CS50P - Lecture 0 - Functions, Variables

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so

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all right this is cs50s introduction to

programming with python my name is david

malin and this is our week on functions

and variables but odds are many of you

most of you have never actually

programmed before so let's start by

doing just that let me go ahead here and

open up my computer and on it a program

called visual studio code or vs code

which is just a very popular program

nowadays for actually writing code now

you don't have to write code using this

particular tool in fact all we need at

the end of the day is a so-called text

editor a program for writing text and

heck if you really want you could even

use something like google docs or

microsoft word you'd have to save it in

the right format but really at the end

of the day all you need is a program for

writing text because that's what code is

text now within this particular program

i'm going to have the ability to create

one or more files via this top portion

of the screen and i'm going to do so by

diving right in and doing this at the

bottom of my screen at the bottom of my

screen is a so-called terminal window

and this is a command line interface or

cli interface to the underlying computer

be it your mac or your pc or even some

server in the cloud and what i'm going

to do here is literally write code and

then the name of the file that i want to

code for instance hello dot pi as we'll

soon see any program that you write in

python generally has a file name that

ends in dot pi to indicate to the

computer that it's indeed a program

written in python now you'll see here at

the top of my screen i have a blinking

cursor a line one which is where the

very first line of my code is going to

go and then just a tab that reminds me

of the name of this file hello.pi and

without even knowing much python i'm

going to write my very first program

here as follows print open parenthesis

quote hello comma world close quote and

close parenthesis and you'll see that at

my keyboard some of my thoughts were

finished for me i only had to type one

parenthesis and the other one

automatically appeared and that's just a

feature that we'll see of tools like

this tool here now even if you've never

programmed before odds are you can guess

infer what this simple program is going

to do and it's only one line print open

parenthesis quote hello world close

quote close parenthesis indeed when i

run this program ultimately it's just

going to say hello to the world and in

fact this is a very famous perhaps the

most canonical program you can write as

your very first program in python or any

other language and so that's what i've

done here

but on my mac my pc even my phone i'm

generally in the habit like you of

running programs by double-clicking an

icon or just tapping on the screen but i

see no such icons here and in fact

that's because my interface to at least

my current mac or pc or some server in

the cloud is again only a cli command

line interface which even though it

might feel like it's a step back from

the menus and buttons and icons that you

and i take for granted every day you'll

find we think that it's ultimately a

much more powerful interface and

incredibly popular to use among

programmers in the real world so to run

this program i'm going to have to use a

command and i'm going to move my cursor

back down to the bottom of the screen

here where previously i already ran one

command the command code which has the

effect of opening vs code in my computer

and then i passed in the name of the

file that i wanted to code up now i have

a chance to type a second command and

you'll see i see a second dollar sign

now the dollar sign here doesn't

indicate any kind of currency or money

it just is the symbol that's generally

used to indicate your prompt where the

command line interface wants you to put

those commands now the command i can run

here is going to be this i'm going to

run python of hello dot pi now why is

that well it turns out that when i

actually write code in a language like

python it's of course stored in that

file hello.pi but i need to interpret

the code top to bottom left to right so

that the computer knows what to do

indeed at the end of the day even if you

don't really know much about computers

you've probably heard that computers

only understand zeros in one the

so-called binary system well if that's

the case then something that says print

and parenthesis and quote-unquote hello

world is not surely zeros and ones we

have to somehow translate it into the

zeros and ones that the computer

understands now fortunately so long as

you've installed such a program in

advance there's a program as well as a

language called python so python is not

only a language in which we're going to

write code it's also a program otherwise

known as an interpreter that you install

for free on your own mac or pc or some

server in the cloud and you can then run

that program that interpreter passing to

it as input the name of your file like

mine here hello.pi and then that program

that interpreter will handle the process

of reading it top to bottom left to

right and translating it effectively

into those zeros and ones that the

computer can understand

so let's do just that let me go back to

vs code here i already typed out python

of hello.pi but i didn't yet hit enter

and that's what's now going to kick off

this command and hopefully if i didn't

mess any of this up i should see my very

first program's output to the screen and

voila hello world so if you two have

typed exactly that same code and have

executed exactly that same command you

will have written your very first

program in this case in python well now

let's take a step back and consider what

is it that we actually just did and what

is it we're looking here on the screen

well first and foremost in most any

programming language you tend to have

access to what are called functions a

function is like an action or a verb

that lets you do something in the

program and generally speaking any

language comes with some predetermined

set of functions some very basic actions

or verbs that the computer will already

know how to do for you that the language

really will know how to do for you and

you the programmer the human can use

those functions at will to get the

computer to do those things now the

program in question here hello.pi

is using one function and you can

perhaps guess what it is that function

is of course going to be this function

print

and that print function of course

doesn't print some pre-ordained string

of text that is to say it prints

whatever it is you want it to print and

here too do we have another piece of

terminology in the world of programming

namely arguments an argument is an input

to a function

that somehow influences its behavior the

people who invented python of course

didn't necessarily know what it is you

and i are going to want to print to the

screen so they designed this print

function using these parentheses with

the ability to take as input some string

of text be it in english or any other

human language that is what you want

this function ultimately to print onto

the screen

and what is it that the program's

ultimately doing on the screen well it's

printing of course it's showing us hello

world on the screen and that's generally

in programming known as a side effect it

can be visual can be audio in this case

it's something that appears on the

screen

and functions therefore can indeed have

these side effects one of the things

they can do as this verb action is to

display on the screen as a side effect

something like those world words that we

wanted

hello world

so that's my first program and you know

i'm feeling pretty good everything

worked as planned i didn't make any

mistakes but honestly when you're

learning how to program and even once

you've learned how to program years

later you're going to make mistakes and

those mistakes of course or refer to a

term you might already know which is

that of a bug a bug is a mistake in a

program and they can take so many forms

and take comfort perhaps in knowing that

over the coming weeks you're going to

make so many mistakes you're going to

have so many bugs in your code just like

i did and just as i still do and those

bugs themselves are just mistakes that

are problems for you to solve and over

the weeks to come we're going to give

you a lot of tools both mental and

technical via which you can solve those

problems but just don't get discouraged

if when writing your program for the

first time it doesn't even work that

first time it will with time with

practice and with experience so let me

deliberately now make a mistake that

there was a non-zero chance i might have

done accidentally already but i got

lucky let me go ahead and just suppose i

forgot to include something like the

closing parenthesis at the end of this

line of code you know the code is almost

correct it's like 99 of the way there

but now that i've pointed it out it's

pretty obvious that it's missing that

closed parenthesis but even little

seemingly minor details like that that

you and i as humans wouldn't really care

about and if you're sending an email or

a text message whatever it's just a typo

it's not that big a deal

it is going to be a big deal to the

computer a computer is going to take you

literally and if you don't finish your

thought in the way the language expects

it's not going to necessarily run at all

so let's do this i'm going to go ahead

here and clear my screen down at the

bottom just so i can start fresh and i'm

going to go ahead and run this version

of my program

after having made that change by

deleting the parenthesis i'm going to go

ahead and type python again of hello.pi

and this time when i hit enter i'm

hoping i'm going to see hello world but

here we have an error on the screen a

so-called syntax error which refers to

my having made a mistake at my keyboard

and it this one fortunately is pretty

straightforward it indeed says that this

open parenthesis was never closed and so

that's probably pretty intuitive now

what i need to do i need of course to

close it unfortunately sometimes the

error messages we'll see in the coming

weeks are not going to be nearly that

user friendly but there too again with

experience with practice will you get

better at debugging such programs let me

now make sure that i indeed fixed it

correctly let me go ahead run now

hello.pi and hit enter and voila

we're back in business well let me pause

here and see if we have any questions

now about python itself writing or

running even the simplest

of these programs

could i write code inside a word or for

example microsoft excel and what's the

barrier to doing that

a really good question and allow me to

very explicitly say to the entire

internet that you should not write code

with microsoft word i mentioned that

only because it's a tool via which you

can write text and code is at the end of

the day just text but it's not the right

tool for the job we don't need bold

facing underlining paragraphs and the

like we generally want something much

simpler than microsoft word or google

docs and so vs code is an example of

just a more general purpose text editor

its purpose in life is to allow you the

human to edit text nowadays these text

editors come with many more features in

fact you'll notice that even in my code

here even though it's just one line

there's a bit of color to it the word

print for me is appearing in blue the

parentheses are black and we'll see as

we might write more lines of code more

and more of the lines will come to life

in various colors now that's just one

feature of a text editor will see too

that it has features like this built-in

terminal window it's going to have a

built-in tool for debugging or finding

problems with code and it's just a very

popular tool nowadays but there are many

many others out there you're welcome to

use them for this course and beyond we

just happen to use this one in large

part two because you can also use vs

code nowadays for free in the cloud how

about one other question here on

programming with python or hello world

or syntax more generally that will spend

to us if

it's not possible to run the computer

using the terminal window

i think i heard is it not if it's

possible to run the program without the

terminal window

are you full

okay you froze for me again but let me

infer what the question is so in this

environment as i've configured my

computer i can only run these python

programs via the terminal window now

that's good for me the programmer or the

person who's trying to learn how to

program but it's not very good if you

want to ship this software and have

other people use your actual code you

can absolutely write programs and then

allow other people to use not a command

line interface but a graphical user

interface or gui gui this is just one

mechanism and perhaps i think the the

best one with which to start writing

code because eventually it's going to

give us a lot more control

allow me to forge ahead here but please

feel free to continue asking questions

along the way if only via the chat let's

consider now how we might go about

improving this program let's go about

improving this program to make it a

little more interactive and not just

assume that everyone is going to want to

be greeted more generically as hello

world let's see if i can't get this

program to say something like hello

david or hello jeremiah or hello horatio

or whatever the actual user's name is

well to do this i'm going to go back up

to hello.pi and i'm going to add another

line of code at the very top that simply

says for instance

what's your name quote unquote with an

extra space at the end so i'm printing

to the user asking them a question for

some input but now i need another

function to actually get input from the

user and perfectly enough python comes

with a function named input so here i'm

going to go ahead and call a function

input open paren close paren and that's

going to prompt the user with just a

blinking cursor waiting for them to type

something in now it turns out if i read

the documentation for the input function

it actually takes an argument itself i

don't need to use print separately and

then prompt the user for input so i can

actually simplify this code before we

even use it i'm going to go ahead here

and take that same string from print put

it as an argument to the input function

and get rid of the print altogether and

in fact that print would have added a

new line anyway so now i've just got a

prompt where the user's cursor is going

to end up blinking at the end of the

line asking them what's your name

in my terminal window i'm going to run

python of hello.pi

enter ok we're making progress it seems

that this new function input is indeed

prompting me the human for input so i'm

going to type in my name david and hit

enter unfortunately it doesn't really do

anything with my name it just outputs it

immediately all right well i could fix

this right i could go up to line two and

i could change world to david and then

back in my terminal window here i can do

python of hello.pi enter what's your

name david enter and there we go all

right now i'm up and running now my

program is working as intended of course

this isn't really working as intended

here let me go ahead and try pretending

to be my colleague carter here well

carter's name is this i'm going to go

ahead and hit enter and i'll see of

course hello carter well obviously not

because i've hard coded so to speak i've

written literally my name inside of the

string so we need some way now of

actually getting back

what the user's input is and doing

something with it ultimately and for

this we're going to leverage another

feature of programming specifically a

feature of some functions which is that

they can have return values as well if

you think of input as being again this

action this verb you can actually

personify it as maybe a person like a

friend of yours that you've asked a

question of and you've asked your friend

to go get input from someone else go ask

that person their name and if your

friend comes back knowing that person's

name well wouldn't it be nice if they

handed that name back to you that's kind

of what we need metaphorically the

function to do is get the user's input

and then hand it back to me so that i

the programmer can do something with it

but if it's going to be handed back to

me i kind of want to put it somewhere so

that i can then print it back on the

screen i need to do the equivalent of

take out like a piece of paper or a

post-it note write down on this piece of

paper what it is the human has said so

that i can then feed it into as input

that print function and to do that we're

going to need one more feature of

programming namely variables and odds

are most everyone's familiar with

variables for math class way back when x

and y and z and the like well

programming has that same capability

this ability to create a variable in

this case in the computer's memory not

just on a piece of paper and that

variable can store a value a number some

text even an image or video or more a

variable is just a container for some

variable

a variable is just a container for some

value inside of a computer or inside of

your own program so how do i go about

expressing myself in this way well i

think what i'm going to do is introduce

a variable that's a little more

interestingly named than x or y i could

just say this x equals input but i'm

going to use a better name than a

typical mathematical variable here and

i'm going to literally call my variable

name why well in programming because i

have a whole keyboard in front of me i

can use more descriptive terms to

describe what it is i'm writing and now

though there's an opportunity to

consider a specific piece of syntax

we've seen parentheses we've seen quotes

all of which are necessary when passing

inputs to a function but this equal sign

here that's in between input on the

right and name on the left is actually

important and it's technically not an

equal sign per se it doesn't mean

equality as much as it means assignment

so in python and many programming

languages a single equal sign is the

assignment operator and what that means

specifically is that you want to assign

from right to left whatever the user's

input is so the equal sign copies from

the right to the left whatever the

return value of the function on the

right is so again the input function

clearly gets input from the user that's

why i was able to type my name or

carters but it also sort of behind the

scenes hands that value that return

value back to me the programmer and if i

use an equal sign and a variable no

matter what i call it i can store that

input in that variable so as to reuse it

later so now sitting in the computer's

memory somewhere is a container

containing david quote-unquote or carter

quote-unquote or whatever the human has

typed in

but here it's easy to make a mistake

suppose i

decide to try to print that name and so

i i kind of on a hunch type in this

hello comma name just kind of plugging

in the name of the variable well let me

go ahead here and run python of hello.pi

and hit enter that's going to prompt me

for my name and let me type in my name

david but i haven't hit enter yet and

perhaps via the chat what's going to

happen here when i now hit enter

i'm hoping it says hello david i'd be

okay if it says hello world but i don't

want it to say what it's actually gonna

say and yep what we're seeing in the

chat is well it's probably gonna say

literally hello comma name so that's not

quite right so we need another way of

printing out the value inside of that

variable rather than just this word name

well let me try this in a couple of

different ways let me try this as

follows let me go ahead and maybe undo

this because i've gotten pretty good

already at saying hello so let's let you

know let's draw that line in the sand

and just say all right let's at least

get hello comma out the door let's now

print name and just on a hunch i'm going

to try this i'm going to use print again

because you can use these functions as

many times as you need and i'm going to

pass to the name to the print function

the variable called name but notice i'm

being a little clever now i'm not

putting it in double quotes because

we've seen already that double quotes

means literally print out n-a-m-e i'm

getting rid of the quotes this time in

hopes that

now by passing the variable called name

to the function called print it will in

fact go about printing the contents of

that variable that is its so-called

value all right let's go ahead and do

this here python of hello.pi enter

what's your name david and now crossing

my finger still i see hello comma david

all right so it's not the best program

i'm kind of cutting some corners here so

to speak i'm saying hello david on two

separate lines so it's not as elegant

it's not as pretty it's not as

grammatically appropriate in english as

just saying it all in one breath on one

line but at least i've solved the

problem just not very well yet but let

me take a step back now and perhaps

introduce a couple of other concepts

with which we should be familiar which

is as our programs get longer and

they're no longer just one line or two

or even three eventually our programs

are going to become dozens of lines

maybe even hundreds of lines long let's

set the stage for success moving forward

it turns out that python and a lot of

programming languages also support

something called comments comments are

notes to yourself in your code and you

include comments by way of a special

symbol in python it's going to be the

hash symbol typically and that allows

you to write the equivalent of a note to

yourself but in a way that's not going

to break your code the computer actually

ignores your comments it's just there

for you it's just there for your teacher

it's just there for your colleague with

whom you're sharing ultimately that code

so if i go back to vs code here and i

just want to add some comments to this

program to explain to my teacher to

myself to my colleagues what this

program is doing well let's go ahead and

do that i'm going to go at the very top

of my program and on line one now i'm

going to move that original line of code

down a bit i'm going to add a hash and

i'm going to say something like this

ask user for their name now i don't have

to use that language i don't have to use

that that text i could use any human

language whatsoever it doesn't have to

be english but i'm going to now below

that just say something like this say

hello to user and you'll notice that vs

code by default is kind of graying out

my comments they're no longer blue

there's no red there's no color in them

and that's just because they're notes to

myself and the computer ultimately is

going to ignore them but what we have

now is two comments ask user for their

name and then a second comment say hello

to user and i've just kind of commented

each chunk of code like each line or

line's plural of code that are doing

something noteworthy why well tomorrow

morning when i wake up having you know

uh slept for quite some time forgotten

what it is i did the previous day it's

convenient with comments to just see in

english or your own human language what

it is this program is doing so that you

don't have to read the code itself and

better yet if there's maybe a mistake

down the road you can read what your

intention was and then you can look at

the code and figure out if your code's

now doing what you intended so this

isn't really necessary for a program

this small it's pretty obvious with just

one or two or three lines what the

program's doing it's just as fast to

read the code than the comments but

getting into this habit is generally a

good thing to comment your code every

one or few lines so as to remind

yourself and others what it is your

intent and your code is doing what's

nice about comments too is this comments

can also serve to be sort of a to-do

list for yourself there's this notion in

uh programming of pseudocode pseudocode

isn't a formal thing it's not one

specific language it's just using

english or your own human language to

express your thoughts succinctly

methodically algorithmically so to speak

but pseudocode therefore because it's

not python and it's not necessarily

english it just kind of allows you to

outline your program even in advance so

for instance if i wasn't sure today

how i wanted to go about writing this

program but i did know what i want to do

i could have started today by just

writing this in hello.pi no code i could

have written just a couple of comments

to myself step one ask user for their

name step two say hello to user then

once i've outlined my program in

pseudocode then i can go in there and

say all right how do i ask the user for

their name well i can do input quote

unquote what's your name question mark

and then on the left here i can maybe

put a variable and assign it to that

okay how do i say hello to the user well

i know i can use print to say things on

the screen let me say hello comma

and let me okay let me now print the

person's name so again pseudocode is a

nice way of structuring your to-do list

especially if you have no idea how to

write the code because it breaks a

bigger program down into small

bite-size tasks all right let me pause

here to see if there are now any

questions on comments pseudo code

return values

or variables

any questions we can clear up here

yeah my question is does the function

input work for any type of information

or only for words yeah really good

question so according to its

documentation and we'll look more at

formal documentation soon input is going

to expect what's called a string that is

a sequence of text be it in english or

any other human language but it's indeed

going to be expecting text with which to

prompt the user

a good question how about another

question from the group if we could

i wanted to ask how i make a several m

comments

oh how do you do many lines of comments

if i'm hearing you correctly sure uh you

would just

keep doing them like this you just

prefix each of the lines with a hash

symbol like i'm doing here there is

another technique for doing multi-line

comments in python that actually tend to

have special meaning you can do three

double quotes like this and then

anything in between here is a comment

that's another technique or you can use

single quotes as well but more on those

i think another time all right well if

you don't mind let me forge ahead here

and see how we might improve this

program further and also introduce a few

other features that we might want to

take into account over time so it turns

out that we can certainly improve on

this program because it's a little

disappointing that i'm cutting this

corner and saying hello comma and then

on a new line printing out name like we

can do better and most programs you use

on your phone or your laptop certainly

keep text together when people want so

how can we go about doing that well

there's a few different ways and in fact

the goal here is not so much to solve

this one problem but to demonstrate and

emphasize that in programming python and

other languages there's so many ways

sometimes to solve the same problem and

here's one way to solve this problem let

me go in here and let me go ahead now

and say hello comma

and let me just add to the end of that

the user's name so i'm using plus in

kind of an interesting way this is not

addition per se i'm not adding numbers

obviously but i do kind of want to add

the person's name to the string of text

hello comma well let me go now down to

my terminal window and run python if

hello.pi again enter what's your name

i'm going to type in david enter

okay it's better

it's better but there's a minor bug

albeit aesthetic here there's missing

space but let's just use some intuition

here well if i'm missing the space after

the comma why don't i go ahead and just

add it manually here let me now rerun

the program python of hello.pi enter

david enter and there we go now we have

something that looks a little prettier

in terms of english grammar hello comma

space david and now if we rewind you

might have noticed before or wondered

why i had this seemingly extra space

after my question mark namely here

there's a space after the question mark

before the double quote and that was

just for aesthetics too i wanted to move

the user's cursor one space to the right

so that when i type their name or they

type their name it's not immediately

next to that same question mark there

but there's other ways we can do this it

turns out that some functions print

among them actually take multiple

arguments and it turns out that if you

separate the inputs to a function the

so-called arguments to a function with a

comma you can pass in not just one but

two three four five onward so let me go

ahead and pass in not just hello comma

space but that

followed by name and this is a little

confusing potentially at first glance

because now i've got two commas but it's

important to note that the first comma

is inside of my quotation marks which is

simply an english grammatical thing the

second comma here is outside of the

quote but between what are now two

separate arguments to print the first

argument is hello comma space the second

argument is the name variable itself so

let's see how this looks

python of hello dot pi enter what's your

name david enter

okay i've kind of over corrected now

i've got two spaces for some reason well

it turns out and this is subtle when you

pass multiple arguments to print it

automatically inserts a space for you

this was not relevant earlier because i

was passing in one big argument to

print all at once by using that plus

operator this time i'm passing in 2

because of the comma so if i don't want

that extra space i don't need to pass in

one myself i can just do this and now

notice if i run this program again

python if hello.pi

type in my name david now it looks

grammatically like i might want now

which of these approaches is better this

approach uses a function print with two

arguments hello comma

and the name variable the previous

version recall technically used one

argument even though it looked a little

curious it's one argument in the sense

that the computer just like

mathematicians are going to do what's

inside of parentheses first so if inside

of parentheses you have this string of

text hello comma and a space which i

need to add back then you have a plus

which means not addition per se but

concatenation to join the thing on the

left and the thing on the right

this ultimately becomes the english

phrase hello comma space david

and then what's being passed ultimately

to the function is technically something

like this but it's doing it all

dynamically it's not me typing in david

as i i secretly as i discretely did

earlier it's figuring out dynamically

what that value is after concatenating

hello with the value of name and then

passing that ultimately to print as the

soul argument

let me pause here to see if there's any

questions on

numbers of arguments now

to functions

can we use a function

many times to solve a certain problem

which we can encounter many times in our

code

you can you can use a function many

different times to solve some problem

what we'll soon see though is if you

find yourself as the programmer solving

a problem the same way again and again

and again it turns out you'll be able to

make your own function so that you don't

have to keep reusing the basic ones that

come with the language

i was curious about the comma and

the plus sign so after plus sign can we

give just one variable and after form

again we give multiple variables like

what is the difference a good question

so in the context of strings and i keep

using that term string is a technical

term in a programming language and again

it means a sequence of text a character

a word a whole paragraph even so the

plus operator is not just used as we'll

see for addition of numbers in python

like we do on paper pencil but it also

is used for concatenation of strings on

the left and the right if you did want

to combine not just two strings left and

right but a third and a fourth you can

absolutely keep using plus plus plus

plus and chain them together just like

in math eventually that's going to start

to look a little ugly i dare say

especially if your line of code gets

long so there's better ways that we'll

actually soon see and a good question as

well well let me come back to the code

here in question and see if we can show

you just a couple of other ways to solve

the same problem along the way

emphasizing that what we're technically

talking about here yes are strings but

there's even a technical term for these

strings in python it's just stir so to

speak str for short for string as you

may know if you've programmed in other

languages people who invent programming

languages like to be very

succinct to the point so we tend to use

fairly short phrases to describe things

not necessarily full words so while you

might say string technically in python

what we're really talking about these

sequences of text are technically

stirrers this is an actual type of data

in a program but we'll soon see that

there's other types of data in programs

as well in fact let's see if we can't

improve this in one other way i like the

progress we've made by keeping

everything on the same line hello comma

david

all on the same line what more though

could we do in terms of solving this

problem well it turns out that we didn't

have to give up entirely with using

print twice let me rewind a little bit

and go back to that earlier version

where i wasn't really sure how to solve

this problem so i was using print once

to print out just the hello in the space

and the comma and then i use print again

to call to print name that strictly

speaking wasn't bad but there was this

visual side effect that i just didn't

like it just looked ugly to have these

two lines of text separate from one

another but there's another way to fix

this clearly it seems to be the case

that the print function is automatically

outputting a blank line it's moving the

cursor automatically for me to the next

line because that's why i'm seeing hello

on one line and david on the next and

then my prompt the dollar sign on the

line below that so print seems to be

presuming automatically that you want it

to move the cursor to the next line

after you pass it some argument but you

can override that behavior again

functions take arguments which influence

their behavior you just have to know

what those arguments are and it turns

out

that if we look at the documentation for

python's print function we can actually

look up at this url here docs.python.org

is where all of python's official

documentation lies if i poke around i

can find my way to more specifically

this url here where i can find all of

the available functions in python

that and the documentation therefore and

if i go a little more precisely i can

even find specific documentation for the

print function itself and rather than

pull that up in a browser i'm going to

go ahead and highlight just one line

from that same url

which is this and this is easily the

most cryptic thing we've seen yet but

this is the official documentation for

the print function and one of the best

things you can do when learning a

programming language is honestly learn

to read the documentation because truly

all of the answers to your questions

will in some way be there even though

admittedly it's not always obvious and i

will say too python's documentation

isn't necessarily the easiest thing

especially for a first time or novice

programmer it too just takes practice so

try not to get overwhelmed if you're not

sure what you're looking at but let me

walk you through this example this again

is a line of text from python's official

documentation for the print function

what this indicates as follows is this

the name of this function is of course

print then there's a parenthesis over

here and another closed parenthesis way

over there everything inside of those

parentheses are the arguments the

potential arguments to the function

however when we're looking at these

arguments in the documentation like this

there's technically a different term

that we would use these are technically

the parameters to the function so when

you're talking about what you can pass

to a function and what those inputs are

called those are parameters when you

actually use the function and pass in

values inside of those parentheses those

inputs those values are arguments so

we're talking about the exact same thing

parameters and arguments are effectively

the same thing but the terms you use

from looking at the problem from

different directions when we're looking

at what the function can take versus

what you're actually passing into the

function so what does this imply well

this syntax is pretty cryptic

but at the moment just know that an

asterisk a star and then the word

objects means that the print function

can take any number of objects you can

pass in zero strings of text one string

like i did two strings like i did or

technically infinitely many if you you

really want though that code's not going

to look very good

after that we see a comma then we see

another parameter here called sep short

for separator in english and notice the

equal sign and the single quote space

single quote so quote unquote space i

don't know what that is yet but i i

think we've seen a hint about it let's

focus though for a moment on this the

print function takes another parameter

called end and the default value of that

parameter is apparently based on this

equal sign and these quotes backslash n

and what is backslash n if you'd like to

chime in the chat anyone who's

programmed before has probably seen this

though if you've never programmed before

this might look quite cryptic

backslash n

means new line and it's a way textually

of indicating if and when you want the

computer effectively to move the cursor

to the next line create a new line of

text and so technically if we read into

the documentation we'll see more detail

on this

the fact that there's a parameter called

end and the documentation for the print

function just means that by default this

print function is going to end every

line with backslash n you don't

literally see backslash n you see a new

line you see the cursor moving to the

next line now by that logic let's move

backwards sep for separator the default

value of separator is apparently a

single blank space well where have we

seen that well recall in an earlier

example when i passed in not just one

but two arguments to the print function

recall that they magically had a space

between them in fact they had that space

plus my own space and that's why i

deleted my space because at that point

it was extra so this just means that

when you pass multiple arguments to

print by default they're going to be

separated by a single space by default

when you pass arguments to print it's

the whole thing is going to be ended

with a new line now just by knowing this

and let me literally wave my hand at the

rest of the documentation for another

day there's more things that print can

do but we're going to focus just on sep

and on end

let's see if we can't leverage this now

to solve that original problem the

original problem was this i don't like

how hello comma david is on two

different lines well that's happening

again because print is automatically

printing out a new line so let's tell it

not to do that let's tell it by passing

a second argument to the first use of

print

to say end equals quote unquote not

backslash n which is the default

automatically let's make it quote

unquote nothing else

let's override the default value so

there is no new line there's literally

nothing there and let's see what happens

let me now go down to my terminal window

and clear it and i'm going to run python

of hello.pi enter i'm going to type in

my name david and i think now

everything's going to stay on the same

line because and it did

this line here 5 is going to print out

hello comma space but then nothing at

the end of it because i changed it to be

quote unquote the second line is going

to print the name david or whatever the

human's name is and it will move the

cursor to the next line because i didn't

override the value of end there just to

see this more explicitly if you do

something cryptic like well i have no

idea what's going on let me just put in

temporarily three question marks here

we'll see the results of this too let me

go back down to my terminal window run

python of hello.pi what's your name

david and now

you see literally really ugly output but

you see literally what's going on hello

comma space

then three question marks end that print

statement and then you see dav id so not

a good outcome but it demonstrates just

how much control we have here too and

let me rewind further recall that in our

other version of this

when i passed in hello comma and name

they were separated by a single space so

python of hello.pi

david enter that just worked well what

if we override the value of sep for

separator instead of being one space we

could say something like uh question

mark question mark question mark just to

wrap our minds around what's going on

there

let me now do python of hello.pi david

enter and you see two these two inputs

hello comma and the name are now

separated in an ugly way by three

question marks because i've overridden

the default behavior of sep and even

though the documentation uses single

quotes i've been in the habit of using

double quotes in python you can use

either strictly speaking it doesn't

matter but you should be consistent and

i generally always use double quotes

python's documentation though always

uses single quotes

questions now on these types of

parameters and allow me to propose that

we give these an official name up until

now when we've been passing values to

print those are called positional

parameters positional in the sense that

the first thing you pass to print gets

printed first the second thing you pass

to print after a comma gets printed

second and so forth but there's also

these things we've now seen called named

parameters named scp separator or end

end for the line ending those are named

parameters because one they're optional

and you can pass them in at the end of

your print statement but you can also

call use them by name

this may be a weird question but i was

wondering uh what if someone wants to

like add actually

quote quotation marks within the

quotation marks yeah i like how you

think this is what we would call a

corner case right just when we've made

right this is this is all sounding great

at least as programming goes but wait a

minute what if you want to print a quote

that's a really good question well let's

see if we can't figure this out suppose

that i want to print out

not just the user's name let me simplify

this further let me go ahead and get rid

of a lot of this and let me just say

something like hello

um

maybe i'm being a little sarcastic here

hello

friend

you know in that kind of tone well this

is not going to work actually because

you were trying to use quotes to be like

friend in finger quotes but you're also

trying to end the sentence and if i try

running this let's do this python if

hello dot pi you'll see that this is

just invalid syntax perhaps you forgot a

comma and this is actually a bit

annoying sometimes the error messages

you see are misleading like the computer

the language doesn't really know what's

going on so it gives its best guess but

it's not necessarily correct but i can

solve this problem in a couple of ways i

can do this i can change my outermost

quotes to single quotes because recall a

moment again i said you could use double

quotes or single quotes so long as

you're consistent so that's fine if you

use single quotes on the outside you can

then use double quotes on the inside and

you'll see them literally so for

instance if i run python if hello.pi

there we go hello friend

but there's another way if you insist on

using double quotes as you might want to

just to be consistent you can also use

that backslash character again we saw

the backslash n a moment ago and that

meant we don't want a literal n to be in

the output we wanted a new line so the

backslash actually represents what's

called an escape character an escape

character is one that you can't just

type necessarily once on your keyboard

you need to express it with multiple

characters so i can actually put

backslashes in front of these inner

double quotes so that the computer

realizes oh wait a minute those aren't

literal those aren't quotes that finish

or start the thought they're literal

quotes so now let me go back to my

terminal window run python of hello.pi

enter and now it's working as well so

escaping is a general technique that

allows us to do that too

and if i may let me rewind now on these

examples and go back to where we left

off with my code i'm just undoing all of

that because i want to get back to the

point ultimately of specifying now a

final way of solving this problem well

it turns out that we have yet another

way we can solve this problem which is

perhaps the most frequently done now or

at least the most elegant when it comes

to setting us up for longer and longer

uses of strings

you can use a relatively new feature of

python that allows you to do this you

can literally put not the name of the

variable like that in your string

because we already saw this is wrong

right if you do this you will literally

see hello comma name but what if i do

this what if i put curly braces or curly

brackets around the variable's name

notice vs code is actually very subtly

changing the color of it so vs code

knows something interesting is going on

here let me run this program but i'm not

done yet python of hello.pi enter david

enter okay obviously not what i want but

i need to tell python that this is a

special string this is what we're going

to call a format string or an f string a

relatively new feature of python in the

past few years that tells python to

actually format stuff in the string in a

special way and the symbol via which you

do this is a little weird but this is

what the world shows if you put a f

at the beginning of the string

right before the first quote mark that's

a clue to python that oh this is a

special string let me format this in a

special way for you let me now rerun the

program pythonhello.pi enter david enter

and now we see the goal this whole time

hello comma david we don't start with

this way because i think if we did this

the first way you'd be like why are we

doing this what are all these magical

symbols but this is just yet another way

to solve the same problem but let me

propose that we consider now

yet other things we can do with strings

and it turns out that even as we've been

doing

some relatively simple operations here

we've generally been trusting that the

user's going to cooperate and that is to

say that they're going to actually type

in what we want them to type now just

because they type a string though

doesn't mean it's going to look the way

we want you and i honestly as humans are

actually in the habit on websites and

apps of like accidentally hitting the

space bar a lot either at the beginning

of our input or at the end maybe because

the space bar tends to be so big it's

pretty common to get accidental spaces

before or after some user's input you

and i are definitely in the habit of not

necessarily capitalizing words like we

should if we're sending text messages

we're probably being a little quick and

just sending everything in lower case

for instance if that's your style if

your phone's not fixing it for you maybe

in a formal letter you would capitalize

things properly but you and i as humans

can't really be trusted to type things

in a nice way necessarily when using

some piece of software be it an app or

website or something else but it turns

out that strings themselves come with a

lot of built-in functionality and you

can see all of that in python's own

documentation here the string

data type that we've been talking about

comes with a lot of functionality built

in that means that we can manipulate the

user's input to do more than just join

it with something else like hello we can

actually clean it up or reformat it in a

way that hopefully looks a little better

for us so let me go back to my code here

and let me just demonstrate what might

happen if the user doesn't cooperate if

i go ahead here and run python of

hello.pi enter let me just sloppily hit

the spacebar a few too many times why i

just wasn't paying attention and i'm

going to type in my name david and i

don't know i hit the spacebar a couple

more times like it's kind of a mess it's

all lowercase that's not going to

necessarily look grammatically right

it's got spaces here and here the

program is going to print exactly that

and that looks really bad at least if

we're prioritizing aesthetics and

grammar like why are there so many

spaces after the comma this is not a

very nice way to greet your users but we

can clean this up

it turns out that built into strings

which again is this data type so to

speak this type of data in python is the

ability to actually do things to that

string so let me do this i can actually

go ahead and do something like this

uh

name equals name dot strip and what does

this do

remove

white space from string

and what do i mean by this well on the

right hand side notice i've written the

variable name called name

i've then used a period or a dot and

then i seem to be doing what's a

function right anytime we've seen this

function thus far we see it's the the

function's name print or input then we

see a parenthesis then another

parenthesis and that's exactly what i

see here but i'm using this function a

little differently technically this

function is in this context called a

method and what do i mean by that well

if name is a string aka stir well it

turns out according to the documentation

there's a lot of functions that come

with strings in python and you can

access that functionality by using the

name of a string like literally name

here then a period then the name of the

function and then an open parenthesis

and a closed parenthesis maybe some

arguments inside of those parentheses

but in this case it doesn't need any

arguments i just want to strip the space

from the left and the space from the

right of the user's input but that's not

enough i want to remember that i've

stripped off that white space on the

left and the right so i'm going to use

the equal sign again here and notice

that just as before this doesn't mean

equality this means assignment from

right to left so when this line of code

here name.strip

returns to me

aka a return value it will return the

same thing that the user typed in but

with no more white space to the left or

to the white

to the right so then the equal sign

assignment is going to copy that value

from the right to the left thereby

updating the value inside of my name

variable so you can not only assign

values to variables you can absolutely

change the value of variables by just

using the assignment operator the equal

sign again and again and again and it

will just keep copying from right to

left whatever the new value should be so

now if i rerun this program python of

hello.pi

enter

i have david let's do it again space

spacebase space space dav id and all

lowercase space space enter

it's better it hasn't fixed my

capitalization so i'm still being a

little sloppy with the first d but it

has stripped off all of that extra space

super minor detail right like this isn't

all that exciting but it just speaks to

the power of what you can do with just a

single line of code now what else can i

do here well i could capitalize the

user's input let me go ahead and try

this it turns out that i could also

do this name dot

capitalize so let me go ahead and

capitalize uh user's name and again i'm

making comments and there's no one right

way to write the comments i'm just using

some short english phrases here to

remind myself of what i'm doing

what's now going on here well let me go

ahead and run python if hello.pi enter

space spacebase spacebase david space

space enter

okay now it's looking prettier right no

matter how the user typed in their name

even a little sloppily i'm now fixing

that but let's let's try something i'm

getting a little curious here how about

this uh spacebase face-to-face space

david spacemailin i'll use my last name

now enter

okay so ironically capitalize is not

really capitalizing everything we want

it's clearly capitalizing what just the

very first letter so it turns out that

again there's other functions in python

that come with strings and if we poke

around the documentation scrolling

through a url like that i bet we'll find

another solution one of which is

actually this

let's actually change this to title

there's yet another function that come

with strings called title that do title

based capitalization just like a book or

a person's name capitalizing the first

letter of each word and this is just

going to do a little more work for us so

let's go ahead and run this and as an

aside i'm kind of tired now at this

point of typing python python python all

the time it turns out that when using a

command line interface like this you can

actually go back through all of your old

commands what i just did a moment ago is

i hit the up arrow that immediately goes

back through my history of all of the

commands i've ever typed so this is just

a faster way now for rep repeat myself

than typing everything manually let me

go ahead and hit enter

space based basically space dav id

mailing space space all lower case enter

now it's it's looking better now i've

capitalized things and cleaned things up

but what about my code i've got like

eight lines of code now four of which

are comments four of which are actual

code do i really need this much well not

necessarily watch what i can also do in

python let me not bother capitalizing

the user's name separately let me say

this and capitalize

capitalize user's name i can chain these

functions together i can add title to

the end of this and now what's happening

well again with a line of code like this

you first focus on what's to the right

of the equal sign then we'll get to the

left of the equal sign what's on the

right of the equal sign this line here

well what does this mean get the value

of the name variable like david space

m-a-l-a-n

then strip off the white space on the

left and the right that is going to

return a value it's going to return

david space m-a-l-a-n without any white

space to the left or right what do you

want to do with that return value you

want python to title case it that is go

through every word in that resulting

string and fix the first letter of the

first word the first letter of the

second word and so forth and then now we

can finish our thought copy the whole

thing

from right to left into that same name

variable and you know what i can take

this even one step further why don't we

go ahead and do this if we want

let me get rid of all that and let me

just do

strip and title all on that first line

and now we've gone from like eight lines

of code to four

it's a lot tighter it's a lot neater and

even though reasonable people might

disagree it's arguably better

because it's just easier to read fewer

lines of code fewer opportunities for

mistakes it just allows me to move on

with my next

problem to solve

all right let me pause here and see if

there's any questions on these methods a

method is a function that's built in to

a type of value like these functions are

or on f strings which we saw a moment

ago

yes hi thanks david um so is there a way

to remove the spaces between

the spaces that i might have added a

short answer no if you read the

documentation at that same url earlier

you'll see that strip removes from the

left and the right but not in between in

fact there's two other functions that

come with strings one's called l-strip

the other is called r strip that allow

you to do one or the other if we want to

start getting rid of space in the middle

we're gonna have to do a different trick

all together

how many

functions can we combine like this dot

strip dot title you have combined so how

many we can combine yeah a really good

question

technically as many as you want but at

some point your code is going to start

to look really really bad right because

the line of code is going to get really

really long it's eventually going to

maybe wrap around again and again so at

some point you just kind of say like uh

that's too many and you start breaking

it up into multiple lines like i did

maybe reassigning the value to the

variable as needed and this is actually

a good question if i can pivot

off your question i mean what do people

think if we could go ahead and put

everyone's hands down for a moment

let me ask this

is the way i've done this now with strip

and title and input all in the same line

better than my previous approach in zoom

you can use the yes icon or the no icon

if you think this version is better say

yes

if you think this previous version was

better

for instance this one here where we had

everything broken out say no

and then we'll see why in just a moment

i proposed earlier that reasonable

people

can disagree and that's absolutely the

case

doing it one way or the other isn't

necessarily best at least if you can

justify it let me go back to the most

recent version here

all right so we're seeing a lot of yeses

and a lot of no's why don't we go ahead

and call on one of the yeses if we could

someone who's voting yes why do you

think the current version of this code

is indeed better than the previous

longer version of the code

i think it's more readable so i can say

hey this is the name from this is the

name variable

it gets some input and then remove the

space and give it a title and there you

go you have a hello name yeah i think

that's pretty reasonable it's very

readable at least if you're in the habit

as you are in english of reading left to

right it just kind of flows very

naturally as a result the lines is not

really that long it's certainly fitting

nicely onto the screen so i think that's

a good argument how about a counterpoint

though someone who voted no if we could

call on someone who thinks this is worse

because it's not

reliable at all

i seems like uh

a

it's a very long

line so i think it's better to separate

yeah i i think that's persuasive too

right it's getting a little longer and

even though my sentence here what's your

name is relatively short you could

imagine that this could get even uglier

quickly if i were asking a longer

question of the user that's going to

make this line of code even longer and

therefore less readable it might be less

obvious to me or my colleagues that i am

calling strip or that i am calling title

it might be kind of a unexpected

surprise so i think that's reasonable

too in short there is no right answer

here and in fact part of the process of

getting better at programming is getting

your own sense of style or working for a

company where they might prescribe which

way is better than the other because

they just want everyone doing the same

thing even though reasonable people

might uh disagree ultimately though so

long as you have what's a pretty good

argument in favor of one way or the

other like ultimately that's what's

important if you're just doing things

because you don't really know which one

is better that's not great but if if and

when you start to acquire opinions and

if your boss if your teacher if your

colleague your friend can challenge you

and say wait why did you do it like this

they might not agree with you but at

least have an answer and that should be

sufficiently persuasive in general now

strings come with a whole bunch of other

methods as well among which is one

called split which can as the name

suggests split a string into multiple

smaller sub strings so to speak for

instance if the human here is in the

habit of typing in their first name then

a space and then their last name and you

want to go ahead and greet them only by

first name well we could actually

leverage that single space between the

first name and last name and split that

string into two smaller sub strings how

can we do this well let me go ahead and

in between these lines proactively

comment that we're about to split user's

name

into first name and last name and then

let's go ahead and take that name

variable which currently contains

something like presumably david

spacemailin and let me go ahead and call

split and pass in as the argument to

split a single white space thereby

indicating that i indeed want to split

on that character now it turns out

split's going to return a sequence of

values ideally a first name and then a

last name and we can actually in python

assign both of those values from that

sequence at once to some variables for

instance first comma last equals and

that's going to have the effect from

right to left of putting the first such

value in the first variable the second

such value in the second variable so now

on my last line of code i can go in and

say hello not to the full name something

like david malen i can just say hello

comma first all right let's go ahead and

clear my terminal window run python of

hello dot pi and hit enter i won't

bother with any leading white space this

time but let me go ahead and type in

david space malin and crossing my

fingers as usual

hello david is what we now see

all right so we've seen so much so many

examples thus far involving strings but

certainly programs and programming

languages can manipulate other types of

data as well let's go ahead and

transition then to another very common

type of data in python in programming

more generally namely integers otherwise

known in python is int int so just as

stir str is short for string so is int

in python short for integer well what's

an integer well just like in math it's a

number like negative 2 negative 1 0 1 2

and all the way toward negative infinity

all the way toward positive infinity but

there's no decimal point in an integer

it's just a number like negative two

negative one zero one and two onward

that's an int of course in the world of

mathematics there's lots of symbols that

we use and we've seen plus before

although we used it for a different

purpose but python supports these

symbols and more and python allows you

to add numbers together plus subtract

numbers uh multiply numbers divide

numbers and the only one here that might

look a little strange to people or

unfamiliar is this percent sign but it

doesn't mean percent in this context if

you use a single percent sign in a

python program that's actually the

so-called modulo operator the operator

that allows you to take the remainder

after dividing one number by another so

we'll see examples of that before long

but the first four of these are perhaps

quite quite familiar well it turns out

that in python you cannot necessarily

you don't necessarily have to keep

writing code in a file like hello.pi and

then running it in a terminal window one

of the features that many people like

about python is that it supports this

so-called interactive mode like you can

start writing python code and

immediately

execute each of those lines

interactively if especially if you don't

care about saving all of your lines of

code you just want to execute code and

get back some answers so for instance

let me go back to vs code here and let

me close hello.pi and let me click on

the little triangle over here in my

terminal window just to make it much

bigger just temporarily for a moment so

i'm not creating any dot pi file now i'm

just going to run python by itself at my

prompt and you'll see when i do this i

get some cryptic looking output and the

date and time at which the program was

last updated and so forth but i

ultimately get three

triple

uh brackets like this this is the

interactive mode for python so i'm

running the python interpreter and

anytime i type a comma a line of code in

the interpreter it's going to execute it

immediately i don't have to keep running

python again and again it's as though in

the human world if you were standing

next to a human who speaks some other

language and you're just having a

conversation with them back and forth

it's all happening the translation

immediately so what might i do in

interactive mode well i could do

something like one plus one enter that's

actually code right you might not think

of it as code but if you know a bit of

arithmetic and you know

numbers and you know plus that's valid

python code and you can use python

really as a fancy calculator but i could

do other things too if i want to print

to myself hello comma world i can also

print out that line of code there too

hello world so it's interactive in the

sense that the moment you execute a line

of code boom you see the result we're

generally not going to do that because

at least when teaching the language we

tend to want to do things incrementally

and we want you to be able to see where

it is we came from and we want to be

able to try things again and again

especially if we make mistakes but know

that this is indeed a feature of python

this so-called interactive mode but

let's focus for a moment now not just on

that interactivity but really on the

fact that python apparently supports

integers and mathematics and some of

those basic operations and let's see if

we can't make maybe our our own little

calculator so let me go ahead and open

up vs code again and i'm going to shrink

down my terminal window and i'm going to

create a new file called calculator.pi

so to do that recall i can type code

down here and the name of the file i

want to create dot pi enter that gives

me a new tab up top so i have already

closed hello.pi i'm now in calculator.pi

and let's just make a simple calculator

that does some addition for me but i'm

going to do it in a file so that we can

iterate on this and make changes for

better for worse over time let me go

ahead and first declare a couple

variables i'm going to do the

mathematical thing of calling my first

variable x

my second variable y

and then i'm going to give myself a

third variable z equals x plus y and

then i'm going to go ahead and print out

z now this program admittedly not very

exciting or interesting in fact it's a

little less interesting than printing

stuff on the screen like before with

strings but we'll build on this and see

what other features exist in python that

we can leverage so hopefully if python

knows its math as well as i do when i

run python of calculator.pi

i should see hopefully that 1 plus 2

equals

indeed 3. all right so not that

surprising and not that interesting and

honestly this isn't the most useful

program because it's always going to

calculate 1 plus 2 equals 3. let's at

least make this program say a little

more interactive right we already know

from previous examples how we can get

input from the user let's bring back

that input function and let's do this

let me go ahead now and at the top of my

code let's change x to not be the number

one always let's change it to be

whatever the return value is of asking

the user for x and i can use any english

or human language i want here i'm going

to say what's x just like i asked before

what's your name and i'm going to do the

same thing for y i'm going to use input

again but this time change the question

to be what's y

all right at this point i think i'm

going to leave the rest of the code the

same z equals x plus y and then print z

but what's nice now is that i think i

have a nice interactive calculator right

now it's not going to do oneplus 2 all

the time it's going to do whatever the

user types plus whatever the user types

so let's try this let me go ahead and

run the program alright let's do it 1 is

going to be x 2 is going to be y and of

course

everyone in agreement 1 plus 2 equals 3

huh

what's going on

there either your math class misled you

or i have misled you

why don't we call on someone here to see

if you can't help us reason through what

the bug is what's the mistake uh anjali

if i'm saying it right

i think the issue is is that it's

concatenating strings because you use

the plus operator instead of adding

perfect so perfect intuition we've seen

that plus is used a little differently

in the context of strings because it

concatenates that as it joins the two

strings and that seems to indeed be

what's happening here even though the

user typed a number but the interesting

thing here is that when you get user

input because they're using a keyboard

on their mac or pc or their phone it is

always going to be text it might look

like a number but by default it's coming

from the keyboard as a string that is as

text and so how do we go about

resolving this if ultimately we don't

want to treat those inputs as strings we

want to treat them as actual numbers

well we need another function and it

turns out in python that you can convert

sometimes from one type of data to

another type of data for instance from

string to int

by doing something like this let me go

back into my code and let me change x

before adding it to y to be whatever the

integer version of x is plus whatever

the integer version of y is so it turns

out that int is not only a type of data

in python it's also a function and it's

a function that if you pass in an input

like a string so long as that string

looks like a number like one or like two

it will convert it to an actual number

that you can perform mathematics on

instead

so if i now go back to my terminal

window and run python and let me show

you another trick calculator is kind of

a long word it's a little tedious to

type notice what i can do in my terminal

window in a command line interface in

general if i start typing cal for

calculator i can actually hit tab to

finish my thought so auto complete is

possible in a terminal window like this

type the first letter or few letters and

then boom with tab it'll finish your

thought for you or you can go back in

your history like i did with the up and

down arrows let me go ahead and execute

this what's x1 what's x2 and there we go

now we have a general purpose calculator

that's going to support not just

addition of one and two but now any two

integers that the user types and let me

now improve this right we've seen how we

can make improvements to code and i

i don't know if it's gonna necessarily

be better but let's try this

do i really need the z variable

it's worth noting that i'm creating a

variable called c z

and then i'm immediately using it on the

next line of code now that's not that

compelling because if you're creating a

variable and then immediately using it

but never again using it did you really

need to take the time to introduce

another symbol and another variable just

to use it once and only once well maybe

not maybe we don't really need z in this

way maybe i should go and do something

like this

maybe i should get rid of z here

maybe i should change this to be int up

here

change this to be int up here

doing something that's pretty

interesting now even though it's a bit

of new syntax notice that

you can nest functions so to speak you

can put one function

call that is the use of a function

inside of the use of another function so

that the return value of the inner

function

becomes the argument to or the input to

the outer function just like in math if

you have parentheses parentheses

parentheses your teacher probably taught

you to focus on what's inside the

innermost parentheses first and then

work your way out same thing with

programming that's what python is going

to do it's going to look at what's

inside of the parentheses first it's

going to get the answer and then it's

going to pass the return value to the

outermost function so what happens on

line 1 now is that the input function

gets called first then the result of

that quote-unquote 1 becomes the input

to the int function and same on line 2.

the output of what's y

becomes the input to this int function

and now there is no z

i could just do print x plus y and

because i've taken the time to convert

each of those strings to an integer i

think we're okay so let me try this

python of calculator.pi enter 1

and 2

and we're still getting 3. not 12 or not

12 1 2 we're indeed getting 3 and we've

additionally gotten rid of the variable

because we didn't necessarily need it it

seems especially if only using it once

well here too let me put everyone's

hands down for just a moment and let me

ask as before

this version now

which uses int

around the invocations of input and does

not use z

is this better than the previous version

if you want to vote yes go ahead or if

you prefer the old way vote no the old

way i'll undo all of this as we vote

instead looked like this

all right and let me go back to now the

newest version let's take a hand of the

yeses someone who thinks this latest

version is better

i think this way is better because it

allows us to immediately see what the x

and y variables are with

integers and so we know what to expect

from them and also the print argument is

more

intuitive we avoid too much clutter

in the codes

i think those are all good reasons it's

nice and succinct the lines of code are

not very long uh i don't need to know

what z is because it doesn't exist it

just c print x plus y i like that but

someone who prefers the older way where

we did have z and we more explicitly

passed individual variables to the in

function yeah hi uh i think but the

earlier version is better because when

i mean if user input something else

other than

let's say i mean let's say they type one

and two like

so will be it will be easier to debug

this version or the this version here or

the old version

okay that's fair and in fact i'm i'm

being very careful today as best i can

not to mess up i have thus far only

inputted integers when i'm expecting

integers and rows actually pointing to

something we'll come back to in the

coming weeks how do we actually handle

errors what if the user doesn't type in

the number one or the number two or a

number at all what if they type in a

word like cat c-a-t that's not a number

and i bet i can't convert it to an

integer but for today i'm not going to

focus on that i'm just going to hope

that the user cooperates but that's not

going to be the case and so perhaps one

way would set us up for more success

when it comes to handling those errors

now for today's purposes which is better

i mean i like both and i think both of

you made very valid arguments and there

too so long as you have a justification

that feels pretty reasonable i mean

that's what ultimately matters but

acquiring again a sense of the

trade-offs here well is this way better

if so why or why not just understanding

what those trade-offs are but generally

speaking prioritizing readability is a

very good thing making your code

readable for someone else is a very good

thing and very good for you too so that

when you wake up the next morning or you

come back the next week or the next year

you too can read your own code without

having to waste time trying to remember

what you did and simplicity tends to be

a good thing too keeping your code

simple so is as you get more comfortable

with programming you might be tempted to

try to like combine an entire program

into one long line for instance let me

do right just that don't technically

speaking we don't really need x in a

variable we don't really need y in a

variable we could also do this i could

just get rid of x and y altogether i

could then now eliminate that and make

it just one line of code okay so on some

sense you might be inclined to think wow

that's really nice you made it one

simple line of code i would argue this

actually isn't that simple now i think

i'm starting to nest too many things i

have to think about print and int and

input i then have to notice that okay

i've opened two parentheses i've closed

two of them there's a plus you're making

me think too much and anytime you make

me think you're wasting time and any

time you complicate the look of the code

like this you're just going to increase

the probability of mistakes and tactical

mistakes or logical errors in your code

so if all the things we've done this is

the only one that i would argue

yes it's one line and it's nice and

compact it's just not readable enough i

would shy away from doing this

especially since two of those function

calls are getting input from the user

but there too reasonable people might

disagree but that's the kind of like

visceral reaction you should have

sometimes when code starts getting a

little too complicated a little too

clever perhaps for its own good

all right well it's not just integers we

have access to let me propose that we

transition from integers to one more

data type here namely a float

so again a string is a sequence of text

an int is an integer like negative one

zero and one a float is a number with a

decimal point properly called a floating

point value and you can think of the

floating point as being the decimal that

might be over here or over here with

some number of digits to the left or the

right mathematically it's a real number

a number that has a decimal point in it

so that's a third type of data that

python supports right now our calculator

is somewhat naively assuming that the

user is only going to type in integers

but if i want to support floating point

values too i think i can just make a

couple of tweaks so i'm going to go back

to vs code here and instead of just

converting the user's input x and y to

integers on line 1 and 2 let's just make

a simple change let's actually convert

it to a float on the first line and a

float on the second line here now i

think if i go down to my terminal window

and run python of calculator.pi let's

type in a number like 1.2 with a decimal

point and 3.4 with the decimal point and

there we go we have 4.6 as the final

answer so that wouldn't have worked

before if i was only expecting integers

from the user but now that i'm support

expecting floating point values and

accommodating it i can actually now do

floating point arithmetic as well but

suppose that i don't really want the

final answer to be

a floating point value like 4.6 i would

be happy if we just round to the nearest

integer so i want to support the user

typing in floating point values with

decimal points but at the end of the day

i just want to round the result to the

nearest possible integer for instance

well it turns out that here too

python comes with some functionality

built in and in fact if we return to

this url from earlier wherein all of the

python built-in functions are listed

there's one called round which does

exactly as we would expect it takes as

input a number and then rounds it for us

for instance to the nearest digit to the

nearest integer

but if we look a little closer to that

documentation as we can here i'll

provide an excerpt this is what the

function looks like in the documentation

and recall that earlier we looked at the

documentation for print and this is

similar in spirit that this shows us not

just the name of the function but its

available parameters that is inputs that

we can provide when using this function

but this is a little cryptic too just

like prince was and it adds some syntax

so let's see the name of this function

here is of course round and it's first

argument is a number notice this times

there's no star there's no star objects

like there was for print the round

function takes just one number as its

first argument period that's its

positional

parameter but notice this syntax and

this is a convention in programming or

technology more generally

generally speaking when you see square

brackets and documentation like this

this means that you're about to see

something optional and so what this

means is that if you want to specify

more precisely the number of digits that

you want the round function to round to

you can specify it here by adding a

comma and then that number so if we read

the documentation if you don't specify a

number of digits you just specify the

number to round it rounds to the nearest

integer but suppose you want to round to

the tenths place or the hundredths place

that is one or two digits after the

decimal point you could additionally

pass in comma one or comma two to be

more precise so that's what the

documentation there is saying let's see

if we can't then translate this to

some actual code for us so if i go back

now to vs code and i consider that i

want to go ahead and round x and y i can

do this in a couple of ways i could do

round x plus y but

you know i'd actually kind of prefer to

break this now out into two lines i

don't have to and reasonable people here

might disagree but i'd like to revert to

a scenario where i'm printing z so that

i can just a little more clearly to

myself to others say z equals the

rounded result of x plus y it's not

necessarily the better way to do it but

i'm a little more comfortable with

breaking out my thoughts one at a time

especially if i want to start commenting

each of these chunks of code all right

let me go down to my terminal window now

and run python of calculator.pi what's x

let's do 1.2 again

then let's do 3.4 and now it was

previously 4.6 but now it's been rounded

up to the nearest integer which of

course is going to be 5.

all right what if i wanted to

change this a little further what if i

wanted to support maybe really big

numbers big numbers irrespective of

rounding let's just do something like

this

let me go ahead and run python i've

calculated at pi again and let me just

add 999

plus 1 and notice i don't have to type

decimal points even though i'm

converting to float my program will just

allow me to type decimal points but i

don't need to oblige the answer of

course here it should be and is in fact

1000 whether or not we round so that's

just arithmetic with integers here but

in uh the us we tend to format long

numbers by putting commas uh after or

before every triple of digits other

countries flip it and they use periods

and commas instead that's a system

setting you can change that on your own

mac or pc or device for python or any

language but for me i'm using the us

approach here which is periods for

decimal points and commas for separators

what if i wanted this to be outputted as

one comma zero zero zero just to make it

a little more clear that it's one

thousand and not something like one

hundred that's even more useful when

it's like one million one comma zero

zero zero comma zero zero zero wouldn't

it be nice if we could automatically

output those numbers as well well it

turns out that we can there is a way

using python

to actually specify that we want to

include commas like this and here we

have an opportunity to bring back our

old friend the f string

first let me do something that's not

that productive first let me do this let

me print out the value of z but wait a

minute i can't just say quote unquote z

because that's literally going to print

z on the screen so let me wrap it with

those curly braces like i did before but

that too was not enough i literally

needed to add an f at the beginning of

my string to tell python that this is an

f string a format string

that now is going to print out not very

interestingly just the value of z itself

so that i'm going to great lengths just

to print z when really i could have just

passed z as the sole argument but just

to ensure that i haven't broken it let's

do this again 999 plus one enter okay

it's still a thousand so i didn't make

anything worse but notice and this

syntax is unfortunately a bit cryptic

notice that i can actually do this i can

put a colon after the z

and i can put a comma thereafter this

looks very cryptic admittedly and even i

have to constantly look things like this

up in the documentation to remember the

syntax but here let me run it again

python of calculator.pi

999

1 and now notice that the number has

been automatically formatted for me if i

were in a different country or locale i

could absolutely override this to use

periods instead of commas or vice versa

but in this case here it's just

happening for me automatically so there

too we see a hint of what it means to

really format a string there's even more

power more powerful capabilities built

into that

all right let me pause here to see if

there's any questions now on floats

on rounding

or on this use of f strings yes so i

have a question so when using floats um

is it like a cap to how many decimal

points it can have

a really good question so floats yes and

this is a problem we'll revisit before

long floats cannot represent numbers

infinitely precisely in a nutshell

because computers only have so much

memory they only have a finite amount of

memory you and i only have a finite

amount of hardware inside of the

computer so at some point they're going

to have to round right now i'm rounding

automatically effectively computers will

eventually have to do that for us but

we'll see that as a fundamental problem

before long

allow me to turn back just for a few

final examples on float before we

introduce a few final examples that

allow us not just to use functions but

to make our own let me propose that we

also try our hand at a bit of division

here let me propose that we modify this

calculator now to still take a couple of

floats but let's now just do something a

little simpler than

a little different from this just doing

x divided by y and let me go ahead and

get rid of my format string and just

keep it simple for now printing out z

instead and what are we going to see

here well just some simple division so

python of calculator dot pi let's do

something like 2 divided by 3 and of

course i get 0.66666 and to ethan's

question a moment ago it does seem to be

finite it's not rounding in a weird way

here but i only seem to see so many

digits that's a

an inevitability of using a float in

this way by contrast just so you know

integers nowadays in python can be as

big as you want them to be unlike other

languages there is no upper bound on how

big an int can be now in python but

there is a bound on just how precise a

floating point value can be all right

now that i've got some simple division

working here let's go ahead and round

this it would be nice to round this

really long number

0.666666 and so forth to maybe just two

decimal places we've seen how to do this

with round though at least in its

documentation

let's just round this not to the nearest

int by passing in just x divided by y

which is one argument once the math is

done inside of the parentheses i don't

want to pass in just one argument i want

to pass in two so that i can specify n

digits number of digits which recall was

the second parameter for round let me go

ahead and run python of calculator to pi

i'll do the same thing 2 and then 3

0.67 so here too we see a way of

rounding now not just to a nearest

integer but to a nearest number of

digits but there's another way to do

this here and in fact this evokes our

f

f string example again let me go ahead

and change this suppose that you didn't

remember the round function or for some

reason you didn't want to use it you

instead want to just use a format string

well let's go there let me do quote

unquote z but let me surround it with

those curly braces let me add the f at

the beginning and again this is not

interesting yet this is just going to

print out z but i'm adding a lot more

complexity to turn it into an f string

but notice i can do something else after

my variable name after the colon

if this were going to be a big integer i

might want to use a comma like before to

separate each triple of numbers with

commas but i don't i'm going to use a

different sequence

of characters i'm going to say 0.2 f and

this too is one of these very cryptic

things i have to constantly look up

because i forget if i don't use it that

often so don't be intimidated if this

looks especially weird but this is

according to the documentation the way

you specify using an f-string how many

digits you want to print so let me run

this version of the calculator type in 2

and then 3 we get the exact same thing

but again this is just consistent with

my claim that in programming we can so

very often solve the same problem in

multiple ways this is just now the f

string approach

to that very same problem

all right which one is better it depends

in this case they're pretty equivalent

you could imagine though it being useful

to use a function sometimes so that you

can pass in an argument like n digits as

that second argument or you can imagine

just deciding in advance that you want

point two and then

writing it like this

let's transition now from focusing on

strings and on integers and on floats to

focusing now on functions themselves we

began today by focusing on how you can

use functions that come with python but

wouldn't it be nice if you could invent

your own functions especially if to our

point earlier you find yourself solving

the same kind of problem again and again

it's nice that python comes with the

print function because it's really

useful to be able to print things on the

screen but wouldn't it be nice if you

could print specific things on the

screen by just calling your own function

well let me propose that we do this let

me go back to vs code here and let me

propose that we go back to hello.pi i'm

going to reopen hello.pi where we left

it before and i'm going to go ahead now

and propose that we consider how we can

start improving this further by making

our own function i have written so many

programs today that just say hello and

each time i'm using print but wouldn't

it have been nice if from the beginning

of today we could just call a function

called hello that just says hello for us

now the authors of python years ago

didn't think that we need a special

function just to say hello but i would

like that to exist i'm saying hello so

many times i just want to be able to

call a function hello so i'm going to

start from scratch here i'm going to

delete all of my code from earlier and

i'm going to pretend for the moment that

a function called hello exists

and i'm going to do just as i did before

i'm going to get the user's name with

the input function asking what's your

name question mark

and now i'm going to call a function

hello

and then i'm going to print out the

user's name

now i will admit hello doesn't exist so

bad things are about to happen but let's

see what let me go down to my terminal

window let me run python of hello.pi

i think the first line is going to be

okay because that worked before and

indeed it's prompting me for my name so

let me type in david the second line of

code is apparently calling a function

that looks like it's called hello

because why is it a function it has a

parenthesis and a closed parenthesis

immediately after it and that's what

every function we've used has looked

like but python's not going to recognize

this one when i hit enter now i get a

name error name hello is not defined did

you mean help

i didn't although it's opportune that's

what i need at this point is some help

but i

am encountering this error because why

the function just doesn't exist so how

do i make this function exist well i

need to create it myself using this

keyword

def

def for define so here too just as stir

is short for string and into short for

integer def is short for define

if and when you want to define create

invent your own functions you can do so

using now this keyword in python so let

me go back to my code here and let me

propose that we define this perhaps in

this way

at the very top of my file i'm going to

first take a moment to define a function

called hello using def

hello open parenthesis close parenthesis

colon

what this means now is that python is

going to treat every line of code that i

indent underneath this one as the

meaning of this new function hello

so def is important as is the space i

get to choose the name of the function

and i'm choosing to call it hello the

parentheses with nothing inside means

that this function at the moment is not

going to take any inputs no arguments

there too the colon means stay tuned for

some indentation everything that's

indented beneath this line of code is

going to be part of this function it's

going to be a super short function one

line of code it's just going to print

out quote unquote hello

but now on lines one and two i have

invented my own function

hello notice these dots that have now

magically appeared here this is just a

setting of my text editor vs code in

this case that's just making super

explicit to me that i've hit the space

bar four times or equivalently the tab

key once which is converted

automatically to four spaces generally

speaking i'm going to need to make sure

that all of my indented code lines up

now so that python knows that it's all

part of the same thing but it's easy in

this case because it's just a single

line but now thanks to lines one and two

the function hello will absolutely exist

when i'm ready to use it on line six so

let me go down to my terminal window and

run python of hello.pi enter

here comes my name again and now when i

hit enter i now see hello david all

right we've kind of regressed though

right this is not nearly as pretty as it

once was i think we can probably do

better than this by improving things

further why don't we consider though how

we might

say parameterize the same function that

is to say can we customize hello to

maybe take the user's name as input so

that we can say not only hello but the

person's name all on one line all in one

breath well i think we can do this let

me propose that we do this as follows

let me go ahead and up in my code let me

inside of these parentheses let me come

up with my own parameter name i have

complete choice here and i'm going to

say that the name of my parameter will

be the word to

why because i want my function to sound

like the verb

it represents hello but who do you want

to say hello to well i'm going to call

my parameter for this function 2 just

because in english it kind of sounds

nice to me hello to who do you want to

say hello to that's why i'm calling this

parameter 2 instead of something simpler

like x or y or z

all right well what do i want to do with

the word two well i can do a couple of

different things we've seen like so many

different ways to implement hello let me

just add a comma there for grammar sake

and then let me put the word to

after that as the second argument to the

function hello there's other ways we can

do this and we've seen so many but this

one looks a little clear to me i'll say

what's going to happen next well i don't

think i need this extra print line here

i think what i'm going to do is this i'm

going to go ahead here and print out

not the person's name manually i'm going

to sense instead say hello

parentheses name

so what am i now doing on lines 1 and 2

i'm defining my very own function called

hello but this time that function has

been designed to take a parameter a

single parameter as input and i'm using

the value of that parameter which i

called 2 to plug into print so that i

see not only hello but also that

person's name what am i doing on line 5.

same as always i'm just getting the

user's name line six i'm not only

calling hello i'm passing as input the

name variable as an argument

so that that's what gets passed into

hello and what's happening here is

essentially this even though the

variable is called name here when the

function itself is called the computer

assumes that that same value is now

called to

so name is essentially copied to another

variable called to

so that in the context of hello i can

say hello to

that variable instead

and we'll see in a moment what happens

if we don't keep those uh straight let

me go ahead and run python if hello.pi

enter what's your name and now i'm

crossing my fingers enter there we go

we're back in business but now i have my

own custom function called hello that's

allowing me to say hello to a specific

person and here's where now things can

get really fancy what if you wanted your

hello function to say hello to someone

specific but you know what if you don't

know who you want to say hello to you

want to say hello to the whole world you

can give parameters default values we've

seen that recall that with print there

was a default value for sep for the

separator there was a default value for

end the line ending we can do that too

and here's the syntax if you want the

value of this parameter

by default if not provided by the