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‫Hello and welcome.

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‫In this lecture, we're going to be learning how we can remove execution pins from our own functions

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‫and how and when it's a good idea to do this.

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‫Let's dive in.

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‫So it's a little bit weird that our has ammo function now has execution pins when previously we didn't

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‫need that and that's kind of made our code a little bit worse.

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‫So what is that?

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‫Why has that happened and why do some function nodes require execution pins and other ones don't?

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‫Well, it all comes down to something called a side effect.

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‫So what is a side effect?

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‫Essentially, a side effect is when a function has some sort of observable effect after it has been

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‫run.

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‫Now, what do I mean by this?

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‫Let's have a look at some examples of functions with side effects so that you can understand prints,

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‫print, string.

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‫After you have run print string.

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‫It is quite clear that something has happened because there is now a string on your screen.

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‫You can see that there is a string add impulse that clearly has an observable effect because suddenly

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‫the thing that you've added impulse to goes flying off into the distance.

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‫Set ammo has an observable effect because after you've set ammo, the value in that variable has changed.

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‫Now at this point, you might be wondering, well, surely every function has an observable effect.

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‫Otherwise, what is the point of it?

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‫Well, not quite.

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‫There are such things as pure functions.

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‫What is a pure function?

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‫It's basically a function with no side effects.

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‫However, it does have return values.

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‫So there are values coming out of that function.

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‫And that is the entire purpose of the function is to calculate and give us those values.

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‫So again, let's have a look at some examples because this will make it a lot clearer.

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‫Get ammo.

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‫That is a pure function.

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‫It has no side effects because you can get ammo as many times as you like.

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‫And unless you do something with the value that you got out of it, your program, your game is not

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‫going to be any different.

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‫You can just keep getting ammo all the time, all all the way through your game and nothing is going

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‫to be observable different about your game.

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‫Get active forward vector.

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‫That's another one where you could get it as many times as you like.

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‫It's not going to change anything until you do something with the value returned and all these mathematical

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‫functions like multiply minus greater.

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‫All of these nodes don't have an effect on your project until their results are used for some greater

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‫purpose.

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‫So these are called pure functions, and you can probably see that these are exactly the nodes that

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‫didn't have any execution pins because they don't need them.

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‫It doesn't matter when I run get ammo or whether I run it 50 times or one time, it's not going to matter

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‫because it's got no observable side effects.

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‫So how can we indicate to blueprint that a function is pure?

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‫Well, let's have a look at my test function here.

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‫So I'm going to select it in the event graph or over in the my blueprint pane.

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‫And you can see that in the details pane there is a checkbox labeled pure.

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‫So if you check that checkbox, notice the execution pins just disappeared.

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‫And all we've got now are the input and outputs of this function.

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‫Now this function will get run whenever we need the output.

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‫So if I plug this output into a print string, then it's going to run this function before the print

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‫string so that it has the result in time to execute the print string.

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‫We can do the same with any function where there are no side effects, where there are no observable

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‫effects such as our has ammo function.

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‫So I'd like to challenge you to make has ammo into a pure function.

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‫So select it and set that checkbox to make it pure.

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‫And what happens to the execution flow now.

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‫What currently it was hooked up through the function.

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‫Where does it go?

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‫Pause the video and have a go.

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‫Okay.

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‫Welcome back.

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‫So let's select has ammo.

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‫We'll check the pure check box and instantly you see that the execution pins disappear and the execution

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‫flow that previously flowed through has ammo is now going directly on to our branch statement, which

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‫is exactly what we want now has ammo is just going to be run as soon as the branch statement is run

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‫to get hold of the return value.

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‫Now a word of caution only use this pure checkbox when you are sure that a function has no side effects.

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‫Common cases and use cases for this are when your function is simply trying to calculate a value such

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‫as has ammo.

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‫And that is why this function doesn't follow the convention of having a verb naming because it isn't

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‫doing something, it is getting something or it is answering a question.

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‫In this case, the question is has ammo.

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‫And in fact, we could even put a little question mark in our function, naming something that you can

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‫do in Blueprint that you can't do in C++, which allows us to see that, yes, this function is answering

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‫a question.

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‫It isn't trying to do something.

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‫So often the distinction between pure and non pure functions are whether it fits with a verb or whether

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‫it fits with a question.

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‫So to review, let's have a look at our glossary.

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‫A side effect is an observable effect of a function we've seen and we've seen examples of those functions.

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‫And a pure function is a function with no side effects, such as these getting functions or question

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‫answering functions.

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‫That's it for this lecture.

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‫I'll see you in the next one.