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All right.

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I hope you're having fun.

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So far in the realm of type annotation, in this lecture, we're going to take a look at generics.

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What is a generic?

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Well, a generic is actually a type parameter that can be replaced by another type during the runtime

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of a script.

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Think of how we can create parameters for functions, and those parameters can represent certain values

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well.

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We can also define parameters for types as well and override those parameters to represent a specific

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type.

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This is incredibly useful when we need to create some custom types, and we don't want to have to create

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a new type for every single thing that could exist in, for example, a table like creating a type that

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represents a table containing strings.

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Then we have to create a type representing a table containing numbers, and so on.

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So how do we create a generic or a type parameter.

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Well I'm going to create a new type.

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I'm going to call it array.

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And before I do anything else I'm going to put a less than and then a greater than symbol.

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And between these two symbols is where we can define type parameters.

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For our custom type I can name the parameters whatever I'd like, just like I can with parameters and

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functions.

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So for simplicity's sake I'm going to name my parameter t t for type.

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And then I can set this type equal to a table.

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And since this table is going to represent an array arrays are ordered by numbers or indexes.

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So I'll type in number here for the index.

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And what's going to be stored at this index is going to be the type T.

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I don't know what type T is going to be, but that's where we're going to use this type later on and

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define what T is going to be during runtime of our script.

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Now, I don't have to go and define types for every single type of arrays, like string arrays, number

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arrays, and so on.

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Instead, I can make a single type and represent whatever type is going to be stored here as a parameter.

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And this is my generic.

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So that means I can create an array like a string array and set it to the type of array.

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And then I'll put the less than and greater than symbol.

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And in here I can define the type.

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So for example in this table I want to store strings.

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So I'll say string.

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And then I can set it equal to a table.

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And in here I can store all my strings like hello and hi.

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And as you can see we don't get any red underlines by Roblox Studio or what they call the linter.

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So all I did is that I replaced our placeholder T with the type string.

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Pretty neat.

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We can also do the same thing with numbers.

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So I can create a number array, set it to the type of array.

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And this one is going to be a number.

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And now I can set it to a table that contains numbers like one, two and three.

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So as you can see it's basically no different than parameters we have in functions.

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But it doesn't stop there.

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In fact, we can define multiple type parameters for a custom type.

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So as an example I'm going to create another new type.

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I'm going to call it pair.

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And in here I'm going to define two type placeholders.

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I'm just going to call one T and I'll call the other one u.

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And then I can set it equal to a table.

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And let's say I want this table to have two key value pairs.

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One will just be called first.

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And this one's going to store the value of t.

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And then I'll have another key value pair I'll call it second.

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And this one's going to store the value of u.

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And I'm going to create a variable.

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I'll just call it sum pair.

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And I'll set the type equal to our custom pair type.

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And I'll override the first parameter with the type let's say string.

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And then for the second type I can do number.

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And now I can set it equal to a table.

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And for the first key value pair I'll just store a string like.

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Hello.

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And then for the second key value pair, I'll just store a number like five.

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And there we go.

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We've overridden these two parameters here with string and number.

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It's that simple.

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Now generics actually even go beyond creating custom types.

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We can actually use generics in functions too.

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And they're called generic functions.

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A generic function is when we define type parameters for the function itself that it can work with.

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The purpose of doing this is to reduce the copying and pasting of our code by creating universal functions

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that can work with multiple data types.

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We don't have to define a function to work with each specific data type.

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So to create a generic function, I'm going to create a function just like I regularly would.

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I'm going to call this function in particular reverse.

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And before I do anything else, I'm going to again put the less than and greater than symbol for this

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function.

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And in here we're going to define again a parameter for the type that this function is going to contain.

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And I'm going to call it t.

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Again for simplicity's sake.

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And then we can put our parentheses.

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And in here I want to let's say I want to pass an array to this function.

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So I'll just call the parameter array.

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And it's going to be of the type table.

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And this table is going to contain types of type t.

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So that's what our T is going to represent here.

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And what I want to do is I want to take all the values in this array and reverse them.

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So whatever is in the front is last and that kind of thing.

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So I can return a table back from this function that also contains all of those types of type T.

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So lots of teas here.

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So basically what I'm going to do is I'll create a table, I'm going to call it reversed, set it to

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the type of a table with tea inside of it, and then set it equal to an empty table.

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And then what I could do is I could loop through every single value that is in my array table.

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So I could do for I is equal to the number of elements in my array.

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So we're going to start from the top.

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And we're going to go to the last item which is at index one.

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And that means we're going to have to decrement by one each time through this for loop.

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So if I pass a table with five indexes in it, it'll start at five and go all the way down to one.

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And then I can insert those values into my reverse table.

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By referencing my array table and getting the value at that specific index.

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And then at the very end I can just return this reverse table.

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So if I pass an array that contain numbers like one, two, three and four, then it'll go start at

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four and then it'll return to me a table that goes four, three, two, one.

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So it's just reversing it.

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And that means I don't have to explicitly define the types that are going to be stored in the array

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that's passed to this function.

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This array could consist of types of strings, could be maybe booleans numbers I don't know.

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All I do know is that this function's main purpose is just to reverse whatever's in an array.

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It doesn't really have to worry about the type, which is why we've set it to be generic.

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This is a generic function, so now we can use this function to our advantage.

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I can create a variable like number array.

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And I can set it to be equal to a table that contains a bunch of numbers.

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And it's going to be returned from the reverse function.

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And then I can pass an array to the reverse function like one, two and three.

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And this function will return back to me an array that has these numbers flipped.

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And again I can do the same thing for strings.

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I can have a string array, and we'll set it to a table that contains strings.

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And we'll set it equal to the reverse function.

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And I'll just pass another array to it that has strings like hi, we could do hello and howdy.

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And it should return back to me a table that says howdy, hello and hi.

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And we can actually confirm this by just printing out number array and then printing out string array

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into the console.

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So if we go and run the game, we should expect that our tables get flipped.

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So if we go ahead and take a look here.

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We see, we get three, two and one.

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See our values have been flipped in order.

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And then for the strings we get howdy, hello and hi instead of hi, hello and howdy.

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And that's the beauty of our generic function in action.

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But guess what?

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Generic functions aren't also limited to only one type.

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We can supply multiple different type parameters to our generic functions.

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So let's make another example.

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I'm going to create a new type.

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I'll call this one pair two.

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And this pair is going to have two type parameters.

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I'll just call one t and we'll call the other one u.

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And it's going to be equal to a table that contains, let's say the t is going to represent the key

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for this table.

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And then the value or the type of u is going to be stored at this key.

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So we're going to have a nice little dictionary here.

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And now using this custom type what I could do is I could create a function I'll call it map and I'll

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define that this generic function will have a t and a u.

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Again you can name these whatever you'd like.

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I'm just doing t and u for simplicity's sake.

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And this function can take an array that contains a bunch of values of type t.

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And then this function will also require us to pass another function to it.

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I'll just call it function or Funct.

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And this function takes that type of t and it returns back to us the type of u.

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So we want this function to take in t and return this u type.

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And once this is finally done, we can say that this function returns a type of pair two using the t

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and u that we've defined here for this function.

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So this is looking a little confusing, but don't worry we're going to work through it.

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So I'm going to create a variable called result.

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Set it equal to pair two with the t and u parameters that we define and set it equal to an empty table

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for now.

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And then I can loop through every single value in the array that gets passed to this function.

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So for every single value in Ipairs array, what I'll do is I will set the result table's key of v equal

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to um, the function that gets passed the return from the function that gets passed to this map function.

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So we'll do function and pass v to it.

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So v is representing our type t.

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And when we pass the type t to our function we should get returned back to us a type U which is being

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stored in our table.

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So again we're having a dictionary storing a bunch of keys with the type T with the values of type u,

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and once that's done looping then we can return result back.

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And I'll show you what we can do with this function.

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I'll create another one.

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I'm just going to call this function double.

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And all it's going to do is take a number and return that number back.

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But it's doubled.

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So we can just return x times two.

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And now what I could do is I could create a variable called num result.

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And I could set it to be of the type pair two.

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And it's going to store, I don't know, we'll just do a number for the key and then a number for the

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value.

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And then I can set it equal to the map function.

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And I'll pass to the map function an array that contains numbers one, two and three.

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And then I can pass a reference to the double function.

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So what's going to happen here is that when this function gets called, it's going to get passed this

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table one, two and three.

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And it's going to see okay this is the type of number.

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It's going to loop through every single number in our array and pass it to our function.

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In this case that's the double function.

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And it's going to return to us a doubling of that number.

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So it's still going to be of the type number.

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But you could have this function return whatever you like depending on what gets passed to the function.

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And then after that's done, it takes that value and stores it in our result table.

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And then we get it returned back to us.

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So this numbers all table should be storing the doubling.

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That's going to be at the first index which is one times two which is two, the doubling of two which

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is four.

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And then the doubling of three which is six.

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And we can also do the same for like maybe we wanted to do something with a string so we could do something

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called string result set it equal to a pair two.

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That'll just be a string and a string.

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We'll just do for this example.

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And we'll set it equal to the map function.

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And we'll pass an array that contains some strings like foo.

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We'll do bar and then baz.

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And then I'll actually create a lambda function in here.

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And this function is going to take a string and it's going to return back.

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And we're going to use a function from the string library called string dot rep, which means repeat

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returns a string that is the concatenation of n copies of the string.

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So we can just pass the string that gets passed to this lambda function and duplicate it three times.

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So for example, if we pass foo, it'll be foo foo foo three times of this string and then put n there.

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So now when the map function gets called on this information, we pass to it.

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It's going to loop through every single string.

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And it's going to call this lambda function.

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And it's going to pass our string to it.

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So for example it'll pass foo to this lambda function.

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And then it'll take that foo string and put it in the string dot repeat function.

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It's going to repeat that string three times and return it back to where the function was called, which

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gets stored in the result table.

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That means this string result table should be stored in a bunch of key value pairs, where foo will

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be foo foo, foo, bar will be bar bar bar, and baz will be baz basbas.

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So we can print out num results and string result to confirm this.

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And let's go ahead and run our game.

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All right, let's take a look.

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Okay.

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First index we get two, which is the doubling of one.

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At two we get four which is the doubling of two.

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And at three we get the doubling of three which is six.

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And then for the strings.

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There we go.

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What's stored at bah is bah bah bah bah is bah bah bah and foo is foo foo foo.

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So this is how you can use generics in Roblox Lua.

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You remember that I always attach the code to the lecture, and I also add in some extra comments in

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that code to explain things further.

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If you didn't fully understand everything here in this video, and don't worry if you're not fully understanding

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all of this.

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But anyways, hopefully you found all the information in this lecture interesting and useful.

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I don't expect you to use generics that often, but there are times when you may want to use generics

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to improve the readability and cohesion of your code.

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See you in the next lecture.