So far, we've just been typing code into Code Sculptor and we typed statements one after another and they get executed immediately when we run our program. This is fine for simple programs. But a very powerful construct in programming is the function. What a function is, is a piece of code that you define that you can execute later. And so you only execute the code inside a function when you call that function. And I can call the function more than once. So this. Becomes a very powerful programming construct. I define a block of code once and then I can use it many times. So in this video, we're going to talk about how to define a function and we're going to show some simple functions and some simple calls to those functions. Alright, let's write some functions. Now, before we actually type any code we have to think about what we want. Beginning programmers often just start typing before they think about it. And before I write any function I should decide what the function is going to do. And a good way of doing that is to put it in a comment. So I am going to write a function that computes area of a triangle. Okay? And I wanna see all your functions have comments like this. You should always put a clear comment that shows, this is what this function does. Now, I'm just gonna type the function, and then we're gonna talk about it. . And there we are. Here's my function for computing the area of a triangle, alright. So I wanna talk about, two parts of the function. The first which is line two is called the header of the function. The second which is the rest of it is the body. Okay so what is the header? The header tells you about the function, so it starts with the keyword Def. Def is short for define and you're telling Python that you want to define a new function. Then I have the word triangle area. That is the name of the function and you can choose any name that you'd like. All names that are valid, variable names are also valid functions names. So you should pick a function name that is indicative of what the function does. So I'm gonna compute the area of a triangle. I want to call my function triangle area. Then. We have a parenthesis list of perameters and in this case this function takes two perameters, the base and the height of the triangle. The functions can take zero or more perameters so, I can have zero, one, two, 100. How many perameters you need to compute whatever does you are computing. Okay. And you should also name your perameters in a way that is obvious to someone who is using your working area program. Here base and height are obvious names for their elements of a triangle. Okay, then we have a special character, at the end of the header is a colon, a colon has huge significance in Python, this means you're about to start a new block of code, and you'll notice. The body of the function, which is the rest, and this is. Which is this new block, after the colon, is indented. It doesn't matter how much it's indented, but it does matter, it's indented by the same amount. And this has significance. When you stop the indentation, the block of code is done, 'kay? So, everything that is indented after that colon is the body of the function. And you can see, I have two statements that are in the body. The first one is actually computing the Area. And you can go to Wikipedia if you've forgotten the Area for the, or the formula for the Area of a triangle. It's just one-half times base times height. Okay. So, I'm computing that to line three. Then at line four, I'm returning that area. Okay, so what does return mean? Return is another key word in Python that says, this is the output of that function. And so, you can output one thing from a function, in this case I'm outputting the value of the local variable inside the function area, okay? So, let's run this programme and see what happens. Hm. I keep hitting the run button here, nothing happens. Alright, this is signifigant. This defined a function, okay? No code executes when I run this program. Why? Because a function is a piece of code that I want to be able to execute later. It only executes when you call it. So let's actually call this function. Let's say, you know, A1 = triangle area. Let's have a base of three and a height of eight. Now. Beginning programmers often will just run this and say hey, great it works cuz you get any old output. Let's think about it first. What is the actual output that's expected here. Let's print A1. I assume that a triangle with a base of three and a height of h and have an area of twelve. Let's see if this actually works. It does, okay. Now, I wanna point out here. I assign this to A1 and then I printed A1, what did A1 get? A1 got the output of the function triangle area, what is the output? The output is whatever the function chose to return after the return statement, so in this case it was it was a variable area which had been assigned to the one half base times height, okay. Let's try it with something else A2 equals triangle area fourteen and. 2.'Kay. Print A2. What do we expect the output to be here? I expect it to be fourteen, and lo and behold, it is fourteen. Now I want you to think of these functions as a box. And this box has a name. In this case, it's Triangle. Area and there's some number of inputs, in this case there are two inputs, and some of them are outputs zero one, this case there's one output okay? The first input. Gets named base, the second input gets named height, something happens here, we create an output, we call it area and then it goes out. Now the key point of this is these names base, height and area, they only exist inside this box, they only exist inside the function, you don't actually use them anywhere else, alright, and what's happening when I call the function, triangle, area I call the function by using the function name in the parenthesis and giving some values three, two. Right. What happens is three goes in here two goes in there. So base gets the value three, height gets the value two. I compute the value of the area okay which becomes three and out goes three on this side. So if I were to print this I'd get three, if I'm gonna assign it to A, A would become three okay. This is valuable for many reasons right. A function is now a black box, I don't care what's inside of it, I don't care what's going on in this squiggle here all I know is that there are two inputs. Base and height and one output and I can just use it. I read the comment and I know what it does. Okay. When I'm implementing the function, I don't have to care how you call it. All I know is these two things are going to come into the input and now I can implement it in here, you know, one half base times height, forgive me for abbreviating base and height for B and H, that computes area. This goes to area Right base goes to here, height goes to here, the output of this goes to area and I get my output. Alright, back to our code. Let's write some more functions. Let's write a function that converts. Fahrenheit. If I can spell two Celsius. Kay, so, let's define the function. We use Def and we give it a name that makes sense Fahrenheit two. We'll be clever with the number two Celsius and we take a variable name, we'll, we'll be Clear about that too that the variable is the, the value in Fahrenheit of the temperature to convert to this I'm going to say Celsius equals five-ninths times Fahrenheit minus 32 and we will then return celsuis. Okay. So now we have a function that takes a single input just the value in Fahrenheit and returns an output in celsuis. Next we should test it we shouldn't just trust that it works so let's say c equals fair in height to Celsius. Let's take some values that we know the answer, okay. See one. To copy paste 2,12. So I know freezing and boiling, and we better make this c2. Let's print this out, c1, c2. I picked these because it's freezing and boiling. So I'm expecting zero and 100 to come out. And they do. So I probably wrote this correctly, Okay? [inaudible]. Maybe I don't like Celsius okay, I'm a big fan of Kelvin. I really feel like you know, absolute zero is my friend. So let's convert Fahrenheit to Kelvin instead of Celsius. So again I'll give it a useful name. I'll name the parameter Correct, I'll spell it correctly okay. Now what do I do. Well. Hang on a second here. I have a function that converts from Fahrenheit to Celsius. I know that Kelvin is Celsius plus 275. Thirteen, or 273.15, sorry. But I, I already know how to convert to Celsius so it would seem silly to try and write that code again, so let's not. Let's call our function. Inside of functions I can write any code, or I can use any code that I would use anywhere else. So if I already have a function. I can use that too, all right? And I know that the Kelvin value. I should be consistent here and not use bad variable names. Kelvin value is simply the Celsius value plus 273.15. And now I can return Kelvin. Alright. Well, I should test this. K1 equals Fahrenheit. Two Kelvin. Again, let's do some of the ones that we know. 212 hit two for in to k one, k two, alright let's run this, and we get the numbers that numbers that I expect here as well. All right, I think this is very important for you to understand. Right inside of a function I called another function. Right so let's think of this in terms of the boxes again, and hopefully you'll forgive me but I am going to use single letters here because I can't write fast enough. So I had a function Fahrenheit to Celsius, right there's some box here. Okay? And it had one input. And that input was my Fahrenheit value. I did some calculations, which, we know what they are, right? They're five-ninths, you know, f-32. Okay? I get out Celsius value, and that comes out. Then I wrote another function. I wrote a function Fahrenheit to Kelvin. Okay? That's also a black box. Alright. Incomes, one input. Something happens. We compute Kelvin, and Kelvin goes out, right? Now, I wanna make a, an interesting point here. I don't care how you implement Fahrenheit to Kelvin if I'm using it. So when I call Fahrenheit to Kelvin. So somewhere, I call Fahrenheit to Kelvin with 32. I just want the right answer. Right? I want the answer. I want this to evaluate to 273.15. Okay. How you do it? That's your business. So when you're in here implementing it, there's absolutely no reason why you can't say, hey, Celsius. I already know how to do that. F to c of f, right? But, you could also recompute it. You could say, you know, I like being messy. And I'll just say, instead of that, I'll say Celsius = five-ninths Fahrenheit -32. Okay and then I go to Calvin from there, alright? So there are many ways to right a function, which is better? I would argue that if I already have a function to do the job, I should not write the code again, you do not want repeated code. If I figure out "Oh I'm screwing this up, it's really nine fifths" or whatever right? You change it in one place and then it would be fixed in both, okay? I think that this is pretty critical alright? But keep in mind I have these boxes I don't need to care how they're implemented as I use them, and once I have a function it's available to me to use anywhere. Alright, I wanna show you one final function here. I wanna show you a function that prints hello world, 'kay. This function, I'm gonna call it hello, takes no arguments, alright. Remember, I said that we don't have to have any parameters or arguments, 'kay. And all it does is print hello world, 'kay. That's it. Alright, let's test it. Cuz we should always test our functions, let's call hello. Alright, oops, I've got to spell it right or that definitely is not going to work, okay so what happens when we run this function? It prints hello world. Okay. That's great. That's what I expected it to do. Now, what's different about this function from all the other functions we've written today? Well, it does not have a return statement. This function, if I drew it with, as a box would have no inputs and no outputs. Nothing comes into hello. There are no parameters. Nothing comes out of hello. There's no return value. So what happens when you try to assign it? Say H equals hello. Print H. Right. Well there's nothing coming back, so what do you think's going to happen? Well, Python has a value none, and none means there's nothing there, okay? And so Python, if you forget the return statement, or if you don't need a return statement, it automatically adds one. So it's exactly like if I typed return none. Okay, this none is a special value. Alright, so H gets the value none. I can print H and it'll say none. Now why do I bring this up? Well, usually you're not writing functions like this, you might write a few. But often, you return a value and you expect it to return a value but you forgot the return statement and then it sh, none will show up here in the console for your variable and I just wanted you to see this is probably the most likely reason. You should go look at your function again and see if you forgot the return. Okay. So this was a quick and dirty introduction to functions. Functions are extremely powerful and extremely useful and every program you write will include functions. I've only scratched the surface here on what you can do with functions and we will see them throughout the course. Pretty much every lecture from now on will use functions, every project that you write will use functions. So you're gonna get familiar with them pretty quickly, all right? I want you to keep in mind the things that I've said here. And hopefully, the model of using a box, with the inputs and outputs inside the box. Those names stay in. Inside the box will be valuable to you. Don't forget the colon. Don't forget the indentation. And don't forget the return. These are critical pieces of the functions. Hopefully, with a little practice. You'll understand and be able to use them
