About

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}]

Originally from Exercism elixir concepts

Lesson Overview

About