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Earlier in the course, we were looking for a

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mean or an average function that Python might have

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to allow us to make the average of the

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items of the numbers of a list.

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And we found out that Python has some more fundamental functions,

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such as len to get the number of items of a

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list, and sum to make the sum of the numbers of

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a list, but it doesn't a mean function.

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So we ended up doing this.

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So we had this list.

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We calculated the sum using the sum function, calculated

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the length so the number of items using the

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len function, and then did the division between sum

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and length to get the mean.

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However, the fact that Python doesn't have a mean function

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doesn't mean we cannot create our own mean function.

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So what we're going to do now is

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we're going to create our own mean function.

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To create functions in Python, you use the def

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keyword and then pick a name for your function.

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The name has to follow the same naming rules

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as with variables, so it cannot start with a

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number, it cannot start with a math operator.

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So let's say mean.

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Then you use round parentheses and inside

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those and the first line ends with

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a colon and inside the parentheses,

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optionally, you can input some

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parameters of your function.

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Parameters, in other words, is input that your

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function will get and that will process that

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input and that will produce an output.

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The input in our case would be a list.

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However, here you have to give a name

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following again the same rules as with variables.

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Let's say mylist, okay?

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Then you want to press Enter there and

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you see that automatically Visual Studio Code,

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so the editor indented the next

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line with four spaces, 1, 2, 3, 4.

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So that's what the syntax requires,

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that after the first line,

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so after you have defined what name you

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want for your function and what parameters you

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want, you have to indent the next lines.

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So either your editor will do it

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automatically, but if it doesn't do it

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1, 2, 3, 4, just indent four spaces in there.

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You can also make it with one space.

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So just one space would also work,

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but it's recommended to use four spaces.

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Everyone uses four spaces because it's

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more visible, the indented code.

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So it will make your code more readable.

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And then you can start to calculate the mean.

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Let's say the_mean.

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I'm going to call it like that so

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I don't confuse it with the function name.

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That would be the sum of mylist

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divided by the length of mylist.

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And next what we want to do, we

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want to use the return keyword and that

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is followed by what you want to return.

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Well, we want to return the mean.

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Now save that and let me open a terminal.

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I want to execute this script now.

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So let me show you what will happen.

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Nothing will actually happen, nothing happens,

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because what Python does is

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it just reads this definition.

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This is like a blueprint.

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You have declared what your function should do, but

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you didn't tell Python to execute your function.

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To execute your function, you want to

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call the function,

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[Author typing]

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pass an input value

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for that parameter that you defined.

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And also you want to print out,

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print open parentheses there, close parentheses there,

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save, don't forget and execute.

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And that's the mean of those three numbers.

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[No audio]

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So like that, I can easily try

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this out with many input values. Just like that.

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I don't have to rewrite anything in here.

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That's the beauty of a function.

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So you can see the similarity here.

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Just like we use the sum function, we are

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using the mean function in the same way.

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So sum gets this input value,

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mean gets this input value.

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This happens to be a variable.

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This happens to be a direct value,

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but it doesn't matter. If you check the type of

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mean, type mean and type of sum, what you're going

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to see is that mean is a function.

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So that was printed out.

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This was printed out by this.

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So mean is a function and sum is again,

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it's a function, but it's a built in function.

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So it's built in in the Python interpreter.

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It was designed by the Python core development team

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and this function was designed by me, the programmer.

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So that is how you create

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and call a function in Python.

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[Outro sound]