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Welcome back, everyone, to this lecture on functions.

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Now, often as you're coding of Python, you're going to have some code that you actually want to use

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multiple times, but you don't always have to want to rewrite that exact same block of code over and

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over again, especially across multiple Python files.

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It would be nice if you could set it up in a block, they can then just call to execute.

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And that's where the function comes into play.

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It allows you to just be able to repeat the use of code.

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Now, creating a function requires a very specific syntax, including the IDF or defining keyword.

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Then you have the correct indentation to concern yourself about as well as the proper structure.

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So I want to give you a quick overview of the Python function, structure and syntax.

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Before we jump to the editor to start coding.

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So it all begins with the death keyword, and this keyword essentially tells Python that you're beginning

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to write a function.

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And then you have the name of the function you should notice.

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It follows snake casing and you can decide on the function name.

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Snake casing just means it's all lowercase of underscores between the words.

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This is by convention.

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You don't have to do it this way, but it is the proper way to do it, so to speak.

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Then you're going to have some parentheses.

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Later on, we're going to discover we can actually pass in arguments or parameters into the function

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using these parentheses.

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Then you have the colon, which indicates an upcoming indented block, and everything indented is then

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inside the function that's going to be executed each time you just call the name of the function.

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Now there's the option of providing a documentation string, which will explain the function if somebody

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passes in the name of your function into the built in help function with Python.

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We should also note that everything inside the function is indented, including in this documentation

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straight.

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And then you're just going to have code that you want to actually execute each time you call the function.

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Here we have a very, very simple function that is simply going to print hello each time you call.

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Name a function.

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So, for example, if I were to call name a function open and close parentheses, I would just see the

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print hello as the result.

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OK.

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We should also note that functions could actually accept arguments to be passed by the user.

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So notice now what's changed?

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I have a name of the function, but now it's going to actually be able to accept an argument and this

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argument is a variable called name.

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And then what's gonna happen is it's going to take that name and then tack it on to hello.

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So if I call name a function, Jose, they'll report back or print back.

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Hello, Jose.

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So functions can accept arguments to be passed by the user.

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So note again, name is being passed in and then we're saying print hello name.

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So in the output input, I can see.

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Name a function, Jose.

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Print out.

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Hello, Jose.

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Typically, however, we're not just printing results with functions, we typically use the return keyword

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to send back the result of the function instead of just printing it out.

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Return allows us to assign the output of the function to a new variable.

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A question I get a lot from students beginning to learn.

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Python is what's the difference between printing in a function and returning in that function?

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And again, return allows us to save the results if we just print the results.

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We do see the output, but I don't get to save them to a variable to then later use.

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And as a quick note, we will have a deeper discussion of the return cured later on in the actual editor

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because it's easier to explain when we're actually coding this out.

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But as a quick example of what this would look like here have a function called ad function, and it

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takes in two arguments.

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Number one and number two and then it returns the result number one plus number two.

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So the return actually allows us to save the result to a variable.

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Notice now how I can say result is equal to add a function of one or two.

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And then if I want it to later on, I could print that result.

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So most functions will use return.

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We are very rarely just using a function to call a print statement.

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It depends on the use case, of course, but pretty much we're always going to add a return statement

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to a function to actually return a result for use later.

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OK, let's start creating functions with Python and the jump to the editor.

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All right, so let's begin by setting up our first function.

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This function is going to be super simple.

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It's actually just going to print something later on.

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We'll talk about return.

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So to start a function, we say DPF.

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And notice we already know it's a function based off the syntax highlighting.

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And I'm going to call this function simply say Underscore hello.

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Open close parentheses colon enter.

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The indentation starts.

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And then every time I call, say hello, I can just print out something like hello and let's.

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In fact, let's print out another line that says another hello.

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So typically, if I wanted to print hello and print another hello, each time I wanted to print both

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of these, I'd have to copy and paste both lines.

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But now that I have a function, what I can do is simply call, say hello and I execute that function

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with open and close parentheses.

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Pay very close attention to my indentation here.

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Say hello is no longer inside the function.

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And what's nice about the code editor of this visual studio?

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When you define the function, you can actually collapse the function by hitting this little dropdown

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arrow key.

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And then you can really see what's inside your function that actually helps you kind of read your code

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and save a little space.

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If you want to collapse the function code, so again, be very careful, don't accidentally do something

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like this.

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In which case you're calling, say hello within, say hello and you can check for doing that by collapsing.

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So again, pull say hello out, which means I'm actually going to execute the block of code inside.

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Say hello.

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So when I run this, I see hello and another hello, and I can call this function now as many times

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as I want, so I can call it twice here.

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Say hello.

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Say hello.

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And so we should see it.

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Hello, another hello.

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Hello another.

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Hello.

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OK, now I didn't mention typically, you're not actually calling print statements like this within

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a function.

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Instead, you like to perform some sort of logic or calculation and then actually return the results.

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So let's talk about this return keyword.

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Return allows you to actually return something to be saved as a variable.

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So I'm actually now just going to return the string.

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Hello.

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And you're going to see what happens when I call say hello.

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So notice now I'm just returning it.

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I'm no longer printing it.

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And when you run the Python code now after saving those changes, you actually don't see anything output

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here.

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And that's because return is just saying, Hey, I'm going to give you this string.

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It's up to you to do whatever you want with it.

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So, for example, I could save it to a variable like this.

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I could say my variable is now equal to say hello and then I could print out.

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My variable save those changes, and now you see hello, print it out, what you cannot do is something

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like this.

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If I were to say.

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Prince, hello.

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And then try to do an assignment like this online for this actually won't work, you can't really assign

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print hello to this and then print out that, well, you're going to end up seeing is hello and then

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you're going to get this weird nun.

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You get hello because you did call say hello, but then say hello is not actually returning anything,

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which is why my variable then becomes not.

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It's just a placeholder there.

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So typically, we want to be able to save the results of all the hard work we do within our functions,

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which is why we end up returning things within the functions in order to save them as variables.

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You may or may not want to print out that variable, but returning it allows you the chance to actually

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save it.

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So let's start adding in some complexity by allowing for arguments.

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So now I'm going to say hello to someone's name, so I'm going to be able to accept an argument name

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and I can accept as many arguments as you want.

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And you can also call this variable whatever you want this argument parameter, you can call it whatever

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you want.

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And now what I'm going to say is return the string and it's going to be an f string literal this time

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that says hello, curly braces and then the name that's passed them by the user.

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All the arguments up here are going to be passed in by the user.

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So now I'm going to say my variable is equal to the result returned by say hello, which is going to

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be this string literal.

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It says hello name.

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If you just run it like this, you're actually going to get an error because it expects a name to be

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passed it.

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So if I run this right now, it's going to say, Hey, say hello is missing a required positional argument

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name.

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So how do I pass in the argument?

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I say name is equal to and then whatever you actually want to pass in as the name.

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So now when you're running this code, you see, Hello, Jose.

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The user is passing in.

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I want name equal to Jose.

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And so it's executing Say hello, it's returning back the string.

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Hello, Jose, save it to the variable and print that out.

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And we can pass in more than just one parameter here, I can say, taken a first name.

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Karma taking a last name, and then I could return something like, hello.

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First name.

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Space.

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And then insert last name, so say hello, and then now I have to make sure these match up, you can

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do it by position or by name.

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So I could just say something like Jose Comma Porsche here.

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Save that.

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And then when you run this code, you should see.

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Hello, Jose Padilla.

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I'm doing it by passing in by position, but it's often better just to be really explicit and say,

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OK, first name is equal to Jose.

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Last name.

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Is equal to Porsche that way, whoever's reading your code later on.

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Maybe you have one hundred lines in between this function call and the sexual assignment, in which

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case it'd be nice to know what these variables or arguments actually stood for.

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So let's start adding in a little bit of logic.

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In fact, let's make a function that is going to check if your a number is less than or equal to 100.

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So I'm going to create a function the f called checker and this simply text if your number is greater

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than or equal to 100.

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So I take in the number and I'm going to say some comparison logic.

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If the number.

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Is greater than 100.

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I'm going to return greater than 100.

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Else I'm going to return.

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Not greater than one hundred.

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The air there, so I have that function checker.

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Go ahead and save those changes and then what I can do here is print out the results of checker and

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let's check 90.

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And again, I'm printing the results so I can see them here in the output.

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And 90 is not greater than 100.

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Perfect.

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So it looks like it's working as expected, let's put in nine hundred there.

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Save that.

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And then it looks like nine hundred is greater than 100.

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Perfect.

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And keep in mind here, we're just showing very simple examples, but we can actually get quite complex

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of this.

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Let's imagine that I have a list like my list and it's filled in.

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If a bunch of random numbers, I'm going to pass in a bunch of random numbers here.

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We can think of this as maybe a data stream that's coming in, and I actually need to know whether or

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not the number 10 is in that list.

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So how can I do that?

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How can I check if 10 is in that list?

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One way I could do it is by actually using a for loop to go through this list.

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I should note there's a better way of doing that.

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You could just use the in operator, but I'm going to pretend I don't know that for right now, just

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to get the idea of logic within the function.

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So I'm going to have another function here that's called check or checker again, and it's going to

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take in the list to check.

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So let's go ahead and just pass in.

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List to check.

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And then what it's going to do is it's going to say for number in the list to check.

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Note that has to match the input variable.

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So for every number enlisted, check.

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If.

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That number is equal to, let's say, 10 is what we wanted to check for.

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We go ahead and return true.

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So what's going to happen here just for this first part of the logic is I'm going to go through every

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number in this list here and then I'm going to say, OK, is that number equal to 10?

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And if at any point this happens to be equal to true, the return hits and then I'm done.

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You'll notice that once you hit return in a function, the function ceases to operate.

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Now let's say I go through this entire for loop and this never actually returns.

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What do I want to return?

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Then I want to return false.

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So a question is where should I put the return false?

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Should I do it here?

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Return false?

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Or should I do it here?

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The indentation can sometimes get confusing.

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It should go here in line with the four.

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And why is that the case?

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Well, because I want to make sure I go through all these numbers first and then if I don't happen to

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return true at the very end, I'll return false.

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A really common beginner mistake to try to solve this sort of problem would to be use an else statement

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here.

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And then they'll put return false inside this else statement.

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But the reason this is incorrect is because at the very first number at one, what's going to happen

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is, well, that's not equal to 10.

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So I'm just simply going to return false.

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It will immediately stop at the first number.

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Instead of having the chance to look through all of them since we want it to have the chance to complete

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that for loop, we don't end up using that statement and we line up the return over here with that for

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a loop.

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So just try to keep that thing in mind.

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It's really helpful to sometimes use prints to decide what's actually happening within your block of

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code.

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But let's go ahead and use checker here.

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I'm going to call prints the result of checker on my list.

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Note my list does have a 10 on it, so I would expect it to return true here.

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And we're going to say Python Example Pi, and let's expand this.

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So Python example pie and it looks like a return, true.

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I'm going to zoom out one level here swing series thing that's going on.

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OK, so let's remove 10 now.

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So there is no longer a 10 on that list.

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We'll go ahead and save that change.

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Let's go ahead and run Python.

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Example Python.

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And now we get false.

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So you can see us beginning to combine many, many different ideas here.

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I'm combining the idea of a function, the idea of Boolean, the idea of a for loop, the idea of if

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but statements of comparison operators.

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It's all coming together to build some sort of functionality.

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What we're going to do next is give you a series of problems where you have to write a function that

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returns something and they're going to get increasingly more complex and more difficult just to make

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sure you understand the building blocks creating functions to end this section of the course.

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OK.

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Thanks.

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And I'll see you at the next lecture.