🦉Keyword lists and maps

Keyword lists and maps are known as "associative data structures" in Elixir. Associative data structures are able to associate a key to a certain value. This is often known as hash maps or dictionaries in other languages. However, there are some minor differences between Elixir's associative data structures and the traditional hash map/dictionary.

Keyword lists

Keyword lists are a common data structure used to pass options to functions. Consider a function call with parameters to configure how the function works such as String.split/3:

iex(11)> String.split("hello world  another ", " ", trim: true)
["hello", "world", "another"]

We are providing the additional options via a keyword list. A keyword list toward the end of a function call can omit the use of the square brackets.

Keyword lists are really just lists with 2-item tuples where the first element is the key (an atom) and the second is any value that corresponds to that key.

You can create keyword lists in two ways:

iex(12)> x = [a: 0, a: 1, b: 2]
[a: 0, a: 1, b: 2]
iex(15)> [{:a, 0}, {:a, 1}, {:b, 2}]
[a: 0, a: 1, b: 2]

Note that keyword lists allow duplicate keys (unlike hash maps).

Accessing elements

When accessing a keyword list, square brackets can be used (much like dictionaries in Python):

iex(16)> x[:b]
2

Note that since keyword lists support duplicate keys, using square brackets on a duplicate key retrieves the first instance:

iex(17)> x[:a]
0

Comparing keyword lists

Another key property of keyword lists is that the keys must be ordered (as defined by the developer, YOU!). This means that [a: 1, b: 2] != [b: 2, a: 1]. Thus, while keyword lists allow you to specify a select number of keywords for the function, it is impractical to set "expectations" on the form that these keywords lists may take.

The Keyword module contains more helper functions to manipulate and interact with keyword lists.

Key properties of keyword lists:

1. Keyword lists must use atoms as keys 2. Keyword lists are order dependent 3. Keyword lists support duplicate keys

Given the order dependence of keyword lists, it is not recommended to use Pattern matching with it. More on this will be covered below.

Maps

Maps are the closer equivalent to hash maps and dictionaries in other languages. Maps essentially store key-value pairs and are defined using %{}:

iex(18)> m = %{:a => 1, "hello" => "world", 2 => :c}
%{2 => :c, :a => 1, "hello" => "world"}

From the above example, you can see that maps are not order dependent nor do they require atoms as keys (in comparison to keyword lists).

Accessing elements

To access elements in a map, the square bracket notation can be used (much like keyword lists):

iex(19)> m[:a]
1
iex(20)> m[2]
:c

Additionally, if a key is an atom, the . operator can also be used (similar to how object access works in Javascript):

iex(21)> m.a
1

There is also a very useful syntax for updating keys in a map via %{map | key: new_value}.

Similar to the Keyword module, the Map module exists to provide extra utility when working with maps.

Maps are the perfect structure to use with Pattern matching as the fields are not order dependent. More on this will be covered below.