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It's time to talk about functions.

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This is where things get really interesting because up until now we've seen functions write functions

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where things like print, like list Bolian, we even saw the input function to get the input of whatever

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the user types and those allowed us to perform actions on our data types.

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But the true power comes when we can start creating our own functions.

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That's right.

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We're not limited to whatever Python gives us.

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We're able to create our own functions and use them in our programs.

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So let's learn how to do that.

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The way we create a function in Python and functions, by the way, exists in all programming languages.

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They're very, very important.

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If we do D.F., that lets the Python interpreter know that we're about to define a function.

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DSF is short for define.

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So the interpreter is going to say, all right, they're about to define a function.

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What's the function going to be?

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Well, we can create whatever we want.

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We use the same name in case, as we do with variables to define our functions.

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So let's create a function again.

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We'll call it say hello, just a variable that I created.

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But this time it's a function because we use the def keyword now in here and say hello.

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We also add the brackets to let the interpreter know that this is something that we're going to take

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action on or this is going to perform an action on a data type.

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We use that, Colin, and then within this block of code we can say print.

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Is a function if I click run.

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Hmm, nothing happened.

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Well, we've created the say hello function, we've defined it, and now it's living somewhere in memory

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on our machine.

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However, in order to use a function, remember, just like we use the print function, we have to call

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it with the brackets.

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So after we define a function, we say, say hello.

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And did you notice how my Reppel as soon as I said, say.

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Actually gives me the say hello, comen, because I've created it and you see this purple box which

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shows that it's a function, well, we now have it available for us to use, just like we had the print

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function available for us to use.

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So if I do say hello and run it with the brackets and I click run.

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I get hello, how cool is that, by the way, what happens if I run it without the brackets and I click

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run?

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Nothing happens because, remember, in order for us to take an action, we have to let the interpreter

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know, hey, I want to run, say hello.

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Now, the reason functions are so powerful is because of the principle that we've talked about, right?

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The idea of dry, which stands for do not repeat yourself functions are really, really useful when

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you have things that you want to do over and over.

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For example, the brain function, we've used it a lot.

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Imagine if we had to code that ourselves every single time and say what print function does.

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Luckily, Python gives us a print because it's such a useful tool.

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But if in our program we want to say hello multiple times and you can imagine this actually being a

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lot more complicated, maybe 10 lines of code, instead of writing those ten lines of code over and

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over, I can just define it as a function and use it anywhere I want.

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In my program, for example, remember this, where we had a picture and we printed a Christmas tree.

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Imagine if we wanted to run this multiple times, well, in order to do that, I would copy and paste

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this code.

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And then added again, and if I click run.

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I now have two Christmas trees, but let's look at that, twenty six lines of code, I just copy and

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paste it the same thing over and over with a function.

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We can do something like this.

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I can say define show.

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Tre.

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Colin, and we now have a function, but remember, the indentation in Python, right indentation is

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important.

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We have the semicolon.

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We're defining a function.

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So we have to create that code block inside saying, hey, whatever is indented here, that's part of

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this function.

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So now that we have the show tree, check this out.

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I can say show tree, run it again.

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Maybe let's run it three times.

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If I click, run.

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How cool is that I'm able to do the same thing over and over by just calling this function and that's

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the power of functions.

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Functions allow us to keep our code dry.

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We don't repeat ourselves and reuse things that our machines can do over and over.

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And the beauty is that this stays a memory for us show tree now means something to this program because

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we've created it.

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So we have our own custom action that we can take.

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Now, what happens if I move show tree here to the top?

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Well, if I run this.

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I'll get an air name, air name, show three is not defined.

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Why is that?

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Well, because our interpreter goes line by line at first, says, all right, picture equals to this

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value.

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And then it goes to line eleven and says, run, show, tree.

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But we haven't defined show tree yet.

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So the python interpreter is going to air out and say, hey, I have no idea what show tree is.

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What are you talking about?

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Instead, with a function, we need to make sure that we define the functions at the beginning.

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So that Python interpreter says, all right, show tree now means this.

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I'm not going to run it.

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I'm not going to use it yet.

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I'm just going to keep it in memory.

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And when I finally come across show tree, I'll know that it means something.

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I'm going to grab it from memory using show tree, and then I'm going to run it using the brackets.

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For example, if I do print show tree without anything, without the brackets.

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If I click run.

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You see that I get function, Shokri, at this location, this is just the location and memory, this

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is the bookshelf where we store that show tree function very, very cool.

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And functions are an important, powerful concept in programming.

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And in the next video, we're going to extend this and explore this a little bit more.

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I'll see in the next one.