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:
[1, "banana", true]
Here, we’ve created a list of three elements. Notice that the elements don’t have to be the same type.
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:
var trees = ["cedar", "birch", "oak", "willow"] System.print(trees[0]) //> cedar System.print(trees[1]) //> birch
Negative indices counts backwards from the end:
System.print(trees[-1]) //> willow System.print(trees[-2]) //> oak
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:
System.print(trees.count) //> 4
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:
System.print(trees[1..2]) //> [birch, oak]
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:
trees[0..-1]
Lists are mutable, meaning their contents can be changed. You can swap out an existing element in the list using the subscript setter:
trees[1] = "spruce" System.print(trees[1]) //> spruce
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:
trees.add("maple")
System.print(trees.count) //> 5
You can insert a new element at a specific position using insert:
trees.insert(2, "hickory")
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:
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]
Lists have the ability to be added together via the + operator. This is often known as concatenation.
var letters = ["a", "b", "c"] var other = ["d", "e", "f"] var combined = letters + other System.print(combined) //> [a, b, c, d, e, f]
The opposite of insert is removeAt. It removes a single element from a
given position in the list.
To remove a specific value instead, use remove. The first value that
matches using regular equality will be removed.
In both cases, all following items are shifted up to fill in the gap.
var letters = ["a", "b", "c", "d"]
letters.removeAt(1)
System.print(letters) //> [a, c, d]
letters.remove("a")
System.print(letters) //> [c, d]
Both the remove and removeAt method return the removed item:
System.print(letters.removeAt(1)) //> c
If remove couldn’t find the value in the list, it returns null:
System.print(letters.remove("not found")) //> null
If you want to remove everything from the list, you can clear it:
trees.clear() System.print(trees) //> []