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[Narrator] I hope you're getting a sense that lists are very powerful.

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That by using mutation, by having lists that could contain other lists

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or any other kind of data we want,

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we can build very complex data structures, and we can do lots of interesting things.

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I'm going to introduce 1 more list operation,

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which will allow us to add a new element at the end of the list.

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We've seen that we can use lists to store complex data,

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that elements of the list can be any type we want including other lists,

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and we've seen that we can use mutation to change the value of a list,

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and that mutation is visible through any reference to the same list object.

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Now I'm going to introduce some other list operations.

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The first one is append, and append is like a procedure but it's a method,

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so we use it similar to the way we use to find on strings.

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We'll have a list first, then a dot followed by append,

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and what we pass in is the element we want to append to the list.

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Append will add a new element to the end of a list,

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and the important thing about append is that it's mutating

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the list that its invoked on.

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It's not creating a new list; it's mutating the old list.

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As an example of the use of append let's assume that instead of

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replacing curly in the 3 stooges, we want to end up with 4 stooges.

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We'll add Shemp and add the other 3 as they are.

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So what we want to do is to append Shemp at the end of the list we have.

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We would do that by invoking append on the stooges,

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passing in the string Shemp as the input.

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Here's what happens after the first assignment,

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the name stooges refers to the list containing the 3 elements,

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Moe, Larry, and Curly.

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When we invoke append it modifies that object,

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adding a new element to it.

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After the append, the list that stooges refers to now has 4 elements.

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We have not created a new list.

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Note that there's no assignment from the result of append.

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What we've done is modify the value that stooges refers to

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by adding a new element to it.