Maps and Keyword Lists

Syntax#

map = %{} // creates an empty map
map = %{:a => 1, :b => 2} // creates a non-empty map
list = [] // creates an empty list
list = [{:a, 1}, {:b, 2}] // creates a non-empty keyword list

Remarks#

Elixir provides two associative data structures: maps and keyword lists.

Maps are the Elixir key-value (also called dictionary or hash in other languages) type.

Keyword lists are tuples of key/value that associate a value to a certain key. They are generally used as options for a function call.

Creating a Map

Maps are the Elixir key-value (also called dictionary or hash in other languages) type. You create a map using the %w{} syntax:

%{} // creates an empty map
%{:a => 1, :b => 2} // creates a non-empty map

Keys and values can use be any type:

%{"a" => 1, "b" => 2}
%{1 => "a", 2 => "b"}

Moreover, you can have maps with mixed types for both keys and values”:

// keys are integer or strings
%{1 => "a", "b" => :foo}
// values are string or nil
%{1 => "a", 2 => nil}

When all the keys in a map are atoms, you can use the keyword syntax for convenience:

%{a: 1, b: 2}

Creating a Keyword List

Keyword lists are tuples of key/value, generally used as options for a function call.

[{:a, 1}, {:b, 2}] // creates a non-empty keyword list

Keyword lists can have the same key repeated more than once.

[{:a, 1}, {:a, 2}, {:b, 2}]
[{:a, 1}, {:b, 2}, {:a, 2}]

Keys and values can be any type:

[{"a", 1}, {:a, 2}, {2, "b"}]

Difference between Maps and Keyword Lists

Maps and keyword lists have different application. For instance, a map cannot have two keys with the same value and it’s not ordered. Conversely, a Keyword list can be a little bit hard to use in pattern matching in some cases.

Here’s a few use cases for maps vs keyword lists.

Use keyword lists when:

you need the elements to be ordered
you need more than one element with the same key

Use maps when:

you want to pattern-match against some keys/values
you don’t need more than one element with the same key
whenever you don’t explicitly need a keyword list