Maps in

About Maps

Maps are a data structure that holds key-value pairs.

• Keys can be of any type, but must be unique.

• Values can be of any type, they do not have to be unique.

• Maps do not guarantee the order of their contents despite appearing to do so.

  • Their underlying implementation gives this misperception:
    • At small sized (<=32 entries), they are implemented as an ordered Keyword list.
    • At larger sizes (>32 entries), they are implemented as a hash array mapped trie [1].

• Maps can be declared with a literal form:

  # An empty map
  %{}
  
  # A map with the atom key :a associated to the integer value 1
  %{a: 1}
  
  # A map with the string key "a" associated to the float value 2.0
  %{"a" => 2.0}
  
  # A map with the map key %{} with the list value [1 ,2, 3]
  %{%{} => [1, 2, 3]}
  
  # A map with keys of different types
  %{:a => 1, "b" => 2}
  

• Maps can also be instantiated using Map.new from the Map module. This might be used if you already have an enumerable collection to turn into a list:

  kw_list = [a: 1, b: 2]
  Map.new(kw_list)
  # => %{a: 1, b: 2}
  

• Values in a map can be accessed in many different ways:

  my_map = %{key: "value"}
  
  # with a dot if the key is an atom
  my_map.key
  # => "value"
  
  # with [], a syntax provided by the Access behaviour
  my_map[:key]
  # => "value"
  
  # with pattern matching
  %{key: x} = my_map
  x
  # => "value"
  
  # with Map.get/2
  Map.get(my_map, :key)
  # => "value"
  

• The Map module, included with the standard library, has many useful functions for using maps.

  Map.delete(%{a: 2, b: 3}, :a)
  # => %{b: 3}
  

• Maps implement the Enumerable protocol, allowing use of Enum module functions.

• Anonymous functions or captured function references are often required as arguments for Map and Enum module functions

  # Increment the value by one, if it is not found, update it with 0
  Map.update(%{a: 1}, :a, 0, &(&1 + 1))
  # => %{a: 2}
  
  # Sorting by a specific sub-value
  list = [{"A", 4}, {"B", 3}, {"C", 2}, {"D", 1}]
  Enum.sort_by(list, &Kernel.elem(&1, 1))
  # => [{"D", 1}, {"C", 2}, {"B", 3}, {"A", 4}]