create this database for you and one of those tables

database for you and one of those tables is called genres and another people

is called genres and another people ratings shows stars and so forth and the

ratings shows stars and so forth and the columns therein even though it's

columns therein even though it's formatted a little more prettily than

formatted a little more prettily than the automatically generated create table

the automatically generated create table statement for favorites whereby we have

statement for favorites whereby we have one column per line of output here uh in

one column per line of output here uh in the for instance people table there's

the for instance people table there's going to be an ID column like unique

going to be an ID column like unique identifier like a Harvard ID a Yale ID

identifier like a Harvard ID a Yale ID or the like uh a name column a birth

or the like uh a name column a birth year and then some other stuff if I

year and then some other stuff if I scroll down to shows every show in the

scroll down to shows every show in the world is going to have a unique ID as

world is going to have a unique ID as well a title of course the year in which

well a title of course the year in which it debuted and the total number of

it debuted and the total number of episodes as of the time we downloaded

episodes as of the time we downloaded the data and then what else is there

the data and then what else is there some of these are a little less obvious

some of these are a little less obvious like ratings here so ratings don't have

like ratings here so ratings don't have an ID column but they have a show ID

an ID column but they have a show ID column and a rating like a fivepoint

column and a rating like a fivepoint scale or 10o scale or the like and then

scale or 10o scale or the like and then the total number of votes that were

the total number of votes that were collected to contribute to that rating

collected to contribute to that rating IMDb allows people to like up vote and

IMDb allows people to like up vote and down vote uh shows and movies and the

down vote uh shows and movies and the like and then similarly is genre

like and then similarly is genre structured there's a show ID and then

structured there's a show ID and then there's a genre which is going to be

there's a genre which is going to be like an English word like comedy or

like an English word like comedy or drama or something else and then what

drama or something else and then what else let's go a little further at the

else let's go a little further at the bottom here for stars and writers if we

bottom here for stars and writers if we go to the very bottom here stars and

go to the very bottom here stars and writers are similarly structured too

writers are similarly structured too they have a show ID and a person ID so

they have a show ID and a person ID so show and person and then this writer's

show and person and then this writer's table has a show ID and a person ID and

table has a show ID and a person ID and there's a whole lot of other words that

there's a whole lot of other words that we'll come to in just a moment but what

we'll come to in just a moment but what are the what is this code hinting at

are the what is this code hinting at well if I go back to the picture from

well if I go back to the picture from earlier here you'll see that this

earlier here you'll see that this picture captures the relationships among

picture captures the relationships among these various tables so for instance if

these various tables so for instance if we focus on shows for just a moment a

we focus on shows for just a moment a show again has a unique ID a title a

show again has a unique ID a title a year in which it debuted and a total

year in which it debuted and a total number of episodes if you want to figure

number of episodes if you want to figure out what genre or genr a show belongs to

out what genre or genr a show belongs to cuz some shows are just comedies some

cuz some shows are just comedies some shows are just just dramas but you know

shows are just just dramas but you know some shows are arguably comedies and

some shows are arguably comedies and dramas depending on the episode or the

dramas depending on the episode or the like so you can imagine wanting to

like so you can imagine wanting to associate two or three or even more

associate two or three or even more genres with a show this line here in

genres with a show this line here in this second table allows us to do that

this second table allows us to do that every Row in the genres table we'll see

every Row in the genres table we'll see has uh two items a show ID which relates

has uh two items a show ID which relates to the ID of a show and that's why these

to the ID of a show and that's why these lines literally line up with that

lines literally line up with that specific column name and genre which is

specific column name and genre which is going to be like quote unquote comedy

going to be like quote unquote comedy quote unquote drama or something else

quote unquote drama or something else now with that said design question why

now with that said design question why have we deliberately not just gotten rid

have we deliberately not just gotten rid of this genre's table and made our lives

of this genre's table and made our lives simpler by just adding a genre column to

simpler by just adding a genre column to this shows table and again a table is

this shows table and again a table is just like a sheet with rows and columns

just like a sheet with rows and columns at the moment shows only have four

at the moment shows only have four columns ID title year episodes why not

columns ID title year episodes why not just add a fifth column called genre and

just add a fifth column called genre and put the show genre

put the show genre there any intuition

there any intuition here why not just keep things simple

here why not just keep things simple like yeah and

back exactly if you add a fifth column here and call it genre then you have to

here and call it genre then you have to pick a genre specifically you have to

pick a genre specifically you have to put in that cell presumably comedy or

put in that cell presumably comedy or drama or music or something else now you

drama or music or something else now you could write multiple words in the cell

could write multiple words in the cell but generally speaking that would be

but generally speaking that would be sloppy bad design like every cell just

sloppy bad design like every cell just like in a spreadsheet should really have

like in a spreadsheet should really have one value it might have multiple words

one value it might have multiple words but it shouldn't be like a weirdly comma

but it shouldn't be like a weirdly comma separated list of multiple things it

separated list of multiple things it should just be in a different cell in

should just be in a different cell in that case so if you instead were to

that case so if you instead were to design this with just a single column

design this with just a single column called genre you're imposing what a

called genre you're imposing what a computer scientist would call a on toone

computer scientist would call a on toone relationship every show has one genre

relationship every show has one genre and that's not necessarily a good thing

and that's not necessarily a good thing or strictly speaking it would be a many

or strictly speaking it would be a many to one because the same genre could

to one because the same genre could belong to multiple shows but each show

belong to multiple shows but each show could only have one genre in that case

could only have one genre in that case what a relational database allows you to

what a relational database allows you to do and relational is indeed the

do and relational is indeed the operative word it allows you to factor

operative word it allows you to factor out some of your information and then

out some of your information and then have maybe one show here in one row but

have maybe one show here in one row but then in this genres table you could have

then in this genres table you could have one row for that one show genre or you

one row for that one show genre or you could have two rows in the genres table

could have two rows in the genres table for comedy and for drama or if it has a

for comedy and for drama or if it has a third genre you could just add another

third genre you could just add another row here so you still have one row for

row here so you still have one row for the show itself with all the juiciest

the show itself with all the juiciest details but a variable number of rows by

details but a variable number of rows by having this relationship with another

having this relationship with another table meanwhile ratings work the same

table meanwhile ratings work the same way at least in this case a show has ID

way at least in this case a show has ID title year and episodes but if you want

title year and episodes but if you want to figure out its rating you have to

to figure out its rating you have to kind of Follow the arrow here so to

kind of Follow the arrow here so to speak and look up the corresponding show

speak and look up the corresponding show ID in this table find the rating of that

ID in this table find the rating of that show and the total number of ratings so

show and the total number of ratings so that's been factored out Two For Better

that's been factored out Two For Better or For Worse um now let's consider

or For Worse um now let's consider people people have just three columns ID

people people have just three columns ID name and birth but there's no mention of

name and birth but there's no mention of the TV show in which people have starred

the TV show in which people have starred or the TV shows that a person has

or the TV shows that a person has written well why is that well if you

written well why is that well if you just had a fourth column here called

just had a fourth column here called show well you would have to decide what

show well you would have to decide what show is that person in and no one could

show is that person in and no one could ever act again in another show because

ever act again in another show because there's no room to store the data but if

there's no room to store the data but if someone of course a popular actor can St

someone of course a popular actor can St star in multiple shows well we could

star in multiple shows well we could have one ID for that person one name one

have one ID for that person one name one birth year obviously like there's only

birth year obviously like there's only one Steve Carell as an actor in the

one Steve Carell as an actor in the world of people but Steve Carell in this

world of people but Steve Carell in this example could have his person ID

example could have his person ID whatever his Harvard ID equivalent Yale

whatever his Harvard ID equivalent Yale ID equivalent is appear in multiple rows

ID equivalent is appear in multiple rows in this table so that it can be

in this table so that it can be associated with multiple shows and this

associated with multiple shows and this allows you to create what's called a one

allows you to create what's called a one to many relationship or technically it's

to many relationship or technically it's bidirectional it's a many to many

bidirectional it's a many to many relationship why well one show can

relationship why well one show can certain certainly have multiple people

certain certainly have multiple people in it and multiple people writing for it

in it and multiple people writing for it just in the real world but conversely

just in the real world but conversely one uh person could certainly act in

one uh person could certainly act in multiple shows or write multiple shows

multiple shows or write multiple shows so this is what you get with relational

so this is what you get with relational databases you put your sort of canonical

databases you put your sort of canonical data for people in one place for for

data for people in one place for for shows in another place and then you use

shows in another place and then you use these additional tables to relate one

these additional tables to relate one thing to another so we won't dwell on

thing to another so we won't dwell on the pictures that's just if you sort of

the pictures that's just if you sort of uh can wrap your mind around the data

uh can wrap your mind around the data set better that way that's one way of

set better that way that's one way of thinking about it but recall that the

thinking about it but recall that the code we just saw for the schema again

code we just saw for the schema again escalated quickly like there's a lot of

escalated quickly like there's a lot of keywords I haven't mentioned yet but

keywords I haven't mentioned yet but some of these are perhaps familiar

some of these are perhaps familiar they're capitalized differently here but

they're capitalized differently here but integer is on the list here null is on

integer is on the list here null is on the list albeit technically not null so

the list albeit technically not null so let's tease apart some of these key

let's tease apart some of these key words and consider what they're actually

words and consider what they're actually doing for your database because now

doing for your database because now we're in exploring features that do not

we're in exploring features that do not exist in the world of spreadsheets alone

exist in the world of spreadsheets alone so it turns out in a SQL database

so it turns out in a SQL database specifically SQL light which is the

specifically SQL light which is the version of SQL we use in cs50 and which

version of SQL we use in cs50 and which is commonly used for things like mobile

is commonly used for things like mobile applications nowadays it's like a

applications nowadays it's like a lightweight version of SQL it's when you

lightweight version of SQL it's when you aren't trying to run Twitter and have

aren't trying to run Twitter and have billions and billions of rows

billions and billions of rows necessarily you've got hundreds

necessarily you've got hundreds thousands tens of thousands maybe even a

thousands tens of thousands maybe even a few million but not crazy uh numbers uh

few million but not crazy uh numbers uh crazy amounts of data in the world of

crazy amounts of data in the world of SQL light specifically there's these

SQL light specifically there's these five data types so just like in C we had

five data types so just like in C we had int and Char and the like in SQL we have

int and Char and the like in SQL we have these uh BL which is kind of funny but

these uh BL which is kind of funny but it just means binary large object so

it just means binary large object so it's like a binary data type zeros and

it's like a binary data type zeros and ones that aren't necessarily uh fitting

ones that aren't necessarily uh fitting into the other categories integer which

into the other categories integer which of course is an integer as we know it

of course is an integer as we know it numeric which is kind of a catchall for

numeric which is kind of a catchall for numbers that are formatted specially so

numbers that are formatted specially so like a date uh would be like year year

like a date uh would be like year year year year-month month Dash day day um

year year-month month Dash day day um and this is actually a wonderful thing

and this is actually a wonderful thing depending on the country you're from you

depending on the country you're from you might think your date system in your

might think your date system in your country is great or it's horrible the US

country is great or it's horrible the US system is horrible because we have month

system is horrible because we have month day and then year which is impossible to

day and then year which is impossible to sort it is the wrong way objectively to

sort it is the wrong way objectively to store data and yet here we are using

store data and yet here we are using this at scale other countries have

this at scale other countries have gotten this better numeric and SQL

gotten this better numeric and SQL itself standardizes that stuff so it

itself standardizes that stuff so it doesn't matter what country you're from

doesn't matter what country you're from you're storing your data in this

you're storing your data in this particular way for instance times are

particular way for instance times are standardized and other types of numeric

standardized and other types of numeric data as well real is synonymous with

data as well real is synonymous with float so something with a decimal point

float so something with a decimal point and some number of uh digits thereafter

and some number of uh digits thereafter and then text is just uh for strings and

and then text is just uh for strings and the like with other even fancier

the like with other even fancier databases like MySQL postgress SQL

databases like MySQL postgress SQL Oracle and other products you might have

Oracle and other products you might have heard of there's even more data types

heard of there's even more data types where you have to make even finer grain

where you have to make even finer grain decisions but for SQL light it's indeed

decisions but for SQL light it's indeed pretty lightweight and you or we just

pretty lightweight and you or we just have to decide the data types for each

have to decide the data types for each column in a table but there's these

column in a table but there's these additional constraints in the world of

additional constraints in the world of SQL you can additionally say that cells

SQL you can additionally say that cells in this column may or may not be null so

in this column may or may not be null so if you want to protect yourself from

if you want to protect yourself from yourself so you don't screw up and

yourself so you don't screw up and insert a null that is a blank value you

insert a null that is a blank value you can explicitly design a table to have a

can explicitly design a table to have a column that cannot be null and so in

column that cannot be null and so in fact someone came up during the break to

fact someone came up during the break to ask me about my having manually inserted

ask me about my having manually inserted SQL quote unquote SQL into our favorites

SQL quote unquote SQL into our favorites database you might recall that I kind of

database you might recall that I kind of cheated I just inserted uh SQL quote

cheated I just inserted uh SQL quote unquote and 50v the name of a new

unquote and 50v the name of a new problem quote unquote but what did I not

problem quote unquote but what did I not insert into the

insert into the database a Tim stamp and I could have I

database a Tim stamp and I could have I could have put like the current day in

could have put like the current day in time a few minutes ago but I didn't and

time a few minutes ago but I didn't and that's fine if it's uh if it's

that's fine if it's uh if it's acceptable to you and the product you're

acceptable to you and the product you're building but I could have prevented that

building but I could have prevented that if we had defined the table to have a

if we had defined the table to have a timestamp column that isn't just text

timestamp column that isn't just text but it's text that's not null SQL would

but it's text that's not null SQL would have complained and would not have let

have complained and would not have let me complete that insertion so there's

me complete that insertion so there's these kinds of built-in defenses that

these kinds of built-in defenses that you don't necessarily get with a

you don't necessarily get with a spreadsheet alone and unique means

spreadsheet alone and unique means exactly that if you want to make sure

exactly that if you want to make sure that every Row in that column is unique

that every Row in that column is unique maybe for email addresses or in the US

maybe for email addresses or in the US Social Security numbers or anything that

Social Security numbers or anything that you want to make sure you don't have two

you want to make sure you don't have two versions of you can specify that the

versions of you can specify that the column is unique and there's other such

column is unique and there's other such constraints as well but again this is

constraints as well but again this is just a list of features that you get

just a list of features that you get from a proper relational database but

from a proper relational database but perhaps the most intellectually

perhaps the most intellectually interesting one and the most powerful

interesting one and the most powerful one is what's called here a primary key

one is what's called here a primary key and a foreign key and let me go back now

and a foreign key and let me go back now to this output if we look at shows

to this output if we look at shows you'll see that a show again has an ID a

you'll see that a show again has an ID a title a year and a number of episodes

title a year and a number of episodes and now the data types might make sense

and now the data types might make sense the ID it turns out just like a Harvard

the ID it turns out just like a Harvard ID a Yale ID is going to be an integer

ID a Yale ID is going to be an integer so a simple number the title of course

so a simple number the title of course is going to be text but not null like it

is going to be text but not null like it would be weird if a TV show had no name

would be weird if a TV show had no name like that can't be like every the whole

like that can't be like every the whole world would break or your TV gu and

world would break or your TV gu and whatnot so that makes sense there to say

whatnot so that makes sense there to say not null uh year is numeric so it's a

not null uh year is numeric so it's a standardized form episodes is an integer

standardized form episodes is an integer like how many episodes have been

like how many episodes have been produced and then lastly notice this the

produced and then lastly notice this the primary key of the show's table is

primary key of the show's table is apparently the column called ID

apparently the column called ID mentioned a few lines earlier this just

mentioned a few lines earlier this just means that the database will use the ID

means that the database will use the ID column as the unique identifier so it's

column as the unique identifier so it's similar to the unique keyword but

similar to the unique keyword but primary key just means the database is

primary key just means the database is going to treat it as special too and

going to treat it as special too and make sure that it is uniquely

make sure that it is uniquely identifying your data but what's

identifying your data but what's interesting is this notice if I scroll

interesting is this notice if I scroll back up to people people were sort of

back up to people people were sort of similarly structured but with different

similarly structured but with different attributes like up here we had a person

attributes like up here we had a person has an ID a name a birth year and a

has an ID a name a birth year and a primary key of ID so a ID is again

primary key of ID so a ID is again integer name is text but not null

integer name is text but not null because it'd be weird to have a human

because it'd be weird to have a human with absolutely no name textually birth

with absolutely no name textually birth is going to be numeric but the primary

is going to be numeric but the primary key of people is ID as well so those are

key of people is ID as well so those are the unique columns that the database

the unique columns that the database will just treat special why well we just

will just treat special why well we just looked at shows we just looked at people

looked at shows we just looked at people let's focus now on this one down here

let's focus now on this one down here Stars how do you determine who star in a

Stars how do you determine who star in a TV show well we had two columns the show

TV show well we had two columns the show ID and the person ID this is the ex the

ID and the person ID this is the ex the Incarnation of a many to- many

Incarnation of a many to- many relationship one person could be in many

relationship one person could be in many shows one show could certainly have many

shows one show could certainly have many people in it or writing for it but

people in it or writing for it but notice this within this table of two

notice this within this table of two columns show ID and person ID there's

columns show ID and person ID there's what's going to be called a foreign key

what's going to be called a foreign key called show ID that references the shows

called show ID that references the shows tables ID column and then another

tables ID column and then another foreign key called person ID though I

foreign key called person ID though I could call these things in parentheses

could call these things in parentheses anything I want that references the

anything I want that references the people tables ID column now you're not

people tables ID column now you're not going to often have to type commands

going to often have to type commands like this again you set the database up

like this again you set the database up once in the beginning typically maybe

once in the beginning typically maybe with some help from a TF maybe with help

with some help from a TF maybe with help of Google or the like but once your

of Google or the like but once your database is designed it's back to the

database is designed it's back to the crud like create read update delete the

crud like create read update delete the selects the inserts the deletions and

selects the inserts the deletions and the like but what's this implying these

the like but what's this implying these keywords like primary key and foreign

keywords like primary key and foreign key are what are doing in code what this

key are what are doing in code what this picture was painting a moment ago these

picture was painting a moment ago these lines here are drawn literally to line

lines here are drawn literally to line up with the corresponding things

up with the corresponding things people's ID lines up with person ID

people's ID lines up with person ID Show's ID lines up with show ID and so

Show's ID lines up with show ID and so you're just seeing graphical version

you're just seeing graphical version code version graphical code that creates

code version graphical code that creates these relationships now given that let's

these relationships now given that let's actually see what these things look like

actually see what these things look like so let me go back to vs code here let me

so let me go back to vs code here let me clear my screen I'm still within SQL

clear my screen I'm still within SQL light with shows. DB let me go ahead and

light with shows. DB let me go ahead and do what I do with like any new database

do what I do with like any new database if I ever download something or I'm

if I ever download something or I'm trying to wrap my mind around a problem

trying to wrap my mind around a problem you know usually it doesn't come with a

you know usually it doesn't come with a pretty picture or uh you know a

pretty picture or uh you know a three-hour lecture to explain what the

three-hour lecture to explain what the data set is rather you just have the

data set is rather you just have the data set in your own knowledge of SQL so

data set in your own knowledge of SQL so let me play around so schema shows me

let me play around so schema shows me all the tables that might be a starting

all the tables that might be a starting point okay this is interesting I know PE

point okay this is interesting I know PE what people are let's go ahead and show

what people are let's go ahead and show me all the people so select star from

me all the people so select star from people I'm just trying to wrap my mind

people I'm just trying to wrap my mind around what this data set looks like in

around what this data set looks like in a more userfriendly way okay okay that's

a more userfriendly way okay okay that's already a lot of people as you see the

already a lot of people as you see the years flying by there's been a lot of

years flying by there's been a lot of people in the TV

people in the TV business so this was maybe not the best

business so this was maybe not the best query to run but this is indicative of

query to run but this is indicative of just how large this data set is from

just how large this data set is from IMDb okay when in doubt and when

IMDb okay when in doubt and when whenever you lose control over your

whenever you lose control over your computer control C is your friend to

computer control C is your friend to interrupt what would have been better

interrupt what would have been better because I don't think I need to know all

because I don't think I need to know all the million people in the world I could

the million people in the world I could do like limit me to 10 people all right

do like limit me to 10 people all right and that's enough now to get us sense of

and that's enough now to get us sense of like Fred a stair has an IDE of one

like Fred a stair has an IDE of one first person ever um birth year of 1899

first person ever um birth year of 1899 Lauren ball and all of these other uh

Lauren ball and all of these other uh people from yesterday year you see that

people from yesterday year you see that they are the first 10 people in the

they are the first 10 people in the database so there's an example of some

database so there's an example of some of the data now if I want to rep my mind

of the data now if I want to rep my mind around what a show is you know I know it

around what a show is you know I know it technically I know it from the picture

technically I know it from the picture but let's just look at some raw data so

but let's just look at some raw data so instead of saying select star from

instead of saying select star from people let me go ahead and select star

people let me go ahead and select star from shows limit 10 and okay I've only

from shows limit 10 and okay I've only heard heard of or seen a couple of these

heard heard of or seen a couple of these but these are older shows at that but I

but these are older shows at that but I see that every show has an ID a title a

see that every show has an ID a title a year in which it debuted and a number of

year in which it debuted and a number of episodes but perhaps most opaque is

episodes but perhaps most opaque is going to be this select star from

going to be this select star from Stars where this is the table that

Stars where this is the table that Associates people with shows am I going

Associates people with shows am I going to see any names or show titles here not

to see any names or show titles here not according to the definition we saw

according to the definition we saw earlier oh I should have done my limit

earlier oh I should have done my limit let me interrupt that let me do that

let me interrupt that let me do that again limit 10

again limit 10 no and this is where now you're

no and this is where now you're definitely in the programmer world

definitely in the programmer world because like this would be the most

because like this would be the most annoying spreadsheet to use on your Mac

annoying spreadsheet to use on your Mac or PC ever if you just had like a sheet

or PC ever if you just had like a sheet with all of these numbers that

with all of these numbers that Associates one thing with the other like

Associates one thing with the other like my God how do you figure out who this is

my God how do you figure out who this is or what this is you have to like

or what this is you have to like manually control F or command F looking

manually control F or command F looking for the data but a database doesn't care

for the data but a database doesn't care once you know SQL you can sort of Stitch

once you know SQL you can sort of Stitch these things back together so what

these things back together so what you're seeing here are foreign Keys

you're seeing here are foreign Keys foreign Keys why because show ID

foreign Keys why because show ID corresponds to the same numbers from

corresponds to the same numbers from that other table called shows that has

that other table called shows that has an proper primary key called ID person

an proper primary key called ID person ID is a foreign key in this context

ID is a foreign key in this context because it refers to numbers that belong

because it refers to numbers that belong to really the people table and its ID

to really the people table and its ID column so this is just a way of somehow

column so this is just a way of somehow linking them and so if you think of I

linking them and so if you think of I always like think of um this in my

always like think of um this in my mind's eye is this if this is like the

mind's eye is this if this is like the people table this is the shows table and

people table this is the shows table and there's this middle table in between the

there's this middle table in between the Stars table there's some way of like

Stars table there's some way of like stitching those two together by lining

stitching those two together by lining up the idas of one with the other and

up the idas of one with the other and getting back some more data so let's

getting back some more data so let's actually play with some of this data how

actually play with some of this data how about we start where we emphasized

about we start where we emphasized earlier genres so let me go ahead and

earlier genres so let me go ahead and take a quick look at all of the genres

take a quick look at all of the genres in this database so select star from

in this database so select star from genres star is usually going to be a

genres star is usually going to be a little overwhelming but it just gives me

little overwhelming but it just gives me a sense of what the data is but let's

a sense of what the data is but let's actually look at um uh let's go look at

actually look at um uh let's go look at all of them there okay that's a lot

all of them there okay that's a lot these are all official genres from IMDb

these are all official genres from IMDb let me oh okay it went okay it wasn't

let me oh okay it went okay it wasn't terribly long let me filter that down so

terribly long let me filter that down so from genres where genre equals comedy uh

from genres where genre equals comedy uh Capital C just based on the data I'm

Capital C just based on the data I'm seeing okay so what am I seeing now and

seeing okay so what am I seeing now and in fact let me limit this arbitrarily to

in fact let me limit this arbitrarily to 10 though I could limit it to anything I

10 though I could limit it to anything I want here are 10 comedies what are they

want here are 10 comedies what are they well who the heck knows like all I know

well who the heck knows like all I know are the 10 show IDs now I could do

are the 10 show IDs now I could do something like this as we've seen before

something like this as we've seen before with SQL I could do all right well let's

with SQL I could do all right well let's figure out what this show idea is Select

figure out what this show idea is Select star from shows where the IDE of the

star from shows where the IDE of the show I'm looking for equals what 62 614

show I'm looking for equals what 62 614 semicolon so I could like manually look

semicolon so I could like manually look it up by cross referencing the other

it up by cross referencing the other table okay so that was the show in

table okay so that was the show in question there the first comedy in the

question there the first comedy in the data set let me look up the second one

data set let me look up the second one so instead of that let's do 6 3881 enter

so instead of that let's do 6 3881 enter okay so that's that show and let's do

okay so that's that show and let's do one more and suffice it to say this is

one more and suffice it to say this is just getting tedious and vulnerable to

just getting tedious and vulnerable to mistakes quickly this is not this surely

mistakes quickly this is not this surely can't be the way to do this and indeed

can't be the way to do this and indeed SQL is going to let us do this a little

SQL is going to let us do this a little more powerfully instead let's do this

more powerfully instead let's do this instead of getting this table

instead of getting this table temporarily with all these show IDs and

temporarily with all these show IDs and all these genres let's refine the query

all these genres let's refine the query so let's just select the show ID from

so let's just select the show ID from the genres table where the genre equals

the genres table where the genre equals quote unquote comedy now I have a big b

quote unquote comedy now I have a big b list of show IDs all of which are comedy

list of show IDs all of which are comedy how many well I can combine ideas from

how many well I can combine ideas from earlier I can just count all of those

earlier I can just count all of those show IDs and or Star if I want to just

show IDs and or Star if I want to just do that too but I can count all those

do that too but I can count all those show IDs

show IDs 4876 comedies and IMDB's database for TV

4876 comedies and IMDB's database for TV shows so feels like a lot but how can I

shows so feels like a lot but how can I now use that information and get back

now use that information and get back the titles of comedies in the database

the titles of comedies in the database without doing it manually well let's do

without doing it manually well let's do this I have a moment ago this query

this I have a moment ago this query select the show ID from genres where the

select the show ID from genres where the current genre is quote unquote

current genre is quote unquote comedy um what if I kind of Nest these

comedy um what if I kind of Nest these queries kind of like grade school math

queries kind of like grade school math in parenthesis what if I combine this

in parenthesis what if I combine this whole thing in parenthesis and now let

whole thing in parenthesis and now let me select what I really want let me go

me select what I really want let me go ahead and select how about uh the title

ahead and select how about uh the title of all shows where the idea of the show

of all shows where the idea of the show is in this list of show

is in this list of show IDs so if you agree that the shows table

IDs so if you agree that the shows table has an ID column which is its otherwise

has an ID column which is its otherwise known as its primary key the unique ID

known as its primary key the unique ID that identifies it just like our Harvard

that identifies it just like our Harvard IDs our Yale IDs and you agree that per

IDs our Yale IDs and you agree that per a moment ago this shorter query will

a moment ago this shorter query will give me back just the show IDs of all of

give me back just the show IDs of all of the comedies in the database you can

the comedies in the database you can actually combine or Nest these queries

actually combine or Nest these queries together uh it's going to respect SQL

together uh it's going to respect SQL light order of operations with

light order of operations with parentheses just like grade school math

parentheses just like grade school math so the thing in parentheses will be

so the thing in parentheses will be executed first that gives it back a list

executed first that gives it back a list of IDs like what 48,000 IDs and then

of IDs like what 48,000 IDs and then this query the outer query is going to

this query the outer query is going to get the title from all of the shows

get the title from all of the shows where the ID of the show is in that big

where the ID of the show is in that big list of 48,000 so if I now execute these

list of 48,000 so if I now execute these together I think the list is still going

together I think the list is still going to be a little long but let me execute

to be a little long but let me execute it together now I see this long list of

it together now I see this long list of outputs a little overwhelming let's go

outputs a little overwhelming let's go ahead and maybe limit it to just 10 as

ahead and maybe limit it to just 10 as before for discussion sake and now I see

before for discussion sake and now I see 10 comedies ordered arbitrarily from

10 comedies ordered arbitrarily from however they're in the database that

however they're in the database that happen to indeed be have comedy as their

happen to indeed be have comedy as their genre if I want to do this a little more

genre if I want to do this a little more cleanly I could do this let's see uh why

cleanly I could do this let's see uh why don't I order by title ascending order

don't I order by title ascending order which is alphabetically or the default

which is alphabetically or the default is also uh ascending limit 10 now I see

is also uh ascending limit 10 now I see the top 10 I mean weirdly named things

the top 10 I mean weirdly named things with hash symbols presumably to get

with hash symbols presumably to get their titles up to the beginning or

their titles up to the beginning or maybe these are hashtags uh here now we

maybe these are hashtags uh here now we have alphabetically the top first 10

have alphabetically the top first 10 shows that are

shows that are comedies any questions on these kinds of

comedies any questions on these kinds of queries it's kind of a lot but at the

queries it's kind of a lot but at the same time it's just like composing the

same time it's just like composing the smaller ideas from before into slightly

smaller ideas from before into slightly more useful

more useful queries

yeah do for Keys have to set the relationship when you create the table

relationship when you create the table the programmer or the database

the programmer or the database administrator would create that

administrator would create that relationship by using those keywords

relationship by using those keywords primary key and foreign key that create

primary key and foreign key that create that teaches the database what is

that teaches the database what is related to what per the picture so you

related to what per the picture so you do that once and now I being the sort of

do that once and now I being the sort of programmer who's familiar with the

programmer who's familiar with the database I am just using these foreign

database I am just using these foreign keys in a way in a manner consistent

keys in a way in a manner consistent with their design but and this is where

with their design but and this is where it's useful at some point even if you no

it's useful at some point even if you no one hands you a picture to make sure you

one hands you a picture to make sure you understand the database because that's

understand the database because that's going to inform literally what you type

going to inform literally what you type in SQL to get the data you care about

in SQL to get the data you care about about well let's do something a little

about well let's do something a little more precise how about very reasonable

more precise how about very reasonable question and honestly this is exactly

question and honestly this is exactly what imdb.com in the app are for what if

what imdb.com in the app are for what if you want to find all of the shows that

you want to find all of the shows that Steve Carell is in like kind of a

Steve Carell is in like kind of a reasonable query like literally

reasonable query like literally something someone might type into Google

something someone might type into Google or more specifically IMDb it's not

or more specifically IMDb it's not really obvious at first glance how to do

really obvious at first glance how to do that though because right like from my

that though because right like from my database if these are my six tables well

database if these are my six tables well I can pretty easily get Steve Carell

I can pretty easily get Steve Carell from here but I can really only get his

from here but I can really only get his ID number whatever that is his name

ID number whatever that is his name which I know already and his birth year

which I know already and his birth year okay interesting but has nothing to do

okay interesting but has nothing to do with the shows that he's in I can look

with the shows that he's in I can look at shows over here but there's no

at shows over here but there's no mention of Steve Carell right because

mention of Steve Carell right because there's no person ID here where is that

there's no person ID here where is that relationship implemented well it's

relationship implemented well it's implemented down here so how do we do

implemented down here so how do we do this well here's the perfect example of

this well here's the perfect example of a a lesson we've trying been trying to

a a lesson we've trying been trying to emphasize for weeks of taking these baby

emphasize for weeks of taking these baby steps like break larger problems down

steps like break larger problems down into smaller ones and let's do something

into smaller ones and let's do something like this let's just get everything I

like this let's just get everything I know about Steve Carell from the

know about Steve Carell from the database let's select star from people

database let's select star from people where the name of the person is quote

where the name of the person is quote unquote Steve Carell I just want to see

unquote Steve Carell I just want to see what data we've got and here's what we

what data we've got and here's what we have okay there's only one Steve Carell

have okay there's only one Steve Carell born in 1962 and his unique ID is 136

born in 1962 and his unique ID is 136 797 according to IMDb this isn't some

797 according to IMDb this isn't some like Global uh actor identifier per se

like Global uh actor identifier per se all right well how do I get now all of

all right well how do I get now all of the shows that Steve Carell is in well I

the shows that Steve Carell is in well I could do this

could do this select star from Stars not to confuse

select star from Stars not to confuse the two one's the symbol one's the table

the two one's the symbol one's the table name uh where person ID equals 13

name uh where person ID equals 13 6797 so I think this will now give me

6797 so I think this will now give me everything from the Stars table that

everything from the Stars table that relates to Steve Carell okay and you'll

relates to Steve Carell okay and you'll see person ID is the same because I'm

see person ID is the same because I'm literally searching for just Steve

literally searching for just Steve Carell but there are what like 20 or so

Carell but there are what like 20 or so shows that he's been in all right well

shows that he's been in all right well here's where things would get tedious

here's where things would get tedious what are those shows well I could do

what are those shows well I could do Select Title from shows where the ID of

Select Title from shows where the ID of the show equals and here's you know

the show equals and here's you know whenever you copy paste you're probably

whenever you copy paste you're probably doing something wrong okay he's was in

doing something wrong okay he's was in The Dana Carvey Show familiar with that

The Dana Carvey Show familiar with that let's do another one we'll copy paste

let's do another one we'll copy paste this uh where ID equals this over the

this uh where ID equals this over the top another and if we keep digging we'll

top another and if we keep digging we'll probably find the office but my God like

probably find the office but my God like that's going to take forever to do 20

that's going to take forever to do 20 queries manually it's not very Dynamic

queries manually it's not very Dynamic but what if we just Nest these queries a

but what if we just Nest these queries a little more dynamically so let me start

little more dynamically so let me start from the beginning again what if we go

from the beginning again what if we go ahead and select everything we want we

ahead and select everything we want we know about uh people whose name

know about uh people whose name equals

equals Steve Carell that gave us earlier this

Steve Carell that gave us earlier this data I don't need all of that data I

data I don't need all of that data I know his name I don't care about his

know his name I don't care about his birth year so let's change this to just

birth year so let's change this to just be give me the ID of Steve Carell and

be give me the ID of Steve Carell and that gives me back now this smaller

that gives me back now this smaller temporary data set all right can I now

temporary data set all right can I now use

use this uh inside of another query well let

this uh inside of another query well let me wrap the whole thing with parentheses

me wrap the whole thing with parentheses and now let me say select star from the

and now let me say select star from the Stars table where the person ID equals

Stars table where the person ID equals this so I'm Del liely not using in

this so I'm Del liely not using in because I'm assuming there's indeed only

because I'm assuming there's indeed only one Steve Carell in the world so I'm not

one Steve Carell in the world so I'm not getting back a list of Steve Carell I'm

getting back a list of Steve Carell I'm getting back the one and only in this

getting back the one and only in this case so equal is fine in is when you

case so equal is fine in is when you have multiple equal is when you have one

have multiple equal is when you have one let me go ahead and hit enter now okay

let me go ahead and hit enter now okay that's more data than I need I don't

that's more data than I need I don't need like 20 copies of Steve Carell's

need like 20 copies of Steve Carell's person ID so let me hit up let me go

person ID so let me hit up let me go back and let me just get show ID from

back and let me just get show ID from Steve Carell and now I have a list of

Steve Carell and now I have a list of just the 20 or so show IDs that he has

just the 20 or so show IDs that he has been in all right how can I now use this

been in all right how can I now use this well let me hit up let me put the whole

well let me hit up let me put the whole thing in parentheses and now let me

thing in parentheses and now let me select what I really want Select Title

select what I really want Select Title from shows where and here's the final

from shows where and here's the final flourish the shows table has an ID has a

flourish the shows table has an ID has a title has a year and has an episode and

title has a year and has an episode and what I really want though is to check

what I really want though is to check which

which shows have ID that is what anyone want

shows have ID that is what anyone want to finish the thought

to finish the thought I just want to

yeah exactly ID in this and this is getting ugly and when you actually write

getting ugly and when you actually write your queries in like a text file you can

your queries in like a text file you can format them nicely and indent them my

format them nicely and indent them my font is just getting I don't want to

font is just getting I don't want to make it too small to fit everything but

make it too small to fit everything but now we have three queries one is in

now we have three queries one is in doubly nested parenthesis then there's

doubly nested parenthesis then there's the middle one then there's the outer

the middle one then there's the outer one so this last query is going to get

one so this last query is going to get me the title from shows where the ID of

me the title from shows where the ID of the show is in this big list of 20 or so

the show is in this big list of 20 or so show IDs that Steve Carell is in and I

show IDs that Steve Carell is in and I knew that because I looked up his name

knew that because I looked up his name here and notice what I did not do this

here and notice what I did not do this time is I didn't manually hardcode his

time is I didn't manually hardcode his ID number there's no need that would be

ID number there's no need that would be kind of a bad way to implement a website

kind of a bad way to implement a website uh if you're using a database underneath

uh if you're using a database underneath the hood you want the IMDb for real to

the hood you want the IMDb for real to search for whatever the human typed in

search for whatever the human typed in and no one's going to know Steve

and no one's going to know Steve Carell's person ID or anything else so

Carell's person ID or anything else so here we've done this all dynamically and

here we've done this all dynamically and now if I hit enter I think I get all of

now if I hit enter I think I get all of his shows let's go ahead and order this

his shows let's go ahead and order this by title just to make it tidy and you

by title just to make it tidy and you probably will see at least one or more

probably will see at least one or more shows that you know and probably the

shows that you know and probably the most popular is dot dot dot the office

most popular is dot dot dot the office so this is literally the kind of query

so this is literally the kind of query that's being executed underneath the

that's being executed underneath the hood when you go to uh websites or apps

hood when you go to uh websites or apps like IMDb your textual query is probably

like IMDb your textual query is probably being plugged into a longer SQL query

being plugged into a longer SQL query like this where some programmer at IMDb

like this where some programmer at IMDb probably wrote this whole query in

probably wrote this whole query in advance weeks months years ago and

advance weeks months years ago and they're just somehow plugging in the

they're just somehow plugging in the value that you the human type into the

value that you the human type into the search box or the like questions

search box or the like questions now on finding this data or any

now on finding this data or any other no okay so where else could we go

other no okay so where else could we go with this well let's

with this well let's consider how else we might combine data

consider how else we might combine data suppose that the next question actually

suppose that the next question actually perhaps appropriately would be focusing

perhaps appropriately would be focusing in on not just like people and shows and

in on not just like people and shows and these stars but how do we kind of like

these stars but how do we kind of like gather more information about the shows

gather more information about the shows themselves like the genres the ratings

themselves like the genres the ratings or the like so indeed let's focus on

or the like so indeed let's focus on just these two tables here recall that

just these two tables here recall that every show has an ID a title a year and

every show has an ID a title a year and episodes but it also might have one or

episodes but it also might have one or more relationships with rose and this

more relationships with rose and this other table is called genres and this is

other table is called genres and this is so that a show can be a comedy can be a

so that a show can be a comedy can be a drama can be any number of other things

drama can be any number of other things one row per so you would see the same

one row per so you would see the same show ID again and again and again with a

show ID again and again and again with a different genre written in English like

different genre written in English like comma comedy drama or the like well how

comma comedy drama or the like well how do I kind of Recon subtitute that data

do I kind of Recon subtitute that data Well turns out there's a few different

Well turns out there's a few different ways to do this and let me propose that

ways to do this and let me propose that we introduce this keyword here join and

we introduce this keyword here join and this is really the most powerful of the

this is really the most powerful of the keywords in SQL itself it doesn't have

keywords in SQL itself it doesn't have to be used we've seen with uh nested

to be used we've seen with uh nested queries that you can still select data

queries that you can still select data across multiple tables but here is

across multiple tables but here is another way so let me do this let me go

another way so let me do this let me go back to my SQL light database and let me

back to my SQL light database and let me select sort of in one uh breath exactly

select sort of in one uh breath exactly the data I want select star from shows

the data I want select star from shows and let's just limit this initially to

and let's just limit this initially to 10 to see what it looks like all right

10 to see what it looks like all right that's again the shows data select star

that's again the shows data select star from genres let's limit that to 102 just

from genres let's limit that to 102 just to wrap our minds around it and now this

to wrap our minds around it and now this is not that useful however the data in

is not that useful however the data in the leftmost column here is the primary

the leftmost column here is the primary key in the shows table these are just

key in the shows table these are just unique IDs the data here in the genres

unique IDs the data here in the genres table recall show ID is the foreign key

table recall show ID is the foreign key so it's the same numbers but just copied

so it's the same numbers but just copied into another table so that we can have

into another table so that we can have this relationship across them how do I

this relationship across them how do I kind of line up these numbers with these

kind of line up these numbers with these numbers to get back like a wider table

numbers to get back like a wider table that has title and year and episodes and

that has title and year and episodes and genre and heck ratings and all of that

genre and heck ratings and all of that too if we want well you can join these

too if we want well you can join these Tables by just telling the database what

Tables by just telling the database what to join on what so let me do this select

to join on what so let me do this select star from shows join that table though

star from shows join that table though on the genres table well how do you want

on the genres table well how do you want to join those two tables and again the

to join those two tables and again the two tables from the picture looked like

two tables from the picture looked like this how do you tell SQL

this how do you tell SQL programmatically to sort of you know put

programmatically to sort of you know put one of them right next to the other line

one of them right next to the other line up all of the ID so that you just get

up all of the ID so that you just get one larger data set well we can use

one larger data set well we can use indeed this this query this uh syntax

indeed this this query this uh syntax called join so back to VSS code here and

called join so back to VSS code here and let me join these two

let me join these two tables sorry typo here join genres on

tables sorry typo here join genres on the shows tables ID column AKA it's

the shows tables ID column AKA it's primary key equaling the genres tables

primary key equaling the genres tables show ID column aka the foreign key so in

show ID column aka the foreign key so in other words it looks a little cryptic

other words it looks a little cryptic but I'm just telling SQL how to line up

but I'm just telling SQL how to line up these two tables and what column to

these two tables and what column to match with the other so that the numbers

match with the other so that the numbers line up and I get essentially a wider

line up and I get essentially a wider table let me go ahead and hit uh

table let me go ahead and hit uh semicolon and enter and this is now

semicolon and enter and this is now going to give me a lot of data we might

going to give me a lot of data we might have to interrupt it but notice even at

have to interrupt it but notice even at a glance we're getting the ID the title

a glance we're getting the ID the title the year the number of episodes the ID

the year the number of episodes the ID again redundantly but that's to be

again redundantly but that's to be expected if I'm joining them and the

expected if I'm joining them and the genre all the way on the right let me

genre all the way on the right let me hit contrl C to interrupt let me just

hit contrl C to interrupt let me just limit this to the office so where title

limit this to the office so where title equals quote unquote the office so we

equals quote unquote the office so we can focus on just one sample uh datum

can focus on just one sample uh datum and here fun fact there's been more than

and here fun fact there's been more than one office the one that you all probably

one office the one that you all probably like is this one that started in 2005

like is this one that started in 2005 with 188 episodes its ID in the shows

with 188 episodes its ID in the shows table is 3866 76 that's confirmed over

table is 3866 76 that's confirmed over here too so again we've just joined the

here too so again we've just joined the two tables how by lining up those fields

two tables how by lining up those fields but now that we can see that almost all

but now that we can see that almost all of the offices produced over the decades

of the offices produced over the decades are comedies except for this one there

are comedies except for this one there was a version of the office produced in

was a version of the office produced in 2001 that was considered more of uh a

2001 that was considered more of uh a drama no unsure if it's related to the

drama no unsure if it's related to the other how can we Link in other data well

other how can we Link in other data well let's go ahead and Link in ratings too

let's go ahead and Link in ratings too or instead so instead of joining this

or instead so instead of joining this with genres let me go ahead and Rewind

with genres let me go ahead and Rewind here and join shows on ratings on shows.

here and join shows on ratings on shows. ID equals ratings. show ID and let's

ID equals ratings. show ID and let's limit it to the office 2 for discussion

limit it to the office 2 for discussion sake where uh title equals quote unquote

sake where uh title equals quote unquote the office semicolon and now you can see

the office semicolon and now you can see that among the various offices it looks

that among the various offices it looks like the one that most of us probably

like the one that most of us probably know and love is the highest rated also

know and love is the highest rated also with a 9.0 with like 585,000 people

with a 9.0 with like 585,000 people having cast votes for whereas these

having cast votes for whereas these other shows seem to have been less

other shows seem to have been less popular perhaps that's why indeed you

popular perhaps that's why indeed you see fewer episodes for them as well so

see fewer episodes for them as well so even though we've put the data in

even though we've put the data in multiple places you can still kind of

multiple places you can still kind of reconstitute it by lining things up in

reconstitute it by lining things up in this way and rejoining the tables

this way and rejoining the tables questions now on

questions now on this like this is the heart of what SQL

this like this is the heart of what SQL does and what relational databases do

does and what relational databases do for

for you

you questions all right a few final features

questions all right a few final features like there's not all that much that uh

like there's not all that much that uh you know SQL takes practice like

you know SQL takes practice like anything else but in terms of syntax and

anything else but in terms of syntax and capabilities let's just introduce you to

capabilities let's just introduce you to a couple of final features here and how

a couple of final features here and how and problems that arise and how we might

and problems that arise and how we might solve them let's do this uh as well so

solve them let's do this uh as well so let me go back into VSS code here and

let me go back into VSS code here and let's just find out um Steve Carell's

let's just find out um Steve Carell's information again last time we did it

information again last time we did it with this nested nested query by getting

with this nested nested query by getting his ID and then the show IDs and then

his ID and then the show IDs and then the titles for those show IDs with join

the titles for those show IDs with join you can do it a little differently and

you can do it a little differently and any of these ways are fine one might

any of these ways are fine one might become EAS easier to you mentally than

become EAS easier to you mentally than another let's go ahead and select the

another let's go ahead and select the titles from what let's select the title

titles from what let's select the title from uh the people table but and I'm

from uh the people table but and I'm going to hit enter and when you're using

going to hit enter and when you're using SQL light 3 interactively if you ever

SQL light 3 interactively if you ever find yourself with a prompt that says

find yourself with a prompt that says dot dot dot angle bracket it means

dot dot dot angle bracket it means you're continuing your thought onto the

you're continuing your thought onto the next line if you didn't intend that you

next line if you didn't intend that you can sometimes hit uh semicolon to just

can sometimes hit uh semicolon to just end the thought and hit enter and just

end the thought and hit enter and just even if it triggers an error but this is

even if it triggers an error but this is one way of formatting my queries now a

one way of formatting my queries now a little more nicely I'm just going to add

little more nicely I'm just going to add some wh space so that it's a little

some wh space so that it's a little easier to read what do I want to select

easier to read what do I want to select well I want to select the title of shows

well I want to select the title of shows from the people table joined with the

from the people table joined with the Stars table on the people tables ID

Stars table on the people tables ID column equaling the Stars tables person

column equaling the Stars tables person ID column so in other words if you think

ID column so in other words if you think back to what people are and what stars

back to what people are and what stars are one has an ID one has a person ID

are one has an ID one has a person ID I'm just now connecting those two tables

I'm just now connecting those two tables I'm joining those two but I want to do

I'm joining those two but I want to do this as well with with another table let

this as well with with another table let me additionally join in so now I only

me additionally join in so now I only have two hands but now I'm putting a

have two hands but now I'm putting a third table joined in together here join

third table joined in together here join shows on stars. show ID equals shows. ID

shows on stars. show ID equals shows. ID so this is now linking three tables

so this is now linking three tables together but I only care about this for

together but I only care about this for one person so where the name of the

one person so where the name of the person equals quote unquote Steve Carell

person equals quote unquote Steve Carell so more cryptic to be to be sure but

so more cryptic to be to be sure but what we're doing with this query is just

what we're doing with this query is just taking all three tables that we care

taking all three tables that we care about and we're joining them them all

about and we're joining them them all together at once using this new join

together at once using this new join syntax literally telling the database

syntax literally telling the database what columns to line up with what and

what columns to line up with what and then we filter at the very end just like

then we filter at the very end just like before to get back if I hit enter the

before to get back if I hit enter the answer we want which in this case is a

answer we want which in this case is a little slower at the moment but that

little slower at the moment but that same list of uh 20 or so shows that he's

same list of uh 20 or so shows that he's been in there's one other way to do this

been in there's one other way to do this and again uh these are all in the slides

and again uh these are all in the slides online so you can repeat them without

online so you can repeat them without having to jot down everything and we'll

having to jot down everything and we'll put them in the notes too but there's

put them in the notes too but there's another way to do this I could also use

another way to do this I could also use an implicit join so that was an explicit

an implicit join so that was an explicit join because I literally typed the word

join because I literally typed the word join multiple times at that but let me

join multiple times at that but let me go ahead and select the title from these

go ahead and select the title from these three tables people stars and shows and

three tables people stars and shows and this might just be nicer because if you

this might just be nicer because if you know what tables you want to select data

know what tables you want to select data from just enumerate them separated by

from just enumerate them separated by commas which you might prefer in your

commas which you might prefer in your mind where the people ID equals the

mind where the people ID equals the Stars person

Stars person ID and the Stars show ID equals the

ID and the Stars show ID equals the shows ID and the name of the person

shows ID and the name of the person equals Steve Carell so this is an

equals Steve Carell so this is an implicit joint and honestly I constantly

implicit joint and honestly I constantly reference my notes for some of this

reference my notes for some of this stuff too it's not the kind of thing

stuff too it's not the kind of thing that's going to come like this to you

that's going to come like this to you after just one day but it's just a

after just one day but it's just a different way of expressing the same

different way of expressing the same thing I want to select data from three

thing I want to select data from three different tables and hey SQL here is how

different tables and hey SQL here is how I want you to line those tables up so

I want you to line those tables up so that I can get like related data for

that I can get like related data for Steve Carell and this now will achieve

Steve Carell and this now will achieve the same results ultimately let me hit

the same results ultimately let me hit enter

enter H and there we go so a little slower and

H and there we go so a little slower and performance might vary based on computer

performance might vary based on computer based on uh implementation of SQL but I

based on uh implementation of SQL but I think I still have the same answers now

think I still have the same answers now suppose as I'm often do and I had to

suppose as I'm often do and I had to look it up again last time suppose you

look it up again last time suppose you forget what uh how to spell Steve

forget what uh how to spell Steve Carell's name is it two RS two L's or

Carell's name is it two RS two L's or the like well I could also do something

the like well I could also do something like this select well let's just keep

like this select well let's just keep this simple select star from people

this simple select star from people where name equals I've been deliberately

where name equals I've been deliberately getting it right so as to not embarrass

getting it right so as to not embarrass myself that's the Steve Carell I keep

myself that's the Steve Carell I keep querying if you forget well you could

querying if you forget well you could try searching for just Steves but there

try searching for just Steves but there interestingly there's a bunch of Steves

interestingly there's a bunch of Steves we don't know when they were born uh but

we don't know when they were born uh but that's probably not the Steve Carell we

that's probably not the Steve Carell we want if we don't have his last name so I

want if we don't have his last name so I could alternatively do well it's Steve

could alternatively do well it's Steve and then it's starts with a C I think

and then it's starts with a C I think well it turns out there's another wild

well it turns out there's another wild card you can use in SQL we used the

card you can use in SQL we used the asterisk to select all of the columns

asterisk to select all of the columns you can in quotes use a percent sign to

you can in quotes use a percent sign to say see something so there's some zero

say see something so there's some zero or more characters after the letter c

or more characters after the letter c and now this doesn't work cuz now I

and now this doesn't work cuz now I would be literally looking for Steve

would be literally looking for Steve space c something but recall earlier I

space c something but recall earlier I mentioned that one other keyword which

mentioned that one other keyword which is for fuzzier matching so to speak

is for fuzzier matching so to speak where it's like not exactly what you're

where it's like not exactly what you're looking for but it's like what you're

looking for but it's like what you're looking for if you instead say where his

looking for if you instead say where his name is like Steve space c something now

name is like Steve space c something now we'll get back a whole bunch of Steves

we'll get back a whole bunch of Steves but I think now I could probably find

but I think now I could probably find the one I'm actually looking for if I

the one I'm actually looking for if I don't remember his name you can use

don't remember his name you can use multiple percent signs if you forget

multiple percent signs if you forget what his first name is you could reverse

what his first name is you could reverse the order but that too is a uh very

the order but that too is a uh very powerful sequel feature at that

powerful sequel feature at that questions on these queries here

questions on these queries here yeah

sorry what about it oh yeah sure so the query I used here

it oh yeah sure so the query I used here there's a lot of Steves whose last name

there's a lot of Steves whose last name starts with C oops too far uh the last

starts with C oops too far uh the last query I executed was this one here so

query I executed was this one here so where the name is like quote unquote

where the name is like quote unquote Steve C percent sign so that's just

Steve C percent sign so that's just another tool for your toolkit here but

another tool for your toolkit here but you'll you'll perhaps have noticed that

you'll you'll perhaps have noticed that those two the prior to that query the

those two the prior to that query the joins I did were sort of slow and

joins I did were sort of slow and honestly this database isn't even that

honestly this database isn't even that big like yes it has tens of thousands of

big like yes it has tens of thousands of rows in it but like in the real world

rows in it but like in the real world and most of the apps you and I use a lot

and most of the apps you and I use a lot every day or websites like there's

every day or websites like there's Millions even billions of rows of data

Millions even billions of rows of data and like if I had to wait on like my

and like if I had to wait on like my computer here or my code space like a

computer here or my code space like a second or two to get the data like

second or two to get the data like that's not going to work for millions of

that's not going to work for millions of users or customers certainly so how can

users or customers certainly so how can we actually improve things well it turns

we actually improve things well it turns out another upside of a proper

out another upside of a proper relational database is that it's not

relational database is that it's not just a spreadsheet where the onus is on

just a spreadsheet where the onus is on like you to like find the data you're

like you to like find the data you're looking for you can also tell the

looking for you can also tell the database to index the data for you an

database to index the data for you an index is an an efficient uh cheat sheet

index is an an efficient uh cheat sheet for Finding data fast like books in the

for Finding data fast like books in the real world often have indices at the end

real world often have indices at the end of the at the end of the book where you

of the at the end of the book where you can look things up alphabetically and

can look things up alphabetically and then you can cross reference it for the

then you can cross reference it for the pages that that topic appears on same

pages that that topic appears on same idea in a database if you tell the

idea in a database if you tell the database in advance that you want to

database in advance that you want to search on a certain column frequently

search on a certain column frequently you can tell it to build a fancy index

you can tell it to build a fancy index that will just allow you to search that

that will just allow you to search that column Faster by default these columns

column Faster by default these columns are going to be searched most likely via

are going to be searched most likely via linear search like not even binary

linear search like not even binary search because the data might not be

search because the data might not be sorted because it came in in any order

sorted because it came in in any order but if you create an index you're

but if you create an index you're probably going to get something closer

probably going to get something closer to logar rmic than linear and that's

to logar rmic than linear and that's going to be a big plus overall so let me

going to be a big plus overall so let me do something simple here first let me

do something simple here first let me turn on a SQL light specific feature

turn on a SQL light specific feature that just is going to time all of my

that just is going to time all of my queries by writing do timer on I just

queries by writing do timer on I just want to keep track of uh how long each

want to keep track of uh how long each of these commands takes this one is not

of these commands takes this one is not a slow command so this is just going to

a slow command so this is just going to be relative but let's just select

be relative but let's just select everything from the shows table where

everything from the shows table where the title thereof is the office let's

the title thereof is the office let's see how long this relatively simple

see how long this relatively simple query takes all right not very long at

query takes all right not very long at all in real terms like less than a

all in real terms like less than a second .035 seconds so not slow by any

second .035 seconds so not slow by any means but if you've got hundreds

means but if you've got hundreds thousands millions of users like every

thousands millions of users like every one of those milliseconds could very

one of those milliseconds could very well add up so can we do better well we

well add up so can we do better well we can if I do this if I use syntax like

can if I do this if I use syntax like this once in the beginning of the design

this once in the beginning of the design of my database I create not a table but

of my database I create not a table but an index with some name on a specific

an index with some name on a specific table on one or more columns I can give

table on one or more columns I can give a clue a hint to the database in advance

a clue a hint to the database in advance saying please optimize with some Secret

saying please optimize with some Secret Sauce searching or selecting on this

Sauce searching or selecting on this column in this table so that my searches

column in this table so that my searches are faster so let me do this let me go

are faster so let me do this let me go back to vs code here let me create an

back to vs code here let me create an index called how about title index I

index called how about title index I could call it anything I want but I want

could call it anything I want but I want to search faster on titles so I'm going

to search faster on titles so I'm going to call this a title index where uh

to call this a title index where uh rather uh title index on the table

rather uh title index on the table called shows and then in parentheses is

called shows and then in parentheses is the syntax the column called title so

the syntax the column called title so again I've just borrowed this canonical

again I've just borrowed this canonical syntax and I've just translated into

syntax and I've just translated into something that's TV show specific all

something that's TV show specific all right what is this going to do for me

right what is this going to do for me once I hit enter this is going to create

once I hit enter this is going to create in the computer's memory the database's

in the computer's memory the database's memory something called a b tree it's

memory something called a b tree it's not a binary tree a b tree is actually a

not a binary tree a b tree is actually a potentially more efficient data

potentially more efficient data structure that we didn't talk about a

structure that we didn't talk about a few weeks back in week five but it looks

few weeks back in week five but it looks a little something like this which looks

a little something like this which looks similar to a binary tree but does anyone

similar to a binary tree but does anyone notice what makes this not a binary tree

exactly binary tree by implying two has no more than two children per node but

no more than two children per node but here's a perfect example one two three

here's a perfect example one two three and there could be four children five

and there could be four children five children or more but the effect of that

children or more but the effect of that if you have a very wide tree the upside

if you have a very wide tree the upside is that it's like very short like it

is that it's like very short like it pulls the data higher up closer to the

pulls the data higher up closer to the node to the root node and recall that

node to the root node and recall that the root node is where we began our

the root node is where we began our searches in the past whether it was a

searches in the past whether it was a BST a binary search tree even a try or

BST a binary search tree even a try or data structures we always began at the

data structures we always began at the top so the higher up you can pull the

top so the higher up you can pull the data even if it makes the data structure

data even if it makes the data structure very wide you're going to be able to do

very wide you're going to be able to do boom boom boom look up queries look up

boom boom boom look up queries look up data probably much faster certainly than

data probably much faster certainly than if it's just a very long list like a

if it's just a very long list like a column by default so with that said let

column by default so with that said let me go back to vs code I didn't create

me go back to vs code I didn't create the index yet let me go ahead and hit

the index yet let me go ahead and hit enter and create it all right it took a

enter and create it all right it took a minute a moment it took like half a

minute a moment it took like half a second which obviously is not that slow

second which obviously is not that slow but with more data that could have been

but with more data that could have been even slower but it's a onetime operation

even slower but it's a onetime operation as of now and now let me hit up and let

as of now and now let me hit up and let me select the same data from shows where

me select the same data from shows where title equals the office last time just a

title equals the office last time just a moment ago it took 0.035 seconds no not

moment ago it took 0.035 seconds no not slow but also that's going to add up if

slow but also that's going to add up if I have lots of users of IMDb let's go

I have lots of users of IMDb let's go ahead now and execute the same query

ahead now and execute the same query again how long did that

again how long did that take

take 0.01 seconds now I mean practically

0.01 seconds now I mean practically nothing and so that's the sort of

nothing and so that's the sort of opportunity now when you've got lots of

opportunity now when you've got lots of data and you want to really speed up

data and you want to really speed up these searches these indexes these

these searches these indexes these indices that just create for you these

indices that just create for you these magical data structures in the databases

magical data structures in the databases memory it allows you to search on

memory it allows you to search on columns that you are pretty sure you

columns that you are pretty sure you want to search on more effectively Now

want to search on more effectively Now by contrast if you've ever used like

by contrast if you've ever used like Google or Bing or some search engine

Google or Bing or some search engine that has advanced search some of those

that has advanced search some of those text boxes that you can search more

text boxes that you can search more precisely in might very well be slower

precisely in might very well be slower why well probably you don't want to go

why well probably you don't want to go crazy and just index every column on

crazy and just index every column on every table why well what might be the

intuition like if logically indexes speed things up why not index everything

speed things up why not index everything there's always going to be a tradeoff

there's always going to be a tradeoff here what might that

here what might that be

be yeah yeah it's going to take a lot of

yeah yeah it's going to take a lot of storage right this is just a slide on

storage right this is just a slide on the screen but like this has to go

the screen but like this has to go somewhere like this needs space in the

somewhere like this needs space in the computer's memory or on the hard drive

computer's memory or on the hard drive or the like and that's fine if you have

or the like and that's fine if you have unlimited space but odds are like you

unlimited space but odds are like you don't and that's going to get expensive

don't and that's going to get expensive for different reasons so maybe you only

for different reasons so maybe you only want to index certain columns and

want to index certain columns and certain tables and not all of them

certain tables and not all of them because you know what even if a user

because you know what even if a user really wants to search maybe VI advanced

really wants to search maybe VI advanced search on some other column or table

search on some other column or table altogether fine if once in a while a

altogether fine if once in a while a query is slow like we're probably

query is slow like we're probably getting the bigger bang for our buck by

getting the bigger bang for our buck by optimizing the common cases the more

optimizing the common cases the more popular queries that people actually

popular queries that people actually care about

care about too all right so let's come full circle

too all right so let's come full circle and bring this now back to uh how how we

and bring this now back to uh how how we actually began which was with some uh

actually began which was with some uh python code so it turns out these are

python code so it turns out these are not either or decisions it turns out in

not either or decisions it turns out in the real world developers are constantly

the real world developers are constantly using one two three languages at once in

using one two three languages at once in fact next week I rattled off HTML CSS

fact next week I rattled off HTML CSS and JavaScript one of which is a proper

and JavaScript one of which is a proper programming language but those languages

programming language but those languages are often used together totally normal

are often used together totally normal and common to use Python and SQL or Java

and common to use Python and SQL or Java in SQL or Swift in SQL or any number of

in SQL or Swift in SQL or any number of different combinations with a database

different combinations with a database language you might use use your

language you might use use your preferred programming language Java

preferred programming language Java python C++ to create the user interface

python C++ to create the user interface and the logic that builds the uh the

and the logic that builds the uh the that implements the program itself but

that implements the program itself but for your data like SQL is a really good

for your data like SQL is a really good candidates and indeed we've seen already

candidates and indeed we've seen already that SQL can just speed up certain

that SQL can just speed up certain operations you can change you can uh

operations you can change you can uh collapse 15 lines of code into just one

collapse 15 lines of code into just one and you can use these things together so

and you can use these things together so let me come back to I'm going to quit

let me come back to I'm going to quit out of SQL light I'm going to minimize

out of SQL light I'm going to minimize my terminal window and here's where we

my terminal window and here's where we left off before with favorite .p with

left off before with favorite .p with favorites. everything was being stored

favorites. everything was being stored in uh favorites. CSV and recall that we

in uh favorites. CSV and recall that we eventually imported that CSV file into

eventually imported that CSV file into favorites. DB automatically with import

favorites. DB automatically with import just so we could start playing around

just so we could start playing around with SQL but we can now tie these two

with SQL but we can now tie these two together and a way to do that is as

together and a way to do that is as follows um cs50 has a library for python

follows um cs50 has a library for python you might recall having available uh get

you might recall having available uh get string get int get float you don't

string get int get float you don't strictly need to use them in Python

strictly need to use them in Python because it's much easier to just use the

because it's much easier to just use the input function and then try accept and

input function and then try accept and convert things to int or float or the

convert things to int or float or the like but it's a lot more work to use SQL

like but it's a lot more work to use SQL in Python without a third-party Library

in Python without a third-party Library a lot of the commercial options or

a lot of the commercial options or popular open source options are actually

popular open source options are actually just complicated to use so cs50 does

just complicated to use so cs50 does have a very useful function inside of

have a very useful function inside of its library for python that you should

its library for python that you should use and must use for the problem set

use and must use for the problem set that just makes it easy to execute

that just makes it easy to execute python uh execute SQL inside of your

python uh execute SQL inside of your python code but it's built on top of a

python code but it's built on top of a very popular open source alternative so

very popular open source alternative so you can use that too in the real world

you can use that too in the real world so the documentation for that at this

so the documentation for that at this URL here but I'll show you what we need

URL here but I'll show you what we need to know here by focusing back on

to know here by focusing back on favorites.i so what I'm going to do here

favorites.i so what I'm going to do here as follows is this let me delete

as follows is this let me delete everything from favorites. py except for

everything from favorites. py except for let's say uh this from cs50 import SQL

let's say uh this from cs50 import SQL in all caps so that's importing a SQL

in all caps so that's importing a SQL feature from cs50's library that's going

feature from cs50's library that's going to allow me to open a DB file in code

to allow me to open a DB file in code how do I do that well let me create a

how do I do that well let me create a variable called DB for database though I

variable called DB for database though I could call it anything I want let me

could call it anything I want let me call this SQL function and pass in using

call this SQL function and pass in using special syntax that's not cs50 specific

special syntax that's not cs50 specific it's an industry thing SQL light colon

it's an industry thing SQL light colon slash slash slash unlike every other URL

slash slash slash unlike every other URL you type this one literally has three in

you type this one literally has three in this context here and then the name of

this context here and then the name of the database which in this case is

the database which in this case is favorites. DB so this is just a way of

favorites. DB so this is just a way of telling this SQL library that we wrote

telling this SQL library that we wrote but that works exactly like third party

but that works exactly like third party Alternatives openen favorites. DB using

Alternatives openen favorites. DB using the SQL light technology if you will all

the SQL light technology if you will all right let's just ask the user a question

right let's just ask the user a question give me your favorite um uh problem so

give me your favorite um uh problem so we're going to use input instead of get

we're going to use input instead of get string but we could use get string but

string but we could use get string but they're pretty much the same for our

they're pretty much the same for our purposes let's ask the user for their

purposes let's ask the user for their favorite and now in Python code let us

favorite and now in Python code let us select from favorites. DB all of the

select from favorites. DB all of the rows where students specify that problem

rows where students specify that problem as their favorite so in SQL alone it

as their favorite so in SQL alone it would be this select uh star from

would be this select uh star from favorites where problem equals and I'll

favorites where problem equals and I'll do um well whatever my favorite's going

do um well whatever my favorite's going to be like problem equals Mario for

to be like problem equals Mario for instance so if I were just using SQL I

instance so if I were just using SQL I would literally write something like

would literally write something like that but I'm in a piy file now like I

that but I'm in a piy file now like I have to use Python syntax but python

have to use Python syntax but python supports strings SQL is just text it's

supports strings SQL is just text it's just a string so I could certainly just

just a string so I could certainly just put my SQL code in a string perhaps and

put my SQL code in a string perhaps and then pass it to a python function and

then pass it to a python function and here's the bridge between the two if you

here's the bridge between the two if you just treat SQL as any old text we can

just treat SQL as any old text we can put it in a string and execute it so let

put it in a string and execute it so let me actually do this let me go ahead and

me actually do this let me go ahead and create a variable called rows which is

create a variable called rows which is eventually going to contain all the rows

eventually going to contain all the rows from the database let me go ahead and uh

from the database let me go ahead and uh select db. execute this is the one

select db. execute this is the one function you need to know about inside

function you need to know about inside of cs50's library and it literally

of cs50's library and it literally executes a SQL statement and then in

executes a SQL statement and then in quotes you pass it literally what you

quotes you pass it literally what you want to execute and let me go ahead and

want to execute and let me go ahead and close the parenthesis at the end there

close the parenthesis at the end there and now let me just try this so for Row

and now let me just try this so for Row in rows let's iterate over all of the

in rows let's iterate over all of the rows let me go ahead and print out how

rows let me go ahead and print out how about uh row quote unquote and what do I

about uh row quote unquote and what do I want here uh let's print out the Tim

want here uh let's print out the Tim stamp of that person for kicks all right

stamp of that person for kicks all right let me open open my terminal window

let me open open my terminal window python of favorites. Pi crossing my

python of favorites. Pi crossing my fingers here for

fingers here for sure

sure enter uh uh there we go favorit so I'll

enter uh uh there we go favorit so I'll type in

type in Mario okay so I got back it's not very

Mario okay so I got back it's not very interesting but I got back all of the

interesting but I got back all of the timestamps of students who typed in

timestamps of students who typed in Mario that we imported into this

Mario that we imported into this database well what I really care about

database well what I really care about is how popular Mario is so let me change

is how popular Mario is so let me change this a little bit let me change this to

this a little bit let me change this to count the number of rows and let me keep

count the number of rows and let me keep it simple let me give an alias like I

it simple let me give an alias like I proposed earlier like as n where N is a

proposed earlier like as n where N is a number so that now down here I can

number so that now down here I can actually just do this print out the

actually just do this print out the value of n all right let me go back to

value of n all right let me go back to my terminal window run python to

my terminal window run python to favorites. py let me type in Mario enter

favorites. py let me type in Mario enter okay 39 now technically I'm cheating

okay 39 now technically I'm cheating like honestly if I'm executing select

like honestly if I'm executing select count we've seen before it only ever

count we've seen before it only ever returns one row not multiple so there's

returns one row not multiple so there's really nothing to iterate over but it's

really nothing to iterate over but it's working fine it's just just iterating

working fine it's just just iterating once but I'm getting lucky so

once but I'm getting lucky so technically what I should probably just

technically what I should probably just do is this I should probably give myself

do is this I should probably give myself like a variable called row set it equal

like a variable called row set it equal to the very first row and only row that

to the very first row and only row that came back and now print out that rows n

came back and now print out that rows n column let me rerun the program I'll

column let me rerun the program I'll type in Mario again enter and I still

type in Mario again enter and I still see

see 39 so this of course I don't strictly

39 so this of course I don't strictly need to do this I don't really need a

need to do this I don't really need a variable I can do Row Bracket Z instead

variable I can do Row Bracket Z instead but let me focus on what this library is

but let me focus on what this library is now doing so per the documentation what

now doing so per the documentation what the C what the cs-50 execute function

the C what the cs-50 execute function always does for you is it returns a list

always does for you is it returns a list of dictionaries so if your query returns

of dictionaries so if your query returns nothing like no matches you get back an

nothing like no matches you get back an empty list like Open Bracket close

empty list like Open Bracket close bracket nothing in it any Loop is not

bracket nothing in it any Loop is not going to execute anything useful because

going to execute anything useful because there's nothing in it if though you get

there's nothing in it if though you get back one row you're going to get back a

back one row you're going to get back a list of size one in inside of which is a

list of size one in inside of which is a single dictionary that dictionary is

single dictionary that dictionary is going to have keys that correspond to

going to have keys that correspond to whatever you selected be it the columns

whatever you selected be it the columns or the count so when I selected star

or the count so when I selected star before I could have like I would have

before I could have like I would have gotten all of the columns that's how I

gotten all of the columns that's how I was able to access timestamp here I'm

was able to access timestamp here I'm just selecting count and I don't want to

just selecting count and I don't want to have to type this down here that would

have to type this down here that would just look kind of atrocious it would

just look kind of atrocious it would work but it would look weird to just

work but it would look weird to just keep retyping count peren star close pen

keep retyping count peren star close pen so I just created an alias called n just

so I just created an alias called n just to like make this my life easier or

to like make this my life easier or cleaner down here so to be clear the SQL

cleaner down here so to be clear the SQL the cs50 execute function returns a list

the cs50 execute function returns a list of dictionaries when you're using select

of dictionaries when you're using select and that is how I can now get back the

and that is how I can now get back the first and only row and then print out

first and only row and then print out that Row's n value it is identical to

that Row's n value it is identical to let me do this let me highlight this

let me do this let me highlight this whole line of text let me in my terminal

whole line of text let me in my terminal window run SQL light three of favorites.

window run SQL light three of favorites. DB like we did before break let me just

DB like we did before break let me just copy paste this query enter that that's

copy paste this query enter that that's the table I got back earlier when we

the table I got back earlier when we played with SQL manually and so when I

played with SQL manually and so when I get back this table here's the key

get back this table here's the key here's the value and I only have one row

here's the value and I only have one row which is why I'm just blindly indexing

which is why I'm just blindly indexing into rows bracket zero because I know

into rows bracket zero because I know there's always going to be an answer

there's always going to be an answer there it's going to be zero or one or

there it's going to be zero or one or more but I know now it's going to be

more but I know now it's going to be called n because of this here so what

called n because of this here so what have I just done well this is SQL down

have I just done well this is SQL down here and this is just me being like a

here and this is just me being like a data scientist asking questions about my

data scientist asking questions about my data just using like black and white SQL

data just using like black and white SQL queries this is me now being a python

queries this is me now being a python programmer who wants to talk to a SQL

programmer who wants to talk to a SQL database using Python and The Bridge

database using Python and The Bridge we're using happens to be the cs-50

we're using happens to be the cs-50 library but again there's third party

library but again there's third party free libraries you can also use as well

free libraries you can also use as well ours is just very simple and indeed the

ours is just very simple and indeed the documentation will explain how execute

documentation will explain how execute behaves a little differently for inserts

behaves a little differently for inserts updates and deletes you don't get back a

updates and deletes you don't get back a list because you're not selecting

list because you're not selecting anything but you do get back some return

anything but you do get back some return values questions on

values questions on this that's the last of our python code

this that's the last of our python code that ties everything together in

that ties everything together in spirit

yeah uh what this this one here yes so db. execute by definition

here yes so db. execute by definition returns a list of rows and each of those

returns a list of rows and each of those rows happens to be a dictionary because

rows happens to be a dictionary because it's convenient key value pairs if I'm

it's convenient key value pairs if I'm selecting the count of rows I just know

selecting the count of rows I just know from Having learned squel an hour ago

from Having learned squel an hour ago that this is always going to give me a

that this is always going to give me a single row whose column in this case is

single row whose column in this case is called n so if I know it's a single row

called n so if I know it's a single row I can just blindly just like in C go

I can just blindly just like in C go into that list or an array in C and go

into that list or an array in C and go to the first location and then treat

to the first location and then treat that as the single row what you don't

that as the single row what you don't want to do is this even if you the human

want to do is this even if you the human know the query returns one row you can't

know the query returns one row you can't just magically change the variable name

just magically change the variable name to be singular and expect to have only

to be singular and expect to have only one value you will always have a list so

one value you will always have a list so even if there is only one value in it

even if there is only one value in it it's up to you to do something like this

it's up to you to do something like this to get at it or if you prefer more

to get at it or if you prefer more succinctness you can do rose bracket I

succinctness you can do rose bracket I bracket n that'll achieve the same thing

bracket n that'll achieve the same thing without a variable

yeah good so I have been misleading this whole time and cheating because this is

whole time and cheating because this is only ever going to return Mario I'm

only ever going to return Mario I'm ignoring the favorite that the human

ignoring the favorite that the human typed in here on line five so let me fix

typed in here on line five so let me fix that and that's going to lead us to some

that and that's going to lead us to some of the problems that arise ultimately

of the problems that arise ultimately with SQL the right way to solve that

with SQL the right way to solve that problem let me get rid of my terminal

problem let me get rid of my terminal window here the right way to solve this

window here the right way to solve this problem is not to use an F string like

problem is not to use an F string like we did in Python generally because SQL

we did in Python generally because SQL queries as we'll see in a moment can be

queries as we'll see in a moment can be dangerous when you want to plug in users

dangerous when you want to plug in users uh data into a query that you've written

uh data into a query that you've written most of in advance you should you must

most of in advance you should you must you had better use a placeholder namely

you had better use a placeholder namely a question mark in this case this is

a question mark in this case this is somewhat specific to cs50's library but

somewhat specific to cs50's library but we just borrowed the convention that

we just borrowed the convention that like every other Library uses too in the

like every other Library uses too in the world of SQL single question marks are

world of SQL single question marks are used as placeholders and the way you do

used as placeholders and the way you do this is as follows if you want to plug

this is as follows if you want to plug in a value for that question mark just

in a value for that question mark just like in print F in C you specify as a

like in print F in C you specify as a second or a third or fourth argument all

second or a third or fourth argument all of the values you want plugged into this

of the values you want plugged into this so in C weeks ago we were using percent

so in C weeks ago we were using percent s same exact idea in SQL it's a question

s same exact idea in SQL it's a question mark that you use instead this now if I

mark that you use instead this now if I open back my terminal window and I run

open back my terminal window and I run uh python of favorites. type in Mario I

uh python of favorites. type in Mario I should still get 39 but now I can also

should still get 39 but now I can also type in scratch perhaps and get 44 for

type in scratch perhaps and get 44 for that very first piece at zero and that

that very first piece at zero and that one is even more popular here so this

one is even more popular here so this now is correct it would work to use an F

now is correct it would work to use an F string here and then plug in a value

string here and then plug in a value like favorite here but you'll see in

like favorite here but you'll see in just a moment don't do that you will

just a moment don't do that you will expose yourself to potential hack or

expose yourself to potential hack or attacks um by trusting the user's input

attacks um by trusting the user's input and so in fact let's transition from

and so in fact let's transition from that to exact some of these kinds of

that to exact some of these kinds of challenges namely two before we wrap up

challenges namely two before we wrap up so in the world of SQL especially when

so in the world of SQL especially when it's used at scale at the Twitter and

it's used at scale at the Twitter and the Googles of the world like a lot of

the Googles of the world like a lot of lots of data is probably coming into the

lots of data is probably coming into the database all at once because multiple

database all at once because multiple people are opening their phones at the

people are opening their phones at the same time around the world they're

same time around the world they're clicking on the same links roughly at

clicking on the same links roughly at the same time around the world when you

the same time around the world when you have thousands of people all using your

have thousands of people all using your site at once like order of operations is

site at once like order of operations is going to be important but unfortunately

going to be important but unfortunately in SQL and in other contexts of

in SQL and in other contexts of computing there's this risk of what's

computing there's this risk of what's known as a race condition so for

known as a race condition so for instance has anyone ever seen or liked

instance has anyone ever seen or liked this this is like yes the world record

this this is like yes the world record egg or it's like this thing that was

egg or it's like this thing that was very popular while back it's still kind

very popular while back it's still kind of going strong but if you go to the

of going strong but if you go to the Instagram profile for world record egg

Instagram profile for world record egg uh the goal was to make the most most

uh the goal was to make the most most liked Instagram post ever and they did

liked Instagram post ever and they did pretty well it's just this it's just a

pretty well it's just this it's just a picture of an egg now at the height of

picture of an egg now at the height of the popularity like there might have

the popularity like there might have been hundreds thousands tens of

been hundreds thousands tens of thousands of people clicking pretty much

thousands of people clicking pretty much at the same time on this egg so it

at the same time on this egg so it actually creates a potential problem

actually creates a potential problem with the Integrity of Instagram's data

with the Integrity of Instagram's data why well if you're have all these

why well if you're have all these requests coming in at once how do you

requests coming in at once how do you possibly keep track of all of them and

possibly keep track of all of them and update your counter in a way that can

update your counter in a way that can keep up with all of that traffic why

keep up with all of that traffic why well let's just hypothesize what meta

well let's just hypothesize what meta formerly Facebook was doing underneath

formerly Facebook was doing underneath the hood with Instagram if this were

the hood with Instagram if this were their code so suppose for the sake of

their code so suppose for the sake of discussion that Instagram servers are

discussion that Instagram servers are using a mix of python and SQL probably

using a mix of python and SQL probably not using the cs50 library but they

not using the cs50 library but they could absolutely be using those two

could absolutely be using those two languages or two others together um

languages or two others together um suppose they do this in order to update

suppose they do this in order to update the number of likes for that post they

the number of likes for that post they first execute a SQL query like select

first execute a SQL query like select the current number of likes from a table

the current number of likes from a table called posts where the ID of the post

called posts where the ID of the post equals whatever the unique identifier is

equals whatever the unique identifier is for that spec specific egg in the table

for that spec specific egg in the table and then they store the result in this

and then they store the result in this row variable just like I did and then

row variable just like I did and then they do this they grab uh they create a

they do this they grab uh they create a variable called likes they set it equal

variable called likes they set it equal to rows bracket Z so the very first row

to rows bracket Z so the very first row in the result set and they get the likes

in the result set and they get the likes key so this is literally what I just did

key so this is literally what I just did with the count let me hypothesize that

with the count let me hypothesize that Instagram does something similar with

Instagram does something similar with the total number of likes why are they

the total number of likes why are they doing this because they then want to

doing this because they then want to execute a third line of code that

execute a third line of code that executes update the posts table set the

executes update the posts table set the new number of likes equal to something

new number of likes equal to something where the ID of the post equals this

where the ID of the post equals this other thing now notice just like in

other thing now notice just like in printf there's the comma separated list

printf there's the comma separated list of values they want to update the

of values they want to update the current number of likes from the current

current number of likes from the current value to the current value plus one so

value to the current value plus one so it's likes plus one and then we plug in

it's likes plus one and then we plug in the ID for this so suppose this is what

the ID for this so suppose this is what Instagram's doing unfortunately whenever

Instagram's doing unfortunately whenever you execute multiple lines of code

you execute multiple lines of code independently and you're so popular like

independently and you're so popular like Instagram that you have thousands

Instagram that you have thousands hundreds of thousands of servers

hundreds of thousands of servers potentially it is quite possible that if

potentially it is quite possible that if you and I and everyone else in the room

you and I and everyone else in the room clicks that egg at the same time it's

clicks that egg at the same time it's not going to be the case statistically

not going to be the case statistically that like three lines of code are

that like three lines of code are executed for me and then three lines for

executed for me and then three lines for you and then three lines for you they're

you and then three lines for you they're probably going to get interspersed like

probably going to get interspersed like this gets executed for me and then this

this gets executed for me and then this gets executed for you and then they get

gets executed for you and then they get back to doing work for me and so forth

back to doing work for me and so forth just to kind of multitask just like a

just to kind of multitask just like a human might but at a super speed here

human might but at a super speed here the problem though is if these lines of

the problem though is if these lines of code get interrupted what could go wrong

code get interrupted what could go wrong well suppose that Carter and I both

well suppose that Carter and I both click the egg at the same time and

click the egg at the same time and suppose the current number of likes back

suppose the current number of likes back in the day is 100 that stores in this

in the day is 100 that stores in this variable the value 100 but if we click

variable the value 100 but if we click so close in time we might get back the

so close in time we might get back the same answer to this select query as of

same answer to this select query as of that moment in time when David and

that moment in time when David and Carter clicked it had a 100 likes but

Carter clicked it had a 100 likes but then this last line of code is executed

then this last line of code is executed for me and then maybe Carter because

for me and then maybe Carter because that answer the state of the database

that answer the state of the database was stored in this variable then both

was stored in this variable then both Carter and I will result in this line of

Carter and I will result in this line of code being executed with the same value

code being executed with the same value update the post table setting the likes

update the post table setting the likes equal to

equal to 101 for that post's ID why because again

101 for that post's ID why because again if each of these lines of code running

if each of these lines of code running on different servers are checking the

on different servers are checking the value of the current number of likes but

value of the current number of likes but then getting interrupted because Carter

then getting interrupted because Carter clicked the darn thing too and then

clicked the darn thing too and then resuming their work on my behalf we

resuming their work on my behalf we might have a race condition where the

might have a race condition where the code is sort of racing to finish but

code is sort of racing to finish but getting interrupted by other users

getting interrupted by other users clicks and the problem with that is that

clicks and the problem with that is that if you are inspecting the value of some

if you are inspecting the value of some variable or in this case a database cell

variable or in this case a database cell and making a decision based on it like

and making a decision based on it like how to update it you might now lose data

how to update it you might now lose data and Instagram probably not good for

and Instagram probably not good for advertising if they're losing likes and

advertising if they're losing likes and so that's probably a problem not to

so that's probably a problem not to retain the value 102 and instead insert

retain the value 102 and instead insert the number 101 twice it's actually

the number 101 twice it's actually similar in spirit to a story that uh was

similar in spirit to a story that uh was told in a databases course I took myself

told in a databases course I took myself years ago

years ago whereby uh it's somewhat analogous to

whereby uh it's somewhat analogous to kind of a contrived scenario involving

kind of a contrived scenario involving like a refrigerator and this is the

like a refrigerator and this is the closest thing to a refrigerator we could

closest thing to a refrigerator we could get on stage but imagine you've got like

get on stage but imagine you've got like one of these little dorm fridges in your

one of these little dorm fridges in your dorm 2 and your roommate and maybe both

dorm 2 and your roommate and maybe both of you as the story was told to me

of you as the story was told to me really like milk and one of you is at

really like milk and one of you is at class but the other of you comes home

class but the other of you comes home and you open your dorm fridge and you're

and you open your dorm fridge and you're like oh darn it we're out of milk and so

like oh darn it we're out of milk and so you close the fridge you walk across the

you close the fridge you walk across the street to CVS or some other store and

street to CVS or some other store and you get in line to buy some milk

you get in line to buy some milk Meanwhile your roommate gets out of

Meanwhile your roommate gets out of class they come back to your dorm room

class they come back to your dorm room they're really thirsty for some milk

they're really thirsty for some milk they open up the fridge they say oh

they open up the fridge they say oh we're out of milk and then they take a

we're out of milk and then they take a different route perhaps to CVS or some

different route perhaps to CVS or some other store nearby get in line to buy

other store nearby get in line to buy some milk fast forward some amount of

some milk fast forward some amount of time in this very contrived story and

time in this very contrived story and what happens oh damn it we now ended up

what happens oh damn it we now ended up with two gallons of milk and there's no

with two gallons of milk and there's no way we can fit gallons of milk in there

way we can fit gallons of milk in there let alone two of them so that's a

let alone two of them so that's a problem but what's the relationship to

problem but what's the relationship to this here well both of us yeah did

what exactly

exactly so to summarize both of us had a very similar thought process made a

very similar thought process made a similar decision based on the same

similar decision based on the same information not realizing that the

information not realizing that the information the fridge was in the

information the fridge was in the process of being updated and of course

process of being updated and of course in the Instagram World happens like this

in the Instagram World happens like this in the fridge World it might take a few

in the fridge World it might take a few minutes but the problem is ultimately

minutes but the problem is ultimately the result of our having made a decision

the result of our having made a decision about the state of the world and the

about the state of the world and the state of the world was in the middle of

state of the world was in the middle of being updated the queries got mingled

being updated the queries got mingled with others or in this case someone was

with others or in this case someone was already on their way to the store so

already on their way to the store so what's the solution in the real world

what's the solution in the real world well you could you know very simply like

well you could you know very simply like take a Post-It note and put like gone

take a Post-It note and put like gone for milk so as to communicate to your

for milk so as to communicate to your roommate that they should not inspect

roommate that they should not inspect the value of that variable and make a

the value of that variable and make a decision on it why because it's not yet

decision on it why because it's not yet consistent with the outcome that's about

consistent with the outcome that's about to happen you could be more traumatic

to happen you could be more traumatic and you could actually lock the fridge

and you could actually lock the fridge somehow put a pad lock around it or the

somehow put a pad lock around it or the like so they can't even get in there and

like so they can't even get in there and that would achieve the result uh the

that would achieve the result uh the same effect too and that is actually

same effect too and that is actually pretty much the solution to this problem

pretty much the solution to this problem in code too it's not safe it's not

in code too it's not safe it's not sufficient to only execute three lines

sufficient to only execute three lines of code like this rather what you

of code like this rather what you probably want to do is use additional

probably want to do is use additional SQL keywords that we won't spend much

SQL keywords that we won't spend much time on in the class itself but these

time on in the class itself but these there are solutions to this problem you

there are solutions to this problem you can begin what's called a transaction

can begin what's called a transaction and you can more explicitly commit to

and you can more explicitly commit to making a decision like updating the

making a decision like updating the database to 10 1001 or 10 1002 or if you

database to 10 1001 or 10 1002 or if you realize wait a minute Carter's query

realize wait a minute Carter's query inist interrupting mine let me roll back

inist interrupting mine let me roll back to the previous state and just uh rewind

to the previous state and just uh rewind let me undo contrl Z if you will there's

let me undo contrl Z if you will there's also another keyword that's not so much

also another keyword that's not so much used anymore in SQL which is locking you

used anymore in SQL which is locking you could literally back in the day lock the

could literally back in the day lock the entire database table preventing anyone

entire database table preventing anyone from updating it or making changes or

from updating it or making changes or even reading it while someone else was

even reading it while someone else was accessing it that was a very

accessing it that was a very heavy-handed solution because it slowed

heavy-handed solution because it slowed everything down but in short

everything down but in short transactions are now a feature of SQL

transactions are now a feature of SQL that you won't necessarily need to use

that you won't necessarily need to use yourselves that do solve this problem by

yourselves that do solve this problem by doing the equivalent of saying while

doing the equivalent of saying while David's like counter is in the process

David's like counter is in the process of being updated keep Carter at Bay

of being updated keep Carter at Bay ideally briefly and then let his data go

ideally briefly and then let his data go through too it's equivalent too to

through too it's equivalent too to putting a note or a lock on the fridge

putting a note or a lock on the fridge and indeed I mean lock litter they they

and indeed I mean lock litter they they were Once Upon a Time called and still

were Once Upon a Time called and still are in some texts called locks on

are in some texts called locks on databases 2 and the last Pro and the

databases 2 and the last Pro and the code for which you might do this is

code for which you might do this is almost the same you simply wrap the

almost the same you simply wrap the three queries uh with a transaction

three queries uh with a transaction statement and a uh commit and the term

statement and a uh commit and the term of art here is that this makes your uh

of art here is that this makes your uh your uh statements Atomic so Atomic

your uh statements Atomic so Atomic means they're either all executed or not

means they're either all executed or not at all that is they're all very tightly

at all that is they're all very tightly coupled together without interruption

coupled together without interruption transactions solves that problem and

transactions solves that problem and avoid having two gallons of milk and the

avoid having two gallons of milk and the last problem that arises that Is

last problem that arises that Is tragically so darn common in the real

tragically so darn common in the real world today is what's called a SQL

world today is what's called a SQL injection attack and it's what I alluded

injection attack and it's what I alluded to earlier with the question mark So

to earlier with the question mark So suppose you're in the habit of logging

suppose you're in the habit of logging in uh to Yale websites with your net ID

in uh to Yale websites with your net ID or password or at Harvard your Harvard

or password or at Harvard your Harvard key and password as well suppose for the

key and password as well suppose for the sake of discussion that the people that

sake of discussion that the people that implemented like Harvard key login allow

implemented like Harvard key login allow you to type in your email address of

you to type in your email address of course and your password but suppose

course and your password but suppose that they are using SQL underneath the

that they are using SQL underneath the hood to check your username and password

hood to check your username and password to make sure that you are David menen or

to make sure that you are David menen or Carter zeni or whoever you claim to be I

Carter zeni or whoever you claim to be I haven't shown you the syntax yet but it

haven't shown you the syntax yet but it turns out that in SQL Das Dash is a

turns out that in SQL Das Dash is a special way of indicating a comment it

special way of indicating a comment it means ignore everything to the right so

means ignore everything to the right so it's just like SL slash and C or the

it's just like SL slash and C or the hash symbol in Python dash dash just

hash symbol in Python dash dash just means ignore everything to the right and

means ignore everything to the right and we've of course seen single quotes So

we've of course seen single quotes So one way to wage a SQL injection attack

one way to wage a SQL injection attack is to try to inject malicious SQL code

is to try to inject malicious SQL code into someone else's database without

into someone else's database without them realizing it how do you do this

them realizing it how do you do this well suppose I log in as M harvard.edu

well suppose I log in as M harvard.edu single quote Das Dash I'm not double

single quote Das Dash I'm not double quoting anything clearly and there's

quoting anything clearly and there's nothing to the right of the dash dash

nothing to the right of the dash dash anyway but it this imbalance is going to

anyway but it this imbalance is going to be useful why because if I'm a hacker

be useful why because if I'm a hacker and I'm presuming you know someone at

and I'm presuming you know someone at Harvard probably is using SQL uh single

Harvard probably is using SQL uh single quotes to wrap the user's email address

quotes to wrap the user's email address and wrap the user's password what if I

and wrap the user's password what if I try to like complete their thought for

try to like complete their thought for them and close one of those quotes for

them and close one of those quotes for them what might happen well we could do

them what might happen well we could do this here for instance let me

this here for instance let me hypothesize is the code that Harvard

hypothesize is the code that Harvard wrote hopefully not underneath the hood

wrote hopefully not underneath the hood so they're using CS library and Python

so they're using CS library and Python and they're using SQL inside suppose

and they're using SQL inside suppose that they have a query like this select

that they have a query like this select star from users where username equals uh

star from users where username equals uh question mark and password equals

question mark and password equals question mark and then suppose they just

question mark and then suppose they just plug in whatever username and password

plug in whatever username and password was typed in and then if they get back

was typed in and then if they get back some number of rows dot dot dot they

some number of rows dot dot dot they assume I am David they assume Carter is

assume I am David they assume Carter is Carter if both the username and password

Carter if both the username and password are in the database just end of story

are in the database just end of story there this is good this has the question

there this is good this has the question mark placeholders we discussed earlier

mark placeholders we discussed earlier but what if you don't quite remember

but what if you don't quite remember that you don't quite take that to heart

that you don't quite take that to heart and you use your more familiar last week

and you use your more familiar last week F strings whereby we use these curly

F strings whereby we use these curly braces to plug in values what if you do

braces to plug in values what if you do this instead so it's almost the same

this instead so it's almost the same idea it's still DB execute but now it's

idea it's still DB execute but now it's select star from users where username

select star from users where username equals and now notice I'm doing the

equals and now notice I'm doing the single quotes which is required by SQL

single quotes which is required by SQL but I'm using F strings with the curly

but I'm using F strings with the curly braces and the password equals single

braces and the password equals single quote password and then close single

quote password and then close single quote the problem is if you're just

quote the problem is if you're just blindly pasting equ effectively the

blindly pasting equ effectively the user's input into that web form into the

user's input into that web form into the username field and the password field

username field and the password field there's nothing stopping a malicious

there's nothing stopping a malicious user student faculty staff from

user student faculty staff from including a single quote in their name

including a single quote in their name or maybe even an uh uh you know

or maybe even an uh uh you know benevolently if their name happens to

benevolently if their name happens to have a single quote as some last names

have a single quote as some last names in particular do so this is very fragile

in particular do so this is very fragile why well suppose that if we plug in my

why well suppose that if we plug in my malicious value Ma at harvard.edu single

malicious value Ma at harvard.edu single quote-- notice what happens to username

quote-- notice what happens to username here the username variable inside of the

here the username variable inside of the curly quotes will get replaced with this

curly quotes will get replaced with this and notice single quote which the

and notice single quote which the Harvard programmer wrote M an

Harvard programmer wrote M an harvard.edu single quote which I wrote

harvard.edu single quote which I wrote dash dash which I wrote single quote

dash dash which I wrote single quote which Harvard wrote and whatever else

which Harvard wrote and whatever else they want after that what's the

they want after that what's the implication though of the dash

dash everything to the right is going to be ignored so the password is never even

be ignored so the password is never even checked in this scenario I'm tricking

checked in this scenario I'm tricking the server into ignoring everything

the server into ignoring everything after the dash dash but I have

after the dash dash but I have constructed very cleverly very

constructed very cleverly very maliciously a syntactically valid query

maliciously a syntactically valid query why because I provided the single quote

why because I provided the single quote that's going to finish the thought of

that's going to finish the thought of that first single quote and now I would

that first single quote and now I would only know how to do this if I saw the

only know how to do this if I saw the code or if I just randomly try putting

code or if I just randomly try putting apostrophes into web forms and see if

apostrophes into web forms and see if things break that's often how

things break that's often how adversaries attack systems they type in

adversaries attack systems they type in potentially dangerous characters hit

potentially dangerous characters hit enter if something breaks they're not

enter if something breaks they're not necessarily into the system but they

necessarily into the system but they know that there might be a vulnerability

know that there might be a vulnerability and then they start trying more

and then they start trying more methodically things like this so This

methodically things like this so This Then is going to be bad because it

Then is going to be bad because it effectively Grays out the rest of the

effectively Grays out the rest of the query and this query is surely going to

query and this query is surely going to return some rows without even knowing my

return some rows without even knowing my password and so this logic here dot dot

password and so this logic here dot dot dot means well if a data came back from

dot means well if a data came back from this query Harvard is presumably going

this query Harvard is presumably going to assume that men logged in show them

to assume that men logged in show them uh show him like uh his account or

uh show him like uh his account or whatever is being protected here so in

whatever is being protected here so in short using F strings bad using any

short using F strings bad using any equivalent like p uh percent s in C bad

equivalent like p uh percent s in C bad when it comes to SQL using question

when it comes to SQL using question marks or whatever a thirdparty library

marks or whatever a thirdparty library like cs50's prescribed is the way to

like cs50's prescribed is the way to solve this why because libraries like

solve this why because libraries like ours are designed to at least be smart

ours are designed to at least be smart and be paranoid and what we will do is

and be paranoid and what we will do is this when you use the question marks and

this when you use the question marks and the values are plugged in we will escape

the values are plugged in we will escape any potentially dangerous characters

any potentially dangerous characters inside of those placeholders and so

inside of those placeholders and so effectively the single quote will no

effectively the single quote will no longer be considered a grammatical

longer be considered a grammatical single quote it will just be literally a

single quote it will just be literally a character in the username or password so

character in the username or password so the library takes care of this for you

the library takes care of this for you because you're plugging in the username

because you're plugging in the username and password as separate arguments and

and password as separate arguments and then we or the third party you're using

then we or the third party you're using actually sanitize that is uh clean up

actually sanitize that is uh clean up the data and prevent those bad

the data and prevent those bad characters now this is kind of an

characters now this is kind of an internet meme that went around for a

internet meme that went around for a while um if you've ever uh driven a car

while um if you've ever uh driven a car been in a car where there's like the

been in a car where there's like the automatic reader for tolls uh this

automatic reader for tolls uh this person thought it might be funny to try

person thought it might be funny to try doing something like this what are they

doing something like this what are they presumably doing the presumption here is

presumably doing the presumption here is whether or not it worked as unclear is

whether or not it worked as unclear is that here's like the end of a actual

that here's like the end of a actual license plate number but here's an

license plate number but here's an interesting single quote and a semicolon

interesting single quote and a semicolon that's especially bad because it means

that's especially bad because it means you can maybe execute a second query on

you can maybe execute a second query on the database this is someone having fun

the database this is someone having fun trying to drop the entire database table

trying to drop the entire database table for whatever municipality is scanning

for whatever municipality is scanning through cameras uh their license plate

through cameras uh their license plate code and I would be remiss if we didn't

code and I would be remiss if we didn't end on this note at least in computer

end on this note at least in computer science circles um there is someone

science circles um there is someone named no relation to the TF name we put

named no relation to the TF name we put in the database earlier um little Bobby

in the database earlier um little Bobby tables which ends with this XKCD

tables which ends with this XKCD comic and if you chuckle if you laugh

comic and if you chuckle if you laugh you're now legit SQL

programmers nice nice like every CS student out there

nice like every CS student out there knows about little Bobby tables so if

knows about little Bobby tables so if you name drop little Bobby tables now

you name drop little Bobby tables now like you're in all right that's it

like you're in all right that's it though for today we will see you next

[Music] [Applause]

all the way to the top and then you're passing

passing down this is for you Yale we love you

down this is for you Yale we love you Yale we're here to Har

Yale we're here to Har go

down it's going to happen it's actually going to happen I

happen it's actually going to happen I can't believe this what do you think of

can't believe this what do you think of Y they don't think

good can't does everyone have it does everyone have their stuff does everyone

everyone have their stuff does everyone have their stuff probably that it's

have their stuff probably that it's going to beable very

small I know what

know what houses how many exra how many

EXT no F forer yeah just make sure everyone

everyone has pass all the car distributed

all right we can do it [Applause]

[Applause] [Music]

what do you think of Y sir go go one more time one

[Applause] more there goes

more there goes [Applause]

[Applause] again

again [Applause]

all right this is cs50 welcome to week 8 last week we learned how to create read

last week we learned how to create read update and delete databases using squl

update and delete databases using squl but this

but this week Adam everyone happy

Halloween all right so this is cs50 and this is week eight already my thanks to

this is week eight already my thanks to Adam on today this happy Halloween uh in

Adam on today this happy Halloween uh in the coming moments we're going to learn

the coming moments we're going to learn all about how the internet itself Works

all about how the internet itself Works which of course is a technology that

which of course is a technology that like we're all take we all use every day

like we're all take we all use every day probably using in some form right now

probably using in some form right now but we'll see that if you start to

but we'll see that if you start to understand some of the underlying

understand some of the underlying building blocks that power the internet

building blocks that power the internet itself we can actually start to build

itself we can actually start to build interesting things on top of it and a

interesting things on top of it and a lot of the apps the websites that you

lot of the apps the websites that you all use every day should become all the

all use every day should become all the more familiar things that you yourself

more familiar things that you yourself VES can create and honestly when things

VES can create and honestly when things go wrong you'll have all the more of a

go wrong you'll have all the more of a mental model for how things work or are

mental model for how things work or are not in fact working so that you can

not in fact working so that you can ultimately diagnose diagnose all the

ultimately diagnose diagnose all the more issues yourselves so if we take a

more issues yourselves so if we take a look at the internet in the early days

look at the internet in the early days it pretty much was just this this

it pretty much was just this this happens to be of course the geography of

happens to be of course the geography of the United States and just some of the

the United States and just some of the first uh points on the internet were

first uh points on the internet were these here this was so-called arpanet

these here this was so-called arpanet back in 1969 and indeed the internet had

back in 1969 and indeed the internet had its Origins here in the United States

its Origins here in the United States with just a few computers interconnect

with just a few computers interconnect somehow initially that of course began

somehow initially that of course began to grow over time such that we

to grow over time such that we eventually had the West Coast connected

eventually had the West Coast connected to the east coast and nowadays what you

to the east coast and nowadays what you can think of these dots on the screen is

can think of these dots on the screen is representing are these things called

representing are these things called routers sort of computers or really

routers sort of computers or really servers that somehow have wires or maybe

servers that somehow have wires or maybe wireless connections between them that

wireless connections between them that allow data to flow from point A to B to

allow data to flow from point A to B to C and then this of course has been now

C and then this of course has been now magnified across the entire Globe um and

magnified across the entire Globe um and even above ground as well so that we can

even above ground as well so that we can connect all the more readily uh to

connect all the more readily uh to systems anywhere now in order to Route

systems anywhere now in order to Route the data from one router to another we

the data from one router to another we need to somehow make routing decisions

need to somehow make routing decisions and this is the kind of thing that the

and this is the kind of thing that the internet service providers the isps of

internet service providers the isps of the world just handle for us you and I

the world just handle for us you and I plug our Macs our PCS into the network

plug our Macs our PCS into the network here at Harvard or equivalently at Yale

here at Harvard or equivalently at Yale or we somehow get online via Wi-Fi or

or we somehow get online via Wi-Fi or cellular technology and then some of

cellular technology and then some of these larger entities these bigger

these larger entities these bigger companies or countries handle most of

companies or countries handle most of the data getting from point A to point B

the data getting from point A to point B and if you think about what these

and if you think about what these routers present they're indeed just

routers present they're indeed just servers somehow interconnected not

servers somehow interconnected not unlike this grid of tiles here for

unlike this grid of tiles here for instance back in the zoom days and in

instance back in the zoom days and in fact here we have I claim a grid of

fact here we have I claim a grid of routers implemented here by the courses

routers implemented here by the courses teaching fellows and course assistants

teaching fellows and course assistants and Tas and if the goal at hand for

and Tas and if the goal at hand for instance is for Phyllis to Route some

instance is for Phyllis to Route some piece of information maybe it's an email

piece of information maybe it's an email maybe it's a request for a web page in

maybe it's a request for a web page in the bottom right hand corner all the way

the bottom right hand corner all the way up to say Brian here in the top left

up to say Brian here in the top left hand corner suffice it to say each each

hand corner suffice it to say each each of these tiles represents a router a

of these tiles represents a router a server that can move the data back forth

server that can move the data back forth left and right that packet of

left and right that packet of information so to speak from Phyllis to

information so to speak from Phyllis to Brian could take any number of different

Brian could take any number of different possible routes up down left right to go

possible routes up down left right to go from the one corner to another so let me

from the one corner to another so let me go ahead and hit play on this video here

go ahead and hit play on this video here we're in the teaching fellows play the

we're in the teaching fellows play the same

same [Music]

[Music] role

all right so in this particular case the data was routed pretty straightforwardly

data was routed pretty straightforwardly up and then to the left but suppose that

up and then to the left but suppose that one or more of the staff were a bit busy

one or more of the staff were a bit busy maybe one of the routers is congested

maybe one of the routers is congested that is to say just got way more

that is to say just got way more envelopes at a moment in time that it

envelopes at a moment in time that it can handle thankfully the design of the

can handle thankfully the design of the internet is such that there's often

internet is such that there's often multiple ways that data can get from

multiple ways that data can get from point A to point B maybe going through

point A to point B maybe going through Point C or Point D instead and so

Point C or Point D instead and so there's a resilience there even as some

there's a resilience there even as some of these servers themselves might go

of these servers themselves might go down so allow me to propose that we use

down so allow me to propose that we use the same grid of routers now to Route

the same grid of routers now to Route the data in a slightly different way

the data in a slightly different way this

this [Music]

[Music] [Applause]

[Applause] [Music]

[Music] [Applause]

[Music] time

so success you'll see perhaps later just how many takes it took us to actually

how many takes it took us to actually get that routing right but it does in

get that routing right but it does in fact manifest that you can uh travel

fact manifest that you can uh travel different paths in order to get the data

different paths in order to get the data from point A to point B so as we talk

from point A to point B so as we talk about routers as you think of the

about routers as you think of the internet I mean think of those humans as

internet I mean think of those humans as just representing these routers points a

just representing these routers points a to p and everywhere in between now how

to p and everywhere in between now how did the teaching staff know to route

did the teaching staff know to route that packet up and then down or left and

that packet up and then down or left and right in order to get get to the

right in order to get get to the destination well all of them were

destination well all of them were programmed so to speak to understand

programmed so to speak to understand protocols it's called TCP and IP

protocols it's called TCP and IP Otherwise Known together typically as

Otherwise Known together typically as tcpip and you've probably seen these

tcpip and you've probably seen these acronyms at some point in the real world

acronyms at some point in the real world on the internet on some kind of

on the internet on some kind of documentation a text even if you haven't

documentation a text even if you haven't really thought hard about it but IP is

really thought hard about it but IP is certainly the more common of the two

certainly the more common of the two perhaps in common culture so what does

perhaps in common culture so what does TCP and IP do for us well really two

TCP and IP do for us well really two primary things any computer or any

primary things any computer or any teaching staff member who understands

teaching staff member who understands tcpip knows how to get data from point A

tcpip knows how to get data from point A to point B but how well let's break down

to point B but how well let's break down what that problem to be solved is IP

what that problem to be solved is IP otherwise known as Internet Protocol is

otherwise known as Internet Protocol is a protocol that computers speak that

a protocol that computers speak that allow them to know how to address

allow them to know how to address computers on the internet and a protocol

computers on the internet and a protocol is just a set of conventions that

is just a set of conventions that computers adhere to so someone wrote

computers adhere to so someone wrote code that probably has a whole lot of

code that probably has a whole lot of conditionals that tells the computer

conditionals that tells the computer what to do if something happens like if

what to do if something happens like if I receive a packet then send it to the

I receive a packet then send it to the next server or something like that in

next server or something like that in the human world we have protocols too

the human world we have protocols too you know pre in healthier times it was

you know pre in healthier times it was quite common to sort of extend your hand

quite common to sort of extend your hand to another human in order to greet them

to another human in order to greet them and if they're following human protocol

and if they're following human protocol they would presumably grab your hand and

they would presumably grab your hand and shake it at least in a a culture like

shake it at least in a a culture like this one here on campus and now that is

this one here on campus and now that is a human protocol in that someone

a human protocol in that someone initiates it someone responds to it and

initiates it someone responds to it and you both sort of know what to do you're

you both sort of know what to do you're programmed to know what to do so same

programmed to know what to do so same idea with Internet Protocol computers

idea with Internet Protocol computers just know what to do when they've been

just know what to do when they've been programmed to do so so what does this

programmed to do so so what does this mean IP had decides that every compter

mean IP had decides that every compter computer in the world will have a unique

computer in the world will have a unique address just like uh a the science

address just like uh a the science center around the corner might have a

center around the corner might have a unique address of one Oxford Street

unique address of one Oxford Street Cambridge Massachusetts 02138 USA IP

Cambridge Massachusetts 02138 USA IP dictates that every computer on the

dictates that every computer on the internet have a unique address of this

internet have a unique address of this form and this too is probably something

form and this too is probably something you've seen in the real world even if

you've seen in the real world even if you haven't thought too hard about it

you haven't thought too hard about it it's a number and what's called dotted

it's a number and what's called dotted decimal notation which means it's a

decimal notation which means it's a decimal number do something do something

decimal number do something do something do something so four digits separated by

do something so four digits separated by convention by decimal points although

convention by decimal points although there are newer and bigger versions of

there are newer and bigger versions of the same and these so-called IP

the same and these so-called IP addresses that might be as simple as

addresses that might be as simple as 1.2.3.4 uniquely identify a computer on

1.2.3.4 uniquely identify a computer on the internet uh the numbers have to

the internet uh the numbers have to range from 0 to 255 each and that's a

range from 0 to 255 each and that's a bit of a hint just as you start to think

bit of a hint just as you start to think more computationally if each of these is

more computationally if each of these is a number from 0 to 255 how many bits

a number from 0 to 255 how many bits does that suggest each number is

does that suggest each number is using feel free to shout it

using feel free to shout it out how many bits gives us 255 256 total

out how many bits gives us 255 256 total possibilities so eight bits that should

possibilities so eight bits that should just be sort of a heuristic in your mind

just be sort of a heuristic in your mind anytime you hear something that's in the

anytime you hear something that's in the range of 0 to 255 or 256 values total

range of 0 to 255 or 256 values total think back to week zero which gives us

think back to week zero which gives us eight bits plus another eight bits

eight bits plus another eight bits another eight bits and another eight

another eight bits and another eight bits which is to say an IP address

bits which is to say an IP address typically is 32 bits in total now if we

typically is 32 bits in total now if we do another bit of quick mental math or

do another bit of quick mental math or think back to week zero if every IP

think back to week zero if every IP address is 32 bits how many computers

address is 32 bits how many computers can we have on the

can we have on the internet at once give or take

internet at once give or take roughly 4 billion is the ballpark and we

roughly 4 billion is the ballpark and we don't need to be super precise for

don't need to be super precise for discussion's sake but roughly 4 billion

discussion's sake but roughly 4 billion is how high you can account assuming no

is how high you can account assuming no negative numbers if you have 32 bits in

negative numbers if you have 32 bits in total now that's not terribly uh many

total now that's not terribly uh many number uh numbers of addresses

number uh numbers of addresses especially considering the number of

especially considering the number of humans in the world the number of us

humans in the world the number of us that do have laptops or desktops or

that do have laptops or desktops or devices more generally phones in our

devices more generally phones in our pockets and the like so let me just

pockets and the like so let me just stipulate for today's purposes that

stipulate for today's purposes that there's even a newer and improved

there's even a newer and improved version of Ip otherwise known as version

version of Ip otherwise known as version 6 this is version four but still super

6 this is version four but still super popular version six uses 128 bits which

popular version six uses 128 bits which is a huge number of possible

is a huge number of possible premutations I dare say I can't even

premutations I dare say I can't even pronounce that number it's so big so

pronounce that number it's so big so there are ways around even this

there are ways around even this limitation already so every computer has

limitation already so every computer has an address like this what does that

an address like this what does that really mean well suppose that I was

really mean well suppose that I was Phyllis in the story told visually

Phyllis in the story told visually earlier and I want to send a message to

earlier and I want to send a message to Brian well both Phyllis and Brian have I

Brian well both Phyllis and Brian have I IP addresses and suppose that Brian's IP

IP addresses and suppose that Brian's IP address happens to be 1.2.3.4 in that

address happens to be 1.2.3.4 in that top left hand corner well phyllis's Mac

top left hand corner well phyllis's Mac or PC or phone would essentially do the

or PC or phone would essentially do the equivalent on this human envelope by

equivalent on this human envelope by writing the two address in the middle of

writing the two address in the middle of the envelope as is our human convention

the envelope as is our human convention like this so this is an envelope a piece

like this so this is an envelope a piece of information an email a text message

of information an email a text message whatever destined for Brian and so she

whatever destined for Brian and so she would have her computer put Brian's IP

would have her computer put Brian's IP address in the middle her IP address is

address in the middle her IP address is Maybe 5.6.7 do8 so just like our human

Maybe 5.6.7 do8 so just like our human convention I might write 5.

convention I might write 5. 6.7.8 at the top of the envelope

6.7.8 at the top of the envelope 5678 thereby indicating what the return

5678 thereby indicating what the return address is and this is helpful because

address is and this is helpful because if Brian's computer needs to acknowledge

if Brian's computer needs to acknowledge receipt if he needs to reply in some

receipt if he needs to reply in some form this way the envelope has all the

form this way the envelope has all the information we need but in the real

information we need but in the real world servers do a lot of things

world servers do a lot of things nowadays not just email but maybe chat

nowadays not just email but maybe chat Maybe video conferencing maybe any

Maybe video conferencing maybe any number of other services as well and so

number of other services as well and so it turns out that an address alone might

it turns out that an address alone might not be sufficient because how do how

not be sufficient because how do how does Brian's computer know when he opens

does Brian's computer know when he opens the envelope so to speak that this

the envelope so to speak that this should be interpreted as an email or

should be interpreted as an email or interpret it as a chat message or

interpret it as a chat message or interpret it as like a video attachment

interpret it as like a video attachment that Phyllis has sent well we need some

that Phyllis has sent well we need some other mechanism some other hint on this

other mechanism some other hint on this envelope to distinguish one type of

envelope to distinguish one type of Internet service from another and so

Internet service from another and so that's where the other acronym in TCI

that's where the other acronym in TCI pip comes in which is TCP so this stands

pip comes in which is TCP so this stands for transmission control protocol which

for transmission control protocol which is just a different set of conventions

is just a different set of conventions that computers adhere to in order to

that computers adhere to in order to solve a couple of different problems one

solve a couple of different problems one is this problem of distinguishing one

is this problem of distinguishing one type of service from another now what

type of service from another now what does that mean well humans decades ago

does that mean well humans decades ago decided as they started inventing all of

decided as they started inventing all of these various internet services the web

these various internet services the web being the first one how they might or

being the first one how they might or the web now being one of the most

the web now being one of the most popular ones they decided to assign

popular ones they decided to assign different services that can be used on

different services that can be used on the internet unique numbers and so two

the internet unique numbers and so two of the most common are these 80 is the

of the most common are these 80 is the number that a bunch of humans decided

number that a bunch of humans decided years ago will represent what you and I

years ago will represent what you and I know as HTTP and we'll talk more

know as HTTP and we'll talk more technically in a bit about what HTTP is

technically in a bit about what HTTP is but obviously it's the thing that's in

but obviously it's the thing that's in the beginning of every URL nowadays or

the beginning of every URL nowadays or https which of course has the S added to

https which of course has the S added to it and that has its own unique number

it and that has its own unique number and for now the S just means secure one

and for now the S just means secure one is encrypted or scrambled somehow for

is encrypted or scrambled somehow for privacy sake and the other is

privacy sake and the other is unencrypted it's a little more

unencrypted it's a little more vulnerable to interception so these two

vulnerable to interception so these two numbers are what the world decided when

numbers are what the world decided when implementing TCP shall uniquely identify

implementing TCP shall uniquely identify those services so what does this mean

those services so what does this mean well this means that if Brian's computer

well this means that if Brian's computer in the story from before is hosting not

in the story from before is hosting not like an email server but maybe he has a

like an email server but maybe he has a website and Phyllis is requesting

website and Phyllis is requesting Brian's homepage or something like that

Brian's homepage or something like that she would have her Mac or PC or phone

she would have her Mac or PC or phone not only write Brian's IP address in the

not only write Brian's IP address in the middle of the envelope but also the

middle of the envelope but also the number otherwise known as a port number

number otherwise known as a port number that she wants this envelope to be

that she wants this envelope to be routed to now 80 would be insecure

routed to now 80 would be insecure nowadays HTTP col is sort of p and we

nowadays HTTP col is sort of p and we almost always see https colon now so I'm

almost always see https colon now so I'm just going to go with best practice and

just going to go with best practice and I'm going to add a colon and then the

I'm going to add a colon and then the number

number 443 at the end of Brian's IP address so

443 at the end of Brian's IP address so now I have an IP address for Brian the

now I have an IP address for Brian the port number for the service that this is

port number for the service that this is relevant to and I'm not going to bother

relevant to and I'm not going to bother writing it but it turns out that

writing it but it turns out that phyllis's computer would also choose a

phyllis's computer would also choose a port number maybe a random port number

port number maybe a random port number so that Brian can conversely reply and

so that Brian can conversely reply and then the computer can know which

then the computer can know which response is in uh coming back for which

response is in uh coming back for which request but the most important one is

request but the most important one is this one in the two field whereby this

this one in the two field whereby this distinguishes this from like an email a

distinguishes this from like an email a chat message a video conference session

chat message a video conference session or a zoom or whatnot from anything else

or a zoom or whatnot from anything else happening on Brian's computer at the

happening on Brian's computer at the same time so all this time if you've

same time so all this time if you've seen these terms TCP and IP those are

seen these terms TCP and IP those are really two of the most important things

really two of the most important things that they do but TCP does one other

that they do but TCP does one other thing that's super useful too it turns

thing that's super useful too it turns out that it's

out that it's super common nowadays especially to

super common nowadays especially to transmit a lot of media on the internet

transmit a lot of media on the internet internet whether it's an image or maybe

internet whether it's an image or maybe it's a movie file and it would be a

it's a movie file and it would be a little obnoxious to say the least if

little obnoxious to say the least if you're downloading a really big file

you're downloading a really big file meant that no one else in your dorm room

meant that no one else in your dorm room or your household could actually

or your household could actually download anything until you're actually

download anything until you're actually done so of course multiple people

done so of course multiple people nowadays can be on the internet at once

nowadays can be on the internet at once even if all of the connections are a

even if all of the connections are a little slower but like one person's

little slower but like one person's usage does not block someone else's now

usage does not block someone else's now how does this work well TCP in

how does this work well TCP in conjunction with IP can also allow you

conjunction with IP can also allow you to take like a really big image of a cat

to take like a really big image of a cat which is the internet of course is

which is the internet of course is filled with and take a big image of a

filled with and take a big image of a cat or a big video file of a cat and

cat or a big video file of a cat and fragment it into multiple pieces so I'm

fragment it into multiple pieces so I'm just going to sort of roughly tear it

just going to sort of roughly tear it down the middle and then maybe tear it

down the middle and then maybe tear it down the middle again so now it's four

down the middle again so now it's four different fragments and I'm I'm sorry

different fragments and I'm I'm sorry but the computer will be reassembling

but the computer will be reassembling these for us and what phyllis's computer

these for us and what phyllis's computer could do now if she's like uploading

could do now if she's like uploading this picture of a cat to Brian's web

this picture of a cat to Brian's web server well she could put one fragment

server well she could put one fragment in this envelope and then have three

in this envelope and then have three separate envelopes for the other three

separate envelopes for the other three fragments and what you could then do on

fragments and what you could then do on the outside of this envelope is just

the outside of this envelope is just kind of number them somehow and in fact

kind of number them somehow and in fact this is something else that TCP and IP

this is something else that TCP and IP together would do for us this first

together would do for us this first envelope now might say something like

envelope now might say something like one out of four in the memo field so to

one out of four in the memo field so to speak of the uh metaphorical envelope

speak of the uh metaphorical envelope here now this should be enough

here now this should be enough information because now if Brian gets

information because now if Brian gets all four of these envelopes he

all four of these envelopes he presumably knows how to reassemble the

presumably knows how to reassemble the picture of the cat in order top to

picture of the cat in order top to bottom left to right but more

bottom left to right but more importantly suppose that one of the

importantly suppose that one of the routers one of the TFS in the video is

routers one of the TFS in the video is sort of distracted and they sort of drop

sort of distracted and they sort of drop one of the packets and that's a metaphor

one of the packets and that's a metaphor actually in practice for when a router

actually in practice for when a router gets really busy it's got way too much

gets really busy it's got way too much data coming in it might metaphorically

data coming in it might metaphorically drop packets what does that mean in

drop packets what does that mean in practice I mean it literally just

practice I mean it literally just ignores the zeros and ones it doesn't

ignores the zeros and ones it doesn't save them to its memory because there's

save them to its memory because there's just no room left so it's equivalent to

just no room left so it's equivalent to sort of dropping the packet so suppose

sort of dropping the packet so suppose now that Brian gets one of four three of

now that Brian gets one of four three of four and four of four what can his

four and four of four what can his computer infer

computer infer now after receiving those three packets

now after receiving those three packets one of four three of four and four of

one of four three of four and four of four what's the use there yeah I think

four what's the use there yeah I think you're you're signaling with your

you're you're signaling with your fingers which one did which one can I

fingers which one did which one can I call on you yeah so he's missing two out

call on you yeah so he's missing two out of four the second of the packets and

of four the second of the packets and this is useful now because you could

this is useful now because you could imagine he can send some message back to

imagine he can send some message back to Phyllis saying hey please retransmit

Phyllis saying hey please retransmit number two of four without having to

number two of four without having to redownload the entirety of the cat so

redownload the entirety of the cat so there's there's an efficiency there as

there's there's an efficiency there as well so tcpip allows data really to go

well so tcpip allows data really to go from point A to point B while solving a

from point A to point B while solving a bunch of these problems along the way so

bunch of these problems along the way so nowadays if you ever see mention on your

nowadays if you ever see mention on your Mac or PC of your so-called IP address

Mac or PC of your so-called IP address that is the sort of problem that's being

that is the sort of problem that's being solved questions now on these protocols

solved questions now on these protocols these conventions called TCP and

these conventions called TCP and IP that's the extent to which we'll need

IP that's the extent to which we'll need to understand them won't have to

to understand them won't have to implement them per se we'll just take

implement them per se we'll just take them Hereafter for

them Hereafter for granted any questions that you've ever

granted any questions that you've ever been wondering wondering about your home

been wondering wondering about your home network

yeah a really good question uh how does TCP know that a user got a message

TCP know that a user got a message another aspect another feature of TCP is

another aspect another feature of TCP is that Brian's computer by design of this

that Brian's computer by design of this protocol will also acknowledge the

protocol will also acknowledge the packets that he's received and it will

packets that he's received and it will do it efficiently if Brian receives all

do it efficiently if Brian receives all four packets in a pretty narrow window

four packets in a pretty narrow window of time his computer will send to

of time his computer will send to phyllis's computer a quick message

phyllis's computer a quick message saying essentially received all four

saying essentially received all four otherwise he'll say the opposite which

otherwise he'll say the opposite which is that I'm missing for for instance two

is that I'm missing for for instance two out of four and that just ensures

out of four and that just ensures ultimately that all of the data is

ultimately that all of the data is indeed uh arrived so that you're not

indeed uh arrived so that you're not missing like uh a quarter a quadrant of

missing like uh a quarter a quadrant of the cat in question all right but th

the cat in question all right but th that's not the only problem that needs

that's not the only problem that needs to be solved ultimately we also need to

to be solved ultimately we also need to make the internet userfriendly if you

make the internet userfriendly if you will and it would be really tedious if

will and it would be really tedious if you had to visit websites for instance

you had to visit websites for instance by way of of their IP addresses right

by way of of their IP addresses right 1.2.3.4 is pretty memorable but there's

1.2.3.4 is pretty memorable but there's like 4 billion other possible addresses

like 4 billion other possible addresses available and it would be super tedious

available and it would be super tedious to remember those it would be bad

to remember those it would be bad marketing to advertise those in fact

marketing to advertise those in fact most of you probably don't even know the

most of you probably don't even know the phone numbers of your closest friends

phone numbers of your closest friends and family members anymore because you

and family members anymore because you instead store them in your contacts

instead store them in your contacts you're in your dress book associating

you're in your dress book associating with numbers that are completely opaque

with numbers that are completely opaque with actual names or strings if you will

with actual names or strings if you will the same is goes for the internet too

the same is goes for the internet too even though every computer does have and

even though every computer does have and must have a unique IP address

must have a unique IP address numerically why well routers or

numerically why well routers or computers computers just crunch numbers

computers computers just crunch numbers very readily but we humans work better

very readily but we humans work better with strings of text we need some system

with strings of text we need some system for converting userfriendly strings like

for converting userfriendly strings like harvard.edu or yale.edu or google.com to

harvard.edu or yale.edu or google.com to the underlying IP addresses and that's

the underlying IP addresses and that's where the next acronym comes in today

where the next acronym comes in today which is DNS domain name system so this

which is DNS domain name system so this is just another technology that's been

is just another technology that's been in use for some time now and it's a

in use for some time now and it's a collection of servers on the internet

collection of servers on the internet that whose purpose in life is to convert

that whose purpose in life is to convert domain names to IP addresses and maybe

domain names to IP addresses and maybe vice versa as well so let me stipulate

vice versa as well so let me stipulate for today's purposes there are some root

for today's purposes there are some root DNS servers in the world that long story

DNS servers in the world that long story short know about all of the Dooms all of

short know about all of the Dooms all of the edus all of the dot dot dot all of

the edus all of the dot dot dot all of the other top level domains around the

the other top level domains around the world as well as in the US and then

world as well as in the US and then there are some smaller DNS servers owned

there are some smaller DNS servers owned by companies owned by universities and

by companies owned by universities and even in your apartments or homes most

even in your apartments or homes most likely indeed if you have a home router

likely indeed if you have a home router plugged into the wall somewhere that's

plugged into the wall somewhere that's not only routing your data in and out of

not only routing your data in and out of your apartment or home or dorm room it's

your apartment or home or dorm room it's also typically serving as a local

also typically serving as a local therefore faster DNS server a cash if

therefore faster DNS server a cash if you will it's sort of locally saving

you will it's sort of locally saving your most frequently accessed websites

your most frequently accessed websites and their IP addresses just to avoid

and their IP addresses just to avoid bothering the bigger more expensive

bothering the bigger more expensive busier servers all day long so there's

busier servers all day long so there's DNS servers all over if you poke around

DNS servers all over if you poke around your settings in Windows or Mac OS or

your settings in Windows or Mac OS or Android or iOS you'll see mention of DNS

Android or iOS you'll see mention of DNS and you'll probably see the IP addresses

and you'll probably see the IP addresses of the servers whose purpose in life is

of the servers whose purpose in life is to do this conversion for you but this

to do this conversion for you but this is a requisite feature if we just want

is a requisite feature if we just want the internet to be user friendly and

the internet to be user friendly and allow us to use words instead of numbers

allow us to use words instead of numbers alone what's inside of these DNS servers

alone what's inside of these DNS servers you know it's essentially a spreadsheet

you know it's essentially a spreadsheet or if we can say it more geeky it's

or if we can say it more geeky it's essentially like a hash table of some

essentially like a hash table of some sort which it has keys and values like

sort which it has keys and values like the key is the domain name harvard.edu

the key is the domain name harvard.edu gale. edug google.com and the value is

gale. edug google.com and the value is the corresponding IP address or in many

the corresponding IP address or in many cases IP addresses plural of the

cases IP addresses plural of the corresponding servers so here already

corresponding servers so here already even though I've drawn it fairly

even though I've drawn it fairly abstractly like you would on a

abstractly like you would on a chalkboard it's really probably

chalkboard it's really probably implemented as some kind of table maybe

implemented as some kind of table maybe a hash table maybe a database table

a hash table maybe a database table maybe SQL or something like that or

maybe SQL or something like that or maybe it's even just a link list or an

maybe it's even just a link list or an array we just have to somehow enable

array we just have to somehow enable this computer to convert one to the

this computer to convert one to the other now just to be super precise

other now just to be super precise DNS servers actually convert what are

DNS servers actually convert what are called fully qualified domain names

called fully qualified domain names which is generally not just harvard.edu

which is generally not just harvard.edu but more verbosely

but more verbosely www.harvard.edu and

www.harvard.edu and www.google.com so the whole thing that

www.google.com so the whole thing that you would see as a substring of the URL

you would see as a substring of the URL so that's what DNS does and that's what

so that's what DNS does and that's what your University your company your home

your University your company your home router are doing for you let me pause

router are doing for you let me pause here to see if there are any

here to see if there are any questions this to is just a technology

questions this to is just a technology now we'll take for granted just

now we'll take for granted just works questions at all all right so

works questions at all all right so let's now transition among our protocols

let's now transition among our protocols really to the last for today which will

really to the last for today which will set the stage for actually solving

set the stage for actually solving problems with these and writing some

problems with these and writing some code ultimately um HTTP this is

code ultimately um HTTP this is something that you see or hear all day

something that you see or hear all day long even though you rarely have to

long even though you rarely have to bother typing it anymore odds are if you

bother typing it anymore odds are if you go to harvard.edu yale.edu google.com

go to harvard.edu yale.edu google.com you don't bother typing HTTP let alone

you don't bother typing HTTP let alone https manually anymore why because your

https manually anymore why because your browser Auto completes that kind of

browser Auto completes that kind of thing just to make life easier but it is

thing just to make life easier but it is officially at the beginning of every URL

officially at the beginning of every URL you visit either HTTP or the more secure

you visit either HTTP or the more secure https whenever you're using your browser

https whenever you're using your browser to access some website so

to access some website so HTTP stands for hyper text transfer

HTTP stands for hyper text transfer protocol and it's uh easily one of the

protocol and it's uh easily one of the most popular dare say one of the most

most popular dare say one of the most powerful features of the internet

powerful features of the internet nowadays but the mental model to have

nowadays but the mental model to have here is that HTTP or the web more

here is that HTTP or the web more generally is kind of a service that runs

generally is kind of a service that runs on top of the internet and maybe Zoom or

on top of the internet and maybe Zoom or Microsoft teams is another service that

Microsoft teams is another service that runs on top of the internet an iMessage

runs on top of the internet an iMessage and Technologies like it is another

and Technologies like it is another service that runs on top of the internet

service that runs on top of the internet so the internet is really like the lower

so the internet is really like the lower level Plumbing the tcpip stuff the DNS

level Plumbing the tcpip stuff the DNS stuff that just gets data from point A

stuff that just gets data from point A to point B but now and we're in a a

to point B but now and we're in a a software development class ultimately

software development class ultimately here in cs50 HTTP is the application

here in cs50 HTTP is the application Level protocol it's sort of what

Level protocol it's sort of what programmers use what companies use what

programmers use what companies use what uh developers use ultimately to use the

uh developers use ultimately to use the underlying Plumbing to build interesting

underlying Plumbing to build interesting and Powerful things so what does this

and Powerful things so what does this mean when it comes to accessing Services

mean when it comes to accessing Services via HTTP or the more secure

via HTTP or the more secure https well here is a representative URL

https well here is a representative URL even though you might not type the whole

even though you might not type the whole thing if you poke around your address

thing if you poke around your address bar this is what's up there with that

bar this is what's up there with that said a lot of browsers nowadays are kind

said a lot of browsers nowadays are kind of simplifying if not dumbing down what

of simplifying if not dumbing down what you see with your human eyes just to

you see with your human eyes just to shorten the strings especially on mobile

shorten the strings especially on mobile devices but almost always if you click

devices but almost always if you click the URL or highlight it then you see the

the URL or highlight it then you see the whole thing but on many browsers you

whole thing but on many browsers you might only ever see example.com but all

might only ever see example.com but all of this information is there it's just

of this information is there it's just getting more and more hidden just for

getting more and more hidden just for user interfaces sake well it turns out

user interfaces sake well it turns out when you visit a URL by default

when you visit a URL by default especially if you type nothing after the

especially if you type nothing after the do com in this case you're technically

do com in this case you're technically implicitly adding a single slash so a

implicitly adding a single slash so a single slash denotes the root of the

single slash denotes the root of the server that is the default page or

server that is the default page or folder in the server and the slash

folder in the server and the slash whether or not you type it or not is

whether or not you type it or not is implicitly going to be there and that

implicitly going to be there and that just means give me the default whatever

just means give me the default whatever is at www.example.com give me that page

is at www.example.com give me that page or that folder but URLs can be longer

or that folder but URLs can be longer than this and more generally there can

than this and more generally there can be a path so to speak and this is a term

be a path so to speak and this is a term of art a path is some sequence of folder

of art a path is some sequence of folder Andor file names after a URL like this

Andor file names after a URL like this and so you might see more specifically

and so you might see more specifically that a URL contains a very specific file

that a URL contains a very specific file this isn't as common nowadays anymore

this isn't as common nowadays anymore though we will begin to today by using

though we will begin to today by using this technique but if there is a file

this technique but if there is a file called literally file. HTML or something

called literally file. HTML or something else on the server that file is going to

else on the server that file is going to be what this URL pulls up on the

be what this URL pulls up on the computer meanwhile you might have slash

computer meanwhile you might have slash folder slash which just means show me

folder slash which just means show me whatever is inside of this folder or you

whatever is inside of this folder or you might have more verbosely folder file.

might have more verbosely folder file. HTML which will show you that file in

HTML which will show you that file in that folder and meanwhile just to give

that folder and meanwhile just to give some other terms of art this is the

some other terms of art this is the so-called fully qualified domain name

so-called fully qualified domain name and again these vocab don't matter all

and again these vocab don't matter all that much but you'll hear or see them

that much but you'll hear or see them over time we generally colloquially just

over time we generally colloquially just refer to this as the domain name which

refer to this as the domain name which is a little less precise but gets the

is a little less precise but gets the job done certainly in conversation and

job done certainly in conversation and this part here I described briefly

this part here I described briefly earlier what's the name for this suffix

earlier what's the name for this suffix at the very end of the fully qualified

at the very end of the fully qualified domain

domain name the yeah yeah top level domain or

name the yeah yeah top level domain or TLD and this is just some form of

TLD and this is just some form of categorization of the URLs now the

categorization of the URLs now the internet in got it start within the

internet in got it start within the United States and a lot of the first

United States and a lot of the first websites of course came from the US and

websites of course came from the US and so For Better or For Worse the sort of

so For Better or For Worse the sort of steak was planted in the ground so

steak was planted in the ground so generally do indicated at least early on

generally do indicated at least early on that it was some kind of commercial

that it was some kind of commercial Enterprise a business that owned a

Enterprise a business that owned a domain name edu is a educational

domain name edu is a educational institution net was some kind of

institution net was some kind of network. goov was the US government now

network. goov was the US government now there are also country code tlds cctlds

there are also country code tlds cctlds like UK or JP every country in the world

like UK or JP every country in the world has its own two letter uh TLD um that

has its own two letter uh TLD um that might very well be restricted to only

might very well be restricted to only servers or companies or people in that

servers or companies or people in that uh in that country many of them can be

uh in that country many of them can be used by anyone you've used a lot of URLs

used by anyone you've used a lot of URLs in this class ending in

in this class ending in cs50. that doesn't mean input output um

cs50. that doesn't mean input output um it actually is a TLD from another

it actually is a TLD from another country that lets anyone on the internet

country that lets anyone on the internet um pay for and on an annual basis using

um pay for and on an annual basis using that domain. TV for instance you might

that domain. TV for instance you might see in some cases like twitch.tv and the

see in some cases like twitch.tv and the like um that too is owned by another

like um that too is owned by another country that allows others in the

country that allows others in the English-speaking World in this case to

English-speaking World in this case to use it as though it connotes TV but

use it as though it connotes TV but those are just different types of TLS

those are just different types of TLS that roughly categorize where the domain

that roughly categorize where the domain lives but it doesn't necessarily mean

lives but it doesn't necessarily mean it's commercial anymore it doesn't

it's commercial anymore it doesn't necessarily mean it's a Network anymore

necessarily mean it's a Network anymore for the most part there are hundreds of

for the most part there are hundreds of tlds now for better for worse most of

tlds now for better for worse most of which are less common than these big

which are less common than these big ones um but most anyone can buy most of

ones um but most anyone can buy most of them with just some restrictions on

them with just some restrictions on things like edu and goov that are still

things like edu and goov that are still very much regulated this meanwhile is

very much regulated this meanwhile is what we might call the host name www

what we might call the host name www it's obviously a super common convention

it's obviously a super common convention like almost every website uses www as

like almost every website uses www as its host name but that's a human

its host name but that's a human convention it's not a requirement and

convention it's not a requirement and indeed some websites don't even bother

indeed some websites don't even bother having a host name they just use their

having a host name they just use their domain to advertise their websites this

domain to advertise their websites this now is going to be the scheme or the

now is going to be the scheme or the protocol and this is just going to

protocol and this is just going to indicate via what protocol the computer

indicate via what protocol the computer your Mac your PC your phone should use

your Mac your PC your phone should use when accessing content at that address

when accessing content at that address because indeed there are other protocols

because indeed there are other protocols you can use but for the most part

you can use but for the most part will'll only focus on HTTP or

will'll only focus on HTTP or equivalently

equivalently https all right any questions now on

https all right any questions now on those just definitions building blocks

those just definitions building blocks of URLs just so we all sort of share a

of URLs just so we all sort of share a common

common vocabulary any questions at all yeah

vocabulary any questions at all yeah what is the

what is the local sure we'll come back to this

local sure we'll come back to this actually later today there's a a

actually later today there's a a technical term known as Local Host which

technical term known as Local Host which is a generic name for your computer your

is a generic name for your computer your Mac your PC your phone especially when

Mac your PC your phone especially when you're doing software development and by

you're doing software development and by convention your own computer has not

convention your own computer has not only whatever IP address you get from

only whatever IP address you get from your University or your internet service

your University or your internet service provider it also has a reflexive IP

provider it also has a reflexive IP address one that just always refers to

address one that just always refers to itself which is 127.0.0.1 and that's

itself which is 127.0.0.1 and that's just a human convention humans decided

just a human convention humans decided that shall refer always to your computer

that shall refer always to your computer and it's actually going to be useful

and it's actually going to be useful today and onward because we can use that

today and onward because we can use that when development on our own computers

when development on our own computers ultimately other questions on URLs IP

ultimately other questions on URLs IP DNS or any of these building

DNS or any of these building blocks all right so what do we mean by

blocks all right so what do we mean by HTTP being a protocol when I extended my

HTTP being a protocol when I extended my hand earlier as a human handshake you

hand earlier as a human handshake you know a typical human in healthy times

know a typical human in healthy times would know to respond in turn well how

would know to respond in turn well how does Brian's computer know to respond to

does Brian's computer know to respond to phyllis's envelope whatever message is

phyllis's envelope whatever message is they're in well assuming that Brian is

they're in well assuming that Brian is indeed still a web server in the store

indeed still a web server in the store and Phyllis was trying to upload a cat

and Phyllis was trying to upload a cat or maybe download a cat from Brian's web

or maybe download a cat from Brian's web server inside of phyllis's envelope

server inside of phyllis's envelope would have been a message literally in

would have been a message literally in text and it's English for the most part

text and it's English for the most part and it would contain at the beginning of

and it would contain at the beginning of that message literally one of these

that message literally one of these English verbs either get which means

English verbs either get which means just that get me the homepage get me a

just that get me the homepage get me a picture of a cat get me a picture of a

picture of a cat get me a picture of a dog or a video or anything else or post

dog or a video or anything else or post which often means post that is um put

which often means post that is um put that is um upload a picture of a cat or

that is um upload a picture of a cat or a dog or something else to the server

a dog or something else to the server instead that's not strictly the only use

instead that's not strictly the only use cases for these but you can generally

cases for these but you can generally think them as one is just getting

think them as one is just getting information and the other is posting

information and the other is posting from the client from Phyllis to the

from the client from Phyllis to the server in this case Brian so those are

server in this case Brian so those are the two keywords that we might see and

the two keywords that we might see and why is this useful well it turns out we

why is this useful well it turns out we can start to see in our own Mac or PC

can start to see in our own Mac or PC some of these very same messages for

some of these very same messages for instance if Phyllis were visiting not

instance if Phyllis were visiting not Brian's but example.com that web server

Brian's but example.com that web server inside of her metaphorical envelope

inside of her metaphorical envelope there would be a textual message that

there would be a textual message that literally starts with get slash then the

literally starts with get slash then the word HTTP then the version she's using

word HTTP then the version she's using 1.1 is very common two and three are

1.1 is very common two and three are becoming more common but HTTP generally

becoming more common but HTTP generally looks like this the next line of text in

looks like this the next line of text in her envelope would probably say host

her envelope would probably say host colon then literally the fully qualified

colon then literally the fully qualified domain name of the server she's

domain name of the server she's accessing just in case and this happens

accessing just in case and this happens super commonly especially on small

super commonly especially on small websites if one server is hosting

websites if one server is hosting multiple domain names multiple websites

multiple domain names multiple websites this just distinguishes which one she

this just distinguishes which one she actually wants and then there's usually

actually wants and then there's usually a whole bunch of other lines of text as

a whole bunch of other lines of text as well so where can you actually see this

well so where can you actually see this well let me actually go ahead and do

well let me actually go ahead and do this give me just a moment and I'm going

this give me just a moment and I'm going to open up on my computer here uh an

to open up on my computer here uh an empty Chrome window in incognito mode

empty Chrome window in incognito mode generally speaking incognito mode or

generally speaking incognito mode or private mode is used when you don't want

private mode is used when you don't want there to be left remnants of what

there to be left remnants of what websites you visited and it has the

websites you visited and it has the effect for software developers of just

effect for software developers of just forgetting any things you might have

forgetting any things you might have tried already within your browser

tried already within your browser including things called cookies more on

including things called cookies more on those another time uh your autocomplete

those another time uh your autocomplete history and the like so for development

history and the like so for development purposes incognito mode is especially

purposes incognito mode is especially helpful because it's sort of like

helpful because it's sort of like starting with a clean slate every time

starting with a clean slate every time you open a new private or incognito mode

you open a new private or incognito mode so there's not going to be like any

so there's not going to be like any remnants of previous testing or code

remnants of previous testing or code that you've been playing with and I'm

that you've been playing with and I'm going to go ahead and do this I'm going

going to go ahead and do this I'm going to go ahead and uh right click or

to go ahead and uh right click or control click on Chrome I'm going to

control click on Chrome I'm going to choose inspect and it's going to pull up

choose inspect and it's going to pull up this window sometimes on the side

this window sometimes on the side sometimes on the bottom I'm going to

sometimes on the bottom I'm going to move it to the bottom just so we can see

move it to the bottom just so we can see it a little more readily and I'm going

it a little more readily and I'm going to zoom in and it's going to look a

to zoom in and it's going to look a little Arcane at first and I'm going to

little Arcane at first and I'm going to just highlight a few of these tabs we'll

just highlight a few of these tabs we'll see here along the top that there's

see here along the top that there's elements console sources Network and

elements console sources Network and whole bunch of other things as well this

whole bunch of other things as well this is sort of the advanced mode in Chrome

is sort of the advanced mode in Chrome and Safari and Firefox and Edge have

and Safari and Firefox and Edge have their own equivalent of these features

their own equivalent of these features they've always been there even if you've

they've always been there even if you've never clicked the right button to enable

never clicked the right button to enable these features and I'm going to focus

these features and I'm going to focus for a moment on network like this

for a moment on network like this this is a feature of the browser that's

this is a feature of the browser that's going to allow me the programmer in this

going to allow me the programmer in this case so the the engineer to just kind of

case so the the engineer to just kind of look at what messages my browser is

look at what messages my browser is actually sending to a server so let me

actually sending to a server so let me go ahead and do something like this let

go ahead and do something like this let me go ahead and visit uh for instance uh

me go ahead and visit uh for instance uh in my browser here and I'm going to

in my browser here and I'm going to shrink the window just a little bit so

shrink the window just a little bit so we can see it exactly I'm going to visit

we can see it exactly I'm going to visit https uh

https uh www.harvard.edu and now I'm going to hit

www.harvard.edu and now I'm going to hit enter

enter and a whole bunch of stuff just happened

and a whole bunch of stuff just happened along the bottom of my screen and I'm

along the bottom of my screen and I'm going to try to pull my window up just a

going to try to pull my window up just a little bit so we can focus on a subset

little bit so we can focus on a subset of this let me pull this up covering up

of this let me pull this up covering up really the content of the page focusing

really the content of the page focusing on these lower level details down here

on these lower level details down here and what I want to see first

and what I want to see first is let me oh sorry let me go ahead and

is let me oh sorry let me go ahead and reload this page here after retaining

reload this page here after retaining the log so that we can see absolutely

the log so that we can see absolutely everything on the screen and to be clear

everything on the screen and to be clear I just checked because I forgot earlier

I just checked because I forgot earlier preserve log because I wanted to

preserve log because I wanted to preserve everything on the screen I want

preserve everything on the screen I want to see everything all at once and we'll

to see everything all at once and we'll see this the very first line of output

see this the very first line of output is completely overwhelming with detail

is completely overwhelming with detail at first glance but what you'll see here

at first glance but what you'll see here if I start to scroll down and down and

if I start to scroll down and down and down and down are the so-called request

down and down are the so-called request headers and let me zoom in here and what

headers and let me zoom in here and what you're seeing inside of chrome inside of

you're seeing inside of chrome inside of its Network tab in its so-called

its Network tab in its so-called developer tools again this is just for

developer tools again this is just for engineering types you'll see all of the

engineering types you'll see all of the headers all of the lines of text that

headers all of the lines of text that magically were sent by my Mac to

magically were sent by my Mac to harvard.edu much like from Phyllis to

harvard.edu much like from Phyllis to Brian server in that story so I can see

Brian server in that story so I can see exactly what messages are being sent and

exactly what messages are being sent and a lot of this we haven't talked about

a lot of this we haven't talked about yet but we do see some mention of get

yet but we do see some mention of get and we see some mention of Slash and a

and we see some mention of Slash and a bunch of other Arcane details but notice

bunch of other Arcane details but notice they're all sort of key value pairs with

they're all sort of key value pairs with the here indicating what the

the here indicating what the corresponding value is now most of this

corresponding value is now most of this is not going to be interesting and we're

is not going to be interesting and we're not going to focus too much on the weeds

not going to focus too much on the weeds of of all of this but it indeed gives us

of of all of this but it indeed gives us a sense of what's inside of that virtual

a sense of what's inside of that virtual envelope now harvard.edu is one thing

envelope now harvard.edu is one thing but there's other uh websites we might

but there's other uh websites we might visit as well um and no matter what they

visit as well um and no matter what they are we're going to expect ultimately an

are we're going to expect ultimately an HTTP response so in addition to a

HTTP response so in addition to a computer like my Mac or phyllis's

computer like my Mac or phyllis's computer sending a request containing

computer sending a request containing gets and host and those details too

gets and host and those details too you'll see here in my slide form just

you'll see here in my slide form just representative response from the server

representative response from the server and notice that key here is that the

and notice that key here is that the server is responding in the same version

server is responding in the same version of HTTP in this example it's sending

of HTTP in this example it's sending back this so-called status code just a

back this so-called status code just a numeric code that indicates in this case

numeric code that indicates in this case that everything's okay and it includes

that everything's okay and it includes this header this HTTP header which again

this header this HTTP header which again is just a key value pair saying that the

is just a key value pair saying that the type of this content that's coming back

type of this content that's coming back from the server is text/html more on

from the server is text/html more on HTML in just a little bit but for our

HTML in just a little bit but for our purposes now this just means that

purposes now this just means that harvard.edu is sending me back a web

harvard.edu is sending me back a web page and indeed if we hide all of this

page and indeed if we hide all of this techn techical stuff that's the web page

techn techical stuff that's the web page that we saw up here with all of the

that we saw up here with all of the usual imagery and the like and in fact I

usual imagery and the like and in fact I can see this if I scroll back up not to

can see this if I scroll back up not to request headers but response headers

request headers but response headers you'll see up here that we get back

you'll see up here that we get back responses including the date that the

responses including the date that the server responded and a whole bunch of

server responded and a whole bunch of other details as well and honestly this

other details as well and honestly this has always been under your fingertips

has always been under your fingertips and it will soon be useful as we start

and it will soon be useful as we start making web-based applications ultimately

making web-based applications ultimately but this very quickly gets overwhelming

but this very quickly gets overwhelming quickly and so better in this might

quickly and so better in this might actually be a tool that we can use

actually be a tool that we can use within our code space itself so let me

within our code space itself so let me go back to vs code here I didn't open

go back to vs code here I didn't open any code tabs I'm just going to use my

any code tabs I'm just going to use my terminal window for a moment and I'm

terminal window for a moment and I'm going to run a couple of commands that

going to run a couple of commands that are going to allow me to actually see

are going to allow me to actually see what is going on when I request one

what is going on when I request one website let me go ahead and use a

website let me go ahead and use a command called curl for connect URL and

command called curl for connect URL and this is like a command line black and

this is like a command line black and white program that's going to pretend to

white program that's going to pretend to be a browser and it's going to connect

be a browser and it's going to connect to the URL show me the headers but it's

to the URL show me the headers but it's not going to show me the images are the

not going to show me the images are the graphics which might very well be useful

graphics which might very well be useful to the humans but not to me right now as

to the humans but not to me right now as the developer so I'm going to do curl

the developer so I'm going to do curl I'm going to do Dashi and then I'm going

I'm going to do Dashi and then I'm going to do

to do https www.harvard.edu as though I'm

https www.harvard.edu as though I'm pretending to be a browser requesting

pretending to be a browser requesting the home page and what's nice about curl

the home page and what's nice about curl is albeit overwhelming too you'll get

is albeit overwhelming too you'll get back a whole response from the server

back a whole response from the server containing only those header values the

containing only those header values the key value pairs inside of the envelope

key value pairs inside of the envelope and we'll ignore almost all of these but

and we'll ignore almost all of these but here is the response from the server it

here is the response from the server it responded using a new and improved

responded using a new and improved version of HTTP in this case version two

version of HTTP in this case version two and it gave me back a 200 there's my

and it gave me back a 200 there's my content type text/html and then this

content type text/html and then this Char set happens to do with the encoding

Char set happens to do with the encoding if it's Unicode or asky or something

if it's Unicode or asky or something else and then there's all this other

else and then there's all this other overwhelming detail for now but this is

overwhelming detail for now but this is the beginnings of my ability to just

the beginnings of my ability to just kind of poke around and see how the

kind of poke around and see how the server works and it turns out too that

server works and it turns out too that we'll be able to see other potential

we'll be able to see other potential responses as well so for instance uh

responses as well so for instance uh HTTP might not only return 100 what if I

HTTP might not only return 100 what if I do this instead let me go ahead and

do this instead let me go ahead and visit c-i

visit c-i HTTP

HTTP colon uh www.harvard.edu so notice I

colon uh www.harvard.edu so notice I deliberately use the insecure version of

deliberately use the insecure version of the URL which maybe Harvard's

the URL which maybe Harvard's Administration system administration

Administration system administration doesn't like anymore well how can they

doesn't like anymore well how can they ensure that I the end user the student

ensure that I the end user the student nonetheless use

nonetheless use https even if I didn't type it myself

https even if I didn't type it myself well let me run just that command with

well let me run just that command with just HTTP not https and you'll see that

just HTTP not https and you'll see that everything is not okay it didn't come

everything is not okay it didn't come back with a 200 it came back with 301 in

back with a 200 it came back with 301 in this message saying Harvard moved

this message saying Harvard moved permanently but here's where you can

permanently but here's where you can look for another clue among all of these

look for another clue among all of these lines most of which I don't care about

lines most of which I don't care about there's a location header colon that's a

there's a location header colon that's a little hint to me that says where

little hint to me that says where Harvard University has apparently moved

Harvard University has apparently moved too on the web and what's different

too on the web and what's different about this URL just to be

about this URL just to be clear it has the S included and what

clear it has the S included and what your browser will do by default because

your browser will do by default because Google and Microsoft and Mozilla

Google and Microsoft and Mozilla programmed it this way whenever it sees

programmed it this way whenever it sees a 301 response instead of 200 it won't

a 301 response instead of 200 it won't show you any web page it will look for a

show you any web page it will look for a location header find that URL and then

location header find that URL and then automatically quote unquote redirect you

automatically quote unquote redirect you there to so this is why it doesn't

there to so this is why it doesn't matter what we type in the browser

matter what we type in the browser Harvard can have its server send these

Harvard can have its server send these semi-secret messages to our browsers and

semi-secret messages to our browsers and then it will just visit a second URL all

then it will just visit a second URL all automatically and you can do this with

automatically and you can do this with host names as well suppose that Harvard

host names as well suppose that Harvard does not want to standardize on

does not want to standardize on harvard.edu why they just want it to

harvard.edu why they just want it to always be www maybe it's a branding

always be www maybe it's a branding thing maybe it's a technical thing we

thing maybe it's a technical thing we can see the exact same response here

can see the exact same response here this first tells me when I visit HTTP

this first tells me when I visit HTTP harvard.edu with no www Harvard

harvard.edu with no www Harvard minimally wants me to be using a secure

minimally wants me to be using a secure connection if I then okay fine cooperate

connection if I then okay fine cooperate let me go ahead and clear my screen let

let me go ahead and clear my screen let me add the s but not the www you can see

me add the s but not the www you can see here that it again responded with 301 up

here that it again responded with 301 up here and the location now adds the www

here and the location now adds the www so it's just a way of bouncing users

so it's just a way of bouncing users from one place to another and this is

from one place to another and this is all thanks to http boiling down to

all thanks to http boiling down to relatively simple messages inside of the

relatively simple messages inside of the envelope that tell the computer the

envelope that tell the computer the browser in this case how to respond now

browser in this case how to respond now odds are you've seen others besides 301

odds are you've seen others besides 301 even though you've probably never seen

even though you've probably never seen that actual number unless you've done

that actual number unless you've done this kind of thing before but there

this kind of thing before but there probably is a number that like everyone

probably is a number that like everyone in this room has seen if if You' never

in this room has seen if if You' never really wondered why is it that number I

really wondered why is it that number I think you're smiling what number are you

think you're smiling what number are you thinking of yeah so 404 why is 404 well

thinking of yeah so 404 why is 404 well 404 indicates by convention not found

404 indicates by convention not found and now why the world decided years ago

and now why the world decided years ago to show us normal humans on the internet

to show us normal humans on the internet 404 is anything significant is unclear

404 is anything significant is unclear that's sort of like bad design like what

that's sort of like bad design like what do I care if the status code is 404 but

do I care if the status code is 404 but it's common enough on the internet that

it's common enough on the internet that probably all of us have seen it but that

probably all of us have seen it but that just means that some server when you

just means that some server when you visit a URL that's incorrect maybe it's

visit a URL that's incorrect maybe it's outdated the URL has been changed if you

outdated the URL has been changed if you see a 404 it just means that the virtual

see a 404 it just means that the virtual envelope that came from the server back

envelope that came from the server back to your Mac or PC or phone contains not

to your Mac or PC or phone contains not 200 okay not 301 moved permanently but

200 okay not 301 moved permanently but 404 not found instead and it's usually

404 not found instead and it's usually accompanied by a technical message maybe

accompanied by a technical message maybe a cute picture of a cat sort of hiding

a cute picture of a cat sort of hiding because it means not found or something

because it means not found or something like that the Aesthetics are entirely up

like that the Aesthetics are entirely up to the server but that's what the 404

to the server but that's what the 404 means and there's other codes too a few

means and there's other codes too a few of which you'll use in the coming weeks

of which you'll use in the coming weeks as we transition from commandline

as we transition from commandline programs in C and python to web-based

programs in C and python to web-based ones we've seen a few of these already

ones we've seen a few of these already and several of them are kind of variants

and several of them are kind of variants of the same thing like these 300 ones

of the same thing like these 300 ones here but we'll see others like 403

here but we'll see others like 403 Forbidden means like you probably forgot

Forbidden means like you probably forgot to log in or you need to log in or

to log in or you need to log in or something like that uh internal server

something like that uh internal server error right now no in the coming weeks

error right now no in the coming weeks you will absolutely encounter 500 errors

you will absolutely encounter 500 errors and they're always your fault it's going

and they're always your fault it's going to be the equivalent of like a

to be the equivalent of like a segmentation fault in C a 500 error

segmentation fault in C a 500 error means like you or I screwed up when

means like you or I screwed up when writing some code so we're going to see

writing some code so we're going to see that but it's just going to be an

that but it's just going to be an opportunity for us to fix it if a

opportunity for us to fix it if a server's overloaded you often see 503

server's overloaded you often see 503 like something's unavailable because

like something's unavailable because something's too popular uh or is maybe

something's too popular uh or is maybe worse getting attacked um this is an old

worse getting attacked um this is an old uh um April Fool's joke 418 is not

uh um April Fool's joke 418 is not actually used in practice but someone

actually used in practice but someone like took the time to write up an entire

like took the time to write up an entire formal text technical proposal so that

formal text technical proposal so that servers can respond saying I'm a teapot

servers can respond saying I'm a teapot so it's kind of part of Internet lore

so it's kind of part of Internet lore and there's other ones of these status

and there's other ones of these status codes as well but this is useful because

codes as well but this is useful because eventually we'll see in code you can use

eventually we'll see in code you can use this understanding uh high level as it

this understanding uh high level as it might be of HTTP to do some interesting

might be of HTTP to do some interesting and Powerful things so for instance we

and Powerful things so for instance we can even send fancier URLs to servers it

can even send fancier URLs to servers it turns out as we'll soon see if you send

turns out as we'll soon see if you send a message like this get/ search question

a message like this get/ search question question mark Q equals cats and then

question mark Q equals cats and then HTTP 1.1 or whatever version and you

HTTP 1.1 or whatever version and you send that message to Google server

send that message to Google server www.google.com this is how you can

www.google.com this is how you can specify not just the path of a web page

specify not just the path of a web page that you want SL search in this case the

that you want SL search in this case the question mark it turns out is going to

question mark it turns out is going to be a convention in the internet in the

be a convention in the internet in the web specifically for passing hum human

web specifically for passing hum human uh user input to the server as well in

uh user input to the server as well in fact you've probably never paid close

fact you've probably never paid close attention to URLs but they very often

attention to URLs but they very often have question question marks they very

have question question marks they very often have equal signs and indeed even

often have equal signs and indeed even google.com supports a certain key Q in

google.com supports a certain key Q in this case for query and you can put

this case for query and you can put anything you want after that in order to

anything you want after that in order to search for actual cats so if I actually

search for actual cats so if I actually go back to Chrome itself here for a

go back to Chrome itself here for a moment let me pull back my uh pull back

moment let me pull back my uh pull back open my Chrome browser here previously I

open my Chrome browser here previously I was using uh incognito mode for

was using uh incognito mode for harvard.edu I've gone ahead and Clos

harvard.edu I've gone ahead and Clos that window and opened a new one so we

that window and opened a new one so we can start fresh by visiting Google

can start fresh by visiting Google normally you and I are in the habit of

normally you and I are in the habit of going to google.com and searching via

going to google.com and searching via the form or nowadays you just type like

the form or nowadays you just type like your search query in the browser itself

your search query in the browser itself and it brings you automatically to

and it brings you automatically to Google or Bing or something else but I

Google or Bing or something else but I can really be pedantic here let me go

can really be pedantic here let me go ahead and zoom in and I'll manually go

ahead and zoom in and I'll manually go to

to https

https www.google.com search question mark Q

www.google.com search question mark Q equals maybe cats now this would not be

equals maybe cats now this would not be a very userfriendly experience if all of

a very userfriendly experience if all of us had to manually type out something

us had to manually type out something crazy like that but that's what the form

crazy like that but that's what the form is redirecting you to when you type in

is redirecting you to when you type in more user friendly cats into like a text

more user friendly cats into like a text box if I hit enter here we'll get back

box if I hit enter here we'll get back indeed a whole bunch of search results

indeed a whole bunch of search results about cats if I zoom back in and maybe I

about cats if I zoom back in and maybe I change it from cats to dogs that too is

change it from cats to dogs that too is going to change and notice it's

going to change and notice it's pre-populating the text box because

pre-populating the text box because Google has written its code in order to

Google has written its code in order to do so as well now appr propo of the

do so as well now appr propo of the video with which we began today from

video with which we began today from yester year one of the better uh Yale

yester year one of the better uh Yale pranks over the years um has anyone one

pranks over the years um has anyone one actually ever been to uh

actually ever been to uh safetyschool.org

safetyschool.org and to our friends at Yale watching live

and to our friends at Yale watching live hi safetyschool.org so it's kind of fun

hi safetyschool.org so it's kind of fun if you actually visit it uh depending on

if you actually visit it uh depending on who you are so if I open up a new window

who you are so if I open up a new window here and I go to

here and I go to https

https safetyschool.org zooming

safetyschool.org zooming in enter my oh my look at where it goes

in enter my oh my look at where it goes now how does that okay okay thank

you this is not a cs50 thing someone out there who I don't know who they are for

there who I don't know who they are for like 20 years has been paying annually

like 20 years has been paying annually for that domain safetyschool.org for

for that domain safetyschool.org for just this joke uh but if I now go back

just this joke uh but if I now go back into VSS code here in my terminal window

into VSS code here in my terminal window and a little more you know

and a little more you know sophisticatedly let me do curl dasi HTTP

sophisticatedly let me do curl dasi HTTP col andsafety

col andsafety school.org the browser of course

school.org the browser of course immediately redirected me to the website

immediately redirected me to the website but what's going on under underneath the

but what's going on under underneath the hood well 20 some years ago someone

hood well 20 some years ago someone bought the domain configured the server

bought the domain configured the server quite simply to spit this out safety

quite simply to spit this out safety school.org for years has moved

school.org for years has moved permanently to

permanently to www.yale.edu so just a little

www.yale.edu so just a little demonstration of what you can do with

demonstration of what you can do with just a little bit of understanding of

just a little bit of understanding of HTTP that's been a lot already we've

HTTP that's been a lot already we've laid the foundation for understanding

laid the foundation for understanding how the internet works now we're going

how the internet works now we're going to use it with a language called HTML

to use it with a language called HTML hypertext markup language CSS cascading

hypertext markup language CSS cascading stylesheets and JavaScript a proper

stylesheets and JavaScript a proper program in language but first it feels

program in language but first it feels opportune to take a 10-minute break we

opportune to take a 10-minute break we have some wonderful Halloween candy in

have some wonderful Halloween candy in the transcept and we'll be back in 10

the transcept and we'll be back in 10 with those

with those languages we're headed up to uh

languages we're headed up to uh Austin checking out the stadium for the

prank a few years ago I was at a math conference and I was sitting around at

conference and I was sitting around at the table at dinner with a few other

the table at dinner with a few other mathematicians and uh one of them went

mathematicians and uh one of them went to Harvard and started telling the story

to Harvard and started telling the story of this amazing prank that was you know

of this amazing prank that was you know against Harvard and at that point I felt

against Harvard and at that point I felt I had to interrupt and said well

I had to interrupt and said well actually I can tell you a lot more about

actually I can tell you a lot more about that okay okay the idea was perfected in

that okay okay the idea was perfected in a dorm room came up with the idea

a dorm room came up with the idea actually to prank them with signs at the

actually to prank them with signs at the football game we threw some ideas out

football game we threw some ideas out there as far as what what the signs

there as far as what what the signs would say we uh eventually settled on we

would say we uh eventually settled on we suck and my immediate reaction was no

suck and my immediate reaction was no this will never work however the problem

this will never work however the problem solver in me started thinking well maybe

solver in me started thinking well maybe we can make this

we can make this work the problem they had to infiltrate

work the problem they had to infiltrate Harvard Stadium without getting caught

Harvard Stadium without getting caught sneak in 1,800 placards distribute them

sneak in 1,800 placards distribute them to unsuspecting Harvard fans and then

to unsuspecting Harvard fans and then convince those fans to prank themselves

convince those fans to prank themselves it's great we thought about basically

it's great we thought about basically every possible thing that could go wrong

every possible thing that could go wrong and tried to come up with a solution for

and tried to come up with a solution for it and then you put two Reds on top of

it and then you put two Reds on top of it they made fake Harvard IDs and fake

it they made fake Harvard IDs and fake backstories

backstories fake placard designs and a 28 member

fake placard designs and a 28 member fake Pep

fake Pep Squad on November 20th 2004 a fake

Squad on November 20th 2004 a fake Harvard student smuggled the placards

Harvard student smuggled the placards into the game what do you think

[Music] [Applause]

[Applause] of but then trouble what houses

of but then trouble what houses how many how many extra

are I you know just showed him the front of this ID and all of a sudden he just

of this ID and all of a sudden he just ran away and he felt so

ran away and he felt so embarrassed having escaped one

embarrassed having escaped one confrontation they couldn't risk another

confrontation they couldn't risk another it was

it was time this just looks like a total mess

time this just looks like a total mess we have absolutely no idea if this is

we have absolutely no idea if this is going to work look at the it's going to

going to work look at the it's going to happen it's actually going to

happen it's actually going to happen I can't believe this what was

happen I can't believe this what was once a

once a prank became a

legend and immediately we started hearing chance from the other side

you and I think it was at that point in time that we knew we had pulled it

time that we knew we had pulled it off on

off on there it goes

there it goes [Applause]

[Applause] again I really think it didn't matter

again I really think it didn't matter that Harvard won because of the prank

that Harvard won because of the prank for a lot of Yale students in alumni we

for a lot of Yale students in alumni we definitely won that

definitely won that [Music]

Hallen thank you all okay thank you oh here oh all right little Halloween candy

here oh all right little Halloween candy all right thank you all so glad I wore

all right thank you all so glad I wore the same thing

the same thing today so in just a

today so in just a moment thank you okay so in just a

moment thank you okay so in just a moment we'll transition to understanding

moment we'll transition to understanding all the now what we can do with this

all the now what we can do with this underlying infrastructure so again HTTP

underlying infrastructure so again HTTP and below it TCP IP is all about just

and below it TCP IP is all about just getting the data from point A to pointb

getting the data from point A to pointb in some standardized way but now let's

in some standardized way but now let's talk about HTML this is the language in

talk about HTML this is the language in which web pages themselves are written

which web pages themselves are written hyper text markup language now some of

hyper text markup language now some of you might have used this before to make

you might have used this before to make personal homepages some of you might

personal homepages some of you might have dabbled even if using some website

have dabbled even if using some website to create your own homepage but

to create your own homepage but understanding this language is useful

understanding this language is useful certainly for creating the Aesthetics of

certainly for creating the Aesthetics of a web page can vying data that's of

a web page can vying data that's of interest but at the end of the day it

interest but at the end of the day it and the language we look at next CSS uh

and the language we look at next CSS uh are not programming languages there's

are not programming languages there's going to be no functions no Loops uh no

going to be no functions no Loops uh no programming logic but we will end today

programming logic but we will end today with a teaser of a proper programming

with a teaser of a proper programming language called JavaScript via which you

language called JavaScript via which you can manipulate all of these various uh

can manipulate all of these various uh other languages as well so ultimately

other languages as well so ultimately HTML has like two features and this is a

HTML has like two features and this is a language that we spend very brief amount

language that we spend very brief amount of time on because it really boils down

of time on because it really boils down to just a couple of basic ideas and then

to just a couple of basic ideas and then vocabulary that you'll build out over

vocabulary that you'll build out over time just by Googling looking up

time just by Googling looking up references looking at other Pages source

references looking at other Pages source code but tags and attributes are what

code but tags and attributes are what characterize HTML now what do I mean by

characterize HTML now what do I mean by that here for instance is the HTML code

that here for instance is the HTML code via which you can make probably the

via which you can make probably the simplest of all web pages one that quite

simplest of all web pages one that quite simply says in the uh browser window

simply says in the uh browser window hello title and hello body for instance

hello title and hello body for instance now what does this actually mean if you

now what does this actually mean if you imagine opening up uh this code in a

imagine opening up uh this code in a browser be it on a Mac or PC or phone

browser be it on a Mac or PC or phone you'll see typically like some kind of

you'll see typically like some kind of rectangular window and there's usually a

rectangular window and there's usually a tab that has the title of that page and

tab that has the title of that page and then most of the rectangular region is

then most of the rectangular region is the web page itself what you're looking

the web page itself what you're looking at then is the code that's going to put

at then is the code that's going to put hello title in the title bar in the tab

hello title in the title bar in the tab at the very top and down at the bottom

at the very top and down at the bottom hello body is going to be all that's in

hello body is going to be all that's in the big black and white box that

the big black and white box that composes the rest of the browser window

composes the rest of the browser window itself now what are the Salient

itself now what are the Salient characteristics here that we'll now

characteristics here that we'll now start to take for granted well first

start to take for granted well first whoops uh first let's go ahead and give

whoops uh first let's go ahead and give me just a moment here um and actually do

me just a moment here um and actually do something with this code so I'm going to

something with this code so I'm going to go ahead and do this back in vs code

go ahead and do this back in vs code here I'm going to first create a file

here I'm going to first create a file called say hello.html and in this tab

called say hello.html and in this tab I'm going to go ahead and really repeat

I'm going to go ahead and really repeat exactly that same code now I had this

exactly that same code now I had this line first DOC type HTML then I had this

line first DOC type HTML then I had this line HTML Lang equals quote unquote n

line HTML Lang equals quote unquote n close quote then I had inside of that

close quote then I had inside of that head then I had inside of that title

head then I had inside of that title then inside of that I had hello title

then inside of that I had hello title and I'm doing this quickly because we'll

and I'm doing this quickly because we'll tease apart in a moment what it actually

tease apart in a moment what it actually all means and then down here below that

all means and then down here below that so-called head I had just the text hello

so-called head I had just the text hello body so at the moment I that I claim is

body so at the moment I that I claim is the entirety of a web page but it

the entirety of a web page but it currently lives in my code space so to

currently lives in my code space so to speak in a file called hello.html that's

speak in a file called hello.html that's fine if I want to create it but how do I

fine if I want to create it but how do I how do you how does anyone on the

how do you how does anyone on the internet actually view it well to serve

internet actually view it well to serve a web page you indeed need a web server

a web page you indeed need a web server and it turns out that codespaces comes

and it turns out that codespaces comes with one of these pre-installed because

with one of these pre-installed because we cs50 staff uh did so for you and what

we cs50 staff uh did so for you and what you can do in a terminal window once you

you can do in a terminal window once you have an HTML file ready to go that you

have an HTML file ready to go that you want the world to see you can literally

want the world to see you can literally run in your terminal window

run in your terminal window http-server single command and what

http-server single command and what that's going to do for you is start a

that's going to do for you is start a web server that is to say a program

web server that is to say a program whose purpose in life is just to serve

whose purpose in life is just to serve web pages and even though probably up

web pages and even though probably up until now for years you probably if

until now for years you probably if you're like me equate server quote

you're like me equate server quote unquote with a physical device server is

unquote with a physical device server is really aie piece of software it just

really aie piece of software it just tends to run on big fancy devices so

tends to run on big fancy devices so when we say server we often all think of

when we say server we often all think of in our Mind's Eye you know big expensive

in our Mind's Eye you know big expensive devices perhaps but a server is just a

devices perhaps but a server is just a program whose purpose in life is to

program whose purpose in life is to respond to requests with responses and

respond to requests with responses and that's the vernacular there now once you

that's the vernacular there now once you run HTTP server and I'm going to do a

run HTTP server and I'm going to do a bit of magic because I set this up

bit of magic because I set this up before class just to make sure it goes

before class just to make sure it goes smoothly you'll see some output like

smoothly you'll see some output like this whereby your server is now

this whereby your server is now available on a very long URL mine here

available on a very long URL mine here uh uh is a very long URL that will be

uh uh is a very long URL that will be different from yours but what this is is

different from yours but what this is is a unique identifier that your codes

a unique identifier that your codes space has temporarily generated so that

space has temporarily generated so that you can now access and ideally only you

you can now access and ideally only you can access that file using your browser

can access that file using your browser now if I flip the URL or you flip the

now if I flip the URL or you flip the your all to public by right clicking or

your all to public by right clicking or control clicking the right features of

control clicking the right features of vs code you can enable anyone in the

vs code you can enable anyone in the world to visit it but we're not going to

world to visit it but we're not going to ultimately host our websites in your

ultimately host our websites in your code space because as soon as you log

code space because as soon as you log off for the night and the thing shuts

off for the night and the thing shuts down like the website will go down but

down like the website will go down but at the end end of the semester

at the end end of the semester particularly for final projects we'll

particularly for final projects we'll show you ways that you can put your own

show you ways that you can put your own website your own code on the actual

website your own code on the actual internet

internet 247365 even with your own domain name if

247365 even with your own domain name if you want to get one so that it lives uh

you want to get one so that it lives uh independent of your own sleep schedule

independent of your own sleep schedule and usage schedule of vs code here so

and usage schedule of vs code here so I'm going to go ahead now and visit um

I'm going to go ahead now and visit um this URL in another tab of my browser

this URL in another tab of my browser and what I'll see here is this this is

and what I'll see here is this this is the output of that program called HTTP

the output of that program called HTTP server and essentially what it is doing

server and essentially what it is doing is it's using TCP and IP in conjunction

is it's using TCP and IP in conjunction with HTTP to just run your very own web

with HTTP to just run your very own web server on gith hubs own servers as well

server on gith hubs own servers as well and that's because of different ports

and that's because of different ports again we won't go too much into the

again we won't go too much into the weeds of the TCP the IP and all of that

weeds of the TCP the IP and all of that stuff but recall that different port

stuff but recall that different port numbers can allow you to distinguish one

numbers can allow you to distinguish one service from another now one of the

service from another now one of the services is of course your code space

services is of course your code space VSS code in the cloud that we've been

VSS code in the cloud that we've been using for weeks but if you want to use

using for weeks but if you want to use the same physical server that GitHub

the same physical server that GitHub controls but actually visit your own web

controls but actually visit your own web server that I just ran in my terminal

server that I just ran in my terminal window in another tab that's fine

window in another tab that's fine they're just going to be using different

they're just going to be using different TCP ports and you and I don't have to

TCP ports and you and I don't have to care what they are but just that this is

care what they are but just that this is a feature that TCP supports so what you

a feature that TCP supports so what you see here is somewhat Arcane this is not

see here is somewhat Arcane this is not like a thing that most people on the

like a thing that most people on the internet should ever see I'm just doing

internet should ever see I'm just doing this for development purposes but this

this for development purposes but this is the index that is the directory the

is the index that is the directory the the folder contents of my code space and

the folder contents of my code space and because I deleted everything from prior

because I deleted everything from prior weeks already all we see right now is

weeks already all we see right now is hello.html

hello.html which I just created so if I click on

which I just created so if I click on hello.html within this folder listing

hello.html within this folder listing you'll be a little underwhelmed and I'll

you'll be a little underwhelmed and I'll zoom in just so there's something more

zoom in just so there's something more interesting there but now you see hello

interesting there but now you see hello comma body but what's interesting

comma body but what's interesting perhaps is that after this long very

perhaps is that after this long very cryptic and uninteresting URL notice at

cryptic and uninteresting URL notice at the very end of it and I'll zoom in in a

the very end of it and I'll zoom in in a moment what do you see slh hello.html

moment what do you see slh hello.html which follows the convention I claimed

which follows the convention I claimed before break is how a browser would

before break is how a browser would allow you to to access a specific file

allow you to to access a specific file on a server by doing slash and then the

on a server by doing slash and then the name of the file name everything before

name of the file name everything before it is very cryptic it would be better if

it is very cryptic it would be better if I you know uh buy a domain name that's a

I you know uh buy a domain name that's a little more easy to remember and set

little more easy to remember and set that up some other time but for now

that up some other time but for now let's just focus on only the file names

let's just focus on only the file names that I'm actually creating all right so

that I'm actually creating all right so the code is up and running underwhelming

the code is up and running underwhelming though it might be with the body in the

though it might be with the body in the middle of the page and let me zoom in up

middle of the page and let me zoom in up here too hello comma title is indeed in

here too hello comma title is indeed in the tab just as promised so what's

the tab just as promised so what's actually going on with code like this

actually going on with code like this that we just created well let's go back

that we just created well let's go back to the slide version of the same and let

to the slide version of the same and let me just highlight a few of these lines

me just highlight a few of these lines the very first line is what's called

the very first line is what's called your document type declaration doesn't

your document type declaration doesn't really matter to remember that by

really matter to remember that by phrasing and this is just something you

phrasing and this is just something you copy paste or do from memory at the top

copy paste or do from memory at the top of any HTML file that you create when

of any HTML file that you create when making your own web page it's a implicit

making your own web page it's a implicit indicator to the browser that you're

indicator to the browser that you're using the very latest version of HTML

using the very latest version of HTML which is version five you don't mention

which is version five you don't mention the number five just browsers now

the number five just browsers now nowadays are program to look for this to

nowadays are program to look for this to know that you're using the very latest

know that you're using the very latest version of the language languages just

version of the language languages just like human languages evolve over time

like human languages evolve over time we're up to version five of HTML but new

we're up to version five of HTML but new features get added every few years so

features get added every few years so indeed this lecture this class has been

indeed this lecture this class has been evolving over time too so let's now

evolving over time too so let's now focus on the next line as well as the

focus on the next line as well as the bottom line and you'll notice some

bottom line and you'll notice some deliberate symmetry here this here is

deliberate symmetry here this here is what we're going to call a tag and it's

what we're going to call a tag and it's technically different from this this is

technically different from this this is a document type declaration it's got the

a document type declaration it's got the weird exclamation point that's the only

weird exclamation point that's the only anomaly everything else follows pattern

anomaly everything else follows pattern this is a tag in HTML and it's the HTML

this is a tag in HTML and it's the HTML tag and a tag generally both starts and

tag and a tag generally both starts and stops or opens and closes at some point

stops or opens and closes at some point so this is the so-called start tag or

so this is the so-called start tag or open tag and this just means essentially

open tag and this just means essentially to the browser hey browser here comes

to the browser hey browser here comes some HTML the language in which web

some HTML the language in which web pages are written this here with the

pages are written this here with the forward slash after the angled bracket

forward slash after the angled bracket means hey browser that's it for the HTML

means hey browser that's it for the HTML of this page so that's what I mean by

of this page so that's what I mean by symmetry I started a thought here

symmetry I started a thought here finished it down here what's in between

finished it down here what's in between those two thoughts well here browser or

those two thoughts well here browser or rather let me clarify one thing this

rather let me clarify one thing this thing here is that other uh keyword an

thing here is that other uh keyword an attribute an attribute is something that

attribute an attribute is something that modifies the behavior of a tag so it's

modifies the behavior of a tag so it's similar to an argument in C or in Python

similar to an argument in C or in Python like a parameter to a function these

like a parameter to a function these aren't functions but it's the same idea

aren't functions but it's the same idea it just modifies the default behavior of

it just modifies the default behavior of something instead Lang equals you can

something instead Lang equals you can probably guess just means that hey

probably guess just means that hey browser assume that everything Hereafter

browser assume that everything Hereafter is in English and that might be useful

is in English and that might be useful for like Google translate or just search

for like Google translate or just search engine optimization so that just the

engine optimization so that just the server the browser know like what human

server the browser know like what human language you have actual content in like

language you have actual content in like hello title hello body even though a

hello title hello body even though a good computer can probably infer from

good computer can probably infer from Context often all right so that's an

Context often all right so that's an attribute that's a tag and the whole

attribute that's a tag and the whole thing here everything in between the

thing here everything in between the start tag and end tag we would also call

start tag and end tag we would also call an HTML element that just means

an HTML element that just means everything related to that open and

everything related to that open and close tag all right now notice indented

close tag all right now notice indented inside of so to speak the HTML open and

inside of so to speak the HTML open and close tag are another pair of tags the

close tag are another pair of tags the head tag and the body tag or the head

head tag and the body tag or the head element collectively and the body

element collectively and the body element collectively and same idea hey

element collectively and same idea hey browser here comes the head of my page

browser here comes the head of my page hey browser that's it for the head hey

hey browser that's it for the head hey browser here comes the body of my page

browser here comes the body of my page hey browser that's it for the body the

hey browser that's it for the body the head is essentially the tiny little

head is essentially the tiny little strip at the very top including the tab

strip at the very top including the tab itself the body is like 95% of

itself the body is like 95% of everything else the big rectangular

everything else the big rectangular region what's inside of your title at

region what's inside of your title at the at of your head of the web page at

the at of your head of the web page at the moment just the title so this

the moment just the title so this indicates hey browser Here Comes My

indicates hey browser Here Comes My Title hey browser that's it for the

Title hey browser that's it for the title the title of course is literally

title the title of course is literally quote unquote hello comma title

quote unquote hello comma title meanwhile if we bounce back out here is

meanwhile if we bounce back out here is the uh second element inside of the HTML

the uh second element inside of the HTML tag uh this says Hey browser here comes

tag uh this says Hey browser here comes the body hey browser that's it for the

the body hey browser that's it for the body and hey browser this is the

body and hey browser this is the contents these are the contents of the

contents these are the contents of the body itself now the indentation is a

body itself now the indentation is a stylistic thing I did it just to be sort

stylistic thing I did it just to be sort of neat and en TIY because it suggests

of neat and en TIY because it suggests what is inside of what but it also

what is inside of what but it also suggests a sort of hierarchy and in fact

suggests a sort of hierarchy and in fact we'll use terminology from like the

we'll use terminology from like the world of family trees if this is like a

world of family trees if this is like a parent so to speak head and body would

parent so to speak head and body would be the child elements of the HTML tag

be the child elements of the HTML tag meanwhile title is a child of the head

meanwhile title is a child of the head tag or equivalently tital is a

tag or equivalently tital is a grandchild of HTML so you can use the

grandchild of HTML so you can use the same sort of vernacular as in the human

same sort of vernacular as in the human world when it comes to uh familial

world when it comes to uh familial relationships too and that just H set

relationships too and that just H set again the same hierarchy so we have tags

again the same hierarchy so we have tags and they include HTML head title body

and they include HTML head title body and that's it for now we have attributes

and that's it for now we have attributes we've seen one example of them Lang but

we've seen one example of them Lang but we'll see many other examples of the

we'll see many other examples of the same idea but these building blocks are

same idea but these building blocks are exactly the same generally you start a

exactly the same generally you start a thought you finish a thought and you

thought you finish a thought and you might do something in between questions

might do something in between questions on this basic structure of any web

on this basic structure of any web page any questions at

page any questions at all no all right so let's now now allow

all no all right so let's now now allow things to ramp up a little more

things to ramp up a little more interestingly and do something with

interestingly and do something with these building blocks but so that you

these building blocks but so that you have a mental model for everything

have a mental model for everything that's going on here on after think of

that's going on here on after think of this same HTML being related in spirit

this same HTML being related in spirit to week five when we talked all about

to week five when we talked all about data structures if I really wanted to I

data structures if I really wanted to I could take to heart this idea of

could take to heart this idea of children and parents and grandparents

children and parents and grandparents and really depict this thing graphically

and really depict this thing graphically and in fact this tree here if you will

and in fact this tree here if you will and it's not a binary tree it's not a

and it's not a binary tree it's not a binary search tree it's just a tree uh

binary search tree it's just a tree uh used week five's terminology if this

used week five's terminology if this special node here represents the whole

special node here represents the whole document well the root element as I

document well the root element as I called it is HTML HTML has two children

called it is HTML HTML has two children head and body the head tag has in turn a

head and body the head tag has in turn a title uh child and in turn has some text

title uh child and in turn has some text just as the body has some text and so

just as the body has some text and so this is what your browser is doing you

this is what your browser is doing you and I the programmers write this stuff

and I the programmers write this stuff the browser reads this code top to

the browser reads this code top to bottom left to right whenever you visit

bottom left to right whenever you visit a website and inside of the computer's

a website and inside of the computer's memory Chrome Edge Firefox Safari what

memory Chrome Edge Firefox Safari what what not they build this data structure

what not they build this data structure in the computer's memory so as to know

in the computer's memory so as to know what it is you have told them to do and

what it is you have told them to do and we'll see over time at the end of today

we'll see over time at the end of today you can write code in an actual

you can write code in an actual programming language JavaScript to maybe

programming language JavaScript to maybe dynamically add or remove things from

dynamically add or remove things from this tree and this is how things like

this tree and this is how things like Gmail work when you open up your Gmail

Gmail work when you open up your Gmail inbox if you're a Gmail user if you just

inbox if you're a Gmail user if you just stay there long enough you'll probably

stay there long enough you'll probably get more and more mail and what happens

get more and more mail and what happens you don't have to like reload the page

you don't have to like reload the page or rebuild the tree per se it just all

or rebuild the tree per se it just all of a sudden appears at the top

of a sudden appears at the top at the top at the top what's happening

at the top at the top what's happening there is that Google wrote some code

there is that Google wrote some code that just keeps adding more nodes to

that just keeps adding more nodes to this tree every time they realize you've

this tree every time they realize you've got a new message again and again so

got a new message again and again so that's the relationship now even with

that's the relationship now even with this world of HTML with all of the

this world of HTML with all of the programmatic ideas we looked at in the

programmatic ideas we looked at in the past all right so let's go ahead and do

past all right so let's go ahead and do something with this that's a little more

something with this that's a little more interesting than just hello itself I'm

interesting than just hello itself I'm going to go ahead and hide my terminal

going to go ahead and hide my terminal window because the server is now running

window because the server is now running and all I want to do now is experiment

and all I want to do now is experiment with uh hello and other examples as well

with uh hello and other examples as well let me go ahead and actually before I do

let me go ahead and actually before I do that let me go ahead and run uh code of

that let me go ahead and run uh code of paragraphs. HTML just so I can keep my

paragraphs. HTML just so I can keep my code separate and now I'll hide the

code separate and now I'll hide the terminal window again um paragraphs.

terminal window again um paragraphs. HTML I'm going to do almost exactly the

HTML I'm going to do almost exactly the same let me go ahead and start with

same let me go ahead and start with something familiar and eventually I'll

something familiar and eventually I'll start copying and pasting just to save

start copying and pasting just to save time so doc type HTML is always there

time so doc type HTML is always there open the HTML tag and now notice I

open the HTML tag and now notice I didn't type the rest of that just like

didn't type the rest of that just like with C just like with python we try to

with C just like with python we try to save you some keystrokes by closing

save you some keystrokes by closing parentheses adding quotes the HTML

parentheses adding quotes the HTML support in VSS code is pretty good too

support in VSS code is pretty good too and it tries to finish your thought when

and it tries to finish your thought when it comes to tags as well it can screw

it comes to tags as well it can screw things up if you if it does something

things up if you if it does something you don't want it to do so sometimes you

you don't want it to do so sometimes you have to delete but it's just

have to delete but it's just autocomplete as we've seen before uh

autocomplete as we've seen before uh let's go ahead and let me add Lang

let's go ahead and let me add Lang equals as all of my examples today will

equals as all of my examples today will be let's add the head tag let's go and

be let's add the head tag let's go and proactively add the body tag and now

proactively add the body tag and now let's go ahead and give this a title tag

let's go ahead and give this a title tag uh which has a I'll just call this

uh which has a I'll just call this paragraphs just so I remember which

paragraphs just so I remember which example is what now notice all of this

example is what now notice all of this white space and all of this neat and

white space and all of this neat and tidy indentation the browser ultimately

tidy indentation the browser ultimately is not going to care about this is just

is not going to care about this is just for us humans to kind of keep ourselves

for us humans to kind of keep ourselves saying when we look at the code it's

saying when we look at the code it's just easier to read but strictly

just easier to read but strictly speaking I could minimally delete all of

speaking I could minimally delete all of this white space and I could just move

this white space and I could just move all of this tag up to the same line both

all of this tag up to the same line both I think are fine I'll just going to

I think are fine I'll just going to follow a certain convention but this too

follow a certain convention but this too would have the exact same meaning but

would have the exact same meaning but we'll see where that detail about whites

we'll see where that detail about whites space could potentially get us into

space could potentially get us into trouble later in my paragraphs tag let's

trouble later in my paragraphs tag let's do this in advance I've written up some

do this in advance I've written up some Latin like text a really long paragraph

Latin like text a really long paragraph of Latin like text like this it's

of Latin like text like this it's actually random nonsense it's not real

actually random nonsense it's not real Latin even though a couple of the words

Latin even though a couple of the words might look familiar and so here we have

might look familiar and so here we have three paragraphs of text and I've

three paragraphs of text and I've deliberately hit enter in between them

deliberately hit enter in between them so that just like an essay in Google

so that just like an essay in Google Docs or Microsoft Word hopefully I'll

Docs or Microsoft Word hopefully I'll see three separate paragraphs let me now

see three separate paragraphs let me now change tabs and I'll close hello.html

change tabs and I'll close hello.html from before I'm going to go back to my

from before I'm going to go back to my other tab here I'm going to click back

other tab here I'm going to click back to go back to that index of all of my

to go back to that index of all of my files which I started at earlier and

files which I started at earlier and you'll see now that I have two files

you'll see now that I have two files because I obviously just created a

because I obviously just created a second file called paragraphs. HTML so

second file called paragraphs. HTML so let's click on this to see our three

let's click on this to see our three paragraphs of Latin like text and voila

paragraphs of Latin like text and voila I'll zoom out all right first bug if you

I'll zoom out all right first bug if you will this just looks like one massive

will this just looks like one massive blob of text not three Blobs of text and

blob of text not three Blobs of text and why might that be borrowing the the hint

why might that be borrowing the the hint I offered a moment

I offered a moment ago why are we not seeing break yeah so

ago why are we not seeing break yeah so we need some kind of line breaks here

we need some kind of line breaks here because the browser turns out is only

because the browser turns out is only going to take us literally and if you

going to take us literally and if you just give it text text text it's just

just give it text text text it's just going to show you text and anytime

going to show you text and anytime there's more than a single white space

there's more than a single white space whether it's two or 20 or 200 it's going

whether it's two or 20 or 200 it's going to just assume that you did that just to

to just assume that you did that just to be neat and tidy and it's going to

be neat and tidy and it's going to collapse them into just one space

collapse them into just one space visually like this so there are in fact

visually like this so there are in fact a couple of solutions one is this here I

a couple of solutions one is this here I could add some explicit line breaks and

could add some explicit line breaks and it turns out that there's a br tag like

it turns out that there's a br tag like this and just for uh visibility sake let

this and just for uh visibility sake let me do two of them so like hitting enter

me do two of them so like hitting enter enter on my keyboard I'll do it here too

enter on my keyboard I'll do it here too BR for break break and now let me go

BR for break break and now let me go back to my other tab nothing's changed

back to my other tab nothing's changed yet but that's because I have to reload

yet but that's because I have to reload I've changed it on the server but now I

I've changed it on the server but now I need to change it in the browser by

need to change it in the browser by reloading and now it looks a little

reloading and now it looks a little better albeit nonsensical but you'll

better albeit nonsensical but you'll know is a curiosity per this BR tag this

know is a curiosity per this BR tag this is kind of poorly designed it's a little

is kind of poorly designed it's a little hackish to just say enter enter and make

hackish to just say enter enter and make the browser do this breaks line breaks

the browser do this breaks line breaks don't actually require Clos tags or end

don't actually require Clos tags or end tags so not all tags need to be closed

tags so not all tags need to be closed at least those that it just makes no

at least those that it just makes no semantic sense to close them right like

semantic sense to close them right like the break is there or it's not you can't

the break is there or it's not you can't imagine like starting to move to the

imagine like starting to move to the next line and then eventually getting

next line and then eventually getting around to finishing like it's either

around to finishing like it's either there or it's not so some tags do not

there or it's not so some tags do not have closed tags as necessary but

have closed tags as necessary but there's a more elegant way here I dare

there's a more elegant way here I dare say not just sort of hackish putting in

say not just sort of hackish putting in these line breaks let me do this instead

these line breaks let me do this instead I'll delete those and let me go ahead

I'll delete those and let me go ahead and as the name of this file suggests

and as the name of this file suggests let me add a paragraph tag now here I

let me add a paragraph tag now here I need to fight with vs codes autocomplete

need to fight with vs codes autocomplete because I don't want to finish the

because I don't want to finish the sentence uh the thought there let me go

sentence uh the thought there let me go ahead and open the paragraph tag and

ahead and open the paragraph tag and close a paragraph tag and just to keep

close a paragraph tag and just to keep things tidy I'll go ahead and indent two

things tidy I'll go ahead and indent two even though the indentation itself

even though the indentation itself doesn't matter let me go ahead and

doesn't matter let me go ahead and create another tag for opening this

create another tag for opening this paragraph and I'll close this one here

paragraph and I'll close this one here and now let me see sometimes it's

and now let me see sometimes it's fighting with my auto complete but

fighting with my auto complete but that's fine because I did this sort of

that's fine because I did this sort of the wrong way at first and now let me go

the wrong way at first and now let me go ahead and finish this thought by closing

ahead and finish this thought by closing this paragraph tag here and I've

this paragraph tag here and I've manually fixed all of my indentation so

manually fixed all of my indentation so now on line 10 I have the equivalent of

now on line 10 I have the equivalent of hey browser start a paragraph and then

hey browser start a paragraph and then it does the Latin like text then on line

it does the Latin like text then on line 12 hey browser that's it for this

12 hey browser that's it for this paragraph and repeat repeat repeat if I

paragraph and repeat repeat repeat if I now go back to my other Tab and reload

now go back to my other Tab and reload again shouldn't be all that different

again shouldn't be all that different but semantically it's a little bit

but semantically it's a little bit better why because just saying break

better why because just saying break break doesn't really mean anything but

break doesn't really mean anything but by saying paragraph paragraph paragraph

by saying paragraph paragraph paragraph now there's some more semantic

now there's some more semantic information there now if like Google is

information there now if like Google is analyzing your page or if the programmer

analyzing your page or if the programmer is trying to understand what it is you

is trying to understand what it is you did in the past when writing this code

did in the past when writing this code you just know semantically oh this is a

you just know semantically oh this is a paragraph This is a paragraph This is a

paragraph This is a paragraph This is a paragraph just like in a book or or an

paragraph just like in a book or or an essay so it's a little more clear

essay so it's a little more clear focusing more on what it is not how you

focusing more on what it is not how you want to display it any questions then on

want to display it any questions then on these

these paragraphs now all right so a few more

paragraphs now all right so a few more tags and indeed these first few examples

tags and indeed these first few examples will really just be like sort of bang

will really just be like sort of bang bang bang just a bunch of different

bang bang just a bunch of different vocabulary words in the form of these

vocabulary words in the form of these new tags but we won't go through the

new tags but we won't go through the entire laundry list of tags this is

entire laundry list of tags this is indeed the thing for which web

indeed the thing for which web references and books and the like are

references and books and the like are ultimately helpful just like a

ultimately helpful just like a dictionary in the real world so I'm

dictionary in the real world so I'm going to go ahead and do this let me go

going to go ahead and do this let me go ahead and copy this let me create a new

ahead and copy this let me create a new file called headings. HTML just so we

file called headings. HTML just so we have a new file for this to save time

have a new file for this to save time I'm just going to paste that exact same

I'm just going to paste that exact same code just to get me started I'm going to

code just to get me started I'm going to change the title of it for clarity for

change the title of it for clarity for the code online to headings and now just

the code online to headings and now just like a book or an essay or a thesis let

like a book or an essay or a thesis let me actually put some actual headings

me actually put some actual headings here now if my first heading like

here now if my first heading like chapter one I could do something like

chapter one I could do something like this up here I could have a paragraph

this up here I could have a paragraph like I just learned and I could say

like I just learned and I could say something like chapter one here but

something like chapter one here but that's not really a paragraph and so

that's not really a paragraph and so it's sort of better designed to tell the

it's sort of better designed to tell the browser and really tell the world what

browser and really tell the world what it is so it turns out there's another

it is so it turns out there's another tag I can use like H1 for heading and

tag I can use like H1 for heading and like most important heading and in here

like most important heading and in here I'm just going to keep it simple and I'm

I'm just going to keep it simple and I'm going to say something like one and in

going to say something like one and in fact this is so short here's a good

fact this is so short here's a good candidate for just keeping this all on

candidate for just keeping this all on the same line but this has no functional

the same line but this has no functional difference but it'll just make it a

difference but it'll just make it a little tur more tur on the screen now

little tur more tur on the screen now let me go ahead down here and I could

let me go ahead down here and I could have multiple headings so H1 2 and down

have multiple headings so H1 2 and down here I could have another one H1

here I could have another one H1 three and if I go back to my other tab I

three and if I go back to my other tab I reload it now we should see just like a

reload it now we should see just like a book or an essay now we have some

book or an essay now we have some proper now we have some oops I have to

proper now we have some oops I have to go to the right file sorry if I go back

go to the right file sorry if I go back to the index now we see the new third

to the index now we see the new third file called headings. HTML and now we

file called headings. HTML and now we indeed see some fairly pretty if simple

indeed see some fairly pretty if simple headings as well now if these aren't

headings as well now if these aren't three chapters 1 two three but maybe

three chapters 1 two three but maybe it's a chapter then a section then a

it's a chapter then a section then a subsection such that just visually you

subsection such that just visually you want things to get smaller and smaller

want things to get smaller and smaller well those exist too and in fact you can

well those exist too and in fact you can do H1 through H6 H1 a little

do H1 through H6 H1 a little paradoxically is the biggest and boldest

paradoxically is the biggest and boldest H6 is the smallest but still bold so it

H6 is the smallest but still bold so it might make sense to make this H2 both

might make sense to make this H2 both open and close and maybe this H3 open

open and close and maybe this H3 open and close if again this is a section or

and close if again this is a section or a subsection inside of that chapter if I

a subsection inside of that chapter if I reload now notice just gets a little

reload now notice just gets a little smaller so it's more similar to what

smaller so it's more similar to what you'd see on the printed page but this

you'd see on the printed page but this now is just another three tags that I

now is just another three tags that I might use in my own code all right well

might use in my own code all right well how about lists of things I have three

how about lists of things I have three paragraphs here but let's do this let me

paragraphs here but let's do this let me go back to vs code I'm going to copy

go back to vs code I'm going to copy this code so I have a starting point I'm

this code so I have a starting point I'm going to create a new file called say

going to create a new file called say list. HTML here I'm going to copy paste

list. HTML here I'm going to copy paste I'm going to change my title to be list

I'm going to change my title to be list just for clarity and in here I'm going

just for clarity and in here I'm going to go ahead and get rid of this whole

to go ahead and get rid of this whole body because let's move away from these

body because let's move away from these massive paragraphs and keep it simpler

massive paragraphs and keep it simpler for now if I want to have a list of

for now if I want to have a list of things uh for instance uh if you haven't

things uh for instance uh if you haven't seen these already a computer scientist

seen these already a computer scientist when they're fishing for just some

when they're fishing for just some arbitrary meaningless words uh they

arbitrary meaningless words uh they often use Foo bar and baz just as their

often use Foo bar and baz just as their go-to just like a mathematician might

go-to just like a mathematician might use XY Z for variables so Fu bar and baz

use XY Z for variables so Fu bar and baz are on three separate lines and maybe

are on three separate lines and maybe this is like my to-do list or my

this is like my to-do list or my shopping list but you can probably

shopping list but you can probably imagine if I go back to my other tab go

imagine if I go back to my other tab go back to the index I now see my new file

back to the index I now see my new file list.

list. HTML but it's probably going to look

HTML but it's probably going to look wrong I think I'm just going to say yeah

wrong I think I'm just going to say yeah Fubar baz all in one breath if you will

Fubar baz all in one breath if you will the not on new line and you can try to

the not on new line and you can try to fight this like you can be like really

fight this like you can be like really want to put some line breaks there go

want to put some line breaks there go back and reload it's still not going to

back and reload it's still not going to make any change how do I want to fix

make any change how do I want to fix this well I can do this in a few ways I

this well I can do this in a few ways I could make them paragraphs but they're

could make them paragraphs but they're not really paragraphs they're a list so

not really paragraphs they're a list so I'm going to use a different tag instead

I'm going to use a different tag instead I'm going to create for instance an

I'm going to create for instance an unordered list using the UL tag open and

unordered list using the UL tag open and close inside of that I'm going to use

close inside of that I'm going to use the list item tag Li I and I'm going to

the list item tag Li I and I'm going to say Foo inside of another tag I'm going

say Foo inside of another tag I'm going to say bar inside of a third tag open

to say bar inside of a third tag open and close I'm going to say baz so it's

and close I'm going to say baz so it's getting a little verbose but it's still

getting a little verbose but it's still relatively succinct Li is all you need

relatively succinct Li is all you need type for list item UL is all you need

type for list item UL is all you need type for unordered list so there's some

type for unordered list so there's some shorthand syntax here that's adopted if

shorthand syntax here that's adopted if I now reload you're going to see a

I now reload you're going to see a so-called unordered list like not sorted

so-called unordered list like not sorted which means by convention to show it as

which means by convention to show it as bullets though it could be displayed in

bullets though it could be displayed in different ways visually as well if you

different ways visually as well if you actually change your mind and you

actually change your mind and you realize oh I'd really like to number

realize oh I'd really like to number this well you could obviously like just

this well you could obviously like just add one and two and three but that's

add one and two and three but that's going to get annoying especially if the

going to get annoying especially if the list grows you want to change something

list grows you want to change something insert something in the middle then you

insert something in the middle then you have to reorder it I mean we using

have to reorder it I mean we using computers here they can do this for us

computers here they can do this for us so we can change the UL to any

so we can change the UL to any guesses o maybe for ordered list which

guesses o maybe for ordered list which is sort of the opposite here so let's

is sort of the opposite here so let's try changing that to O let me go back to

try changing that to O let me go back to my browser and I'm just hitting command

my browser and I'm just hitting command r or controlr to reload the page instead

r or controlr to reload the page instead of clicking the button every time now I

of clicking the button every time now I automatically get 1 2 and three and you

automatically get 1 2 and three and you can even override the Aesthetics using

can even override the Aesthetics using different numera bles or symbology

different numera bles or symbology instead but that would be perhaps the

instead but that would be perhaps the most common there as well all right it's

most common there as well all right it's a lot of tags quickly but any questions

a lot of tags quickly but any questions on lists paragraphs headings or the like

on lists paragraphs headings or the like no all right so let me go ahead and

no all right so let me go ahead and propose this here um let's go ahead and

propose this here um let's go ahead and create what we'll call a table so let me

create what we'll call a table so let me copy and paste this into a new file code

copy and paste this into a new file code of table. HTML and in table. HTML let me

of table. HTML and in table. HTML let me again rename the title to table let me

again rename the title to table let me get rid of that ordered list from before

get rid of that ordered list from before and let me now use a table tag open and

and let me now use a table tag open and close this one's a little weird but

close this one's a little weird but inside of a table you typically have a

inside of a table you typically have a head of the table so uh I'll say well

head of the table so uh I'll say well let's say the first row we'll keep this

let's say the first row we'll keep this one simple a table row or TR inside of a

one simple a table row or TR inside of a table row you would ideally have columns

table row you would ideally have columns but that's not the nomenclature instead

but that's not the nomenclature instead you have data so TD for table data and

you have data so TD for table data and let me go ahead and just have the first

let me go ahead and just have the first datm be one I'm just going to

datm be one I'm just going to arbitrarily do 1 two three just so we

arbitrarily do 1 two three just so we have something to play with and you know

have something to play with and you know what just for demonstration sake I am

what just for demonstration sake I am going to deliberately copy paste this

going to deliberately copy paste this twice and I'm just going to manually

twice and I'm just going to manually change the numbers just so we can see

change the numbers just so we can see what I'm creating 789 and then maybe

what I'm creating 789 and then maybe just for good measure if you're seeing

just for good measure if you're seeing where this is going let me copy this one

where this is going let me copy this one more time and give myself a final row

more time and give myself a final row with an asterisk a zero and a pound sign

with an asterisk a zero and a pound sign if maybe you see where this is going let

if maybe you see where this is going let me go back to my other tab let me go

me go back to my other tab let me go back to the index there's my new file

back to the index there's my new file table. HTML I'll click that and while

table. HTML I'll click that and while it's not very pretty I'll zoom in it's

it's not very pretty I'll zoom in it's indeed a table of data I happen to mimic

indeed a table of data I happen to mimic like a key uh a telephone keypad but you

like a key uh a telephone keypad but you could imagine this being much juicier

could imagine this being much juicier much more interesting scientific or

much more interesting scientific or financial data or the like laid out into

financial data or the like laid out into these rows TRS and these columns AKA

these rows TRS and these columns AKA table data as well so we have that

table data as well so we have that ability as well for structured data now

ability as well for structured data now of course the internet um has lots of

of course the internet um has lots of images on it and in fact this is all

images on it and in fact this is all just text how can we introduce images

just text how can we introduce images well let me go ahead and do this let me

well let me go ahead and do this let me first uh sort of semi secretly copy an

first uh sort of semi secretly copy an image file that I brought from uh

image file that I brought from uh earlier just so we have something to

earlier just so we have something to play with and I have in my account here

play with and I have in my account here now an image called harbor. JPEG and I

now an image called harbor. JPEG and I uploaded this semi- secretly a second

uploaded this semi- secretly a second ago into my account so that I can

ago into my account so that I can reference a second file let me go ahead

reference a second file let me go ahead and copy this HTML just to save myself

and copy this HTML just to save myself some keystrokes

some keystrokes let me go ahead and do code of image.

let me go ahead and do code of image. HTML and let me paste that code and hide

HTML and let me paste that code and hide my terminal window I'm going to get rid

my terminal window I'm going to get rid of all of this table as just

of all of this table as just uninteresting now we're going to make

uninteresting now we're going to make this even simpler by changing my title

this even simpler by changing my title to image to keep all these

to image to keep all these demonstrations uh separate and now if I

demonstrations uh separate and now if I want to make a web page that when

want to make a web page that when visited shows us a picture of Harvard

visited shows us a picture of Harvard well there's an image tag abbreviated

well there's an image tag abbreviated IMG for short I can specify what the

IMG for short I can specify what the source of that image is and if my file a

source of that image is and if my file a JPEG in this case is literally in the

JPEG in this case is literally in the same folder I can just say quote unquote

same folder I can just say quote unquote harvard. jpeg if it's in a folder I

harvard. jpeg if it's in a folder I should mention the folder name in a

should mention the folder name in a slash or something like that if the

slash or something like that if the image is on the internet somewhere with

image is on the internet somewhere with a URL I could also have a whole URL

a URL I could also have a whole URL https colon and then the URL of the

https colon and then the URL of the image but I upload it in in advance now

image but I upload it in in advance now this is just going to visually display

this is just going to visually display on the screen but not everyone of course

on the screen but not everyone of course can see images screen readers might need

can see images screen readers might need a bit of assistance and even search

a bit of assistance and even search engines might want to analyze the page

engines might want to analyze the page and know what this is an image of now

and know what this is an image of now machine learning and artificial

machine learning and artificial intelligence are maybe getting better to

intelligence are maybe getting better to be fair at figuring out just by

be fair at figuring out just by analyzing images what they are but

analyzing images what they are but they're certainly imperfect I am a human

they're certainly imperfect I am a human I know pretty well what I took a photo

I know pretty well what I took a photo of for instance so maybe what I should

of for instance so maybe what I should do proactively which would be good for

do proactively which would be good for accessibility is have this alt tag for

accessibility is have this alt tag for alternative text and then literally say

alternative text and then literally say like Harvard University so that someone

like Harvard University so that someone who can't see or so that a a server can

who can't see or so that a a server can actually know with higher probability

actually know with higher probability what it is uh they're looking at and I

what it is uh they're looking at and I could be even more detailed than just a

could be even more detailed than just a phrase I could describe the image as

phrase I could describe the image as well all right let me go back to my

well all right let me go back to my index in the second tab let me go back

index in the second tab let me go back and zoom back out there's my new file

and zoom back out there's my new file and there's my new JPEG that I quickly

and there's my new JPEG that I quickly uploaded before I can click now on

uploaded before I can click now on image. HTML and albeit a little

image. HTML and albeit a little overwhelming that is a really big image

overwhelming that is a really big image of harvor now apparently it's too big to

of harvor now apparently it's too big to fit on the screen so this isn't the best

fit on the screen so this isn't the best user experience to have to scroll up

user experience to have to scroll up okay so there's the image horrible

okay so there's the image horrible horrible design if you will at least in

horrible design if you will at least in terms of my code but there's going to be

terms of my code but there's going to be ways where I can sort of rein that in

ways where I can sort of rein that in and affect the height or the width as

and affect the height or the width as well but for now it's just deliberately

well but for now it's just deliberately a little overwhelming instead now we can

a little overwhelming instead now we can do something a little more fun and

do something a little more fun and topical today uh which might be to use

topical today uh which might be to use an IM a video instead so let me go ahead

an IM a video instead so let me go ahead here and very quickly grab another file

here and very quickly grab another file today which is uh I brought in advance

today which is uh I brought in advance and that you might have seen briefly

and that you might have seen briefly earlier which is an MP4 an actual video

earlier which is an MP4 an actual video file and what I'm going to do here by

file and what I'm going to do here by revealing vs code again is I'm going to

revealing vs code again is I'm going to code a file called video. HTML get

code a file called video. HTML get another demonstration here I'm going to

another demonstration here I'm going to change my title to video just to keep

change my title to video just to keep these things straight and instead of the

these things straight and instead of the image tag you might imagine using now

image tag you might imagine using now indeed a video tag and this is a

indeed a video tag and this is a relatively newer tag that has increasing

relatively newer tag that has increasing support among browsers so it's good now

support among browsers so it's good now to use and inside of this the syntax is

to use and inside of this the syntax is a little different you specify and this

a little different you specify and this is weirdly annoyingly in consistent not

is weirdly annoyingly in consistent not SRC for Source you literally say source

SRC for Source you literally say source and then in Source you use a source

and then in Source you use a source attribute horrible design semantically

attribute horrible design semantically but like this is what we're stuck with

but like this is what we're stuck with halloween. MP4 is the name of the video

halloween. MP4 is the name of the video we uploaded in advance made by some of

we uploaded in advance made by some of harder Harvard's digital artists and the

harder Harvard's digital artists and the type of this video so that the browser

type of this video so that the browser knows for sure is video/ MP4 that's a

knows for sure is video/ MP4 that's a so-called content type that you just

so-called content type that you just know or you look up to figure it out and

know or you look up to figure it out and just so that this is as animated as

just so that this is as animated as possible I'm going to tell the video tag

possible I'm going to tell the video tag with a few attributes to autoplay and it

with a few attributes to autoplay and it turns out that attributes often have key

turns out that attributes often have key value pairs whereby it's the key the

value pairs whereby it's the key the attribute name equals quote unquote some

attribute name equals quote unquote some value just like Lang equals quote

value just like Lang equals quote unquote for English but not all

unquote for English but not all attributes need values in fact if you

attributes need values in fact if you read the documentation for html's video

read the documentation for html's video tag there's an autoplay attribute where

tag there's an autoplay attribute where you can literally just say the key and

you can literally just say the key and it needs no value it's just going to

it needs no value it's just going to mean autoplay and if you don't want to

mean autoplay and if you don't want to autoplay you just omit it alt together

autoplay you just omit it alt together so so you don't necessarily need a value

so so you don't necessarily need a value on or off uh I want the thing to Loop

on or off uh I want the thing to Loop just so it keeps going I want it to be

just so it keeps going I want it to be muted so that we don't hear any sound in

muted so that we don't hear any sound in fact there is no sound but browsers

fact there is no sound but browsers nowadays for anti-spam and advertising

nowadays for anti-spam and advertising reasons often will not play a video if

reasons often will not play a video if it has sound because it's just kind of

it has sound because it's just kind of obnoxious if you visit a page and all of

obnoxious if you visit a page and all of a sudden your speakers start blaring so

a sudden your speakers start blaring so I know this from having read up on this

I know this from having read up on this that I should mute it so if I want it to

that I should mute it so if I want it to actually autoplay for real and then I'll

actually autoplay for real and then I'll set the width manually for now to be

set the width manually for now to be like 128 pixels across just from some

like 128 pixels across just from some trial and error earlier and that with

trial and error earlier and that with attribute does have a value now I'm

attribute does have a value now I'm being a little um a little uptight here

being a little um a little uptight here by alphabetizing all of my attributes

by alphabetizing all of my attributes not at all necessary I do it just so I

not at all necessary I do it just so I can skim things faster and know if

can skim things faster and know if something is there or not so for me it's

something is there or not so for me it's just a matter of style let me go back to

just a matter of style let me go back to my other tab go back to my index and

my other tab go back to my index and you'll see two new files again the mp4

you'll see two new files again the mp4 file and video. HTML I'll click on the

file and video. HTML I'll click on the ladder and if I did this well here we

ladder and if I did this well here we have thank thanks to our friends in

have thank thanks to our friends in Harvard U our artistic friends at

Harvard U our artistic friends at Harvard very like an oo would help with

Harvard very like an oo would help with the drama here but okay but we have a

the drama here but okay but we have a very dramatic nice Halloween type view

very dramatic nice Halloween type view here as well so we have videos embedded

here as well so we have videos embedded as well and suffice it to say there's

as well and suffice it to say there's ways to embed YouTube videos or Vimeo or

ways to embed YouTube videos or Vimeo or other services as well using yet more

other services as well using yet more tags too but the web is of course all

tags too but the web is of course all about hyperlink Hyper text markup

about hyperlink Hyper text markup language where you click on something

language where you click on something and you end up somewhere else and this

and you end up somewhere else and this is how the web is so powerfully

is how the web is so powerfully interconnected

interconnected so how do we start creating links from

so how do we start creating links from one website or web page to another

one website or web page to another either that I made or someone else well

either that I made or someone else well let me go ahead and open back up my

let me go ahead and open back up my terminal window and let's create a file

terminal window and let's create a file called link. HTML just to demonstrate

called link. HTML just to demonstrate what you and I know as a link I'll hide

what you and I know as a link I'll hide my terminal window now let me copy paste

my terminal window now let me copy paste just to save myself some keystrokes and

just to save myself some keystrokes and let me get rid of the video tag so we

let me get rid of the video tag so we can focus now on links suppose that I

can focus now on links suppose that I want you to visit Harvard virtually well

want you to visit Harvard virtually well I could say something like visit uh

I could say something like visit uh Harvard period this is uninteresting

Harvard period this is uninteresting because it's just going to be text I

because it's just going to be text I probably want you to actually visit

probably want you to actually visit harvard.edu instead more specifically

harvard.edu instead more specifically and I'll lower case it just to be

and I'll lower case it just to be consistent with what browsers do in the

consistent with what browsers do in the address bar all right let me go now to

address bar all right let me go now to the video uh back to this video Tab and

the video uh back to this video Tab and go back where we now see my index I'll

go back where we now see my index I'll Zoom back in and there's link. HTML

Zoom back in and there's link. HTML unfortunately when I click this and I'll

unfortunately when I click this and I'll zoom in you literally just see the text

zoom in you literally just see the text that I wrote and yet on every social

that I wrote and yet on every social media platform nowadays except like

media platform nowadays except like Instagram when you type a URL or what

Instagram when you type a URL or what looks like a URL even if you didn't

looks like a URL even if you didn't bother with the HTTP or https it usually

bother with the HTTP or https it usually automatically links it for you on

automatically links it for you on Facebook on Twitter and other sites as

Facebook on Twitter and other sites as well that's just a convenience Discord

well that's just a convenience Discord and slack do that too but they're just

and slack do that too but they're just doing it to make things more

doing it to make things more userfriendly but they have to generate

userfriendly but they have to generate HTML with the proper tags and attributes

HTML with the proper tags and attributes so to get this to actually work it's not

so to get this to actually work it's not even good enough to say

even good enough to say https www.harvard.edu because if I go

https www.harvard.edu because if I go back now and reload now you'll literally

back now and reload now you'll literally just see all of that as text if you want

just see all of that as text if you want the browser to treat this as a link you

the browser to treat this as a link you need to use the anchor tag it'd be great

need to use the anchor tag it'd be great if it were called the link tag but it's

if it were called the link tag but it's not it's called The Anchor tag or a for

not it's called The Anchor tag or a for short and the way you reference the URL

short and the way you reference the URL to which you want to lead the user is

to which you want to lead the user is via href for hyper reference this is one

via href for hyper reference this is one of the earliest tags perhaps among the

of the earliest tags perhaps among the most Arcane now but if I then put that

most Arcane now but if I then put that whole URL in quotes and close my tag I

whole URL in quotes and close my tag I now have to opportunity to finish my

now have to opportunity to finish my thought in between the start tag and the

thought in between the start tag and the end tag for this anchor and what I put

end tag for this anchor and what I put in between the start and end tag is

in between the start and end tag is whatever the human's going to see so

whatever the human's going to see so here I can say Harvard I can go back to

here I can say Harvard I can go back to my other tab I can reload the page and

my other tab I can reload the page and now you see the familiar blue underline

now you see the familiar blue underline this now is an actual link and if I

this now is an actual link and if I click it I'll be whisked away to the

click it I'll be whisked away to the actual Harvard websites but there's a

actual Harvard websites but there's a risk here can anyone imagine pretty

risk here can anyone imagine pretty simply after like what 60 seconds of the

simply after like what 60 seconds of the link tag of the anchor tag how could

link tag of the anchor tag how could someone an adversary misuse this tag

someone an adversary misuse this tag alone How could a website run by an

alone How could a website run by an adversary How could a spammer misuse

adversary How could a spammer misuse this tag do you think

yeah yeah absolutely you could have it say one thing but lead another lead

say one thing but lead another lead elsewhere so I could say Yale in here

elsewhere so I could say Yale in here nothing stopping me as the developer go

nothing stopping me as the developer go back to the page reload now it says

back to the page reload now it says visit Yale you click on Yale and voila

visit Yale you click on Yale and voila you end up applying to the wrong place

you end up applying to the wrong place instead now there's some hints of this I

instead now there's some hints of this I could hover over this and super small

could hover over this and super small like this isn't very good for your

like this isn't very good for your anti-hacking techniques but way down

anti-hacking techniques but way down here you can actually see the URL that

here you can actually see the URL that it's going to go to in most browsers

it's going to go to in most browsers indeed do this at least on desktops and

indeed do this at least on desktops and laptops so it's a little bit of a hint

laptops so it's a little bit of a hint but what you're seeing here even though

but what you're seeing here even though this is kind of a silly um uh playful

this is kind of a silly um uh playful example this is exactly how fishing

example this is exactly how fishing attacks work p i sh iing work whereby an

attacks work p i sh iing work whereby an adversary tells you to log into your

adversary tells you to log into your PayPal account but it doesn't go to

PayPal account but it doesn't go to paypal.com it goes to some other random

paypal.com it goes to some other random website that they bought and built that

website that they bought and built that then tries to collect your username and

then tries to collect your username and password and stored in their database so

password and stored in their database so now they can log into your PayPal

now they can log into your PayPal account as you and it boils down to that

account as you and it boils down to that simple primitive and you can be even

simple primitive and you can be even more manipulative to you can even say

more manipulative to you can even say the whole URL for Yale like yale.edu or

the whole URL for Yale like yale.edu or Worse htps www.yale.edu reload that and

Worse htps www.yale.edu reload that and now who I mean who among you and people

now who I mean who among you and people in your lives are necessarily going to

in your lives are necessarily going to be so paranoid as to not just blindly

be so paranoid as to not just blindly click on that URL this is why just being

click on that URL this is why just being a defensive real world person nowadays

a defensive real world person nowadays digitally is just ever more so important

digitally is just ever more so important so these same things that can be used

so these same things that can be used for good or uh benign use cases can also

for good or uh benign use cases can also be used for ill purposes too and it is

be used for ill purposes too and it is literally that simple questions now on

literally that simple questions now on any of these tags thus

any of these tags thus far just a few more to offer up any

far just a few more to offer up any questions on this

questions on this here no well let me open up a couple

here no well let me open up a couple that I brought in advance just so we

that I brought in advance just so we don't have to type all of them here um

don't have to type all of them here um if you for instance have a web page

if you for instance have a web page that's got quite a bit of code let me go

that's got quite a bit of code let me go ahead and grab from the website a couple

ahead and grab from the website a couple of examples real fast here namely one

of examples real fast here namely one that we'll call how about uh meta

that we'll call how about uh meta HTML and in this example here give me

HTML and in this example here give me just a moment to full screen it we're

just a moment to full screen it we're going to

going to have a file So codem Meta HTML I'll open

have a file So codem Meta HTML I'll open this up next no relationship to what we

this up next no relationship to what we now know as meta the company but rather

now know as meta the company but rather this is going to be a page that I copied

this is going to be a page that I copied and pasted the same chunk of Latin like

and pasted the same chunk of Latin like text from earlier so it's going to be a

text from earlier so it's going to be a really big paragraph of text and this is

really big paragraph of text and this is an example where if you were to open

an example where if you were to open this web page not on my own Mac or your

this web page not on my own Mac or your PC but on your phone the font might

PC but on your phone the font might actually be really annoying and

actually be really annoying and difficult to read why because your

difficult to read why because your phone's going to try to squeeze all of

phone's going to try to squeeze all of the content onto the tiny viewport the

the content onto the tiny viewport the rectangular region of your phone instead

rectangular region of your phone instead so it turns out there are ways pretty

so it turns out there are ways pretty easy ways to make your website mobile

easy ways to make your website mobile friendly as well Otherwise Known

friendly as well Otherwise Known technically as responsive and the

technically as responsive and the easiest way to do this is to include

easiest way to do this is to include this tag here a meta tag again no

this tag here a meta tag again no relationship to Facebook this has been

relationship to Facebook this has been here much longer and this case here this

here much longer and this case here this meta tag online five has its own sort of

meta tag online five has its own sort of approach to key value pairs this is a

approach to key value pairs this is a good example of where it'd be nice if it

good example of where it'd be nice if it looked just like everything else but

looked just like everything else but this is what we have historically

this is what we have historically you can have a meta tag with an

you can have a meta tag with an attribute called name that refers to the

attribute called name that refers to the name of some feature of the browser in

name of some feature of the browser in this case viewport is the technical term

this case viewport is the technical term for like the big rectangular region to

for like the big rectangular region to which I keep referring the body really

which I keep referring the body really of your page the content for the

of your page the content for the viewport you can say some AR esoteric

viewport you can say some AR esoteric details like this the initial scale

details like this the initial scale should be one that is no matter who

should be one that is no matter who visits your site it shouldn't start

visits your site it shouldn't start zoomed in it shouldn't start zoomed out

zoomed in it shouldn't start zoomed out it should start at just the default

it should start at just the default sizing and then this here with equals

sizing and then this here with equals device width is a very arcane way of

device width is a very arcane way of saying if the user has a small screen

saying if the user has a small screen show the text proportional to that size

show the text proportional to that size don't just try to cram it all into a

don't just try to cram it all into a tiny little window so it's super simple

tiny little window so it's super simple but if for the next problem set or

but if for the next problem set or future projects as well you find that

future projects as well you find that just things look really bad on mobile

just things look really bad on mobile like this kind of tag is the place to

like this kind of tag is the place to start meta there aren't terribly many of

start meta there aren't terribly many of these that you'll use but they're useful

these that you'll use but they're useful for other mechanisms as well in fact let

for other mechanisms as well in fact let me go ahead and semi secretly pull up

me go ahead and semi secretly pull up one other example as well whereby I'm

one other example as well whereby I'm going to grab another example that uses

going to grab another example that uses more of these tags and in just a moment

more of these tags and in just a moment I'll reveal it here give me just one

I'll reveal it here give me just one second here I'll have I'll propose that

second here I'll have I'll propose that in this

in this example of

example of meta we now add these properties instead

meta we now add these properties instead so I copy pasted this from an existing

so I copy pasted this from an existing file just so as to not waste time typing

file just so as to not waste time typing all of these out if you've ever shared a

all of these out if you've ever shared a URL on Facebook or Twitter or slack or

URL on Facebook or Twitter or slack or Discord order any number of websites

Discord order any number of websites nowadays that automatically show a nice

nowadays that automatically show a nice preview where you sudden instantly see

preview where you sudden instantly see like the default image of the page maybe

like the default image of the page maybe a few sentences or words of text or

a few sentences or words of text or something like that sometimes those

something like that sometimes those applications those websites will just

applications those websites will just choose like the first image it finds on

choose like the first image it finds on the website or the first sentence it

the website or the first sentence it sees and show that but that might not be

sees and show that but that might not be very user friendly or search engine

very user friendly or search engine friendly and so a prog a developer might

friendly and so a prog a developer might want to control what it is that slack

want to control what it is that slack Discord Facebook Twitter and other such

Discord Facebook Twitter and other such sites show by default for that can use

sites show by default for that can use nowadays what are called open graph tags

nowadays what are called open graph tags which is to say there's other uses of

which is to say there's other uses of the meta tag and you just look these

the meta tag and you just look these things up even I had to look this up to

things up even I had to look this up to remember what the key value pairs are

remember what the key value pairs are the meta tag can also have a property

the meta tag can also have a property attribute that can be these very

attribute that can be these very specific strings OG title OG description

specific strings OG title OG description OG image which denotes open graph which

OG image which denotes open graph which again is this standard that's evolved in

again is this standard that's evolved in recent years and what you can do here is

recent years and what you can do here is tell browsers and in turn servers what

tell browsers and in turn servers what you want them to show as the default

you want them to show as the default title of the page P the description of

title of the page P the description of the page and even the default image just

the page and even the default image just so you can exercise more control when

so you can exercise more control when sharing things socially nowadays as well

sharing things socially nowadays as well again it just boils down to these key

again it just boils down to these key value pairs this is absolutely the kind

value pairs this is absolutely the kind of thing you look up as needed to cross

of thing you look up as needed to cross check but those capabilities are there

check but those capabilities are there and so literally the next time you paste

and so literally the next time you paste a link into slack or Discord or any

a link into slack or Discord or any online site that then displays it in

online site that then displays it in embedded fashion just know that all this

embedded fashion just know that all this time a little bit of textual code like

time a little bit of textual code like this in HTML has been there by whoever

this in HTML has been there by whoever authored the site

authored the site all right let's do one final example in

all right let's do one final example in HTML alone before we transition to just

HTML alone before we transition to just cleaning up the Aesthetics and improving

cleaning up the Aesthetics and improving the visuals of everything we've been

the visuals of everything we've been creating let me go ahead here and close

creating let me go ahead here and close meta HTML let me code up a new file

meta HTML let me code up a new file called how about search. HTML and see if

called how about search. HTML and see if we can't draw some inspiration from our

we can't draw some inspiration from our cursory understanding earlier of how

cursory understanding earlier of how URLs work to see if we can't reinvent

URLs work to see if we can't reinvent google.com itself so recall that a

google.com itself so recall that a canonical URL might look like this here

canonical URL might look like this here and in particular if you want to pass in

and in particular if you want to pass in user input to that URL again you can

user input to that URL again you can potentially have a question mark and

potentially have a question mark and then a key equals value pair or for that

then a key equals value pair or for that matter you can even have multiple key

matter you can even have multiple key value pairs that by convention are

value pairs that by convention are separated by ampers sense these things

separated by ampers sense these things are everywhere like later today when you

are everywhere like later today when you pull up almost any website in your

pull up almost any website in your browser look at the URL and you'll just

browser look at the URL and you'll just see a lot of this a lot of noise too and

see a lot of this a lot of noise too and distractions but there's going to be

distractions but there's going to be some equal signs most likely Andor some

some equal signs most likely Andor some ampersands as well and those are just

ampersands as well and those are just separating key value pairs now what can

separating key value pairs now what can I do here well if you think back to how

I do here well if you think back to how we manually searched for cats earlier

we manually searched for cats earlier let me quickly do this I'll do this one

let me quickly do this I'll do this one manually doc type HTML as my very first

manually doc type HTML as my very first line HTML tag with how about my Lang

line HTML tag with how about my Lang attribute for English up here and then

attribute for English up here and then inside of this I'll have a head tag

inside of this I'll have a head tag inside of this I'll have a title I'll

inside of this I'll have a title I'll call this example uh search and then

call this example uh search and then down here I'll have my beginning of a

down here I'll have my beginning of a body tag and now let me introduce you to

body tag and now let me introduce you to really a final tag for now uh a form tag

really a final tag for now uh a form tag which will create a web form the thing

which will create a web form the thing with text boxes and buttons that you and

with text boxes and buttons that you and I use every day on any number of

I use every day on any number of websites inside of this form I'm going

websites inside of this form I'm going to have an input like a text box whose

to have an input like a text box whose name is going to be Q for query because

name is going to be Q for query because I'm trying to re-implement Google here

I'm trying to re-implement Google here uh the uh type of that I want to be a

uh the uh type of that I want to be a text box or if I know I'm using this for

text box or if I know I'm using this for search I can actually change this to a

search I can actually change this to a search box and it's going to let me it's

search box and it's going to let me it's going to generally put a little X there

going to generally put a little X there so you can clear it quickly that's a

so you can clear it quickly that's a nice little enhancement as well and then

nice little enhancement as well and then I'm going to give myself a submit button

I'm going to give myself a submit button by doing input whoops I'm going to give

by doing input whoops I'm going to give myself a submit button by doing input

myself a submit button by doing input type equals submit and then I'll leave

type equals submit and then I'll leave that as such here all right now I need

that as such here all right now I need to do a little bit more but let's see

to do a little bit more but let's see how this looks let me go over to my

how this looks let me go over to my other tab let me go back to my index and

other tab let me go back to my index and if I zoom

if I zoom out there is search. HTML I'll click it

out there is search. HTML I'll click it and there's not much going on here even

and there's not much going on here even if I zoom in but I do indeed have a

if I zoom in but I do indeed have a really big text box and a submit button

really big text box and a submit button but I haven't in my HTML told anyone

but I haven't in my HTML told anyone anywhere that I want this input whether

anywhere that I want this input whether I type cat or dog to go to google.com so

I type cat or dog to go to google.com so for that I need a couple of more

for that I need a couple of more attributes and I know this from having

attributes and I know this from having done this before and any online

done this before and any online reference will say the same you can add

reference will say the same you can add an action attribute like what do you

an action attribute like what do you want the action of this form to be and

want the action of this form to be and you can put the URL to which you want

you can put the URL to which you want this form to be submitted and I know

this form to be submitted and I know from tinkering that it should be hdps

from tinkering that it should be hdps www.google.com search I don't need to

www.google.com search I don't need to put any question marks here myself but I

put any question marks here myself but I do want the uh browser to do that for me

do want the uh browser to do that for me so let me go back to my other tab let me

so let me go back to my other tab let me reload and nothing visually has happened

reload and nothing visually has happened but watch this when I now type in cats

but watch this when I now type in cats but before I hit enter notice that I'm

but before I hit enter notice that I'm currently at some long crazy URL search.

currently at some long crazy URL search. HTML is expected if I now go down to the

HTML is expected if I now go down to the submit button and click submit watch

submit button and click submit watch what happens to the URL and the page

what happens to the URL and the page itself I'm whisked away to the actual

itself I'm whisked away to the actual google.com and indeed there are those

google.com and indeed there are those same cats and if I zoom in here you'll

same cats and if I zoom in here you'll see that my URL has changed to be indeed

see that my URL has changed to be indeed SL search question mark Q equals cats so

SL search question mark Q equals cats so this is just how web forms work when you

this is just how web forms work when you submit any form on the web in this way

submit any form on the web in this way the browser automatically goes to that

the browser automatically goes to that action URL adds a question mark puts any

action URL adds a question mark puts any key value pairs that you manually typed

key value pairs that you manually typed into the text boxes and lets the server

into the text boxes and lets the server do its thing now here's where Chrome is

do its thing now here's where Chrome is starting to simplify things Safari does

starting to simplify things Safari does this too if you double click on your url

this too if you double click on your url now you see the full URL but if any

now you see the full URL but if any parts are missing that's just a UI thing

parts are missing that's just a UI thing to eliminate visual distractions

to eliminate visual distractions nowadays meanwhile if I go back to my

nowadays meanwhile if I go back to my own form if I search this time for dogs

own form if I search this time for dogs and hit enter now again the URL changes

and hit enter now again the URL changes to be Q equals dogs and it all reduces

to be Q equals dogs and it all reduces to this basic building block of using an

to this basic building block of using an a form tag now I can be more explicit if

a form tag now I can be more explicit if I know I want to use get it which is

I know I want to use get it which is actually the default I can literally say

actually the default I can literally say quote unquote get in all lowercase even

quote unquote get in all lowercase even though the verb earlier was by Design in

though the verb earlier was by Design in uppercase but here now I'm just being

uppercase but here now I'm just being ever more explicit if I don't want the

ever more explicit if I don't want the label of this button to be very

label of this button to be very generically submit maybe I want it to be

generically submit maybe I want it to be Google search quote unquote well if you

Google search quote unquote well if you read the documentation for forms you can

read the documentation for forms you can actually change the value of the button

actually change the value of the button to be quote unquote Google search and if

to be quote unquote Google search and if I now go back here and reload I get a

I now go back here and reload I get a fresh form and now I get a button that

fresh form and now I get a button that literally says Google Search and if I

literally says Google Search and if I tinker with this further because this

tinker with this further because this isn't very user friendly there's even

isn't very user friendly there's even more attributes I can do I can add on my

more attributes I can do I can add on my t uh search input a uh autoc complete

t uh search input a uh autoc complete equals uh off if I don't want to see my

equals uh off if I don't want to see my own history for whatever reason I don't

own history for whatever reason I don't want people knowing I'm searching for

want people knowing I'm searching for cats and dogs on this page I can

cats and dogs on this page I can autofocus on the text box so that it

autofocus on the text box so that it shows the cursor blinking in that box by

shows the cursor blinking in that box by default and I can even do something like

default and I can even do something like this I can have a placeholder attribute

this I can have a placeholder attribute that says something like query or some

that says something like query or some other documentation for the user and if

other documentation for the user and if I now go back and reload you'll see

I now go back and reload you'll see notice it says query and it's subtle but

notice it says query and it's subtle but my cursor is already positioned there it

my cursor is already positioned there it gave it focus and I can type cats now

gave it focus and I can type cats now without having to click in the Box

without having to click in the Box manually which is just marginally better

manually which is just marginally better for the user's experience any questions

for the user's experience any questions now on all of this

now on all of this here any

here any questions all right that too was a lot

questions all right that too was a lot why don't we take a casual five minute

why don't we take a casual five minute break and when we resume we'll take a

break and when we resume we'll take a look at CSS add in some JavaScript and

look at CSS add in some JavaScript and then wrap up so 5 minutes only for now

then wrap up so 5 minutes only for now all right we are back so that's

all right we are back so that's technically it for HTML like here on out

technically it for HTML like here on out it'll be up to like online resources and

it'll be up to like online resources and references we point you to just to fill

references we point you to just to fill in your vocabulary for more tags and

in your vocabulary for more tags and attributes but like conceptually that's

attributes but like conceptually that's it there's attributes uh there are tags

it there's attributes uh there are tags and there are attributes and the rest of

and there are attributes and the rest of it really is just kind of a laundry list

it really is just kind of a laundry list of capable uh possible features but it

of capable uh possible features but it turns out too you'll see over time that

turns out too you'll see over time that you can even see the HTML for websit so

you can even see the HTML for websit so for instance if I go over to harvard.edu

for instance if I go over to harvard.edu in my browser which I'll go ahead in

in my browser which I'll go ahead in just a moment here here and do here and

just a moment here here and do here and do

do https

https www.harvard.edu

www.harvard.edu enter it again will pull up today's

enter it again will pull up today's version of Harvard's website and if I

version of Harvard's website and if I rightclick or control click on it again

rightclick or control click on it again and go to inspect you'll see those

and go to inspect you'll see those so-called developer tools and earlier we

so-called developer tools and earlier we focused on the network tab just so we

focused on the network tab just so we could see the HTTP stuff going on but

could see the HTTP stuff going on but what I glossed over earlier was the

what I glossed over earlier was the so-called elements tab which actually

so-called elements tab which actually shows you the HTML underlying any web

shows you the HTML underlying any web page on the internet and so for instance

page on the internet and so for instance here is the underlying HTML for

here is the underlying HTML for Harvard's homepage as of right now and

Harvard's homepage as of right now and aesthetically some of it's been

aesthetically some of it's been collapsed so if I click on these various

collapsed so if I click on these various triangles I'll see what's actually

triangles I'll see what's actually inside of that is the children of some

inside of that is the children of some of these HTML tags but here on out if

of these HTML tags but here on out if you're ever curious as to like how a web

you're ever curious as to like how a web page uh made some feature visually you

page uh made some feature visually you can just literally use these developer

can just literally use these developer tools built into your own browser just

tools built into your own browser just to see what the uh web developer

to see what the uh web developer actually did and you can do things too

actually did and you can do things too like this like if you really like maybe

like this like if you really like maybe uh let's see if you really like this

uh let's see if you really like this menu in the top right hand corner of

menu in the top right hand corner of Harvard's website you can even

Harvard's website you can even rightclick that or control click that

rightclick that or control click that specifically choose inspect and what

specifically choose inspect and what browsers will do is jump to the HTML

browsers will do is jump to the HTML corresponding to that visual element on

corresponding to that visual element on the page and here you can see though

the page and here you can see though we've not talked about this tag before

we've not talked about this tag before there's a button tag there's an ID

there's a button tag there's an ID attribute and there's some other

attribute and there's some other attributes as well that Define that

attributes as well that Define that button um you can do other things too in

button um you can do other things too in the web page let me scroll down for

the web page let me scroll down for instance here and let's go actually

instance here and let's go actually let's go to another one like yale.edu

let's go to another one like yale.edu here in today's theme and suppose we

here in today's theme and suppose we want to do something like uh change the

want to do something like uh change the Aesthetics of this website well let's do

Aesthetics of this website well let's do how about this over here life at Yale

how about this over here life at Yale let's rightclick on this choose inspect

let's rightclick on this choose inspect that's going to jump to that part of the

that's going to jump to that part of the page and notice what you can do here in

page and notice what you can do here in this elements tab we can be a little a

this elements tab we can be a little a little playful in return today life at

little playful in return today life at Harvard and voila we've now changed

Harvard and voila we've now changed Yale's website it would seem so have we

Yale's website it would seem so have we really like hopefully hacking is not

really like hopefully hacking is not actually this

actually this easy what did we actually do based on

easy what did we actually do based on today's mental

today's mental model like I have changed the page

model like I have changed the page but yeah just changed how it is for me

but yeah just changed how it is for me right because my browser just like with

right because my browser just like with Phyllis and Brian from the GetGo

Phyllis and Brian from the GetGo requested Yale's web page I got back a

requested Yale's web page I got back a virtual envelope containing that HTML as

virtual envelope containing that HTML as we've now called it my browser has a

we've now called it my browser has a local copy it's got its own tree

local copy it's got its own tree otherwise known as a Dom document object

otherwise known as a Dom document object model built up in its memory and yeah I

model built up in its memory and yeah I went to town and changed my copy of it

went to town and changed my copy of it but of course hopefully I've not changed

but of course hopefully I've not changed the actual server and in fact if I

the actual server and in fact if I reload Yale's website now hopefully it

reload Yale's website now hopefully it will revert back to indeed yep what it

will revert back to indeed yep what it should be instead life at Yale but this

should be instead life at Yale but this ability in your own browser be it Chrome

ability in your own browser be it Chrome or Firefox or Edge or Safari to have

or Firefox or Edge or Safari to have these built-in developer tools are very

these built-in developer tools are very powerful because it's going to enable

powerful because it's going to enable you to not only diagnose problems that

you to not only diagnose problems that will invariably arise in the coming

will invariably arise in the coming weeks with your own code but is also

weeks with your own code but is also going to allow you to learn from other

going to allow you to learn from other sites like how you can do things and

sites like how you can do things and Tinker as well but up until now we

Tinker as well but up until now we focused only on tags and attributes and

focused only on tags and attributes and on the structure of a web page let's now

on the structure of a web page let's now focus more on the Aesthetics and

focus more on the Aesthetics and fine-tuning that it turns out that HTML

fine-tuning that it turns out that HTML has very limited support for anything

has very limited support for anything aesthetic like font sizes and colors and

aesthetic like font sizes and colors and so forth and in recent years people have

so forth and in recent years people have used necessarily a second language

used necessarily a second language called CSS not a programming language

called CSS not a programming language again a markup language if you will to

again a markup language if you will to just fine-tune the Aesthetics of a page

just fine-tune the Aesthetics of a page font sizes colors margins and all of

font sizes colors margins and all of that so CSS is going to allow us to

that so CSS is going to allow us to define a whole bunch of properties which

define a whole bunch of properties which is just another group of people's

is just another group of people's terminology for key value pairs indeed

terminology for key value pairs indeed ever since week five like key value

ever since week five like key value pairs are everywhere in the world not

pairs are everywhere in the world not just at like sweet green and restaurants

just at like sweet green and restaurants but indeed under in line code and

but indeed under in line code and languages and Technologies like these so

languages and Technologies like these so properties is the new word in CSS for

properties is the new word in CSS for what a moment ago we called attributes

what a moment ago we called attributes in H but it's the same idea just

in H but it's the same idea just different vocabulary that you get used

different vocabulary that you get used to over time a few phrases I might use

to over time a few phrases I might use now and you'll hear in the coming days

now and you'll hear in the coming days would be these type selector class

would be these type selector class selector ID selector attribute selector

selector ID selector attribute selector which just refer to different techniques

which just refer to different techniques we're about to see that are going to

we're about to see that are going to allow you to control more precisely the

allow you to control more precisely the Aesthetics of specific things on the

Aesthetics of specific things on the page and the way we're going to do this

page and the way we're going to do this is we're going to take our basic HTML

is we're going to take our basic HTML like we saw earlier and we're going to

like we saw earlier and we're going to introduce in the next few minutes just a

introduce in the next few minutes just a couple of more tags and or attributes

couple of more tags and or attributes one we're going to introduce you to a

one we're going to introduce you to a tag called style which nicely named

tag called style which nicely named allows you to um allows you to control

allows you to um allows you to control the style the Aesthetics the visuals of

the style the Aesthetics the visuals of the web page or we're going to introduce

the web page or we're going to introduce you to a link tag which very confusingly

you to a link tag which very confusingly does not give you a link that you can

does not give you a link that you can click on it just links to another file

click on it just links to another file that then gets automatically included or

that then gets automatically included or imported to borrow our language from C

imported to borrow our language from C or in Python but same idea this will

or in Python but same idea this will allow us to include secondary files and

allow us to include secondary files and we're going to ultimately show you how

we're going to ultimately show you how you can leverage third third party

you can leverage third third party Frameworks libraries that other people

Frameworks libraries that other people wrote so as to not get stuck in the

wrote so as to not get stuck in the weeds of all the fine tuning of

weeds of all the fine tuning of Aesthetics and just make pretty things

Aesthetics and just make pretty things fast so you can focus really on the

fast so you can focus really on the intellectually interesting part if

intellectually interesting part if that's your choice of building the

that's your choice of building the content the site out the application out

content the site out the application out yourself all right so with that said let

yourself all right so with that said let me go back to vs code here and let me go

me go back to vs code here and let me go ahead and create a simple example called

ahead and create a simple example called home.html like a very simple homepage

home.html like a very simple homepage for John Harvard for instance let me

for John Harvard for instance let me give myself three paragraphs initially

give myself three paragraphs initially the first of which is just going to have

the first of which is just going to have uh the person name the second paragraph

uh the person name the second paragraph is going to say something like welcome

is going to say something like welcome to my

to my homepage uh to greet visitors and the

homepage uh to greet visitors and the third is going to be like a little

third is going to be like a little footer like copyright uh how about John

footer like copyright uh how about John Harvard 1636 or something like that all

Harvard 1636 or something like that all right let me go back to my other tab as

right let me go back to my other tab as before reload my index there's my new

before reload my index there's my new file home.html and I'll click that and

file home.html and I'll click that and you'll see okay I mean this is sort of

you'll see okay I mean this is sort of 1636 style web page super simple all

1636 style web page super simple all text nothing really interesting going on

text nothing really interesting going on there

there but we can start to style it a little

but we can start to style it a little differently like if the title of the

differently like if the title of the page is John Harvard and then it's

page is John Harvard and then it's welcome to my homepage and then this

welcome to my homepage and then this less important footer why don't we have

less important footer why don't we have the text be large then medium then small

the text be large then medium then small so something arbitrary but a little more

so something arbitrary but a little more nuanced so let me go back to vs code

nuanced so let me go back to vs code here and in my home.html file let me

here and in my home.html file let me introduce not yet the style tag but what

introduce not yet the style tag but what I'm going to call temporarily the style

I'm going to call temporarily the style attribute both indeed exist this one's

attribute both indeed exist this one's simpler and it's going to be correct but

simpler and it's going to be correct but we'll see in a moment not as well

we'll see in a moment not as well designed arguably as is often our

designed arguably as is often our narrative so inside of the style tag you

narrative so inside of the style tag you can put this language called CSS key

can put this language called CSS key value pairs otherwise known as

value pairs otherwise known as properties the only way you know what

properties the only way you know what properties exist what keys exist is by

properties exist what keys exist is by taking a class reading a book looking at

taking a class reading a book looking at an online reference and we're going to

an online reference and we're going to give you just a sampling of what's out

give you just a sampling of what's out there so suppose I want to control the

there so suppose I want to control the font size of this first paragraph I can

font size of this first paragraph I can literally say font Das size in all lower

literally say font Das size in all lower case colon and then a word like large or

case colon and then a word like large or I can specify 12p point or 18 point or

I can specify 12p point or 18 point or something more precise like that like

something more precise like that like from Google Docs or Microsoft Word and

from Google Docs or Microsoft Word and suppose I want to make this text down

suppose I want to make this text down here uh medium well uh I'll do quote

here uh medium well uh I'll do quote unquote font size colon medium and down

unquote font size colon medium and down here I'll do style equals uh font Das

here I'll do style equals uh font Das size small so I'm going to start with

size small so I'm going to start with just these three key value pairs same

just these three key value pairs same key but different values I'll go back to

key but different values I'll go back to my page and in a moment I'll reload and

my page and in a moment I'll reload and it's going to be somewhat subtle but

it's going to be somewhat subtle but watch how the font size do change when I

watch how the font size do change when I reload now all right so got a little

reload now all right so got a little bigger middle one's about the same and

bigger middle one's about the same and the last one is a little smaller what if

the last one is a little smaller what if I want to center it just like many web

I want to center it just like many web pages have the text like this centered

pages have the text like this centered well I can separate these key value

well I can separate these key value pairs with semicolons and I'm sorry

pairs with semicolons and I'm sorry semicolons are kind of sort of back with

semicolons are kind of sort of back with CSS but I can do text-align Colon Center

CSS but I can do text-align Colon Center strictly speaking I don't need the last

strictly speaking I don't need the last semicolon if there's no more key value

semicolon if there's no more key value pairs but I'll just do it to be

pairs but I'll just do it to be consistent uh text align Colon Center

consistent uh text align Colon Center and then down here after another

and then down here after another semicolon text align Colon Center all

semicolon text align Colon Center all right let's go back reload now it's

right let's go back reload now it's going to be much more obvious the change

going to be much more obvious the change and we now have the beginnings of a

and we now have the beginnings of a homepage still pretty basic but at least

homepage still pretty basic but at least it's a little more interesting turns out

it's a little more interesting turns out we can do a little better with the

we can do a little better with the copyright symbol like most computers

actually have support for a circle with a c in it but you can't just do that

a c in it but you can't just do that with uh text like this there's different

with uh text like this there's different ways to do this you could copy paste it

ways to do this you could copy paste it from like a website that already has it

from like a website that already has it so you don't have to figure out the mag

so you don't have to figure out the mag iCal keystroke on your Mac or PC but

iCal keystroke on your Mac or PC but there's also in HTML what are called

there's also in HTML what are called entities and you can actually specify

entities and you can actually specify using heximal or decimal codes numbers

using heximal or decimal codes numbers like this H1 169 semicolon after an

like this H1 169 semicolon after an Amper sand and this is a special symbol

Amper sand and this is a special symbol that you can look up in any online

that you can look up in any online reference for like special characters

reference for like special characters that are hard or impossible to type

that are hard or impossible to type manually at your keyboard and this let

manually at your keyboard and this let me zoom in just so it's obvious if I

me zoom in just so it's obvious if I reload now instead of being two

reload now instead of being two parentheses and a c character

parentheses and a c character now it's a proper Copyright symbol so

now it's a proper Copyright symbol so you'll see these out there they're not

you'll see these out there they're not necessarily that frequently used

necessarily that frequently used nowadays but it's good to know that they

nowadays but it's good to know that they exist but let me go back now to my code

exist but let me go back now to my code and propose that while correct uh this

and propose that while correct uh this is arguably not very welld designed and

is arguably not very welld designed and even if you've never seen HTML never

even if you've never seen HTML never seen CSS before what Instinct might you

seen CSS before what Instinct might you have for why this is poorly

have for why this is poorly designed

designed yeah there's repetition right in general

yeah there's repetition right in general in the past several weeks see python SQL

in the past several weeks see python SQL like repetition generally bad and and

like repetition generally bad and and sloppy and it's not going to scale well

sloppy and it's not going to scale well so the repetition I think you're

so the repetition I think you're probably alluding to is textalign Center

probably alluding to is textalign Center textalign Center textalign Center well

textalign Center textalign Center well we can factor that out in CSS the C in

we can factor that out in CSS the C in CSS means cascading and this means that

CSS means cascading and this means that if you move some properties to like a

if you move some properties to like a parent or a grandparent uh the children

parent or a grandparent uh the children or grandchildren will inherit those

or grandchildren will inherit those properties that is they will Cascade

properties that is they will Cascade down the family tree so to speak and so

down the family tree so to speak and so so let me go ahead and remove all of

so let me go ahead and remove all of these since I claim they're indeed

these since I claim they're indeed redundant and let me preserve just one

redundant and let me preserve just one of them by well let me do this it's not

of them by well let me do this it's not quite right to put a a paragraph inside

quite right to put a a paragraph inside of a paragraph like that's just not a

of a paragraph like that's just not a thing in English writing or in writing

thing in English writing or in writing more generally so I'm going to do one

more generally so I'm going to do one thing first it turns out that these two

thing first it turns out that these two are arguably not paragraphs right this

are arguably not paragraphs right this is like a header the body the essence of

is like a header the body the essence of the page and then the footer so if a

the page and then the footer so if a paragraph isn't quite the right English

paragraph isn't quite the right English semantics you can actually use more

semantics you can actually use more generically a tag that's all over the

generically a tag that's all over the internet called div for division of the

internet called div for division of the page and this is just a very generic

page and this is just a very generic term for like a big rectangular region

term for like a big rectangular region that divides the page again and again

that divides the page again and again just so that you can think about

just so that you can think about different regions now that I have div

different regions now that I have div which really has no more meaning than

which really has no more meaning than that it's a division of the page

that it's a division of the page interpret as you will now I can have

interpret as you will now I can have multiple ones of these and let me go

multiple ones of these and let me go ahead and open a div tag here let me

ahead and open a div tag here let me close a new div tag here and then just

close a new div tag here and then just to keep everything tidy I'm going to

to keep everything tidy I'm going to highlight everything in between and hit

highlight everything in between and hit Tab and that just automatically indents

Tab and that just automatically indents everything for me now I have a three

everything for me now I have a three divs inside of another div and that's

divs inside of another div and that's totally fine this is very commonly done

totally fine this is very commonly done now I'm going to do this style equals

now I'm going to do this style equals quote unquote text align Colon Center

quote unquote text align Colon Center semicolon or not and now I have some

semicolon or not and now I have some cascading capabilities now the parent of

cascading capabilities now the parent of those three children John Harvard welome

those three children John Harvard welome to my homepage and the copyright will

to my homepage and the copyright will now all inherit that property so when I

now all inherit that property so when I hit reload nothing aesthetically has

hit reload nothing aesthetically has changed whoops sorry

changed whoops sorry um I should have done reload slightly

um I should have done reload slightly earlier when you use a div instead of a

earlier when you use a div instead of a paragraph it actually gets rid of the

paragraph it actually gets rid of the par uh the space between those

par uh the space between those paragraphs it just sandwiches them a

paragraphs it just sandwiches them a little closer together I can fix this in

little closer together I can fix this in another way but that aside everything is

another way but that aside everything is still centered and the text is still

still centered and the text is still large medium and small but I should have

large medium and small but I should have called out that change in the paragraph

called out that change in the paragraph spacing but we could bring that back

spacing but we could bring that back before long if we wanted now what more

before long if we wanted now what more could I do to maybe improve this well

could I do to maybe improve this well strictly speaking I don't really need

strictly speaking I don't really need that parent div right because these

that parent div right because these three divs inside already had a parent

three divs inside already had a parent so let me actually get rid of that new

so let me actually get rid of that new div just undo what I did I'll highlight

div just undo what I did I'll highlight this and if you haven't seen this trick

this and if you haven't seen this trick shift tab will unindent nicely which is

shift tab will unindent nicely which is perhaps helpful I could just put that

perhaps helpful I could just put that text align Center on the body tag so

text align Center on the body tag so text align Colon Center quote unquote

text align Colon Center quote unquote this two would work as well so long as

this two would work as well so long as you go up the family tree so to speak

you go up the family tree so to speak reload and now indeed there's nothing

reload and now indeed there's nothing aesthetically that has changed this time

aesthetically that has changed this time but it turns out nowadays the web is

but it turns out nowadays the web is getting a little more sophisticated and

getting a little more sophisticated and even though you will see so many

even though you will see so many examples online and tutorials and books

examples online and tutorials and books using div div div div all over the place

using div div div div all over the place there are newer semantic tags semantic

there are newer semantic tags semantic just means they have more meaning than

just means they have more meaning than this generic notion of a division and if

this generic notion of a division and if you look up the a documentation for HTML

you look up the a documentation for HTML you'll see that if you want to have a

you'll see that if you want to have a header on a page not a heading like H1

header on a page not a heading like H1 H2 but a header there's literally

H2 but a header there's literally nowadays a header tag and this is

nowadays a header tag and this is marginally better because it now says

marginally better because it now says what it is search engines like Google

what it is search engines like Google and Bing can detect oh that's the header

and Bing can detect oh that's the header of the page maybe we should use this and

of the page maybe we should use this and give it more prominence in the search

give it more prominence in the search results you can then have a main part of

results you can then have a main part of the page so literally a tag called main

the page so literally a tag called main nowadays you can literally have a footer

nowadays you can literally have a footer of the page and again these are often

of the page and again these are often useful for screen readers to help recite

useful for screen readers to help recite things verbally for folks who might

things verbally for folks who might otherwise not be able to read them and

otherwise not be able to read them and probably these screen readers might

probably these screen readers might highlight the header and the main part

highlight the header and the main part but maybe not might not spend time for

but maybe not might not spend time for the user on the footer which is arguably

the user on the footer which is arguably a little less important semantically

a little less important semantically usually um or search engines again now

usually um or search engines again now know what's the header what's the footer

know what's the header what's the footer what's the main part of the page so they

what's the main part of the page so they know what to search and analyze so this

know what to search and analyze so this would arguably be a a better design

would arguably be a a better design nowadays as

nowadays as well but what else remains as a problem

well but what else remains as a problem well this is now getting a little bit

well this is now getting a little bit more subtle and takes some experience

more subtle and takes some experience but this practice of putting HTML and

but this practice of putting HTML and CSS all in the same file it's a little

CSS all in the same file it's a little sloppy why because it means I'm

sloppy why because it means I'm co-mingling my data with the

co-mingling my data with the presentation thereof like the juicy

presentation thereof like the juicy stuff I care about like John Harvard and

stuff I care about like John Harvard and the phrase welcome to my homepage and

the phrase welcome to my homepage and all of the Aesthetics that I might want

all of the Aesthetics that I might want to change over time and honestly because

to change over time and honestly because everything is currently in one big file

everything is currently in one big file it's going to make it really hard for me

it's going to make it really hard for me to collaborate with a classmate or a

to collaborate with a classmate or a colleague at work so that maybe I do the

colleague at work so that maybe I do the HTML they do the CSS like uhuh not if

HTML they do the CSS like uhuh not if you're all working in the same file it

you're all working in the same file it would be a nightmare even if you use vs

would be a nightmare even if you use vs code sharing feature like Google Docs

code sharing feature like Google Docs and both are typing at the same time

and both are typing at the same time like you're going to mess up somehow

like you're going to mess up somehow it'd be nice if we could separate these

it'd be nice if we could separate these two languages well one way to do that

two languages well one way to do that would be as follows let me get rid of

would be as follows let me get rid of all of the style tags sorry style

all of the style tags sorry style attributes that I've added up until

attributes that I've added up until now on all four now of these tags and

now on all four now of these tags and let me introduce the style tag that we

let me introduce the style tag that we saw on the slide earlier instead I'm

saw on the slide earlier instead I'm going to go up here into the head of the

going to go up here into the head of the page which is where technically these

page which is where technically these style tags must go so that they're

style tags must go so that they're already loaded into memory before the

already loaded into memory before the body is even analyzed by the browser and

body is even analyzed by the browser and inside of the the style tag I'm actually

inside of the the style tag I'm actually going to select the HTML elements that I

going to select the HTML elements that I want to stylize if you will so if I want

want to stylize if you will so if I want to change the body's Aesthetics I'm

to change the body's Aesthetics I'm going to literally type the name of that

going to literally type the name of that tag body and then I'm sorry curly braces

tag body and then I'm sorry curly braces are back also from c u inside of these

are back also from c u inside of these curly braces I'm going to put text align

curly braces I'm going to put text align Center so the key value pairs are the

Center so the key value pairs are the same the only new thing I've done is

same the only new thing I've done is I've moved some of the syntax up to this

I've moved some of the syntax up to this new style tag in the head if I want to

new style tag in the head if I want to now control the header tag as well I can

now control the header tag as well I can use the same curly braces this is

use the same curly braces this is convention to put the open curly brace

convention to put the open curly brace on the same line the closed curly brace

on the same line the closed curly brace on another the browser doesn't really

on another the browser doesn't really care but this is a common CSS style

care but this is a common CSS style convention I'm going to do font size

convention I'm going to do font size large semicolon then for the main tag

large semicolon then for the main tag I'm going to do font size medium and

I'm going to do font size medium and then for the footer tag I'm going to do

then for the footer tag I'm going to do font size small so same exact thing and

font size small so same exact thing and it's admittedly a little bit more

it's admittedly a little bit more verbose it's taking up more lines of

verbose it's taking up more lines of code it doesn't all quite fit on the

code it doesn't all quite fit on the screen but if you scroll back down now

screen but if you scroll back down now and you'll acquire an eye for this this

and you'll acquire an eye for this this is just better like it's just more

is just better like it's just more compact it's more readable the the

compact it's more readable the the content the data jumps out and there's

content the data jumps out and there's no visual distractions like the CSS

no visual distractions like the CSS properties as before upside here too is

properties as before upside here too is that we don't actually need to uh this

that we don't actually need to uh this doesn't actually change the Aesthetics

doesn't actually change the Aesthetics if I reload the same page it still looks

if I reload the same page it still looks the same but I've taken a step toward

the same but I've taken a step toward some slightly better design but let me

some slightly better design but let me propose that there's other ways to do

propose that there's other ways to do this too we just selected things by way

this too we just selected things by way of their uh type so that was a so-called

of their uh type so that was a so-called type selector when I literally just

type selector when I literally just specifi the type of tag body header main

specifi the type of tag body header main footer but there's other ways that now

footer but there's other ways that now we can lay the foundation for making

we can lay the foundation for making reusable CSS that you and colleagues and

reusable CSS that you and colleagues and classmates can use and reuse in multiple

classmates can use and reuse in multiple files and even in multiple projects so

files and even in multiple projects so let me actually go ahead and do this

let me actually go ahead and do this instead of just very explicitly saying I

instead of just very explicitly saying I want the body to be centered let me

want the body to be centered let me invent an adjective if you will and let

invent an adjective if you will and let me change this to do centered and this

me change this to do centered and this new uh this new vocabulary word centered

new uh this new vocabulary word centered will literally mean texal Center let me

will literally mean texal Center let me go ahead here and I'm just going to

go ahead here and I'm just going to create a new adjective called large a

create a new adjective called large a new adjective called medium and a new

new adjective called medium and a new adjective called small they are

adjective called small they are deliberately consistent with what the

deliberately consistent with what the properties do but these are now my own

properties do but these are now my own vocabulary words and they are called

vocabulary words and they are called classes so a class is just a collection

classes so a class is just a collection of key value

of key value a collection of properties that you get

a collection of properties that you get to invent for yourself and what it lets

to invent for yourself and what it lets you do now is this now if I want the

you do now is this now if I want the whole body to be centered I can add this

whole body to be centered I can add this tag which we actually saw briefly in

tag which we actually saw briefly in Yale's HTML class equals centered down

Yale's HTML class equals centered down here in the header if I want this to be

here in the header if I want this to be large I can say class equals quote

large I can say class equals quote unquote large down here on Main I can

unquote large down here on Main I can say class equals quote unquote medium

say class equals quote unquote medium and down here I can have class equals

and down here I can have class equals quote unquote small now I have taken one

quote unquote small now I have taken one step backward by read addding some of

step backward by read addding some of the Aesthetics to the page but it's not

the Aesthetics to the page but it's not the actual properties it's not the key

the actual properties it's not the key value pairs it's now more semantically

value pairs it's now more semantically nice because now I just know from

nice because now I just know from reading the HTML what these things are

reading the HTML what these things are going to look like whereas the

going to look like whereas the implementation details for all four of

implementation details for all four of those adjectives is now relegated up

those adjectives is now relegated up above and these are literally my words I

above and these are literally my words I could change it to Fu and use class

could change it to Fu and use class equals quote unquote Foo but obviously

equals quote unquote Foo but obviously that would not be the best choice of

that would not be the best choice of words in this case all right any

words in this case all right any questions on this this now is what we

questions on this this now is what we would call a class selector by using

would call a class selector by using literally the dot even though the dot

literally the dot even though the dot does not appear elsewhere but dot means

does not appear elsewhere but dot means this is a

this is a class these are not always the best

class these are not always the best syntactic design decisions that the

syntactic design decisions that the world makes all right well one last

world makes all right well one last trick then notice that this is a little

trick then notice that this is a little Annoying that I'm still working in the

Annoying that I'm still working in the same file and if my classmate wants to

same file and if my classmate wants to clean up my Aesthetics make my homepage

clean up my Aesthetics make my homepage look way better if my colleague wants to

look way better if my colleague wants to do the same wouldn't it be nice if we

do the same wouldn't it be nice if we could actually move all of this code to

could actually move all of this code to a different function file like a python

a different function file like a python library or a c header file well you can

library or a c header file well you can let me go ahead and delete that whole

let me go ahead and delete that whole style tag let me add a confusingly named

style tag let me add a confusingly named link tag the href of which let's call a

link tag the href of which let's call a new file styles.css and let's say that

new file styles.css and let's say that the relationship of that file is that of

the relationship of that file is that of stylesheet so this is a term of Art in

stylesheet so this is a term of Art in the world of web development a

the world of web development a stylesheet is a text file that contains

stylesheet is a text file that contains lots of styles lots of CSS properties

lots of styles lots of CSS properties let me open my terminal real fast

let me open my terminal real fast and let me do code of

and let me do code of styles.css

styles.css enter and in this file I'm going to

enter and in this file I'm going to paste all of those same lines as earlier

paste all of those same lines as earlier but now they're in a separate file and

but now they're in a separate file and indeed if I hide my terminal window and

indeed if I hide my terminal window and I give this file to a colleague they can

I give this file to a colleague they can now work on the Aesthetics of the page

now work on the Aesthetics of the page and make things a lot prettier than this

and make things a lot prettier than this maybe use specific font sizes maybe add

maybe use specific font sizes maybe add colors and the like whereas I can focus

colors and the like whereas I can focus entirely on the HTML because this file

entirely on the HTML because this file now will reference that other and if I

now will reference that other and if I go back to my other Tab and reload Cod

go back to my other Tab and reload Cod the content's going to be exactly the

the content's going to be exactly the same but now I'm using some separate

same but now I'm using some separate file

file instead any questions now about these

instead any questions now about these techniques

techniques here no all right so with that said let

here no all right so with that said let me show just one example now of what I

me show just one example now of what I called a moment ago Frameworks and this

called a moment ago Frameworks and this is where web development gets kind of

is where web development gets kind of fun at least if you like this especially

fun at least if you like this especially if you like the sort of logical design

if you like the sort of logical design the presentation of information you care

the presentation of information you care about but you really don't want to

about but you really don't want to struggle with like font sizes and colors

struggle with like font sizes and colors and getting everything Pixel Perfect so

and getting everything Pixel Perfect so to speak let me propose that I open up

to speak let me propose that I open up here an example in just a moment in vs

here an example in just a moment in vs code that I prepared in advance and this

code that I prepared in advance and this one is going to be an opportunity to

one is going to be an opportunity to consider how you might take some of the

consider how you might take some of the data from last week which we had wherein

data from last week which we had wherein we collected everyone's favorites and

we collected everyone's favorites and lay it out in a really big HTML table so

lay it out in a really big HTML table so I wrote this out in advance because it

I wrote this out in advance because it was a huge amount of data but it's the

was a huge amount of data but it's the same data from the Google form from last

same data from the Google form from last week and you'll see already the hints of

week and you'll see already the hints of a table tag and these TRS and I added a

a table tag and these TRS and I added a few other tags for Aesthetics it turns

few other tags for Aesthetics it turns out when you have a more visually

out when you have a more visually interesting header for your table

interesting header for your table there's another tag called T head

there's another tag called T head there's another tag called T body these

there's another tag called T body these are not all that intellectually

are not all that intellectually interesting I just read the

interesting I just read the documentation and realized oh to make

documentation and realized oh to make things prettier I need a t head a t body

things prettier I need a t head a t body and so forth but what's interesting here

and so forth but what's interesting here is that if I go to my index here and

is that if I go to my index here and open this file called favorites. HTML

open this file called favorites. HTML here is all of the data from last week's

here is all of the data from last week's Google spreadsheet which we exported as

Google spreadsheet which we exported as CS and I manually before class converted

CS and I manually before class converted to just HTML it's indeed a table but

to just HTML it's indeed a table but it's really not pretty like the columns

it's really not pretty like the columns are really close together it's kind of

are really close together it's kind of hard to distinguish one row from another

hard to distinguish one row from another but this is just raw HTML written by me

but this is just raw HTML written by me now I could use CSS and some of the

now I could use CSS and some of the tricks we just saw to maybe change font

tricks we just saw to maybe change font size there's ways to change color

size there's ways to change color background color and a lot of things

background color and a lot of things like that but honestly other surely

like that but honestly other surely other people in the world have presented

other people in the world have presented tabular data in pretty ways right I've

tabular data in pretty ways right I've been to many websites that have prettier

been to many websites that have prettier tables than M can I maybe use someone

tables than M can I maybe use someone else's framework someone else's CSS

else's framework someone else's CSS include it in my page but then stand on

include it in my page but then stand on their shoulders and just make my stuff

their shoulders and just make my stuff look prettier well I dare say I can let

look prettier well I dare say I can let me go ahead here and semi secretly open

me go ahead here and semi secretly open up vs code again and let me grab a

up vs code again and let me grab a slightly different version of favorites.

slightly different version of favorites. HTML that I also opened in advance

HTML that I also opened in advance wherein I add this line of code instead

wherein I add this line of code instead give me just a moment to foreground this

give me just a moment to foreground this version and the data is is all the same

version and the data is is all the same as before but I've added one of these

as before but I've added one of these link tags and I'm not linking to my own

link tags and I'm not linking to my own styles.css I'm using a popular Library

styles.css I'm using a popular Library called bootstrap and bootstrap is just

called bootstrap and bootstrap is just one of many popular libraries out there

one of many popular libraries out there free at that that has a whole bunch of

free at that that has a whole bunch of CSS files and soon JavaScript files that

CSS files and soon JavaScript files that you can just use for free in your own

you can just use for free in your own projects personally or professionally

projects personally or professionally that just make things look and behave

that just make things look and behave better without you having to reinvent

better without you having to reinvent Wheels now to access their CSS I had to

Wheels now to access their CSS I had to read their documentation and grab this

read their documentation and grab this very long URL here but it's the same

very long URL here but it's the same idea link a forre equals quote unquote

idea link a forre equals quote unquote something and I read their documentation

something and I read their documentation and they told me to add this they told

and they told me to add this they told me that if I want my tables to be

me that if I want my tables to be prettier I have to add a class attribute

prettier I have to add a class attribute to my own table tag and specify a little

to my own table tag and specify a little weirdly but this is what bootstrap told

weirdly but this is what bootstrap told me to do a class called table and that

me to do a class called table and that will make it a prettier bootstrap table

will make it a prettier bootstrap table and if I want to stripe it like every

and if I want to stripe it like every other row is gray instead of white just

other row is gray instead of white just to make it pop a little more visually I

to make it pop a little more visually I can also add a second class separated by

can also add a second class separated by a space called table striped that's all

a space called table striped that's all I did I added line five and I changed

I did I added line five and I changed line nine and that is it the rest of the

line nine and that is it the rest of the hundreds of lines in favorites. HTML are

hundreds of lines in favorites. HTML are the same but if I go back here now and

the same but if I go back here now and reload the browser now thanks to

reload the browser now thanks to bootstrap voila like it's much prettier

bootstrap voila like it's much prettier now I can zoom out and that changes the

now I can zoom out and that changes the font size just locally for me and even

font size just locally for me and even if you don't love their Aesthetics I

if you don't love their Aesthetics I mean this is easily better than my own

mean this is easily better than my own there and it turns out we can can do

there and it turns out we can can do even better by adding interactivity to

even better by adding interactivity to this too but to do that we're going to

this too but to do that we're going to need one final language for today and

need one final language for today and this one is an actual programming

this one is an actual programming language and we won't use it all that

language and we won't use it all that much in cs50 but we introduce it here as

much in cs50 but we introduce it here as we begin web stuff because there's just

we begin web stuff because there's just so many free libraries and professional

so many free libraries and professional libraries that you can use just to make

libraries that you can use just to make your web applications fancier and more

your web applications fancier and more interactive mobile applications as well

interactive mobile applications as well increasingly use HTML CSS and JavaScript

increasingly use HTML CSS and JavaScript to power our iPhones and Android devices

to power our iPhones and Android devices as well so a quick tour some syntax and

as well so a quick tour some syntax and then we'll conclude with just some

then we'll conclude with just some hopefully inspiring examples to give you

hopefully inspiring examples to give you a taste of what JavaScript can do so

a taste of what JavaScript can do so JavaScript supports conditionals just

JavaScript supports conditionals just like C and python before it if we rewind

like C and python before it if we rewind to our scratch days here of course is a

to our scratch days here of course is a conditional here is the corresponding

conditional here is the corresponding JavaScript code as of today it's pretty

JavaScript code as of today it's pretty much identical to see with the syntax

much identical to see with the syntax here uh if we had an if an if else in

here uh if we had an if an if else in scratch it looked like this in

scratch it looked like this in JavaScript it's going to look like this

JavaScript it's going to look like this instead so it's a bit of a regression V

instead so it's a bit of a regression V ofv python like the the parentheses are

ofv python like the the parentheses are back the curly braces are back the

back the curly braces are back the semicolons I mentioned in CSS are also

semicolons I mentioned in CSS are also back in JavaScript potentially but it's

back in JavaScript potentially but it's familiar is the point here and it's a

familiar is the point here and it's a different language that's frequently

different language that's frequently used for the web whereas you can't use

used for the web whereas you can't use python in the ways we're about to use

python in the ways we're about to use JavaScript it just wasn't designed for

JavaScript it just wasn't designed for that purpose meanwhile if you have an if

that purpose meanwhile if you have an if El it's else if else in scratch well in

El it's else if else in scratch well in JavaScript just like in C it's going to

JavaScript just like in C it's going to look like this instead variables in

look like this instead variables in JavaScript of course are a thing too and

JavaScript of course are a thing too and in scratch we might have initialized a

in scratch we might have initialized a counter variable to Z in JavaScript a

counter variable to Z in JavaScript a few different ways to do this and just

few different ways to do this and just the for now the keyword is let it's sort

the for now the keyword is let it's sort of a polite way of asking for a variable

of a polite way of asking for a variable let uh counter equal zero semicolon so

let uh counter equal zero semicolon so you don't mention the type but you do

you don't mention the type but you do use a keyword here in this case called

use a keyword here in this case called let if you want to increment counter by

let if you want to increment counter by one few different ways in JavaScript you

one few different ways in JavaScript you can do this just like in C in JavaScript

can do this just like in C in JavaScript you can do this just like in C and in

you can do this just like in C and in Python in JavaScript you can also get

Python in JavaScript you can also get this so plus plus is back so maybe that

this so plus plus is back so maybe that counterbalances the other syntax as well

counterbalances the other syntax as well that was was not the case in Python

that was was not the case in Python Loops are back of course in JavaScript

Loops are back of course in JavaScript whereas in scratch you could repeat

whereas in scratch you could repeat three times like this in JavaScript it's

three times like this in JavaScript it's pretty much just like C the only

pretty much just like C the only difference here is that you say let

difference here is that you say let instead of int for an example like this

instead of int for an example like this meanwhile if you want to do something

meanwhile if you want to do something forever in scratch in JavaScript just

forever in scratch in JavaScript just like in C you say while true in this

like in C you say while true in this case so this is to say we're sort of

case so this is to say we're sort of comfortable spending relatively little

comfortable spending relatively little time on JavaScript at least for today's

time on JavaScript at least for today's purposes because syntactically it's

purposes because syntactically it's really the same as we've seen before

really the same as we've seen before with maybe a slight variance here or

with maybe a slight variance here or there but what's interesting today

there but what's interesting today arguably is just what kinds of things

arguably is just what kinds of things you can do with it so with that said

you can do with it so with that said what kinds of things can we do it's all

what kinds of things can we do it's all comes back to this picture if this is a

comes back to this picture if this is a simple web page on the left and this is

simple web page on the left and this is the corresponding tree or Dom document

the corresponding tree or Dom document object model on the right that is the

object model on the right that is the tree the browser automatically creates

tree the browser automatically creates in memory or RAM for you JavaScript is

in memory or RAM for you JavaScript is now a proper programming language that

now a proper programming language that lets us dynamically manipul ulate like

lets us dynamically manipul ulate like read data from this change this and this

read data from this change this and this is how Google for instance implements

is how Google for instance implements your inbox they might have uh in your

your inbox they might have uh in your inbox it's like a table so TR TR TR TR

inbox it's like a table so TR TR TR TR probably something like that or heck

probably something like that or heck maybe div div div div using JavaScript

maybe div div div div using JavaScript anytime they realize someone sent you

anytime they realize someone sent you new mail they can create a new node a

new mail they can create a new node a new rectangle in memory and you the

new rectangle in memory and you the human see a new div or a new TR again

human see a new div or a new TR again and again and again so with JavaScript

and again and again so with JavaScript you just have the ability to control the

you just have the ability to control the user's experience and instead of like

user's experience and instead of like I've been doing constantly hitting

I've been doing constantly hitting reload in the page to see some new

reload in the page to see some new content to see some new content

content to see some new content JavaScript can be running 247 so that

JavaScript can be running 247 so that you can actually see all of these

you can actually see all of these changes live all right let's go about

changes live all right let's go about writing some JavaScript code now instead

writing some JavaScript code now instead of writing it on the server and

of writing it on the server and executing it on the server we're going

executing it on the server we're going to actually use a very common Paradigm

to actually use a very common Paradigm whereas JavaScript is actually executed

whereas JavaScript is actually executed in the browser client side that is to

in the browser client side that is to say we can actually start writing some

say we can actually start writing some JavaScript code inside of our own HTML

JavaScript code inside of our own HTML file so that when a user visits that web

file so that when a user visits that web page with their browser not only is the

page with their browser not only is the HTML and any CSS downloaded to the

HTML and any CSS downloaded to the user's browser so is that JavaScript

user's browser so is that JavaScript code so that it's executed indeed client

code so that it's executed indeed client side on the browser rather than server

side on the browser rather than server side as has been the case with python in

side as has been the case with python in previous weeks well where do we go about

previous weeks well where do we go about writing some JavaScript code let's go

writing some JavaScript code let's go ahead and revisit hello.html which

ahead and revisit hello.html which previously was a completely static

previously was a completely static example that literally just said hello

example that literally just said hello title hello body indeed if I open this

title hello body indeed if I open this up using HTTP server and view it now in

up using HTTP server and view it now in a separate tab all it said was exactly

a separate tab all it said was exactly that hello title in the tab and hello

that hello title in the tab and hello body in the main part of the viewport so

body in the main part of the viewport so to speak well let's make this example a

to speak well let's make this example a little more Dynamic so it doesn't just

little more Dynamic so it doesn't just say hello body but maybe says hello to

say hello body but maybe says hello to an actual person so let's go ahead and

an actual person so let's go ahead and do this let's go ahead and remove the

do this let's go ahead and remove the hardcoded hello body and let's actually

hardcoded hello body and let's actually go ahead here and use a form tag but

go ahead here and use a form tag but we're not going to use this form in the

we're not going to use this form in the usual way whereby the data gets sent all

usual way whereby the data gets sent all the way back to the server we're going

the way back to the server we're going to Leverage control over this form

to Leverage control over this form client side instead so I'm going to go

client side instead so I'm going to go ahead and create this open form tag

ahead and create this open form tag close form tag inside of that let me

close form tag inside of that let me give myself a text input that's going to

give myself a text input that's going to have autocomplete equals quote unquote

have autocomplete equals quote unquote off just to ensure that what I

off just to ensure that what I previously type in my examples doesn't

previously type in my examples doesn't reappear accidentally we're going to

reappear accidentally we're going to autofocus it so that the cursor is

autofocus it so that the cursor is blinking right there in that text box

blinking right there in that text box and this time I'm going to go ahead and

and this time I'm going to go ahead and give it how about a uh placeholder of

give it how about a uh placeholder of quote unquote name to make clear that

quote unquote name to make clear that prompting the user for their name and

prompting the user for their name and then the type of this text box will be

then the type of this text box will be the default or more explicitly here text

the default or more explicitly here text and then I'm going to have as we've seen

and then I'm going to have as we've seen before a button the type of which is

before a button the type of which is submit also our typical default and then

submit also our typical default and then inside of this button is going to be the

inside of this button is going to be the label how about something like uh how

label how about something like uh how about we'll call this greet so that's

about we'll call this greet so that's what the button will actually say well

what the button will actually say well let me actually go back into my browser

let me actually go back into my browser tab let me reload this page and we

tab let me reload this page and we should now see a relatively simple form

should now see a relatively simple form whereby I have the cursor blinking on a

whereby I have the cursor blinking on a text inp input prompting the user for

text inp input prompting the user for their name and then a greet button that

their name and then a greet button that I can click but if I click this button

I can click but if I click this button now it's not going to do anything useful

now it's not going to do anything useful because I haven't written any code to

because I haven't written any code to tell the browser what to do when I click

tell the browser what to do when I click that button but it turns out there's all

that button but it turns out there's all sorts of events in the world of

sorts of events in the world of JavaScript that you can listen for so to

JavaScript that you can listen for so to speak in fact here's just a list of some

speak in fact here's just a list of some of them anytime something changes in a

of them anytime something changes in a form field anytime the user clicks or

form field anytime the user clicks or drags on something anytime the user

drags on something anytime the user presses a key and maybe lifts their

presses a key and maybe lifts their finger up anytime the mouse goes down or

finger up anytime the mouse goes down or over or up on top of something or

over or up on top of something or anytime a form is submitted those are

anytime a form is submitted those are events in the same way that we talked

events in the same way that we talked about events back in week zero in

about events back in week zero in scratch and in JavaScript just like in

scratch and in JavaScript just like in scratch where you can do something when

scratch where you can do something when green flag clicked in JavaScript you can

green flag clicked in JavaScript you can write code that actually listens for any

write code that actually listens for any of these events or more so with that

of these events or more so with that said let's go back to vs code here and

said let's go back to vs code here and let's make a couple of changes instead

let's make a couple of changes instead let's go ahead and add to this form a

let's go ahead and add to this form a new attribute that's not the best way to

new attribute that's not the best way to do it but it's perhaps the simplest for

do it but it's perhaps the simplest for version one here and let's say onsubmit

version one here and let's say onsubmit do the following so onsubmit is an HTML

do the following so onsubmit is an HTML attribute and curiously its value inside

attribute and curiously its value inside of the quotes there can actually be some

of the quotes there can actually be some JavaScript code and let's go ahead now

JavaScript code and let's go ahead now and let's assume there exists a function

and let's assume there exists a function in the world called greet and what I

in the world called greet and what I want to do is call that function right

want to do is call that function right then and there well now in JavaScript

then and there well now in JavaScript how do I go about making that function

how do I go about making that function exist it doesn't come out of the box

exist it doesn't come out of the box just like print might or say might in

just like print might or say might in python or scratch respectively but I can

python or scratch respectively but I can do this let me go up into the head of

do this let me go up into the head of this page inside of a script tag here

this page inside of a script tag here both open and close let me actually

both open and close let me actually write some JavaScript code and just so

write some JavaScript code and just so it stands out I'm going to give myself a

it stands out I'm going to give myself a couple of blank lines though not

couple of blank lines though not strictly necessary and let me Define a

strictly necessary and let me Define a new function in JavaScript called greet

new function in JavaScript called greet and this is the syntax in JavaScript for

and this is the syntax in JavaScript for creating your own function similar in

creating your own function similar in Python instead of saying defa in

Python instead of saying defa in JavaScript you just say function then

JavaScript you just say function then the name of the function and any

the name of the function and any arguments within the parentheses

arguments within the parentheses thereafter but I'm not going to pass in

thereafter but I'm not going to pass in any here then inside of curly braces

any here then inside of curly braces what I'm going to do is use a built-in

what I'm going to do is use a built-in JavaScript function that comes with any

JavaScript function that comes with any browser called alert it's not the best

browser called alert it's not the best or prettiest user interface but for now

or prettiest user interface but for now it's going to get the job done what do I

it's going to get the job done what do I want to say to the user well let's first

want to say to the user well let's first just say something simple like hello

just say something simple like hello comma World close quote semicolon

comma World close quote semicolon thereby alerting the user with precisely

thereby alerting the user with precisely that message now what I'm going to do

that message now what I'm going to do down here is make one other change I

down here is make one other change I don't want this form to actually get

don't want this form to actually get submitted to the server just like we've

submitted to the server just like we've seen in the past when you submit a form

seen in the past when you submit a form it often goes to something like

it often goes to something like google.com/ search I actually want my

google.com/ search I actually want my JavaScript code to take control over the

JavaScript code to take control over the entire user experience of this form so

entire user experience of this form so that I'm just using the form as a user

that I'm just using the form as a user input mechanism not to actually send via

input mechanism not to actually send via get or post this data to some other

get or post this data to some other server including my own so this is going

server including my own so this is going to look a little ugly but after calling

to look a little ugly but after calling greet I'm actually going to do this

greet I'm actually going to do this which I've read the documentation about

which I've read the documentation about and I know that if you actually hardcode

and I know that if you actually hardcode return false here that just tells the

return false here that just tells the browser please don't actually submit the

browser please don't actually submit the form only call the Greet function all

form only call the Greet function all right well let me go back to my browser

right well let me go back to my browser here let me reload this because I need

here let me reload this because I need to download the latest version of the

to download the latest version of the JavaScript code and I'm just going to go

JavaScript code and I'm just going to go ahead without even typing my name I'm

ahead without even typing my name I'm going to click on the Greet button and

going to click on the Greet button and you'll see that Al be it a little

you'll see that Al be it a little cryptically at the top we see an alert

cryptically at the top we see an alert that says hello world there's my ugly

that says hello world there's my ugly URL of my code space there at the moment

URL of my code space there at the moment but we do indeed see that string but

but we do indeed see that string but what I haven't of course done is taken

what I haven't of course done is taken any actual name from the user so how can

any actual name from the user so how can we go about doing that well ideally I

we go about doing that well ideally I want to alert the user with hello comma

want to alert the user with hello comma David or hello comma Carter whatever a

David or hello comma Carter whatever a name I type into that box so how can I

name I type into that box so how can I go about doing that well let me create a

go about doing that well let me create a variable called name and let me set it

variable called name and let me set it equal to this function call

equal to this function call document. query

document. query selector that comes with JavaScript

selector that comes with JavaScript itself let me then in parenthesis pass

itself let me then in parenthesis pass in an argument that is going to be huh

in an argument that is going to be huh the ID the unique I need a unique

the ID the unique I need a unique identifier for the thing I want to

identifier for the thing I want to select so let me actually go back to my

select so let me actually go back to my HTML code here and instead of giving

HTML code here and instead of giving this form field a name like Q for query

this form field a name like Q for query let me actually use another HTML

let me actually use another HTML attribute called ID where now I can call

attribute called ID where now I can call this anything I want and for clarity I'm

this anything I want and for clarity I'm just going to call this input element

just going to call this input element uniquely name now up here in query

uniquely name now up here in query selector just like in CSS where you can

selector just like in CSS where you can use hashes and dots and other symbology

use hashes and dots and other symbology in order to select certain nodes in your

in order to select certain nodes in your Dom that is rectangles in that memory

Dom that is rectangles in that memory tree well I can go ahead and select hash

tree well I can go ahead and select hash name which again is just the Syntax for

name which again is just the Syntax for uniquely selecting the element whose ID

uniquely selecting the element whose ID is in this case name so you have the

is in this case name so you have the hash up here you don't need the hash as

hash up here you don't need the hash as the value of the attribute down here on

the value of the attribute down here on line 20 and now if I want to actually

line 20 and now if I want to actually get the value of that text box I

get the value of that text box I literally just say do value so document

literally just say do value so document refers to the whole web page itself

refers to the whole web page itself query selector is a function that's

query selector is a function that's built into that object so to speak and

built into that object so to speak and the value accessible via value just like

the value accessible via value just like a c stru or even a python class allows

a c stru or even a python class allows me to go inside of that text field and

me to go inside of that text field and get whatever the value the user has

get whatever the value the user has typed in now as I've uh been able to do

typed in now as I've uh been able to do in languages like python pretty Le

in languages like python pretty Le fairly readily I can concatenate this

fairly readily I can concatenate this name onto the string hello comma space

name onto the string hello comma space so as to form a complete phrase and

so as to form a complete phrase and you'll notice here that I'm actually

you'll notice here that I'm actually using single quotes in my JavaScript

using single quotes in my JavaScript double quotes in my HTML this is perhaps

double quotes in my HTML this is perhaps a common convention in JavaScript the

a common convention in JavaScript the language does not care if you use double

language does not care if you use double quotes or single quotes but I dare say

quotes or single quotes but I dare say single quotes are just more common and

single quotes are just more common and so that's what I've done here all right

so that's what I've done here all right now as always I'm going to cross my

now as always I'm going to cross my fingers go back to this page I'm going

fingers go back to this page I'm going to reload because I've changed the

to reload because I've changed the JavaScript and I need my browser to

JavaScript and I need my browser to download it and now I'm going to type in

download it and now I'm going to type in my name for instance David click greets

my name for instance David click greets with fingers crossed and voila now I see

with fingers crossed and voila now I see hello comma David all right so it turns

hello comma David all right so it turns out that while functional this isn't the

out that while functional this isn't the best design and co-mingling your uh HTML

best design and co-mingling your uh HTML with your JavaScript script code AS with

with your JavaScript script code AS with this onsubmit attribute isn't

this onsubmit attribute isn't particularly clean it's better as with

particularly clean it's better as with CSS to keep your HTML over here your CSS

CSS to keep your HTML over here your CSS over here and your JavaScript now over

over here and your JavaScript now over here so to speak and better still

here so to speak and better still perhaps even in some separate files so

perhaps even in some separate files so how can I go about changing this a

how can I go about changing this a little bit well let me go ahead and

little bit well let me go ahead and actually let's go ahead and delete all

actually let's go ahead and delete all of this code for just a moment and let

of this code for just a moment and let me go and get rid of this on submit

me go and get rid of this on submit Handler down here and really just

Handler down here and really just distill my HTML only into the HTML and

distill my HTML only into the HTML and the attributes therefore and what I'm

the attributes therefore and what I'm instead going to do now is do this I can

instead going to do now is do this I can use JavaScript to achieve the listening

use JavaScript to achieve the listening for that submit event or that onsubmit

for that submit event or that onsubmit event I don't need to actually use HTML

event I don't need to actually use HTML for that I can use JavaScript entirely

for that I can use JavaScript entirely so it turns out I can access some other

so it turns out I can access some other uh member of this document by doing

uh member of this document by doing document. query selector again but this

document. query selector again but this time let's select the actual form tag

time let's select the actual form tag and it doesn't have an ID because it has

and it doesn't have an ID because it has no ID in its HTML but it does have a tag

no ID in its HTML but it does have a tag name so just like in CSS when you can

name so just like in CSS when you can Target Elements by way of their name I'm

Target Elements by way of their name I'm just going to select the one and only

just going to select the one and only form on this page by using that same

form on this page by using that same query selector function and now I'm

query selector function and now I'm going to use another function that just

going to use another function that just comes with JavaScript in the context of

comes with JavaScript in the context of browsers whereby once you select an

browsers whereby once you select an element like that form I can call add

element like that form I can call add event listener which is similar in

event listener which is similar in spirit to scratches when green flag

spirit to scratches when green flag clicked or any block like that you can

clicked or any block like that you can then tell the browser what event you

then tell the browser what event you want to listen for I want to listen for

want to listen for I want to listen for the submit event so you don't say

the submit event so you don't say onsubmit here now that we're in pure

onsubmit here now that we're in pure JavaScript you just say submit and now I

JavaScript you just say submit and now I can do something like this I can go

can do something like this I can go ahead and say call the following

ahead and say call the following function and I'm not even going to

function and I'm not even going to bother giving this function a name and

bother giving this function a name and that is allowed to in JavaScript as we

that is allowed to in JavaScript as we saw briefly in Python and what I'm going

saw briefly in Python and what I'm going to do now inside of curly braces after

to do now inside of curly braces after that keyword function is the same kind

that keyword function is the same kind of code as before I'm going to do let

of code as before I'm going to do let name equals document. query selector I'm

name equals document. query selector I'm going to select that same IDE uh same

going to select that same IDE uh same Name ID as before and get its value and

Name ID as before and get its value and then I'm going to do alert and then pass

then I'm going to do alert and then pass in hello comma a single quote again

in hello comma a single quote again after that concatenate with that the

after that concatenate with that the name and then semicolon but I need to do

name and then semicolon but I need to do one other thing it turns out that this

one other thing it turns out that this function and if you read the

function and if you read the documentation for this technique

documentation for this technique actually takes automatically a special

actually takes automatically a special argument called by convention event and

argument called by convention event and this is just an a variable if you will

this is just an a variable if you will that refers to whatever event just

that refers to whatever event just happened in this case it's of course

happened in this case it's of course going to be submit but in other contexts

going to be submit but in other contexts it might be a click event a mouse down

it might be a click event a mouse down event or something else entirely so this

event or something else entirely so this allows me now to do this which is a

allows me now to do this which is a little cryptic but you get used to these

little cryptic but you get used to these conventions I can use that event

conventions I can use that event whatever it is and then prevent whatever

whatever it is and then prevent whatever the default behavior is by calling a

the default behavior is by calling a special function called prevent default

special function called prevent default with a capital D this is the alternative

with a capital D this is the alternative to that Messier return fall semicolon

to that Messier return fall semicolon that I had inside of my HTML before so

that I had inside of my HTML before so all I've done here now is I've left all

all I've done here now is I've left all of my HTML as pure HTML down here and

of my HTML as pure HTML down here and I've put all of my JavaScript code as

I've put all of my JavaScript code as pure JavaScript up here this sort of

pure JavaScript up here this sort of separation of concerns similar to what

separation of concerns similar to what we started doing with CSS just a bit ago

we started doing with CSS just a bit ago in order to keep those two languages

in order to keep those two languages separate too well let me go back to my

separate too well let me go back to my browser here reload the page and

browser here reload the page and unfortunately there's a subtle mistake

unfortunately there's a subtle mistake I've made here let me go ahead and type

I've made here let me go ahead and type in David and click greet and

in David and click greet and unfortunately nothing actually seems to

unfortunately nothing actually seems to happen well maybe it's just my name

happen well maybe it's just my name Carter greet and nothing seems to happen

Carter greet and nothing seems to happen that alert does not come up well why is

that alert does not come up well why is that well let me go back to vs code here

that well let me go back to vs code here and point out that order of operations

and point out that order of operations in zscript matters similar in spirit to

in zscript matters similar in spirit to C because on line seven I'm selecting

C because on line seven I'm selecting the form and trying to add an event

the form and trying to add an event listener for submitting that form

listener for submitting that form unfortunately the form had better exist

unfortunately the form had better exist at that moment in time but it doesn't

at that moment in time but it doesn't because just like in C and in some cases

because just like in C and in some cases python where the compiler or The

python where the compiler or The Interpreter reads the code top to bottom

Interpreter reads the code top to bottom notice that the form doesn't actually

notice that the form doesn't actually exist and therefore get loaded into the

exist and therefore get loaded into the computer's memory until line 19 so we've

computer's memory until line 19 so we've got to kind of reorder these somehow now

got to kind of reorder these somehow now maybe the simplest way to do this would

maybe the simplest way to do this would just be to perhaps do something like

just be to perhaps do something like this let me scroll back up to my script

this let me scroll back up to my script tag and perhaps a little more explicitly

tag and perhaps a little more explicitly move it into the order in which I want

move it into the order in which I want it to be executed so I'm going to go

it to be executed so I'm going to go below my form and inside of my body

below my form and inside of my body which is actually okay for JavaScript

which is actually okay for JavaScript here and just use that same code and

here and just use that same code and assuming I didn't make any typos let's

assuming I didn't make any typos let's go back to the browser click reload

go back to the browser click reload again to get the latest typee in my name

again to get the latest typee in my name again using that purely JavaScript

again using that purely JavaScript solution and the only change I made was

solution and the only change I made was I move the code from up here to down

I move the code from up here to down here clicking greet now and wow it's now

here clicking greet now and wow it's now back we get the alert with hello comma

back we get the alert with hello comma David so those kinds of things those

David so those kinds of things those kinds of principles matter at least when

kinds of principles matter at least when we're back in this world but there's

we're back in this world but there's other Solutions too and just so that

other Solutions too and just so that you've seen it because it's a common

you've seen it because it's a common convention in libraries as well let me

convention in libraries as well let me undo that change and put that script tag

undo that change and put that script tag back in the head or really anywhere else

back in the head or really anywhere else in the page where it might be

in the page where it might be and let me propose that there's one

and let me propose that there's one other way to solve this problem to

other way to solve this problem to postpone that code on line 7 through 11

postpone that code on line 7 through 11 getting executed until really the whole

getting executed until really the whole Dom the tree is ready to go and the

Dom the tree is ready to go and the Syntax for this might be as follows I

Syntax for this might be as follows I can do document and I can add to the

can do document and I can add to the document a an event listener that's

document a an event listener that's going to listen for something a little

going to listen for something a little special and I always have to look this

special and I always have to look this up myself to remember the spelling and

up myself to remember the spelling and the capitalization but it turns out that

the capitalization but it turns out that the browser itself once it's done

the browser itself once it's done loading all of your HTML top to bottom

loading all of your HTML top to bottom left right it will raise an event called

left right it will raise an event called Dom content loaded capitalized exactly

Dom content loaded capitalized exactly as such and if you want to call some

as such and if you want to call some function and I don't even need an event

function and I don't even need an event argument in this case you can open curly

argument in this case you can open curly braces just as before and put inside of

braces just as before and put inside of those curly braces the code that you

those curly braces the code that you want to execute only once the dumbs

want to execute only once the dumbs content has been loaded top to bottom

content has been loaded top to bottom and now let me just finish my thought

and now let me just finish my thought with a closed curly brace Clos

with a closed curly brace Clos parenthesis and semicolon it gets a

parenthesis and semicolon it gets a little Annoying to visually line all of

little Annoying to visually line all of this up but I think I'm still good and

this up but I think I'm still good and now even though this code is at the top

now even though this code is at the top of my file or really above the form tag

of my file or really above the form tag itself I think we're okay so let's go

itself I think we're okay so let's go back to the browser here reload the page

back to the browser here reload the page type in David and click greet and we

type in David and click greet and we still get the same correct behavior and

still get the same correct behavior and so this is just a very common Paradigm

so this is just a very common Paradigm to use these kinds of events to listen

to use these kinds of events to listen and listen and listen for something to

and listen and listen for something to happen and then only do something once

happen and then only do something once that thing has transpired all right well

that thing has transpired all right well let's take one more step with JavaScript

let's take one more step with JavaScript code before we take a look at what's

code before we take a look at what's really fun about this language and what

really fun about this language and what you can do with browsers in particular

you can do with browsers in particular by just cleaning things up a little bit

by just cleaning things up a little bit further I'm going to go back into the

further I'm going to go back into the code here and I'm actually going to

code here and I'm actually going to remove uh or cut all of this code out of

remove uh or cut all of this code out of the hello.html file itself and I'm going

the hello.html file itself and I'm going to change my script tag to have nothing

to change my script tag to have nothing in between the open and close tag but I

in between the open and close tag but I am going to give it a source attribute

am going to give it a source attribute and let's go ahead and call this for

and let's go ahead and call this for instance hello.js sojs would be the

instance hello.js sojs would be the convention for the file extension for a

convention for the file extension for a Javascript file and even though this is

Javascript file and even though this is a little weird that we have the script

a little weird that we have the script tag and a source attribute then nothing

tag and a source attribute then nothing in between the open and close tag this

in between the open and close tag this is indeed the convention when you want

is indeed the convention when you want to put all of your code in a separate

to put all of your code in a separate file and let me go ahead and do that let

file and let me go ahead and do that let me go ahead and open my terminal window

me go ahead and open my terminal window create a new file called

create a new file called hello.js and then in that file I'm just

hello.js and then in that file I'm just going to paste the very code that I just

going to paste the very code that I just cut from the previous file so no changes

cut from the previous file so no changes to the code all I'm doing is factoring

to the code all I'm doing is factoring it out and now I'm doing something just

it out and now I'm doing something just like our CSS factorization before which

like our CSS factorization before which confusingly use the link tag this uses

confusingly use the link tag this uses the script tag this just now allows me

the script tag this just now allows me to collaborate with someone like Carter

to collaborate with someone like Carter or someone else so that they can do the

or someone else so that they can do the JavaScript code I can do the HTML maybe

JavaScript code I can do the HTML maybe a third person can do the CSS and indeed

a third person can do the CSS and indeed maybe we can build even grander things

maybe we can build even grander things by uh designing things in this way all

by uh designing things in this way all right well let me go back to my browser

right well let me go back to my browser again reload the page I shouldn't see

again reload the page I shouldn't see any visual changes but if I type in my

any visual changes but if I type in my name again David and click greet this

name again David and click greet this still now works and what my browser has

still now works and what my browser has just done underneath the hood is not

just done underneath the hood is not only download the uh hello.html file as

only download the uh hello.html file as always because there's now this script

always because there's now this script tag that's referencing the source of

tag that's referencing the source of another file just like an image tag

another file just like an image tag might reference the source of an image

might reference the source of an image the browser is automatically helping me

the browser is automatically helping me out by loading that into its memory as

out by loading that into its memory as well and now how about one final example

well and now how about one final example and for this one I'm going to go ahead

and for this one I'm going to go ahead and not write it live but open it up as

and not write it live but open it up as prepared in advance just to show you

prepared in advance just to show you what you can do by listening for some of

what you can do by listening for some of these other events as well like the key

these other events as well like the key up the finger going down the finger

up the finger going down the finger going up and listening for exactly that

going up and listening for exactly that so as the user is typing something you

so as the user is typing something you can do something interesting as well I'm

can do something interesting as well I'm going to go back into my directory

going to go back into my directory listing here and I click on this Source

listing here and I click on this Source a directory which has all of the

a directory which has all of the examples that I wrote here in advance

examples that I wrote here in advance and I'm going to scroll down to one

and I'm going to scroll down to one called hello 5. HTML and in hello 5 now

called hello 5. HTML and in hello 5 now we've gotten rid of the button and we

we've gotten rid of the button and we just have this text box but notice now

just have this text box but notice now what happens if I start typing my name

what happens if I start typing my name as d a v i d D I'm not typing enter at

as d a v i d D I'm not typing enter at all and in fact if I start deleting and

all and in fact if I start deleting and I change my mind and start typing

I change my mind and start typing Carter's name notice now that the web

Carter's name notice now that the web page the Dom inside of the computer's

page the Dom inside of the computer's memory is now automatically updating

memory is now automatically updating itself so it's not even listening it

itself so it's not even listening it would seem for a submit event anymore

would seem for a submit event anymore but maybe for a key up event instead so

but maybe for a key up event instead so let me go back to VSS code here open my

let me go back to VSS code here open my terminal window and in my source a

terminal window and in my source a directory let me open up hello 5. HTML

directory let me open up hello 5. HTML inside of the script tag you'll see some

inside of the script tag you'll see some code that's similar in spirit to before

code that's similar in spirit to before whereby I'm adding an event listener to

whereby I'm adding an event listener to the document waiting for the whole Dom's

the document waiting for the whole Dom's content to be loaded but then inside of

content to be loaded but then inside of that function I'm now doing this I'm

that function I'm now doing this I'm creating a variable called input and

creating a variable called input and selecting from the document the one and

selecting from the document the one and only input tag that we saw just a moment

only input tag that we saw just a moment ago I'm then adding on line 11 in event

ago I'm then adding on line 11 in event listener for key up which is exactly

listener for key up which is exactly that gesture so that I can execute some

that gesture so that I can execute some additional code anytime the human lists

additional code anytime the human lists their finger from the keyboard after

their finger from the keyboard after typing a key what do I then do well I'm

typing a key what do I then do well I'm going to go ahead it seems and declare

going to go ahead it seems and declare another variable called name and I'm

another variable called name and I'm just going to select some P tag on the

just going to select some P tag on the page and now we didn't really see a P

page and now we didn't really see a P tag so I think it's time to look at the

tag so I think it's time to look at the HTML if I scroll down to the bottom of

HTML if I scroll down to the bottom of the page where my actual HTML is you'll

the page where my actual HTML is you'll see that there's just a form tag and no

see that there's just a form tag and no onsubmit Handler anymore as before

onsubmit Handler anymore as before there's just an input tag and no button

there's just an input tag and no button at all but there is on line 29 here an

at all but there is on line 29 here an open and close P tag just so I have an

open and close P tag just so I have an empty placeholder in which to put

empty placeholder in which to put something like hello David or hello

something like hello David or hello Carter so that's why now on line 12 I

Carter so that's why now on line 12 I can define a variable called name and I

can define a variable called name and I can select that P tag so that what do I

can select that P tag so that what do I want to do well if inside of the input

want to do well if inside of the input there's a value so this is essentially

there's a value so this is essentially checking for null or the absence of a

checking for null or the absence of a string so if input. value does not equal

string so if input. value does not equal nothing that is there is a value in that

nothing that is there is a value in that text box well this syntax here on line

text box well this syntax here on line 14 is a very clever way of doing the

14 is a very clever way of doing the following go into that name tag that is

following go into that name tag that is to say the empty par paragraph change

to say the empty par paragraph change the inner HTML of it the HTML inside of

the inner HTML of it the HTML inside of it to be literally this hello comma and

it to be literally this hello comma and then just so you've seen an additional

then just so you've seen an additional piece of syntax this is similar to

piece of syntax this is similar to Python's F strings the syntax is a

Python's F strings the syntax is a little weird in JavaScript you actually

little weird in JavaScript you actually use not single quotes not double quotes

use not single quotes not double quotes but back ticks which on a US English

but back ticks which on a US English keyboard are typically the top leftand

keyboard are typically the top leftand key of your keyboard or thereabouts and

key of your keyboard or thereabouts and using dollar sign curly braces just like

using dollar sign curly braces just like the curly braces alone in Python allows

the curly braces alone in Python allows us to plug in whatever the value is of

us to plug in whatever the value is of that input however if there's no value

that input however if there's no value there it looks like I'm just going to

there it looks like I'm just going to say hello whoever you are and in fact we

say hello whoever you are and in fact we can see that behavior now is I delete

can see that behavior now is I delete delete delete delete delete delete and

delete delete delete delete delete and nothing's there now that if condition is

nothing's there now that if condition is no longer true and so we see this

no longer true and so we see this default value instead so this is only to

default value instead so this is only to say that by harnessing these various

say that by harnessing these various events that are constantly happening on

events that are constantly happening on most any web page we can now register

most any web page we can now register code just like we did way back in

code just like we did way back in scratch to actually listen for those

scratch to actually listen for those events and do something with them now it

events and do something with them now it turns out we can do some interesting

turns out we can do some interesting things even using third party coat and

things even using third party coat and just as we used bootstrap a bit ago to

just as we used bootstrap a bit ago to make our table prettier allow me to

make our table prettier allow me to propose that we also take a look at this

propose that we also take a look at this version of favorites as well let me go

version of favorites as well let me go back into my source a directory and open

back into my source a directory and open up favorites 2 which I made in advance

up favorites 2 which I made in advance which looks almost the same though I've

which looks almost the same though I've zoomed in here a bit but you'll notice

zoomed in here a bit but you'll notice somewhat subtly over the leftmost column

somewhat subtly over the leftmost column in this table you'll see now this arrow

in this table you'll see now this arrow in Gray pointing up and pointing down

in Gray pointing up and pointing down previously those were not there all I

previously those were not there all I had was a static HTML table with all of

had was a static HTML table with all of this data sorted in whatever order it

this data sorted in whatever order it was inputed the other day in that form

was inputed the other day in that form but now notice what I can do if I want

but now notice what I can do if I want to sort in uh one order I can click this

to sort in uh one order I can click this Arrow or the other order I can sort in

Arrow or the other order I can sort in this Arrow so essentially doing it

this Arrow so essentially doing it chronologically forward or backward now

chronologically forward or backward now how is that sorting happening it's

how is that sorting happening it's presumably based on all of the

presumably based on all of the timestamps that were registered when we

timestamps that were registered when we submitted that Google form just a bit

submitted that Google form just a bit ago um uh when it was live but now using

ago um uh when it was live but now using JavaScript it turns out that we can use

JavaScript it turns out that we can use some logic somehow and sort this data by

some logic somehow and sort this data by the same and you don't get that

the same and you don't get that automatically just by using HTML alone

automatically just by using HTML alone now how did I achieve that well it turns

now how did I achieve that well it turns out if I go ahead and close these hello

out if I go ahead and close these hello files and in vs code let's open up uh

files and in vs code let's open up uh favorites 2. HTML you'll see that all of

favorites 2. HTML you'll see that all of the HTML is actually the same if I

the HTML is actually the same if I scroll down and down through this file

scroll down and down through this file but I added a little something

but I added a little something interesting at top I copied and pasted

interesting at top I copied and pasted the appropriate URLs and HTML tags from

the appropriate URLs and HTML tags from bootstraps documentation and you'll see

bootstraps documentation and you'll see here that I have a file called not only

here that I have a file called not only bootstrap.css but also

bootstrap.css but also bootstrap.min.js as well as a couple of

bootstrap.min.js as well as a couple of other things as well that allows me

other things as well that allows me ultimately using third-party libraries

ultimately using third-party libraries to add some special HTML attributes that

to add some special HTML attributes that those libraries told me to add and then

those libraries told me to add and then as soon as those libraries detect the

as soon as those libraries detect the presence of these attributes now on my

presence of these attributes now on my own raw data they do their thing and

own raw data they do their thing and javascrip defy not a technical term the

javascrip defy not a technical term the entire table and now make it interactive

entire table and now make it interactive and not static so you'll see here that

and not static so you'll see here that the Aesthetics of the table are as

the Aesthetics of the table are as before table and table striped but I'm

before table and table striped but I'm adding now another HTML attribute called

adding now another HTML attribute called Data toggle whose value is table and I

Data toggle whose value is table and I know that only from the documentation of

know that only from the documentation of these libraries indicating that's that's

these libraries indicating that's that's how I can now enable this table to be

how I can now enable this table to be interactive as I can too by adding data

interactive as I can too by adding data sortable equals quote unquote true on

sortable equals quote unquote true on specifically the timestamp column and

specifically the timestamp column and the only thing unfamiliar here perhaps

the only thing unfamiliar here perhaps is I'm using t P for table heading as

is I'm using t P for table heading as opposed to TD as I do elsewhere but

opposed to TD as I do elsewhere but that's all that it takes to now focus on

that's all that it takes to now focus on the raw data you want to present and let

the raw data you want to present and let someone else do the heavy lifting of

someone else do the heavy lifting of actually implementing the logic well

actually implementing the logic well let's end with just a look at what more

let's end with just a look at what more you can do with Java script and just how

you can do with Java script and just how powerful it is when you combine a

powerful it is when you combine a language like this with the data and the

language like this with the data and the uh user interface you want to convey

uh user interface you want to convey let's go ahead and open up within Source

let's go ahead and open up within Source a directory something called background.

a directory something called background. HTML now this interface here is quite

HTML now this interface here is quite simple it just has three buttons RG and

simple it just has three buttons RG and B and the background of course noticed

B and the background of course noticed by default is just white but when I

by default is just white but when I click on the r the background turns red

click on the r the background turns red when the I click on the G it turns green

when the I click on the G it turns green and the blue it turns blue and again and

and the blue it turns blue and again and again so how is this working well if you

again so how is this working well if you think back again to the available events

think back again to the available events perhaps I'm just listening for a click

perhaps I'm just listening for a click on those buttons and then doing

on those buttons and then doing something with maybe the CSS of the page

something with maybe the CSS of the page to allow me to see those different

to allow me to see those different colors so in fact let's go back to vs

colors so in fact let's go back to vs code here and let's open up background.

code here and let's open up background. HTML and in here you'll see some simple

HTML and in here you'll see some simple HTML at the top just three buttons but

HTML at the top just three buttons but I've given each a unique ID so that I

I've given each a unique ID so that I can reference it in code and then inside

can reference it in code and then inside of a script tag here below because I

of a script tag here below because I didn't bother with the uh Dom content

didn't bother with the uh Dom content loaded event here notice that I'm doing

loaded event here notice that I'm doing the following I'm creating a variable

the following I'm creating a variable called body that lets me select the body

called body that lets me select the body tag I then have in these three line some

tag I then have in these three line some code that handles red what am I doing

code that handles red what am I doing well I'm selecting from the document

well I'm selecting from the document whatever h HTML tag has unique ID of red

whatever h HTML tag has unique ID of red and then I'm adding an event listener

and then I'm adding an event listener for any click on that button and anytime

for any click on that button and anytime someone clicks on that red button I call

someone clicks on that red button I call this function anonymously it doesn't

this function anonymously it doesn't even have or need a name and this syntax

even have or need a name and this syntax here is powerful because now in

here is powerful because now in JavaScript I can alter the CSS of my

JavaScript I can alter the CSS of my page by doing body which is the tag that

page by doing body which is the tag that I selected two lines ago accessing its

I selected two lines ago accessing its style accessing its background color

style accessing its background color property and setting it equal to quote

property and setting it equal to quote unquote red and I do the same down below

unquote red and I do the same down below for green I do the same down below for

for green I do the same down below for blue and the only thing worth noting

blue and the only thing worth noting here is that in CSS it turns out it's

here is that in CSS it turns out it's the case that the CSS property for the

the case that the CSS property for the background color of a page is actually

background color of a page is actually background Das color in all lower case

background Das color in all lower case with a hyphen in between unfortunately

with a hyphen in between unfortunately in the world of JavaScript a hyphen

in the world of JavaScript a hyphen would be mistaken for subtraction like

would be mistaken for subtraction like background minus color which would be

background minus color which would be wrong so the convention in JavaScript is

wrong so the convention in JavaScript is when you're trying to manipulate CSS you

when you're trying to manipulate CSS you take whatever the property name is uh

take whatever the property name is uh font size background color and you

font size background color and you change it into so-called camel case here

change it into so-called camel case here you get rid of The Hyphen and you

you get rid of The Hyphen and you capitalize the subsequent words like

capitalize the subsequent words like color in this case here all right how

color in this case here all right how about another well it turns out back in

about another well it turns out back in the day back in my day there was a HTML

the day back in my day there was a HTML tag that would actually allow you to do

tag that would actually allow you to do this create blinking text on a screen

this create blinking text on a screen it's rather unpleasant at this rate

it's rather unpleasant at this rate certainly but how might this work well

certainly but how might this work well it turns out in JavaScript if we take a

it turns out in JavaScript if we take a look at the blink. HTML file here we'll

look at the blink. HTML file here we'll see that you can in your HTML do

see that you can in your HTML do something literally as simple as hello

something literally as simple as hello world in the body but then you can call

world in the body but then you can call this function here turns out just like

this function here turns out just like document there's another Global special

document there's another Global special variable you can use in JavaScript and

variable you can use in JavaScript and browsers called window which refers to

browsers called window which refers to all things related to the window itself

all things related to the window itself the window comes with a set interval

the window comes with a set interval function which lets you do exactly that

function which lets you do exactly that set an interval in milliseconds and

set an interval in milliseconds and after every expiration of that interval

after every expiration of that interval some function will be called for you so

some function will be called for you so in this case it's saying every 50

in this case it's saying every 50 milliseconds but let's actually slow

milliseconds but let's actually slow that down now to 500 milliseconds or for

that down now to 500 milliseconds or for one half a second call a function called

one half a second call a function called blink notice I do not have parentheses

blink notice I do not have parentheses after the blink name because I don't

after the blink name because I don't want to call blink now I want to tell

want to call blink now I want to tell JavaScript to call the function called

JavaScript to call the function called blink every 500 milliseconds now we'll

blink every 500 milliseconds now we'll see in a moment what that code looks

see in a moment what that code looks like but let's go back to the page and

like but let's go back to the page and reload and you'll see now that it's a

reload and you'll see now that it's a more pleasant blinking if that's even

more pleasant blinking if that's even the case every half second because I'm

the case every half second because I'm now firing that event that is I'm uh

now firing that event that is I'm uh calling that function now every 500

calling that function now every 500 milliseconds instead how am I doing that

milliseconds instead how am I doing that well this same script tag I've invented

well this same script tag I've invented my own blink function this is has the

my own blink function this is has the distinction back in the day of actually

distinction back in the day of actually being an HTML tag and among the few tags

being an HTML tag and among the few tags that the world removed and got rid of so

that the world removed and got rid of so that it's no longer used because it's

that it's no longer used because it's not all that user friendly but down here

not all that user friendly but down here what am I doing I'm getting the body of

what am I doing I'm getting the body of the document itself with this variable

the document itself with this variable and then I'm checking two CSS properties

and then I'm checking two CSS properties that we didn't talk about before but it

that we didn't talk about before but it turns out that you can check and set the

turns out that you can check and set the visibility of an element in JavaScript

visibility of an element in JavaScript by going into that tag checking its

by going into that tag checking its style and getting its visibility uh

style and getting its visibility uh property and if it happens to equal

property and if it happens to equal hidden the next line of code here 13

hidden the next line of code here 13 sets it equal to visible instead else if

sets it equal to visible instead else if it's not hidden it must already be

it's not hidden it must already be visible and so line 17 flips it the

visible and so line 17 flips it the other way and changes it to Hidden here

other way and changes it to Hidden here left hand right hand clearly not talking

left hand right hand clearly not talking no idea why the opposite of visible Is

no idea why the opposite of visible Is Not Invisible it's indeed visible and

Not Invisible it's indeed visible and hidden but this just allows you every

hidden but this just allows you every time this function is called to change

time this function is called to change the property from one value to another

the property from one value to another achieving that blinking effect you can

achieving that blinking effect you can do even more powerful things that you

do even more powerful things that you and I take for granted every day let's

and I take for granted every day let's go into Source 8 and go to autocom

go into Source 8 and go to autocom complete. HTML which I wrote in advance

complete. HTML which I wrote in advance and this is a page that also loads into

and this is a page that also loads into memory a really big dictionary that you

memory a really big dictionary that you might recall from problem set five and

might recall from problem set five and if I go ahead and type in something like

if I go ahead and type in something like c a t you'll notice dynamically an

c a t you'll notice dynamically an unordered bulleted list appearing below

unordered bulleted list appearing below the text box that shows you all of the

the text box that shows you all of the words in that dictionary from pet 5 that

words in that dictionary from pet 5 that start with c and then CA and then T just

start with c and then CA and then T just like the autocomplete you see every day

like the autocomplete you see every day on your phone in Google or websites like

on your phone in Google or websites like it how is that working well probably I'm

it how is that working well probably I'm listening for the key press going up as

listening for the key press going up as soon as that key is pressed I'm probably

soon as that key is pressed I'm probably searching through a big array uh really

searching through a big array uh really of all of those words maybe using linear

of all of those words maybe using linear search maybe binary search if uh faster

search maybe binary search if uh faster than that and then looking for any

than that and then looking for any string in that array that starts with C

string in that array that starts with C or CA or c a t and then I'm generating

or CA or c a t and then I'm generating automattic IC Ally the HTML therefore

automattic IC Ally the HTML therefore but perhaps most familiar nowadays is

but perhaps most familiar nowadays is just how much your phone and your laptop

just how much your phone and your laptop know about you and let me go into a

know about you and let me go into a final example here in Source 8 called

final example here in Source 8 called geolocation HTML which is a term of art

geolocation HTML which is a term of art that just refers to figuring out your

that just refers to figuring out your geography for instance your GPS

geography for instance your GPS coordinates now here we have a third and

coordinates now here we have a third and final Global variable that your browser

final Global variable that your browser provides you with called Navigator and

provides you with called Navigator and in Navigator there's a special object

in Navigator there's a special object called geolocation that comes with a

called geolocation that comes with a function called get current position

function called get current position so if you call Navigator geolocation dog

so if you call Navigator geolocation dog getcurrent position and then pass in a

getcurrent position and then pass in a function of your own that doesn't need a

function of your own that doesn't need a name but does take an argument in this

name but does take an argument in this case I called it position as soon as the

case I called it position as soon as the browser or as soon as the phone has

browser or as soon as the phone has figured out where in the world that user

figured out where in the world that user is with that browser at some latitude

is with that browser at some latitude and longitude the browser will

and longitude the browser will automatically call that function for you

automatically call that function for you and you can do anything you want with

and you can do anything you want with the position that comes back the

the position that comes back the latitude and longitude respectively so

latitude and longitude respectively so I'm going to use a function that's not

I'm going to use a function that's not often that help but for our purposes

often that help but for our purposes today it's just going to write to the

today it's just going to write to the document itself to my big rectangular

document itself to my big rectangular region whatever the latitude is then a

region whatever the latitude is then a comma and then the longitude as well so

comma and then the longitude as well so if we go back with this final flourish

if we go back with this final flourish into Source 8 open up geolocation HTML

into Source 8 open up geolocation HTML you'll see that my browser first wants

you'll see that my browser first wants my permission to let this website my own

my permission to let this website my own know my location I'm going to go ahead

know my location I'm going to go ahead and click allow crossing my fingers

and click allow crossing my fingers because it might take a moment for the

because it might take a moment for the phone or the laptop to figure it out and

phone or the laptop to figure it out and it looks like according to Google I am

it looks like according to Google I am right this moment with my Mac at

right this moment with my Mac at latitude longitude 42375 comma

latitude longitude 42375 comma 7111 let's go ahead and highlight and

7111 let's go ahead and highlight and copy that let's go to a website like

copy that let's go to a website like Googl maps.com crossing our fingers if

Googl maps.com crossing our fingers if you've never done this you can search

you've never done this you can search for GPS coordinates too let's hit enter

for GPS coordinates too let's hit enter and amazing we are indeed in Sanders

and amazing we are indeed in Sanders Theater roughly there standing on this

Theater roughly there standing on this stage on Halloween and that then is week

stage on Halloween and that then is week eight we will see you next time happy

eight we will see you next time happy Halloween

Halloween nothing

nothing go buffering

go buffering okay Josh

[Music] nice it's Moy no oh

Josh uh um Sophie [Music]

[Music] amazing that was

amazing that was [Music]

[Music] perfect I think I

perfect I think I hey to

hey to you

[Music] oh that was amazing thank you all so

oh that was amazing thank you all so good

all right this is cs50 and this is already week nine which is our second to

already week nine which is our second to last indeed this is really the last week

last indeed this is really the last week where you'll learn in this class how to

where you'll learn in this class how to program but indeed it's this week that's

program but indeed it's this week that's really meant to be the the pedagogical

really meant to be the the pedagogical climax of like all of these various

climax of like all of these various languages we've been looking at all of

languages we've been looking at all of these various techniques all of this

these various techniques all of this syntax so that at the end of cs50 in

syntax so that at the end of cs50 in just a few weeks you indeed feel that

just a few weeks you indeed feel that you didn't take a class on C and you

you didn't take a class on C and you didn't take a class on python but you

didn't take a class on python but you really more generally took a class on

really more generally took a class on programming because indeed we know

programming because indeed we know already about half of you uh will go on

already about half of you uh will go on to study computer science further but

to study computer science further but half of you will not and indeed all of

half of you will not and indeed all of your programming chops here on out

your programming chops here on out theoretically will have a foundation in

theoretically will have a foundation in what we been doing these past many weeks

what we been doing these past many weeks but here on out it's really going to be

but here on out it's really going to be up to you to learn some new fangled

up to you to learn some new fangled language when it comes out or to follow

language when it comes out or to follow some new trend when some language

some new trend when some language eclipses the ones we've been using as

eclipses the ones we've been using as more popular as more appropriate for

more popular as more appropriate for problems you want to solve and so today

problems you want to solve and so today really is about synthesizing so many of

really is about synthesizing so many of the past few weeks but doing it in the

the past few weeks but doing it in the context of web programming which For

context of web programming which For Better or For Worse is so very much

Better or For Worse is so very much invogue nowadays both on our laptops and

invogue nowadays both on our laptops and phones and indeed the languages we

phones and indeed the languages we looked at in recent weeks are used not

looked at in recent weeks are used not only to make websites but also full

only to make websites but also full fledged applications and app stores and

fledged applications and app stores and the like so this really will be the

the like so this really will be the culmination of those past several weeks

culmination of those past several weeks and indeed we'll even talk about some

and indeed we'll even talk about some familiar Concepts like shopping carts

familiar Concepts like shopping carts when you're on Amazon and these things

when you're on Amazon and these things called cookies when you're visiting

called cookies when you're visiting websites all of those topics too will

websites all of those topics too will come into play and you'll have an

come into play and you'll have an understanding of what all that means

understanding of what all that means from the ground up so how did we get

from the ground up so how did we get here well just last week we focused on

here well just last week we focused on HTML and CSS primarily which are not

HTML and CSS primarily which are not programming languages they're just about

programming languages they're just about Aesthetics structuring your data

Aesthetics structuring your data presenting your data and so forth um and

presenting your data and so forth um and we served the web pages we wrote using

we served the web pages we wrote using this program HTTP server this is just

this program HTTP server this is just one such program there's dozens hundreds

one such program there's dozens hundreds of different web servers that you can

of different web servers that you can use out there this is just a super

use out there this is just a super simple one we pre-installed in your

simple one we pre-installed in your codes space for you in vs code so that

codes space for you in vs code so that you can just serve up web pages at the

you can just serve up web pages at the end of last week two though we teased

end of last week two though we teased JavaScript a full-fledged programming

JavaScript a full-fledged programming language that you can use to manipulate

language that you can use to manipulate the users experience for the better to

the users experience for the better to make things more Dynamic and interactive

make things more Dynamic and interactive by actually running code in the user

by actually running code in the user users's browser on their Mac their PC

users's browser on their Mac their PC their phone as opposed to server side

their phone as opposed to server side which up until now is where all of our

which up until now is where all of our code in C and python has been written so

code in C and python has been written so you're writing code on a server you're

you're writing code on a server you're serving code from a server but now with

serving code from a server but now with HTML CSS and JavaScript it's getting

HTML CSS and JavaScript it's getting executed in a browser but today we're

executed in a browser but today we're going to give you one final feature of

going to give you one final feature of python or really languages like it that

python or really languages like it that you can also use code on the server to

you can also use code on the server to generate automatically dynamically the

generate automatically dynamically the HTML uh the JavaScript the CSS that you

HTML uh the JavaScript the CSS that you actually want the user to rece receive

actually want the user to rece receive you don't have to hardcode everything as

you don't have to hardcode everything as you have when making your own homepage

you have when making your own homepage well let's consider what what some of

well let's consider what what some of the building blocks were last week so

the building blocks were last week so here's a sample URL and over here slash

here's a sample URL and over here slash is sort of the default page on any web

is sort of the default page on any web server it might be index.html it might

server it might be index.html it might be something else that's just a

be something else that's just a convention but it refers to whatever the

convention but it refers to whatever the default actually is you can visit of

default actually is you can visit of course when any browser like a URL that

course when any browser like a URL that ends in file. HTML or something else.

ends in file. HTML or something else. HTML and that literally means your

HTML and that literally means your browser wants this file on this server

browser wants this file on this server or of course we saw that it can be a

or of course we saw that it can be a folder and inside of that folder is

folder and inside of that folder is presumably some default file name like

presumably some default file name like again index.html or you can be more

again index.html or you can be more explicit like folder file. HTML and

explicit like folder file. HTML and these more generally we just called

these more generally we just called paths and indeed a path is just a

paths and indeed a path is just a location on your Mac your PC or on a

location on your Mac your PC or on a server of some piece of information but

server of some piece of information but today we're just going to rename this

today we're just going to rename this only to use other common terminology but

only to use other common terminology but they're really just synonyms today we're

they're really just synonyms today we're going to refer to those same things as

going to refer to those same things as routes because now today we're going to

routes because now today we're going to ultimately replace HT PTP server which

ultimately replace HT PTP server which just serves up static content that you

just serves up static content that you all write with your own web server like

all write with your own web server like now you will be the ones controlling

now you will be the ones controlling what it is the server does in response

what it is the server does in response to the user in uh requests so that you

to the user in uh requests so that you can respond interactively and

can respond interactively and dynamically but we're still going to see

dynamically but we're still going to see techniques like this these were our

techniques like this these were our so-called HTTP parameters they're

so-called HTTP parameters they're everything that comes after a question

everything that comes after a question mark in a URL and it can be like key

mark in a URL and it can be like key equals value and an example was what

equals value and an example was what when we played with Google what was the

when we played with Google what was the key and what was the value

key and what was the value that I first tried any

recollection I was searching for cats and so the key I figured that was Q

and so the key I figured that was Q because that's what Larry and Sergey who

because that's what Larry and Sergey who created Google years ago decided the

created Google years ago decided the name would be of the HTML text box that

name would be of the HTML text box that you type your query into and if I type

you type your query into and if I type cat for cat the value of that would end

cat for cat the value of that would end up in the URL for Google as being

up in the URL for Google as being question mark cat equals value and I

question mark cat equals value and I mentioned that it's often the case that

mentioned that it's often the case that you want to send two different inputs to

you want to send two different inputs to a server and this is why I propose that

a server and this is why I propose that you just keep an eye out for Amper Sands

you just keep an eye out for Amper Sands and and Ampersand separate these key

and and Ampersand separate these key value pairs but again this is the same

value pairs but again this is the same darn Paradigm as before and we've seen

darn Paradigm as before and we've seen this so many times right key value pairs

this so many times right key value pairs in dictionaries in Python we've seen uh

in dictionaries in Python we've seen uh HTML attributes and their values we've

HTML attributes and their values we've seen CSS properties and their values

seen CSS properties and their values it's all the same thing associating

it's all the same thing associating something with something else even

something with something else even though every language every person seems

though every language every person seems to have their own uh vernacular for it

to have their own uh vernacular for it it really is just the same idea this

it really is just the same idea this associating of something with something

associating of something with something else we'll continue to see and here to

else we'll continue to see and here to be concrete were the

be concrete were the HTTP lines of text that were in those

HTTP lines of text that were in those virtual envelopes if you will if I were

virtual envelopes if you will if I were indeed selecting trying to search for

indeed selecting trying to search for something like cats on Google this

something like cats on Google this recall was the message that got sent to

recall was the message that got sent to the server by my browser in order to

the server by my browser in order to tell Google to please search for not

tell Google to please search for not dogs but in this case cats now what is

dogs but in this case cats now what is HTTP server been doing for us well it's

HTTP server been doing for us well it's just been serving up HTML files CSS

just been serving up HTML files CSS files maybe some js or JavaScript files

files maybe some js or JavaScript files but it has been ignoring any HTTP

but it has been ignoring any HTTP parameters like HTTP server does not

parameters like HTTP server does not take user input why well what's it going

take user input why well what's it going to do with it because you already wrote

to do with it because you already wrote the HTML you already wrote the CSS like

the HTML you already wrote the CSS like there's no decisions to be made until we

there's no decisions to be made until we introduce a proper programming language

introduce a proper programming language on the server and so we're going to move

on the server and so we're going to move away now from this simple HTTP server

away now from this simple HTTP server program and introduce you to your own

program and introduce you to your own server that's going to handle the

server that's going to handle the parsing that is the extraction of these

parsing that is the extraction of these key value pairs so that you and I don't

key value pairs so that you and I don't have to write python code all of a

have to write python code all of a sudden that like analyzes this stuff

sudden that like analyzes this stuff figures out what pages requested the key

figures out what pages requested the key value pairs all of that were still going

value pairs all of that were still going to get for free by just using the right

to get for free by just using the right framework and so today we revisit python

framework and so today we revisit python uh which we've now used in some form the

uh which we've now used in some form the past few weeks and indeed it's kind of

past few weeks and indeed it's kind of been the glue that allows us to stitch

been the glue that allows us to stitch together some of our own logic we saw it

together some of our own logic we saw it with SQL we're going to now see it with

with SQL we're going to now see it with HTML CSS and even JavaScript if we want

HTML CSS and even JavaScript if we want and we're also going to see another

and we're also going to see another language today not a programming

language today not a programming language called Ginga and this is going

language called Ginga and this is going to be a common Paradigm in the real

to be a common Paradigm in the real world whereby different languages

world whereby different languages different libraries different Frameworks

different libraries different Frameworks often like borrow from each other or

often like borrow from each other or they use uh technologies that someone

they use uh technologies that someone else wrote just so they don't have to

else wrote just so they don't have to reinvent that wheel so flask is just a

reinvent that wheel so flask is just a framework that is a third party Library

framework that is a third party Library it's pretty popular nowadays it's

it's pretty popular nowadays it's relatively simple which is why we use it

relatively simple which is why we use it in cs50 if you've programmed before cs50

in cs50 if you've programmed before cs50 Jango is another popular framework or

Jango is another popular framework or library in the python sta space but it's

library in the python sta space but it's a little more complicated so we focus on

a little more complicated so we focus on flask and Ginga we'll see is not a

flask and Ginga we'll see is not a programming language it's just going to

programming language it's just going to be some syntax thankfully familiar with

be some syntax thankfully familiar with curly braces that allow us to use

curly braces that allow us to use placeholders in our actual web pages so

placeholders in our actual web pages so again you'll wrap your minds all

again you'll wrap your minds all eventually around where the lines are

eventually around where the lines are among these various Technologies but

among these various Technologies but these are not the interesting ideas the

these are not the interesting ideas the interesting ideas are the ones we'll

interesting ideas are the ones we'll focus on in code but starting today

focus on in code but starting today instead of running HTTP server to serve

instead of running HTTP server to serve up a static website we'll have you start

up a static website we'll have you start running literally flask space run in

running literally flask space run in your terminal window to run your own web

your terminal window to run your own web server that's implemented in Python

server that's implemented in Python using this flask framework so bootstrap

using this flask framework so bootstrap was a library for making your CSS and

was a library for making your CSS and JavaScript prettier and more interactive

JavaScript prettier and more interactive flask is a framework or library for just

flask is a framework or library for just making your python code more pleasant to

making your python code more pleasant to use since you're borrowing features from

use since you're borrowing features from someone else all right so how can we go

someone else all right so how can we go about doing this well if you are to

about doing this well if you are to write your very own web application your

write your very own web application your own amazon.com your own google.com in

own amazon.com your own google.com in Python using flask minimally you need to

Python using flask minimally you need to have a file called app.py by convention

have a file called app.py by convention which is where all your python code goes

which is where all your python code goes and then a folder called templates which

and then a folder called templates which is where all of your templates go and

is where all of your templates go and for now your templates are just your

for now your templates are just your HTML files so if we're going to now

HTML files so if we're going to now start building more interesting

start building more interesting interactive things like google.com or

interactive things like google.com or amazon.com we need to be able to execute

amazon.com we need to be able to execute code on the server and so this is the

code on the server and so this is the convention it's not index.html anymore

convention it's not index.html anymore necessarily it's these two things at the

necessarily it's these two things at the top level with that said we'll quickly

top level with that said we'll quickly see that there's some other conventions

see that there's some other conventions and in my examples online and in the uh

and in my examples online and in the uh problem set nine you'll see another file

problem set nine you'll see another file called requirements.txt which is just a

called requirements.txt which is just a text file that allows you to enumerate

text file that allows you to enumerate all of the thirdparty libraries your

all of the thirdparty libraries your application might want to use it's a

application might want to use it's a convention so that the server can like

convention so that the server can like automatically install things for you

automatically install things for you without you having to do it manually and

without you having to do it manually and then static is going to be where

then static is going to be where literally your static content goes so if

literally your static content goes so if you've got images for your web

you've got images for your web application if you've got JavaScript

application if you've got JavaScript files CSS files by convention that goes

files CSS files by convention that goes in static these are just conventions

in static these are just conventions like all of this can be changed but this

like all of this can be changed but this is like the way to do things so we'll

is like the way to do things so we'll introduce you to the defaults all right

introduce you to the defaults all right so what does this mean how for instance

so what does this mean how for instance could I go about implementing my own web

could I go about implementing my own web application using python that somehow

application using python that somehow spits out a message like Hello World all

spits out a message like Hello World all right well turns out just this now we'll

right well turns out just this now we'll tease this apart in just a moment but

tease this apart in just a moment but this is the content of a sample app.py

this is the content of a sample app.py file that apparently uses some Library

file that apparently uses some Library stuff like familiar syntax from

stuff like familiar syntax from something import something else we've

something import something else we've seen that before with csvs and other

seen that before with csvs and other libraries this is somewhat new syntax

libraries this is somewhat new syntax but it's kind of copy paste for now this

but it's kind of copy paste for now this is definitely new syntax and kind of

is definitely new syntax and kind of weird with the at sign here but we'll

weird with the at sign here but we'll see this again and again today and it's

see this again and again today and it's just copy paste initially until you

just copy paste initially until you understand what it's doing for you but

understand what it's doing for you but at least there's some familiar stuff

at least there's some familiar stuff here like index.html is still going to

here like index.html is still going to be with us but it's going to be up to us

be with us but it's going to be up to us when and how to show it to the user so

when and how to show it to the user so let's make this more real let me go over

let's make this more real let me go over to vs code here and let me go ahead and

to vs code here and let me go ahead and create a how about we'll do this in

create a how about we'll do this in hello let me do makeer hello to make a

hello let me do makeer hello to make a new folder called hello and I'm going to

new folder called hello and I'm going to CD into it just to isolate all of these

CD into it just to isolate all of these files to the same directory so that we

files to the same directory so that we have different apps today and different

have different apps today and different folders and now I'm going to do code of

folders and now I'm going to do code of let's do this actually let's do our

let's do this actually let's do our maker templates. HTM ah sorry not

maker templates. HTM ah sorry not templates. HTML let me rename that to

templates. HTML let me rename that to templates using the MV command this has

templates using the MV command this has nothing to do with web programming this

nothing to do with web programming this is me making typos so if I type LS now

is me making typos so if I type LS now I've got a folder called templates all

I've got a folder called templates all right in there let's create a file

right in there let's create a file called index.html that is going to be

called index.html that is going to be super simple and pretty much copy paste

super simple and pretty much copy paste from last week let me hide my terminal

from last week let me hide my terminal window and let me just very quickly whip

window and let me just very quickly whip up a simp simple hello world page using

up a simp simple hello world page using my HTML tag Lang will equal English then

my HTML tag Lang will equal English then inside of this I'm going to have a head

inside of this I'm going to have a head tag inside of this I'm going to have a

tag inside of this I'm going to have a title tag and I'm just going to call

title tag and I'm just going to call this thing hello uh I'm going to then

this thing hello uh I'm going to then have a body and in this I'm only going

have a body and in this I'm only going to say something simple like hello comma

to say something simple like hello comma world and just so this is mobile

world and just so this is mobile friendly recall that we touched on these

friendly recall that we touched on these meta tags so just in case you after

meta tags so just in case you after class play with your mobile device

class play with your mobile device instead of your laptop I'll do name

instead of your laptop I'll do name equals quote unquote

equals quote unquote viewport uh viewport and and content

viewport uh viewport and and content equals and I never remember this I'm

equals and I never remember this I'm literally reading it off of a cheat

literally reading it off of a cheat sheet initial scale equals 1 width

sheet initial scale equals 1 width equals device width and this is just

equals device width and this is just this magical incantation that says to

this magical incantation that says to the browser like size things

the browser like size things appropriately for the size of the device

appropriately for the size of the device it blows up the font sizes a bit all

it blows up the font sizes a bit all right so that's what I would have done

right so that's what I would have done last week and I would have served this

last week and I would have served this web page by running HTTP server in the

web page by running HTTP server in the same directory and boom I would see that

same directory and boom I would see that HTML but let's now start to take some

HTML but let's now start to take some control over the user's experience and

control over the user's experience and for now it's going to be underwhelming

for now it's going to be underwhelming it's just going to always say hello

it's just going to always say hello world but in a moment version two is

world but in a moment version two is going to say hello David or hello Carter

going to say hello David or hello Carter a bit more dynamically and we'll quickly

a bit more dynamically and we'll quickly escalate from there to just more

escalate from there to just more interesting applications as well

interesting applications as well culminating with things like cookies and

culminating with things like cookies and shopping carts and the like so let me go

shopping carts and the like so let me go back into my terminal window and as

back into my terminal window and as promised let me create another file

promised let me create another file called

called app.py and this is where now I need to

app.py and this is where now I need to implement the web server I'm going to

implement the web server I'm going to run using this flask framework and for

run using this flask framework and for now I'm just going to kind of do some

now I'm just going to kind of do some copy paste from uh what we saw on the

copy paste from uh what we saw on the slide a moment ago from the flask

slide a moment ago from the flask Library which we've pre-installed for

Library which we've pre-installed for you I'm going to import a uh function

you I'm going to import a uh function called flask capital F it's subtle but

called flask capital F it's subtle but it's important there and I'm also going

it's important there and I'm also going to import a few other things a function

to import a few other things a function called render template and another

called render template and another variable called request and the only way

variable called request and the only way I know this is from having taught this

I know this is from having taught this before read the documentation followed a

before read the documentation followed a tutorial like you wouldn't know this

tutorial like you wouldn't know this unless someone told you or you read how

unless someone told you or you read how to do this this but what this means is

to do this this but what this means is that this Library called flask has three

that this Library called flask has three things in it a function called flask

things in it a function called flask capital f a function called render

capital f a function called render template and a variable built into it

template and a variable built into it called request and this is going to be

called request and this is going to be all the building blocks I need to

all the building blocks I need to implement my own web server the

implement my own web server the convention in flask when you want to

convention in flask when you want to create a web app in Python is you create

create a web app in Python is you create a variable by convention called app and

a variable by convention called app and then you assign it the return value of

then you assign it the return value of that flask function capital F and pass

that flask function capital F and pass into it underscore uncore name uncore

into it underscore uncore name uncore underscore which is weird but we have

underscore which is weird but we have seen this before a few weeks ago anyone

seen this before a few weeks ago anyone recall when and why we mentioned uncore

recall when and why we mentioned uncore uncore name uncore

uncore name uncore uncore yeah I think was

it name or something yeah if we wanted to check if the name of the file was

to check if the name of the file was itself main so that we avoided a

itself main so that we avoided a situation where if you're writing your

situation where if you're writing your own Library code you don't want your

own Library code you don't want your code to be executed automatically you

code to be executed automatically you want to potentially execute the main

want to potentially execute the main function and that was a solution to that

function and that was a solution to that problem here for today's purposes this

problem here for today's purposes this is just the way you do it underscore

is just the way you do it underscore underscore name underscore uncore refers

underscore name underscore uncore refers to the current file and so this is just

to the current file and so this is just a little trick that says turn this file

a little trick that says turn this file into a flask application that's all it

into a flask application that's all it is and for now uh that line suffices all

is and for now uh that line suffices all right what do I want to do after that

right what do I want to do after that well now I'm in charge of the web server

well now I'm in charge of the web server I need to write the code that decides

I need to write the code that decides based on the browsers request what file

based on the browsers request what file or files I'm going to send from the

or files I'm going to send from the server to the browser last week http

server to the browser last week http server did all of this for us just based

server did all of this for us just based on the file name but today I'm going to

on the file name but today I'm going to take over control over that process and

take over control over that process and the way I do that is as follows I say

the way I do that is as follows I say app. route with weirdly an at sign in

app. route with weirdly an at sign in front of it this is known in python as a

front of it this is known in python as a decorator and it's a feature of python

decorator and it's a feature of python not a flask that we just didn't

not a flask that we just didn't introduce in weeks past but it's a

introduce in weeks past but it's a special it's a handy trick to do what

special it's a handy trick to do what we're about to do the route I want to

we're about to do the route I want to Define is quote unquote slash so that is

Define is quote unquote slash so that is here is code I want the server to

here is code I want the server to execute whenever I user visits forward

execute whenever I user visits forward slash the default page of the website

slash the default page of the website well what code do I want them to execute

well what code do I want them to execute well I want them to execute a function

well I want them to execute a function and I can therefore Define in Python a

and I can therefore Define in Python a function I can technically call this

function I can technically call this thing anything I want X or Y or Z but

thing anything I want X or Y or Z but because they're accessing the default

because they're accessing the default page otherwise known as the index of the

page otherwise known as the index of the site I'm going to just more reasonably

site I'm going to just more reasonably call this function index but just a

call this function index but just a convention you could call it anything

convention you could call it anything you want but X Y or Z is probably a bad

you want but X Y or Z is probably a bad stylistic choice it doesn't need to take

stylistic choice it doesn't need to take any arguments in this case and the only

any arguments in this case and the only thing this code this function is going

thing this code this function is going to do is for now let's go ahead and have

to do is for now let's go ahead and have it return hello world quote unquote and

it return hello world quote unquote and that's it all right now let me go into

that's it all right now let me go into my terminal window let me go ahead and

my terminal window let me go ahead and do flask run in the same directory that

do flask run in the same directory that has appy and hit enter I'm going to see

has appy and hit enter I'm going to see some cryptic output but including a URL

some cryptic output but including a URL of my code space and if I open that URL

of my code space and if I open that URL after hovering over it I'll indeed see

after hovering over it I'll indeed see hello world as you might hope but let me

hello world as you might hope but let me do this let me go ahead and rightclick

do this let me go ahead and rightclick on the page and click view page source

on the page and click view page source which if you haven't done before shows

which if you haven't done before shows you all of the HTML for a page however

you all of the HTML for a page however pretty or messy it is and that's it

pretty or messy it is and that's it there's no HTML that I've spit out it's

there's no HTML that I've spit out it's just quote unquote hello world well if I

just quote unquote hello world well if I actually want to spit out a full web

actually want to spit out a full web page which is not a big deal here

page which is not a big deal here because who cares it's just the text

because who cares it's just the text anyway but if I want to spit out a whole

anyway but if I want to spit out a whole file let me do this I want to return

file let me do this I want to return essentially the contents of index.html

essentially the contents of index.html which have all of the tags I want the

which have all of the tags I want the mobile friend stuff and all of that well

mobile friend stuff and all of that well I can't just return index.html but I can

I can't just return index.html but I can return this render template quote

return this render template quote unquote

unquote index.html and per the documentation for

index.html and per the documentation for flask this render template function will

flask this render template function will go find that file for me in my templates

go find that file for me in my templates folder by convention it will open it up

folder by convention it will open it up and then it will spit the whole thing

and then it will spit the whole thing out to the browser for me so I can keep

out to the browser for me so I can keep all my HTML in one place and all my

all my HTML in one place and all my python code in this one place so now if

python code in this one place so now if I go back to my browser and reload I

I go back to my browser and reload I don't think I'll really see a difference

don't think I'll really see a difference because it's the same text ultimately

because it's the same text ultimately but if I view page Source now notice

but if I view page Source now notice that ah there is all of the HTML that

that ah there is all of the HTML that was just sent to the browser so this is

was just sent to the browser so this is only to say we have the building blocks

only to say we have the building blocks the puzzle pieces if you will via which

the puzzle pieces if you will via which to now store all of our HTML in one

to now store all of our HTML in one place and presumably CSS JavaScript and

place and presumably CSS JavaScript and so forth but then serve up whatever we

so forth but then serve up whatever we want even though I'm just blindly

want even though I'm just blindly spitting out

spitting out index.html so before we proceed any

index.html so before we proceed any questions on this which again I claim is

questions on this which again I claim is like my manual version of what HTTP

like my manual version of what HTTP server was doing for us automatically

server was doing for us automatically last week but this is how you do it

last week but this is how you do it yourself any

yourself any questions all right well let's make it

questions all right well let's make it more interesting which we could not do

more interesting which we could not do with HTTP server and HTML alone why

with HTTP server and HTML alone why don't we go ahead and do this let me

don't we go ahead and do this let me visit the same URL and I'm going to zoom

visit the same URL and I'm going to zoom in and your url will differ from my code

in and your url will differ from my code space but it's going to end similarly

space but it's going to end similarly here I'm going to do slash question mark

here I'm going to do slash question mark name equals David for instance or Q

name equals David for instance or Q equals cats or name equals Carter any

equals cats or name equals Carter any key value pair I want I'm going to

key value pair I want I'm going to append after a slash and a question mark

append after a slash and a question mark thereby providing user input to the

thereby providing user input to the server albe it in a very user unfriendly

server albe it in a very user unfriendly way no one's going to normally do this

way no one's going to normally do this in their browser enter nothing changes

in their browser enter nothing changes here it just says hello world but

here it just says hello world but wouldn't it be nice if it says hello

wouldn't it be nice if it says hello David or equivalently if I zoom in here

David or equivalently if I zoom in here again and change David to Carter and hit

again and change David to Carter and hit enter wouldn't it be nice if it says

enter wouldn't it be nice if it says hello Carter instead so we need some

hello Carter instead so we need some dynamism there and here's now python is

dynamism there and here's now python is going to be our friend if I want to

going to be our friend if I want to access the HTTP parameters that the user

access the HTTP parameters that the user has provided via the URL be it Q equals

has provided via the URL be it Q equals cats or name equals David I can use this

cats or name equals David I can use this special variable I already preemptively

special variable I already preemptively imported earlier and I can do this if

imported earlier and I can do this if there is an HTTP parameter called name

there is an HTTP parameter called name in what I'm going to call request. args

in what I'm going to call request. args then I'm going to go ahead and create a

then I'm going to go ahead and create a variable called name and I'm going to

variable called name and I'm going to set it equal to request. args bracket

set it equal to request. args bracket name else if there is no quote unquote

name else if there is no quote unquote name key in this special variable called

name key in this special variable called request. args I'm going to just assume

request. args I'm going to just assume that the user's name is World by default

that the user's name is World by default now what's going on here well it turns

now what's going on here well it turns out that flask provides us with this

out that flask provides us with this special variable called request. ARs and

special variable called request. ARs and in there is all of the key value pairs

in there is all of the key value pairs that might have come in via the URL so

that might have come in via the URL so if you had to guess what type of data or

if you had to guess what type of data or what data type is request. args that's

what data type is request. args that's its name and here is in context line n

its name and here is in context line n might provide a clue in Python what data

might provide a clue in Python what data type might request. args

type might request. args be

be yeah uh it's not going to be an array or

yeah uh it's not going to be an array or a list because those are always in every

a list because those are always in every language we've seen numerically indexed

language we've seen numerically indexed but you're

but you're close someone else it's a dictionary so

close someone else it's a dictionary so a dictionary is similar syntactically to

a dictionary is similar syntactically to a list in Python but instead of numeric

a list in Python but instead of numeric indices like 012 you can literally use

indices like 012 you can literally use strings like quote unquote name now

strings like quote unquote name now that's a bit of a white lie it is a

that's a bit of a white lie it is a dictionary but it's flask's special

dictionary but it's flask's special fancy version of a dictionary but the

fancy version of a dictionary but the syntax via which you can access it is

syntax via which you can access it is exactly the same and I actually this is

exactly the same and I actually this is a typo I didn't mean to say names there

a typo I didn't mean to say names there I meant to say name singular but

I meant to say name singular but otherwise I think the code is correct

otherwise I think the code is correct this is going to on line eight check if

this is going to on line eight check if there's a key called name in request.

there's a key called name in request. ARs and if so it's going to set it equal

ARs and if so it's going to set it equal to that value otherwise it's going to

to that value otherwise it's going to default to world I deliberately did not

default to world I deliberately did not do this I added this IFL

do this I added this IFL and did not do this why what error might

and did not do this why what error might happen if I just blindly grab

name exactly if there was nothing at the end of the URL that was of the form

end of the URL that was of the form question mark name equals someone then

question mark name equals someone then there would be no name key and this is

there would be no name key and this is uh you know a couple weeks back but this

uh you know a couple weeks back but this would give you one of those annoying key

would give you one of those annoying key errors when you get a trace back because

errors when you get a trace back because you screwed up because you used a string

you screwed up because you used a string that doesn't exist that's why I'm just

that doesn't exist that's why I'm just proactively trying to avoid that

proactively trying to avoid that situation just like I'm might have a

situation just like I'm might have a couple of weeks ago so even though it's

couple of weeks ago so even though it's more verbose this is just much more

more verbose this is just much more defensive so that I don't accidentally

defensive so that I don't accidentally index into a dictionary where there is

index into a dictionary where there is no key but we'll see how we can tighten

no key but we'll see how we can tighten this up to be not four lines but one but

this up to be not four lines but one but I think now I can do this wouldn't it be

I think now I can do this wouldn't it be nice if now in my index.html file which

nice if now in my index.html file which recall is in my templates folder

recall is in my templates folder wouldn't it be nice if I could do the

wouldn't it be nice if I could do the equivalent in C of like a percent s here

equivalent in C of like a percent s here for instance or in Python something like

for instance or in Python something like this name well it's close and this is

this name well it's close and this is just because different humans invent

just because different humans invent different languages invent different uh

different languages invent different uh Frameworks the Syntax for this in flask

Frameworks the Syntax for this in flask is to actually do whoops two curly

is to actually do whoops two curly braces and then name of the variable

braces and then name of the variable inside of it why it's just probably

inside of it why it's just probably someone figured what are the odds that a

someone figured what are the odds that a normal person is ever going to use two

normal person is ever going to use two curly braces at once versus just one so

curly braces at once versus just one so this is probably decreasing the

this is probably decreasing the probability that people actually want to

probability that people actually want to Output literal curly braces like this so

Output literal curly braces like this so it's similar in spirit to Python's F

it's similar in spirit to Python's F strings it's similar in spirit to C's

strings it's similar in spirit to C's percent s it's similar in spirit to

percent s it's similar in spirit to sql's question marks same idea slightly

sql's question marks same idea slightly different syntax and this there is

different syntax and this there is ginger so it's not programming code per

ginger so it's not programming code per se it's just a template and indeed

se it's just a template and indeed that's why this folder is called

that's why this folder is called templates it is sort of like a a

templates it is sort of like a a blueprint for what I want to be spit out

blueprint for what I want to be spit out to the user but I've got these

to the user but I've got these placeholders like this variable that I

placeholders like this variable that I want to plug into that value now this

want to plug into that value now this alone is not enough watch what happens

alone is not enough watch what happens if I go back to my other browser and I

if I go back to my other browser and I reload the page after changing up here

reload the page after changing up here let's do name equals David again enter

let's do name equals David again enter nothing outputs after the hello comma so

nothing outputs after the hello comma so it seems that the name variable doesn't

it seems that the name variable doesn't exist yet and that's why indeed if I do

exist yet and that's why indeed if I do view page Source you can see what was

view page Source you can see what was sent to the browser something's wrong

sent to the browser something's wrong with my placeholder but I just need to

with my placeholder but I just need to be a little more explicit as to what I

be a little more explicit as to what I want to send where so it turns out that

want to send where so it turns out that the render template function takes not

the render template function takes not just one argument the name of the

just one argument the name of the template you want to spit out but it

template you want to spit out but it takes after that with commas all of the

takes after that with commas all of the placeholders you want to plug in so for

placeholders you want to plug in so for instance if you want the placeholder to

instance if you want the placeholder to be this literally placeholder inside of

be this literally placeholder inside of those curly braces you can then specify

those curly braces you can then specify as the second argument to rep uh render

as the second argument to rep uh render template a placeholder named argument

template a placeholder named argument equals whatever the name is so name is

equals whatever the name is so name is the variable in the lines above

the variable in the lines above placeholder is the name of my literal

placeholder is the name of my literal placeholder in the curly braces and so

placeholder in the curly braces and so now if I go back to my browser and

now if I go back to my browser and reload this with still quote unquote

reload this with still quote unquote with still question mark name equals

with still question mark name equals David in the URL now I indeed see hello

David in the URL now I indeed see hello comma David and if I zoom in here and

comma David and if I zoom in here and let me move over here let me type in

let me move over here let me type in Carter and hit enter now I see Hello

Carter and hit enter now I see Hello Carter instead now this is a little

Carter instead now this is a little unnecessary to explicitly call the

unnecessary to explicitly call the placeholder placeholder especially if

placeholder placeholder especially if you want to have two or three of them so

you want to have two or three of them so you can actually call this anything you

you can actually call this anything you want and I'm going to change it back to

want and I'm going to change it back to name which is a little more

name which is a little more straightforward the only weird thing

straightforward the only weird thing here is that now you'll see that you're

here is that now you'll see that you're writing code like this and this is

writing code like this and this is correct and this is the norm it just

correct and this is the norm it just looks weird but the thing on the left of

looks weird but the thing on the left of the equal sign is the placeholder you're

the equal sign is the placeholder you're using in the template the thing on the

using in the template the thing on the right can be any value you want

right can be any value you want including a variable so even though I'm

including a variable so even though I'm naming them exactly the same which looks

naming them exactly the same which looks stupid admittedly like this is what

stupid admittedly like this is what people tend to do just because it's uh

people tend to do just because it's uh simpler than introducing another word

simpler than introducing another word like placeholder any questions now on

like placeholder any questions now on this

this any questions on these

any questions on these placeholders no all right well let's

placeholders no all right well let's tighten this up a little bit and see if

tighten this up a little bit and see if we can't get things to be more Dynamic

we can't get things to be more Dynamic still let me propose now that instead of

still let me propose now that instead of outputting instead of using this

outputting instead of using this condition which made a very simple idea

condition which made a very simple idea like very verbose with four different

like very verbose with four different lines it turns out there's an easier way

lines it turns out there's an easier way to do this you can actually still create

to do this you can actually still create a variable called name and you can set

a variable called name and you can set it equal to request. args but instead of

it equal to request. args but instead of just blindly indexing into to that

just blindly indexing into to that dictionary it turns out that request.

dictionary it turns out that request. ARS comes with a function as well called

ARS comes with a function as well called get you can pass it an argument that

get you can pass it an argument that tells you what value you want to get and

tells you what value you want to get and by default if there is no key called

by default if there is no key called name in that dictionary this function

name in that dictionary this function will not throw a key error it's just

will not throw a key error it's just going to return none an O the special V

going to return none an O the special V the Special Value in Python so it avoids

the Special Value in Python so it avoids a bug in your code but it tightens up

a bug in your code but it tightens up four lines into one but even nicer if

four lines into one but even nicer if you read the documentation

you read the documentation the get function can also take a

the get function can also take a explicit default value so if you don't

explicit default value so if you don't want none to be on the screen like hello

want none to be on the screen like hello comma blank or I mean that would be

comma blank or I mean that would be weird too you can just put in a default

weird too you can just put in a default value per the documentation of this

value per the documentation of this function like world so now we've gone

function like world so now we've gone from four lines to just one so arguably

from four lines to just one so arguably it's better designed and if I go back to

it's better designed and if I go back to the browser now still with Carter in the

the browser now still with Carter in the URL and hit reload same thing happens

URL and hit reload same thing happens but we notice this suppose I uh get rid

but we notice this suppose I uh get rid of the name parameter altogether and hit

of the name parameter altogether and hit enter now it goes to the default instead

enter now it goes to the default instead world so it's just a little better a

world so it's just a little better a little better designed than doing it the

little better designed than doing it the other way instead all right how about we

other way instead all right how about we take things up one more Notch and how

take things up one more Notch and how about we introduce multiple routes and

about we introduce multiple routes and actually introduce perhaps a form to the

actually introduce perhaps a form to the mix because again no normal person is

mix because again no normal person is going to like visit a URL and add a

going to like visit a URL and add a slash and a question mark and their name

slash and a question mark and their name like that's not how browsers work uh

like that's not how browsers work uh well that's how browsers work that's not

well that's how browsers work that's not how humans interact with browsers you

how humans interact with browsers you and I use a form to quickly instead so

and I use a form to quickly instead so now things can get a little more

now things can get a little more interesting when making our own web

interesting when making our own web application cuz maybe we could do

application cuz maybe we could do something like this let me go and zoom

something like this let me go and zoom out again let me go back to my code here

out again let me go back to my code here and let me move this around and focus

and let me move this around and focus now on the index.html file instead of

now on the index.html file instead of just this placeholder why don't we go

just this placeholder why don't we go ahead and give ourselves a form like

ahead and give ourselves a form like we've played with a little bit in the

we've played with a little bit in the past be it for Google or something else

past be it for Google or something else and let's do this uh form and inside of

and let's do this uh form and inside of this form let's have an input and the

this form let's have an input and the name of this input will be quote unquote

name of this input will be quote unquote name so that too is confusing but inputs

name so that too is confusing but inputs have name attributes but this is a

have name attributes but this is a person's name so I'm saying name equals

person's name so I'm saying name equals name here so just a messy world of

name here so just a messy world of semantics and let me go ahead and make

semantics and let me go ahead and make this a text box by default and then let

this a text box by default and then let me give myself a button whose default

me give myself a button whose default type will be submit and the name of this

type will be submit and the name of this button will be greet for instance so

button will be greet for instance so let's see what happens here but let me

let's see what happens here but let me change app.py to just be the original

change app.py to just be the original simpler I'm not passing in any

simpler I'm not passing in any placeholders now and I'm going to even

placeholders now and I'm going to even get rid of this I'm just going to rewind

get rid of this I'm just going to rewind to the first version of this for

to the first version of this for Simplicity let's now change the url to

Simplicity let's now change the url to get rid of Carter and myself so we just

get rid of Carter and myself so we just go to slash and hit enter and now we

go to slash and hit enter and now we have a super simple form again all right

have a super simple form again all right this is not super userfriendly but

this is not super userfriendly but there's some nice enhancements we can

there's some nice enhancements we can make for instance like we can uh for

make for instance like we can uh for instance turn off autocomplete

instance turn off autocomplete especially if I want to type David and

especially if I want to type David and Carter manually and I don't want it

Carter manually and I don't want it finishing my thought during class uh we

finishing my thought during class uh we can do autofocus which puts the cursor

can do autofocus which puts the cursor there BL linking by default which is

there BL linking by default which is nice cuz then the human doesn't have to

nice cuz then the human doesn't have to deal with that um and then we can even

deal with that um and then we can even have a placeholder attribute placeholder

have a placeholder attribute placeholder equals name so that it's like built in

equals name so that it's like built in instructions for this thing and so now

instructions for this thing and so now if I go back to the other tab nothing's

if I go back to the other tab nothing's changed yet because I have to download

changed yet because I have to download the HTML again reload okay now it's a

the HTML again reload okay now it's a little more user friendly it says name

little more user friendly it says name and light gray the cursor is blinking

and light gray the cursor is blinking and I'm sort of ready to go but this

and I'm sort of ready to go but this form hasn't been wired up to go anywhere

form hasn't been wired up to go anywhere yet and so let's do this let's for

yet and so let's do this let's for instance say that the action of this

instance say that the action of this form

form is not going to be something like

is not going to be something like google.com which we did last time with

google.com which we did last time with for cats I am now going to be both the

for cats I am now going to be both the front end and the back end of this

front end and the back end of this website the front end is what the human

website the front end is what the human sees the web page the graphics the forms

sees the web page the graphics the forms the back end is the stuff the human

the back end is the stuff the human typically doesn't see the python code

typically doesn't see the python code the SQL code the server itself but now

the SQL code the server itself but now I'm in control of both sides of the

I'm in control of both sides of the experience the HTML and also the routes

experience the HTML and also the routes so let's just propose that we invent our

so let's just propose that we invent our own route and instead of calling it SL

own route and instead of calling it SL search like Google does let's call it

search like Google does let's call it SLG greet and let me specify that the

SLG greet and let me specify that the method this form will use which is

method this form will use which is technically the default will be get and

technically the default will be get and confusingly it is lowercase get even

confusingly it is lowercase get even though in the envelope we keep talking

though in the envelope we keep talking about virtually it's actually capitals

about virtually it's actually capitals again left hand wasn't talking to right

again left hand wasn't talking to right hand when these things were decided all

hand when these things were decided all right so all I've done is create a web

right so all I've done is create a web form that's going to submit whatever the

form that's going to submit whatever the text box value is to a route called SLG

text box value is to a route called SLG greet by default because there's no HTTP

greet by default because there's no HTTP or htps or no domain name SLG greet is

or htps or no domain name SLG greet is going to be assumed to be not at

going to be assumed to be not at google.com but whatever my own serers

google.com but whatever my own serers URL is so whatever my code spaces URL is

URL is so whatever my code spaces URL is that's going to be the implicit prefix

that's going to be the implicit prefix this SLG greet is just the route so now

this SLG greet is just the route so now let's go back to VSS codes app.py file

let's go back to VSS codes app.py file how do I now Stitch this together well I

how do I now Stitch this together well I think we're good to go with index.html

think we're good to go with index.html if index. html's purpose in life is just

if index. html's purpose in life is just to spit out this form we're done with

to spit out this form we're done with one of my routes but if I want to have a

one of my routes but if I want to have a second route greet that actually spits

second route greet that actually spits out some greeting to the user well let's

out some greeting to the user well let's prepare that template too let me go

prepare that template too let me go ahead and highlight all of this HTML let

ahead and highlight all of this HTML let me go back into my terminal window and

me go back into my terminal window and into my hello directory and then into my

into my hello directory and then into my templates directory and let me create

templates directory and let me create another template called greet HTML whose

another template called greet HTML whose purpose in life will not be to show a

purpose in life will not be to show a form but to greet the user with hello so

form but to greet the user with hello so and so so in this file I'm going to

and so so in this file I'm going to paste all that same HTML but I'm going

paste all that same HTML but I'm going to get rid of the form and essentially

to get rid of the form and essentially revert to our previous version hello

revert to our previous version hello comma and then using the ginger syntax

comma and then using the ginger syntax name so one template index.html is for

name so one template index.html is for the form the second template now is for

the form the second template now is for the greeting of hello comma so and so

the greeting of hello comma so and so but otherwise these files notice are

but otherwise these files notice are almost the same except one has the form

almost the same except one has the form one has just the hello so now let's

one has just the hello so now let's finish this up in app doop High let me

finish this up in app doop High let me go down here after a couple of blank

go down here after a couple of blank lines stylistically let me do app. route

lines stylistically let me do app. route quote unquote SLG greet but I could call

quote unquote SLG greet but I could call this route anything I want I'm just

this route anything I want I'm just using a a reasonable verb then let's

using a a reasonable verb then let's define another function I could call the

define another function I could call the function anything I want X Y or Z I'm

function anything I want X Y or Z I'm going to call it more reasonably greet

going to call it more reasonably greet no arguments and then now is the code

no arguments and then now is the code where I want to render the template so I

where I want to render the template so I do return render template greet HTML but

do return render template greet HTML but but I need to do one more

but I need to do one more thing what else do I want to do if I

thing what else do I want to do if I want greet HTML to have access to the

want greet HTML to have access to the human's name just to

human's name just to recap I think we solved this already but

recap I think we solved this already but I deleted it but what do I have to add

I deleted it but what do I have to add back

back yeahh yeah so I got to pass in the

yeahh yeah so I got to pass in the placeholder somehow so I can do this a

placeholder somehow so I can do this a couple of different ways I I'm going to

couple of different ways I I'm going to keep it a little more elegant this time

keep it a little more elegant this time I'm just going to put my name uh

I'm just going to put my name uh argument there and I'm going to set it

argument there and I'm going to set it equal to request. ar. get quote unquote

equal to request. ar. get quote unquote name comma world before I used a

name comma world before I used a separate variable but I only used it in

separate variable but I only used it in one place so that's not strictly

one place so that's not strictly necessary so this is fine too but if

necessary so this is fine too but if this gets a little overwhelming notice

this gets a little overwhelming notice that I can alternatively do this I can

that I can alternatively do this I can create an actual variable called name

create an actual variable called name and then I can pass in an argument

and then I can pass in an argument called name with a value that is that

called name with a value that is that variable but again what's really the

variable but again what's really the point here it was kind of prettier all

point here it was kind of prettier all on one line so these are the exact same

on one line so these are the exact same things I'm just trying to tighten things

things I'm just trying to tighten things up further here all right so what just

up further here all right so what just happened if I go back to my form this is

happened if I go back to my form this is still index.html if I reload it nothing

still index.html if I reload it nothing has changed if I type in my name to this

has changed if I type in my name to this form notice again the URL I'm currently

form notice again the URL I'm currently at this is Chrome hiding things it's

at this is Chrome hiding things it's technically slash by default even though

technically slash by default even though many browsers are just hiding

many browsers are just hiding unnecessary uh characters these days but

unnecessary uh characters these days but Watch What Happens now if I scroll over

Watch What Happens now if I scroll over here and I click greet on this new

here and I click greet on this new form notice my URL my route changed to

form notice my URL my route changed to SL greet question mark name equals David

SL greet question mark name equals David and the body of the page at top left

and the body of the page at top left says hello comma David so this is

says hello comma David so this is exactly how google.com works and it's

exactly how google.com works and it's how we implemented search. HTML last

how we implemented search. HTML last time but instead of submitting the form

time but instead of submitting the form to Google via the form I'm submitting it

to Google via the form I'm submitting it to myself my very own route so I'm

to myself my very own route so I'm implementing my own backend for this

implementing my own backend for this same front end all right any questions

same front end all right any questions just yet much less interesting than

just yet much less interesting than Google certainly but we kind of have all

Google certainly but we kind of have all of the wiring

of the wiring now any

now any questions no

questions no all right so what can we do to further

all right so what can we do to further uh tighten this up and adhere to some

uh tighten this up and adhere to some conventions well let me propose that in

conventions well let me propose that in this version we solve one problem and

this version we solve one problem and even if you've never done this sort of

even if you've never done this sort of thing before I dare say we have enough

thing before I dare say we have enough weeks of cs-50 where if I show you

weeks of cs-50 where if I show you index.html again and greet HTML again

index.html again and greet HTML again odds are to someone's mind there's a

odds are to someone's mind there's a opportunity for

opportunity for improvement why is this web app super

improvement why is this web app super simple though it is arguably poorly

simple though it is arguably poorly designed at the

designed at the moment and the answer lies somewhere in

moment and the answer lies somewhere in these two templates index.html and

these two templates index.html and greet.tolowercase

probably did necessarily for your homepage why because when you have HTML

homepage why because when you have HTML only maybe CSS and even JavaScript

only maybe CSS and even JavaScript that's all you can do is copy paste copy

that's all you can do is copy paste copy paste and just make sure that you have

paste and just make sure that you have the same structure maybe you have the

the same structure maybe you have the same CSS file the same Javascript file

same CSS file the same Javascript file the same third party libraries but it

the same third party libraries but it makes it very very annoying as you might

makes it very very annoying as you might have realized already to just m make a

have realized already to just m make a change that affects everything so

change that affects everything so wouldn't it be nice to like factor out

wouldn't it be nice to like factor out all of this and all of this and just let

all of this and all of this and just let the body change so here too is something

the body change so here too is something that flask and really other equivalent

that flask and really other equivalent Frameworks let us do it allows us to

Frameworks let us do it allows us to create what we're going to call

create what we're going to call conventionally a layout instead so I'm

conventionally a layout instead so I'm going to go ahead and do this I'm going

going to go ahead and do this I'm going to copy one last time all of the same

to copy one last time all of the same HTML I'm going to go into uh my terminal

HTML I'm going to go into uh my terminal window and I'm going to create by

window and I'm going to create by convention a file called layout. HTML

convention a file called layout. HTML this is truly going to be a blueprint of

this is truly going to be a blueprint of sorts and in layout. HTML I'm going to

sorts and in layout. HTML I'm going to paste all of that same code but I'm

paste all of that same code but I'm going to use now some ginger syntax to

going to use now some ginger syntax to indicate that I don't want to plug in

indicate that I don't want to plug in just a variable like name here I want to

just a variable like name here I want to actually plug the contents of a whole

actually plug the contents of a whole other file so instead of just using

other file so instead of just using curly braces two of them left and right

curly braces two of them left and right I have to use slightly different syntax

I have to use slightly different syntax to say I want a whole block of HTML here

to say I want a whole block of HTML here from some other file and the way to do

from some other file and the way to do this even though the syntax is a little

this even though the syntax is a little non-obvious is you use open curly brace

non-obvious is you use open curly brace percent sign block then you can call the

percent sign block then you can call the next word anything you want it just has

next word anything you want it just has to be a special type of placeholder for

to be a special type of placeholder for an actual file not for just a variable

an actual file not for just a variable I'm going to call it body only because

I'm going to call it body only because I'm in the body so I'm want a

I'm in the body so I'm want a placeholder to be the entire body and

placeholder to be the entire body and then outside of this you then say in one

then outside of this you then say in one word no space end block so it looks kind

word no space end block so it looks kind of stupid honestly and why do we have

of stupid honestly and why do we have yet more ugly syntax again just

yet more ugly syntax again just different software developers in the

different software developers in the world are all choosing their own Syntax

world are all choosing their own Syntax for their own libraries so they all kind

for their own libraries so they all kind of look different but are all kind of

of look different but are all kind of similar in spirit and you just get used

similar in spirit and you just get used to seeing the different syntax this now

to seeing the different syntax this now is not nearly as pretty as the pair of

is not nearly as pretty as the pair of curly braces for variables but this is

curly braces for variables but this is how I can say plug the contents of an

how I can say plug the contents of an entire file Here and Now what does this

entire file Here and Now what does this let me do I can now go back into my

let me do I can now go back into my index.html file which at the moment

index.html file which at the moment still looks like this but almost all of

still looks like this but almost all of this is copy paste the only lines that

this is copy paste the only lines that are interesting and different are these

are interesting and different are these four lines here in the body so what I

four lines here in the body so what I can actually do now is I'm going to

can actually do now is I'm going to highlight that and cut it and then I'm

highlight that and cut it and then I'm going to highlight everything else and

going to highlight everything else and just delete the entire file and I'm

just delete the entire file and I'm going to use some of that same syntax

going to use some of that same syntax and say curly brace percent sign extends

and say curly brace percent sign extends quote unquote layout. HTML and then I

quote unquote layout. HTML and then I close my thought with a percent sign and

close my thought with a percent sign and close curly brace so this syntax as you

close curly brace so this syntax as you might just be inferring is now saying

might just be inferring is now saying please extend whatever layout. HTML

please extend whatever layout. HTML looks like that's the original blueprint

looks like that's the original blueprint the mold out of which I want to make

the mold out of which I want to make this web page and now here the syntax is

this web page and now here the syntax is a little weird too but similar at least

a little weird too but similar at least from before I can now say the block the

from before I can now say the block the body block that I want you to plug into

body block that I want you to plug into that layout is going to be everything

that layout is going to be everything between these two tags which we already

between these two tags which we already saw earlier but in layout. HTML they're

saw earlier but in layout. HTML they're sort of giving a placeholder in

sort of giving a placeholder in index.html this is what I'm going to

index.html this is what I'm going to plug in to those other placeholders as

plug in to those other placeholders as well so I'm just going to give myself

well so I'm just going to give myself some extra white space I'm going to

some extra white space I'm going to paste the HTML that was there if I want

paste the HTML that was there if I want to make clear what's going on I can

to make clear what's going on I can indent it although this has no no

indent it although this has no no functional impact but it just makes

functional impact but it just makes clear that just like in HTML you can

clear that just like in HTML you can open a ginger tag and close it but in

open a ginger tag and close it but in ginger here we have this here hey uh Hey

ginger here we have this here hey uh Hey python here comes the body of this page

python here comes the body of this page hey python that's it for the body of

hey python that's it for the body of this page and all of this stuff should

this page and all of this stuff should be plugged into this main parent layout

be plugged into this main parent layout if you will so super ugly admittedly but

if you will so super ugly admittedly but now at least things get way less

now at least things get way less redundant because I'm going to do the

redundant because I'm going to do the exact same thing over here in greet HTML

exact same thing over here in greet HTML it looks like this but now I'm going to

it looks like this but now I'm going to do this extends layout. HTML also just

do this extends layout. HTML also just as before uh the body the uh the body

as before uh the body the uh the body that I want to plug in is going to be

that I want to plug in is going to be everything inside of these tags here and

everything inside of these tags here and this body is just going to be hello

this body is just going to be hello comma name in curly braces like that so

comma name in curly braces like that so again ugly syntax got really ugly fast

again ugly syntax got really ugly fast but it's really just following these

but it's really just following these patterns now and we have two types of

patterns now and we have two types of placeholders two curly braces for

placeholders two curly braces for variables and now this kind of syntax

variables and now this kind of syntax with the percent signs and the single

with the percent signs and the single curly braces for like contents of actual

curly braces for like contents of actual files and so now in this world or in the

files and so now in this world or in the world of a homepage if you were using

world of a homepage if you were using flask and python to make your personal

flask and python to make your personal homepage with all of those various Pages

homepage with all of those various Pages you would probably design one main

you would probably design one main layout with all of your pretty logos and

layout with all of your pretty logos and colors and fonts and like what you want

colors and fonts and like what you want the site to look like and then each of

the site to look like and then each of your smaller Pages would now be

your smaller Pages would now be distilled into just these smaller

distilled into just these smaller fragments and whether you're using

fragments and whether you're using python or Java or Javas script or other

python or Java or Javas script or other languages too all different programming

languages too all different programming languages have popular Frameworks that

languages have popular Frameworks that do things like this the idea is the same

do things like this the idea is the same across all of them all right let's see

across all of them all right let's see if it works let's go back into the

if it works let's go back into the browser let me go back to my slash route

browser let me go back to my slash route there's that same form let me type in

there's that same form let me type in David and type and click greet and

David and type and click greet and indeed I see hello comma David I see

indeed I see hello comma David I see that greet was automatically added to

that greet was automatically added to the URL by the browser when I submitted

the URL by the browser when I submitted the form followed by the key value Pairs

the form followed by the key value Pairs and if I view the page source as I did

and if I view the page source as I did earlier you'll see that you have the

earlier you'll see that you have the entirety of that layout with hello David

entirety of that layout with hello David plugged in meanwhile if I go back to the

plugged in meanwhile if I go back to the form and view this page Source you'll

form and view this page Source you'll see the exact same layout but with the

see the exact same layout but with the form tag plugged in and here's where you

form tag plugged in and here's where you can be a little less uh nitpicky with

can be a little less uh nitpicky with styling okay yes this isn't technically

styling okay yes this isn't technically indented inside of the body but it was

indented inside of the body but it was relative to the original file so at this

relative to the original file so at this point in the game you don't need to

point in the game you don't need to worry about your outputed HTML looking

worry about your outputed HTML looking super pretty you want your source code

super pretty you want your source code that the see to be pretty not the

that the see to be pretty not the browser this is not a stylistic

browser this is not a stylistic concern okay questions on these

concern okay questions on these capabilities then a

capabilities then a flask or problems that we've just solved

flask or problems that we've just solved and why

yeah uh okay so if the files in question are in different folders for instance if

are in different folders for instance if I go back into my uh index page which

I go back into my uh index page which has the form um the routes here are

has the form um the routes here are entirely dependent on what is an app.py

entirely dependent on what is an app.py there's no notion of a folder when it

there's no notion of a folder when it comes to implementing a web application

comes to implementing a web application anymore they are more generically routes

anymore they are more generically routes however and we've not done this yet you

however and we've not done this yet you can put your static content your images

can put your static content your images your video files your CSS files in a

your video files your CSS files in a folder called Static and there can be

folder called Static and there can be subfolders therein and that would affect

subfolders therein and that would affect what you use as your Source attributes

what you use as your Source attributes for images or your Source uh tags for

for images or your Source uh tags for video or any of those kinds of assets

video or any of those kinds of assets and we'll see that eventually in the

and we'll see that eventually in the home in the uh the problem set next

home in the uh the problem set next other questions on what we've just done

other questions on what we've just done here

here yeah good question how do I how did I

yeah good question how do I how did I ensure that the web app starts on the

ensure that the web app starts on the form and then goes to the hello page so

form and then goes to the hello page so whatever you decide your default index

whatever you decide your default index route is like the implicit slash that is

route is like the implicit slash that is what is going to be pulled up when a

what is going to be pulled up when a user visits the domain name where your

user visits the domain name where your website is hosted so if I go back over

website is hosted so if I go back over here to app Pi because my slash route is

here to app Pi because my slash route is designed to return index.html that's

designed to return index.html that's exactly why that response came back good

exactly why that response came back good question all right so beyond this let's

question all right so beyond this let's consider now issues of privacy and we'll

consider now issues of privacy and we'll touch on this too as we get to issues

touch on this too as we get to issues like shopping carts and cookies if I

like shopping carts and cookies if I zoom in on the URL here even though the

zoom in on the URL here even though the URL itself is a little long and ugly and

URL itself is a little long and ugly and cryptic but when I type in my name and

cryptic but when I type in my name and hit uh greet of course we keep seeing

hit uh greet of course we keep seeing name equals David in the URL in what

name equals David in the URL in what sense might this be bad

sense might this be bad design or in what kinds of web apps

design or in what kinds of web apps might you not want the name to show up

might you not want the name to show up in the URL like that because this is

in the URL like that because this is what Google does this is what my app

what Google does this is what my app does

does yeah yeah so if I'm logging in with a

yeah yeah so if I'm logging in with a username and password I I could imagine

username and password I I could imagine that they show up in the URL after the

that they show up in the URL after the question mark where username equals

question mark where username equals mailin and password equals 1 2 3 4 5 but

mailin and password equals 1 2 3 4 5 but then all my you know like nosy siblings

then all my you know like nosy siblings need to do is go through through my

need to do is go through through my browser history and boom like it's right

browser history and boom like it's right there for them to copy paste so that

there for them to copy paste so that doesn't seem particularly uh secure or

doesn't seem particularly uh secure or if someone's walking past you in a cafe

if someone's walking past you in a cafe they can just look at your url if it's

they can just look at your url if it's revealed by the browser and they see it

revealed by the browser and they see it too so get is not necessarily the best

too so get is not necessarily the best verb to use even though it's the default

verb to use even though it's the default when submitting forms typically when

when submitting forms typically when you've got anything remotely sensitive

you've got anything remotely sensitive or anything large so be it a password or

or anything large so be it a password or credit card number or like an image or a

credit card number or like an image or a video that you're uploading to uh

video that you're uploading to uh Instagram or to YouTube or any such site

Instagram or to YouTube or any such site like that you don't want the data going

like that you don't want the data going into the URL and thankfully there's

into the URL and thankfully there's actually an easy way to fix this I can

actually an easy way to fix this I can go into my form which is currently in

go into my form which is currently in index.html and I can just change the

index.html and I can just change the method from get to post in lower case

method from get to post in lower case here but the verb in the virtual

here but the verb in the virtual envelope we discussed last week would

envelope we discussed last week would itself be capital p o St now

itself be capital p o St now unfortunately Watch What Happens here

unfortunately Watch What Happens here let me go over to my original form by

let me go over to my original form by going back to the Slash rout and I'm

going back to the Slash rout and I'm reloading the page to make sure I get

reloading the page to make sure I get the latest freshest HTML and just as a

the latest freshest HTML and just as a um just to confirm here yep if I view

um just to confirm here yep if I view Source method is now post so let me go

Source method is now post so let me go ahead and type in David now and click

ahead and type in David now and click greet and before we saw hello David but

greet and before we saw hello David but now I get method not allowed and this is

now I get method not allowed and this is somewhat subtle but in the title of the

somewhat subtle but in the title of the tab notice that it's a 405 error which

tab notice that it's a 405 error which is not familiar probably almost all of

is not familiar probably almost all of us have seen 404 file not found turns

us have seen 404 file not found turns out 405 a little more Arcane is the

out 405 a little more Arcane is the method the HTTP verb is not allowed why

method the HTTP verb is not allowed why because by default my app.py only

because by default my app.py only currently supports get by default how do

currently supports get by default how do I support post well I just need a little

I support post well I just need a little bit more syntax so let me go back into

bit more syntax so let me go back into vs code here let me go into app.py now

vs code here let me go into app.py now and after changing the form I just need

and after changing the form I just need to inform flask that you know what the

to inform flask that you know what the method I want this GRE route to use

method I want this GRE route to use should not be the default which is only

should not be the default which is only get I want it to use these methods and

get I want it to use these methods and it takes a second argument called

it takes a second argument called Methods the value of which is a list the

Methods the value of which is a list the default of which is quote unquote get so

default of which is quote unquote get so that's the default this has not made any

that's the default this has not made any changes but if I want to support post

changes but if I want to support post instead I can explicitly pass a list

instead I can explicitly pass a list with one string in it P instead and now

with one string in it P instead and now what does this mean we didn't talk about

what does this mean we didn't talk about this in any detail last week but inside

this in any detail last week but inside of this virtual envelope typically is

of this virtual envelope typically is that line like get slash search Q equals

that line like get slash search Q equals cat after the ex after the question mark

cat after the ex after the question mark if you want to hide that kind of

if you want to hide that kind of information for privacy sake or because

information for privacy sake or because you want to upload like an image which

you want to upload like an image which just doesn't make sense to put in the

just doesn't make sense to put in the URL essentially the part of the story

URL essentially the part of the story would be well the computer looks deeper

would be well the computer looks deeper inside of that virtual envelope and

inside of that virtual envelope and anything submitted via post goes below

anything submitted via post goes below the htdp headers like deeper in that

the htdp headers like deeper in that envelope so they're still there they're

envelope so they're still there they're just not obviously visible uh for prying

just not obviously visible uh for prying eyes in the user's own browser so just

eyes in the user's own browser so just by making that change in the HTML

by making that change in the HTML telling the browser to submit the data

telling the browser to submit the data via post and changing app.py to tell the

via post and changing app.py to tell the route to expect the data via post I can

route to expect the data via post I can now go back to my other tab let me go

now go back to my other tab let me go back to the original page let me reload

back to the original page let me reload just so I've got the latest HTML and

just so I've got the latest HTML and indeed view page Source it's still yep

indeed view page Source it's still yep it's still post but now when I type in

it's still post but now when I type in DAV ID and click greet now it works but

DAV ID and click greet now it works but but but notice the Privacy implication

but but notice the Privacy implication I'm at the SLG greet route but where's

I'm at the SLG greet route but where's my name it's not actually there it's

my name it's not actually there it's still went to the server but it's not in

still went to the server but it's not in your autocomplete or your history now

your autocomplete or your history now for privacy sake questions now on

for privacy sake questions now on post yeah oh No Just scratching all

post yeah oh No Just scratching all right can you the programmer see this

right can you the programmer see this well let me show you a couple of other

well let me show you a couple of other features of Chrome's uh Chrome and

features of Chrome's uh Chrome and Safari and other browsers as well I keep

Safari and other browsers as well I keep going to view page source which just

going to view page source which just shows you like a readon version of your

shows you like a readon version of your HTML but recall that last time I

HTML but recall that last time I actually right-clicked and went to

actually right-clicked and went to inspect or viewed developer tool tools

inspect or viewed developer tool tools and this brings up a much fancier

and this brings up a much fancier version of the developer tools and under

version of the developer tools and under elements here you see everything and

elements here you see everything and it's nice and pretty printed it's

it's nice and pretty printed it's hierarchical it collapses things into

hierarchical it collapses things into these clickable triangles but it's the

these clickable triangles but it's the exact same thing it's just more

exact same thing it's just more interactive but notice what I can do

interactive but notice what I can do today is this if I go to the network tab

today is this if I go to the network tab here and let me zoom out a little bit

here and let me zoom out a little bit let me go ahead and re uh load the form

let me go ahead and re uh load the form here and type in David again and click

here and type in David again and click greet notice now in the network tab of

greet notice now in the network tab of Chrome's developer tools I see a few

Chrome's developer tools I see a few things as we saw before one I see that

things as we saw before one I see that the request method is post two I see

the request method is post two I see that the server automatically without me

that the server automatically without me writing any code for this returns 200

writing any code for this returns 200 when it's successful but I can scroll

when it's successful but I can scroll down down down down down and you'll see

down down down down down and you'll see that eventually after all these cookies

that eventually after all these cookies more on those later if I click on

more on those later if I click on payload the second tab next to headers

payload the second tab next to headers you can see as the developer what was

you can see as the developer what was actually sent to the server so indeed

actually sent to the server so indeed this is going to be super useful like

this is going to be super useful like when doing problem set 9 maybe your

when doing problem set 9 maybe your final projects if you want to see what's

final projects if you want to see what's going from browser to server you have

going from browser to server you have complete control over all of that

complete control over all of that information even if you're using htps

information even if you're using htps because your browser and you the

because your browser and you the developer can certainly see all of this

developer can certainly see all of this so again these developer tools even

so again these developer tools even though there's a lot of tabs and buttons

though there's a lot of tabs and buttons you probably won't need anytime soon

you probably won't need anytime soon some of them like elements and network

some of them like elements and network and with JavaScript console are going to

and with JavaScript console are going to be super useful to start to get familiar

be super useful to start to get familiar with all right any questions now on this

with all right any questions now on this implication of

implication of post anything at

post anything at all no okay how about one final hello

all no okay how about one final hello example that ties a few of these things

example that ties a few of these things together how about now we try to tighten

together how about now we try to tighten things up further only in anticipation

things up further only in anticipation of something like problem set 9 or

of something like problem set 9 or really more complicated web apps where

really more complicated web apps where you might have not two but 20 or maybe

you might have not two but 20 or maybe even more different routes it might

even more different routes it might might be ideal to just minimize how many

might be ideal to just minimize how many total routes we have so we don't get a

total routes we have so we don't get a little too overwhelmed and I dare say

little too overwhelmed and I dare say that these two routes are so short maybe

that these two routes are so short maybe I can combine them into one and maybe I

I can combine them into one and maybe I can keep the user at what seems to be

can keep the user at what seems to be the same URL but just a kind of Tidy

the same URL but just a kind of Tidy things up so let me propose that we do

things up so let me propose that we do this instead let me get rid of my greet

this instead let me get rid of my greet route and let me go into my form in

route and let me go into my form in index.html and let me go ahead and just

index.html and let me go ahead and just have the action of this form still slash

have the action of this form still slash so I want the form to be visible at

so I want the form to be visible at slash the index of the site but I also

slash the index of the site but I also want the form to submit to itself if

want the form to submit to itself if only because I don't want to introduce

only because I don't want to introduce another route like SLG greet which

another route like SLG greet which eventually indeed will be compelling so

eventually indeed will be compelling so you don't have one route for everything

you don't have one route for everything you want your website to do so

you want your website to do so technically this is the default to and

technically this is the default to and if I omit action the exact same thing

if I omit action the exact same thing would happen as well but let me rewind

would happen as well but let me rewind and let me now go into app.py to see how

and let me now go into app.py to see how we can make this happen well if I want

we can make this happen well if I want my one and now only route to support

my one and now only route to support both methods I can say methods equals

both methods I can say methods equals and then a list with both get and post

and then a list with both get and post in any order but I'll keep them

in any order but I'll keep them alphabetical like this this now tells

alphabetical like this this now tells python hey this route should handle both

python hey this route should handle both get and post requests at the same place

get and post requests at the same place let's now go into this function I kind

let's now go into this function I kind of want to say the equivalent of this if

of want to say the equivalent of this if get then I want to return the form else

get then I want to return the form else if post I want to then return render

if post I want to then return render template of greet Doh HTML with the

template of greet Doh HTML with the user's name but this is not yet complete

user's name but this is not yet complete code but I think I can do this I'm going

code but I think I can do this I'm going to go ahead and say the following I'm

to go ahead and say the following I'm going to go ahead and say if

going to go ahead and say if request. method equals equals get then

request. method equals equals get then indeed return

indeed return index.html L if request. method equals

index.html L if request. method equals equals post then go ahead and return

equals post then go ahead and return greet HTML this isn't quite enough

greet HTML this isn't quite enough though because I still want to pass in

though because I still want to pass in that placeholder so let me again add

that placeholder so let me again add back name equals request. args doget

back name equals request. args doget quote unquote name and then a default

quote unquote name and then a default value of world what does this Now do for

value of world what does this Now do for me well let me go back to my other tab

me well let me go back to my other tab here let me close the developer tools

here let me close the developer tools let me go back to the form here let me

let me go back to the form here let me reload to make sure I have the latest

reload to make sure I have the latest let me view page Source just to make

let me view page Source just to make sure I have the latest and yep I have

sure I have the latest and yep I have the latest because it still says post

the latest because it still says post but it now says slash and let's see what

but it now says slash and let's see what happens now if I type in my name David

happens now if I type in my name David previously this submitted bya post so I

previously this submitted bya post so I didn't see any name or value thereof in

didn't see any name or value thereof in the URL but I did end up at SLG greet

the URL but I did end up at SLG greet but if the action is now slash and I

but if the action is now slash and I click greet notice that it still kind of

click greet notice that it still kind of works I see hello comma World although

works I see hello comma World although that didn't quite work so we'll come

that didn't quite work so we'll come back to that issue in a moment but

back to that issue in a moment but notice the URL ends in just slash and

notice the URL ends in just slash and again Chrome is hiding the slash because

again Chrome is hiding the slash because that's all that's there but it does not

that's all that's there but it does not end in name equals David in this case or

end in name equals David in this case or name and equals world now notice this

name and equals world now notice this too if I reload I'm going to get this

too if I reload I'm going to get this warning do you want to confirm form

warning do you want to confirm form resubmission the page you're looking for

resubmission the page you're looking for used information that you entered

used information that you entered returning to that page might cause any

returning to that page might cause any action you took to be repeated do you

action you took to be repeated do you want to continue you might have seen

want to continue you might have seen this on websites you've actually visited

this on websites you've actually visited where you hit reload and you're prompted

where you hit reload and you're prompted wait a minute do you want to do that

wait a minute do you want to do that odds are you've been prompted to reload

odds are you've been prompted to reload explicitly because why whatever you just

explicitly because why whatever you just did was post instead of get and by

did was post instead of get and by convention besides post being used for

convention besides post being used for privacy to like hide your username your

privacy to like hide your username your password your credit card number or the

password your credit card number or the like besides being used to upload bigger

like besides being used to upload bigger files like images or videos post is also

files like images or videos post is also used by convention to make changes to

used by convention to make changes to the server to add something to your

the server to add something to your shopping cart to add something to the

shopping cart to add something to the database whereas get is the name

database whereas get is the name suggests is all about getting

suggests is all about getting information not posting or sending

information not posting or sending information instead so this is Chrome

information instead so this is Chrome being a little careful because you know

being a little careful because you know if you just checked out on Amazon and

if you just checked out on Amazon and then you hit reload you don't want to

then you hit reload you don't want to accidentally like buy the same book

accidentally like buy the same book again so to speak even though Amazon and

again so to speak even though Amazon and fancy websites have other defenses for

fancy websites have other defenses for this too to avoid this issue now there

this too to avoid this issue now there is a bug though here it says hello comma

is a bug though here it says hello comma World instead of hello comma David and

World instead of hello comma David and it actually would have said the same a

it actually would have said the same a moment ago and I just didn't retest the

moment ago and I just didn't retest the code and reveal as much to you or if I

code and reveal as much to you or if I did I didn't even notice it said hello

did I didn't even notice it said hello world instead of hello David it turns

world instead of hello David it turns out that request. RGS is only used for

out that request. RGS is only used for get when using get request. args is a

get when using get request. args is a dictionary that contains all of your key

dictionary that contains all of your key value pairs but somewhat confusingly

value pairs but somewhat confusingly when using post with flask you have to

when using post with flask you have to go into request. form I have no idea why

go into request. form I have no idea why these are not sort of more obvious

these are not sort of more obvious opposites like request.get or request.

opposites like request.get or request. form and sorry request.get and request.

form and sorry request.get and request. poost would be sort of sensible names in

poost would be sort of sensible names in this case though we have request. args

this case though we have request. args for get and request. form for post all

for get and request. form for post all right that's an easy fix though if I go

right that's an easy fix though if I go back to vs code here let's change

back to vs code here let's change request. ARS to request. form let's go

request. ARS to request. form let's go back to my other tab let me just reload

back to my other tab let me just reload and you know what I'm going to say okay

and you know what I'm going to say okay continue to resubmit the same form

continue to resubmit the same form because the form was okay it was my

because the form was okay it was my python code that was buggy hitting enter

python code that was buggy hitting enter now it's accessing David okay but watch

now it's accessing David okay but watch this again if I hit reload command r or

this again if I hit reload command r or control r i get the same warning are you

control r i get the same warning are you sure you want to submit the form yes if

sure you want to submit the form yes if I do it manually with the reload icon I

I do it manually with the reload icon I get the same warning as before but if I

get the same warning as before but if I want to manually induce a get request

want to manually induce a get request well that's fine don't hit reload and

well that's fine don't hit reload and send the same request instead go up to

send the same request instead go up to your URL and just put the cursor up

your URL and just put the cursor up there and hit enter and now notice same

there and hit enter and now notice same URL is a get by default so anytime you

URL is a get by default so anytime you and I have typed URLs into browsers get

and I have typed URLs into browsers get is always the default only when you

is always the default only when you click on a button typically that the

click on a button typically that the programmer has configured to use post

programmer has configured to use post are you actually adding things to your

are you actually adding things to your shopping cart or the like all right so

shopping cart or the like all right so we are back and if I go way back in time

we are back and if I go way back in time myself like this is actually like the

myself like this is actually like the first web application I made back in

first web application I made back in 1997 I believe uh so at the time this

1997 I believe uh so at the time this was would have been what my sophomore or

was would have been what my sophomore or so year I had taken cs50 I took a

so year I had taken cs50 I took a follow- on class called cs51 which is a

follow- on class called cs51 which is a different type of programming and then I

different type of programming and then I pretty much taught myself a language

pretty much taught myself a language called Pearl which is somewhat less

called Pearl which is somewhat less popular nowadays but it's another

popular nowadays but it's another language like python like Java like

language like python like Java like JavaScript like others that can be used

JavaScript like others that can be used to make web-based applications and the

to make web-based applications and the web was very young at the time and the

web was very young at the time and the process via which students my classmates

process via which students my classmates could register uh for the first year

could register uh for the first year intral sports program AKA Frost's was to

intral sports program AKA Frost's was to grab a piece of paper and like write

grab a piece of paper and like write your name and email address on it and

your name and email address on it and walk it across across the yard to

walk it across across the yard to Wigglesworth I believe where the Proctor

Wigglesworth I believe where the Proctor lived and you'd Slide the piece of paper

lived and you'd Slide the piece of paper under the door and like that was how we

under the door and like that was how we submitted forms in my day um so this was

submitted forms in my day um so this was an opportunity even back in 1997 is to

an opportunity even back in 1997 is to like move things online and the website

like move things online and the website went on to live on until I think like

went on to live on until I think like 2007 I found this online and then it's

2007 I found this online and then it's become something else since um but this

become something else since um but this was a website via which people could

was a website via which people could register for sports and people could log

in the scores for various games and whatnot and so underneath the hood I

whatnot and so underneath the hood I didn't even know anything anything about

didn't even know anything anything about databases at the time it was just like

databases at the time it was just like CSV files that I was storing the data in

CSV files that I was storing the data in but there were HTML forms and there was

but there were HTML forms and there was with pearl the language at the time the

with pearl the language at the time the way to do the exact kind of stuff that

way to do the exact kind of stuff that we've just been doing already with flask

we've just been doing already with flask and so what I thought we'd do is

and so what I thought we'd do is Implement a slightly less ugly version

Implement a slightly less ugly version of this um repeating uh graphical

of this um repeating uh graphical backgrounds were invogue in like 1997 as

backgrounds were invogue in like 1997 as you can see here um but this is where

you can see here um but this is where these were the Aesthetics of the day uh

these were the Aesthetics of the day uh including the the so-called blink tag so

including the the so-called blink tag so let's at least focus on the

let's at least focus on the functionality of this website and not so

functionality of this website and not so much the Aesthetics and see if we can't

much the Aesthetics and see if we can't Implement some of the plumbing for

Implement some of the plumbing for actually solving like a real world

actually solving like a real world representative problem be it for

representative problem be it for freshman inal sports or something else

freshman inal sports or something else like it where you're getting data from

like it where you're getting data from users and processing it somehow so let

users and processing it somehow so let me go over here to VSS code let me

me go over here to VSS code let me create a new directory called fros IMS

create a new directory called fros IMS just so we can keep all of this code in

just so we can keep all of this code in its own directory let me CD into fros

its own directory let me CD into fros IMS let me proactively make another

IMS let me proactively make another directory called templates in which our

directory called templates in which our templates our HTML files do need to live

templates our HTML files do need to live and eventually I'm going to go ahead and

and eventually I'm going to go ahead and create a two files minimally app.py and

create a two files minimally app.py and index.html so let's do the first of

index.html so let's do the first of those app.py will live in my Frost im's

those app.py will live in my Frost im's directory and I'm just going to recreate

directory and I'm just going to recreate something very simple like we have

something very simple like we have previously so from flask in lowercase

previously so from flask in lowercase import flask capitalized render template

import flask capitalized render template and also request so same first line is

and also request so same first line is before let me then give myself a

before let me then give myself a variable called app set it equal to

variable called app set it equal to calling the flask function capital f

calling the flask function capital f with underscore uncore name underscore

with underscore uncore name underscore uncore and then let me give myself a

uncore and then let me give myself a route for slash as before with an index

route for slash as before with an index function though again I could call that

function though again I could call that anything I want and just for now let's

anything I want and just for now let's return render template of quote unquote

return render template of quote unquote index.html as though that exists so this

index.html as though that exists so this is not really a web application as much

is not really a web application as much as it is at the moment just a recreation

as it is at the moment just a recreation of HTTP server for one file let's now in

of HTTP server for one file let's now in another tab create a templates file uh

another tab create a templates file uh called called

called called index.html and I'm going to save myself

index.html and I'm going to save myself a few keystrokes I let me copy paste

a few keystrokes I let me copy paste from earlier almost all of the layout

from earlier almost all of the layout from before I've change the title in

from before I've change the title in advance to frosts instead of hello but

advance to frosts instead of hello but this is essentially the same template

this is essentially the same template and for now though because I'm in an

and for now though because I'm in an index.html I'm not going to use extends

index.html I'm not going to use extends or any of that fancy block stuff yet I'm

or any of that fancy block stuff yet I'm just going to go ahead and create a

just going to go ahead and create a relatively simple form via which back in

relatively simple form via which back in the day my classmates could have

the day my classmates could have registered for interal Sports so let's

registered for interal Sports so let's go ahead here and I'll propose that we

go ahead here and I'll propose that we do this um in this page we'll have a

do this um in this page we'll have a form the action of which will be a route

form the action of which will be a route called SL register though I could call

called SL register though I could call that anything I want it'll be somewhat

that anything I want it'll be somewhat private so I'm going to use post instead

private so I'm going to use post instead of get just so that people don't

of get just so that people don't accidentally maybe register twice by

accidentally maybe register twice by hitting reload uh without warning uh

hitting reload uh without warning uh inside of this form let's go ahead and

inside of this form let's go ahead and give them an input uh where autocomplete

give them an input uh where autocomplete will be off as always for demonstration

will be off as always for demonstration sake autofocus so the cursor goes there

sake autofocus so the cursor goes there initially the name of this field will be

initially the name of this field will be literally named because I want my

literally named because I want my classmates's name if they want to

classmates's name if they want to register for some sport the placeholder

will again be quote unquote Nam just so they see some gray instructions and the

they see some gray instructions and the type of this field will indeed be text

type of this field will indeed be text as before and then I need to give them

as before and then I need to give them the ability to register for a few Sports

the ability to register for a few Sports why don't we keep it simple like back in

why don't we keep it simple like back in the day basketball soccer and Ultimate

the day basketball soccer and Ultimate Frisbee were three of the sports that we

Frisbee were three of the sports that we supported and so let me do this and you

supported and so let me do this and you might not have seen this before uh

might not have seen this before uh unless you dabbled further on with forms

unless you dabbled further on with forms on your own but I can create a select

on your own but I can create a select menu otherwise known as a drop-down menu

menu otherwise known as a drop-down menu in HTML inside of which are a whole

in HTML inside of which are a whole bunch of options and each option

bunch of options and each option typically follows this Paradigm the

typically follows this Paradigm the value of the option and then the actual

value of the option and then the actual text that the human sees so the value of

text that the human sees so the value of these options will be how about we do uh

these options will be how about we do uh basketball as one and I want the human

basketball as one and I want the human to see literally the same thing though

to see literally the same thing though just like with a link in HTML they could

just like with a link in HTML they could be different but I'm going to keep them

be different but I'm going to keep them the same another option will be

the same another option will be uh let's say

uh let's say soccer and whoops let me fix my quotes

soccer and whoops let me fix my quotes and this human will see the exact same

and this human will see the exact same thing though it could say something else

thing though it could say something else and then lastly the value will be quote

and then lastly the value will be quote unquote Ultimate Frisbee and the humans

unquote Ultimate Frisbee and the humans will see the same thing there ultimate

will see the same thing there ultimate frisbe all right so this is going to

frisbe all right so this is going to create as we'll soon see just a drop-

create as we'll soon see just a drop- down menu with three separate options if

down menu with three separate options if I want the students to be able to submit

I want the students to be able to submit this now let me give them a button the

this now let me give them a button the type of which is submit and this button

type of which is submit and this button will be like the word register on it so

will be like the word register on it so I think we're pretty much good to go

I think we're pretty much good to go like this is all just HTML no python no

like this is all just HTML no python no flask per se except for the rendering of

flask per se except for the rendering of the same template so let me go into my

the same template so let me go into my terminal window let me do flask run

terminal window let me do flask run inside of this directory because I need

inside of this directory because I need to serve this app instead I'm going to

to serve this app instead I'm going to see some ugly output including my own

see some ugly output including my own URL and if I hover over that and then

URL and if I hover over that and then open that URL I should now see a more

open that URL I should now see a more interesting form it's got not only a

interesting form it's got not only a field for their name but also this

field for their name but also this drop-down menu with all three Sports now

drop-down menu with all three Sports now this isn't maybe the best user uh

this isn't maybe the best user uh experience thus far because I feel like

experience thus far because I feel like I'm biasing people to registering for

I'm biasing people to registering for basketball maybe because it's checked by

basketball maybe because it's checked by default I mean a lot of forms nowadays

default I mean a lot of forms nowadays have like a blank placeholder for the

have like a blank placeholder for the form so this is just an aesthetic thing

form so this is just an aesthetic thing but I can do this let me go back to the

but I can do this let me go back to the same form and let me give myself just a

same form and let me give myself just a a blank option at the top that in fact

a blank option at the top that in fact I'm going to disable so you technically

I'm going to disable so you technically can't select it proactively but I am

can't select it proactively but I am going to select it by default and so we

going to select it by default and so we probably haven't seen those HTML

probably haven't seen those HTML attributes before but if I want to

attributes before but if I want to create the equivalent of like a a a

create the equivalent of like a a a title for this dropdown I'm going to

title for this dropdown I'm going to literally create a disabled option

literally create a disabled option that's automatically selected called

that's automatically selected called sport so that you can't select it per se

sport so that you can't select it per se but it is there at the top so if I go

but it is there at the top so if I go back now to my other tab reload you know

back now to my other tab reload you know it's just marginally prettier than

it's just marginally prettier than before and I'm not biasing people toward

before and I'm not biasing people toward accidentally registering for basketball

accidentally registering for basketball alone and if I click on this you'll see

alone and if I click on this you'll see that sport is great out and therefore

that sport is great out and therefore not manually selectable but I can select

not manually selectable but I can select any of these other three still all right

any of these other three still all right well un unfortunately if I type in David

well un unfortunately if I type in David and I try registering for instance for

and I try registering for instance for soccer and click register I do end up

soccer and click register I do end up atreg and there's no question mark or

atreg and there's no question mark or name or sport so it's probably indeed

name or sport so it's probably indeed post instead of get those are hints but

post instead of get those are hints but not found notice the tab here very uh uh

not found notice the tab here very uh uh uh succinctly says 404 not found well

uh succinctly says 404 not found well why is that just to be clear why did SL

why is that just to be clear why did SL register give me a

404 what's the logic here perhaps just State the obvious

here perhaps just State the obvious or it doesn't exist right we haven't

or it doesn't exist right we haven't done that step yet all right so

done that step yet all right so something as simple as that and so I

something as simple as that and so I actually U sort of belabor that point

actually U sort of belabor that point because as you're learning like a lot of

because as you're learning like a lot of these conventions and some of this new

these conventions and some of this new syntax like honestly you're just going

syntax like honestly you're just going to make stupid mistakes something's not

to make stupid mistakes something's not going to work but again go back to First

going to work but again go back to First principles why is it not found all

principles why is it not found all right/ register should be a template

right/ register should be a template maybe called register. HTML oh I forgot

maybe called register. HTML oh I forgot my app. route so that should be the kind

my app. route so that should be the kind of of thinking as you try to diagnose

of of thinking as you try to diagnose these problems moving forward all right

these problems moving forward all right so let me go into app. and let me give

so let me go into app. and let me give myself a second route here uh so app.

myself a second route here uh so app. route quote unquote SL route then let me

route quote unquote SL route then let me Define a function called anything I want

Define a function called anything I want but I'm going to call it oh sorry not SL

but I'm going to call it oh sorry not SL route SL register let me call the

route SL register let me call the function just to be consistent register

function just to be consistent register so but I could call that anything I want

so but I could call that anything I want and just for now let's not do anything

and just for now let's not do anything too interesting let's just return the

too interesting let's just return the rendering of a template called success

rendering of a template called success .html let's just pretend for now that

.html let's just pretend for now that registration is successful no matter who

registration is successful no matter who you are or what you do now I need that

you are or what you do now I need that template and I only have index.html at

template and I only have index.html at this point so let me actually now do my

this point so let me actually now do my best practices let me copy all of that

best practices let me copy all of that let me in a separate terminal window let

let me in a separate terminal window let me do code uh let me go into my Frost

me do code uh let me go into my Frost im's directory and let me create a new

im's directory and let me create a new template called layout. HTML just like

template called layout. HTML just like before let me paste all that same code

before let me paste all that same code let me delete the form and just put in

let me delete the form and just put in that big placehold folder so block body

that big placehold folder so block body and then end block is all I did earlier

and then end block is all I did earlier this is just kind of boiler plate now

this is just kind of boiler plate now convention everything else I'm going to

convention everything else I'm going to leave the same but if I wanted to make

leave the same but if I wanted to make it prettier I could add my CSS up top if

it prettier I could add my CSS up top if I wanted to add like this crazy

I wanted to add like this crazy repeating background I could probably do

repeating background I could probably do that up top too so I could make every

that up top too so I could make every page look as ugly as it did back in my

page look as ugly as it did back in my day but we'll focus just today on the

day but we'll focus just today on the text all right so now that I have

text all right so now that I have layout. HTML let me clean up index.html

layout. HTML let me clean up index.html I don't need all this redundancy I don't

I don't need all this redundancy I don't need all of these tags at the top

need all of these tags at the top instead recall I think I just need

instead recall I think I just need extends quote unquote layout. HTML with

extends quote unquote layout. HTML with the appropriate percent signs and curly

the appropriate percent signs and curly braces I then have the appropriate block

braces I then have the appropriate block body though I could call Body anything I

body though I could call Body anything I want but I'm going to stick with my

want but I'm going to stick with my convention earlier and I'm going to

convention earlier and I'm going to delete the tags down here that I no

delete the tags down here that I no longer need why because if I go into

longer need why because if I go into layout. HTML I already have all my open

layout. HTML I already have all my open tags all my Clos tags the only stuff I

tags all my Clos tags the only stuff I want in index.html is is going to be

want in index.html is is going to be which belongs in the body so end block

which belongs in the body so end block down here and just to be pedantic let me

down here and just to be pedantic let me go ahead and highlight all that hit

go ahead and highlight all that hit shift Tab and that will like unindent it

shift Tab and that will like unindent it just to line things up just to be tidy

just to line things up just to be tidy all right so better even though it looks

all right so better even though it looks a little cryptic now but now I've laid

a little cryptic now but now I've laid the foundation for making a third page a

the foundation for making a third page a fourth page that don't have all of that

fourth page that don't have all of that same copy paste all right so now let's

same copy paste all right so now let's go back into app.py success. HTML is

go back into app.py success. HTML is where I left off so okay let me open my

where I left off so okay let me open my terminal window let me code up a

terminal window let me code up a template called success. HTML whose

template called success. HTML whose purpose in life is literally just going

purpose in life is literally just going to be like to say you are registered

to be like to say you are registered just so that we see some informative

just so that we see some informative message on the screen so this part I do

message on the screen so this part I do still need extends layout. HTML so

still need extends layout. HTML so there's a little bit of copy paste still

there's a little bit of copy paste still which is a little ugly but so be it

which is a little ugly but so be it block body for this template and I'm

block body for this template and I'm just going to say you are registered

just going to say you are registered exclamation point all right and then end

exclamation point all right and then end block so super simple it's just an

block so super simple it's just an informative message claiming that the

informative message claiming that the Stu the student is registered all right

Stu the student is registered all right let's go back to the original form which

let's go back to the original form which is this let me reload to make sure my

is this let me reload to make sure my HTML has reloaded type in David I'm

HTML has reloaded type in David I'm going to register again for soccer and

going to register again for soccer and click register and oh interesting method

click register and oh interesting method not allowed so I'm not getting a 404

not allowed so I'm not getting a 404 anymore I'm getting

anymore I'm getting 405 at SL

405 at SL register what's the deduction here how

time 405 is progress yeah the placeholder uh so it's not the

placeholder uh so it's not the placeholder I think is okay this is now

placeholder I think is okay this is now about the underlying HTTP stuff the

about the underlying HTTP stuff the method was disallow was not

method was disallow was not allowed say again so get purposes post

allowed say again so get purposes post thing too so by default all of these

thing too so by default all of these routes in flask just by default assume

routes in flask just by default assume get because it's safe it doesn't allow

get because it's safe it doesn't allow you to send information to the server in

you to send information to the server in quite the same way but if I do want to

quite the same way but if I do want to support post recall that we changed this

support post recall that we changed this to be methods equals and then a list

to be methods equals and then a list with quote quote Post in it so I just

with quote quote Post in it so I just need to enable support for that method

need to enable support for that method that that is that HTTP verb all right

that that is that HTTP verb all right let's go back to the form reload just to

let's go back to the form reload just to make sure I haven't screwed up type in

make sure I haven't screwed up type in my name David select soccer from the

my name David select soccer from the dropdown click register and now I'm not

dropdown click register and now I'm not only at SL register in the URL it claims

only at SL register in the URL it claims that I am indeed registered now of

that I am indeed registered now of course I'm not I've done nothing

course I'm not I've done nothing interesting there's no database there's

interesting there's no database there's no CSV file we'll get to that in a bit

no CSV file we'll get to that in a bit but at least I now have the plumbing in

but at least I now have the plumbing in place to do something Dynamic based on

place to do something Dynamic based on that sport all right well how can I now

that sport all right well how can I now improve upon this how about we go ahead

improve upon this how about we go ahead and implement store the actual

and implement store the actual registrant in a dictionary in the

registrant in a dictionary in the computer's memory so instead of just

computer's memory so instead of just claiming that they're registered let's

claiming that they're registered let's actually make a notation and the

actually make a notation and the simplest way as we did weeks ago in

simplest way as we did weeks ago in Python is just store things in like a

Python is just store things in like a variable in memory like a list or a

variable in memory like a list or a dictionary a set anything like that all

dictionary a set anything like that all right well let me go back into vs code

right well let me go back into vs code and in app.py and I think what I'm going

and in app.py and I think what I'm going to have to do here is change my register

to have to do here is change my register route to actually do some useful

route to actually do some useful information but before I register the

information but before I register the user let's consider where I want to

user let's consider where I want to actually put them and so let me propose

actually put them and so let me propose that how about we do this at the top of

that how about we do this at the top of my file let me go ahead and declare a

my file let me go ahead and declare a global variable called registrant uh and

global variable called registrant uh and set that equal to an empty dictionary so

set that equal to an empty dictionary so we've done this before when we were

we've done this before when we were playing around previously with using

playing around previously with using dictionaries to store key value Pairs

dictionaries to store key value Pairs and I'm going to propose that we store

and I'm going to propose that we store the registrant as a dictionary why cuz

the registrant as a dictionary why cuz I'm going to keep it simple like the

I'm going to keep it simple like the name is going to be the student's name

name is going to be the student's name sorry the key is going to be the

sorry the key is going to be the student's name and the value is going to

student's name and the value is going to be whatever sport they registered for so

be whatever sport they registered for so David and soccer and Carter and

David and soccer and Carter and basketball and so it kind of makes sense

basketball and so it kind of makes sense for like a two column dictionary so to

for like a two column dictionary so to speak as we often depict it on screen so

speak as we often depict it on screen so how can I use this dictionary well let

how can I use this dictionary well let me go ahead and do this down here under

me go ahead and do this down here under SL register let me go ahead and

SL register let me go ahead and initially do this how about we get the

initially do this how about we get the user's name from request. form. Get and

user's name from request. form. Get and set it equal to whatever the value of

set it equal to whatever the value of name is and I'm not going to give a

name is and I'm not going to give a default value now because I don't want

default value now because I don't want to call the student world or something

to call the student world or something strange like that I'm just going to

strange like that I'm just going to assume for now that it's there let's

assume for now that it's there let's then create another variable called

then create another variable called Sport and do request. form doget quote

Sport and do request. form doget quote unquote sport to get these students

unquote sport to get these students Sport and then let's go ahead and do

Sport and then let's go ahead and do this in the

this in the registrant dictionary let's index into

registrant dictionary let's index into it using the student's name and let's

it using the student's name and let's set it equal to whatever the sport is so

set it equal to whatever the sport is so I've got these variables just to keep my

I've got these variables just to keep my code tidy and I'm now putting a key

code tidy and I'm now putting a key value pair in that uh into that

value pair in that uh into that dictionary all right well what do I want

dictionary all right well what do I want to now do and I'll I'll go ahead and say

to now do and I'll I'll go ahead and say success. HTML sure let's go ahead and do

success. HTML sure let's go ahead and do that but now I think success. HTML means

that but now I think success. HTML means that so let me go back to the form

that so let me go back to the form reload let me type in David and soccer

reload let me type in David and soccer register okay let me go back and say

Carter and basketball registered okay now let's see what I want to do next how

now let's see what I want to do next how about I go

about I go into um Let me give myself another route

into um Let me give myself another route and let's play around here so app. route

and let's play around here so app. route let's give myself another third route

let's give myself another third route called registrant whose purpose in life

called registrant whose purpose in life is just to show me who all of those

is just to show me who all of those registrants are just like you would

registrants are just like you would expect from a website like this and then

expect from a website like this and then let me Define a function called

let me Define a function called registrants or anything else and then

registrants or anything else and then let me return the rendering of a

let me return the rendering of a template called

template called registrant H ML and let me pass in this

registrant H ML and let me pass in this is kind of neat I can do registrant

is kind of neat I can do registrant equals registrant which again looks

equals registrant which again looks weird but what am I doing I'm presuming

weird but what am I doing I'm presuming to pass in a placeholder called

to pass in a placeholder called registrant the value of which is this

registrant the value of which is this dictionary that I've been collecting all

dictionary that I've been collecting all of the registrations in so similar to

of the registrations in so similar to the name placeholder before but it's a

the name placeholder before but it's a little more powerful because now it's a

little more powerful because now it's a whole dictionary not just a single

whole dictionary not just a single string so I think now let me be creative

string so I think now let me be creative here let me go into my

here let me go into my templates under uh my templates folder

templates under uh my templates folder and let's do this uh let's go into uh my

and let's do this uh let's go into uh my terminal window let's create another

terminal window let's create another template called registrant HTML that's

template called registrant HTML that's actually going to do this displaying of

actually going to do this displaying of all of the registrant for us so extends

all of the registrant for us so extends layout. HTML just so I can borrow all of

layout. HTML just so I can borrow all of the same HTML as

the same HTML as before and let's define block body just

before and let's define block body just like before and inside of this and block

like before and inside of this and block I want to put I don't know like a

I want to put I don't know like a bulleted list or an ordered list list of

bulleted list or an ordered list list of all of the registrant so how can I do

all of the registrant so how can I do this well let's do uh an unordered list

this well let's do uh an unordered list ul and here's where Ginger and flask

ul and here's where Ginger and flask more generally get kind of interesting

more generally get kind of interesting like I want there to be something like

like I want there to be something like this An

this An Li uh and then like the students's name

Li uh and then like the students's name and then an L uh maybe Li yeah like that

and then an L uh maybe Li yeah like that and then maybe like sport something like

and then maybe like sport something like this but I didn't pass in a name I

this but I didn't pass in a name I didn't pass in a sport I passed in the

didn't pass in a sport I passed in the entire dictionary of registrants now in

entire dictionary of registrants now in Python if we were just doing something

Python if we were just doing something at the black and white terminal window

at the black and white terminal window and doing a command line program you

and doing a command line program you know I'd probably have some kind of for

know I'd probably have some kind of for Loop in Python Ginger does allow you to

Loop in Python Ginger does allow you to do this so a templating language tends

do this so a templating language tends to come with very lightweight mechanisms

to come with very lightweight mechanisms for doing placeholders doing Simple

for doing placeholders doing Simple Loops doing Simple conditions so python

Loops doing Simple conditions so python like syntax and it's almost identical so

like syntax and it's almost identical so watch what I can do inside of this

watch what I can do inside of this unordered list let me not start to

unordered list let me not start to manually output a single Li let me use

manually output a single Li let me use this syntax the same Ginger syntax that

this syntax the same Ginger syntax that I used for Block so curly brace percent

I used for Block so curly brace percent sign and I'm going to say this for name

sign and I'm going to say this for name in registrant so this is just like

in registrant so this is just like python Syntax for iterating over a

python Syntax for iterating over a dictionary and now this is going to look

dictionary and now this is going to look stupid but the opposite of that is end4

stupid but the opposite of that is end4 so in HTML you use the slash in ginger

so in HTML you use the slash in ginger you literally use the word end no space

you literally use the word end no space and then the name of the keyword so end4

and then the name of the keyword so end4 is how you close this but this is where

is how you close this but this is where templating gets really cool you can now

templating gets really cool you can now do Li and in here I can do something

do Li and in here I can do something like that student's name and that's it

like that student's name and that's it I'm going to leave it like that and what

I'm going to leave it like that and what I'm doing here is using really a

I'm doing here is using really a template as templates are intended I've

template as templates are intended I've got like the basic building blocks of

got like the basic building blocks of what I want this output to look like but

what I want this output to look like but thanks to this little for Loop here

thanks to this little for Loop here thanks to Ginger syntax the curly brace

thanks to Ginger syntax the curly brace and the percent sign I'm going to

and the percent sign I'm going to iterate over every dictionary printing

iterate over every dictionary printing out name name name name and so if I've

out name name name name and so if I've got two kids registered now I'm going to

got two kids registered now I'm going to see two liis David and Carter

see two liis David and Carter respectively so let's see let me go back

respectively so let's see let me go back to uh my Frost IM tab here and I don't

to uh my Frost IM tab here and I don't have a link yet so I got to do this

have a link yet so I got to do this manually uh like a developer would let

manually uh like a developer would let me go to slash registrant and I'll zoom

me go to slash registrant and I'll zoom out and hit

out and hit enter and you'll see what you'll

enter and you'll see what you'll probably see two when making mistakes

probably see two when making mistakes for the first time in this world so

for the first time in this world so where is the error message unfortunately

where is the error message unfortunately internal Ser server error is not all

internal Ser server error is not all that useful but we do tell you you see

that useful but we do tell you you see terminal window so if I go to the

terminal window so if I go to the terminal window I haven't been paying

terminal window I haven't been paying attention to this for quite some time

attention to this for quite some time and in fact I have two terminal windows

and in fact I have two terminal windows open so that I can still use commands at

open so that I can still use commands at the prompt but if I go back to my first

the prompt but if I go back to my first terminal window AKA bash there you'll

terminal window AKA bash there you'll see in your terminal window When

see in your terminal window When developing web applications like all of

developing web applications like all of the mistakes you made in the terminal

the mistakes you made in the terminal itself this is one of those python

itself this is one of those python tracebacks that's related to me screwing

tracebacks that's related to me screwing up here now let me go ahead here and

up here now let me go ahead here and let's see uh type error function is not

let's see uh type error function is not iterable

iterable and block for name function is not

iterable all right so what mistake did I make well this is what happens when I

make well this is what happens when I don't follow my notes and make changes

don't follow my notes and make changes on the fly so I have this variable on

on the fly so I have this variable on line five called registrant in all lower

line five called registrant in all lower case but what did I then do on the Fly

case but what did I then do on the Fly here in line

22 I defined a function called registrant so like newbie mistake like I

registrant so like newbie mistake like I shouldn't have done this I can't have a

shouldn't have done this I can't have a variable and a function of the same name

variable and a function of the same name name because the symbols are literally

name because the symbols are literally identical so just to make clear that

identical so just to make clear that this variable up here is actually Global

this variable up here is actually Global we'll use our convention like we did in

we'll use our convention like we did in C often when we had a global variable

C often when we had a global variable we'll capitalize it all just to make it

we'll capitalize it all just to make it stand out like a a constant value up

stand out like a a constant value up there and so down here what I'm going to

there and so down here what I'm going to do is pass in registrants in all cap so

do is pass in registrants in all cap so that was stupid didn't mean to confuse

that was stupid didn't mean to confuse there but the reason for that error to

there but the reason for that error to be clear is that you can't have a

be clear is that you can't have a function that's the same name as a

function that's the same name as a variable I could just change the

variable I could just change the variable name alt together I'm going to

variable name alt together I'm going to go ahead and just capitalize it to make

go ahead and just capitalize it to make it really stand out that this is in fact

it really stand out that this is in fact a global variable up top all right now

a global variable up top all right now I'm going to go back to my browser let's

I'm going to go back to my browser let's do David and

socer all right but there's going to be some other mistakes here so on line 17

some other mistakes here so on line 17 let me go ahead and change this variable

let me go ahead and change this variable to be capitalized there because indeed I

to be capitalized there because indeed I want to put the key and the value in

want to put the key and the value in this newly named variable as all

this newly named variable as all capitals registrant let me now go back

capitals registrant let me now go back to vs code here let me go back to the

to vs code here let me go back to the form and let me start adding some data

form and let me start adding some data fresh let me register David for soccer

fresh let me register David for soccer clicking register now and we should see

clicking register now and we should see you are registered but hopefully now

you are registered but hopefully now it's indeed in the computer's memory let

it's indeed in the computer's memory let me go back and register now Carter for

me go back and register now Carter for basketball clicking register again and

basketball clicking register again and hopefully it's now registered if I now

hopefully it's now registered if I now change my route manually to B SL

change my route manually to B SL registrant which is this newly added

registrant which is this newly added route that I made and hit enter now I

route that I made and hit enter now I see thank God now I see the unordered

see thank God now I see the unordered list containing everything in the

list containing everything in the computer's memory so when I say you are

computer's memory so when I say you are registered I kind of mean it now because

registered I kind of mean it now because the server is still running and in the

the server is still running and in the computer's memory is in this registrant

computer's memory is in this registrant Global variable a dictionary of key

Global variable a dictionary of key value pairs of course we're only seeing

value pairs of course we're only seeing the keys at the moment so it might be

the keys at the moment so it might be nice to actually see the values in as

nice to actually see the values in as well so let me go back to VSS code and

well so let me go back to VSS code and let me go into registr trans. HTML and

let me go into registr trans. HTML and I'll just do something a little messy

I'll just do something a little messy I'll just say uh how about let just make

I'll just say uh how about let just make it a sentence is registered

it a sentence is registered four and now another placeholder I'm

four and now another placeholder I'm going to say registrant bracket name so

going to say registrant bracket name so just like in Python if registr is itself

just like in Python if registr is itself a dictionary registr bracket and then

a dictionary registr bracket and then the key you want to index into is

the key you want to index into is perfectly valid syntax as well so now

perfectly valid syntax as well so now let me go back to SL registrant let me

let me go back to SL registrant let me click reload again so why isn't it

click reload again so why isn't it working everyone what's the bug that I

working everyone what's the bug that I introduced earlier if David is

introduced earlier if David is registered for none and Carter is

registered for none and Carter is registered for none but David and Carter

registered for none but David and Carter are in the dictionary like that's a good

are in the dictionary like that's a good thing so some of the data is in there so

thing so some of the data is in there so why are there no Sports Associated well

why are there no Sports Associated well the first thing I literally just did in

the first thing I literally just did in front of you all was I went to app.py

front of you all was I went to app.py and I stared at line 17 thinking like

and I stared at line 17 thinking like how did I screw this up I'm putting

how did I screw this up I'm putting sport as the value of the key which is

sport as the value of the key which is the student's name all right line 17

the student's name all right line 17 looked fine to me a few seconds ago so I

looked fine to me a few seconds ago so I looked then with my eyes at line 16 and

looked then with my eyes at line 16 and this too looked okay my first thought

this too looked okay my first thought was oh did I use request. args instead

was oh did I use request. args instead of request. form instead because that

of request. form instead because that would have assumed get instead of post

would have assumed get instead of post but no like that looks okay too so then

but no like that looks okay too so then my final Instinct was oh my god did I

my final Instinct was oh my god did I screw up the HTML form and so that's why

screw up the HTML form and so that's why I went back over to my tab here I went

I went back over to my tab here I went to the original form here I then view

to the original form here I then view page source and this might not be as

page source and this might not be as obvious to you if you've never seen the

obvious to you if you've never seen the select menu before what is apparently

select menu before what is apparently missing here that might explain my

missing here that might explain my mistake yeah

mistake yeah yeah I didn't name this form field quote

yeah I didn't name this form field quote unquote sport now to be fair you haven't

unquote sport now to be fair you haven't seen me do this as a select menu before

seen me do this as a select menu before and it's different from this input when

and it's different from this input when you have an input tag you literally say

you have an input tag you literally say name equals whatever on the input tag it

name equals whatever on the input tag it turns out I don't know why I skipped

turns out I don't know why I skipped this earlier I probably meant to come

this earlier I probably meant to come back to it the select tag also can take

back to it the select tag also can take a name parameter so if I go back to the

a name parameter so if I go back to the name

name parameter here and go back and add the

parameter here and go back and add the name parameter let me go into that

name parameter let me go into that template which is

template which is index.html let me add name equals quote

index.html let me add name equals quote unquote sport in all lowercase which is

unquote sport in all lowercase which is different from the visual aesthetic of

different from the visual aesthetic of this temporary disabled option that's

this temporary disabled option that's just there to make things prettier for

just there to make things prettier for the human now let me go ahead here and

the human now let me go ahead here and first I'm going to go into my terminal

first I'm going to go into my terminal window and I'm actually going to hit

window and I'm actually going to hit control C to stop the server alt

control C to stop the server alt together because I want to throw away

together because I want to throw away the contents of memory and therefore get

the contents of memory and therefore get rid of that dictionary that had David

rid of that dictionary that had David and Carter and those nun values so this

and Carter and those nun values so this is sort of me clearing the computer's

is sort of me clearing the computer's memory I'm going to rerun flask run I

memory I'm going to rerun flask run I get that same URL as before so I'm going

get that same URL as before so I'm going to hover over that and open the new tab

to hover over that and open the new tab and just to be sure I'm going to do view

and just to be sure I'm going to do view page source and here I see now okay now

page source and here I see now okay now the form has both a name and a sport in

the form has both a name and a sport in it all right now I'm really going to

it all right now I'm really going to cross my fingers because I intend for

cross my fingers because I intend for this now to work David will register

this now to work David will register again for soccer register claims we are

again for soccer register claims we are registered I'm going to go back and do

registered I'm going to go back and do it again for Carter and basketball

it again for Carter and basketball register we still don't have a link so

I'm going to manually go up to the URL and change/ register to registrant as

and change/ register to registrant as before zooming out and hit enter and

before zooming out and hit enter and thank God now I'm actually registered

thank God now I'm actually registered properly for this so oh thank

you so what is it like 20 years later I'm still struggling to implement this

I'm still struggling to implement this site okay so um so here now we have for

site okay so um so here now we have for the first time in Python and web stuff

the first time in Python and web stuff like now we have a proper web

like now we have a proper web application and it's not just echoing

application and it's not just echoing back hello David hello Carter this could

back hello David hello Carter this could now work for any of you and it's

now work for any of you and it's currently served privately but if I made

currently served privately but if I made this URL public I could put this on the

this URL public I could put this on the web now and let anyone in the world

web now and let anyone in the world register but there's kind of some issues

here there's some security flaws potentially and so for instance let me

potentially and so for instance let me go back to the web form here and let me

go back to the web form here and let me open up the inspect tab the developer

open up the inspect tab the developer tools and just remind you that anyone on

tools and just remind you that anyone on the internet not only you the developer

the internet not only you the developer but a an adversary can see all of your

but a an adversary can see all of your HTML see all of your CSS see all of your

HTML see all of your CSS see all of your JavaScript but more importantly because

JavaScript but more importantly because this is all client side in the browser

this is all client side in the browser there is literally nothing technically

there is literally nothing technically stopping them from changing the HTML or

stopping them from changing the HTML or at least their copy of it and I did that

at least their copy of it and I did that last week with Yale I changed their

last week with Yale I changed their website but no I changed my copy of

website but no I changed my copy of their website but when forms get

their website but when forms get involved you could maybe be actually

involved you could maybe be actually malicious now because even though this

malicious now because even though this drop-down menu only has basketball

drop-down menu only has basketball soccer and Ultimate Frisbee suppose I

soccer and Ultimate Frisbee suppose I really want to register for how about uh

really want to register for how about uh let's say uh name your favorite sport

let's say uh name your favorite sport volleyball we really want to register

volleyball we really want to register for volleyball but like this website

for volleyball but like this website won't let me well there's nothing

won't let me well there's nothing stopping me from going under the

stopping me from going under the elements tab in my browser going into

elements tab in my browser going into this select menu here and you know what

this select menu here and you know what no one ultimate for me let's change this

no one ultimate for me let's change this to volleyball and let's change this to

to volleyball and let's change this to volleyball enter I'm going to close the

volleyball enter I'm going to close the inspector now and as requested now we

inspector now and as requested now we support volleyball in the form now now

support volleyball in the form now now it's not changed on the server to be

it's not changed on the server to be fair but think about how HTTP works when

fair but think about how HTTP works when I fill out this with uh say let's see

I fill out this with uh say let's see Bernie's name Bernie really wants to

Bernie's name Bernie really wants to register for volleyball as well at the

moment my code is just going to trust that what's in request. form is what was

that what's in request. form is what was in the original form itself no matter

in the original form itself no matter whether the human adversarially actually

whether the human adversarially actually changed it so if I actually submit this

changed it so if I actually submit this form and click register for Bernie and

form and click register for Bernie and volleyball even though that's not one of

volleyball even though that's not one of the supported available Sports if I now

the supported available Sports if I now go to registrant my website nonetheless

go to registrant my website nonetheless has trusted that uh Bernie and perhaps

has trusted that uh Bernie and perhaps you are registered for volleyball so

you are registered for volleyball so what's the implication of this this has

what's the implication of this this has surely happened in the past when like

surely happened in the past when like really poorly implemented websites um

really poorly implemented websites um allow you to uh specify the price of an

allow you to uh specify the price of an item for instance in your shopping cart

item for instance in your shopping cart and they just trust that when you click

and they just trust that when you click submit or add to cart it adds the price

submit or add to cart it adds the price to the backend server if you're not

to the backend server if you're not validating the price and making sure as

validating the price and making sure as with a database that wait a minute that

with a database that wait a minute that price is valid valid or wait a minute

price is valid valid or wait a minute those sports are valid like who knows

those sports are valid like who knows what people are going to do to your site

what people are going to do to your site and it's that simple to actually hack a

and it's that simple to actually hack a website accordingly now we can very

website accordingly now we can very easily fix this with some um some week

easily fix this with some um some week six style python we really just need to

six style python we really just need to do a bit of logic here and so let me

do a bit of logic here and so let me propose this let me go into app.py here

propose this let me go into app.py here and at the very top let me also create

and at the very top let me also create how about a uh Global variable called

how about a uh Global variable called Sports in all caps and I'm going to set

Sports in all caps and I'm going to set that equal to in square brackets the

that equal to in square brackets the list of Sports I actually want to

list of Sports I actually want to support so I'm going to put in

support so I'm going to put in basketball here I'm going to put in

basketball here I'm going to put in soccer here and I'm sorry no volleyball

soccer here and I'm sorry no volleyball officially I'm going to put in ultimate

officially I'm going to put in ultimate frisbee here so I've got this Global

frisbee here so I've got this Global list of supported Sports now think about

list of supported Sports now think about how I made this form a while ago I just

how I made this form a while ago I just hardcoded these Sports here well I don't

hardcoded these Sports here well I don't have to do that I can sort of draw upon

have to do that I can sort of draw upon my own official list of sports instead

my own official list of sports instead so let me scroll down to my

so let me scroll down to my index.html rendering template here let

index.html rendering template here let me say that the sports I want to support

me say that the sports I want to support are are these so just using the same

are are these so just using the same placeholder trick as before but I'm now

placeholder trick as before but I'm now telling the template what sports we

telling the template what sports we currently support now if I go back into

currently support now if I go back into index.html I don't have to manually do

index.html I don't have to manually do any of this let me get rid of all three

any of this let me get rid of all three of those options which I manually

of those options which I manually inputed earlier let me use my new trick

inputed earlier let me use my new trick with ginger syntax and say for sport in

with ginger syntax and say for sport in sports then let me proactively say end

sports then let me proactively say end for just to finish that thought and then

for just to finish that thought and then in here let me do option value equals in

in here let me do option value equals in curly BRAC is Sport and then so that the

curly BRAC is Sport and then so that the human also sees the same words I'm going

human also sees the same words I'm going to say sport out here so I've completely

to say sport out here so I've completely changed what was hardcoded manually

changed what was hardcoded manually typed to something now that's completely

typed to something now that's completely Dynamic so now it's not going to stop

Dynamic so now it's not going to stop someone adversar like me from changing

someone adversar like me from changing the HTML but watch this the behavior on

the HTML but watch this the behavior on the form if we go back is still now the

the form if we go back is still now the same drop down as before so

same drop down as before so aesthetically it looks the same but you

aesthetically it looks the same but you know what why don't we be clever now and

know what why don't we be clever now and let's go into app.py and the/ register

let's go into app.py and the/ register route and why don't we say this uh if

route and why don't we say this uh if how about sport not in sports then let's

how about sport not in sports then let's return render template uh failure. HTML

return render template uh failure. HTML now this template doesn't exist yet so

now this template doesn't exist yet so let me just quickly make this real fast

let me just quickly make this real fast I'm going to copy that code from before

I'm going to copy that code from before let me create a code file in uh failure.

let me create a code file in uh failure. HTML I'm just going to paste this here

HTML I'm just going to paste this here so I have a super simple error message

so I have a super simple error message and I'm going to say you are not

and I'm going to say you are not registered just to that's what we mean

registered just to that's what we mean by failure and now in app.py consider

by failure and now in app.py consider what logic I've added Sports in all caps

what logic I've added Sports in all caps on line 22 is that same Global list as

on line 22 is that same Global list as before by asking pythonic if sport not

before by asking pythonic if sport not in sports well then you hacked me like

in sports well then you hacked me like you tried to inject volleyball or some

you tried to inject volleyball or some other sport into request. form so I'm

other sport into request. form so I'm just going to say no failure not letting

just going to say no failure not letting you register and I can do this a little

you register and I can do this a little more uh verbosely too why don't I also

more uh verbosely too why don't I also say this if if not name so if the name

say this if if not name so if the name is blank let's similarly return a render

is blank let's similarly return a render template of failure. HTML in other words

template of failure. HTML in other words if you didn't give me a name you left it

if you didn't give me a name you left it blank that's not useful for me running

blank that's not useful for me running the sports program let's also consider

the sports program let's also consider that to be a failure so if I go back to

that to be a failure so if I go back to this tab now I'm going to reload just to

this tab now I'm going to reload just to make sure I have the latest client side

make sure I have the latest client side let me be lazy and just click register

let me be lazy and just click register enter you are not registered because I

enter you are not registered because I didn't give it an actual name all right

didn't give it an actual name all right well let's go back how about I now type

well let's go back how about I now type David but no no I'm not going to choose

David but no no I'm not going to choose a sport I just want register myself nope

a sport I just want register myself nope that did not work now let me go ahead

that did not work now let me go ahead here and choose soccer this I think does

here and choose soccer this I think does work let me go back now and try this

work let me go back now and try this hacker trick whereby I go into the drop-

hacker trick whereby I go into the drop- down menu I go into the select menu I

down menu I go into the select menu I change as before Ultimate Frisbee to

change as before Ultimate Frisbee to volleyball and I'll change this one here

volleyball and I'll change this one here to

to volleyball let me close the tab now this

volleyball let me close the tab now this looks like it's available now but when I

looks like it's available now but when I click register this this time it says

click register this this time it says you are not registered and this is much

you are not registered and this is much better than relying on other techniques

better than relying on other techniques you might see or have seen online with

you might see or have seen online with regard to HTML because there's also this

regard to HTML because there's also this trick let me go back to the screen here

trick let me go back to the screen here let me go back to index.html and you

let me go back to index.html and you might have seen online or you might

might have seen online or you might eventually see online that there's other

eventually see online that there's other attributes you can use like required you

attributes you can use like required you can literally tell the browser uhuh this

can literally tell the browser uhuh this field is required you cannot leave it

field is required you cannot leave it blank if I go back to the browser now

blank if I go back to the browser now reload and I again presume to be lazy

reload and I again presume to be lazy and I don't type in any name and click

and I don't type in any name and click register okay so that's kind of nice

like now the browser is being a little more helpful for me saying no no no this

more helpful for me saying no no no this is required you have to fill this out

is required you have to fill this out but again if you know what you're doing

but again if you know what you're doing okay well I disagree with your requiring

okay well I disagree with your requiring a name of me let me go in here let me go

a name of me let me go in here let me go over to this tag let me delete the

over to this tag let me delete the required attribute and now I slip

required attribute and now I slip through but I didn't slip through on the

through but I didn't slip through on the server and so there's a difference here

server and so there's a difference here and an important distinction and so many

and an important distinction and so many people in the real world still screw

people in the real world still screw this up there's client side validation

this up there's client side validation like actually checking that the data is

like actually checking that the data is as you expect on the client side the

as you expect on the client side the browser and there server side validation

browser and there server side validation and even though client side validation

and even though client side validation like adding that required attribute

like adding that required attribute makes things more user friendly right

makes things more user friendly right like that was a pretty little popup it

like that was a pretty little popup it tells me that it's required it just

tells me that it's required it just looks better than the previous version

looks better than the previous version it is not trustable you cannot trust any

it is not trustable you cannot trust any input that ever comes from the user

input that ever comes from the user because clearly with like a an hour or

because clearly with like a an hour or so of cs50 like they can learn how to

so of cs50 like they can learn how to turn all of these defenses off so even

turn all of these defenses off so even if you like the user interface better

if you like the user interface better client side you have to have to have to

client side you have to have to have to do server side validation always users

do server side validation always users are not to be trusted and as soon as any

are not to be trusted and as soon as any app or website you make becomes popular

app or website you make becomes popular unfortunately then you have to deal with

unfortunately then you have to deal with all of the adversarial possibilities as

well oh good question could the adversary potentially access things

adversary potentially access things sensitive like app.py theoretically no

sensitive like app.py theoretically no like if flask itself is buggy then sure

like if flask itself is buggy then sure maybe if you're running some other

maybe if you're running some other software on your server on your laptop

software on your server on your laptop then sure uh it it's possible however if

then sure uh it it's possible however if your server is properly configured

your server is properly configured theoretically they should not be able to

theoretically they should not be able to get access to that with that said we'll

get access to that with that said we'll soon see or You might with your final

soon see or You might with your final project if you do something web based

project if you do something web based you're never going to want to write like

you're never going to want to write like usernames and passwords in your actual

usernames and passwords in your actual code you can put them in what are called

code you can put them in what are called environment variables so sort of in the

environment variables so sort of in the computer's memory but not in your code

computer's memory but not in your code just in case you or someone screws up

just in case you or someone screws up there are uh still ways to defend

there are uh still ways to defend against those kinds of possibilities

against those kinds of possibilities however slim

yeah a good question and this comes back to First principles just like in C and

to First principles just like in C and in python as soon as you return from a

in python as soon as you return from a function that's it nothing below that

function that's it nothing below that line of code executes and so to

line of code executes and so to summarize the question even though I'm

summarize the question even though I'm returning this failure. HTML template

returning this failure. HTML template how am I making sure we still don't

how am I making sure we still don't accidentally put volleyball in that

accidentally put volleyball in that Global dictionary it's because for

Global dictionary it's because for instance if you don't give me a name on

instance if you don't give me a name on line 22 at the moment I'm returning the

line 22 at the moment I'm returning the failure template and that's it lines 23

failure template and that's it lines 23 24 25 26 27 never execute in particular

24 25 26 27 never execute in particular 26 never executes and that's where I

26 never executes and that's where I would have been saving the name

would have been saving the name similarly if we do we get a invalid

similarly if we do we get a invalid sport that is either blank or not in the

sport that is either blank or not in the original authoritative list we return

original authoritative list we return failure. HTML is a template in line 25

failure. HTML is a template in line 25 we never get to line 26 so it just boils

we never get to line 26 so it just boils down to return and what that means in

down to return and what that means in Python 2 good question other questions

Python 2 good question other questions as

as well no all right how about we make one

well no all right how about we make one more enhancement here or so because the

more enhancement here or so because the problem with storing everything in this

problem with storing everything in this Global dictionary might be what we've

Global dictionary might be what we've got it all working finally but why is

got it all working finally but why is probably a global dictionary not the

probably a global dictionary not the place to store Frost's registration

place to store Frost's registration data what's the implication of

data what's the implication of this okay might slow the whole process

this okay might slow the whole process down but that actually Ram is actually

down but that actually Ram is actually good memory is actually generally a good

good memory is actually generally a good thing so not going to be a deal breaker

thing so not going to be a deal breaker here why might I not want to store that

here why might I not want to store that data though in that

variable you can perhaps infer how I fix something

something earlier yeah and

earlier yeah and back yeah it gets it's the memory gets

back yeah it gets it's the memory gets deleted garbage collected if you will as

deleted garbage collected if you will as soon as flask stops running so if you so

soon as flask stops running so if you so much as hit control C like you've just

much as hit control C like you've just lost all of your freshmen who registered

lost all of your freshmen who registered for the sport probably not a good thing

for the sport probably not a good thing I did this deliberately a moment ago and

I did this deliberately a moment ago and I hit control C because I did want to

I hit control C because I did want to clear the dictionary but trusting that

clear the dictionary but trusting that your server will never crash and your

your server will never crash and your code will always work and the power will

code will always work and the power will never go out like that's not the right

never go out like that's not the right way to build any kind of web application

way to build any kind of web application with persistent data so what we probably

with persistent data so what we probably want to do is reintroduce csvs and we've

want to do is reintroduce csvs and we've played with those in C and in Python

played with those in C and in Python could totally use csvs but we also now

could totally use csvs but we also now have p uh SQL at our disposal and let me

have p uh SQL at our disposal and let me propose that we do this in SQL in

propose that we do this in SQL in instead and for this let me go ahead and

instead and for this let me go ahead and open up a version of the program that I

open up a version of the program that I wrote in advance so let me go ahead and

wrote in advance so let me go ahead and close these templates which will look

close these templates which will look very similar but a little different from

very similar but a little different from the ones I wrote in advance and let me

the ones I wrote in advance and let me go ahead and open up uh in today's let

go ahead and open up uh in today's let me go into Source 9 let me go into Frost

me go into Source 9 let me go into Frost im's how about version four technically

im's how about version four technically in the versions online and let me go

in the versions online and let me go ahead and open up app.py as follows so

ahead and open up app.py as follows so here is an already made version that

here is an already made version that does just a little something different

does just a little something different at the very top I'm importing cs50 SQL

at the very top I'm importing cs50 SQL Library which you might recall we used a

Library which you might recall we used a couple of weeks past just to write

couple of weeks past just to write python that talks to a SQL database and

python that talks to a SQL database and this feels like an opportune moment to

this feels like an opportune moment to bring that idea back down here on line

bring that idea back down here on line eight I'm creating a DB variable that

eight I'm creating a DB variable that opens up a file called frost. DB using

opens up a file called frost. DB using syntax that we've seen before I did

syntax that we've seen before I did create this FRS DB file in advance of

create this FRS DB file in advance of class just so that we have a couple of

class just so that we have a couple of columns in which to store names and

columns in which to store names and sports and such here's that same Global

sports and such here's that same Global array a global list called Sports and

array a global list called Sports and let's just see what's going on down

let's just see what's going on down below if I scroll down to Index this is

below if I scroll down to Index this is the same as before as we wrote together

the same as before as we wrote together on the Fly let's skip deregister for a

on the Fly let's skip deregister for a moment and go now into register so this

moment and go now into register so this one's a little different but let's see

one's a little different but let's see what I've done I've got some comments in

what I've done I've got some comments in here because I wrote it in advance and I

here because I wrote it in advance and I think this logic is pretty much the same

think this logic is pretty much the same though I tightened it up and I'm asking

though I tightened it up and I'm asking two questions at once using on line 38

two questions at once using on line 38 the or keyword here just to say if

the or keyword here just to say if there's not a name or the sport is not

there's not a name or the sport is not in sports that is what we'll call now a

in sports that is what we'll call now a failure but what's fun now is that on

failure but what's fun now is that on line 42

line 42 I'm using the cs50 SQL library to

I'm using the cs50 SQL library to execute some actual SQL and I'm going to

execute some actual SQL and I'm going to insert into a table called registrant

insert into a table called registrant two columns name and Sport what names

two columns name and Sport what names and Sport well these two values with

and Sport well these two values with placeholders plugging in name and Sport

placeholders plugging in name and Sport notice I'm using the question marks

notice I'm using the question marks absolutely necessary so we don't get one

absolutely necessary so we don't get one of those SQL injection attacks because

of those SQL injection attacks because that too could be possible if someone

that too could be possible if someone typed in some dangerous words or

typed in some dangerous words or keywords like delete or single quotes or

keywords like delete or single quotes or semicolons in the form here I'm letting

semicolons in the form here I'm letting the library sanitize the data and then

the library sanitize the data and then this is a trick we haven't yet seen and

this is a trick we haven't yet seen and it's really going to start tying things

it's really going to start tying things together I can also use a redirect

together I can also use a redirect function in flask that has the effect of

function in flask that has the effect of doing the the if you will uh

doing the the if you will uh safetyschool.org trick again whereby

safetyschool.org trick again whereby after the user registers if I want to

after the user registers if I want to automatically show them now everyone who

automatically show them now everyone who is registered atreg I don't have to

is registered atreg I don't have to manually expect that they'll change the

manually expect that they'll change the url like I've been doing for the past

url like I've been doing for the past few minutes I can just redirect them

few minutes I can just redirect them anywhere I want on my app or heck I

anywhere I want on my app or heck I could redirect them to any URL on the

could redirect them to any URL on the internet using this function call and

internet using this function call and it's just a nice way to send them to a

it's just a nice way to send them to a different route if you want them to see

different route if you want them to see in this case those registrant so let me

in this case those registrant so let me do this in the same directory let me

do this in the same directory let me increase my terminal window size let me

increase my terminal window size let me do SQL light three of frost. DB and let

do SQL light three of frost. DB and let me type schema and you can indeed see it

me type schema and you can indeed see it wraps on to two lines here that each

wraps on to two lines here that each registrant has an ID which will be

registrant has an ID which will be automatically assigned one two three on

automatically assigned one two three on up a name which is not null text and a

up a name which is not null text and a sport which is also the same and the

sport which is also the same and the primary key is just going to be this

primary key is just going to be this unique identifier so that I made in

unique identifier so that I made in advance but if I do select star from

advance but if I do select star from registrant semicolon there's no one

registrant semicolon there's no one currently registered for any sports but

currently registered for any sports but let's try now running this let me go

let's try now running this let me go ahead and close my old version which we

ahead and close my old version which we wrote together and I'll close that tab

wrote together and I'll close that tab let me do flask run in this version four

let me do flask run in this version four here all right I'm going to see some

here all right I'm going to see some similar output I'm going to open the URL

similar output I'm going to open the URL now and you'll see that I made a couple

now and you'll see that I made a couple of Chang changes before instead of using

of Chang changes before instead of using a select menu I used what are called

a select menu I used what are called radio buttons now which is a reference

radio buttons now which is a reference to Old School radio buttons that were

to Old School radio buttons that were mutually exclusive in cars back in the

mutually exclusive in cars back in the day and we'll see how to do this but

day and we'll see how to do this but it's just an alternative to a select

it's just an alternative to a select menu and I'm going to go ahead and type

menu and I'm going to go ahead and type in my name again here so I'll do David

in my name again here so I'll do David I'll do soccer by selecting this radio

I'll do soccer by selecting this radio button and I'm going to click register

button and I'm going to click register now and notice what happened it's a

now and notice what happened it's a little ugly the formatting but so again

little ugly the formatting but so again was this 20 years ago here I have now

was this 20 years ago here I have now now at the slash registrant route

now at the slash registrant route instead of an unordered list I'm just

instead of an unordered list I'm just using a simple HTML table so I'll show

using a simple HTML table so I'll show you what this looks like in just a

you what this looks like in just a moment too and I'll show you this

moment too and I'll show you this deregister button which is sort of

deregister button which is sort of unnecessarily large I also have

unnecessarily large I also have functionality we'll soon see for how you

functionality we'll soon see for how you can unregister someone from a sport as

can unregister someone from a sport as well so take your name out of contention

well so take your name out of contention well let me go back to my terminal

well let me go back to my terminal window here and I'm going to click the

window here and I'm going to click the plus to give myself s a second terminal

plus to give myself s a second terminal so I can go back into Source 9 Frost IMS

so I can go back into Source 9 Frost IMS 4 I'm going to do SQL light of frost. DB

4 I'm going to do SQL light of frost. DB I'm going to do select star from

I'm going to do select star from registrant now and now you'll see that

registrant now and now you'll see that indeed there's David registered for

indeed there's David registered for soccer and in fact if I quit the flask

soccer and in fact if I quit the flask program with control C and rerun it

program with control C and rerun it again no big deal because that next

again no big deal because that next version of flask will just use the

version of flask will just use the database as well so I'm persisting

database as well so I'm persisting keeping the data in SQL light whereas

keeping the data in SQL light whereas I'm actually grabbing it using my python

I'm actually grabbing it using my python code in flask all right let's put one

code in flask all right let's put one more person in here so we can delete one

more person in here so we can delete one of us too Carter for basketball register

of us too Carter for basketball register and now we see both of us here all right

and now we see both of us here all right so let's see how we did this let's go

so let's see how we did this let's go back over to VSS code let me shrink down

back over to VSS code let me shrink down my terminal window let me go into the

my terminal window let me go into the actually let's go into the templates

actually let's go into the templates directory and let's look at for instance

directory and let's look at for instance index.html so previously we were using a

index.html so previously we were using a select menu turns out radio buttons use

select menu turns out radio buttons use the input tag but instead of having

the input tag but instead of having input of uh type equals text like for

input of uh type equals text like for the human's name you have type equals

the human's name you have type equals radio and so long as each of your radio

radio and so long as each of your radio buttons has the same name the same name

buttons has the same name the same name the same name that's what makes them

the same name that's what makes them mutually exclusive so checking one radio

mutually exclusive so checking one radio button turns off the others because they

button turns off the others because they have the same name the value I want to

have the same name the value I want to assign to each of these radio buttons is

assign to each of these radio buttons is just the sport placeholder this is what

just the sport placeholder this is what the human sees on the screen so it's

the human sees on the screen so it's almost the same as the select menu it

almost the same as the select menu it just looks aesthetically different but

just looks aesthetically different but there's my same button so that's all the

there's my same button so that's all the difference I made there and I added a

difference I made there and I added a heading tag H1 just to say register to

heading tag H1 just to say register to make clear what it is but let's take a

make clear what it is but let's take a look at another file uh this one now

look at another file uh this one now being how about the SL registrant route

being how about the SL registrant route so if I open up registr trans. HTML here

so if I open up registr trans. HTML here now it's way more verbose than my

now it's way more verbose than my unordered list but this is just kind of

unordered list but this is just kind of boring HTML here's my table tag table

boring HTML here's my table tag table head table row table heading this makes

head table row table heading this makes things bold as the first row of the

things bold as the first row of the table name sport are my two columns I've

table name sport are my two columns I've got a third empty column just so I can

got a third empty column just so I can fit that button as we'll soon see again

fit that button as we'll soon see again T body for table body here's the same

T body for table body here's the same for Loop trick again so that I can

for Loop trick again so that I can output for every registrant a whole

output for every registrant a whole table row and there's this weird form in

table row and there's this weird form in there but we'll come back to that but

there but we'll come back to that but there's the registrant's name there's

there's the registrant's name there's the registrant sport but notice the

the registrant sport but notice the slightly different syntax here recall

slightly different syntax here recall that cs50's select uh cs50's execute

that cs50's select uh cs50's execute function when it returns to you a list

function when it returns to you a list of dictionaries you can then get at the

of dictionaries you can then get at the individual columns by way of those keys

individual columns by way of those keys so let's go to the/ registrant Route let

so let's go to the/ registrant Route let me go back to app.py scroll down here

me go back to app.py scroll down here and it's actually super simple here I

and it's actually super simple here I have a SL registr route that first

have a SL registr route that first executes select star from registrant so

executes select star from registrant so just old SQL stuff give me everyone from

just old SQL stuff give me everyone from the registr table let me then render the

the registr table let me then render the template called registr trans. HTML and

template called registr trans. HTML and just pass in this list of dictionaries

just pass in this list of dictionaries and we haven't quite done this yet but

and we haven't quite done this yet but if you go back to register. HTML how do

if you go back to register. HTML how do you iterate over each dictionary in that

you iterate over each dictionary in that list well the syntax is just for

list well the syntax is just for registrant in registrants that makes

registrant in registrants that makes this a dictionary one at a time in the

this a dictionary one at a time in the list just like in Python so registrant

list just like in Python so registrant name and registrants sport is just

name and registrants sport is just another Syntax for using the square

another Syntax for using the square bracket notation it's just a little

bracket notation it's just a little cleaner and slightly more succinct than

cleaner and slightly more succinct than having quotes and square brackets

having quotes and square brackets everywhere and then the rest of this is

everywhere and then the rest of this is just

just HTML so what happens now if I want to uh

HTML so what happens now if I want to uh like Carter has been cut from the

like Carter has been cut from the basketball team if you will so how do we

basketball team if you will so how do we do that well we want to click this

do that well we want to click this button deregister next to Carter's name

button deregister next to Carter's name but how does this work and think about

but how does this work and think about now any website you visited that has

now any website you visited that has something like a shopping cart uh where

something like a shopping cart uh where you can remove things from your cart or

you can remove things from your cart or update quantities or add more quantities

update quantities or add more quantities to your shopping cart on Amazon or

to your shopping cart on Amazon or anything else well let's actually look

anything else well let's actually look at the HTML that my app has spit out

at the HTML that my app has spit out let's actually look at this here and

let's actually look at this here and we'll see the following we'll see that

we'll see the following we'll see that we have here in the HTML that reached

we have here in the HTML that reached the user not only is David in the first

the user not only is David in the first column socer in the second notice that

column socer in the second notice that my register. HTML form is also spitting

my register. HTML form is also spitting out a tiny little web form of its own

out a tiny little web form of its own it's ugly but I only care about its

it's ugly but I only care about its functionality for now and notice what

functionality for now and notice what I'm doing here every registrant in this

I'm doing here every registrant in this database gets their very own deregister

database gets their very own deregister button and that form has a button that

button and that form has a button that says deregister but notice what else

says deregister but notice what else each of those forms have there's no text

each of those forms have there's no text box there's no drop down menu there's no

box there's no drop down menu there's no radio buttons rather you have a hidden

radio buttons rather you have a hidden input field here so there is a way with

input field here so there is a way with HTML to have a form that will submit

HTML to have a form that will submit information but you don't have to give

information but you don't have to give the user the ability to change that

the user the ability to change that information you can just go ahead and

information you can just go ahead and tuck it inside of the form invisibly if

tuck it inside of the form invisibly if you will hidden in fashion and so what's

you will hidden in fashion and so what's going to happen is if I click the D

going to happen is if I click the D register button next to Carter his

primary key is two mine is instead one so what's going to

mine is instead one so what's going to happen if I click his deregister button

happen if I click his deregister button it submits a form with a

it submits a form with a ID parameter whose value is two and it

ID parameter whose value is two and it submits it to the deregister route so

submits it to the deregister route so what do that mean well if I go to VSS

what do that mean well if I go to VSS code and I go to app.py let's look at

code and I go to app.py let's look at the deregister route that I skipped over

the deregister route that I skipped over so if you access the deregister route

so if you access the deregister route via post this code gets called I grab

via post this code gets called I grab from request form the ID that was

from request form the ID that was submitted in Hidden fashion if there's

submitted in Hidden fashion if there's indeed an ID that is it's not blank it's

indeed an ID that is it's not blank it's not zero it's an actual number like one

not zero it's an actual number like one 2 three or more I execute delete from

2 three or more I execute delete from registrant where ID equals that value

registrant where ID equals that value with a question mark Place holder and

with a question mark Place holder and then I redirect the user back to

then I redirect the user back to registrant now if I go back to this form

registrant now if I go back to this form here I click deregister we'll see that

here I click deregister we'll see that in action gone is now Carter and in fact

in action gone is now Carter and in fact if I go back to my terminal window here

if I go back to my terminal window here I open open up uh SQL light 3 of frost.

I open open up uh SQL light 3 of frost. DB and rerun select star from registrant

DB and rerun select star from registrant Carter is now gone so again using very

Carter is now gone so again using very simple HTML forms you can get buttons

simple HTML forms you can get buttons and links and other such UI mechanisms

and links and other such UI mechanisms to like do things on the server that you

to like do things on the server that you want but there is a danger here this now

want but there is a danger here this now is really meant this example is like an

is really meant this example is like an administrative website like it was some

administrative website like it was some 20 years ago just for us internal staff

20 years ago just for us internal staff to be doing things technically this is

to be doing things technically this is dangerous what I've just done

dangerous what I've just done two even though Carter's ID is two and

two even though Carter's ID is two and hidden and mine is one in Hidden what

hidden and mine is one in Hidden what could this allow an adversary to do if

could this allow an adversary to do if they had admin access to the same

site any thoughts

thoughts yeah

yeah yeah yeah they could change the value of

yeah yeah they could change the value of that hidden attri by opening up chrom's

that hidden attri by opening up chrom's like developer tools change the number

like developer tools change the number in the HTML they could delete anyone

in the HTML they could delete anyone deregister anyone want from the database

deregister anyone want from the database now in this case I claim this is fine

now in this case I claim this is fine because this is only meant for us staff

because this is only meant for us staff who were running Sports back in the day

who were running Sports back in the day but it's indeed a risk so wouldn't it be

but it's indeed a risk so wouldn't it be nice if we could actually ensure that

nice if we could actually ensure that only those users who are authorized are

only those users who are authorized are allowed to execute certain actions I

allowed to execute certain actions I think for this capability we're actually

think for this capability we're actually going to need to introduce something a

going to need to introduce something a bit more and so here of course is an

bit more and so here of course is an opportunity to talk briefly about really

opportunity to talk briefly about really what you and I do all day long every day

what you and I do all day long every day we log into one or more websites or apps

we log into one or more websites or apps or at least until uh you're logged out

or at least until uh you're logged out automatically and you have to do it

automatically and you have to do it again so here for instance is a

again so here for instance is a screenshot of Gmail when you type in

screenshot of Gmail when you type in your username you type in your password

your username you type in your password maybe your to factor code that gets

maybe your to factor code that gets texted or sent to your phone then you're

texted or sent to your phone then you're logged in and thankfully you're not

logged in and thankfully you're not prompted to log in again typically for

prompted to log in again typically for number of hours or days or weeks

number of hours or days or weeks depending on the website like Gmail

depending on the website like Gmail keeps you logged in for ages your bank

keeps you logged in for ages your bank probably logs you out within an hour or

probably logs you out within an hour or so for safety sake so that is completely

so for safety sake so that is completely configurable on the server but how does

configurable on the server but how does g Emil know how does Google know that

g Emil know how does Google know that even as you're checking different mails

even as you're checking different mails again and again and again how do they

again and again and again how do they know that you're still the same person

know that you're still the same person who logged in well it turns out that

who logged in well it turns out that using these same building blocks as

using these same building blocks as today HTTP and HTML and more you can

today HTTP and HTML and more you can actually implement the notion of a login

actually implement the notion of a login feature by doing the equivalent of

feature by doing the equivalent of something with something called cookies

something with something called cookies essentially what happens when you first

essentially what happens when you first log into a website for the very first

log into a website for the very first time successfully with your username and

time successfully with your username and password a cookie so to speak is planted

password a cookie so to speak is planted on your computer and metaphorically this

on your computer and metaphorically this is kind of like taking a hand stamp and

is kind of like taking a hand stamp and your hand is now stamped in this case a

your hand is now stamped in this case a smiley face so that every other time you

smiley face so that every other time you click on a link on that same website

click on a link on that same website google.com gmail.com whatever

google.com gmail.com whatever unbeknownst to you your browser is

unbeknownst to you your browser is constantly presenting that handstamp

constantly presenting that handstamp just like going into a club or an

just like going into a club or an amusement park or something like that

amusement park or something like that where they don't want to check your ID

where they don't want to check your ID again they don't want to check your

again they don't want to check your ticket again they just want to quickly

ticket again they just want to quickly see the same hand stamp and so that is

see the same hand stamp and so that is one of the things that a browser is

one of the things that a browser is always doing for you once you're logged

always doing for you once you're logged in any cookies that have been planted so

in any cookies that have been planted so to speak in your Mac or PC or phone are

to speak in your Mac or PC or phone are constantly represented to the site every

constantly represented to the site every time you click a link or make another

time you click a link or make another request to that website and mechanically

request to that website and mechanically how this works not just metaphorically

how this works not just metaphorically and ink essentially this is what happens

and ink essentially this is what happens here is a example of an HTTP request to

here is a example of an HTTP request to something like Gmail and suppose for

something like Gmail and suppose for instance that you've logged in typically

instance that you've logged in typically as of last week we said that coming back

as of last week we said that coming back from the server would be another virtual

from the server would be another virtual envelope containing like a 200 okay

envelope containing like a 200 okay message and then like the actual web

message and then like the actual web page or the picture of a cat or whatever

page or the picture of a cat or whatever it may be but Google can also if they

it may be but Google can also if they verified that you have some username and

verified that you have some username and password correctly inputed they can do

password correctly inputed they can do the equivalent of stamping your hand and

the equivalent of stamping your hand and the way they do this is they send an

the way they do this is they send an additional line of text in that virtual

additional line of text in that virtual envelope from the server to your browser

envelope from the server to your browser literally using another HTTP header not

literally using another HTTP header not content type which just mundanely tells

content type which just mundanely tells you what kind of content has come back

you what kind of content has come back they literally send an http header

they literally send an http header called set-cookie and then they set a

called set-cookie and then they set a key value pair on your Mac or PC this is

key value pair on your Mac or PC this is the technical equivalent of this smiley

the technical equivalent of this smiley face handstamp and what your computer is

face handstamp and what your computer is designed to do because your computer and

designed to do because your computer and intern browser are supposed to implement

intern browser are supposed to implement HTTP anytime you click another link on

HTTP anytime you click another link on Gmail or click on another mail or the

Gmail or click on another mail or the like your browser unbeknownst to you

like your browser unbeknownst to you presents that handstamp and how it does

presents that handstamp and how it does it technically is in the envelope it

it technically is in the envelope it sends to Google from your browser it

sends to Google from your browser it doesn't send set cookie it just sends

doesn't send set cookie it just sends cookie colon and the exact same thing

cookie colon and the exact same thing and so long as Google is smart and they

and so long as Google is smart and they have a database or something of all of

have a database or something of all of the session Valu session is the

the session Valu session is the technical term for this maintenance of

technical term for this maintenance of information across HTTP so long as

information across HTTP so long as Google has a big database that knows

Google has a big database that knows that my cookie value is one 123 and your

that my cookie value is one 123 and your cookie value is 456 they can infer from

cookie value is 456 they can infer from this virtual handstamp whose emails they

this virtual handstamp whose emails they should be showing mine or yours or the

should be showing mine or yours or the like so this is just scratching the

like so this is just scratching the surface but if I really wanted to enable

surface but if I really wanted to enable only Carter to deregister himself I just

only Carter to deregister himself I just have to make sure that I log him in

have to make sure that I log him in somehow username password and all of

somehow username password and all of that I stamp his hand or really put a

that I stamp his hand or really put a cookie on his computer and only if his

cookie on his computer and only if his cookie lines up with the user ID he's

cookie lines up with the user ID he's trying to deregister should he be

trying to deregister should he be allowed to in fact do so so all of this

allowed to in fact do so so all of this is quite possible and indeed the

is quite possible and indeed the technical term for this is session and

technical term for this is session and what we thought we'd do in our remaining

what we thought we'd do in our remaining time Today show you some examples of

time Today show you some examples of exactly how some of the most familiar

exactly how some of the most familiar web functionality is implemented today

web functionality is implemented today some of which you'll use in your own uh

some of which you'll use in your own uh problem set 9 uh which itself will be a

problem set 9 uh which itself will be a web app or perhaps even your final

web app or perhaps even your final project so let me go ahead and do this

project so let me go ahead and do this let me close my previous tabs and all

let me close my previous tabs and all things for ashs and let's move on to

things for ashs and let's move on to implementing some notion of login so in

implementing some notion of login so in just a moment I'll switch over here to

just a moment I'll switch over here to VSS code and what I'm going to do is

VSS code and what I'm going to do is indeed in my source 9 directory I'm

indeed in my source 9 directory I'm going to go into a login directory and

going to go into a login directory and if I type LS here you'll see app.py

if I type LS here you'll see app.py requirements. text which just refers to

requirements. text which just refers to libraries I want to automatically

libraries I want to automatically install and a templates folder as well

install and a templates folder as well I'm going to go ahead and stop the

I'm going to go ahead and stop the previous server and close that terminal

previous server and close that terminal window and I'm going to open up this

window and I'm going to open up this version of

version of app.py so there's a few new lines here

app.py so there's a few new lines here and we'll give you these lines for

and we'll give you these lines for problems at nine but I've got some of

problems at nine but I've got some of the familiar stuff up here including

the familiar stuff up here including this new redirect function we just used

this new redirect function we just used and I have a session variable that comes

and I have a session variable that comes with flask 2 so what's nice again about

with flask 2 so what's nice again about flask is that it deals with all of this

flask is that it deals with all of this cookie stuff for you it sets the cookie

cookie stuff for you it sets the cookie it checks the cookie and what flask does

it checks the cookie and what flask does for you is it gives you the abstraction

for you is it gives you the abstraction of a variable called session so that

of a variable called session so that anything you put in the session variable

anything you put in the session variable which itself is a dictionary will be

which itself is a dictionary will be there again and again and again whenever

there again and again and again whenever that same user comes back a session is

that same user comes back a session is how you implement essentially like the

how you implement essentially like the proverbial shopping cart uh if I'm

proverbial shopping cart uh if I'm logged into Amazon you're logged into

logged into Amazon you're logged into Amazon Amazon knows which of us is which

Amazon Amazon knows which of us is which by way of that cookie and Amazon if

by way of that cookie and Amazon if they're using flask provides the

they're using flask provides the programmer with a dictionary called

programmer with a dictionary called session and flasks make sure that when

session and flasks make sure that when Carter is visiting the site the code

Carter is visiting the site the code uses his session object when I'm

uses his session object when I'm visiting the site it uses my session

visiting the site it uses my session object but it's all implemented with

object but it's all implemented with those same cookies this is the same as

those same cookies this is the same as before these lines are new and you'll

before these lines are new and you'll see these in problem Set n this is how

see these in problem Set n this is how we enable sessions in a web application

we enable sessions in a web application and I'll just wave my hands at the

and I'll just wave my hands at the detail there's different ways to

detail there's different ways to implement sessions whether you use

implement sessions whether you use cookies on the server cookies on the

cookies on the server cookies on the browser or other things these just

browser or other things these just ensure that we're storing the the

ensure that we're storing the the session information the shopping cart on

session information the shopping cart on the server itself now down here let's go

the server itself now down here let's go ahead and do this let's go ahead and run

ahead and do this let's go ahead and run flask run so I can see what this app

flask run so I can see what this app does if I do this and visit the URL that

does if I do this and visit the URL that gets outputed you'll see a very simple

gets outputed you'll see a very simple web page here and if I type in for

web page here and if I type in for instance my name I'm not going to bother

instance my name I'm not going to bother with a password and click log in you'll

with a password and click log in you'll see that you are logged in as David and

see that you are logged in as David and now I can log out so I'm going to go

now I can log out so I'm going to go ahead and click log out and now it seems

ahead and click log out and now it seems to know that I'm not logged in again I

to know that I'm not logged in again I can log in as Carter because I didn't

can log in as Carter because I didn't bother implementing passwords for

bother implementing passwords for Simplicity but now the site knows I'm

Simplicity but now the site knows I'm logged in as Carter better yet if I

logged in as Carter better yet if I reload reload reload or click this

reload reload reload or click this button again and again notice it still

button again and again notice it still knows that I'm Carter uh until such time

knows that I'm Carter uh until such time as I log out all right well how is this

as I log out all right well how is this working well let's go back to vs code

working well let's go back to vs code here and let me scroll down to first

here and let me scroll down to first this route this is a very common

this route this is a very common Paradigm here whereby I'm checking for

Paradigm here whereby I'm checking for the index route if there is not a name

the index route if there is not a name in the session redirect the user to SL

in the session redirect the user to SL login now what does that mean well let

me go back to vs code here let me go to the slash route so again your url will

the slash route so again your url will be different but I'm just going to go to

be different but I'm just going to go to slash and hit enter notice that I got

slash and hit enter notice that I got automatically redirected to login and so

automatically redirected to login and so many websites do this if you go to a

many websites do this if you go to a website and you're not logged in you're

website and you're not logged in you're very often redirected to SL login or SL

very often redirected to SL login or SL account or something like that where

account or something like that where you're prompted the code for doing that

you're prompted the code for doing that is right here if there's no name in the

is right here if there's no name in the session if there's no name in the

session if there's no name in the shopping cart if you will go ahead and

shopping cart if you will go ahead and return the user to login that route

return the user to login that route otherwise implicitly if they are logged

otherwise implicitly if they are logged in show them index.html so let's go down

in show them index.html so let's go down that rabbit hole let me open up in VSS

that rabbit hole let me open up in VSS code a second terminal window let me go

code a second terminal window let me go into Source 9 and go into this same

into Source 9 and go into this same login demonstration and let me open up

the template called index.html and here's all this is

index.html and here's all this is there's some layout but who cares at

there's some layout but who cares at this point it's just the boiler plate

this point it's just the boiler plate generic HTML here's the body block I

generic HTML here's the body block I have this and this two is ginger because

have this and this two is ginger because of the curly brace and the percent sign

of the curly brace and the percent sign if there's a name in the session this is

if there's a name in the session this is just python syntax then say this

just python syntax then say this sentence you are logged in as whatever

sentence you are logged in as whatever name is in the session in the shopping

name is in the session in the shopping cart if you will and then I just have

cart if you will and then I just have this HTML link for logging the user out

this HTML link for logging the user out else if there is no name in the session

else if there is no name in the session logically just say you are not logged in

logically just say you are not logged in and give them a manual link for logging

and give them a manual link for logging in instead so that's all this particular

in instead so that's all this particular template does but how does the slash

template does but how does the slash login work well let's go into this other

template code of login. HTML to which I'm redirected super simple this is just

I'm redirected super simple this is just copy paste from HTML before I've got a

copy paste from HTML before I've got a login form that's going to have an

action of Slash login submits for privacy sake just via post and then the

privacy sake just via post and then the rest of this is just a simple form and

rest of this is just a simple form and I'm using using an input type equals

I'm using using an input type equals submit instead of button equals type

submit instead of button equals type equals submit but same idea here too and

equals submit but same idea here too and if I go back to app.py well let's see

if I go back to app.py well let's see how login works all right it's a lot all

how login works all right it's a lot all at once but they're relatively simple

at once but they're relatively simple reapplications of the same idea so if

reapplications of the same idea so if the user visits SL login via get or post

the user visits SL login via get or post call this function login if the user has

call this function login if the user has submitted via post and we saw this

submitted via post and we saw this technique before go ahead and do this on

technique before go ahead and do this on line 23 store in this special session

line 23 store in this special session variable that comes with flask a name

variable that comes with flask a name key and store in it the user's own name

key and store in it the user's own name so quote unquote name will have a value

so quote unquote name will have a value of David or Carter or the like and as

of David or Carter or the like and as soon as you do that redirect the user

soon as you do that redirect the user back to slash just so they see the

back to slash just so they see the homepage again and this is how Amazon

homepage again and this is how Amazon and all these other websites work too

and all these other websites work too otherwise if they visit this page

otherwise if they visit this page implicitly via get and even though I

implicitly via get and even though I didn't say equals equals get anywhere

didn't say equals equals get anywhere that's sort of the implication because

that's sort of the implication because if you can only get here via get or post

if you can only get here via get or post and we already handled post logically

and we already handled post logically All That Remains is get well then just

All That Remains is get well then just show them the login screen instead but

show them the login screen instead but there's half a dozen ways we could

there's half a dozen ways we could express that same logic and then for log

express that same logic and then for log out this is kind of straightforward if

out this is kind of straightforward if the user clicks that log out link and

the user clicks that log out link and ends up at SL logout this route well

ends up at SL logout this route well just change the value of that key in the

just change the value of that key in the session to be none effectively no Carter

session to be none effectively no Carter is gone David's gone there's no one

is gone David's gone there's no one logged in so that is all that's required

logged in so that is all that's required to actually implement the notion of

to actually implement the notion of logging in and logging out of website

logging in and logging out of website plus the password thing which should

plus the password thing which should probably involve a database but one

probably involve a database but one thing at a time and really session is

thing at a time and really session is sort of like the code version of a

sort of like the code version of a shopping cart whereby if I visit the

shopping cart whereby if I visit the same code I get my own session object if

same code I get my own session object if Carter visits the website he gets his

Carter visits the website he gets his own session object and the way flask

own session object and the way flask Keeps Us straight is they put one cookie

Keeps Us straight is they put one cookie on my computer a different cookie on his

on my computer a different cookie on his computer and uses those to line up with

computer and uses those to line up with making sure the right session gets shown

making sure the right session gets shown to the right actual

to the right actual user questions on this notion of

sessions no all right how about a couple final examples just to tie this all

final examples just to tie this all together let me go back into vs code

together let me go back into vs code here let me quit my previous version of

here let me quit my previous version of flask let's go into Source 9 and go into

flask let's go into Source 9 and go into store which is a separate app Al

store which is a separate app Al together and let's start by just running

together and let's start by just running flask to see what it does let's hover

flask to see what it does let's hover over the URL and open it in another Tab

over the URL and open it in another Tab and this is pretty ugly too let me zoom

and this is pretty ugly too let me zoom in but it's a very simple bookstore like

in but it's a very simple bookstore like an early amazon.com for each of these

an early amazon.com for each of these seven books here Each of which seems to

seven books here Each of which seems to be like maybe this is H1 this is H2 H2

be like maybe this is H1 this is H2 H2 H2 H2 and then there's a button

H2 H2 and then there's a button underneath each well now let's use this

underneath each well now let's use this as an opportunity to kind of infer like

as an opportunity to kind of infer like for any website how this thing works let

for any website how this thing works let me go ahead and do view page source and

me go ahead and do view page source and you can do this for any website on the

you can do this for any website on the internet let's try to figure out how

internet let's try to figure out how this bookstore adds things to a cart

this bookstore adds things to a cart well here's the H1 tag uninteresting H2

well here's the H1 tag uninteresting H2 H2 H2 so the juicy part is in these

H2 H2 so the juicy part is in these forms each of these forms has an action

forms each of these forms has an action of SL cart so that's the route that's

of SL cart so that's the route that's going to be interesting in a moment and

going to be interesting in a moment and it uses post for privacy sake each of

it uses post for privacy sake each of these forms like the deregister feature

these forms like the deregister feature for Carter has an ID attribute an ID

for Carter has an ID attribute an ID parameter that's hidden visually that

parameter that's hidden visually that has a value of one or two or three so

has a value of one or two or three so like the unique uh uh like barcodes for

like the unique uh uh like barcodes for the books if you will but super small

the books if you will but super small numbers in our case and then each of

numbers in our case and then each of these other forms just had each of these

these other forms just had each of these other books has an identical form except

other books has an identical form except for the value of this year now in this

for the value of this year now in this case this isn't such a big deal that a

case this isn't such a big deal that a user could technically hack the HTML of

user could technically hack the HTML of this bookstore amazon.com and change the

this bookstore amazon.com and change the IDS because whoa what's the worst

IDS because whoa what's the worst they're going to do like buy more books

they're going to do like buy more books by adding more IDs to their shopping

by adding more IDs to their shopping cart like that's not a problem there's

cart like that's not a problem there's no prices here it's just the unique

no prices here it's just the unique ideas of books so whereas the deregister

ideas of books so whereas the deregister was maybe worrisome because you're

was maybe worrisome because you're changing the server I think it's okay

changing the server I think it's okay because the user can only at worst buy

because the user can only at worst buy more books than then uh they might via

more books than then uh they might via the buttons alone so how does this now

the buttons alone so how does this now work well let's go into vs code again

work well let's go into vs code again here let me give myself another terminal

here let me give myself another terminal window and in Source 9/ store let me

window and in Source 9/ store let me open up app.py which is where all the

open up app.py which is where all the logic is so I'll flip through most of

logic is so I'll flip through most of this quickly because we've seen this

this quickly because we've seen this before these Imports are pretty much the

before these Imports are pretty much the same as before this line is the same

same as before this line is the same this line is almost the same but the

this line is almost the same but the database now is called store. DB instead

database now is called store. DB instead of frost. DB this is the boilerplate

of frost. DB this is the boilerplate code for just enabling sessions this

code for just enabling sessions this notion of a shopping cart and so let's

notion of a shopping cart and so let's see how the index Works how is that I'm

see how the index Works how is that I'm seeing all seven books at once well in

seeing all seven books at once well in this index function I'm using on line 19

this index function I'm using on line 19 select star from books to get all the

select star from books to get all the books from the database and then I'm

books from the database and then I'm rendering a template called books. HTML

rendering a template called books. HTML passing in as a placeholder all of those

passing in as a placeholder all of those books all right let's go down that

books all right let's go down that rabbit hole for a second let me open up

rabbit hole for a second let me open up books. HTML in my templates directory

books. HTML in my templates directory and here again even though it's you know

and here again even though it's you know new today it's probably increasingly

new today it's probably increasingly familiar syntactically here's the H1

familiar syntactically here's the H1 here is my Loop here's the H2 which is

here is my Loop here's the H2 which is going to Output the current book's title

going to Output the current book's title in that Loop here's the form here's the

in that Loop here's the form here's the ID of that book that's going to be

ID of that book that's going to be outputed in the form so I didn't

outputed in the form so I didn't manually type out seven long forms I

manually type out seven long forms I just did one in this template so that it

just did one in this template so that it gets generated automatically literally

gets generated automatically literally what a website CL Amazon would do to

what a website CL Amazon would do to show you 10 books at a time or 10 search

show you 10 books at a time or 10 search results or more at a time all right well

results or more at a time all right well let's go to SLC cart which was the juicy

let's go to SLC cart which was the juicy one and this one's longer but let's see

one and this one's longer but let's see if we can reason through it if the user

if we can reason through it if the user submits via get or post to/ cart well we

submits via get or post to/ cart well we first do this and this is just necessary

first do this and this is just necessary in some boilerplate rewrote we're going

in some boilerplate rewrote we're going to use the session object to store a

to use the session object to store a variable called cart that in this case

variable called cart that in this case is going to be a list so session again

is going to be a list so session again is just a dictionary you can put

is just a dictionary you can put anything in it you want previously we

anything in it you want previously we put students names and sports now uh

put students names and sports now uh what I want to do is actually sorry prev

what I want to do is actually sorry prev previously we put uh the student name

previously we put uh the student name the user's name in it now I'm going to

the user's name in it now I'm going to actually store a cart key whose value is

actually store a cart key whose value is a list why because I want to aggregate

a list why because I want to aggregate more and more books in this list all

more and more books in this list all right so that just makes sure that I

right so that just makes sure that I have at least an empty shopping cart the

have at least an empty shopping cart the very first time the user does this if

very first time the user does this if they visit this form via post let's go

they visit this form via post let's go ahead and get the ID of the book that

ahead and get the ID of the book that they posted if it is not empty if there

they posted if it is not empty if there is a number like one or two or three

is a number like one or two or three let's go ahead and go into the shopping

let's go ahead and go into the shopping C cart which is a list per this line and

C cart which is a list per this line and just append that ID so this list of

just append that ID so this list of books in your shopping cart is going to

books in your shopping cart is going to contain like one comma 2 comma four

contain like one comma 2 comma four comma six whatever books you're actually

comma six whatever books you're actually buying and then the user gets redirected

buying and then the user gets redirected to cart what if though the user got here

to cart what if though the user got here via get and not post well this one's

via get and not post well this one's relatively straightforward if you just

relatively straightforward if you just visit slart we select star from books

visit slart we select star from books where ID is in okay so this is

where ID is in okay so this is interesting this list and and this is a

interesting this list and and this is a syntax you might not have seen before

syntax you might not have seen before but if you read the documentation for

but if you read the documentation for cs50's Library if you select something

cs50's Library if you select something and use a question mark placeholder and

and use a question mark placeholder and the placeholder itself is a list we

the placeholder itself is a list we output a comma separated list of values

output a comma separated list of values just like you would use maybe in problem

just like you would use maybe in problem Set uh seven for doing SQL queries on

Set uh seven for doing SQL queries on your own so this just means show me only

your own so this just means show me only those books in my shopping cart not

those books in my shopping cart not Carters not someone else's not in the

Carters not someone else's not in the whole database only show me the books in

whole database only show me the books in my shopping cart and then

my shopping cart and then render it as such so we only saw what

render it as such so we only saw what the catalog here looks like at slash

the catalog here looks like at slash books Let's go ahead and in slash let's

books Let's go ahead and in slash let's go ahead and add maybe the first book to

go ahead and add maybe the first book to my cart and now I see at SLC cart only

my cart and now I see at SLC cart only that first book whose ID is one let me

that first book whose ID is one let me now go back to the bookstore here scroll

now go back to the bookstore here scroll down to maybe the seventh book and add

down to maybe the seventh book and add that to my cart and now I see this here

that to my cart and now I see this here too meanwhile all of this information is

too meanwhile all of this information is stored in my session and so when I

stored in my session and so when I reload this cart again and again the

reload this cart again and again the reason I'm only seeing my two is because

reason I'm only seeing my two is because we're checking only the list in my

we're checking only the list in my session and flasks make sure again that

session and flasks make sure again that my session is different from your

my session is different from your session is different from Carter's

session is different from Carter's session as well but you write the code

session as well but you write the code once and it works for thousands millions

once and it works for thousands millions of people in

of people in parallel any questions on

parallel any questions on this this yes in the

this this yes in the back sorry say a little louder

uh so to recap so users will never have the same session values theoretically

the same session values theoretically the cookie that gets planted does not

the cookie that gets planted does not look like a smiley face for everyone

look like a smiley face for everyone each of us gets a big random number

each of us gets a big random number that's assigned to us so it' be like

that's assigned to us so it' be like each of us gets a completely unique hand

each of us gets a completely unique hand stamp that no one else can see the

stamp that no one else can see the reason no one else can see it is because

reason no one else can see it is because if the website's using https every time

if the website's using https every time this hand stamp is shown every time this

this hand stamp is shown every time this cookie is sent back and forth It's all

cookie is sent back and forth It's all encrypted as well so each of us can even

encrypted as well so each of us can even if we have the same contents by

if we have the same contents by coincidence because we like the same

coincidence because we like the same books they will be separate cookies

books they will be separate cookies separate memory separate sessions behind

separate memory separate sessions behind you

yeah really good question when does the session end totally configurable

session end totally configurable typically it ends when you close the tab

typically it ends when you close the tab or when you quit the browser or you can

or when you quit the browser or you can also configure cookies to themselves be

also configure cookies to themselves be persistent for a day for a week for

persistent for a day for a week for longer so for instance when you log into

longer so for instance when you log into um uh say Gmail they plant a cookie on

um uh say Gmail they plant a cookie on your computer probably for a week a

your computer probably for a week a month a year something like that because

month a year something like that because it would be annoying and probably drive

it would be annoying and probably drive you to like Outlook or something else if

you to like Outlook or something else if you kept having to log into your account

you kept having to log into your account whereas your bank account might actually

whereas your bank account might actually wait for you to just close the tab and

wait for you to just close the tab and then for your own Financial safety they

then for your own Financial safety they just automatically delete the session

just automatically delete the session far sooner but totally configurable by

far sooner but totally configurable by default as I'm using it it will

default as I'm using it it will typically be thrown away when the

typically be thrown away when the browser itself quits and here too is

browser itself quits and here too is another reason to develop websites using

another reason to develop websites using incognito mode because if you want to

incognito mode because if you want to just throw throw away all of your

just throw throw away all of your cookies you close the incognito window

cookies you close the incognito window mode open a new one and now you're

mode open a new one and now you're starting from scratch you don't have to

starting from scratch you don't have to manually delete all your cookies which

manually delete all your cookies which could log you out of websites you

could log you out of websites you actually care about

yeah a good question when using sessions if someone maliciously changes the value

if someone maliciously changes the value of sub forms could it affect other

of sub forms could it affect other people theoretically no because the

people theoretically no because the worst you can do is like add books to

worst you can do is like add books to your own shopping cart that you don't

your own shopping cart that you don't want there so at that point even though

want there so at that point even though it's on the server it doesn't affect you

it's on the server it doesn't affect you or Carter or anyone else unless there is

or Carter or anyone else unless there is something more globally happening like

something more globally happening like registering or deregistering for a sport

registering or deregistering for a sport or removing books from the Amazon

or removing books from the Amazon database that would be problematic but

database that would be problematic but in this case we're removing things only

in this case we're removing things only from my own session that the website is

from my own session that the website is giving me all right the last topic for

giving me all right the last topic for today is this thing here which is sort

today is this thing here which is sort of everywhere nowadays these things

of everywhere nowadays these things called apis or application programming

called apis or application programming interfaces and this is a very generic

interfaces and this is a very generic term in fact because any function you've

term in fact because any function you've used in C in scratch in Python in SQL

used in C in scratch in Python in SQL are all apis like there is a standard

are all apis like there is a standard way of interfacing with those functions

way of interfacing with those functions they all have names they sometimes have

they all have names they sometimes have return values they sometimes have

return values they sometimes have arguments and an API is just how you use

arguments and an API is just how you use a function or more generally an API just

a function or more generally an API just specifies how you interact with some

specifies how you interact with some service and so nowadays there's a lot of

service and so nowadays there's a lot of web-based services that you can use to

web-based services that you can use to get back data like the weather or the

get back data like the weather or the current time or the database of Amazon

current time or the database of Amazon books for instance all might have apis

books for instance all might have apis often web-based that allow you using

often web-based that allow you using URLs or some other technology to just

URLs or some other technology to just get data from someone else as though

get data from someone else as though it's a function you're calling remotely

it's a function you're calling remotely but HTTP is very often the mechanism

but HTTP is very often the mechanism that's used to actually get data from

that's used to actually get data from servers and the way the data can come

servers and the way the data can come back can be as follows let me end with

back can be as follows let me end with one final example using some of our

one final example using some of our familiar shows from uh weeks past let me

familiar shows from uh weeks past let me go ahead and close the old flask version

go ahead and close the old flask version version go back into Source 9 and go

version go back into Source 9 and go into how about an example called shows

into how about an example called shows and the first version of this zero I'm

and the first version of this zero I'm just going to go ahead and run with

just going to go ahead and run with flask run I'll hover over my URL and

flask run I'll hover over my URL and open it here and you'll see now that I

open it here and you'll see now that I have a very simple form as we keep doing

have a very simple form as we keep doing today I'm going to type in like o f f i

today I'm going to type in like o f f i c office into this search box and click

c office into this search box and click search and you'll see now that I ended

search and you'll see now that I ended up at a URL ending in/ search question

up at a URL ending in/ search question mark Q equals office so this is like my

mark Q equals office so this is like my own baby version of google.com but I

own baby version of google.com but I implemented it myself and for any title

implemented it myself and for any title of a TV show from a couple of weeks past

of a TV show from a couple of weeks past that matches o i I spit it out into an

that matches o i I spit it out into an unordered list how is this working you

unordered list how is this working you can maybe imagine even if you might not

can maybe imagine even if you might not be able to program this off the top of

be able to program this off the top of your head certainly so soon let me go

your head certainly so soon let me go into Source 9 let me go into uh show

into Source 9 let me go into uh show zero let me open up

zero let me open up app.py and in this file you'll see that

app.py and in this file you'll see that that I'm grabbing a uh file called

that I'm grabbing a uh file called shows. DB which is like a simpler

shows. DB which is like a simpler version of the one from a couple of

version of the one from a couple of weeks past uh here is why I see the web

weeks past uh here is why I see the web form my first route my index is super

form my first route my index is super simple it just spits out that form and

simple it just spits out that form and my search route like you can think of

my search route like you can think of this as google.com there only like four

this as google.com there only like four lines of code so if the user sends data

lines of code so if the user sends data to SL search this function called search

to SL search this function called search is called I declare a variable called

is called I declare a variable called shows I execute a SQL command that is

shows I execute a SQL command that is Select star from shows where title like

Select star from shows where title like question mark and the syntax here is a

question mark and the syntax here is a little crazy but I want to prefix to the

little crazy but I want to prefix to the user's input percent sign and suffix it

user's input percent sign and suffix it with a percent sign as well putting in

with a percent sign as well putting in the between those two values the actual

the between those two values the actual input why in SQL what is it mean if you

input why in SQL what is it mean if you have a percent sign to the left and to

have a percent sign to the left and to the right nothing to do with Ginger

the right nothing to do with Ginger today yeah it's a it's a wild card so it

today yeah it's a it's a wild card so it means match zero or more characters on

means match zero or more characters on the left or match zero or more

the left or match zero or more characters on the right you have to do

characters on the right you have to do the concatenation

the concatenation as the second argument to this function

as the second argument to this function you can't do something clever like put

you can't do something clever like put it here around the like the is the

it here around the like the is the placeholder that you plug these values

placeholder that you plug these values into but this just means hey SQL show me

into but this just means hey SQL show me all of the titles that have o f i

all of the titles that have o f i somewhere in them that gives me back an

somewhere in them that gives me back an a list of dictionaries I pass that in as

a list of dictionaries I pass that in as a placeholder for a variable called

a placeholder for a variable called shows and if we look at search. HTML

shows and if we look at search. HTML let's look at that in my templates

let's look at that in my templates directory there's something called

directory there's something called search. HTML super simple I mean this is

search. HTML super simple I mean this is like the essence of google.com search

like the essence of google.com search results I'm using an unordered list to

results I'm using an unordered list to keep things simple but I iterate over

keep things simple but I iterate over every show in the shows list that came

every show in the shows list that came back and I output An Li with each of

back and I output An Li with each of those shows titles and that's it now

those shows titles and that's it now Google has like Blue Links and like

Google has like Blue Links and like little previews and other text the first

little previews and other text the first sentence or so from each page but like

sentence or so from each page but like that's the idea like this is really

that's the idea like this is really similar in spirit to what google.com

similar in spirit to what google.com search does for you now how is this

search does for you now how is this working there's no API involved here yet

working there's no API involved here yet this is just very basic HTTP I submit

this is just very basic HTTP I submit the the form I go to another route and I

the the form I go to another route and I get back the results but check out this

get back the results but check out this version Let me close these tabs here and

version Let me close these tabs here and open my first terminal window let me go

open my first terminal window let me go into shows one from today's Source 9

into shows one from today's Source 9 directory and do flask run this time let

directory and do flask run this time let me go ahead and hover over that URL and

me go ahead and hover over that URL and open it here and gone now is the submit

open it here and gone now is the submit button now I'm going to make an user

button now I'm going to make an user interface that uses a technique called

interface that uses a technique called Ajax for asynchronous uh JavaScript and

Ajax for asynchronous uh JavaScript and XML which is somewhat of a data term

XML which is somewhat of a data term because we're not using something called

because we're not using something called ml anymore but Ajax is a technique

ml anymore but Ajax is a technique whereby you don't have to submit forms

whereby you don't have to submit forms anymore to get more data from the server

anymore to get more data from the server you can use JavaScript per last week

you can use JavaScript per last week listen for an event like the key press

listen for an event like the key press coming down or up and as soon as you

coming down or up and as soon as you hear such an event you can secretly in

hear such an event you can secretly in JavaScript code send a request to the

JavaScript code send a request to the server to get back more data and then

server to get back more data and then plug it into the Dom the tree in the

plug it into the Dom the tree in the computer's memory and this just makes

computer's memory and this just makes for more seamless experiences like

for more seamless experiences like autocomplete on any website so now let

autocomplete on any website so now let me try typing o okay we got auto

me try typing o okay we got auto complete super fast f f i c e and you'll

complete super fast f f i c e and you'll see every time I add more keys to my

see every time I add more keys to my input I'm doing another search another

input I'm doing another search another search another search and the data is

search another search and the data is changing now how is this working well

changing now how is this working well let me go back to vs code here and in my

let me go back to vs code here and in my other terminal window let me open up uh

other terminal window let me open up uh app.py and in app.py you'll see that

app.py and in app.py you'll see that there's still a

there's still a search route down below that turns a

search route down below that turns a search template but watch this let me go

search template but watch this let me go into templates search. HTML and notice

into templates search. HTML and notice here that we're indeed getting back an

here that we're indeed getting back an unordered list of shows again and again

unordered list of shows again and again and again and this HTML that's coming

and again and this HTML that's coming back let me go here let me open my

back let me go here let me open my terminal oh sorry this is the wrong

terminal oh sorry this is the wrong version uh sorry I was in the wrong

version uh sorry I was in the wrong folder let's fix this and shows one code

folder let's fix this and shows one code of app.py it's almost the same thing

of app.py it's almost the same thing inserch

inserch here okay well I changed this slightly

here okay well I changed this slightly let me show you this version of search

let me show you this version of search if I open up app.py here's my search

if I open up app.py here's my search route I'm getting a variable called Q

route I'm getting a variable called Q giving it the value of whatever request.

giving it the value of whatever request. RX has from the user like Q equals

RX has from the user like Q equals office and then I'm checking if the user

office and then I'm checking if the user actually typed something in execute this

actually typed something in execute this SQL query select star from shows where

SQL query select star from shows where title is like that using the same and

title is like that using the same and this time just to keep things efficient

this time just to keep things efficient I limited the total results to 50

I limited the total results to 50 instead of an infinite number otherwise

instead of an infinite number otherwise if the user type nothing just to be

if the user type nothing just to be super safe here I'm setting shows equal

super safe here I'm setting shows equal to an empty list so if you don't type

to an empty list so if you don't type anything there's nothing to show and no

anything there's nothing to show and no matter what I render this template

matter what I render this template called search. HTML well let's look at

called search. HTML well let's look at that if I open up templat search. HTML

that if I open up templat search. HTML this time there's no layout there's no

this time there's no layout there's no inheritance of that layout. HTML I am

inheritance of that layout. HTML I am literally generating just a whole bunch

literally generating just a whole bunch of Li fragments why well let's see what

of Li fragments why well let's see what what's happening in the browser and this

what's happening in the browser and this is the beginning of an API if I wanted

is the beginning of an API if I wanted to make this API available this

to make this API available this application programming interface I

application programming interface I could tell the world that if you want to

could tell the world that if you want to search for TV shows in my database go to

search for TV shows in my database go to URL that's something something/ search

URL that's something something/ search question mark Q equals office or cat or

question mark Q equals office or cat or dog or anything else and what I will

dog or anything else and what I will return to you is this enter I will just

return to you is this enter I will just give you a whole bunch of Li tags which

give you a whole bunch of Li tags which almost looks the same but let me view

almost looks the same but let me view the page source only am I going to hand

the page source only am I going to hand you back a fragment of HTML I'm not

you back a fragment of HTML I'm not giving you an HTML tag a body tag a

giving you an HTML tag a body tag a title tag a head tag I'm not giving you

title tag a head tag I'm not giving you a web page I'm giving you a fragment of

a web page I'm giving you a fragment of HTML that you can now do whatever you

HTML that you can now do whatever you want including insert this into your own

want including insert this into your own unordered list so notice what happens in

unordered list so notice what happens in this actual app if I go back to vs code

this actual app if I go back to vs code here let me open up my index template

here let me open up my index template here and you'll see some JavaScript

here and you'll see some JavaScript magic so in JavaScript here in my form

magic so in JavaScript here in my form that only had the text box and no button

that only had the text box and no button what am I doing in a script tag here I

what am I doing in a script tag here I am creating a variable called input and

am creating a variable called input and I'm using this function called query

I'm using this function called query selector that just gets me a reference

selector that just gets me a reference to the input text box on the form so I

to the input text box on the form so I can see what the human typed this is a

can see what the human typed this is a little different today but I'm using

little different today but I'm using input. atevent listener which is a way

input. atevent listener which is a way in JavaScript to tell it just like in

in JavaScript to tell it just like in scratch listen for something to happen

scratch listen for something to happen like the green flag being clicked but in

like the green flag being clicked but in this case listen for an event that

this case listen for an event that involves input that is like typing on

involves input that is like typing on the keyboard whether it's by a copy

the keyboard whether it's by a copy paste manual input or anything else then

paste manual input or anything else then whenever that happens call this function

whenever that happens call this function and async stands for asynchronous this

and async stands for asynchronous this is a term of art which means that this

is a term of art which means that this this function might take like a split

this function might take like a split second maybe even a second or two to

second maybe even a second or two to execute so it's going to do it behind

execute so it's going to do it behind the scenes like in the background so to

the scenes like in the background so to speak and what is it going to do well

speak and what is it going to do well it's going to call a JavaScript function

it's going to call a JavaScript function that all browsers now Support called

that all browsers now Support called Fetch which is a function that uses HTTP

Fetch which is a function that uses HTTP to go fetch more data via the from the

to go fetch more data via the from the server it's going to fetch data from a

server it's going to fetch data from a route called SL search question mark Q

route called SL search question mark Q equals and whatever the value is that

equals and whatever the value is that the user typed in so I'm just sort of

the user typed in so I'm just sort of manually creating my own mini URL and

manually creating my own mini URL and telling JavaScript go fetch me that HTML

telling JavaScript go fetch me that HTML when it comes back via this line of text

when it comes back via this line of text here called response. text and let me

here called response. text and let me wave my hand at await await just means

wave my hand at await await just means this might not come back immediately

this might not come back immediately let's await the response and when it

let's await the response and when it does come and then let's execute this

does come and then let's execute this code I'm going to do this I'm going to

code I'm going to do this I'm going to search the document the whole web page

search the document the whole web page for this UL tag which is somewhere in

for this UL tag which is somewhere in this page that we'll see in a moment

this page that we'll see in a moment change its inner HTML to be that

change its inner HTML to be that fragment of Li Li Li of all of those

fragment of Li Li Li of all of those matching shows and where does this all

matching shows and where does this all go well if we scroll up here you'll

go well if we scroll up here you'll notice that there's my usual HTML up at

notice that there's my usual HTML up at top head tag body tag and all of that

top head tag body tag and all of that there's the text box that we've talked

there's the text box that we've talked about already there's no button for

about already there's no button for submitting cuz it all happens

submitting cuz it all happens automatically but there is by default

automatically but there is by default this empty UL that has nothing by

this empty UL that has nothing by default until we start using that API

default until we start using that API and the final flourish here is this this

and the final flourish here is this this is kind of a uh ugly sloppy way to get

is kind of a uh ugly sloppy way to get data from a server to just get back like

data from a server to just get back like a fragment of HTML like what if I'm not

a fragment of HTML like what if I'm not using HTML I want to store these uh TV

using HTML I want to store these uh TV shows in a PDF or in some other wet tag

shows in a PDF or in some other wet tag in a table or something like that it

in a table or something like that it doesn't really make sense for the server

doesn't really make sense for the server to be presuming that I want Li tags

to be presuming that I want Li tags surrounding each of my data better would

surrounding each of my data better would be to get a more generic format back and

be to get a more generic format back and that format is almost always nowadays

that format is almost always nowadays called this our final acronym Json

called this our final acronym Json JavaScript object notation and let me do

JavaScript object notation and let me do this let me close these two tabs here

this let me close these two tabs here and open my terminal window and C cancel

and open my terminal window and C cancel the previous version of flask that was

the previous version of flask that was running let me close this version and

running let me close this version and look at our final version called shows

look at our final version called shows two and do flask run on I'm going to

two and do flask run on I'm going to hover over that URL and open it in a

hover over that URL and open it in a browser and I'm going to manually visit

browser and I'm going to manually visit after zooming in let's do again slash uh

after zooming in let's do again slash uh search question mark uh Q equals Office

search question mark uh Q equals Office enter and this is what Json looks like

enter and this is what Json looks like now at a glance this does not seem like

now at a glance this does not seem like an improvement like this looks crazy

an improvement like this looks crazy that it's just this Big Blob of text but

that it's just this Big Blob of text but it's just enough text for the computer

it's just enough text for the computer to be able to process it reliably notice

to be able to process it reliably notice that there's a cur a square bracket here

that there's a cur a square bracket here and if I actually scroll to the Bott

and if I actually scroll to the Bott botom there' be a closed square bracket

botom there' be a closed square bracket like way down there inside of that

like way down there inside of that square bracket is a curly brace then ID

square bracket is a curly brace then ID colon and then a number then a title

colon and then a number then a title quote unquote colon and then the title

quote unquote colon and then the title and then the closed curly brace so what

and then the closed curly brace so what you're seeing in JavaScript object

you're seeing in JavaScript object notation is a very standard super

notation is a very standard super popular format that's just text that

popular format that's just text that still uses square brackets for lists AKA

still uses square brackets for lists AKA arrays that still uses curly braces for

arrays that still uses curly braces for dictionaries key value pairs so what you

dictionaries key value pairs so what you see here is a massive list up to 50 I

see here is a massive list up to 50 I think think shows that came back from

think think shows that came back from this API Each of which has a dictionary

this API Each of which has a dictionary if you will an object of key value pairs

if you will an object of key value pairs what keys and values an ID key and a

what keys and values an ID key and a title key Each of which has a value

title key Each of which has a value respectively and this is the same data

respectively and this is the same data from IMDb some of which you might be

from IMDb some of which you might be recalling visually this is just a very

recalling visually this is just a very raw computer friendly way of returning a

raw computer friendly way of returning a whole bunch of data that we humans don't

whole bunch of data that we humans don't need to see but I can use this data by

need to see but I can use this data by going back into vs code let me open

going back into vs code let me open another terminal window and go into

another terminal window and go into Source

Source 9/ shows 2 and in here let me go ahead

9/ shows 2 and in here let me go ahead and open up how about uh templates

and open up how about uh templates index.html which previously just used

index.html which previously just used that inner HTML trick and this is not

that inner HTML trick and this is not going to impress you're not going to be

going to impress you're not going to be pleased with this syntax but let me just

pleased with this syntax but let me just at least explain what we're doing it

at least explain what we're doing it turns out that Json is just the better

turns out that Json is just the better way in general the more generic

way in general the more generic multi-purpose user agnostic language

multi-purpose user agnostic language agnostic way of returning data from a

agnostic way of returning data from a server because it's just text so it

server because it's just text so it doesn't matter if you're using python or

doesn't matter if you're using python or C or C++ or JavaScript or Ruby or PHP or

C or C++ or JavaScript or Ruby or PHP or something else like all of those

something else like all of those languages can process Json information

languages can process Json information and indeed here is some JavaScript that

and indeed here is some JavaScript that does just that same code as before

does just that same code as before initially I declare a variable called

initially I declare a variable called input that gives me access to the user's

input that gives me access to the user's text box I listen for input like key

text box I listen for input like key strokes going up and down and when they

strokes going up and down and when they happen I call this Anonymous function uh

happen I call this Anonymous function uh I fetch data from the server using the

I fetch data from the server using the exact same code as before search

exact same code as before search question mark Q equals office or

question mark Q equals office or anything else and then this is just now

anything else and then this is just now new code that I use to convert that Json

new code that I use to convert that Json data into my own HTML format be it an

data into my own HTML format be it an unordered list an ordered list a table

unordered list an ordered list a table or anything else what am I doing I've

or anything else what am I doing I've got a variable called HTML initialized

got a variable called HTML initialized to nothing so I've got no HTML initially

to nothing so I've got no HTML initially I then iterate over every ID in those

I then iterate over every ID in those shows so every one of IMDB's unique

shows so every one of IMDB's unique identifiers I iterate over them one at a

identifiers I iterate over them one at a time and then I go into the show at its

time and then I go into the show at its ID location and I grab its title and

ID location and I grab its title and this

this is forget this for just a moment I then

is forget this for just a moment I then take this HTML variable concatenate or

take this HTML variable concatenate or join onto it my own Li tag plus the

join onto it my own Li tag plus the title plus the close Li tag and I

title plus the close Li tag and I skipped this because it got scary pretty

skipped this because it got scary pretty fast but it turns out that if some TV

fast but it turns out that if some TV shows have actually angled brackets in

shows have actually angled brackets in that could break my HTML entirely so it

that could break my HTML entirely so it turns out you might recall super briefly

turns out you might recall super briefly last week we had the copyright symbol

last week we had the copyright symbol using an HTML entity using the Ampersand

using an HTML entity using the Ampersand and the hash symbol and 169 semicolon it

and the hash symbol and 169 semicolon it turns out there are other such cryptic

turns out there are other such cryptic sequences of characters that represent

sequences of characters that represent otherwise dangerous or untypable

otherwise dangerous or untypable characters like this which could confuse

characters like this which could confuse the computer into thinking it's at the

the computer into thinking it's at the beginning of a tag and an Amper sand

beginning of a tag and an Amper sand which could similarly trick the computer

which could similarly trick the computer into thinking that it's a entity which

into thinking that it's a entity which it isn't but long story short there are

it isn't but long story short there are libraries thankfully that handle much of

libraries thankfully that handle much of this for you for our purposes the

this for you for our purposes the takeaway is that now that you understand

takeaway is that now that you understand a bit of

a bit of HTTP uh now that you understand a bit of

HTTP uh now that you understand a bit of HTML CSS and JavaScript all of which

HTML CSS and JavaScript all of which they have their roles you can use them

they have their roles you can use them ultimately to start assembling your own

ultimately to start assembling your own web applications as you will for problem

web applications as you will for problem set nine um stitching together all of

set nine um stitching together all of those languages and building

those languages and building full-fledged web applications mobile

full-fledged web applications mobile applications or anything more and for

applications or anything more and for that I think we are all set and if the

that I think we are all set and if the first one up here uh can have these

first one up here uh can have these cookies as well we'll see you next time

cookies as well we'll see you next time for our very last cs50

[Music] lecture

good afternoon my name is Sarah and my name is Grant and we are the Harvard

name is Grant and we are the Harvard crocodillos and pitches now Sarah and I

crocodillos and pitches now Sarah and I understand that today is the final

understand that today is the final lecture of cs50 it's been a tough

lecture of cs50 it's been a tough semester we made it through pets four

semester we made it through pets four five and even Finance now I know this is

five and even Finance now I know this is an unpopular opinion but I particularly

an unpopular opinion but I particularly enjoyed Finance I spent a lot of time

enjoyed Finance I spent a lot of time with my

with my flask the P set there was a p

flask the P set there was a p set well um at least things are looking

set well um at least things are looking up um today is our last lecture and look

up um today is our last lecture and look how far we've come if I were an emoji

how far we've come if I were an emoji right now I'd be the face with tears of

right now I'd be the face with tears of joy sorry about that we're just trying

joy sorry about that we're just trying to work some cs50 references into the uh

to work some cs50 references into the uh intro I I mean uh boy I sure hope this

intro I I mean uh boy I sure hope this uh tide man doesn't run off my Mario

uh tide man doesn't run off my Mario filter you could say that for Loop x

filter you could say that for Loop x equals Open Bracket one comma 2 close

equals Open Bracket one comma 2 close bracket boy I sure wish we had checked

bracket boy I sure wish we had checked to see if these jokes were funny when we

to see if these jokes were funny when we wrote

wrote them any who we hope that you'll enjoy

them any who we hope that you'll enjoy this brief

this brief [Music]

serenade your boot

KN House of well there's friers Andy truck barbecue ribs Tri

the boy boy bo boy bo bo boy boy down at the

boy bo boy bo bo boy boy down at the house the house of well your root and

house the house of well your root and you walk on down to a knock down Shack

you walk on down to a knock down Shack on the edge of town There's a l

on the edge of town There's a l there just quit you see

there just quit you see fall down the house the house

[Applause] of up your boots and you walk on down

at the house the House of Blue

[Applause] good afternoon everyone we are the

good afternoon everyone we are the Harvard crocodillos and it is such an

Harvard crocodillos and it is such an honor to be here with the Radcliff

honor to be here with the Radcliff pitches performing for cs50's final

pitches performing for cs50's final lecture congratulations to everyone and

lecture congratulations to everyone and we hope you'll enjoy this our tribute to

4 C is for the language I once knew O is for all notation I must do D is for

for all notation I must do D is for dynamic flask run and finance it is even

dynamic flask run and finance it is even more than David May in canor so code is

more than David May in canor so code is all that I can give to you to youe

all that I can give to you to youe debugging it since

debugging it since PE soon deadlines I'll make it hit

PE soon deadlines I'll make it hit compile and please don't break it code

compile and please don't break it code was made by

was made by [Music]

I for notation I must do is

more is [Music]

I is for the language 0 1 0 1 0 oh is for otation 0 1 1 D is for dynamic flas

for otation 0 1 1 D is for dynamic flas run and finance e is even more than

run and finance e is even more than David mail in canor so C is all that I

David mail in canor so C is all that I can give to

can give to you debugging it since

you debugging it since PE soon deadlines I'll make it hit

PE soon deadlines I'll make it hit compile and please don't break it code

compile and please don't break it code was made by me for COD was made by me

was made by me for COD was made by me for COD was

for COD was [Music]

all right this is cs50 and cs50 this was the Harvard crocodillos and the Radcliff

the Harvard crocodillos and the Radcliff pitches if one more time we could thank

pitches if one more time we could thank them for joining us

today so this is already week 10 our last and indeed among the goals for

last and indeed among the goals for today are to hopefully give you all the

today are to hopefully give you all the more of appreciation of truly just how

more of appreciation of truly just how far you've come recall that in week zero

far you've come recall that in week zero we began with this this visual here

we began with this this visual here whereby it was described this class as a

whereby it was described this class as a bit of a fire hose whereby drinking from

bit of a fire hose whereby drinking from that fire hose or really a fire hose

that fire hose or really a fire hose from a water fountain uh is not unlike

from a water fountain uh is not unlike getting an education down the road too

getting an education down the road too and so this is to say that if you're

and so this is to say that if you're feeling like you didn't quite get it all

feeling like you didn't quite get it all down like that's actually okay and

down like that's actually okay and that's to be expected and even if you

that's to be expected and even if you felt that with each passing week 0 1 two

felt that with each passing week 0 1 two all the way up now till 10 it never

all the way up now till 10 it never really ever got easier perhaps just

really ever got easier perhaps just consider that what was once hard like

consider that what was once hard like Mario and like getting hello world to

Mario and like getting hello world to compile is indeed the right measure of

compile is indeed the right measure of the Delta between week Zer and now in

the Delta between week Zer and now in week 10 in fact you might recall that

week 10 in fact you might recall that again in week zero 2third of your

again in week zero 2third of your classmates had never taken a CS course

classmates had never taken a CS course before now of course you all have and

before now of course you all have and indeed if you think back too to this

indeed if you think back too to this final sentiment from week zero that

final sentiment from week zero that indeed what ultimately matters in this

indeed what ultimately matters in this course is not where you end up relative

course is not where you end up relative to your classmates but where you end up

to your classmates but where you end up relative to where you yourself began so

relative to where you yourself began so I would take some pride take some

I would take some pride take some satisfaction take some relief even

satisfaction take some relief even though a little bit more work does

though a little bit more work does remain at really just how far you've

remain at really just how far you've come since that week zero and recall

come since that week zero and recall that in week zero we literally started

that in week zero we literally started with just zeros and ones and by now many

with just zeros and ones and by now many of you might have gleaned that these 64

of you might have gleaned that these 64 zeros and ones have been spelling

zeros and ones have been spelling something week by week in fact today is

something week by week in fact today is our very last message here in binary uh

our very last message here in binary uh encoded on stage but then quickly we

encoded on stage but then quickly we introduced scratch and we started to

introduced scratch and we started to assemble some building blocks of

assemble some building blocks of programming Loops conditions uh uh Loops

programming Loops conditions uh uh Loops conditions functions and the like but

conditions functions and the like but without all the distractions of

without all the distractions of semicolons and curly braces and all of

semicolons and curly braces and all of that which admittedly we introduce the

that which admittedly we introduce the next week when we introduce you to C but

next week when we introduce you to C but even now that we've transitioned to

even now that we've transitioned to python hopefully even those kinds of

python hopefully even those kinds of Curiosities or confusions are hopefully

Curiosities or confusions are hopefully starting to just get more familiar and

starting to just get more familiar and so you finally start to see the missing

so you finally start to see the missing semicolon as opposed to spending time on

semicolon as opposed to spending time on that kind of struggle recall too that in

that kind of struggle recall too that in week two we started talking already

week two we started talking already about memory and like how you can manage

about memory and like how you can manage things in arrays that later became of

things in arrays that later became of course in Python lists uh the week after

course in Python lists uh the week after we talked not only about uh debugging uh

we talked not only about uh debugging uh bugs in code but how to debug those same

bugs in code but how to debug those same programs um thereafter we started

programs um thereafter we started talking talking about algorithms and we

talking talking about algorithms and we took a step back from code and looked at

took a step back from code and looked at the bubble sorts and the selection sorts

the bubble sorts and the selection sorts and the merge sorts and all of the

and the merge sorts and all of the searches as well that go hand in hand

searches as well that go hand in hand with that and indeed this ultimately is

with that and indeed this ultimately is what a lot of problem solving moving

what a lot of problem solving moving forward is going to be about just

forward is going to be about just solving problems with some form of

solving problems with some form of algorithm and you have so many different

algorithm and you have so many different languages um in your toolkit now with

languages um in your toolkit now with which to approach problems like those we

which to approach problems like those we talk thereafter about pointers which are

talk thereafter about pointers which are not likely to come back in any modern

not likely to come back in any modern languages that you now use but hopefully

languages that you now use but hopefully you have an all the better of a sense

you have an all the better of a sense underneath the hood of like what's going

underneath the hood of like what's going on inside of the computer so that when

on inside of the computer so that when you're designing something you're using

you're designing something you're using something something crashes you at least

something something crashes you at least have a mental model for what's going on

have a mental model for what's going on and it's no longer that week zero black

and it's no longer that week zero black box as it once was I mean you built

box as it once was I mean you built things like this think back to week five

things like this think back to week five when you built your own hash table and

when you built your own hash table and those things are everywhere key value

those things are everywhere key value pairs whether it's in python or in C or

pairs whether it's in python or in C or if it's now in CSS and JavaScript and

if it's now in CSS and JavaScript and even HTML like that principle of key

even HTML like that principle of key value pairs is really everywhere and so

value pairs is really everywhere and so of course now code doesn't necessarily

of course now code doesn't necessarily have to look like this it now

have to look like this it now wonderfully looks a little something

wonderfully looks a little something more like this but eventually you're

more like this but eventually you're probably not I'm not going to use Python

probably not I'm not going to use Python anymore something new and better is

anymore something new and better is going to come along but odds are like a

going to come along but odds are like a lot of the building blocks from these

lot of the building blocks from these past 11 weeks are still going to be

past 11 weeks are still going to be useful for wrapping your mind around

useful for wrapping your mind around those new worlds and indeed SQL we

those new worlds and indeed SQL we introduced you a little bit too and even

introduced you a little bit too and even if you don't feel yourself yet an expert

if you don't feel yourself yet an expert hopefully have a sense of like what you

hopefully have a sense of like what you can do with it and what problems you can

can do with it and what problems you can solve uh it's of course a better

solve uh it's of course a better alternative to something like something

alternative to something like something simple like a spreadsheet and now of

simple like a spreadsheet and now of course like web stuff is everywhere

course like web stuff is everywhere whether it's on your laptop or desktop

whether it's on your laptop or desktop or a lot of the mobile apps that you use

or a lot of the mobile apps that you use on your phone even though they're native

on your phone even though they're native applications like you install them from

applications like you install them from Google Play or the Apple App Store like

Google Play or the Apple App Store like they're implemented increasingly with

they're implemented increasingly with HTML CSS and JavaScript but they're put

HTML CSS and JavaScript but they're put in a little rectangular window so you

in a little rectangular window so you don't even notice that that's actually

don't even notice that that's actually really just an embedded browser and then

really just an embedded browser and then of course you can build things as you

of course you can build things as you might for your final project that to

might for your final project that to might very well be web based if you go

might very well be web based if you go that route I mean I'm still clinging to

that route I mean I'm still clinging to like the very first like web app I ever

like the very first like web app I ever made years ago um but honestly I do that

made years ago um but honestly I do that in part because I was just so darn proud

in part because I was just so darn proud that like I taught myself how to do

that like I taught myself how to do something and it actually worked and was

something and it actually worked and was used by other people so whether it's

used by other people so whether it's just used by you or your classmates or

just used by you or your classmates or your roommates or your family or your

your roommates or your family or your company down the line there's a great

company down the line there's a great sense of satisfaction that comes despite

sense of satisfaction that comes despite all of the the pain that might be along

all of the the pain that might be along the way when you just can't see or fix

the way when you just can't see or fix that bug now of course we'll transition

that bug now of course we'll transition as you'll see in the coming days to try

as you'll see in the coming days to try empowering you to code client side as

empowering you to code client side as well up until now you've been using our

well up until now you've been using our own vs code installation in the cloud

own vs code installation in the cloud which is nice cuz you got up in running

which is nice cuz you got up in running super fast in week one focusing only on

super fast in week one focusing only on code challenges not on technical

code challenges not on technical difficulties but among the goals now if

difficulties but among the goals now if you so choose and want to program after

you so choose and want to program after this class even if you never take

this class even if you never take another CS course before you can use

another CS course before you can use these same real world deao standard

these same real world deao standard tools on your own Mac or PC and so

tools on your own Mac or PC and so pictured here is a screenshot of like vs

pictured here is a screenshot of like vs code on the Mac and even though yes

code on the Mac and even though yes you're going to have to like jump

you're going to have to like jump through a couple of more Hoops to just

through a couple of more Hoops to just get python or some other language

get python or some other language working on your own Mac or PC like

working on your own Mac or PC like that's what programming is ultimately

that's what programming is ultimately going to be about and we deliberately

going to be about and we deliberately transition you to this at terms end so

transition you to this at terms end so that now you have 11 weeks of more

that now you have 11 weeks of more Comfort under your belt with which to

Comfort under your belt with which to solve sort of silly technical support

solve sort of silly technical support headaches that might have been deal

headaches that might have been deal breakers so many weeks ago so there's

breakers so many weeks ago so there's still more to be done uh in the coming

still more to be done uh in the coming weeks and indeed a support structure

weeks and indeed a support structure there for the cs50 hackathon of course

there for the cs50 hackathon of course will be this 700 p.m. till 7 a.m.

will be this 700 p.m. till 7 a.m. opportunity to dive into your final

opportunity to dive into your final project well really continue diving into

project well really continue diving into your final project ideally at that point

your final project ideally at that point uh alongside classmates perhaps your

uh alongside classmates perhaps your project Partners if you're working

project Partners if you're working collaboratively

collaboratively uh and awaiting you will be such

uh and awaiting you will be such memories and excitement hopefully as

memories and excitement hopefully as these even as you then turn your

these even as you then turn your attention back to your final project

attention back to your final project there of course will be several meals

there of course will be several meals during the day culminating with 500 a.m.

during the day culminating with 500 a.m. uh shuttles to IHOP the local Pancake

uh shuttles to IHOP the local Pancake Place uh if you are so awake at that

Place uh if you are so awake at that point or even if you get there uh this

point or even if you get there uh this is not an uncommon site as you might

is not an uncommon site as you might recall from week zero and then lastly is

recall from week zero and then lastly is the cs-50 fair which is finally back

the cs-50 fair which is finally back after a couple of years now of it not

after a couple of years now of it not being on campus and this will be an

being on campus and this will be an opportunity for everyone to present

opportunity for everyone to present their final projects to passers by

their final projects to passers by classmates faculty and staff and really

classmates faculty and staff and really just Delight in what it is you created

just Delight in what it is you created on your Mac your PC your phone in the

on your Mac your PC your phone in the cloud or anywhere else and indeed it's

cloud or anywhere else and indeed it's just going to be an opportunity to bring

just going to be an opportunity to bring your laptop to a shared space or your

your laptop to a shared space or your phone and introduce your project to

phone and introduce your project to passers by such as might appear and

passers by such as might appear and ultimately celebrate what you all uh

ultimately celebrate what you all uh accomplished and indeed will you be

accomplished and indeed will you be handed at the cs50 fair your very own I

handed at the cs50 fair your very own I took cs50 t-shirt which I dare say I'm

took cs50 t-shirt which I dare say I'm still wearing all of these years later

still wearing all of these years later and so you

and so you to

to we'll have that uh ahead of you as well

we'll have that uh ahead of you as well so for what's on the agenda today we

so for what's on the agenda today we thought we would not only look back but

thought we would not only look back but look forward but first we wanted to

look forward but first we wanted to thank so many of the team members that

thank so many of the team members that have been helping both on stage and off

have been helping both on stage and off who've made this course and these

who've made this course and these sections and so much more about cs50

sections and so much more about cs50 possible of course um the building that

possible of course um the building that we are now in there's a whole team

we are now in there's a whole team downstairs in Memorial Hall who helps us

downstairs in Memorial Hall who helps us get set up and organized each day our

get set up and organized each day our thanks to them there's the education

thanks to them there's the education Support Services team who makes

Support Services team who makes everything look and sound so well down

everything look and sound so well down here especially when we have all of the

here especially when we have all of the more microphones as well our friends the

more microphones as well our friends the harbard crocodillos and the redcliff

harbard crocodillos and the redcliff pitches most recently and then of course

pitches most recently and then of course cs50's own team uh but butter cs50's own

cs50's own team uh but butter cs50's own favorite restaurant Chang show down the

favorite restaurant Chang show down the road indeed if you find yourself in

road indeed if you find yourself in Cambridge for the next one two 3 four

Cambridge for the next one two 3 four years or visiting from out of town uh do

years or visiting from out of town uh do pay a visit to our friends just down the

pay a visit to our friends just down the road and in fact we'll have our very

road and in fact we'll have our very last cs50 lunch this Friday if you're

last cs50 lunch this Friday if you're able locally to partake and then there

able locally to partake and then there cs50's own team um both on stage and off

cs50's own team um both on stage and off in thanks truly because not only do they

in thanks truly because not only do they make the everything run so smoothly they

make the everything run so smoothly they capture it for students here who might

capture it for students here who might not be physically present here for our

not be physically present here for our friends down in New Haven at Yale and

friends down in New Haven at Yale and certainly for anyone online who might be

certainly for anyone online who might be tuning in as well and then lastly wanted

tuning in as well and then lastly wanted to thank of course the huge team of your

to thank of course the huge team of your classmates your peers that make cs50

classmates your peers that make cs50 possible in sections and office hours

possible in sections and office hours tutorials and more allow me to share

tutorials and more allow me to share with you the outtakes so that even we

with you the outtakes so that even we the teaching staff sometimes struggle

the teaching staff sometimes struggle with computer science here are some of

with computer science here are some of the clips that we captured when just

the clips that we captured when just passing packets via tcpip a while back

passing packets via tcpip a while back you saw the finally the the nicely

you saw the finally the the nicely polished version but here are if I may

polished version but here are if I may if we could dim the lights are some of

if we could dim the lights are some of the

the outakes nothing

outakes nothing go buffering

go buffering okay Josh

okay Josh nice

[Music] Helen no oh wait

Josh uh um [Music]

Sophie amazing that was

amazing that was perfect

B I think I what

I what you oh nice

you oh nice [Music]

[Music] guy that was amazing thank you all so

good indeed and that moment if we could just one round of applause for everyone

just one round of applause for everyone who's helped out this

semester so back in week zero uh we introduced you of course to this idea of

introduced you of course to this idea of computational thinking which is to think

computational thinking which is to think a a little more methodically a little

a a little more methodically a little more algorithmically and by way of these

more algorithmically and by way of these various languages hopefully that is

various languages hopefully that is something you notice maybe not in the

something you notice maybe not in the moment but in the months and the years

moment but in the months and the years to come that you do find that your

to come that you do find that your thoughts are indeed a little more

thoughts are indeed a little more cleaned up and you're just able to

cleaned up and you're just able to express yourself a little more precisely

express yourself a little more precisely and even spot illogic in someone else's

and even spot illogic in someone else's document or statements as well but at

document or statements as well but at the end of the day really a course like

the end of the day really a course like this is also about critical thinking and

this is also about critical thinking and indeed rewind again to week zero when we

indeed rewind again to week zero when we frame the entirety of computer science

frame the entirety of computer science is really just this like problem solving

is really just this like problem solving and any problem in the world be it CS or

and any problem in the world be it CS or otherwise has some input and we decided

otherwise has some input and we decided how to represent those inputs it needs

how to represent those inputs it needs some output the solution there too and

some output the solution there too and then all of what you focused on doing

then all of what you focused on doing and learning and applying these past

and learning and applying these past several weeks is in that proverbial

several weeks is in that proverbial black box which hopefully is not such an

black box which hopefully is not such an abstraction anymore but is indeed

abstraction anymore but is indeed something that you know how to harness

something that you know how to harness and know what could be going on

and know what could be going on underneath the hood even if it's in some

underneath the hood even if it's in some technology or some language that maybe

technology or some language that maybe we ourselves didn't cover because a lot

we ourselves didn't cover because a lot of those first principles remain the

of those first principles remain the same now along the way we talked about

same now along the way we talked about the quality of solutions to those

the quality of solutions to those problems we happen to focus on

problems we happen to focus on correctness just does it work design

correctness just does it work design which is a bit more qualitative and

which is a bit more qualitative and subjective and then style the Aesthetics

subjective and then style the Aesthetics of it all and these two are

of it all and these two are characteristic maybe not with these same

characteristic maybe not with these same words of just how you might write or

words of just how you might write or evaluate other creations in life be it

evaluate other creations in life be it physical or written or the like so think

physical or written or the like so think about too as you solve problems just how

about too as you solve problems just how you can sort of frame for yourself like

you can sort of frame for yourself like am I doing a good job or not by

am I doing a good job or not by quantizing it along these or perhaps

quantizing it along these or perhaps other axes as well and we thought we'd

other axes as well and we thought we'd highlight just two topics from that week

highlight just two topics from that week zero that have really been manifest for

zero that have really been manifest for the past several weeks namely

the past several weeks namely abstraction like taking complicated

abstraction like taking complicated things and ideas and trying to simplify

things and ideas and trying to simplify them so that we can sort of operate at

them so that we can sort of operate at this level and like solve problems we

this level and like solve problems we care about without getting into the

care about without getting into the weeds of implementation detail so to

weeds of implementation detail so to speak but there's this tension because

speak but there's this tension because you know now from all of these different

you know now from all of these different languages that code is fairly

languages that code is fairly unforgiving I mean even omitting a

unforgiving I mean even omitting a stupid semicolon sometimes breaks

stupid semicolon sometimes breaks everything and so Precision is sort of

everything and so Precision is sort of at odd sometimes with this idea of

at odd sometimes with this idea of leveraging abstraction and so we thought

leveraging abstraction and so we thought we would try to tease this apart

we would try to tease this apart especially all these Weeks Later here uh

especially all these Weeks Later here uh with a bit of a uh live demonstration so

with a bit of a uh live demonstration so on the way in you probably all received

on the way in you probably all received a sheet of paper if not but someone near

a sheet of paper if not but someone near you did just tear theirs in half and

you did just tear theirs in half and borrow half a sheet if you can or any

borrow half a sheet if you can or any piece of loose leaf paper or the like

piece of loose leaf paper or the like will suffice so long as you have a pen

will suffice so long as you have a pen or pencil and for this allow me to

or pencil and for this allow me to propose that we invite up maybe two

propose that we invite up maybe two final cs50 volunteers this semester and

final cs50 volunteers this semester and like a lot of hands are going up in this

like a lot of hands are going up in this okay a lot of hands how about I saw the

okay a lot of hands how about I saw the first hand there uh yes yes who's yes

first hand there uh yes yes who's yes who's pointing at herself now come on

who's pointing at herself now come on down we just need the one hand for now

down we just need the one hand for now but oh wait oh wait uh you'll be our

but oh wait oh wait uh you'll be our number two well okay we have way too

number two well okay we have way too many volunteers now no no please please

many volunteers now no no please please come down yes in the black shirt and if

come down yes in the black shirt and if you guys we will okay we'll do pair

you guys we will okay we'll do pair programming in just a bit if you want to

programming in just a bit if you want to hang out in the wings here we'll have

hang out in the wings here we'll have our second demonstration as well so okay

our second demonstration as well so okay now maybe a round of applause for our

now maybe a round of applause for our three

volunteers oh come on up first oh second and third okay you come first we'll do

and third okay you come first we'll do order no uh this is a q okay Q here okay

order no uh this is a q okay Q here okay what's your name I'm Danny Danny okay

what's your name I'm Danny Danny okay take this mic okay so we will DQ you

take this mic okay so we will DQ you momentarily all right so Danny come on

momentarily all right so Danny come on over to the middle here and in a moment

over to the middle here and in a moment I'm going to hand to Danny a sheet of

I'm going to hand to Danny a sheet of paper that has a picture on it and this

paper that has a picture on it and this picture is going to be something that

picture is going to be something that I'd like you to verbally program the

I'd like you to verbally program the audience to draw you can use any words

audience to draw you can use any words any abstractions any PR level of

any abstractions any PR level of precision that you want but you just

precision that you want but you just can't make hand gestures or sort of show

can't make hand gestures or sort of show them what to draw But first you want to

them what to draw But first you want to tell us a little something about

tell us a little something about yourself including everyone here I'm

yourself including everyone here I'm daddy and I took ts50 okay wonderful

daddy and I took ts50 okay wonderful wonderful so I'm going to reveal the

wonderful so I'm going to reveal the picture only to Danny and if each of you

picture only to Danny and if each of you would like to take out that sheet of

would like to take out that sheet of paper and just make sure that no one

paper and just make sure that no one else can see this if you want to hold it

else can see this if you want to hold it up this way everyone here is now holding

up this way everyone here is now holding their pen or pencil and in some number

their pen or pencil and in some number of steps give them a verbal algorithm

of steps give them a verbal algorithm for drawing what you see and you can say

for drawing what you see and you can say anything you want but no gestures okay

anything you want but no gestures okay so you're going to want to

so you're going to want to draw a square in the center of the paper

draw a square in the center of the paper with the

with the diagonal pointing to

diagonal pointing to the center of the

edge wait no actually I scratched that draw a rbus in the center of your

paper and for those who forget what a rhombus is for um a d

rhombus is for um a d Diamond a a square that's on its

Diamond a a square that's on its side and then from the bottom vertex

side and then from the bottom vertex draw a straight line down but not all

draw a straight line down but not all the way to the edge of the

paper okay and then keep your pencil or pen at that point and you're going to

pen at that point and you're going to want to draw a line that's parallel to

want to draw a line that's parallel to the line of the original rumus to the

right and then keep your pencil or pen at that point and draw a line straight

at that point and draw a line straight up connecting to the side

vertex yes and then go back to the line that you drew from the bottom vertex to

that you drew from the bottom vertex to the bottom of the paper and then draw a

the bottom of the paper and then draw a line parallel to the left edge of the

line parallel to the left edge of the rumus and then then keep your L your

rumus and then then keep your L your paper your pencil at that point and draw

paper your pencil at that point and draw a line up to the vertex of the rumus

again the end the end all right well thank you to Danny hang on to your paper

thank you to Danny hang on to your paper thank you so much and if you want to

thank you so much and if you want to step off to the stage there we will

step off to the stage there we will reveal thank you a round of applause if

reveal thank you a round of applause if we could for Danny that is not an easy

we could for Danny that is not an easy task I'm sure and if Carter wouldn't

task I'm sure and if Carter wouldn't mind just grabbing a few samples here

mind just grabbing a few samples here let's actually take a look on the

let's actually take a look on the overhead if we could I'm going to pop

overhead if we could I'm going to pop down over here real fast we don't need

down over here real fast we don't need to collect them all but if you're

to collect them all but if you're feeling either good or bad with what you

feeling either good or bad with what you drew happy to collect a few of them

drew happy to collect a few of them okay okay thank you thank

okay okay thank you thank you okay hope you won't mind if I can't

you okay hope you won't mind if I can't reach everyone just a couple more okay

reach everyone just a couple more okay over here okay all right that's that's

over here okay all right that's that's okay this one's really funny okay I'm

okay this one's really funny okay I'm going to go with this one if I may and

going to go with this one if I may and Carter has some

Carter has some too

okay thank you so much okay so just a random assortment here let me turn on a

random assortment here let me turn on a camera so I can show you what I see here

camera so I can show you what I see here for instance is one classmate drawing

for instance is one classmate drawing which might resemble perhaps what you

which might resemble perhaps what you drew here uh here is the beginnings of a

drew here uh here is the beginnings of a house it

house it seems nice this

seems nice this one okay this one is larger

one okay this one is larger and how about a couple of others that

and how about a couple of others that were getting closer I think okay so more

were getting closer I think okay so more edges and vertices there this one seems

edges and vertices there this one seems a little similar in

a little similar in spirit if not

spirit if not proportional this is uh Zach's the best

proportional this is uh Zach's the best one but it turns out if I may Zach

one but it turns out if I may Zach you're not all that far off here Denny

you're not all that far off here Denny is what you were reciting to everyone

is what you were reciting to everyone algorithmically indeed it was this here

algorithmically indeed it was this here Cube and so Danny can you come on back

Cube and so Danny can you come on back up for a

up for a moment so if you'd like to share for

moment so if you'd like to share for just a moment like what were some of the

just a moment like what were some of the thoughts going through your head and why

thoughts going through your head and why did you choose the words that you did

did you choose the words that you did okay so what was going through my head

okay so what was going through my head when I saw the cube um I didn't know if

when I saw the cube um I didn't know if I could say draw Cube so I decided to

I could say draw Cube so I decided to start with the top and so draw a Remus

start with the top and so draw a Remus in the center of your paper and then

in the center of your paper and then draw a line down and just like do the

draw a line down and just like do the first part then the second part then the

first part then the second part then the third part and then you would get a cube

third part and then you would get a cube like Zach yeah and so had you said and

like Zach yeah and so had you said and you could have said draw a cube which

you could have said draw a cube which would be more of an abstraction even

would be more of an abstraction even that's not necessarily sufficiently

that's not necessarily sufficiently precise right because you don't

precise right because you don't necessarily know what the orientation of

necessarily know what the orientation of that cube is the size of it the

that cube is the size of it the positioning on the paper so you instead

positioning on the paper so you instead took a lower level approach which is not

took a lower level approach which is not unlike scratch if you think about

unlike scratch if you think about scratch being able to move up down left

scratch being able to move up down left right turn 90 degrees turn 15 degrees

right turn 90 degrees turn 15 degrees and the like I mean that is ultimately

and the like I mean that is ultimately how a lot of graphical programs and

how a lot of graphical programs and games even might be implemented by

games even might be implemented by really focusing at the level you do but

really focusing at the level you do but of course there's this tension with us

of course there's this tension with us humans whereby we prefer often to think

humans whereby we prefer often to think at this level but even that might not be

at this level but even that might not be sufficient which is to say here in week

sufficient which is to say here in week 10 like these are still going to be in

10 like these are still going to be in our hard problems uh but a hand if we

our hard problems uh but a hand if we could one more time for Danny for

could one more time for Danny for getting us that far along thank you so

getting us that far along thank you so much let me give you a stress ball here

much let me give you a stress ball here all right and if we could have both of

all right and if we could have both of our volunteers come up here we're going

our volunteers come up here we're going to have yes come on up come on up and

to have yes come on up come on up and let me uh have you guys introduce

let me uh have you guys introduce yourselves in the middle

yourselves in the middle here hi I'm

here hi I'm hey there I'm sadik from Turkey nice to

hey there I'm sadik from Turkey nice to meet you all wonderful welcome and this

meet you all wonderful welcome and this time we're going to flip it around so as

time we're going to flip it around so as to have the audience do what Danny just

to have the audience do what Danny just did for us the only catch here is that

did for us the only catch here is that the only means we have for showing the

the only means we have for showing the audience what they need to tell you to

audience what they need to tell you to draw is like literally right above the

draw is like literally right above the chalkboard so on our system here that

chalkboard so on our system here that your eyes must stay on the chalkboard

your eyes must stay on the chalkboard and not look up and in just a moment if

and not look up and in just a moment if you guys want to both stand in front of

you guys want to both stand in front of the chalkboard back to the audience and

the chalkboard back to the audience and as you're talking with each other other

as you're talking with each other other verbalize it through the microphone if

verbalize it through the microphone if you will I'm going to show everyone else

you will I'm going to show everyone else in the room a second and final drawing

in the room a second and final drawing and we'll just go rapid fire around the

and we'll just go rapid fire around the room give us one step at a time

room give us one step at a time collectively and we'll see if these guys

collectively and we'll see if these guys can't draw exactly that same outcome so

can't draw exactly that same outcome so is there another chunk what's that is

is there another chunk what's that is there another chunk or just just the one

there another chunk or just just the one so you'll have to collaborate and let's

so you'll have to collaborate and let's give you a clean slate here

give you a clean slate here literally all right so no looking up

literally all right so no looking up that's the only rule for you guys here

that's the only rule for you guys here we go for the audience here is what we'd

we go for the audience here is what we'd like them ironically to

like them ironically to draw step one from anyone in the

draw step one from anyone in the audience yes draw a circle draw a circle

audience yes draw a circle draw a circle anywhere not anywhere not

anywhere okay that's step one step two someone else yeah in the

middle draw a line down from the bottom of the

draw a line down from the bottom of the circle about halfway down I think there

circle about halfway down I think there was a hand in front of you too number

three

[Music] okay okay the overarching goal here for

okay okay the overarching goal here for those unable here is to draw a person

those unable here is to draw a person fig okay it may be a SI figure draw the

fig okay it may be a SI figure draw the left leg of the person of this person

left leg of the person of this person okay good job all right next step four

okay good job all right next step four yeah

yeah of line up Circle

of line up Circle okay the left oh sorry to the right draw

okay the left oh sorry to the right draw a v okay to the right of the vertex at

a v okay to the right of the vertex at the bottom of the circle draw a v draw a

the bottom of the circle draw a v draw a v um like what V nope not interactive

v um like what V nope not interactive draw a v um no no

well yeah it seems Weir weird let's get ready for maybe

weird let's get ready for maybe something like step

something like step five okay we'll go with that step five

five okay we'll go with that step five someone else step five someone else

someone else step five someone else someone else

someone else yeah draw the right side of the leg okay

yeah draw the right side of the leg okay nice step six step six happy face six

nice step six step six happy face six six yes erase the line

erase the line that you have on the left on the

on the okay okay step seven yes instead of like

okay okay step seven yes instead of like that line that was before going up make

that line that was before going up make it go down instead of that line before

it go down instead of that line before going up make it go down mhm okay step

going up make it go down mhm okay step eight step eight step eight yes connect

eight step eight step eight yes connect that line to the hip connect that line

that line to the hip connect that line to the hip not like not touching not

to the hip not like not touching not touching something like this

touching something like this maybe okay compromise not touching okay

maybe okay compromise not touching okay not touching okay all right step nine

not touching okay all right step nine almost there I think step

back uh write the word high on the top left of the circle

left of the circle here okay

here okay and step 10 almost there line draw a

and step 10 almost there line draw a line pointing to high so like a spee

line pointing to high so like a spee bubble basically

bubble basically yeah okay and step uh 10

yeah okay and step uh 10 11

11 yeah erase the exclamation point nice 12

yeah erase the exclamation point nice 12 do we want to give them one

do we want to give them one more 12 or we good yeah last one erase

more 12 or we good yeah last one erase the erase erace the right arm okay I

the erase erace the right arm okay I think we're going to need a 13

think we're going to need a 13 then and

then and then yeah

then yeah repat repeat the left arm but rotate it

repat repeat the left arm but rotate it by

wrong wait how would you like as an organic human being how put your

organic human being how put your arms like would you put your would you

arms like would you put your would you ever structure your arm like that that

ever structure your arm like that that would not be a stick

would not be a stick figure CU would you do this or would you

figure CU would you do this or would you do that or a little hint maybe get a get

do that or a little hint maybe get a get a give me a step 14 step 14 and final

a give me a step 14 step 14 and final step

step 14 I think we just got to tell them what

14 I think we just got to tell them what to do step 14

yes think of a walking man and and have the left the right hand

and and have the left the right hand walking to your right all right so like

walking to your right all right so like it's like where could the hand go where

it's like where could the hand go where should the hand go on that

should the hand go on that arm but yeah yeah okay yes

arm but yeah yeah okay yes no yeah I mean look look look like right

no yeah I mean look look look like right here look right here look right I yes

here look right here look right I yes sorry thank you 14

St that's pretty close so congratulations to you guys and thank

congratulations to you guys and thank you as well all right so I mean these

you as well all right so I mean these these things too are not yes Round of

these things too are not yes Round of Applause then sure

so so this is to say that these ideas of abstraction and precision and really

abstraction and precision and really every other term of art that we explored

every other term of art that we explored this term are sort of omnipresent and

this term are sort of omnipresent and can be easier or harder to implement

can be easier or harder to implement depending on exactly what the problem is

depending on exactly what the problem is but what we thought we' do now in our uh

but what we thought we' do now in our uh final day is try to now similarly

final day is try to now similarly prepare prepare you for life after cs50

prepare prepare you for life after cs50 and this is really going to be a list of

and this is really going to be a list of really potential to-dos so that you can

really potential to-dos so that you can stand on your own after the class after

stand on your own after the class after the class's infrastructure write actual

the class's infrastructure write actual code and then we'll come full circle one

code and then we'll come full circle one final time with our friend Jennifer 8

final time with our friend Jennifer 8 Lee to look at the world of emojis and

Lee to look at the world of emojis and how they relate to all forms of

how they relate to all forms of representation that we've talked to uh

representation that we've talked to uh talked about up until now so one how can

talked about up until now so one how can you go about programming after cs50 so

you go about programming after cs50 so one you can actually install command

one you can actually install command line tools on your own Mac or PC perhaps

line tools on your own Mac or PC perhaps unbeknownst to you Windows has what's

unbeknownst to you Windows has what's generally called a command prompt Mac OS

generally called a command prompt Mac OS literally comes with a terminal program

literally comes with a terminal program in your applications utilities folder

in your applications utilities folder and so even if you've never run those

and so even if you've never run those programs you've actually had a sort of

programs you've actually had a sort of blinking cursor black and white prompt

blinking cursor black and white prompt available to you might not have all of

available to you might not have all of the same software installed as your code

the same software installed as your code space in the cloud but you have that

space in the cloud but you have that command line interface even within

command line interface even within today's graphical tools and among the

today's graphical tools and among the tools you can install within that

tools you can install within that command line interface would be

command line interface would be something called xcode on the Mac which

something called xcode on the Mac which comes not only with a guey IDE

comes not only with a guey IDE integrated development environment but

integrated development environment but also those command line tools and

also those command line tools and Microsoft for Windows has something

Microsoft for Windows has something similar as well learning git so we've

similar as well learning git so we've used git only unbeknownst to you

used git only unbeknownst to you underneath the hood for the most part

underneath the hood for the most part but git is a very very popular tool if

but git is a very very popular tool if challenging to pick up for the first

challenging to pick up for the first time that makes it easy to push code to

time that makes it easy to push code to a website called GitHub or any

a website called GitHub or any equivalent and then collaborate more

equivalent and then collaborate more effectively with classmates there's

effectively with classmates there's definitely a bit of a learning curve but

definitely a bit of a learning curve but thanks to cs50's own Brian youu you can

thanks to cs50's own Brian youu you can start for instance with a video like

start for instance with a video like this and this indeed is going to be one

this and this indeed is going to be one of these deao standards in the real

of these deao standards in the real world at least for the next several

world at least for the next several years that you'll probably encounter if

years that you'll probably encounter if you work in Tech or really any company

you work in Tech or really any company where you're doing some programming vs

where you're doing some programming vs code itself will walk you through this

code itself will walk you through this process in the coming days but you can

process in the coming days but you can indeed install it on your own Mac or PC

indeed install it on your own Mac or PC and what can you do when you write code

and what can you do when you write code well you can certainly write software

well you can certainly write software for your Mac for your PC for your phone

for your Mac for your PC for your phone or of course per week 10 uh week nine

or of course per week 10 uh week nine you can host uh your own website be it

you can host uh your own website be it static as in week uh week eight um

static as in week uh week eight um hosting it at websites like these which

hosting it at websites like these which gener generally have free or

gener generally have free or student-friendly accounts via which you

student-friendly accounts via which you can put something statically on the web

can put something statically on the web at a real domain name that you might

at a real domain name that you might choose or you can host a full-fledged

choose or you can host a full-fledged web app and using uh student tiers on

web app and using uh student tiers on Amazon and Microsoft and Google's cloud

Amazon and Microsoft and Google's cloud services or others you can sign up for

services or others you can sign up for being a student certainly a whole lot of

being a student certainly a whole lot of free software free hosting so as to if

free software free hosting so as to if nothing else um experiment and uh

nothing else um experiment and uh perhaps maximally get your own app or

perhaps maximally get your own app or website up and running so know that

website up and running so know that those are resources available to you and

those are resources available to you and this is by uh certainly a non-exhaustive

this is by uh certainly a non-exhaustive list if you'd like to geek out in the

list if you'd like to geek out in the coming months in the coming years these

coming months in the coming years these are just some of the places that people

are just some of the places that people who take computer science classes who

who take computer science classes who write code might tend to hang out and

write code might tend to hang out and ask and answer questions of each other

ask and answer questions of each other um so keep an eye for instance on these

um so keep an eye for instance on these here and then cs50 has its own

here and then cs50 has its own communities as you'll see if you go to

communities as you'll see if you go to this URL here via the open coreware

this URL here via the open coreware version of cs50 which is open to the

version of cs50 which is open to the world do uh is there a Vibrant Community

world do uh is there a Vibrant Community uh thanks to time zones that's pretty

uh thanks to time zones that's pretty much active 247 365 talking about not

much active 247 365 talking about not only cs50 going on in problem sets and

only cs50 going on in problem sets and projects but really technology more

projects but really technology more generally as well so certainly feel

generally as well so certainly feel welcome to partake either asking or

welcome to partake either asking or answering questions now in speaking of

answering questions now in speaking of asking and answering questions a couple

asking and answering questions a couple of weeks ago you kindly gave us a whole

of weeks ago you kindly gave us a whole bunch of review questions which we

bunch of review questions which we called through and picked out our

called through and picked out our favorite 20 of them these of course were

favorite 20 of them these of course were multiple choice questions and in

multiple choice questions and in preparation for this week uh in

preparation for this week uh in preparation for life ahead we thought we

preparation for life ahead we thought we would choreograph a bit of a a quiz show

would choreograph a bit of a a quiz show here and ceed as you came in at the

here and ceed as you came in at the start of class you might recall being

start of class you might recall being invited to go to this URL here

invited to go to this URL here cs50. either here in person or if you're

cs50. either here in person or if you're watching live from home at this URL here

watching live from home at this URL here you can use a phone or a laptop and if

you can use a phone or a laptop and if it's easier on a phone you can point

it's easier on a phone you can point your camera at this 2D barcode here

your camera at this 2D barcode here we'll give folks a moment to pull that

we'll give folks a moment to pull that up and again that URL was cs50.

l/p and once it looks like most folks have

and once it looks like most folks have it up and running our friend Carter here

it up and running our friend Carter here will help us dive into this uh review

will help us dive into this uh review session if you will with a bit of fun

session if you will with a bit of fun along the way all right Carter if you'd

along the way all right Carter if you'd like to take it away what do we have as

like to take it away what do we have as our first question you should see on

our first question you should see on your phone or laptop this same question

your phone or laptop this same question being asked the first question is how do

being asked the first question is how do you print quote un quote hello world in

you print quote un quote hello world in Python so among the possible answers are

Python so among the possible answers are these here

these here buzz in on your phone or your

laptop we've got a few hundred responses are

are ready 7 Seconds to make your

decision this is question one of 20 go to it with some confidence I think we're

to it with some confidence I think we're down to zero on the clock and Carter it

down to zero on the clock and Carter it looks like 98% of you uh indeed said

looks like 98% of you uh indeed said hello world and Carter per the check

hello world and Carter per the check mark That's indeed the correct answer

mark That's indeed the correct answer here now to make things interesting in

here now to make things interesting in know that you'll see some number of

know that you'll see some number of points and we've deliberately anonymized

points and we've deliberately anonymized it so only you know what number you are

it so only you know what number you are so a whole lot of guests have a perfect

so a whole lot of guests have a perfect score of 1,000 at the moment hopefully

score of 1,000 at the moment hopefully we'll see over the next several

we'll see over the next several questions things start to bridge out uh

questions things start to bridge out uh but know that the speed with which you

but know that the speed with which you buzz in will also factor into how many

buzz in will also factor into how many points you now get So the faster you

points you now get So the faster you move the more points you get question

move the more points you get question two if we could what does DNS stand for

two if we could what does DNS stand for from just a couple of weeks back domain

from just a couple of weeks back domain number system domain name system data

number system domain name system data numbering structure or there's no such

numbering structure or there's no such thing as

thing as DNS few hundred responses are

DNS few hundred responses are in 8 seconds

in 8 seconds remain fewer points now but still a

remain fewer points now but still a chance to buzz in and now as we hit zero

chance to buzz in and now as we hit zero the responses are these domain name

the responses are these domain name system which is indeed correct and 84%

system which is indeed correct and 84% of you got that one correctly and indeed

of you got that one correctly and indeed exists we talked about it a couple of

exists we talked about it a couple of weeks ago so we're still seeing a whole

weeks ago so we're still seeing a whole lot lot of ties at 2,000 we'll see if

lot lot of ties at 2,000 we'll see if someone starts to pull away before long

someone starts to pull away before long question three what is the upper bound

question three what is the upper bound of merge sorts

of merge sorts runtime so that escalated

runtime so that escalated quickly Big O of n log n Big O of log n

quickly Big O of n log n Big O of log n Theta or Omega of log n or Big O of

Theta or Omega of log n or Big O of one what is the upper bound of merge

one what is the upper bound of merge sorts

sorts runtime that was the last of the

runtime that was the last of the algorithms we solve for sorting and in 1

algorithms we solve for sorting and in 1 second we'll see that the correct answer

second we'll see that the correct answer is is just edging out everyone else

is is just edging out everyone else indeed 46% it is n I know it's n log now

indeed 46% it is n I know it's n log now if I may as the teacher it can't be

if I may as the teacher it can't be login because login is strictly less

than n and you can't possibly sort n elements unless you minimally look at or

elements unless you minimally look at or touch each of them so it's got to be at

touch each of them so it's got to be at least greater than n intuitively we

least greater than n intuitively we still have a whole bunch of ties let's

still have a whole bunch of ties let's move on to number four what is stored in

move on to number four what is stored in ARG

ARG C back to the language C is it in Array

C back to the language C is it in Array of arguments the maximum size of an

of arguments the maximum size of an array the count of arguments given to a

array the count of arguments given to a program when first run or how much

program when first run or how much memory is allocated to a

memory is allocated to a function again you wrote all of these

function again you wrote all of these questions and we have 5 seconds for the

questions and we have 5 seconds for the reveal ARG C

reveal ARG C is indeed the count of arguments given

is indeed the count of arguments given to a program when first run think back

to a program when first run think back to C when we did command line arguments

to C when we did command line arguments there was argc and arv argv was the

there was argc and arv argv was the array but Arc was indeed the count the

array but Arc was indeed the count the CN AR save all right we still have a

CN AR save all right we still have a whole bunch of ties at the top here but

whole bunch of ties at the top here but let's move on then to number five what

let's move on then to number five what is the duck debugger favorite hobby

is the duck debugger favorite hobby according to one of your classmates

according to one of your classmates dressing up like Dracula swimming across

dressing up like Dracula swimming across the stage filling up the entire bathroom

the stage filling up the entire bathroom of a guy's house and sitting quietly on

of a guy's house and sitting quietly on stage the third of course is a reference

stage the third of course is a reference to a YouTube video that was on the

to a YouTube video that was on the course's website that week but according

course's website that week but according to your class mate number two seconds

to your class mate number two seconds remaining sitting quietly on stage is

remaining sitting quietly on stage is its favorite pastime so a little harder

its favorite pastime so a little harder perhaps than the

perhaps than the others 5,000 now we're starting to see

others 5,000 now we're starting to see some spread so we only have six six

some spread so we only have six six guests in contention for first place and

guests in contention for first place and the next question now is six what is the

the next question now is six what is the function used to open a file in

function used to open a file in C F open open file open

C F open open file open file

file what is the function used to open a file

what is the function used to open a file in

C 7 Seconds there's some differences between

Seconds there's some differences between C and python here and the reveal it is

C and python here and the reveal it is indeed fop it's 77% correct too all

indeed fop it's 77% correct too all right let's see the rankings now if you

right let's see the rankings now if you are guest 15 9715 6171 3753 or 3273

are guest 15 9715 6171 3753 or 3273 you're now in the lead as we move on on

you're now in the lead as we move on on to se question seven how does Sterling

to se question seven how does Sterling compute the average sorry how does

compute the average sorry how does Sterling compute the length of a string

Sterling compute the length of a string in C it looks at how much memory the

in C it looks at how much memory the string uses it counts the number of

string uses it counts the number of characters until it reaches back0 it

characters until it reaches back0 it counts the number of bits in the string

counts the number of bits in the string or it creates pointers for each

or it creates pointers for each character and counts

character and counts them 10 seconds Sterling in

them 10 seconds Sterling in C recall that we implemented this

C recall that we implemented this ourselves in class but then we used the

ourselves in class but then we used the library thereafter and in indeed with

library thereafter and in indeed with 85% it simply counts the number of

85% it simply counts the number of characters until it reaches that

characters until it reaches that Sentinel back sl0 AKA null and in this

Sentinel back sl0 AKA null and in this case we

case we have five four four of you tied now for

have five four four of you tied now for first all right question eight where

first all right question eight where does Malo allocate memory

does Malo allocate memory from the stack the Heap the pointers or

from the stack the Heap the pointers or the

the temp where does malok allocate memory

temp where does malok allocate memory from responses are coming in 8

seconds a good review question at that in two seconds we'll see that malok

in two seconds we'll see that malok allocates memory from woo close one the

allocates memory from woo close one the Heap is correct the Heap is correct the

Heap is correct the Heap is correct the stack recall is where functions store

stack recall is where functions store their local variables and their

their local variables and their arguments and that just happens

arguments and that just happens automatically the Heap represented in

automatically the Heap represented in our pictures up top is where malok draws

our pictures up top is where malok draws from now we have guest

from now we have guest 15 has

15 has made its way to the top here but others

made its way to the top here but others can catch up if they don't buzz in fast

can catch up if they don't buzz in fast enough so number nine how many people

enough so number nine how many people flew from 50v to New York on the day of

flew from 50v to New York on the day of the

the crime 16 29 8 or

three anyone with a laptop perhaps has an advantage

here 5 Seconds and the answers are but the answer is

and the answers are but the answer is 16 let's see if guest 15 got

16 let's see if guest 15 got this they did not goodbye to guest 15 at

this they did not goodbye to guest 15 at the top all right question 10 we're

the top all right question 10 we're about halfway there what are meta tags

about halfway there what are meta tags used for in

used for in HTML to describe a web page to Define

HTML to describe a web page to Define parameters for an element to group

parameters for an element to group elements together to translate content

elements together to translate content into machine readable format

into machine readable format at what are meta tags used for in HTML

at what are meta tags used for in HTML we saw a few of

we saw a few of them for different use

them for different use cases and with one second we see that

cases and with one second we see that indeed the number one answer was to

indeed the number one answer was to describe the web page be it for a mobile

describe the web page be it for a mobile device be it for screen scrapers like

device be it for screen scrapers like Facebook and Twitter and other such apps

Facebook and Twitter and other such apps that grab images and descriptions

that grab images and descriptions thereof all right we're in the second

thereof all right we're in the second half now guest nine guest four 669 has

half now guest nine guest four 669 has edged ahead guess 15 is now in sixth

edged ahead guess 15 is now in sixth place all right number 11 is how do you

place all right number 11 is how do you find the address of a variable in C

find the address of a variable in C think back a few weeks star dollar sign

think back a few weeks star dollar sign Ampersand or

ask from one of your own classmates how do you find the address of a variable in

C and the number one answer is ersan which is indeed the address of operator

which is indeed the address of operator at 62% nicely done let's see who's the

at 62% nicely done let's see who's the top of the list now guess 4669 has

top of the list now guess 4669 has retained their lead so we move on to 12

retained their lead so we move on to 12 what does the arrow operator mean in C a

what does the arrow operator mean in C a hyphen and a greater than sign nothing

hyphen and a greater than sign nothing starts a comment replaces a star and Dot

starts a comment replaces a star and Dot operator declares a

operator declares a pointer what does this Arrow operator

pointer what does this Arrow operator mean in

mean in C

C again from a few weeks back 3 seconds

again from a few weeks back 3 seconds harder assortment perhaps and it's oh

harder assortment perhaps and it's oh replaces a star and Dot operator the

replaces a star and Dot operator the number two answer was indeed correct

number two answer was indeed correct this was just a cleaner way syntactic

this was just a cleaner way syntactic sugar for collapsing what would be a

sugar for collapsing what would be a star and then some parentheses and then

star and then some parentheses and then a DOT into quite simply something that

a DOT into quite simply something that looks like an arrow itself all right

looks like an arrow itself all right Carter who's in the lead now still that

Carter who's in the lead now still that same guest and let's see what 13 has for

same guest and let's see what 13 has for us which of these is not a data type in

us which of these is not a data type in SQL light blob string integer

text we used a few of these more commonly than others but not all of

commonly than others but not all of these are for

these are for real 5 Seconds to make your

real 5 Seconds to make your decision and the results are blob is a

decision and the results are blob is a thing string is not in SQL light it's of

thing string is not in SQL light it's of course called text as we've seen it blob

course called text as we've seen it blob as goofy as it sounds is just binary

as goofy as it sounds is just binary large object but indeed it's how you

large object but indeed it's how you might store a binary file in your

might store a binary file in your database all right the rankings now oh

database all right the rankings now oh guest 8444 has Eed ahead so we move on

guest 8444 has Eed ahead so we move on to 14 which of the following is a valid

to 14 which of the following is a valid way to print Exclamation point

way to print Exclamation point Exclamation point Exclamation point

Exclamation point Exclamation point Exclamation point in

Exclamation point in Python I'll let you read these

Python I'll let you read these yourselves

yourselves which is a valid

which is a valid way everyone got quiet in is

thinking all right 6 seconds few hundred responses in so

responses in so far all right and yes nicely done 78% is

far all right and yes nicely done 78% is correct you can use the star operator to

correct you can use the star operator to essentially multiply the character at

essentially multiply the character at left all right who's in the lead Carter

left all right who's in the lead Carter we now have still G guest

we now have still G guest 8444 and so we move on now to 15 what

8444 and so we move on now to 15 what does the free function do D allocates

does the free function do D allocates memory from A Primitive deallocates

memory from A Primitive deallocates memory at the given pointer terminates a

memory at the given pointer terminates a loop or returns a value from the

loop or returns a value from the function what does the free function

do all right 5 Seconds what does the free function do call it's the opposite

free function do call it's the opposite essentially of malok and it deallocates

essentially of malok and it deallocates the memory at the given pointer as well

the memory at the given pointer as well and in C that's on you in Python you

and in C that's on you in Python you don't need to worry about allocating or

don't need to worry about allocating or freeing aik all right we now have at the

freeing aik all right we now have at the top still guest 8444 and seeing more and

top still guest 8444 and seeing more and more spread 16 which is not a step of

more spread 16 which is not a step of compiling think back now to week two

compiling think back now to week two compiling pre-processing linking or

compiling pre-processing linking or threading which is not a step of

threading which is not a step of compiling we use it as a catchall but it

compiling we use it as a catchall but it technically means a few different things

collectively all right and threading is indeed not on the list that is a

indeed not on the list that is a technical thing generally meaning a

technical thing generally meaning a program can do multiple things at once

program can do multiple things at once but that is not related here to

but that is not related here to compiling all right guest 8444 is still

compiling all right guest 8444 is still at the top we have just a few questions

at the top we have just a few questions left and so 17 what was the surprise at

left and so 17 what was the surprise at the beginning of the Halloween lecture

the beginning of the Halloween lecture according to your classmates someone

according to your classmates someone scared me someone hid candy under every

scared me someone hid candy under every chair someone dressed up as me the

chair someone dressed up as me the entire staff dressed up as

Carter interesting litmus test of who came to or watch lecture

came to or watch lecture perhaps let's see which guests got this

perhaps let's see which guests got this correct someone someone's really dressed

correct someone someone's really dressed up as indeed me at 64% so I think we

up as indeed me at 64% so I think we have attendance here essentially from

have attendance here essentially from that lecture so let's move on now to the

that lecture so let's move on now to the final few questions after seeing that

final few questions after seeing that guest a 8444 8444 is still doing well

guest a 8444 8444 is still doing well can anyone Dethrone them why is it

can anyone Dethrone them why is it incorrect to use the equals equals

incorrect to use the equals equals operator in C to compare

operator in C to compare strings it's computationally inefficient

strings it's computationally inefficient you comparing the locations of the

you comparing the locations of the strings strings don't exist using equal

strings strings don't exist using equal equals will lead to buffer

overflow why is it incorrect to use equal equals in C to compare strings 3

equal equals in C to compare strings 3 seconds we saw this live and it

seconds we saw this live and it motivated like an entire week because

motivated like an entire week because you're comparing the locations that is

you're comparing the locations that is the addresses in memory of those same

the addresses in memory of those same strings all right let's see if this

strings all right let's see if this leaves the rankings the same guest 8444

leaves the rankings the same guest 8444 is still at the top and pulling ahead

is still at the top and pulling ahead two final questions 19 what is the

two final questions 19 what is the difference between null one L and null 2

difference between null one L and null 2 L's they null and null mean the same

L's they null and null mean the same thing nulls refers to back sl0 whereas

thing nulls refers to back sl0 whereas null to L's is the zero address null is

null to L's is the zero address null is the zero address whereas null to L's

the zero address whereas null to L's refers to back sl0 null is null but

refers to back sl0 null is null but lazier 5

lazier 5 Seconds subtle not the best design

Seconds subtle not the best design perhaps to have in technical terms but

perhaps to have in technical terms but indeed 62% of you got that NL is the

indeed 62% of you got that NL is the first thing we talked about when we

first thing we talked about when we talked about back sl0 and N is a pointer

talked about back sl0 and N is a pointer it's the zero pointer same thing same

it's the zero pointer same thing same number but different context all right

number but different context all right Carter guest 8444 is the person to beat

Carter guest 8444 is the person to beat with our final 20th questions what do

with our final 20th questions what do the binary bulbs on stage spell

today and these are your four choices different from usual we usually use 8

different from usual we usually use 8 bit asky today we are using utf8 which

bit asky today we are using utf8 which is a form of Unicode which is the larger

is a form of Unicode which is the larger subset that uses one or two or three or

subset that uses one or two or three or even four bites to spell a single

even four bites to spell a single character and the answer wow close close

character and the answer wow close close is indeed a cupcake indeed a cupcake

is indeed a cupcake indeed a cupcake well done and let's see the final

well done and let's see the final results 8444 is the winner are they here

results 8444 is the winner are they here in person perhaps

in person perhaps 8444 you're 844 come on

8444 you're 844 come on [Applause]

down thank you here you go congratulations oh you're all right so

congratulations oh you're all right so today if we

today if we may give me just one

right all right so today we are so pleased to be joined by uh Jennifer 8

pleased to be joined by uh Jennifer 8 Lee who's an of the college a dear

Lee who's an of the college a dear friend and is actually really the reason

friend and is actually really the reason why there's evidence of Muppets in cs50

why there's evidence of Muppets in cs50 in fact some years ago I was visiting

in fact some years ago I was visiting her and she had on her shelf like this

her and she had on her shelf like this custom Muppet it wasn't one that appears

custom Muppet it wasn't one that appears on TV but she had somehow gone on a

on TV but she had somehow gone on a website former toy store called FAO

website former toy store called FAO Schwarz at the time and you're allowed

Schwarz at the time and you're allowed to configure your own Muppet whatnot

to configure your own Muppet whatnot choose the eyes the nose the face and

choose the eyes the nose the face and the Torso and I just thought this was

the Torso and I just thought this was the coolest thing and so in the taxi on

the coolest thing and so in the taxi on the way home I was like going on the

the way home I was like going on the website trying to purchase our very

website trying to purchase our very first Muppet I then woke up the next

first Muppet I then woke up the next morning thinking why did I just buy a

morning thinking why did I just buy a puppet in the back of a taxi and so it

puppet in the back of a taxi and so it sat on the shelf for really 2 years and

sat on the shelf for really 2 years and then a colleague of mine within cs50's

then a colleague of mine within cs50's team decided after I brought it into the

team decided after I brought it into the office to sit on a shelf there to

office to sit on a shelf there to actually bring it to life and indeed if

actually bring it to life and indeed if you Google around cs50 Muppet and

you Google around cs50 Muppet and puppetry online you'll see in fact these

puppetry online you'll see in fact these as characters not only over the past

as characters not only over the past couple of years in coid times when

couple of years in coid times when really there was next to no one actually

really there was next to no one actually here and so they were instead um but

here and so they were instead um but indeed she's brought not only this this

indeed she's brought not only this this educational element this pedagogical

educational element this pedagogical element this playful element El to cs50

element this playful element El to cs50 and we have her here today to speak to

and we have her here today to speak to exactly the sorts of encodings that are

exactly the sorts of encodings that are here on stage Jenny is the former Vice

here on stage Jenny is the former Vice chair of the Unicode subcommittee on

chair of the Unicode subcommittee on emoji which is to say that she and her

emoji which is to say that she and her colleagues have been influential in

colleagues have been influential in taking emoji from what was a very

taking emoji from what was a very limited character set early on and by

limited character set early on and by far unrepresentative of much uh human

far unrepresentative of much uh human emotion in speech into really an

emotion in speech into really an initiative now to capture digitally all

initiative now to capture digitally all of the world's languages past present

of the world's languages past present and future as well as the range of

and future as well as the range of emotions that might see here in the form

emotions that might see here in the form of that pillow or even in the cake that

of that pillow or even in the cake that awaits so allow me to introduce Jennifer

awaits so allow me to introduce Jennifer 8 Lee thank

8 Lee thank [Music]

[Music] you I much drink okay clicker hi all

you I much drink okay clicker hi all right hold on I have to hide my drinks I

right hold on I have to hide my drinks I might need more water all right um I'm

might need more water all right um I'm really excited to speak to speak here

really excited to speak to speak here last time last year I was here uh one I

last time last year I was here uh one I was wearing a mask which is like a real

was wearing a mask which is like a real bummer if you're lecturing and then the

bummer if you're lecturing and then the entire like front part was all Muppet so

entire like front part was all Muppet so I'm really happy to see humans actually

I'm really happy to see humans actually um and it's always an honor to speak at

um and it's always an honor to speak at Sanders and and then Dave and I were

Sanders and and then Dave and I were actually classmates way back when so I

actually classmates way back when so I do remember him when he was an undergrad

do remember him when he was an undergrad much like you so I um I'm going to give

much like you so I um I'm going to give a talk on the world of emoji and how I

a talk on the world of emoji and how I kind of became an emoji activist so it

kind of became an emoji activist so it kind of all starts um with my friend e

kind of all starts um with my friend e inl who is a designer who's well known

inl who is a designer who's well known for doing the Twitter fail whale which

for doing the Twitter fail whale which was kind of laid to rest except now this

was kind of laid to rest except now this week it seems like it may be like may be

week it seems like it may be like may be necessary again um so she and I one day

necessary again um so she and I one day were texting this is back in 2015 we're

were texting this is back in 2015 we're texting about dumplings because we are

texting about dumplings because we are Chinese is women and we like like to

Chinese is women and we like like to text about food and so I sent her this

text about food and so I sent her this picture of dumplings she was like yum

picture of dumplings she was like yum yum yum yum yum yum yum yum um and then

yum yum yum yum yum yum yum um and then she was like oh Apple doesn't have a

she was like oh Apple doesn't have a dumpling emoji and I was like oh that's

dumpling emoji and I was like oh that's kind of interesting and didn't really

kind of interesting and didn't really think anything about it because like you

think anything about it because like you know people Point things out to you all

know people Point things out to you all the time and then you just like forget

the time and then you just like forget you just like move on but then half an

you just like move on but then half an hour later um on my phone appears this

hour later um on my phone appears this like dumpling with hard eyes and you

like dumpling with hard eyes and you don't see it in in because it's a still

don't see it in in because it's a still shot but it actually had like blinking

shot but it actually had like blinking eyes so she liked to call it like bling

eyes so she liked to call it like bling bling dumpling so she as a designer had

bling dumpling so she as a designer had decided to like go in and like make her

decided to like go in and like make her own dumpling Emoji um because she was

own dumpling Emoji um because she was like I'm a designer I can fix it but

like I'm a designer I can fix it but that actually got me thinking I was like

that actually got me thinking I was like where do Emoji come from and like how is

where do Emoji come from and like how is there not a dumpling Emoji cuz from my

there not a dumpling Emoji cuz from my perspective dumplings are this kind of

perspective dumplings are this kind of universal food right so and there are a

universal food right so and there are a lot of Japanese Foods

lot of Japanese Foods on the emoji keyboard and I I was not

on the emoji keyboard and I I was not this was like back in 2015 I was like

this was like back in 2015 I was like not a big Emoji user like at all so I

not a big Emoji user like at all so I mean you have things like Ramen you have

mean you have things like Ramen you have Bento boxes you have Curry you have

Bento boxes you have Curry you have tempura you even have like kind of

tempura you even have like kind of obscure um kind of foods like this thing

obscure um kind of foods like this thing things on a stick turns out to be fish

things on a stick turns out to be fish fish things on a stick then this pink

fish things on a stick then this pink and white white swirly thing is also a

and white white swirly thing is also a fish thing and there's even like that

fish thing and there's even like that triangle rice ball that looks like it's

triangle rice ball that looks like it's had a bikini wax all well represented on

had a bikini wax all well represented on the Emoji Keyboard but no dumplings and

the Emoji Keyboard but no dumplings and it's very strange cuz like all cultures

it's very strange cuz like all cultures kind of have their dumpling right

kind of have their dumpling right whether or not it's um kinali or ravioli

whether or not it's um kinali or ravioli or aanas like essentially everyone sort

or aanas like essentially everyone sort of like discover the idea of like um

of like discover the idea of like um yummy goodness inside a carbo hydrate

yummy goodness inside a carbo hydrate carbohydrate shell whether not baked or

carbohydrate shell whether not baked or fried or um steamed so I was like okay I

fried or um steamed so I was like okay I literally Google I was like who controls

literally Google I was like who controls emoji and you discover that they're

emoji and you discover that they're actually regulated by a nonprofit called

actually regulated by a nonprofit called the Unicode Consortium and um it is you

the Unicode Consortium and um it is you know and I just like went on their

know and I just like went on their website and I discovered that they had

website and I discovered that they had 12 full voting members as of 2015 so

12 full voting members as of 2015 so this is 2015 and they were like mostly

this is 2015 and they were like mostly us multinational tech companies it was

us multinational tech companies it was Oracle it was IBM Microsoft Adobe Google

Oracle it was IBM Microsoft Adobe Google Apple Facebook and Yahoo and of the

Apple Facebook and Yahoo and of the three that were not multinational US

three that were not multinational US tech companies they were let's see a

tech companies they were let's see a German company called sap a Chinese

German company called sap a Chinese Telecom company called Huawei and then

Telecom company called Huawei and then like the government of Oman like those

like the government of Oman like those were basically the 12 full voting

were basically the 12 full voting members of the US multi-national tech

members of the US multi-national tech companies so they at that point paid um

companies so they at that point paid um $118,000 a year to have full voting

$118,000 a year to have full voting power on the you know Unicode committee

power on the you know Unicode committee and I was like oh that's a that's a lot

and I was like oh that's a that's a lot of money and I kind of felt indignant

of money and I kind of felt indignant about this and uh but then like if you

about this and uh but then like if you kind of keep on digging on their website

kind of keep on digging on their website you found like there was this kind of

you found like there was this kind of interesting loophole which is you could

interesting loophole which is you could join as an individual for

join as an individual for $75 um you don't get voting power but it

$75 um you don't get voting power but it gave you the right to put yourself on

gave you the right to put yourself on the email list and also to like attend

the email list and also to like attend the quarterly Unicode meeting so I was

the quarterly Unicode meeting so I was like I'll do that I had no idea what I

like I'll do that I had no idea what I was doing but I'm like I'm going to go

was doing but I'm like I'm going to go fight um for this dumpling Emoji because

fight um for this dumpling Emoji because from my perspective dumplings are

from my perspective dumplings are Universal Emoji are kind of universal so

Universal Emoji are kind of universal so uh the fact there was no dumpling Emoji

uh the fact there was no dumpling Emoji meant like something was wrong in the

meant like something was wrong in the universe and I was I was determined to

universe and I was I was determined to fix this so um you know I was on this

fix this so um you know I was on this email list and then a couple of like

email list and then a couple of like maybe even like a couple weeks later I

maybe even like a couple weeks later I got you know they they they kind of sent

got you know they they they kind of sent out this note that's like hey who's

out this note that's like hey who's coming to the quarterly meeting and I

coming to the quarterly meeting and I was like um I looked at the calendar I

was like um I looked at the calendar I looked at my schedule I was like oh I'll

looked at my schedule I was like oh I'll be in like you know Silicon Valley that

be in like you know Silicon Valley that time so I basically like rsvpd and I was

time so I basically like rsvpd and I was like I will be there and took um cow

like I will be there and took um cow train to an Apple building it's a legal

train to an Apple building it's a legal building in I think it was sunny Veil so

building in I think it was sunny Veil so I just like show up and I don't know

I just like show up and I don't know what I was sort of expecting like with

what I was sort of expecting like with like you know the Unicode I think maybe

like you know the Unicode I think maybe thought it was going to be like a baby

thought it was going to be like a baby Congress like you know like with a

Congress like you know like with a little you know very formal seats people

little you know very formal seats people with gabbles um that is not what I found

with gabbles um that is not what I found uh basically it is a it was a conference

uh basically it is a it was a conference room full of people who skewed wider

room full of people who skewed wider skewed older skewed Mal skewed engineers

skewed older skewed Mal skewed engineers and this is basically the room where it

and this is basically the room where it happen so this is 2015 these were the

happen so this is 2015 these were the people who decided your Emoji um all

people who decided your Emoji um all very nice and um there was you know one

very nice and um there was you know one one even had a daughter who had a sense

one even had a daughter who had a sense of humor and made him a shirt that said

of humor and made him a shirt that said shadowy Emoji Overlord um so I just kind

shadowy Emoji Overlord um so I just kind of listen to them debate things like

of listen to them debate things like milk emoji and beans emoji and it just

milk emoji and beans emoji and it just seemed like not quite right to me that

seemed like not quite right to me that like it would be uh This Global visual

like it would be uh This Global visual language that were basically decided

language that were basically decided decided by like a a small group of

decided by like a a small group of people inside a conference room in

people inside a conference room in Silicon Valley so I decided to former

Silicon Valley so I decided to former group called Emoji nation whose motto is

group called Emoji nation whose motto is like Emoji by the people for the people

like Emoji by the people for the people and it basically advocates for more kind

and it basically advocates for more kind of Representative inclusive Emoji um you

of Representative inclusive Emoji um you know we we we we start with a

know we we we we start with a Kickstarter campaign uh dumpling Emoji

Kickstarter campaign uh dumpling Emoji process trying to like you know write

process trying to like you know write the wrong in this world and uh made this

the wrong in this world and uh made this little cute video sort of advocating

little cute video sort of advocating dink for one of the most universal

dink for one of the most universal crosscultural Foods in the world Georgia

crosscultural Foods in the world Georgia has Kali Japan has giosa Korea has Mandu

has Kali Japan has giosa Korea has Mandu Italy has ravioli Poland has barog

Italy has ravioli Poland has barog Russia has pelman Argentina has

Russia has pelman Argentina has empanadas Jewish people have kler China

empanadas Jewish people have kler China has pot stickers Nepal and Tibet have

has pot stickers Nepal and Tibet have Momos yet somehow despite their

Momos yet somehow despite their popularity there is no dumpling Emoji in

popularity there is no dumpling Emoji in the standard set why is that emoji

the standard set why is that emoji exists for pizza tempora Sushi spaghetti

exists for pizza tempora Sushi spaghetti hot dog and now tacos which Taco Bell

hot dog and now tacos which Taco Bell takes credit for we need to write this

takes credit for we need to write this disparity dumplings are Global Emoji a

disparity dumplings are Global Emoji a global isn't it time we brought them

global isn't it time we brought them together

together oh yeah and while we're at it how about

oh yeah and while we're at it how about an emoji for Chinese

takeout so uh I did put together a dumpling Emoji proposal I wrote this uh

dumpling Emoji proposal I wrote this uh I remember Thanksgiving Day 2015 on a

I remember Thanksgiving Day 2015 on a plane and uh actually and we got it past

plane and uh actually and we got it past basically dumpling takeout box

basically dumpling takeout box Chopsticks and uh fortune cookie I have

Chopsticks and uh fortune cookie I have to say I don't think fortune cookie

to say I don't think fortune cookie would have made it on its own merits but

would have made it on its own merits but it kind of like slid in on the clo tails

it kind of like slid in on the clo tails of the other ones um and so these were

of the other ones um and so these were the proposals as we submitted them and

the proposals as we submitted them and then these are the ones that kind of uh

then these are the ones that kind of uh exist now on the Apple keyboard and I

exist now on the Apple keyboard and I have to say the dumpling looks really

have to say the dumpling looks really really realistic um oddly realistic and

really realistic um oddly realistic and whereas like the fortune cookie is think

whereas like the fortune cookie is think it's like a big fail because first of

it's like a big fail because first of all there's like it has no it has no Gap

all there's like it has no it has no Gap it looks like a dead 3D Pac-Man so I'm

it looks like a dead 3D Pac-Man so I'm very

very disappointed in uh the manifestation of

disappointed in uh the manifestation of that but that's okay that's okay um and

that but that's okay that's okay um and so it's kind of interesting like what is

so it's kind of interesting like what is the process of getting an emoji um ped

the process of getting an emoji um ped and I will sort of walk you through it

and I will sort of walk you through it so first of all you come up with your

so first of all you come up with your idea right and then you kind of write

idea right and then you kind of write this proposal and then you submit it to

this proposal and then you submit it to the Unicode Emoji subcommittee who then

the Unicode Emoji subcommittee who then kind of gives you comments and then

kind of gives you comments and then sends it back to you and you kind of go

sends it back to you and you kind of go around and around in the circle um and

around and around in the circle um and when so these are things that we

when so these are things that we consider so somewhere in there I also

consider so somewhere in there I also like fought my way on to the Emoji

like fought my way on to the Emoji committee and then also became a vice

committee and then also became a vice chair became a vice chair sort of a

chair became a vice chair sort of a extracurricular that's like completely

extracurricular that's like completely run a muck in my life um um so things

run a muck in my life um um so things that matter uh popular demand is a

that matter uh popular demand is a frequently requested Emoji um multiple

frequently requested Emoji um multiple usages usages and meaning so that's

usages usages and meaning so that's actually kind of very important for

actually kind of very important for something like you know certain animals

something like you know certain animals have meaning so we did you know sloth a

have meaning so we did you know sloth a while ago and that also has not only the

while ago and that also has not only the literal meaning but sort of like um like

literal meaning but sort of like um like um connotations there are visually

um connotations there are visually distinctiveness it can be recognized so

distinctiveness it can be recognized so this was a blocker for one of the uh

this was a blocker for one of the uh emojis that I have worked on before

emojis that I have worked on before which is cave which is kind of hard to

which is cave which is kind of hard to do I think you know meaning wise is

do I think you know meaning wise is pretty good but it's really hard to get

pretty good but it's really hard to get a cave down in emoji sizes and then it

a cave down in emoji sizes and then it filling a gap completeness so a good

filling a gap completeness so a good example for that is like for some reason

example for that is like for some reason for many years we had Red Heart yellow

for many years we had Red Heart yellow heart green heart blue heart purple

heart green heart blue heart purple heart and uh no orange heart so so

heart and uh no orange heart so so somewhat so you know you would do the

somewhat so you know you would do the rainbow but people would substitute the

rainbow but people would substitute the pumpkin in so You' like you know have a

pumpkin in so You' like you know have a have a rainbow heart thing with a little

have a rainbow heart thing with a little pumpkin stuck in the middle so orange

pumpkin stuck in the middle so orange heart obviously should be added and give

heart obviously should be added and give a sense of completeness um and then

a sense of completeness um and then something else is existing vendor cap

something else is existing vendor cap cap compatibility and so a good example

cap compatibility and so a good example for that was um many years ago what's

for that was um many years ago what's app decided to uh add the gender

app decided to uh add the gender non-binary emoji and then once it did

non-binary emoji and then once it did that then all the other kind of vendors

that then all the other kind of vendors um jumped on um so what kind of knocks

um jumped on um so what kind of knocks out an emoji so too specific or narrow

out an emoji so too specific or narrow so we'll often see that with like very

so we'll often see that with like very specific animals or a very specific

specific animals or a very specific group it's redundant so one

group it's redundant so one year oh my God who makes that Butterball

year oh my God who makes that Butterball Butterball makes the turkeys Butterball

Butterball makes the turkeys Butterball submitted um a an emoji proposal that

submitted um a an emoji proposal that was like a cooked turkey but we already

was like a cooked turkey but we already had a live turkey so it seem kind of

had a live turkey so it seem kind of redundant have both like a cooked turkey

redundant have both like a cooked turkey for Thanksgiving and a live turkey so so

for Thanksgiving and a live turkey so so not visually discernible um this is a

not visually discernible um this is a struggle for things like I don't my

struggle for things like I don't my friends have kind of proposed

friends have kind of proposed kimchi kimchi is is really hard on emoji

kimchi kimchi is is really hard on emoji sizes for many reasons and part of that

sizes for many reasons and part of that but part of that tension is because it's

but part of that tension is because it's not visually discernable then there are

not visually discernable then there are no logos Brands deities or celebrities

no logos Brands deities or celebrities so no Nike swish no McDonald's M and

so no Nike swish no McDonald's M and then this is one that we kind of decided

then this is one that we kind of decided in the last uh year or so which is no

in the last uh year or so which is no more flags flags are a very complicated

more flags flags are a very complicated thing um and as a result Unico does not

thing um and as a result Unico does not want to be in the business of deciding

want to be in the business of deciding what is a country or not a country uh so

what is a country or not a country uh so like you know when you get a proposal

like you know when you get a proposal from like Kurdistan you're like yeah so

from like Kurdistan you're like yeah so right now the way that the Emoji flags

right now the way that the Emoji flags are decided is they kind of depend on

are decided is they kind of depend on what the UN recognizes and then those

what the UN recognizes and then those get passed down to the international

get passed down to the international standards organization and then and then

standards organization and then and then Uno just does that like it does not want

Uno just does that like it does not want to be in the business of kind of you

to be in the business of kind of you know geopolitical

know geopolitical Affairs um so once it comes out of the

Affairs um so once it comes out of the subcommittee it goes to the full Unicode

subcommittee it goes to the full Unicode technical committee UTC those were the

technical committee UTC those were the people that were in the room that I

people that were in the room that I showed you um and they vote once a year

showed you um and they vote once a year basically to pass all the Emoji and

basically to pass all the Emoji and takes a while there's a lot of like

takes a while there's a lot of like coordinating with the international

coordinating with the international standards organization and like a lot of

standards organization and like a lot of working with the companies and

working with the companies and eventually it takes a long time but it

eventually it takes a long time but it ends up on your on your devices and that

ends up on your on your devices and that historically is taken about 18 to 24

historically is taken about 18 to 24 months from when you put in the propose

months from when you put in the propose to when you get it in it's going to be a

to when you get it in it's going to be a little bit longer um going forward

little bit longer um going forward probably is my sense as we were're going

probably is my sense as we were're going to Unicode has decided to sort of space

to Unicode has decided to sort of space out some of the Unicode releases so yeah

out some of the Unicode releases so yeah Emoji Nation you know kind of has done

Emoji Nation you know kind of has done its thing so of the weird things is like

its thing so of the weird things is like how did Unicode this like kind of

how did Unicode this like kind of nonprofit organization based in Mountain

nonprofit organization based in Mountain View California end up controlling this

View California end up controlling this like Global visual language um so a lot

like Global visual language um so a lot of it has to start with has has to do

of it has to start with has has to do with the fact that emoji started in

with the fact that emoji started in Japan uh back in the late 1990s one of

Japan uh back in the late 1990s one of the this set from dok 1999 is considered

the this set from dok 1999 is considered is widely considered sort of like the

is widely considered sort of like the first like color Emoji set it has been

first like color Emoji set it has been collected by the Museum of Modern Art

collected by the Museum of Modern Art and um so these the Japanese telecom

and um so these the Japanese telecom companies would use

companies would use basically would have their own sets of

basically would have their own sets of emoji and then they were different

emoji and then they were different companies so they would have different

companies so they would have different sets so you could basically only send

sets so you could basically only send these like visual characters with

these like visual characters with someone who is on your same carrier so

someone who is on your same carrier so it's like it's like basically equivalent

it's like it's like basically equivalent of if you were on like Verizon you can

of if you were on like Verizon you can only text people on Verizon with like

only text people on Verizon with like Emoji or if you were on um T-Mobile you

Emoji or if you were on um T-Mobile you could only do that so at a certain point

could only do that so at a certain point they um decided they were they the Apple

they um decided they were they the Apple and Google came into Japan they wanted

and Google came into Japan they wanted to start selling smartphones and they

to start selling smartphones and they realized that it was a hodgepodge of

realized that it was a hodgepodge of systems and they wanted to unify it and

systems and they wanted to unify it and so in 2007 they went to unic code and

so in 2007 they went to unic code and they're like okay help us unify the

they're like okay help us unify the Emoji um like kind of like basically all

Emoji um like kind of like basically all the Emojis so that we have one standard

the Emojis so that we have one standard system and part of the reason is why

system and part of the reason is why Unicode because Unicode basically has

Unicode because Unicode basically has this mission is to enable everyone

this mission is to enable everyone speaking every language on Earth to be

speaking every language on Earth to be able to use your languages on computer

able to use your languages on computer and smartphone so it basically unifies

and smartphone so it basically unifies um all written languages into one

um all written languages into one ginormous set and that was not the case

ginormous set and that was not the case actually when I was uh growing up there

actually when I was uh growing up there was a point where like if you you were

was a point where like if you you were Japanese on Apple that would be

Japanese on Apple that would be different than Japanese on um dos or

different than Japanese on um dos or like Chinese or Arabic so it drove

like Chinese or Arabic so it drove everyone crazy and they basically

everyone crazy and they basically decided um around the late 80s early

decided um around the late 80s early 1990s that they were going to come up

1990s that they were going to come up with one standardized system that sort

with one standardized system that sort of encoded all characters in in one

of encoded all characters in in one ginormous set so um there's three main

ginormous set so um there's three main projects for Unicode if you care so one

projects for Unicode if you care so one is encoding characters including Emoji

is encoding characters including Emoji now they're about 100,000 characters

now they're about 100,000 characters aside assigned so that includes like

aside assigned so that includes like like Chinese Japanese Korean uh Arabic

like Chinese Japanese Korean uh Arabic cilic um actually all the hieroglyphics

cilic um actually all the hieroglyphics all of the Emoji um a lot of things like

all of the Emoji um a lot of things like the Bitcoin symbol or like copy left or

the Bitcoin symbol or like copy left or whatever those all assigned were about

whatever those all assigned were about 100,000 uh characters even those

100,000 uh characters even those languages that are basically out of use

languages that are basically out of use so the other thing it does it creates

so the other thing it does it creates um localization resources so things

um localization resources so things things so that you know like oh if

things so that you know like oh if you're in this country this is this is

you're in this country this is this is uh you're using the Euro or you're using

uh you're using the Euro or you're using the pesos or something so there's a lot

the pesos or something so there's a lot of localization that is that data that

of localization that is that data that is needed depending on which um

is needed depending on which um geography you're using your device from

geography you're using your device from or like that you know you you know that

or like that you know you you know that the time is used this way or the dates

the time is used this way or the dates are shown that way so that is called um

are shown that way so that is called um the common local data repository or cldr

the common local data repository or cldr as they call it and the other thing they

as they call it and the other thing they do is they kind of maintain libraries

do is they kind of maintain libraries for developers in order to do all um to

for developers in order to do all um to kind of so they are not building

kind of so they are not building everything from scratch and that is

everything from scratch and that is called ICU so it's very funny cuz like

called ICU so it's very funny cuz like it's called cldr and then one day one of

it's called cldr and then one day one of our friends girlfriends made him this

our friends girlfriends made him this thing which she called a seal deer

thing which she called a seal deer because whenever he talked about

because whenever he talked about cldr he thought he thought she thought

cldr he thought he thought she thought he was talking about seal deers so now

he was talking about seal deers so now this is his little like mascot for like

this is his little like mascot for like Unicode related things so what is um how

Unicode related things so what is um how does Unicode do this so it basically

does Unicode do this so it basically asss code points and each code point is

asss code points and each code point is a unique number assigned to each Unicode

a unique number assigned to each Unicode character so uh face with tears of joy

character so uh face with tears of joy can be written like this with a code

can be written like this with a code point or it can be written like this in

point or it can be written like this in terms of decimal numbers and it can also

terms of decimal numbers and it can also be written this in binary numbers so

be written this in binary numbers so these are all basically the same and the

these are all basically the same and the key thing to know at least about Emoji

key thing to know at least about Emoji is that when your device if you're

is that when your device if you're talking your iPhone with um someone

talking your iPhone with um someone who's on a laptop or an iPad or like um

who's on a laptop or an iPad or like um an Android device it's not sending the

an Android device it's not sending the images back and forth it's sending just

images back and forth it's sending just a number back and forth and then locally

a number back and forth and then locally your phone or like your laptop decides

your phone or like your laptop decides like oh this number correlates with

like oh this number correlates with which image um in terms of our emoji

which image um in terms of our emoji font and then pulls it up so this is

font and then pulls it up so this is really key to know why different Emoji

really key to know why different Emoji look different on different

look different on different platforms so 2007 to 2010 it took about

platforms so 2007 to 2010 it took about three years but Unicode 6.0 came out

three years but Unicode 6.0 came out with a first our first little baby set

with a first our first little baby set of emoji um and it just kind of hung out

of emoji um and it just kind of hung out there for a year like no one it wasn't

there for a year like no one it wasn't doing anything so 2011 though Apple

doing anything so 2011 though Apple starts adding the emoji keyboard and it

starts adding the emoji keyboard and it it just like explodes like I feel like

it just like explodes like I feel like in some ways Emoji are were not like

in some ways Emoji are were not like invented they were discovered they like

invented they were discovered they like obviously touch something very very

obviously touch something very very Primal to um to like our human desire to

Primal to um to like our human desire to like communicate in like little colorful

like communicate in like little colorful glyphs on electronic devices and you

glyphs on electronic devices and you kind of have what's kind of really

kind of have what's kind of really interesting is like the ambiguity that

interesting is like the ambiguity that comes with what emoji kind of mean and

comes with what emoji kind of mean and so one of the you know this one my

so one of the you know this one my favorite emoji is sort of like an upside

favorite emoji is sort of like an upside down smiley face very very ambiguous

down smiley face very very ambiguous clearly very ambiguous because if you

clearly very ambiguous because if you start typing into Google like the top

start typing into Google like the top hits are like what does it mean from a

hits are like what does it mean from a guy what does it mean from a girl like

guy what does it mean from a girl like it's clearly something that a lot of

it's clearly something that a lot of people are are using in like complex

people are are using in like complex situationships between each other so um

situationships between each other so um so so one of the fun things is who can

so so one of the fun things is who can propose Emoji technically anyone can

propose Emoji technically anyone can anyone here can normal normal humans can

anyone here can normal normal humans can we have basically Google doc uh or

we have basically Google doc uh or Google form that we throw up uh

Google form that we throw up uh historically between August sorry April

historically between August sorry April and August in the last two years um and

and August in the last two years um and uh so this is one of our my our favorite

uh so this is one of our my our favorite examples this is Ru aumed she was a

examples this is Ru aumed she was a 15-year-old Saudi Arabian girl who was

15-year-old Saudi Arabian girl who was living in Vienna at the time that she

living in Vienna at the time that she proposed the um hijab emoji and then you

proposed the um hijab emoji and then you know then she was like Time Magazine

know then she was like Time Magazine like you know coolest teens she got like

like you know coolest teens she got like a whole bunch of different things she

a whole bunch of different things she got into Harvard and Stanford and she

got into Harvard and Stanford and she went to Stanford um and uh so this is a

went to Stanford um and uh so this is a PR we got then there was a group of

PR we got then there was a group of folks from Argentina who got the mate

folks from Argentina who got the mate Emoji kind of they you know similar

Emoji kind of they you know similar their national drink then we worked with

their national drink then we worked with um there was a a nonprofit that really

um there was a a nonprofit that really wanted to like get a menstruation emoji

wanted to like get a menstruation emoji and so what they proposed to begin with

and so what they proposed to begin with was like bloody underwear and I was just

was like bloody underwear and I was just like no no no so many different reasons

like no no no so many different reasons why this is a terrible Emoji uh but so

why this is a terrible Emoji uh but so we did get blood drops so then you can

we did get blood drops so then you can do Moon and blood drop or underwear and

do Moon and blood drop or underwear and blood drop there's a lot of different

blood drop there's a lot of different ways actually it's really funny cuz I

ways actually it's really funny cuz I was ping my friends beforehand what they

was ping my friends beforehand what they would use to indicate menstruation

would use to indicate menstruation before there you know there was a blood

before there you know there was a blood drop so there was like red wine there

drop so there was like red wine there was like kind of that Rose um with the

was like kind of that Rose um with the like falling petal and then my favorite

like falling petal and then my favorite is actually um my friend who used a

is actually um my friend who used a Japanese flag as a way to indicate that

Japanese flag as a way to indicate that she was having her

she was having her period so um one of the biggest

period so um one of the biggest contributors this uh skin tone Emoji

contributors this uh skin tone Emoji appeared I think in 2015 it was amazing

appeared I think in 2015 it was amazing it is and it was proposed by a mom

it is and it was proposed by a mom Katrina parrot she is a entrepreneur and

Katrina parrot she is a entrepreneur and a mom who is just like at home one day

a mom who is just like at home one day and her daughter comes home and is

and her daughter comes home and is like you know I wish there were emoji

like you know I wish there were emoji that looked like me and her mom was like

that looked like me and her mom was like that's great honey what's an emoji and

that's great honey what's an emoji and so she like me I guess Googled and just

so she like me I guess Googled and just figured out that uh Unicode controlled

figured out that uh Unicode controlled emoji and she just came up with a

emoji and she just came up with a proposal saying we should not only have

proposal saying we should not only have the yellow you know skin tones at that

the yellow you know skin tones at that time everything was sort of Simpsons

time everything was sort of Simpsons yellow um it's really interesting to see

yellow um it's really interesting to see how race and like nationality are like

how race and like nationality are like depicted in different parts of the world

depicted in different parts of the world so originally in Japan everyone was

so originally in Japan everyone was yellow um but this these were the non

yellow um but this these were the non you know by default everyone was just

you know by default everyone was just like like you know human or Japanese but

like like you know human or Japanese but they had like a couple things that were

they had like a couple things that were like not like one was you had a blonde

like not like one was you had a blonde person so there's an emoji called like

person so there's an emoji called like blonde blonde woman or whatever that

blonde blonde woman or whatever that represents all westerners okay so that

represents all westerners okay so that was one and then they have one that's

was one and then they have one that's like an Indian guy with a turban so

like an Indian guy with a turban so that's supposed to represent Indian

that's supposed to represent Indian people and then there's like a like a

people and then there's like a like a guy with a little like um little hat

guy with a little like um little hat that's supposed to represent like

that's supposed to represent like Chinese people so that was like that was

Chinese people so that was like that was the view Japanese view of race which was

the view Japanese view of race which was like default then you were I then you

like default then you were I then you were like blonde Western Chinese or

were like blonde Western Chinese or Indian and and that is all there was and

Indian and and that is all there was and um obviously in the United States we

um obviously in the United States we care a lot about race and then so she

care a lot about race and then so she came up with this system with five skin

came up with this system with five skin tones um like just like normal people

tones um like just like normal people some guy in Germany decided that he

some guy in Germany decided that he wanted to do a a a face with one eyebrow

wanted to do a a a face with one eyebrow raised or as we call it the coar Emoji

raised or as we call it the coar Emoji um and oh this one's fun so woman's flat

um and oh this one's fun so woman's flat shoe I have to say not highly used

shoe I have to say not highly used statistically at this point but I really

statistically at this point but I really kind of appreciate it because it was a

kind of appreciate it because it was a mom who was very offended that all

mom who was very offended that all women's shoes had heels even the sandals

women's shoes had heels even the sandals so this is her she had like three kids

so this is her she had like three kids at the time now is four she was very

at the time now is four she was very fertile um and she also did um the women

fertile um and she also did um the women women's flat okay she also did um um one

women's flat okay she also did um um one piece bathing suit because she was also

piece bathing suit because she was also um offended by the fact that the only

um offended by the fact that the only kind of bathing suit you had was like

kind of bathing suit you had was like this like little itsy-bitsy polka dot

this like little itsy-bitsy polka dot bikini thing which is not great if

bikini thing which is not great if you're like taking your six-year-old so

you're like taking your six-year-old so I have to say that got passed but like

I have to say that got passed but like it didn't go over like super well with

it didn't go over like super well with everyone um so you know unic Cod because

everyone um so you know unic Cod because it's very public submits things for

it's very public submits things for comments and um we got this comment back

comments and um we got this comment back so one piece bathing suit why a person

so one piece bathing suit why a person want to indicate the use of swimwear

want to indicate the use of swimwear can't use existing bikini is this really

can't use existing bikini is this really necessary what about a Victorian bathing

necessary what about a Victorian bathing costume or a wet suit or water rings

costume or a wet suit or water rings this is like literally in in like uh the

this is like literally in in like uh the records and like do not encode um and so

records and like do not encode um and so the person who did it's actually very

the person who did it's actually very impressive he's actually the person who

impressive he's actually the person who created the middle finger emoji and

created the middle finger emoji and actually you ever seen the Vulcan Emoji

actually you ever seen the Vulcan Emoji he he the Vulcan hand emoji he's

he he the Vulcan hand emoji he's actually very active and I have to say

actually very active and I have to say this is actually I think one of the more

this is actually I think one of the more impressive Emoji so so obviously

impressive Emoji so so obviously obviously we have a lot of active debate

obviously we have a lot of active debate um sometimes you get like whole

um sometimes you get like whole countries submitting so literally the

countries submitting so literally the government of Finland as in like their

government of Finland as in like their equivalent in the state department their

equivalent in the state department their diplomacy kind of thing uh submitted a

diplomacy kind of thing uh submitted a proposal for SAA and um these were so

proposal for SAA and um these were so creepy and so

creepy and so weird so much is wrong with this and but

weird so much is wrong with this and but I I I felt where they were coming from

I I I felt where they were coming from first of all like they're naked they

first of all like they're naked they have no they have club feet and so we

have no they have club feet and so we decided to help them we're like okay we

decided to help them we're like okay we see where you're going with this let's

see where you're going with this let's see if we can like help you like come up

see if we can like help you like come up you know like we had the spoon like

you know like we had the spoon like should there be like steam around them

should there be like steam around them like should they be naked or wearing a

like should they be naked or wearing a tow towel it's like super like dicey but

tow towel it's like super like dicey but we wanted to help them because it was

we wanted to help them because it was like literally a foreign government

like literally a foreign government coming from an Unicode advocating for

coming from an Unicode advocating for the sauna emoji on behalf of their ENT

the sauna emoji on behalf of their ENT country so then this is um is sort of

country so then this is um is sort of evolved into just basically person in

evolved into just basically person in stey room which is which is the most

stey room which is which is the most sort of like the PG version of sauna no

sort of like the PG version of sauna no and there's no spoon they're all dressed

and there's no spoon they're all dressed it's very odd but um so you can see the

it's very odd but um so you can see the evolution of what it started out what we

evolution of what it started out what we submitted and what it ended up so

submitted and what it ended up so there's a lot of like Evolution

there's a lot of like Evolution throughout the entire process um and

throughout the entire process um and like companies can submit Emoji

like companies can submit Emoji proposals too so Google actually worked

proposals too so Google actually worked on this one I love this one okay so just

on this one I love this one okay so just to give you some context as of 2015

to give you some context as of 2015 there are many ways you could be or have

there are many ways you could be or have an occupation as a male on the Emoji

an occupation as a male on the Emoji Keyboard right like like for example you

Keyboard right like like for example you could be a police officer you could be a

could be a police officer you could be a detective you could be a Buckingham

detective you could be a Buckingham Palace guard you could even be Santa

Palace guard you could even be Santa Claus like these are so many jobs that

Claus like these are so many jobs that you could have but if you were a woman

you could have but if you were a woman as of 2015 there were four things that

as of 2015 there were four things that you could be you could be a princess you

you could be you could be a princess you could be a bride you could be a dancer

could be a bride you could be a dancer or you could be a Playboy Bunny these

or you could be a Playboy Bunny these were the sum total of all the

were the sum total of all the occupations that we can have so so we

occupations that we can have so so we there was sort of this movement at that

there was sort of this movement at that time there was like this like video on

time there was like this like video on YouTube that like went viral there was

YouTube that like went viral there was like a New York Times op that was like

like a New York Times op that was like where are the women with professions so

where are the women with professions so um basically they came up with a set of

um basically they came up with a set of emoji for professions and what's nice is

emoji for professions and what's nice is not only did women have these

not only did women have these professions now men have them too so of

professions now men have them too so of emoji Nation Emoji these are some of the

emoji Nation Emoji these are some of the ones that we've worked on I think about

ones that we've worked on I think about 130 of the Emoji on your keyboard

130 of the Emoji on your keyboard probably came through touched our system

probably came through touched our system in in some way including I I have to say

in in some way including I I have to say uh microbe or virus I think I have the

uh microbe or virus I think I have the opinion that every Emoji has its day

opinion that every Emoji has its day right like it might not be like today it

right like it might not be like today it might not be next year but I have to say

might not be next year but I have to say virus was not doing anything then came

virus was not doing anything then came 2020 and that was like such a good

2020 and that was like such a good moment for it um along with soap we had

moment for it um along with soap we had also done soap so um you know among the

also done soap so um you know among the other emoji that we have worked on are

other emoji that we have worked on are sari moon cake llama like uh teddy bear

sari moon cake llama like uh teddy bear there were no toys I felt really sad for

there were no toys I felt really sad for toys we have like giraffe um there was

toys we have like giraffe um there was Hut bubble te bubble tea was very

Hut bubble te bubble tea was very controversial actually I have to say it

controversial actually I have to say it kind we tried to slide it in originally

kind we tried to slide it in originally with the um takeout box and the

with the um takeout box and the dumplings and people were not not having

dumplings and people were not not having it I understand that CU there's not a

it I understand that CU there's not a lot like compared to like beer or wine

lot like compared to like beer or wine like bubble tea does not is not long

like bubble tea does not is not long does not have a long history on this

does not have a long history on this planet um but I will say that um they

planet um but I will say that um they submitted again actually kind of

submitted again actually kind of originally proposing that it was not

originally proposing that it was not just bubble tea but but like a black

just bubble tea but but like a black ball and milk and tea it was It was kind

ball and milk and tea it was It was kind of cool and I I have to say there was

of cool and I I have to say there was definitely a generational divide between

definitely a generational divide between like the Asian women who sit in that

like the Asian women who sit in that room and are like this absolutely is a

room and are like this absolutely is a thing that we consume like almost like

thing that we consume like almost like every week of our life and people who

every week of our life and people who are a little bit older who are like that

are a little bit older who are like that looks like a parfait how do you not know

looks like a parfait how do you not know that's a parfet and we're like we

that's a parfet and we're like we absolutely know it is uh not a parfait

absolutely know it is uh not a parfait and so so it got in eventually so it

and so so it got in eventually so it does sort of influence it kind of shows

does sort of influence it kind of shows like who is in the room influences you

like who is in the room influences you know the decisions um that get made or

know the decisions um that get made or sometimes in the room now sometimes more

sometimes in the room now sometimes more more likely in the zoom um I actually I

more likely in the zoom um I actually I just say beaver Emoji if you see Beaver

just say beaver Emoji if you see Beaver Emoji that's one of one of the ones I'm

Emoji that's one of one of the ones I'm most proud of so that is actually

most proud of so that is actually co-authored by a professor here at

co-authored by a professor here at Harvard um who is both lesbian and was

Harvard um who is both lesbian and was married to a woman from Canada so it was

married to a woman from Canada so it was very important to her to get um the

very important to her to get um the beaver Emoji passed and she promised me

beaver Emoji passed and she promised me it would always be the first line of her

it would always be the first line of her bio so and indeed if you go to her

bio so and indeed if you go to her Twitter handle uh it's like Joan Donovan

Twitter handle uh it's like Joan Donovan creator of the Beaver Emoji comma is

creator of the Beaver Emoji comma is headed research at the shoren scene

headed research at the shoren scene Center at the Harvard Kennedy School

Center at the Harvard Kennedy School It's Kind it's pretty impressive um and

It's Kind it's pretty impressive um and then we did greens actually greens was

then we did greens actually greens was really interesting because people this

really interesting because people this was also like a generational cultural

was also like a generational cultural thing people were like why do we need

thing people were like why do we need greens we have salad and I was like

greens we have salad and I was like we're Chinese we don't eat raw greens

we're Chinese we don't eat raw greens cuz like you don't know where it's been

cuz like you don't know where it's been or if it's clean so we cook our greens

or if it's clean so we cook our greens so salad is not something that we have

so salad is not something that we have So eventually I got my greens and um so

So eventually I got my greens and um so that was kind of fun and then so these

that was kind of fun and then so these are some of the people who sort of have

are some of the people who sort of have contributed to our little Emoji Nation

contributed to our little Emoji Nation things including a number of Native

things including a number of Native Americans who help get feather so why do

Americans who help get feather so why do I care so part of it is because I'm

I care so part of it is because I'm Chinese I grew up speaking Chinese and

Chinese I grew up speaking Chinese and English at the same time and it's really

English at the same time and it's really interesting to see like Chinese and

interesting to see like Chinese and English characters in terms of emoji and

English characters in terms of emoji and Chinese together right so this is fire

Chinese together right so this is fire you have like Fire characters and now

you have like Fire characters and now you have the Emoji things and it's kind

you have the Emoji things and it's kind of It kind of shows that there is a

of It kind of shows that there is a longevity in The Human Experience of

longevity in The Human Experience of something that was designed 4,000 years

something that was designed 4,000 years ago as the same visual cues in the human

ago as the same visual cues in the human experiences as now so mouth Tre tree

experiences as now so mouth Tre tree Moon Sun you can mix and match them

Moon Sun you can mix and match them which is super fun um so you know two

which is super fun um so you know two trees kind of make a forest oops sorry

trees kind of make a forest oops sorry oh well okay then the um Moon and Sun

oh well okay then the um Moon and Sun together means bright which I like um I

together means bright which I like um I like this one so if you stop and you

like this one so if you stop and you think about this so this is a basically

think about this so this is a basically a pig under a roof and what does that

a pig under a roof and what does that mean

mean it does not mean Farm as you might think

it does not mean Farm as you might think it um it actually means home or family

it um it actually means home or family so in the Chinese kind of structure and

so in the Chinese kind of structure and outlook on The View it's like where you

outlook on The View it's like where you keep your pigs is actually where your

keep your pigs is actually where your home is and what your farm is um so it

home is and what your farm is um so it gets kind of weird in all kinds of ways

gets kind of weird in all kinds of ways so one of my favorite radicals so this

so one of my favorite radicals so this character means woman uh KN

character means woman uh KN and as I was learning Chinese you kind

and as I was learning Chinese you kind of notice like how it shows up so so

of notice like how it shows up so so this is a woman underneath a roof and

this is a woman underneath a roof and you're like oh it means mom or wife or

you're like oh it means mom or wife or or like whatever like home um it does

or like whatever like home um it does not it means peace because things are at

not it means peace because things are at home when the woman is or things are at

home when the woman is or things are at peace when the woman is at home

peace when the woman is at home underneath a roof which I always thought

underneath a roof which I always thought was a little bit odd um then there is

was a little bit odd um then there is also uh woman plus child so you're like

also uh woman plus child so you're like oh and actually specifically Boy Child

oh and actually specifically Boy Child the connotation there is a little

the connotation there is a little unclear so you're like a woman plus

unclear so you're like a woman plus child family mom you know whatever um it

child family mom you know whatever um it is not it means good so the standard for

is not it means good so the standard for goodness in ancient China was a woman

goodness in ancient China was a woman who had a male child child so that kind

who had a male child child so that kind of kind of just like kind of irked me um

of kind of just like kind of irked me um growing up and then you know three women

growing up and then you know three women together means evil which is like very

together means evil which is like very like

like MCB this character means greed this

MCB this character means greed this character means slave this marriage

character means slave this marriage let's see I think this one is jealousy

let's see I think this one is jealousy and this means means adultery or

and this means means adultery or betrayal so like definitely not loving

betrayal so like definitely not loving the way women were P you know portrayed

the way women were P you know portrayed um on the emoji keyboard so uh in case

um on the emoji keyboard so uh in case you're wondering we just came out with a

you're wondering we just came out with a kids book called had emoji that kind of

kids book called had emoji that kind of Compares emoji and uh Chinese and I

Compares emoji and uh Chinese and I think I I think they sent a bunch of

think I I think they sent a bunch of books so that you guys can do some kind

books so that you guys can do some kind of contest some like later on with cs50

of contest some like later on with cs50 um so but the mixing and matching is

um so but the mixing and matching is really interesting right for example the

really interesting right for example the skin tones are actually the same yellow

skin tones are actually the same yellow character plus a layer of skin tone on

character plus a layer of skin tone on top of this so I kind of took my lessons

top of this so I kind of took my lessons from Chinese in terms of seeing how

from Chinese in terms of seeing how things can be binded so there's

things can be binded so there's something you should know about which is

something you should know about which is zge this is also an invisible emoji

zge this is also an invisible emoji character it stands for zero with Joiner

character it stands for zero with Joiner and it was actually originally created

and it was actually originally created mostly for I think Arabic where you

mostly for I think Arabic where you would you would basically kind of force

would you would basically kind of force something to have like be in the

something to have like be in the beginning of of a word or an end of the

beginning of of a word or an end of the word by kind of having this like

word by kind of having this like invisible character so the rainbow flag

invisible character so the rainbow flag for example is actually a rainbow plus

for example is actually a rainbow plus the white flag

the white flag um and we we could have all kinds of fun

um and we we could have all kinds of fun combinations if you look at polar bear

combinations if you look at polar bear it actually is if you have an older

it actually is if you have an older device or it breaks apart it is bear

device or it breaks apart it is bear plus snow which is really cute it was

plus snow which is really cute it was originally I had bear plus white and

originally I had bear plus white and then we decided that bear plus snow made

then we decided that bear plus snow made a lot more sense so uh another one this

a lot more sense so uh another one this is new I I think if you guys should have

is new I I think if you guys should have it if you've updated your phones in the

it if you've updated your phones in the last year or so so mending heart is

last year or so so mending heart is heart plus

heart plus Band-Aid um um what is this one oh this

Band-Aid um um what is this one oh this is interesting um there was a

is interesting um there was a breastfeeding woman for a long time and

breastfeeding woman for a long time and people felt like there was not gender

people felt like there was not gender par was actually really interesting all

par was actually really interesting all the people who wrote it and were like I

the people who wrote it and were like I want to I want to be a dad showing that

want to I want to be a dad showing that I'm holding my baby why is there only a

I'm holding my baby why is there only a woman holding my baby so so we kind of

woman holding my baby so so we kind of created a whole set so this is man plus

created a whole set so this is man plus bottle Tada um and so this is and all of

bottle Tada um and so this is and all of these occupations are actually often

these occupations are actually often times a woman plus like you know the

times a woman plus like you know the fried egg or like um a school or a

fried egg or like um a school or a tractor and that's how you got the

tractor and that's how you got the occupations if you if you if you send an

occupations if you if you if you send an emoji over into an older system

emoji over into an older system sometimes you'll it'll break apart so

sometimes you'll it'll break apart so one of my kind of favorite kind of

one of my kind of favorite kind of contributions in this in this world is

contributions in this in this world is interracial couple Emoji which we did

interracial couple Emoji which we did with um Tinder which is super fun

with um Tinder which is super fun because then you could have you know

because then you could have you know different couples that are and and like

different couples that are and and like so many combin this is like this is a

so many combin this is like this is a fun if you guys ever have to do a

fun if you guys ever have to do a combinatoric test um this is really fun

combinatoric test um this is really fun because you have two genders plus a

because you have two genders plus a third like gender plus two people plus

third like gender plus two people plus five skin tones plus yellow like how

five skin tones plus yellow like how many Emoji couples can you come come up

many Emoji couples can you come come up with when you introduce this Factor so

with when you introduce this Factor so this is um and underneath it it's just a

this is um and underneath it it's just a wige sequence it's like two people

wige sequence it's like two people standing together that are like glued

standing together that are like glued together now this actually gets

together now this actually gets interesting from a cs-50 perspective

interesting from a cs-50 perspective because in many cases even though you

because in many cases even though you only see one character underneath the

only see one character underneath the hood depending on how your system works

hood depending on how your system works they're counting each one of these as an

they're counting each one of these as an individual character so your string

individual character so your string length actually might be five and

length actually might be five and instead of one and this this kind of

instead of one and this this kind of became a problem with things like

became a problem with things like Twitter where things had a hard skin um

Twitter where things had a hard skin um length so gender inclusivity um is

length so gender inclusivity um is actually one of the things that we've

actually one of the things that we've been dealing most with um in the last

been dealing most with um in the last couple years so it's kind of interesting

couple years so it's kind of interesting if you think about both what a pictorial

if you think about both what a pictorial language looks like versus the

language looks like versus the abstractness of a of a spoken language

abstractness of a of a spoken language so because you know we had boy and we

so because you know we had boy and we had

had girl but there was no way to say a

girl but there was no way to say a generic child right like if you were on

generic child right like if you were on you wanted to say child you had to pick

you wanted to say child you had to pick a boy or a girl but not a way to say

a boy or a girl but not a way to say like just some little person and that's

like just some little person and that's really key cuz in English at least there

really key cuz in English at least there is no gender implied by

is no gender implied by child um so how do we mimic that and it

child um so how do we mimic that and it also is key for something like doctor

also is key for something like doctor right doctor and teacher those are those

right doctor and teacher those are those have don't have gender implied but when

have don't have gender implied but when we have them on on the Emoji Keyboard

we have them on on the Emoji Keyboard you to pick a male teacher or um you

you to pick a male teacher or um you know a a a female doctor or whatnot so

know a a a female doctor or whatnot so there was actually uh a guy at Adobe who

there was actually uh a guy at Adobe who can considers himself non-gender binary

can considers himself non-gender binary also The Man Behind the orange um the

also The Man Behind the orange um the orange heart he fought and got basically

orange heart he fought and got basically uh the first three non-gender uh

uh the first three non-gender uh non-gender binary emojis so child adult

non-gender binary emojis so child adult and old older adults so those are

and old older adults so those are creative and then we started having to

creative and then we started having to propagate them through actually all the

propagate them through actually all the occupations so these are the gender

occupations so these are the gender neutral versions of many of those um but

neutral versions of many of those um but then uh we got into this whole thing

then uh we got into this whole thing where every emoji that had a gender

where every emoji that had a gender originally had to be mirrored so

originally had to be mirrored so originally we had bearded man and then

originally we had bearded man and then we're like okay well we actually have to

we're like okay well we actually have to get bearded woman so that is on your

get bearded woman so that is on your keyboard there is pregnant woman there

keyboard there is pregnant woman there is now pregnant man which is interesting

is now pregnant man which is interesting um there is you know woman in a bridal

um there is you know woman in a bridal gown there's now man in a bridal gown um

gown there's now man in a bridal gown um and then there were ones that actually

and then there were ones that actually had to be created that were neither man

had to be created that were neither man or woman so this is a mer person so

or woman so this is a mer person so there was merman there was mermaid and

there was merman there was mermaid and there was M person it was really

there was M person it was really interesting to figure out like how do

interesting to figure out like how do you draw a gender neutral M person like

you draw a gender neutral M person like a bunch of them in the beginning

a bunch of them in the beginning actually had the arms crossed around

actually had the arms crossed around sort of the chest um Monarch so there

sort of the chest um Monarch so there was prince and princess and now there is

was prince and princess and now there is monarch and one of my favorite actually

monarch and one of my favorite actually is there was Santa Claus and there was

is there was Santa Claus and there was Mrs Claus and now there's MX Claus like

Mrs Claus and now there's MX Claus like the name of this character literally in

the name of this character literally in Unicode is MX claw so I feel like it's

Unicode is MX claw so I feel like it's sort of like a very official enshrining

sort of like a very official enshrining of gender non biner in like the world um

of gender non biner in like the world um not everyone loved it New York Post did

not everyone loved it New York Post did not love this that they like you know

not love this that they like you know we're we're cing into like Emoji woke

we're we're cing into like Emoji woke Wars um so some Emoji stats for you this

Wars um so some Emoji stats for you this is very fascinating this is sort of like

is very fascinating this is sort of like the Gen the General Distribution by far

the Gen the General Distribution by far the single emoji that used more than

the single emoji that used more than anything else is the face with tears of

anything else is the face with tears of joy about 10% of all Emoji scent is that

joy about 10% of all Emoji scent is that one character and then number two is

one character and then number two is heart red heart and then it kind of goes

heart red heart and then it kind of goes down so um there's a frequency of emoji

down so um there's a frequency of emoji use these this is sort of done by um

use these this is sort of done by um order magnitude so one is half of two

order magnitude so one is half of two two is half of one so it's really

two is half of one so it's really interesting it's a very very steep drop

interesting it's a very very steep drop off after the first couple um in case

off after the first couple um in case you ever want to go onto the the Emoji

you ever want to go onto the the Emoji kind of Unicode website you can you too

kind of Unicode website you can you too can see all the frequency things so I

can see all the frequency things so I think it's really funny so basically

think it's really funny so basically it's green going this way it increased

it's green going this way it increased in usage between 2019 and uh 2021 and

in usage between 2019 and uh 2021 and it's red going this way it drop and so

it's red going this way it drop and so pleading face which is a relatively new

pleading face which is a relatively new emoe you just sort of shut up on the

emoe you just sort of shut up on the charts um and whereas actually like

charts um and whereas actually like smiling face with heart eyes

smiling face with heart eyes like kind of kind of slipped which is

like kind of kind of slipped which is interesting so we just closed our Emoji

interesting so we just closed our Emoji proposal round for 2022 these were sort

proposal round for 2022 these were sort of the breakdowns people love submitting

of the breakdowns people love submitting Smileys and food and beverages animals

Smileys and food and beverages animals and nature da da da I don't I mean these

and nature da da da I don't I mean these are very googly colors um so what is the

are very googly colors um so what is the future of emoji I will um I will tell

future of emoji I will um I will tell you because we just had a meeting two

you because we just had a meeting two weeks ago so I can now publicly talk

weeks ago so I can now publicly talk about it so historically there was this

about it so historically there was this whole idea like Unico doesn't want to be

whole idea like Unico doesn't want to be in the world of like encoding glyphs for

in the world of like encoding glyphs for like devices everywhere like there was

like devices everywhere like there was very controversial when it started doing

very controversial when it started doing that because mostly what Unicode used to

that because mostly what Unicode used to do was you take an existing language

do was you take an existing language could be dead you know and then it would

could be dead you know and then it would just take it and digitize it right it

just take it and digitize it right it took languages that existed and just

took languages that existed and just digitize them and uh then when it kind

digitize them and uh then when it kind of wandered into Emoji World Suddenly

of wandered into Emoji World Suddenly It's like deciding what deserves to be

It's like deciding what deserves to be like an emoji decides to be digitized so

like an emoji decides to be digitized so um trying to get kind of get out of it

um trying to get kind of get out of it and they have proposals over time where

and they have proposals over time where it's like oh maybe we should like come

it's like oh maybe we should like come up with a a way to just send pictures

up with a a way to just send pictures back and forth where you you it's a

back and forth where you you it's a fixed picture and you like use a hash so

fixed picture and you like use a hash so that you know like we would look at the

that you know like we would look at the picture and then like go do a lookup

picture and then like go do a lookup somewhere like that did not go over well

somewhere like that did not go over well then there was actually a really

then there was actually a really interesting proposal I kind of like uh

interesting proposal I kind of like uh didn't go over well which is using

didn't go over well which is using something called the qid which is in

something called the qid which is in Wiki in the Wikipedia world so in

Wiki in the Wikipedia world so in Wikipedia World items all have uh

Wikipedia World items all have uh numbers across the different language

numbers across the different language Wikipedia so Obama human Earth they will

Wikipedia so Obama human Earth they will have an ID number so that the page in

have an ID number so that the page in English and the page in German the page

English and the page in German the page in Chinese all know that they're

in Chinese all know that they're pointing to the same thing so the

pointing to the same thing so the question so one idea came up like why

question so one idea came up like why don't we use the numbering system so we

don't we use the numbering system so we can use like Eiffel Tower in you know

can use like Eiffel Tower in you know see the number and then like oh people

see the number and then like oh people know like oh you're trying to say Eiffel

know like oh you're trying to say Eiffel Tower that did not go over well so those

Tower that did not go over well so those are both both of those proposals seem

are both both of those proposals seem dead as of yet um and it's too bad cuz

dead as of yet um and it's too bad cuz and you'll see kind of what's happening

and you'll see kind of what's happening okay so what's coming in 22 so these are

okay so what's coming in 22 so these are the Emoji that I actually sort of

the Emoji that I actually sort of thought they would be on your phones by

thought they would be on your phones by now but cuz we're in mid November and

now but cuz we're in mid November and they usually update early November so

they usually update early November so three more Hearts people love hearts

three more Hearts people love hearts like uh wing blackbird Goose Birds also

like uh wing blackbird Goose Birds also purple flowers jellyfish moose face

purple flowers jellyfish moose face donkey donkey was a little bit late for

donkey donkey was a little bit late for the kind of Elections um Ginger peep pod

the kind of Elections um Ginger peep pod Wireless cond shaking face um folding

Wireless cond shaking face um folding hand pan that one was interesting cuz

hand pan that one was interesting cuz when people first proposed it they

when people first proposed it they proposed it as like an electric fan and

proposed it as like an electric fan and that didn't like who knows what electric

that didn't like who knows what electric fans will look like in 100 years because

fans will look like in 100 years because the thing is Once An Emoji always an

the thing is Once An Emoji always an emoji they never retire so they're

emoji they never retire so they're always looking for things that have a

always looking for things that have a long visual longevity floppy disc did

long visual longevity floppy disc did not actually do that so there's always

not actually do that so there's always like we don't want another floppy disc

like we don't want another floppy disc um and then hairpick is interesting so

um and then hairpick is interesting so there was a whole debate about how to

there was a whole debate about how to convey um like afro African hair like

convey um like afro African hair like the the curly hair that they introduced

the the curly hair that they introduced a couple years ago was supposed to do

a couple years ago was supposed to do that and most of the vendors actually

that and most of the vendors actually have it in a sort of Afro way except for

have it in a sort of Afro way except for Apple so there's a lot of complaints but

Apple so there's a lot of complaints but um hairpick was sort of a an interesting

um hairpick was sort of a an interesting way it means both comb but also has sort

way it means both comb but also has sort of an interesting historic connotation

of an interesting historic connotation and it's been around for about 2,000

and it's been around for about 2,000 years uh a couple music things maracus

years uh a couple music things maracus and flute uh Beyond 2022 one of the

and flute uh Beyond 2022 one of the things that's going to die oops can I go

things that's going to die oops can I go back no I can't oh well uh we're going

back no I can't oh well uh we're going to retire the the family Emoji um they

to retire the the family Emoji um they didn't go over so well there were so

didn't go over so well there were so many of them combinatorically if you had

many of them combinatorically if you had all the everything in in uh you know all

all the everything in in uh you know all the race all the races all because you

the race all the races all because you wanted to have skin tones because you

wanted to have skin tones because you didn't want to imply that families can

didn't want to imply that families can only be one race it was such an ordeal

only be one race it was such an ordeal uh essentially we're all like no one

uh essentially we're all like no one uses them and there's so many and it's

uses them and there's so many and it's like the fonts like in terms of the load

like the fonts like in terms of the load is like too large so they're just going

is like too large so they're just going to make them all into like basically

to make them all into like basically little like bathroom symbol Type U folks

little like bathroom symbol Type U folks um so I think that is those will

um so I think that is those will disappear what what's actually really

disappear what what's actually really interesting about the family Emoji is

interesting about the family Emoji is they had gay when they introduced the

they had gay when they introduced the family they had gay family emoji and the

family they had gay family emoji and the Russian government went berserk and

Russian government went berserk and actually you can Google this in 2015

actually you can Google this in 2015 you'll see a bunch of articles about

you'll see a bunch of articles about like the Russian government considering

like the Russian government considering it homosexual propaganda to Youth and

it homosexual propaganda to Youth and there was a big debate about whether or

there was a big debate about whether or not they were going to ban Apple devices

not they were going to ban Apple devices and what so you can see a lot of the

and what so you can see a lot of the media coverage from that time but I

media coverage from that time but I thought it was really interesting how

thought it was really interesting how upset a national government can get

upset a national government can get about little pictures on your phone um

about little pictures on your phone um another thing that's going to that's on

another thing that's going to that's on the agenda as of you know a couple of

the agenda as of you know a couple of weeks ago is directionality in terms of

weeks ago is directionality in terms of emoji so as you know most emojis kind of

emoji so as you know most emojis kind of just flip one way or another and the

just flip one way or another and the reason why it matters is because not all

reason why it matters is because not all languages run in the same direction so

languages run in the same direction so for example Arabic so we are used to

for example Arabic so we are used to left to right but a lot of languages go

left to right but a lot of languages go right to left um so and this kind of It

right to left um so and this kind of It kind of changes the meaning of emoji for

kind of changes the meaning of emoji for example right to left I send this a lot

example right to left I send this a lot to my friends when I'm going flying from

to my friends when I'm going flying from the Bay Area to New York um if you do it

the Bay Area to New York um if you do it from left to right however that is what

from left to right however that is what it looks like so it looks like um you

it looks like so it looks like um you know you're in NE Bay but the plane is

know you're in NE Bay but the plane is still going that you know kind of up and

still going that you know kind of up and to the right and then now it looks like

to the right and then now it looks like you're going from New York to the Bay

you're going from New York to the Bay Area the other place is like oh it's

Area the other place is like oh it's it's a girl and she's running really

it's a girl and she's running really fast right uh that is left to right to

fast right uh that is left to right to left in our world wait left to right

left in our world wait left to right sorry sorry that was that's that's

sorry sorry that was that's that's supposed to be left to right and in here

supposed to be left to right and in here it would be she's like behind like

it would be she's like behind like pollution or something like that so

pollution or something like that so sorry about so so an example this is

sorry about so so an example this is actually in The Proposal like in one

actually in The Proposal like in one case if it's left or right you're

case if it's left or right you're running away from a line of cars and the

running away from a line of cars and the other one it's a warning to not run

other one it's a warning to not run behind car fumes so they are trying to

behind car fumes so they are trying to figure out like how do we mirror a bunch

figure out like how do we mirror a bunch of the Emoji um but the LA the main

of the Emoji um but the LA the main thing that I think sort of I don't know

thing that I think sort of I don't know when it's going to happen I'm really

when it's going to happen I'm really hopeful is going to happen is trying to

hopeful is going to happen is trying to come up with a system that supports

come up with a system that supports little stickers in line that don't need

little stickers in line that don't need Unicode so this is like slack or on um

Unicode so this is like slack or on um twitch you can embed little pictures on

twitch you can embed little pictures on in line and all all all the vendors have

in line and all all all the vendors have to get together and agree to come up

to get together and agree to come up with a standard way to do that they have

with a standard way to do that they have not yet come up with that but that is

not yet come up with that but that is sort of one of the ways that unic coded

sort of one of the ways that unic coded like want it wants to back away from

like want it wants to back away from actually being like a global regulator

actually being like a global regulator for like little colorful glyphs and so

for like little colorful glyphs and so if you ever need to reach me um on in my

if you ever need to reach me um on in my emoji world you can find me um Jenny

emoji world you can find me um Jenny EmojiNation org there will

EmojiNation org there will be um the it will be actually a while I

be um the it will be actually a while I think before we see a next generation of

think before we see a next generation of emoji showing up in it used to be like

emoji showing up in it used to be like every year they would get new code

every year they would get new code points it might be a little bit less

points it might be a little bit less than every year now as they work on

than every year now as they work on things like directionality over time so

things like directionality over time so that uh if anyone has questions you can

that uh if anyone has questions you can ask questions you can find me afterwards

ask questions you can find me afterwards I think I've I feel like there's

I think I've I feel like there's supposed to be some some hubub right now

supposed to be some some hubub right now about maybe maybe

about maybe maybe um microphones but maybe not but maybe

um microphones but maybe not but maybe I'm just done and if if there are

I'm just done and if if there are questions or if David is around I'm

questions or if David is around I'm happy you know he can I'm I'm happy to

happy you know he can I'm I'm happy to to answer any questions that folks have

to answer any questions that folks have yes hi yes I was wondering what were

yes hi yes I was wondering what were your thoughts on the

your thoughts on the emo so the question is what are my

emo so the question is what are my thoughts on the Emoji Movie you're

thoughts on the Emoji Movie you're talking about the Sony animated one yes

talking about the Sony animated one yes okay my thought on that is it is better

okay my thought on that is it is better than a 6% rating on um Rotten Tomatoes

than a 6% rating on um Rotten Tomatoes would lead you to believe so that's my

would lead you to believe so that's my one thought and my my next thought is

one thought and my my next thought is that um that was a rush job from an

that um that was a rush job from an animation perspective that was that was

animation perspective that was that was was about 18 months of whereas a typical

was about 18 months of whereas a typical animated movie takes four years so in my

animated movie takes four years so in my spare time I also produce movies and

spare time I also produce movies and documentaries so one thing that is key

documentaries so one thing that is key to know

to know about about movies and animated movies

about about movies and animated movies and this is very important they take a

and this is very important they take a very long time but you can always fix it

very long time but you can always fix it because you haven't shot anything and a

because you haven't shot anything and a very good example of that is I assume

very good example of that is I assume you guys have seen Frozen if you haven't

you guys have seen Frozen if you haven't seen you seen of the age you would have

seen you seen of the age you would have seen Frozen um I do not understand like

seen Frozen um I do not understand like how huge and a phenomenon or why it was

how huge and a phenomenon or why it was such a huge phenomenon but

such a huge phenomenon but um they actually did a original cut of

um they actually did a original cut of Frozen and it did so I don't know you

Frozen and it did so I don't know you guys know the the Eye the the sort of

guys know the the Eye the the sort of Snow Queen thing but she's like super

Snow Queen thing but she's like super dark and like not fun and like kind of

dark and like not fun and like kind of evil and like not someone you want to

evil and like not someone you want to like get behind as a character so they

like get behind as a character so they actually did sort of a rough cut of that

actually did sort of a rough cut of that of Frozen and they came out of that with

of Frozen and they came out of that with um it's just storyboarding and they're

um it's just storyboarding and they're like that is not good and they killed it

like that is not good and they killed it so they were like we can't go with this

so they were like we can't go with this and then started from scratch more or

and then started from scratch more or less again starting with the song um Let

less again starting with the song um Let It Go which is actually written by a kid

It Go which is actually written by a kid from my elementary school Bobby Lopez or

from my elementary school Bobby Lopez or co-written by Bobby Lopez um I also

co-written by Bobby Lopez um I also actually fun fact I also went I would

actually fun fact I also went I would took the school bus with uh Lyn mmel

took the school bus with uh Lyn mmel Miranda so I was a fourth grader when he

Miranda so I was a fourth grader when he was like a kindergartener so we had a

was like a kindergartener so we had a very musical Elementary School in New

very musical Elementary School in New York City but the thing is they could

York City but the thing is they could fix it because they had enough time and

fix it because they had enough time and have enough money not like movies where

have enough money not like movies where you shoot humans much harder to fix so

you shoot humans much harder to fix so you have the footage that you have and

you have the footage that you have and you can do little pickups but you can't

you can do little pickups but you can't fix it so essentially what happened in

fix it so essentially what happened in that case I think um it's 18 months and

that case I think um it's 18 months and it could have gotten better and a lot of

it could have gotten better and a lot of the movies that you see with Pixar like

the movies that you see with Pixar like it's very it's actually sort emotionally

it's very it's actually sort emotionally similar to the movie called inside out

similar to the movie called inside out and uh but inside they just have more

and uh but inside they just have more time and so it's better so as opposed to

time and so it's better so as opposed to 18 months which is not long enough to

18 months which is not long enough to make a animated movie good but uh the

make a animated movie good but uh the other fun thing is is it was the it it

other fun thing is is it was the it it was so weird cuz they sold sponsorships

was so weird cuz they sold sponsorships it was like like oh my God here comes

it was like like oh my God here comes the Bots and the malware let's go into

the Bots and the malware let's go into Dropbox and protect ourselves and was so

Dropbox and protect ourselves and was so I think that it got a lot of like bad

I think that it got a lot of like bad kind of um kind of uh vibes from the

kind of um kind of uh vibes from the from the press for doing things like

from the press for doing things like that but from a kids perspective it's

that but from a kids perspective it's it's fine I think if um I don't know

it's fine I think if um I don't know that I would like put into my top 10 of

that I would like put into my top 10 of animated pictures but it's better than

animated pictures but it's better than 6% on Rotten Tomatoes

6% on Rotten Tomatoes and then actually if you guys ever care

and then actually if you guys ever care we we have done uh I did a documentary

we we have done uh I did a documentary about Emoji so and all the people that

about Emoji so and all the people that create helped create emoji and uh we did

create helped create emoji and uh we did a cs50x movie night I think during the

a cs50x movie night I think during the pandemic was it during the pandemic

pandemic was it during the pandemic everything sort of like blurred together

everything sort of like blurred together but it was during the pandemic yeah Are

but it was during the pandemic yeah Are we more questions yes um I wanted to

we more questions yes um I wanted to know you mentioned that one of the

know you mentioned that one of the criteria for

criteria for AC do or there's demand

yeah yeah that's a very good question yeah so the question is one of the

yeah so the question is one of the propos one of our criterias of of um

propos one of our criterias of of um getting an emoji accepted is to sort of

getting an emoji accepted is to sort of to demonstrate demand and how do we

to demonstrate demand and how do we demonstrate demand and I would say in a

demonstrate demand and I would say in a in a pretty um clumsy way actually at

in a pretty um clumsy way actually at this point so the main thing that you

this point so the main thing that you have in our in our current proposal

have in our in our current proposal process is we have um a median Emoji

process is we have um a median Emoji which is elephant so elephant is like if

which is elephant so elephant is like if you stack ranked all the emoji for

you stack ranked all the emoji for popularity elephant is like right there

popularity elephant is like right there in the middle and it's also a concept

in the middle and it's also a concept that's like universally understood

that's like universally understood across all

across all languages so um elephant shows up

languages so um elephant shows up somewhere between 500 million and 700

somewhere between 500 million and 700 million in Google search results like if

million in Google search results like if you type it into a laptop you'll see you

you type it into a laptop you'll see you know elephant 500,000 uh 500 million

know elephant 500,000 uh 500 million search results and generally you're

search results and generally you're you're trying to when you're comparing

you're trying to when you're comparing your term to elephant you want to see

your term to elephant you want to see very roughly how many Google search

very roughly how many Google search results B search results sometimes

results B search results sometimes Instagram so actually something that was

Instagram so actually something that was really surprising to me was someone

really surprising to me was someone proposed hummingbird I think hummingbird

proposed hummingbird I think hummingbird is uh a good proposal and um but if you

is uh a good proposal and um but if you look at Hummingbird it's only like 21

look at Hummingbird it's only like 21 million in terms of the stat so which I

million in terms of the stat so which I thought was like very surprisingly low

thought was like very surprisingly low so that's one of the main ways that we

so that's one of the main ways that we kind of we kind of see like is it also

kind of we kind of see like is it also visually used and all of that yeah any

visually used and all of that yeah any other questions are we good I didn't

other questions are we good I didn't even need my water or anything oh can I

even need my water or anything oh can I take I'm going to take a picture I'm

take I'm going to take a picture I'm going to take a picture for because now

going to take a picture for because now you guys are actually human and not

you guys are actually human and not Muppets so I'm very very excited about

Muppets so I'm very very excited about this so I will send this to like my

this so I will send this to like my block mates and be like I just lectured

block mates and be like I just lectured at CS you know in Sanders Theater my

at CS you know in Sanders Theater my thanks to Jenny Lee thank

thanks to Jenny Lee thank you yeah you can stay up here for give

you yeah you can stay up here for give me one

me one second

so if up until now thought it would be appropriate to toss this up on the board

appropriate to toss this up on the board if up until now you've not yet gotten

if up until now you've not yet gotten cs50 stress ball on the way out please

cs50 stress ball on the way out please do grab one we got some extras as well

do grab one we got some extras as well off to the side but I would also keep in

off to the side but I would also keep in mind back in week zero where again we

mind back in week zero where again we began we asked you to categorize

began we asked you to categorize yourselves as to whether you are among

yourselves as to whether you are among those less comfortable those more

those less comfortable those more comfortable or those somewhere in

comfortable or those somewhere in between uh please know now that you are

between uh please know now that you are officially all some uh please know now

officially all some uh please know now that you are all officially among those

that you are all officially among those more comfortable and indeed even though

more comfortable and indeed even though a couple of more Milestones await we

a couple of more Milestones await we cannot wait to see what you accomplish

cannot wait to see what you accomplish with your final projects in the meantime

with your final projects in the meantime as always this is and now this was

[Laughter] [Music]

Mr when I in CS I'll say truth be told I thought

I in CS I'll say truth be told I thought i' be real coding hero because I did

so it picked up so fast turn out that harder scratch hell world and

harder scratch hell world and goodby

goodby [Applause]

[Applause] [Music]

because Mr I was on

Mr I was on the trying so hard not fall behind

the trying so hard not fall behind attending section and wanting a stuck in

attending section and wanting a stuck in the in void of my

computer to pyth in why on because this is

s Mr s we completed JavaScript sequ CSS

JavaScript sequ CSS and every language our to reset don't

get again I repli obviously because

obviously because cie

cie [Applause]

all right so this is cs50 my name is David May and this is Harvard

David May and this is Harvard University's introduction to the

University's introduction to the intellectual Enterprises of computer

intellectual Enterprises of computer science and the Art of programming and

science and the Art of programming and this of course is our special family

this of course is our special family weekend wherein not only are cs50's own

weekend wherein not only are cs50's own students here in the audience but also

students here in the audience but also some family members as well now you're

some family members as well now you're showing up in the semester a little bit

showing up in the semester a little bit late we've just tackled week eight which

late we've just tackled week eight which is really our ninth week since computer

is really our ninth week since computer scientists start counting from zero so

scientists start counting from zero so we've done a whole lot of work over the

we've done a whole lot of work over the past few weeks as you might have heard

past few weeks as you might have heard via emails or text messages home

via emails or text messages home including a language known here as binar

including a language known here as binar so on the screen here of course is a lot

so on the screen here of course is a lot of zeros and ones and suffice it to say

of zeros and ones and suffice it to say let me sum up the past nine weeks with

let me sum up the past nine weeks with this is what's going on underneath the

this is what's going on underneath the hood but of course today we thought we'd

hood but of course today we thought we'd make things a little more accessible a

make things a little more accessible a little more broadly applicable and

little more broadly applicable and indeed our Focus today will not be on

indeed our Focus today will not be on what these patterns of zeros and ones

what these patterns of zeros and ones represent which in astute eye might

represent which in astute eye might notice are replicated visually with

notice are replicated visually with these light bulbs being in a pattern on

these light bulbs being in a pattern on and off and as your child might have

and off and as your child might have hinted uh before class or perhaps now

hinted uh before class or perhaps now this might very well spell a word up to

this might very well spell a word up to eight characters long because you can

eight characters long because you can encode even in the real world things

encode even in the real world things digital too but today we'll focus on

digital too but today we'll focus on things much more high level this notion

things much more high level this notion of cyber security like our the security

of cyber security like our the security of our data our privacy of our systems

of our data our privacy of our systems particularly on the internet nowadays

particularly on the internet nowadays because presumably all of us are

because presumably all of us are carrying Technologies around in our

carrying Technologies around in our pocket using laptops and desktops every

pocket using laptops and desktops every day and so the goal today is to

day and so the goal today is to stipulate that this is what's going on

stipulate that this is what's going on underneath the hood but let's solve some

underneath the hood but let's solve some problems at a higher level so that your

problems at a higher level so that your homework when you go back to wherever

homework when you go back to wherever you're visiting from can actually be to

you're visiting from can actually be to apply some of today's Lessons Learned so

apply some of today's Lessons Learned so with that said perhaps the most common

with that said perhaps the most common familiar defense uh of one's systems and

familiar defense uh of one's systems and data phone and laptops and desktops

data phone and laptops and desktops would just be these simple passwords

would just be these simple passwords unfortunately you and I are frankly as

unfortunately you and I are frankly as humans not all that good at choosing

humans not all that good at choosing passwords and this is in itself a

passwords and this is in itself a relatively weak form of Defense even

relatively weak form of Defense even though each of us has dozens hundreds of

though each of us has dozens hundreds of passwords nowadays or at least dozens or

passwords nowadays or at least dozens or hundreds of accounts Maybe fives or tens

hundreds of accounts Maybe fives or tens of dozens of passwords indeed if you're

of dozens of passwords indeed if you're in the habit of reusing passwords we'll

in the habit of reusing passwords we'll see today probably among our first

see today probably among our first Lessons Learned so for instance if we

Lessons Learned so for instance if we look back at the past year 2021 thanks

look back at the past year 2021 thanks to security researchers who take a look

to security researchers who take a look at data that has been hacked or leaked

at data that has been hacked or leaked online by way of public databases we

online by way of public databases we have a sense as computer scientists of

have a sense as computer scientists of what the most popular or equivalently

what the most popular or equivalently what some of the worst passwords are

what some of the worst passwords are that you and I are choosing for our

that you and I are choosing for our system so as of this past year according

system so as of this past year according to one measure the most commonly used

to one measure the most commonly used password in systems everywhere was 1 2 3

password in systems everywhere was 1 2 3 4 5 6 all right number two password in

4 5 6 all right number two password in our top 10 here list was only slightly

our top 10 here list was only slightly longer 1 2 3 4 5 6 7 8 9 after that we

longer 1 2 3 4 5 6 7 8 9 after that we took a turn in the other direction 1 2 3

took a turn in the other direction 1 2 3 4 five alone after that got a little

4 five alone after that got a little more interesting quiry which might sound

more interesting quiry which might sound pretty cryptic but not if you look down

pretty cryptic but not if you look down at your US keyboard and it's the top

at your US keyboard and it's the top leftand row of the keys on a an American

leftand row of the keys on a an American keyboard so also not all that hard uh

keyboard so also not all that hard uh perhaps not surprisingly uh a little

perhaps not surprisingly uh a little disconcertingly number five was

disconcertingly number five was password meanwhile number six returns us

password meanwhile number six returns us to digits 1 2 3 4 5 6 7 8 after that

to digits 1 2 3 4 5 6 7 8 after that really less effort 111 111 uh after that

really less effort 111 111 uh after that a little more variation but not all that

a little more variation but not all that much 1 two 3 one two 3 after that it's

much 1 two 3 one two 3 after that it's getting even less interesting 1 2 3 4 5

getting even less interesting 1 2 3 4 5 6 7

6 7 890 and then lastly topping the list is

890 and then lastly topping the list is just 1 2 3 4 5 6 7 so this is not a good

just 1 2 3 4 5 6 7 so this is not a good top 10 list to be on so among today's

top 10 list to be on so among today's first takeaways is if you see your

first takeaways is if you see your password on the screen like you didn't

password on the screen like you didn't make the list in a good way this means

make the list in a good way this means hundreds thousands millions of other

hundreds thousands millions of other people probably have that password of

people probably have that password of yours now in of itself that's not

yours now in of itself that's not necessarily worrisome because I don't

necessarily worrisome because I don't know who has these passwords in a room

know who has these passwords in a room as large as this but just intuitively

as large as this but just intuitively why is this a bad thing either parent or

why is this a bad thing either parent or child welcome to raise a hand here why

child welcome to raise a hand here why might this be

might this be intuitively yeah access to it so access

intuitively yeah access to it so access to it like I mean we literally as

to it like I mean we literally as computer scientists now have a database

computer scientists now have a database of really common passwords and your your

of really common passwords and your your thoughts password and can just find it

thoughts password and can just find it out quickly yeah you can just find it

out quickly yeah you can just find it out quickly I mean you could imagine

out quickly I mean you could imagine trying to guess someone's password by

trying to guess someone's password by just typing in random letters random

just typing in random letters random numbers random words but not if you have

numbers random words but not if you have a top 10 list like the the adversaries

a top 10 list like the the adversaries in the world might as well just start

in the world might as well just start with this list now you'll notice that

with this list now you'll notice that even absent from this are slight

even absent from this are slight variants like some of you might be

variants like some of you might be thinking I'm not on the list cuz I do

thinking I'm not on the list cuz I do something clever like I use an

something clever like I use an exclamation point for the number one or

exclamation point for the number one or a three for an e or a five for an S and

a three for an e or a five for an S and based on the smiles in the room right

based on the smiles in the room right now you're not all that clever it turns

now you're not all that clever it turns out because other people are smiling too

out because other people are smiling too which is to say that an adversary can

which is to say that an adversary can take those same fistic that you might

take those same fistic that you might think are making things more secure by

think are making things more secure by just tweaking some letters to numbers or

just tweaking some letters to numbers or vice versa but if you're doing it and

vice versa but if you're doing it and other people are doing it the bad guys

other people are doing it the bad guys so to speak are going to be doing it as

so to speak are going to be doing it as well so unfortunately when it comes to

well so unfortunately when it comes to passwords better is longer and random

passwords better is longer and random and really unguessable but that's not

and really unguessable but that's not what most of us have in fact case in

what most of us have in fact case in point on our phones whether you have an

point on our phones whether you have an Android device or an iPhone nowadays you

Android device or an iPhone nowadays you know odds are you have something

know odds are you have something relatively simplistic protecting it if

relatively simplistic protecting it if you have anything at all but at least

you have anything at all but at least Apple and Google are pretty good at

Apple and Google are pretty good at least nudging us to choose these kinds

least nudging us to choose these kinds of passcodes now and a four-digit

of passcodes now and a four-digit passcode is quite common nowadays and so

passcode is quite common nowadays and so here's where we have an opportunity

here's where we have an opportunity thanks to uh the URL that you saw on the

thanks to uh the URL that you saw on the screen earlier to conjecture as a group

screen earlier to conjecture as a group just how long might it take an adversary

just how long might it take an adversary someone out there who's out to get us or

someone out there who's out to get us or get one of us uh how long might it take

get one of us uh how long might it take an adversary to figure out your phone's

an adversary to figure out your phone's 4digit passcode this is a cs50 is on

4digit passcode this is a cs50 is on Carter Carter if we could switch over

Carter Carter if we could switch over and poll the audience here if you take

and poll the audience here if you take out your phone or laptop whatever device

out your phone or laptop whatever device you might have used a few minutes ago to

you might have used a few minutes ago to scan that QR code or to visit that same

scan that QR code or to visit that same URL you can see these questions on your

URL you can see these questions on your browser and if you can't that's fine

browser and if you can't that's fine we'll share some aggregate data

we'll share some aggregate data nonetheless but you should have an

nonetheless but you should have an opportunity to tap one of your answers

opportunity to tap one of your answers and we'll give folks a few more

and we'll give folks a few more seconds if you'd like to play along at

seconds if you'd like to play along at home and here in just a

home and here in just a moment probably have many people

moment probably have many people reporting but why don't we go ahead and

reporting but why don't we go ahead and take a look at some percentages it looks

take a look at some percentages it looks like most of you 67% are proposing just

like most of you 67% are proposing just a few seconds so that's not all that

a few seconds so that's not all that good news if it's a four-digit passcode

good news if it's a four-digit passcode some of you are hoping it's a few

some of you are hoping it's a few minutes uh 8% are hoping a few hours

minutes uh 8% are hoping a few hours four more than 4% of you are really

four more than 4% of you are really hoping perhaps it's a few days well

hoping perhaps it's a few days well let's actually consider how we can

let's actually consider how we can answer this question and make today not

answer this question and make today not just conceptual but a little

just conceptual but a little quantitative too and see if we can't

quantitative too and see if we can't slap some numbers on questions like

slap some numbers on questions like these so ultimately you can make more

these so ultimately you can make more informed decisions with your system

informed decisions with your system security so for instance when it comes

security so for instance when it comes to four digigit passcodes rather than

to four digigit passcodes rather than just consider how secure it is well

just consider how secure it is well let's make it a more precise question

let's make it a more precise question like what are the forms of attack well

like what are the forms of attack well the simplest attack might be just

the simplest attack might be just someone grabbing your phone be it in

someone grabbing your phone be it in your family or maybe at Starbucks or the

your family or maybe at Starbucks or the airport or the like and just starting

airport or the like and just starting all possible combinations maybe 00000000

all possible combinations maybe 00000000 then 00001 and 00002 we could maybe

then 00001 and 00002 we could maybe automate this a little bit so for

automate this a little bit so for instance I might potentially be able to

instance I might potentially be able to do something like uh roboticize this

do something like uh roboticize this here let me go ahead and full screen a

here let me go ahead and full screen a quick video here that's just going to

quick video here that's just going to paint a picture in just a moment on the

paint a picture in just a moment on the screen of how if we're a really clever

screen of how if we're a really clever adversary and know how to build things

adversary and know how to build things well at least maybe we could automate

well at least maybe we could automate some of that process so here's an

some of that process so here's an Android phone on a counter here's a very

Android phone on a counter here's a very simple tripod and a little touch device

simple tripod and a little touch device robotically doing all of that hacking

robotically doing all of that hacking for you starting at 0000 Z probably all

for you starting at 0000 Z probably all the way up to 9999 now that too wasn't

the way up to 9999 now that too wasn't necessarily all of that all that fast

necessarily all of that all that fast but at least you the adversary can step

but at least you the adversary can step away and doesn't actually have to be

away and doesn't actually have to be bothered with the time involved the cost

bothered with the time involved the cost involved in actually hacking that

involved in actually hacking that particular device well let's go one

particular device well let's go one level deeper a little more interestingly

level deeper a little more interestingly and consider here um the how much much

and consider here um the how much much time really this so-called Brute Force

time really this so-called Brute Force attack would take and that's actually a

attack would take and that's actually a term of art much like in yester year

term of art much like in yester year when maybe there was a battering ram

when maybe there was a battering ram trying to brute force their way into a

trying to brute force their way into a castle or something like that a Brute

castle or something like that a Brute Force attack digitally is just someone

Force attack digitally is just someone trying manually all possible codes or

trying manually all possible codes or maybe robotically trying all possible

maybe robotically trying all possible codes but generally automating the

codes but generally automating the process in some way to go through all

process in some way to go through all possibilities well if you've got for

possibilities well if you've got for instance um a 4digit passcode let's ask

instance um a 4digit passcode let's ask maybe a follow-up question here not how

maybe a follow-up question here not how long will take but how many possible

long will take but how many possible 4digit passcodes are there because then

4digit passcodes are there because then maybe we can do some quick math and if

maybe we can do some quick math and if every passcode takes me a second or a

every passcode takes me a second or a few milliseconds or the like then I

few milliseconds or the like then I think we can try to extrapolate from

think we can try to extrapolate from that whether the first answer was

that whether the first answer was seconds or minutes or days or hours or

seconds or minutes or days or hours or something else so how many four-digit

something else so how many four-digit passcodes are possible if you take out

passcodes are possible if you take out your same device it should have just

your same device it should have just changed automatically if it doesn't seem

changed automatically if it doesn't seem to have maybe reload your browser with

to have maybe reload your browser with some menu option and then tap in here

some menu option and then tap in here how many four digigit passcodes are

how many four digigit passcodes are possible four total 40 99,999 10,000 or

possible four total 40 99,999 10,000 or unsure is okay

unsure is okay too so let's see we'll give you a few

too so let's see we'll give you a few more moments how many four digigit

more moments how many four digigit passcodes are possible and shall we

passcodes are possible and shall we reveal the results so now it looks like

reveal the results so now it looks like uh a a few of you uh 2% of you are

uh a a few of you uh 2% of you are saying just four passcodes 40 99,999

saying just four passcodes 40 99,999 there's definitely some contention here

there's definitely some contention here and 6% are un sure well how do we wrap

and 6% are un sure well how do we wrap our minds around this well let's just

our minds around this well let's just kind of do this real simply here let me

kind of do this real simply here let me switch back over to doing a bit of math

switch back over to doing a bit of math and if we have here 10 possibilities for

and if we have here 10 possibilities for each digit if there's four digits each

each digit if there's four digits each digit can be 0 1 2 3 4 5 6 7 8 nine so

digit can be 0 1 2 3 4 5 6 7 8 nine so that's 10 possibilities so if you think

that's 10 possibilities so if you think about the number of permutations that's

about the number of permutations that's 10 possibilities for the first digit

10 possibilities for the first digit times 10 for the next time 10 to the for

times 10 for the next time 10 to the for the next times 10 for the next and so if

the next times 10 for the next and so if we do that out 10 * 10 * 10 * 10 or 10

we do that out 10 * 10 * 10 * 10 or 10 the 4th there are indeed as 66% of you

the 4th there are indeed as 66% of you uh found uh 10,000 possibilities and so

uh found uh 10,000 possibilities and so now we can kind of work backwards and

now we can kind of work backwards and decide how long is it going to take for

decide how long is it going to take for an adversary to hack into this phone

an adversary to hack into this phone because if it's one attack one guess per

because if it's one attack one guess per second well that's going to map out to

second well that's going to map out to 10,000 seconds but maybe not if the

10,000 seconds but maybe not if the adversary isn't a roboticist or a human

adversary isn't a roboticist or a human what if they're like at a software

what if they're like at a software programmer someone who's taken even a

programmer someone who's taken even a class introductory like cs50 and learned

class introductory like cs50 and learned a little bit of programming well a

a little bit of programming well a little bit frighteningly it's not all

little bit frighteningly it's not all that hard to hack into systems if you

that hard to hack into systems if you just know how to code to and really have

just know how to code to and really have the computer do your work for you so in

the computer do your work for you so in fact let me go ahead and change over to

fact let me go ahead and change over to another screen on my computer here um

another screen on my computer here um this is different for students in the

this is different for students in the group from vs code this is just a black

group from vs code this is just a black and white version of it that we've used

and white version of it that we've used briefly in the past and I'm just going

briefly in the past and I'm just going to go ahead and create a program called

to go ahead and create a program called crack.pie to crack something just

crack.pie to crack something just technically means to figure out what it

technically means to figure out what it is figure out a password in this case

is figure out a password in this case and dop means I'm going to use a

and dop means I'm going to use a programming language that we here in

programming language that we here in cs50 have been dabbling in in the past

cs50 have been dabbling in in the past couple of weeks with more to come next

couple of weeks with more to come next week as well so it turns out and you

week as well so it turns out and you need not understand each of these lines

need not understand each of these lines of code if I want to try maybe

of code if I want to try maybe generating all 10,000 possible codes I'm

generating all 10,000 possible codes I'm not going to bother with a robot I've

not going to bother with a robot I've got all these cables coming out of my

got all these cables coming out of my computer and odds are one of them is a

computer and odds are one of them is a USB cable or a lightning cable surely we

USB cable or a lightning cable surely we could figure out how to connect like

could figure out how to connect like laptop or desktop to phone and just like

laptop or desktop to phone and just like automate the process nowadays by just

automate the process nowadays by just sending all of the numbers into the

sending all of the numbers into the phone until one uh unlocks the trick

phone until one uh unlocks the trick just like in the movies or TV well in

just like in the movies or TV well in Python I could write a program that does

Python I could write a program that does this as follows I can import so to speak

this as follows I can import so to speak all of the decimal digits 0 through n

all of the decimal digits 0 through n and this for students in the room is

and this for students in the room is just a slightly better version of typing

just a slightly better version of typing out 10 different numbers manually I can

out 10 different numbers manually I can also import from a library so to speak

also import from a library so to speak called iter tools for iteration tools

called iter tools for iteration tools which means to do something again and

which means to do something again and again I can import a function called

again I can import a function called Product which means the cross product

Product which means the cross product like combine this with this some number

like combine this with this some number of times and then it's just two more

of times and then it's just two more lines of code I can use what's called a

lines of code I can use what's called a loop and programming so for every

loop and programming so for every passcode in the cross product of all 10

passcode in the cross product of all 10 of those digits repeated a total of four

of those digits repeated a total of four times let me go ahead and rather than

times let me go ahead and rather than bother connecting my phone and hacking

bother connecting my phone and hacking my own phone let me just print out every

my own phone let me just print out every one of those 10,000 codes on the screen

one of those 10,000 codes on the screen and we'll see how fast the hacker could

and we'll see how fast the hacker could do this let me go ahead and print and

do this let me go ahead and print and with an asterisk which is a little trick

with an asterisk which is a little trick to format it nicely I'm going to print

to format it nicely I'm going to print out each of those passcodes and that's

out each of those passcodes and that's it four lines of code maybe 40 seconds

it four lines of code maybe 40 seconds of talking but maybe really four seconds

of talking but maybe really four seconds of code if I actually did this without

of code if I actually did this without the audience and now let me go ahead and

the audience and now let me go ahead and save the file and I'm going to run as we

save the file and I'm going to run as we do every in class of late python of

do every in class of late python of crack.pie and when I hit enter I should

crack.pie and when I hit enter I should see on the screen all 10,000

see on the screen all 10,000 possibilities from 000000 to 9999 so

possibilities from 000000 to 9999 so let's see is it a few seconds minutes

let's see is it a few seconds minutes hours or days done so barely even

hours or days done so barely even seconds plural if that so that should be

seconds plural if that so that should be a little disconcerting because all that

a little disconcerting because all that adversary needs to do is grab your phone

adversary needs to do is grab your phone off the counter plug in a cable and boom

off the counter plug in a cable and boom they're done like there's no ticking

they're done like there's no ticking clock or worries as in the movies or TV

clock or worries as in the movies or TV that maybe you're going to come into the

that maybe you're going to come into the room you don't need that much of a

room you don't need that much of a window of time so what would be better

window of time so what would be better than this well let's consider what our

than this well let's consider what our options might be if we don't want to

options might be if we don't want to just use four-digit passcode some of you

just use four-digit passcode some of you indeed might have better passcodes than

indeed might have better passcodes than that and maybe you use four letter

that and maybe you use four letter passcodes instead so A through Z maybe

passcodes instead so A through Z maybe uppercase and lowercase that starts to

uppercase and lowercase that starts to make things a little more interesting so

make things a little more interesting so should we pull this question too if we

should we pull this question too if we upgrade from four digits to just four

upgrade from four digits to just four letters English letters a through z

letters English letters a through z uppercase and lowercase why don't we go

uppercase and lowercase why don't we go ahead and pull the grip here and ask how

ahead and pull the grip here and ask how many four letter passcodes are there

many four letter passcodes are there instead so this time the range starts at

instead so this time the range starts at four still not the right answer though

four still not the right answer though this time how many four-letter passcodes

this time how many four-letter passcodes are

possible not just just take a couple more

more seconds

seconds all right almost a couple hundred

all right almost a couple hundred responses in already few more

seconds and why don't we go ahead and reveal now

and why don't we go ahead and reveal now the answers which are Okay so we've

the answers which are Okay so we've solved a couple problems at least so we

solved a couple problems at least so we well we well okay someone's just messing

well we well okay someone's just messing with us now all right so it looks like

with us now all right so it looks like most of you 76% of you have claimed it's

most of you 76% of you have claimed it's 7 million plus possibilities so that's

7 million plus possibilities so that's encouraging cuz that's a whole order of

encouraging cuz that's a whole order of magnitude more than before well let's

magnitude more than before well let's figure out how we might do this

figure out how we might do this mathematically so if we've got 26

mathematically so if we've got 26 lowercase 26 uppercase that's 52

lowercase 26 uppercase that's 52 possibilities now for each of those four

possibilities now for each of those four digits so that's 52 times itself four

digits so that's 52 times itself four times which indeed either off the top of

times which indeed either off the top of your head a good guess a calculator on

your head a good guess a calculator on the same device you're using right now

the same device you're using right now indeed gives us 7 million instead well

indeed gives us 7 million instead well what might be slightly better than that

what might be slightly better than that well maybe four characters and this

well maybe four characters and this indeed is what your Macs PCS and phones

indeed is what your Macs PCS and phones are urging us to do nowadays not just

are urging us to do nowadays not just numbers not just letters but like really

numbers not just letters but like really annoying punctuation so it really looks

annoying punctuation so it really looks cryptic not just to the adversary but

cryptic not just to the adversary but also to you and me unfortunately and

also to you and me unfortunately and that's the downside but here now we have

that's the downside but here now we have a mental model and really a

a mental model and really a computational framework via which we can

computational framework via which we can evaluate the security of these and I'll

evaluate the security of these and I'll go ahead and spoil some of the math here

go ahead and spoil some of the math here if we've got 52 uh letters of the

if we've got 52 uh letters of the alphabet uppercase and lowercase 10

alphabet uppercase and lowercase 10 digits and if I count them out on my

digits and if I count them out on my keyboard about 32 punctuation symbols in

keyboard about 32 punctuation symbols in typical English grammar that actually

typical English grammar that actually gives us 94 possibilities now which is

gives us 94 possibilities now which is up from 52 which is up from 10 so now

up from 52 which is up from 10 so now we're really moving and now that would

we're really moving and now that would give us 78 million possibility so

give us 78 million possibility so another order of magnitude now it's

another order of magnitude now it's still going to be relatively fast

still going to be relatively fast because you know what I can actually do

because you know what I can actually do this let me go back into my code here

this let me go back into my code here let me reopen the same program and I can

let me reopen the same program and I can point out just how easy it is to make

point out just how easy it is to make these changes instead of importing

these changes instead of importing digits as before I can import as your uh

digits as before I can import as your uh child might know asky letters which are

child might know asky letters which are a through z uppercase lowercase and I

a through z uppercase lowercase and I can just change this here asky letters

can just change this here asky letters and so this was that first version where

and so this was that first version where we just changed to letters let me now

we just changed to letters let me now rerun the code and instead of seeing

rerun the code and instead of seeing numbers we'll see letters flying across

numbers we'll see letters flying across the screen and if I walk over here to

the screen and if I walk over here to the screen we'll see that by the time I

the screen we'll see that by the time I get here we're halfway through the

get here we're halfway through the entire alphabet lowercase if I now start

entire alphabet lowercase if I now start walking away I think yep we're already

walking away I think yep we're already done now with uppercase as well if I

done now with uppercase as well if I upgrade this slightly further let's go

upgrade this slightly further let's go ahead and take it one more level and

ahead and take it one more level and perhaps do let's say ask letters and

perhaps do let's say ask letters and digits and punctuation and this would be

digits and punctuation and this would be the pythonic way to say that and I'm

the pythonic way to say that and I'm going to add to those letters those same

going to add to those letters those same digits those same punctuation symbols

digits those same punctuation symbols let me shrink my font just so the code

let me shrink my font just so the code still fits on the screen and what we now

still fits on the screen and what we now have is with a two seconds of changes a

have is with a two seconds of changes a program that if I run this version

program that if I run this version whoops without the typographical error

whoops without the typographical error this is what we call in cs50 a bug so

this is what we call in cs50 a bug so now we run the same this is what we call

now we run the same this is what we call in cs50 a second

in cs50 a second bug

bug punctuation okay this is where I cross

punctuation okay this is where I cross my fingers okay so now it's going to be

my fingers okay so now it's going to be a little hard to see as it flies across

a little hard to see as it flies across the screen but you probably are seeing

the screen but you probably are seeing glimpses of some weird punctuation

glimpses of some weird punctuation characters as well and I won't waste our

characters as well and I won't waste our time trying to talk through this because

time trying to talk through this because this is going to take longer we're still

this is going to take longer we're still in the lower case I'm still over here

in the lower case I'm still over here already we've not even gotten to n now O

already we've not even gotten to n now O then P so this is going to run longer

then P so this is going to run longer but let's end with one final question on

but let's end with one final question on the security of all these systems I'm

the security of all these systems I'm going to cancel that by hitting contrl C

going to cancel that by hitting contrl C on my keyboard and let's ask the

on my keyboard and let's ask the question instead if we use eight

question instead if we use eight character passwords so twice as many

character passwords so twice as many characters but even that is not terribly

characters but even that is not terribly long right this is eight characters

long right this is eight characters alone on the stage eight characters

alone on the stage eight characters using letters numbers and punctuation

using letters numbers and punctuation might be better let's do one final vote

might be better let's do one final vote here if we could on your same device how

here if we could on your same device how many eight character possibilities are

many eight character possibilities are there now

there now for these

for these passcodes and now for didn't even make

passcodes and now for didn't even make the list this

the list this time all right few more seconds about

time all right few more seconds about 100 responses so

100 responses so far how about we go ahead and Carter if

far how about we go ahead and Carter if you would't mind let's reveal the

you would't mind let's reveal the results based on the vote a pretty

results based on the vote a pretty decent spread here although the

decent spread here although the quadrillions are quickly buzzing in and

quadrillions are quickly buzzing in and they're contending with the others here

they're contending with the others here looks like 44% of you said quintilian

looks like 44% of you said quintilian 34% said quadrillion and this time for

34% said quadrillion and this time for the first time uh you overbid so indeed

the first time uh you overbid so indeed if we go back to the math here at least

if we go back to the math here at least the majority overbid if we have eight

the majority overbid if we have eight character passcodes that gives us 94

character passcodes that gives us 94 times itself 8 times or 94 to the eth

times itself 8 times or 94 to the eth power and in fact that gives us roughly

power and in fact that gives us roughly 6 quadrillion 95 trillion 689 Bill 385

6 quadrillion 95 trillion 689 Bill 385 mil 410,000 and

mil 410,000 and 86 possible passcodes now what does that

86 possible passcodes now what does that mean well the adversary's algorithm the

mean well the adversary's algorithm the step-by-step code that they write to try

step-by-step code that they write to try to hack into your phone is no different

to hack into your phone is no different and honestly if your passcode is eight

and honestly if your passcode is eight characters long but there're zeros

characters long but there're zeros 00000000000000 you're no more secure

00000000000000 you're no more secure fundamentally you really want to be

fundamentally you really want to be somewhere in The Sweet Spot of that

somewhere in The Sweet Spot of that massive range of values so that if the

massive range of values so that if the adversary tries this Brute Force attack

adversary tries this Brute Force attack just running through all possibilities

just running through all possibilities they will eventually reach your passcode

they will eventually reach your passcode just mathematically it will be there

just mathematically it will be there hopefully though well maybe not

hopefully though well maybe not hopefully you and I and they will be

hopefully you and I and they will be gone from this world because that much

gone from this world because that much time will have passed and if we fact if

time will have passed and if we fact if we do out the math here uh this number

we do out the math here uh this number of seconds for instance is long past uh

of seconds for instance is long past uh when uh uh we will uh no longer be here

when uh uh we will uh no longer be here so that's the sort of measure is we

so that's the sort of measure is we don't sort of fundamentally change the

don't sort of fundamentally change the equation for the adversary it's still

equation for the adversary it's still the same risk it's still the same attack

the same risk it's still the same attack but you significantly drive down the

but you significantly drive down the probability of success on their part or

probability of success on their part or conceptually you drive up the cost to

conceptually you drive up the cost to the adversary and indeed even in the

the adversary and indeed even in the physical world this is true you just

physical world this is true you just want your passcode in the digital world

want your passcode in the digital world really to be better than someone else's

really to be better than someone else's because you want someone else's passcode

because you want someone else's passcode to be the one that the adversary does

to be the one that the adversary does something with with just like in the

something with with just like in the physical world even though it's a bit uh

physical world even though it's a bit uh uncomfortable to consider your house

uncomfortable to consider your house doesn't need to be 100% secure and

doesn't need to be 100% secure and indeed it's difficult to make it such

indeed it's difficult to make it such there's always going to be a a point of

there's always going to be a a point of weakness maybe it's that window the door

weakness maybe it's that window the door or something like that but if your home

or something like that but if your home is more secure than the next door home

is more secure than the next door home just probabilistically you are more

just probabilistically you are more secure you're not secure and indeed any

secure you're not secure and indeed any website you see down the road that says

website you see down the road that says we are secure because we do X Y or Z

we are secure because we do X Y or Z like that's nonsense security is really

like that's nonsense security is really about comparisons and Val ating things

about comparisons and Val ating things if quantitatively relative to some other

if quantitatively relative to some other system relative to some other code so

system relative to some other code so what's the takeaway here well hopefully

what's the takeaway here well hopefully a non-trivial number of you we'll go

a non-trivial number of you we'll go home this weekend on Monday and change

home this weekend on Monday and change at least one passcode um but there's

at least one passcode um but there's going to be a trade-off here and we talk

going to be a trade-off here and we talk about this all the time in cs50 anytime

about this all the time in cs50 anytime we improve something we pay some price

we improve something we pay some price in time and performance and cost

in time and performance and cost somewhere else so what's the downside

somewhere else so what's the downside then of this advice that you should use

then of this advice that you should use minimally eight character

minimally eight character passcodes why might you want to say nay

passcodes why might you want to say nay and not do

and not do this say again you have to remember you

this say again you have to remember you have to remember it right and so here

have to remember it right and so here there's sort of some sociology there's

there's sort of some sociology there's some human behavior you know some of you

some human behavior you know some of you might have colleagues if you're working

might have colleagues if you're working in the real world at least back in

in the real world at least back in healthier times when you had colleagues

healthier times when you had colleagues with desks and cubicles and there's

with desks and cubicles and there's probably one person in the office with

probably one person in the office with like a Post-It note on their monitor

like a Post-It note on their monitor with their passcode you know it's a bit

with their passcode you know it's a bit of a cyber security offense but it's

of a cyber security offense but it's also a sort of real world side effect

also a sort of real world side effect maybe of corporate policies that aren't

maybe of corporate policies that aren't really calibrated for human behavior so

really calibrated for human behavior so we'll see if there's some other defenses

we'll see if there's some other defenses and indeed let me propose that we talk

and indeed let me propose that we talk briefly about one that actually tends to

briefly about one that actually tends to kick in automatically even if your

kick in automatically even if your passcode is not as strong as we've just

passcode is not as strong as we've just seen one of these six quadrillion

seen one of these six quadrillion possibilities well what could we do

possibilities well what could we do instead well as anyone and I'll zoom in

instead well as anyone and I'll zoom in on this here accidentally locked themsel

on this here accidentally locked themsel out of their own phone before like when

out of their own phone before like when does that happen yeah when you try the

does that happen yeah when you try the password too many times yeah so too many

password too many times yeah so too many times maybe your finger's slightly off

times maybe your finger's slightly off maybe you're slightly off and you just

maybe you're slightly off and you just don't input the same passcode correctly

don't input the same passcode correctly after like five times 10 times there's

after like five times 10 times there's some reasonable threshold and why does

some reasonable threshold and why does that happen well Apple and Google

that happen well Apple and Google equivalently figure just

equivalently figure just probabilistically if after 10 guesses

probabilistically if after 10 guesses you still haven't typed in the right

you still haven't typed in the right passcode probably you're not you you're

passcode probably you're not you you're someone else who's picked up your phone

someone else who's picked up your phone so we're just going to go ahead and lock

so we're just going to go ahead and lock you out now what's the effect of this

you out now what's the effect of this well this means now that each of those

well this means now that each of those possible passcodes no longer takes

possible passcodes no longer takes roughly 1 second now it takes roughly 1

roughly 1 second now it takes roughly 1 minute so the attack is still the same

minute so the attack is still the same but if it's now one passcode or 10

but if it's now one passcode or 10 guesses per minute we have significantly

guesses per minute we have significantly by a factor of 60 in this story slowed

by a factor of 60 in this story slowed things down and unfortunately does

things down and unfortunately does anyone know what happens if you screw up

anyone know what happens if you screw up again after a

again after a minute yeah it goes longer it's like

minute yeah it goes longer it's like five minutes and then 10 minutes and uh

five minutes and then 10 minutes and uh Google is kind of obnoxious about it

Google is kind of obnoxious about it they don't even give you a time frame

they don't even give you a time frame they just say try again later and so

they just say try again later and so that keeps not only the adversary out

that keeps not only the adversary out but also potentially you so there in

but also potentially you so there in lies that trade-off if you've forgotten

lies that trade-off if you've forgotten your code if nowadays your finger is

your code if nowadays your finger is slightly wet so the screen isn't

slightly wet so the screen isn't responding correctly these could be

responding correctly these could be usability downsides too so security is

usability downsides too so security is really just about finding The Sweet Spot

really just about finding The Sweet Spot among these various tradeoffs here but

among these various tradeoffs here but there's other mechanisms too and some of

there's other mechanisms too and some of you might recognize this screen from

you might recognize this screen from Gmail via which of course you log in but

Gmail via which of course you log in but after you log into Gmail or similar

after you log into Gmail or similar websites or apps or systems at work

websites or apps or systems at work nowadays especially you might be

nowadays especially you might be presented with uh what's called

presented with uh what's called two-factor authentication and what is

two-factor authentication and what is this in a nutshell in Lay person

this in a nutshell in Lay person terms most many of you if you do

terms most many of you if you do anything digitally at work might have to

anything digitally at work might have to do this now

do this now yeah exactly you get texted at your

yeah exactly you get texted at your phone an additional code that's not your

phone an additional code that's not your same password it's typically a numeric

same password it's typically a numeric code maybe six digits long it expires

code maybe six digits long it expires after a minute or 10 minutes but why is

after a minute or 10 minutes but why is this a good thing well one it's no

this a good thing well one it's no longer just a piece of information that

longer just a piece of information that you know or that you might have written

you know or that you might have written down it's information that changes every

down it's information that changes every time you try to log in but more

time you try to log in but more importantly it's a fundamentally second

importantly it's a fundamentally second factor which means it's not just

factor which means it's not just something you know now it's something

something you know now it's something you have so you for instance are the

you have so you for instance are the only one theoretically that should be

only one theoretically that should be receiving that code and so now the

receiving that code and so now the adversary if they want to get into your

adversary if they want to get into your account not only have to guess or brute

account not only have to guess or brute force or maybe read off of a Post-It

force or maybe read off of a Post-It note your password they also have to

note your password they also have to physically have access now to that phone

physically have access now to that phone so there's still a threat absolutely but

so there's still a threat absolutely but it's not everyone on the internet with

it's not everyone on the internet with an internet connection now it's only the

an internet connection now it's only the people in Starbucks now it's only the

people in Starbucks now it's only the people at work now it's only the people

people at work now it's only the people in your home who might have access to

in your home who might have access to that second Factor so there too it just

that second Factor so there too it just raises the bar to the adversary making

raises the bar to the adversary making it harder more timec consuming more

it harder more timec consuming more geographically impossible for them to

geographically impossible for them to attack you but what's the downside of

attack you but what's the downside of two-factor authentication whether it's a

two-factor authentication whether it's a device or even nowadays it's in software

device or even nowadays it's in software whether it's on your keychain or on your

whether it's on your keychain or on your phone where you're prompted for this

phone where you're prompted for this code what's a downside as some of us

code what's a downside as some of us have probably experienced

have probably experienced too forg you forget your cell phone

too forg you forget your cell phone absolutely right the the factor that you

absolutely right the the factor that you have you don't have with you or maybe

have you don't have with you or maybe you're in a basement somewhere don't

you're in a basement somewhere don't have reception you're on a plane you

have reception you're on a plane you can't get the code and so there too are

can't get the code and so there too are these tradeoffs and even it departments

these tradeoffs and even it departments need to keep that in mind because what

need to keep that in mind because what does that mean for them well if you

does that mean for them well if you don't have your phone with you and you

don't have your phone with you and you are in the habit of calling it to help

are in the habit of calling it to help you fix this now there's a a cost a

you fix this now there's a a cost a human cost maybe even a financial cost

human cost maybe even a financial cost and so it policy nowadays is really just

and so it policy nowadays is really just about finding the right balance and

about finding the right balance and where we want to spend our resources but

where we want to spend our resources but at least raise the bar to the adversary

at least raise the bar to the adversary but of course there's other ways too and

but of course there's other ways too and this is going to be one of our homework

this is going to be one of our homework assignments if you will after today

assignments if you will after today there's this software called password

there's this software called password managers and no need to buzz in on your

managers and no need to buzz in on your phone but maybe with a physical hand how

phone but maybe with a physical hand how many folks here use a password manager

many folks here use a password manager okay let me ballpark this at 10 20%

okay let me ballpark this at 10 20% perhaps Okay so we've got 80% upside

perhaps Okay so we've got 80% upside here and a lesson learned potentially so

here and a lesson learned potentially so a password manager is just a piece of

a password manager is just a piece of software on your Mac your PC or your

software on your Mac your PC or your phone nowadays that manages your

phone nowadays that manages your passwords well what is that mean when

passwords well what is that mean when you go to a website for the first time

you go to a website for the first time or you download an app for the first

or you download an app for the first time and you have to create an account

time and you have to create an account you can can still use your email address

you can can still use your email address or David as your username or whatever

or David as your username or whatever your name might be so you don't have to

your name might be so you don't have to change that methodology but instead of

change that methodology but instead of typing in one 2 3 4 5 six as your same

typing in one 2 3 4 5 six as your same password for that website or app as well

password for that website or app as well as for every other now you use the

as for every other now you use the password manager software to generate

password manager software to generate something difficult to guess for you

something difficult to guess for you that is you tell the password manager

that is you tell the password manager give me an8 character random passcode

give me an8 character random passcode not 0000 but something with punctuation

not 0000 but something with punctuation with numbers with letters and better yet

with numbers with letters and better yet the password manager is the name

the password manager is the name suggests remembers that password for you

suggests remembers that password for you and the next time you go to another

and the next time you go to another website you do it again with a

website you do it again with a completely different password maybe same

completely different password maybe same username maybe two-factor authentication

username maybe two-factor authentication but different password different

but different password different password different password and it

password different password and it doesn't have to be eight I mean I'm in

doesn't have to be eight I mean I'm in the habit of using a dozen two dozen

the habit of using a dozen two dozen characters in total and at that point I

characters in total and at that point I can't even pronounce the number of

can't even pronounce the number of possibilities because it goes well be

possibilities because it goes well be Beyond uh the quadrillions so the

Beyond uh the quadrillions so the probability that someone's going to get

probability that someone's going to get into one of those accounts for me now is

into one of those accounts for me now is very very very low and they're going to

very very very low and they're going to take less interest in me and maybe more

take less interest in me and maybe more interest in someone else that's not

interest in someone else that's not using as good of a password now what

using as good of a password now what does this mean in real terms well when

does this mean in real terms well when you go to log into that managed site you

you go to log into that managed site you don't manually type your password

don't manually type your password anymore in fact you don't generally even

anymore in fact you don't generally even need to know it nowadays I probably

need to know it nowadays I probably don't know 90 plus 99% of my passwords I

don't know 90 plus 99% of my passwords I entrust them to this password manager

entrust them to this password manager now of course you'd like to think that

now of course you'd like to think that the password manager itself is secure so

the password manager itself is secure so what might that mean well those of you

what might that mean well those of you who do use a password manager how do you

who do use a password manager how do you access that software itself what's

access that software itself what's protecting your data in your

protecting your data in your understanding so maybe Biometrics like

understanding so maybe Biometrics like your your face ID or maybe your

your your face ID or maybe your fingerprint or maybe more simply what

fingerprint or maybe more simply what else password maybe just a password and

else password maybe just a password and hopefully that password that primary

hopefully that password that primary password that gatekeeper is not itself 1

password that gatekeeper is not itself 1 2 3 4 5 6 otherwise it doesn't matter

2 3 4 5 6 otherwise it doesn't matter how secure all of the others are but if

how secure all of the others are but if you're willing to put in the effort and

you're willing to put in the effort and pick one pretty long somewhat random

pick one pretty long somewhat random very unguessable password that you just

very unguessable password that you just promise to commit to memory and maybe

promise to commit to memory and maybe for backup you literally print it out

for backup you literally print it out and put it in a safe deposit box or a

and put it in a safe deposit box or a safe or just kind of hide it somewhere

safe or just kind of hide it somewhere physically that there's very low

physically that there's very low probability someone's going to find the

probability someone's going to find the backup copy that might be Al loan but of

backup copy that might be Al loan but of course the flip side is now if you

course the flip side is now if you forget that Ma that primary password

forget that Ma that primary password you've now lost all of the eggs in the

you've now lost all of the eggs in the basket if someone gets that primary

basket if someone gets that primary password now they have access to

password now they have access to everything so that's rather the

everything so that's rather the trade-off but I dare say you're probably

trade-off but I dare say you're probably less threatened depending on your family

less threatened depending on your family uh uh by the people immediately around

uh uh by the people immediately around you than the billions of other people on

you than the billions of other people on the internet that have access

the internet that have access potentially to those same systems so

potentially to those same systems so there to it's a trade-off and it's up to

there to it's a trade-off and it's up to you to decide whether or not to manage

you to decide whether or not to manage your passwords in this way but if you

your passwords in this way but if you were on that top 10 list or even if

were on that top 10 list or even if you're not but you can think of several

you're not but you can think of several accounts that all have the same password

accounts that all have the same password you're probably going to benefit from

you're probably going to benefit from something like this and why is it bad to

something like this and why is it bad to be clear to use the same password on

be clear to use the same password on multiple sites in case that's never sort

multiple sites in case that's never sort of dawned

of dawned in thought why is that a bad thing to

in thought why is that a bad thing to reuse a password on different websites

reuse a password on different websites different apps any intuition yeah and

back exactly once it's attacked you can the adversary presumably by transitivity

the adversary presumably by transitivity can see oh well if this user's username

can see oh well if this user's username is mailin harvard.edu on this website

is mailin harvard.edu on this website and their password is foolishly 1 2 3 4

and their password is foolishly 1 2 3 4 56 or even something way more

56 or even something way more complicated they can probably just

complicated they can probably just assume with high probability that if I'm

assume with high probability that if I'm being a little reckless let's try access

being a little reckless let's try access mailin at harvard.edu other accounts

mailin at harvard.edu other accounts other apps using that exact same

other apps using that exact same password and so by transitivity

password and so by transitivity essentially you're putting your other uh

essentially you're putting your other uh Accounts at risk so what's maybe a

Accounts at risk so what's maybe a takeaway minimally here I would start to

takeaway minimally here I would start to reconsider your passcodes on your most

reconsider your passcodes on your most important data Maybe it's medical maybe

important data Maybe it's medical maybe it's Financial maybe it's email anything

it's Financial maybe it's email anything remotely personal that you really

remotely personal that you really wouldn't want to have access uh do you

wouldn't want to have access uh do you necessarily need the same form level of

necessarily need the same form level of security on eCommerce sites or sites

security on eCommerce sites or sites that you don't really care about or that

that you don't really care about or that you signed up for once and after that H

you signed up for once and after that H that's it probably not so you can decide

that's it probably not so you can decide for yourself but again software like a

for yourself but again software like a password manager and these are just some

password manager and these are just some of the possibilities out there um are

of the possibilities out there um are probably to be your friend a couple of

probably to be your friend a couple of these are free they come with Windows or

these are free they come with Windows or Mac OS a couple are commercial Harvard

Mac OS a couple are commercial Harvard has a site license for students for uh

has a site license for students for uh one of these as well so there are

one of these as well so there are options out there but what else do

options out there but what else do people use what else can people use to

people use what else can people use to keep their system secure so most of us

keep their system secure so most of us nowadays have probably heard of

nowadays have probably heard of encryption this technique for just kind

encryption this technique for just kind of scrambling information so when you

of scrambling information so when you want to send a message an email or

want to send a message an email or upload a a photograph or use your credit

upload a a photograph or use your credit card hopefully it's not just being sent

card hopefully it's not just being sent out for all to see but there's some kind

out for all to see but there's some kind of scrambling going on and some fancy

of scrambling going on and some fancy mathematics ensure that encryption

mathematics ensure that encryption ensures that only you the sender and

ensures that only you the sender and someone else the receiver can

someone else the receiver can theoretically see what that credit card

theoretically see what that credit card number is what that message is what that

number is what that message is what that photograph is instead so encryption is

photograph is instead so encryption is sort of Common Place nowadays both in

sort of Common Place nowadays both in websites and apps and ATMs and other

websites and apps and ATMs and other such devices but how does it work well

such devices but how does it work well back in week two of cs50 uh your child

back in week two of cs50 uh your child learned learned a little something about

learned learned a little something about encryption otherwise known as

encryption otherwise known as cryptography and one of the algorithms

cryptography and one of the algorithms we talked about was quite simply

we talked about was quite simply something like this this is what we

something like this this is what we might call uh not only cs50 but plain

might call uh not only cs50 but plain text so very plain text that in this

text so very plain text that in this case is English and obviously everyone

case is English and obviously everyone in the room can read it but what if I

in the room can read it but what if I wanted to send this message out to

wanted to send this message out to someone in this room or out on the

someone in this room or out on the Internet or maybe equivalently back in

Internet or maybe equivalently back in the day maybe write a message down on a

the day maybe write a message down on a scrap of paper in grade school and pass

scrap of paper in grade school and pass a secret note a secret love note to

a secret note a secret love note to someone in class with hopes that the

someone in class with hopes that the teach or any other students in the class

teach or any other students in the class can't intercept it and read it well you

can't intercept it and read it well you probably don't want to say this is cs50

probably don't want to say this is cs50 or I love you or anything remotely

or I love you or anything remotely sensitive but rather maybe you want to

sensitive but rather maybe you want to encrypt it and let's change the the T to

encrypt it and let's change the the T to a u maybe change the H to an i the I to

a u maybe change the H to an i the I to a j the S to a t the I to a j the S to a

a j the S to a t the I to a j the S to a t again the C to a d the S to a T and

t again the C to a d the S to a T and we'll just leave the numbers alone even

we'll just leave the numbers alone even though I worry someone could probably

though I worry someone could probably guess what this now does say nonetheless

guess what this now does say nonetheless but what was the algorithm as I rattled

but what was the algorithm as I rattled those ch changes off whether student

those ch changes off whether student from week two or parent from week now

from week two or parent from week now yeah a on shift just a onlet shift and

yeah a on shift just a onlet shift and this is more sophisticatedly called a

this is more sophisticatedly called a rotational Cipher or a Caesar Cipher

rotational Cipher or a Caesar Cipher after Caesar back in the day it's

after Caesar back in the day it's relatively simplistic but back in the

relatively simplistic but back in the day it's not so simplistic if you're the

day it's not so simplistic if you're the first person in the world to ever use it

first person in the world to ever use it or think of it but nowadays this is not

or think of it but nowadays this is not actually what we use but it's similarly

actually what we use but it's similarly mathematical in nature it's not quite as

mathematical in nature it's not quite as simple as just adding one or subtracting

simple as just adding one or subtracting one to go from now what we'd call text

one to go from now what we'd call text to plain test but it's similarly math

to plain test but it's similarly math that's involved and let me just

that's involved and let me just stipulate that the way the math works is

stipulate that the way the math works is that the sender and the receiver just

that the sender and the receiver just have to have in mind some kind of secret

have to have in mind some kind of secret and the Secret in this case would very

and the Secret in this case would very trivially be one but it could be a much

trivially be one but it could be a much bigger much more unguessable number or

bigger much more unguessable number or maybe some other secret we share the

maybe some other secret we share the presumption being that my classmates my

presumption being that my classmates my teacher in that grade school classroom

teacher in that grade school classroom if they don't know what that secret is

if they don't know what that secret is that number is yeah they could try to

that number is yeah they could try to brute force it and try all possible

brute force it and try all possible mathematics plus one plus 2 plus three

mathematics plus one plus 2 plus three but that's going to take them some time

but that's going to take them some time and they probably don't care enough and

and they probably don't care enough and so my data might be therefore relatively

so my data might be therefore relatively secure but we use encryption all the

secure but we use encryption all the time nowadays and so for instance this

time nowadays and so for instance this is at the start of most URLs nowadays

is at the start of most URLs nowadays even if you don't type it yourself with

even if you don't type it yourself with that said Safari and even Chrome now or

that said Safari and even Chrome now or kind of simplifying if not dumbing down

kind of simplifying if not dumbing down user interfaces to just hide details

user interfaces to just hide details that you and I as sort of normal users

that you and I as sort of normal users don't need to see 24/7 but it is there

don't need to see 24/7 but it is there and if in fact on your phone or laptop

and if in fact on your phone or laptop you click on the URL even if it's super

you click on the URL even if it's super short initially you'll probably see the

short initially you'll probably see the whole thing starting with this and the s

whole thing starting with this and the s means secure the s means that encryption

means secure the s means that encryption is being used but there's other forms of

is being used but there's other forms of this not just when you visit websites

this not just when you visit websites there's this endtoend encryption which

there's this endtoend encryption which is being talked about more nowadays

is being talked about more nowadays especially during coid times with so

especially during coid times with so many more of us on video and talking

many more of us on video and talking about uh more sensitive things tele

about uh more sensitive things tele medicine talking to doctors things that

medicine talking to doctors things that you also wouldn't want to verbally or

you also wouldn't want to verbally or visually get out into the wild just like

visually get out into the wild just like text what's different about endtoend

text what's different about endtoend encryption versus HTT GPS and the type

encryption versus HTT GPS and the type of encryption that most of us use every

of encryption that most of us use every day on websites

day on websites alone end to end encryption is sort of a

alone end to end encryption is sort of a better feature that you want to

better feature that you want to increasingly seek when using services

increasingly seek when using services like Zoom or Microsoft teams or Whatsapp

like Zoom or Microsoft teams or Whatsapp or the

or the like any instincts here yeah over on the

right good so the encryption the scrambling of information happens in The

scrambling of information happens in The Source the sender and the destination

Source the sender and the destination the receiver without a so-called

the receiver without a so-called middleman in between and this is

middleman in between and this is actually very different from most

actually very different from most contexts nowadays that use just https

contexts nowadays that use just https because when you're using https to buy

because when you're using https to buy something on Amazon securely with your

something on Amazon securely with your credit card well of course Amazon needs

credit card well of course Amazon needs to be able to decrep the message at the

to be able to decrep the message at the end of the day and so that's fine but

end of the day and so that's fine but even when you're using services like

even when you're using services like video conferencing or maybe text

video conferencing or maybe text messaging nowadays well if you're using

messaging nowadays well if you're using Whatsapp that's owned by meta and if

Whatsapp that's owned by meta and if you're using Instagram that's owned by

you're using Instagram that's owned by meta there's a lot of middlemen in these

meta there's a lot of middlemen in these apps that we're using and if they were

apps that we're using and if they were only using encryption period or only

only using encryption period or only using something like https yes your en

using something like https yes your en your connection from you to WhatsApp and

your connection from you to WhatsApp and in turn to the recipient might very well

in turn to the recipient might very well be secure on each end of that channel

be secure on each end of that channel but meta in the between the company and

but meta in the between the company and any other company in between could

any other company in between could theoretically For Better or For Worse be

theoretically For Better or For Worse be looking at that data whether it's to

looking at that data whether it's to mine it for advertising purposes whether

mine it for advertising purposes whether it's to Snoop on data that you're

it's to Snoop on data that you're sending that is not end to-end

sending that is not end to-end encryption if the middleman a company

encryption if the middleman a company typically has technically access to that

typically has technically access to that data now zoom and Microsoft teams and

data now zoom and Microsoft teams and WhatsApp and iMessage and other services

WhatsApp and iMessage and other services with which you're familiar increasingly

with which you're familiar increasingly are offering stronger guarantees of

are offering stronger guarantees of encryption whereby it's indeed between

encryption whereby it's indeed between parties A and B and not the one in the

parties A and B and not the one in the middle now there's downsides here and

middle now there's downsides here and you can actually see this kind of

you can actually see this kind of functionality manifest in certain

functionality manifest in certain settings for instance besides iMessage

settings for instance besides iMessage uh which just does this for you on

uh which just does this for you on iPhones or Macs besides Zoom um you can

iPhones or Macs besides Zoom um you can actually fine-tune these settings indeed

actually fine-tune these settings indeed within Zoom itself s so here's a

within Zoom itself s so here's a screenshot that I took last night of

screenshot that I took last night of just what the user interface looks like

just what the user interface looks like today to create a new Zoom meeting with

today to create a new Zoom meeting with the latest version of Zoom software and

the latest version of Zoom software and maybe unbeknownst to you there's a

maybe unbeknownst to you there's a choice of buttons down here and most

choice of buttons down here and most likely yours is by default on enhanced

likely yours is by default on enhanced encryption which is brilliant marketing

encryption which is brilliant marketing speak because it's just encryption it's

speak because it's just encryption it's not enhanced it actually ironically

not enhanced it actually ironically means worse than this um but they want

means worse than this um but they want you using it most likely why well it's a

you using it most likely why well it's a little easier to implement it's a little

little easier to implement it's a little less expensive for them computationally

less expensive for them computationally and to be fair enhanced encryption does

and to be fair enhanced encryption does scramble the data but not in a way that

scramble the data but not in a way that Zoom can't see it Zoom can indeed see it

Zoom can't see it Zoom can indeed see it but that's actually a plus in some

but that's actually a plus in some context because if you want to do like

context because if you want to do like Cloud recordings and you want a meeting

Cloud recordings and you want a meeting recorded not on your Mac or PC but like

recorded not on your Mac or PC but like let Zoom deal with that if you want

let Zoom deal with that if you want automatic transcription nowadays so the

automatic transcription nowadays so the words to appear whether it's English or

words to appear whether it's English or something else on the screen well you

something else on the screen well you can't really lock Zoom or any other

can't really lock Zoom or any other middleman out of that because someone

middleman out of that because someone needs to save it to the cloud someone

needs to save it to the cloud someone needs to translate the voice to those

needs to translate the voice to those English or some other language words so

English or some other language words so enhanced encryption enables those

enhanced encryption enables those features but they also allow a bad actor

features but they also allow a bad actor malicious employee someone who's just

malicious employee someone who's just nosy at Zoom or the equivalent middleman

nosy at Zoom or the equivalent middleman to just kind of poke around your video

to just kind of poke around your video conference and hear what you've said or

conference and hear what you've said or see what you've typed as well unless you

see what you've typed as well unless you instead check this box as well so

instead check this box as well so increasingly look for mentions of end

increasingly look for mentions of end to-end encryption or give that some

to-end encryption or give that some thought when you choose a technology via

thought when you choose a technology via which to communicate with someone

which to communicate with someone whether it's within your family or

whether it's within your family or without as well now last but not least

without as well now last but not least there's other applications of encryption

there's other applications of encryption too and this too might be a lesson

too and this too might be a lesson learned as well full dis encryption so a

learned as well full dis encryption so a dis is like where your data stored in

dis is like where your data stored in your Mac or PC or even your phone and

your Mac or PC or even your phone and full dis encryption just means ideally

full dis encryption just means ideally that all of your data is encrypted that

that all of your data is encrypted that is somehow scrambled now hopefully your

is somehow scrambled now hopefully your password for your computer or phone is

password for your computer or phone is good enough that even though the device

good enough that even though the device is encrypted with that password at least

is encrypted with that password at least you'll remember it and your phone or

you'll remember it and your phone or your Mac or PC will automatically

your Mac or PC will automatically decrypt it for you of course you can't

decrypt it for you of course you can't scramble the information and hide it

scramble the information and hide it from from ourselves one of us at least

from from ourselves one of us at least for these devices needs to have access

for these devices needs to have access but full dis encryption typically means

but full dis encryption typically means that at least when you close the laptop

that at least when you close the laptop lid or power down for the night that

lid or power down for the night that even if someone else steals that device

even if someone else steals that device opens the lid unless they don't unless

opens the lid unless they don't unless they have your passcode they can't even

they have your passcode they can't even plug in fancy cables to the device and

plug in fancy cables to the device and just rip the zeros and ones off of the

just rip the zeros and ones off of the device and see what's actually there

device and see what's actually there full dis encryption means they could do

full dis encryption means they could do that but they would just see seemingly

that but they would just see seemingly random zeros and ones now there's a

random zeros and ones now there's a downside here too this might slow things

downside here too this might slow things down potentially but it is a feature

down potentially but it is a feature increasingly that's offered and is

increasingly that's offered and is absolutely something you should consider

absolutely something you should consider enabling um in general especially if

enabling um in general especially if your laptop or phone travels with you

your laptop or phone travels with you and certainly your phone does or if you

and certainly your phone does or if you plan to donate or sell or give away a

plan to donate or sell or give away a device you don't want to leave all of

device you don't want to leave all of the zeros and ones the remnants of your

the zeros and ones the remnants of your own sensitive data passed on there so

own sensitive data passed on there so Windows has a feature called bit Locker

Windows has a feature called bit Locker Mac OS has a feature called file Vault

Mac OS has a feature called file Vault there's commercial options as well but

there's commercial options as well but generally we're at the point now in 2022

generally we're at the point now in 2022 we're clicking a button is suff

we're clicking a button is suff efficient to enable these features with

efficient to enable these features with that said don't rush into all of these

that said don't rush into all of these decisions I would make backups of your

decisions I would make backups of your data and don't maybe email cs50 if

data and don't maybe email cs50 if something goes wrong with that process

something goes wrong with that process but I would do your own due diligence

but I would do your own due diligence but this too would be a menu of

but this too would be a menu of possibilities and now the bad side the

possibilities and now the bad side the downside of what seems to be great this

downside of what seems to be great this notion of full dis encryption

notion of full dis encryption unfortunately just as we can encrypt our

unfortunately just as we can encrypt our data to protect it from the adversaries

data to protect it from the adversaries so can the adversaries if they get into

so can the adversaries if they get into our devices encrypt our data and do what

our devices encrypt our data and do what not tell us that secret key and so this

not tell us that secret key and so this is generally applied in the context of

is generally applied in the context of ransomware which tragically you

ransomware which tragically you increasingly hear about in Hospital

increasingly hear about in Hospital Systems school systems municipalities

Systems school systems municipalities where systems are getting attacked and

where systems are getting attacked and the data is not just getting stolen

the data is not just getting stolen because what is the adversary typically

because what is the adversary typically need with like local municipal or even

need with like local municipal or even Hospital data the value to the adversary

Hospital data the value to the adversary is encrypting all of the hospital all of

is encrypting all of the hospital all of the mpali data preventing them from

the mpali data preventing them from accessing it if they have no backups or

accessing it if they have no backups or the like and so ransomware is liter

the like and so ransomware is liter Lally about trying to convince someone

Lally about trying to convince someone to pay you money or pay you Bitcoin or

to pay you money or pay you Bitcoin or something like that to give you that

something like that to give you that secret key and this key in this case is

secret key and this key in this case is surely more sophisticated than the

surely more sophisticated than the number one but it's really the same idea

number one but it's really the same idea so here too yet again a trade-off just

so here too yet again a trade-off just as we sort of invent something for good

as we sort of invent something for good it can also be used for evil and so to

it can also be used for evil and so to speak as well but it's really the same

speak as well but it's really the same underlying principles even though we

underlying principles even though we keep seeing it and hearing about it in

keep seeing it and hearing about it in these different forms and lastly if only

these different forms and lastly if only because folks are generally familiar but

because folks are generally familiar but don't necessarily know what it is that

don't necessarily know what it is that it's doing for them browsers nowadays

it's doing for them browsers nowadays have what's often called incognito mode

have what's often called incognito mode or private mode which has nothing to do

or private mode which has nothing to do with encryption but does have to do with

with encryption but does have to do with cyber security or really cyber privacy

cyber security or really cyber privacy keeping your data from prying eyes uh

keeping your data from prying eyes uh incognito mode if you open it in Chrome

incognito mode if you open it in Chrome for instance looks a little something

for instance looks a little something like this and we use it in cs50 when

like this and we use it in cs50 when introducing students as we did last week

introducing students as we did last week to web programming because it in effect

to web programming because it in effect lets you start with a clean slate like a

lets you start with a clean slate like a brand new browser that has never visited

brand new browser that has never visited any websites before which is good for

any websites before which is good for just diagnosing problems but it's often

just diagnosing problems but it's often commonly used if you want to log into

commonly used if you want to log into maybe your Gmail account on someone

maybe your Gmail account on someone else's computer and you don't want your

else's computer and you don't want your password being saved or you want to

password being saved or you want to visit some website where you don't want

visit some website where you don't want the URL or the Search terms ending up in

the URL or the Search terms ending up in your autocomplete history so there's

your autocomplete history so there's multiple uses for incognito mode but

multiple uses for incognito mode but what does it really do well it doesn't

what does it really do well it doesn't stop your company it doesn't stop your

stop your company it doesn't stop your University your internet service

University your internet service provider be it Comcast Verizon or the

provider be it Comcast Verizon or the like from knowing what websites you go

like from knowing what websites you go to because ask your student a couple

to because ask your student a couple weeks ago we talked about actually a

weeks ago we talked about actually a week ago we talked about how the

week ago we talked about how the internet works and unfortunately

internet works and unfortunately computer has an IP address which is

computer has an IP address which is unique identifier which goes out anytime

unique identifier which goes out anytime you go anywhere incognito mode or not so

you go anywhere incognito mode or not so this isn't really covering your tracks

this isn't really covering your tracks outside of your office or outside of

outside of your office or outside of your home or outside of your company but

your home or outside of your company but it is at least throwing away local

it is at least throwing away local information and so we'll talk in fact in

information and so we'll talk in fact in cs50 is week nine this coming Monday

cs50 is week nine this coming Monday about cookies which you might generally

about cookies which you might generally know about and what are called sessions

know about and what are called sessions and so long story short what incognito

and so long story short what incognito mode does is it throws away when you

mode does is it throws away when you close the window any locally stored

close the window any locally stored information to the these things called

information to the these things called cookies which are sort of like virtual

cookies which are sort of like virtual handstamps that just remember what

handstamps that just remember what you've logged in as or what's in your

you've logged in as or what's in your shopping cart or the like but it doesn't

shopping cart or the like but it doesn't hide any information from anyone outside

hide any information from anyone outside of your own Mac or PC it only prevents

of your own Mac or PC it only prevents those local prying eyes so there too

those local prying eyes so there too even though we have tools that many of

even though we have tools that many of you are probably in the habit of using

you are probably in the habit of using or thinking you should use to um be more

or thinking you should use to um be more private be more secure on the internet

private be more secure on the internet what we do really in cs50 boast week's

what we do really in cs50 boast week's uh past and future is talk about how

uh past and future is talk about how these Technologies work so that

these Technologies work so that ultimately we have all the more of an

ultimately we have all the more of an educated citizenry um here among

educated citizenry um here among undergrads and here as well as on line

undergrads and here as well as on line so that you can apply these same Lessons

so that you can apply these same Lessons Learned to problems you'll encounter in

Learned to problems you'll encounter in the future so as promised the homework

the future so as promised the homework one should probably use a password

one should probably use a password manager doesn't have to be one of those

manager doesn't have to be one of those ones on the list but at least starting

ones on the list but at least starting that conversation maybe with someone who

that conversation maybe with someone who does maybe the you know it's often the

does maybe the you know it's often the you know the the the student in your

you know the the the student in your family perhaps who can advise you on

family perhaps who can advise you on some of these Technologies consider

some of these Technologies consider using a password manager to using

using a password manager to using two-factor authentication whether it's

two-factor authentication whether it's your phone or some key fob or or the

your phone or some key fob or or the like but at least seeking out that

like but at least seeking out that feature at least for accounts that you

feature at least for accounts that you really care about your email social

really care about your email social media Financial medical anything where

media Financial medical anything where you'd be embarrassed at best or really

you'd be embarrassed at best or really violated at worst if that kind of

violated at worst if that kind of information got out and then

information got out and then increasingly using not just encryption

increasingly using not just encryption which you kind of get automatically for

which you kind of get automatically for most Technologies today but increasingly

most Technologies today but increasingly choosing technologies that offer

choosing technologies that offer stronger guarantees that keep those

stronger guarantees that keep those middlemen those companies out of the way

middlemen those companies out of the way if only so that you can trust with

if only so that you can trust with higher probability that only party B

higher probability that only party B knows what party a has said or sent now

knows what party a has said or sent now this of course was a whirlwind tour

this of course was a whirlwind tour there's so much more that you can do

there's so much more that you can do online indeed this course cs50 can be

online indeed this course cs50 can be taken for free online via platforms like

taken for free online via platforms like edex at edx.org cs50 I thought it might

edex at edx.org cs50 I thought it might be appropriate to end on this note if

be appropriate to end on this note if anyone would like to conjecture before

anyone would like to conjecture before we start playing music and adjourn for

we start playing music and adjourn for lunch what our final message here

lunch what our final message here is if we reverse the plus one and maybe

is if we reverse the plus one and maybe start minus one here minus one here and

start minus one here minus one here and indeed thank you so much for coming this

indeed thank you so much for coming this was

was [Applause]

[Applause] [Music]

[Music] cs50

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