CS50P - Lecture 0 - Functions, Variables

[Music] so [Music] 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 programmer to be equal to quote-unquote world you

literally do that in the same line you're defining the function and i'll admit it's starting to look more cryptic but i'm still just defining a function called hello it takes a parameter called 2 but i'm assigning it with the equal sign a default value of quote-unquote world just in case the programmer doesn't call hello with an argument and we can see this here let me change my code to use hello in two ways on line five i'm gonna very simply call hello no arguments then on line six i'm going to get the name line seven

i'm going to call hello with an argument so you'll see hello now being used in two ways let me go ahead and run pythonflo.pi i'll type in my name oh interesting notice i already see hello world but that's expected because line five happens before line six but once i type my name now the program is going to be a little more polite and say hello to me personally so there too we see with relatively simple but new syntax how you can implement functionality very similar in spirit to what the print function gave us automatically now

you have control over doing that yourself but let me now make this point too one of the whole points of defining your own functions is one just to avoid having to repeat yourself again and again you don't have to actually keep reinventing the wheel and keep using the print function again and again and again if you just want to say hello wouldn't it be nice now if i could kind of move this code that i wrote for defining the hello function and just to be dramatic i'm going to hit enter a whole lot of times

50 lines down and put my definition of hello way further down in this file why well just from the spirit of out of sight out of mind because if i now rewind to the start of my program now you can sort of take for granted that oh hello is a function y because it's there on line one and it has an open parenthesis and a closed parenthesis which up until now has meant call this function and then on line two we're getting a variable from the user by typing in their name and then we're calling

hello passing in that value well at this point i can just take for granted that hello exists even if it's way down further in the file or as we'll see in future weeks even if it's in a different file altogether but there's a problem here and let me go ahead and run this version of hello dot pi notice that as soon as i run the interpreter python of hello.pi i see a name error name hello is not defined again did you mean help well again fitting i do need some help here but i didn't mean

to call the function help the problem here though is that python is just taking me literally i have defined my function hello all the way down here but i'm trying to use it way up here and that's not allowed python's interpreter is going to take you literally and if you use a function it must already exist by the time you are calling it so how do i fix this well apparently i can't do that i have to define any functions i want at the very top of my file but that too could get me into

a bit of trouble eventually because if i constantly have to define a function above where i want to use it you're kind of writing code in reverse you're constantly writing functions up here up here up here as opposed to like writing your code logically top to bottom so let me fix this in a more standard way which is to do this generally speaking you do want to put the main part of your code at the top of your file and in fact i'm going to go so far as to define my function called main it's

not a requirement but it's indeed a convention and this just connotes to the reader that this is the main part of my program i'm going to get rid of my empty hello call now and only pass in one version with hello name and then down here a couple lines further down i'll actually define my hello function unfortunately now that i've reordered the functions in this way by putting the main part of my code at the top and hello at the bottom so that my logic kind of flows top to bottom if i go ahead and

run python of hello.pi enter nothing whatsoever happens if i do it again nothing whatsoever happens well why in the world is this well just because i've defined a function called main and i've defined a function called hello doesn't mean that i've actually called that is used either of them yes i'm using hello inside of main but no one is telling python to actually use or call main so in order to tidy this up the last thing i need to do in this file it seems is actually call my main function and in fact by calling

my main function in this way it gets me out of trouble because now i'm defining main first but i'm not calling hello yet i'm defining hello next but i'm not calling hello next i only at the very end of this file call main which has the effect of running this code up here which has the effect of running this code down here and it allows me therefore to organize my file and order my functions in any way i want including main at the very top and solving ultimately that problem of python not knowing what's going

on now it's important to note that i defined my function hello as taking an argument too and then i passed into that function the value of the variable that i wanted to say hello to that is the variable called name because suppose i had done something a little bit differently suppose that i hadn't defined hello as taking an argument so i just remove mention of two and its default value help world and i go back up to my main function and i just call hello itself without passing in any argument and now let me go

ahead and make one more change one more mistake technically let me go ahead and just try to naively print out the value of name in the hello function so now to be clear in my main function on line two i'm defining my variable called name and assigning it the return value of the input function from the user i'm then just calling hello in my hello function which now no longer takes any arguments i am calling print passing in hello comma and then immediately passing in name the variable into which i got the user's input but

the catch is that name exists now only in main and so watch what happens when i try to run this version of the program with python hello.pi i hit enter i'm prompted for my name david enter and ah a name error name name quote unquote is not defined so it turns out that this is actually an issue of what's called scope scope refers to a variable only existing in the context in which you defined it so insofar as i define this variable name in my main function i can only use that variable in my name

function i can't use it as i've tried to here in my hello function it doesn't exist in that so-called scope and so this is why now if i rewind and undo all of those changes you'll see that i'm deliberately passing main from my main function into my hello function and now in the hello function it technically has a different name it's called 2 in that context but that's fine it's completely up to each individual function to name its own variables or name its own arguments but this is a way now that i'm handing to the

hello function the value of that variable so it can be printed by hello as well and there's one final flourish we can add here now that we've implemented hello you'll notice that hello only has a so-called side effect it only prints out something to the screen well what if i also want my function to not have a side effect per se but actually hand me back a value recall that the input function returns a value the string that the user typed in recall that the int function returns a value the float function returns a value

that was passed into it well you can use one final keyword here literally return to return a value explicitly yourself in fact let me go back to vs code here and i think we'll return our attention to calculator.pi and see if we can't implement one other version of calculate.calculator.pi that actually has our own function that even returns a value so i'm going to go ahead and open up calculator.pi and i think this time i'm going to throw everything away as before and i'm just going to start practicing what we're preaching here define a function called

main which is now going to be the main part of my function let's go ahead and now declare a variable called x and assign it to the converted version of the user's input after asking them what's x so again a line of code quite like we've done before and suppose now that what i want to do is square this value i want to take the number that the users typed in and raise it to the power of two so two squared would be four three squared would be nine four squared would be sixteen and so

forth well how do i go about implementing a function literally called square which actually doesn't come with python built in well let me assume for the moment that it does exist and let me say something like this let me go ahead and say that printing how about x squared is comma square of x so what have i done i've defined a function called main and i've implemented two lines the first of these lines prompts the user for a value x and converts it to an int and stores it in a variable called x on line

three i then say x squared is and then i pass a second argument to the print function whatever the return value is of a square function but square doesn't exist and i'll show you this here if i now call main at the bottom and i run python of calculator.pi i'll see that x is 2 and then i see a whole bunch of errors a name error name square is not defined so this isn't a typo here it's just the function doesn't exist but i think i can make it exist here let me go ahead and

define another function called square this one's going to take in a number and i'm going to call it generically n as many a programmer would just to represent any old number and then what do i want to do in order to square n well a number squared is really just itself times itself so i'm going to do this n times n but it's not enough just to do the math yourself n times n you're going to have to return the actual value n times n and that's our new keyword here when i now do this

watch what happens python of calculator dot pi enter x say shall be 2 x squared is 4. let me go ahead now and say x is now 3 x squared is now 9. so i've implemented my very own function that returns the square of a value and because i'm using the return keyword that ensures that i can pass the return value of this just like the return value of input or int or float to another function like print instead and here too there's going to be so many ways to solve the same problem i can

actually raise n to the power of two we've not seen the syntax before but if you use stu two asterisks like this two stars that raises the thing on the left to the power on the right or it turns out there is in python a function called pow for raising something to the power that takes two arguments the first of which is the number the second of which is the exponent so there too there's just so many ways to actually solve that same problem as well so ultimately what we have we done here we first

introduced functions these actions are verbs many of which come built into python that you can just use in your own code we then introduced variables by which you could store those return values and then maybe do something more with it at the end of the day too you now have the ability to create to invent your own functions to solve simple problems like hello or in the weeks to come much more sophisticated more challenging more fun problems as well