Maps

Maps in Elixir provide a quick key-value data store. Maps can be created by using the %{} syntax:

iex> map = %{:a => 1, 2 => :b}
%{2 => :b, :a => 1}
iex> map[:a]
1
iex> map[2]
:b
iex> map[:c]
nil

We can use the . syntax to access a specific key in the map:

iex> map.a
1

But not for an element that doesn’t exist…

iex> map.c
** (KeyError) key :c not found in: %{2 => :b, :a => 1}

In comparison to keyword lists, maps do not follow any ordering and allow any value to be set as a key. However, it’s recommended to stick to literals such as strings, atoms, and numbers.

Nested data structures

Elixir provides a way for us to hold variables inside of maps. We’ll use this a lot in the Phoenix framework, where we’ll have variables that represent data structures in our database.

For example, we can create a list of users that is a key-value map to another nested map:

iex> users = [
  john: %{name: "John", age: 27, languages: ["Erlang", "Ruby", "Elixir"]},
  mary: %{name: "Mary", age: 29, languages: ["Elixir", "F#", "Clojure"]}
]

We can then get variables using [] (dynamic access) and . (strict access) syntax:

iex> users[:john].age
27

We can then manipulate variables by using the put_in/2 function to create a new value:

iex> users = put_in users[:john].age, 31
iex> users
[john: %{age: 31, languages: ["Erlang", "Ruby", "Elixir"], name: "John"},
 mary: %{age: 29, languages: ["Elixir", "F#", "Clojure"], name: "Mary"}]

Check out the Kernel module documentation for other available functions when using nested data structures.

Executing scripts

So far we’ve used IEx to execute Elixir code in real-time which is convenient for small, one-line expressions. Rather than typing in the REPL, we can write our Elixir code to a file and execute it as a script using the elixir command:

$ echo "IO.puts \"Hello world\"" > test.exs
$ elixir test.exs
Hello world