wren/doc/site/lists.markdown

^title Lists

A list is a compound object that holds a collection of elements identified by integer index. You can create a list by placing a sequence of comma-separated expressions inside square brackets:

:::wren
[1, "banana", true]

Here, we've created a list of three elements. Notice that the elements don't have to be the same type.

Accessing elements

You can access an element from a list by calling the subscript operator on it with the index of the element you want. Like most languages, indexes start at zero:

:::wren
var hirsute = ["sideburns", "porkchops", "'stache", "goatee"]
System.print(hirsute[0]) //> sideburns
System.print(hirsute[1]) //> porkchops

Negative indices counts backwards from the end:

:::wren
System.print(hirsute[-1]) //> goatee
System.print(hirsute[-2]) //> 'stache

It's a runtime error to pass an index outside of the bounds of the list. If you don't know what those bounds are, you can find out using count:

:::wren
System.print(hirsute.count) //> 4

Slices and ranges

Sometimes you want to copy a chunk of elements from a list. You can do that by passing a range to the subscript operator, like so:

:::wren
System.print(hirsute[1..2]) //> [porkchops, 'stache]

This returns a new list containing the elements of the original list whose indices are within the given range. Both inclusive and exclusive ranges work and do what you expect.

Negative bounds also work like they do when passing a single number, so to copy a list, you can just do:

:::wren
hirsute[0..-1]

Adding elements

Lists are mutable, meaning their contents can be changed. You can swap out an existing element in the list using the subscript setter:

:::wren
hirsute[1] = "muttonchops"
System.print(hirsute[1]) //> muttonchops

It's an error to set an element that's out of bounds. To grow a list, you can use add to append a single item to the end:

:::wren
hirsute.add("goatee")
System.print(hirsute.count) //> 5

You can insert a new element at a specific position using insert:

:::wren
hirsute.insert(2, "soul patch")

The first argument is the index to insert at, and the second is the value to insert. All elements following the inserted one will be pushed down to make room for it.

It's valid to "insert" after the last element in the list, but only right after it. Like other methods, you can use a negative index to count from the back. Doing so counts back from the size of the list after it's grown by one:

:::wren
var letters = ["a", "b", "c"]
letters.insert(3, "d")   // OK: inserts at end.
System.print(letters)    //> [a, b, c, d]
letters.insert(-2, "e")  // Counts back from size after insert.
System.print(letters)    //> [a, b, c, e, d]

Adding lists together

Lists have the ability to be added together via the + operator. This is often known as concatenation.

:::wren
var letters = ["a", "b", "c"]
var other = ["d", "e", "f"]
var combined = letters + other
System.print(combined)  //> [a, b, c, d, e, f]

Removing elements

The opposite of insert is removeAt. It removes a single element from a given position in the list. All following items are shifted up to fill in the gap:

:::wren
var letters = ["a", "b", "c", "d"]
letters.removeAt(1)
System.print(letters) //> [a, c, d]

The removeAt method returns the removed item:

:::wren
System.print(letters.removeAt(1)) //> c

If you want to remove everything from the list, you can clear it:

:::wren
hirsute.clear()
System.print(hirsute) //> []

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Maps → ← Values