Lists

In this chapter we will learn two of the most used collection data-types in Elixir: lists and tuples. Let’s start with lists.

Elixir uses square brackets to specify a list of values. Values can be of any type, so you can mix different types within one list.

A list of various elements:

# 💡 Try nesting another list within the list.
[1, true, 3, "hello"]

Getting the length of a list:

length([1, 2, 3])

Accessing List Elements

Throughout the tutorial, we will talk a lot about the head and tail of a list. The head is the first element of a list and the tail is the remainder of the list. They can be retrieved with the functions hd and tl.

Head of the list:

# 💡 Try changing the list to an empty list.
hd([1, 2, 3])

Tail of the list:

tl([1, 2, 3])

Prepending to list

Lists are stored in memory as linked lists, meaning that each element in a list holds its value and points to the following element until the end of the list is reached. Therefore, prepending to a list is a fast (constant time) operation, while appending requires traversing entire list. That’s why prepending an element is a common operation in Elixir and there’s a special syntax for it: [element | list]

Prepending an element:

# 💡 Try expressing it using the `++` operator.
list = [1, 2, 3]
[0 | list]

A list constructed only with [head | tail] syntax:

[0 | [1 | [2 | [3 | []]]]]

List Operations in Elixir

Two lists can be concatenated or subtracted using the ++ and -- operators respectively.

List operators never modify the existing list. Concatenating to or removing elements from a list returns a new list. We say that Elixir data structures are immutable.

One advantage of immutability is that it leads to clearer code. You can freely pass the data around with the guarantee no one will mutate it in memory - only transform it.

Concatenating lists

list ++ [4, 5, 6]

# 💡 Let's check `list` after concatenating. Did it change?
list

Subtracting lists:

[1, true, 2, false, 3, true] -- [true, false]

Charlists

When Elixir sees a list of printable ASCII numbers, it will print that as a charlist (a list of characters). That’s for better interoperability with Erlang, where strings are represented as charlists instead of binaries.

~c"" syntax lets you create a charlist similarly to strings.

Charlist with control characters:

# 💡 Try appending `0` to the list.
[11, 12, 13]

Charlist representing “hello”:

[104, 101, 108, 108, 111]

Using the ~c"" syntax:

~c"hello"