View Source Map (Elixir v1.12.3)

Maps are the "go to" key-value data structure in Elixir.

Maps can be created with the %{} syntax, and key-value pairs can be expressed as key => value:

iex> %{}
%{}
iex> %{"one" => :two, 3 => "four"}
%{3 => "four", "one" => :two}

Key-value pairs in a map do not follow any order (that's why the printed map in the example above has a different order than the map that was created).

Maps do not impose any restriction on the key type: anything can be a key in a map. As a key-value structure, maps do not allow duplicated keys. Keys are compared using the exact-equality operator (===/2). If colliding keys are defined in a map literal, the last one prevails.

When the key in a key-value pair is an atom, the key: value shorthand syntax can be used (as in many other special forms):

iex> %{a: 1, b: 2}
%{a: 1, b: 2}

If you want to mix the shorthand syntax with =>, the shorthand syntax must come at the end:

iex> %{"hello" => "world", a: 1, b: 2}
%{:a => 1, :b => 2, "hello" => "world"}

Keys in maps can be accessed through some of the functions in this module (such as Map.get/3 or Map.fetch/2) or through the map[] syntax provided by the Access module:

iex> map = %{a: 1, b: 2}
iex> Map.fetch(map, :a)
{:ok, 1}
iex> map[:b]
2
iex> map["non_existing_key"]
nil

To access atom keys, one may also use the map.key notation. Note that map.key will raise a KeyError if the map doesn't contain the key :key, compared to map[:key], that would return nil.

map = %{foo: "bar", baz: "bong"}
map.foo
#=> "bar"
map.non_existing_key
** (KeyError) key :non_existing_key not found in: %{baz: "bong", foo: "bar"}

Note: do not add parens when accessing fields, such as in data.key(). If parenthesis are used, Elixir will expect data to be an atom representing a module and attempt to call the function key/0 in it.

The two syntaxes for accessing keys reveal the dual nature of maps. The map[key] syntax is used for dynamically created maps that may have any key, of any type. map.key is used with maps that hold a predetermined set of atoms keys, which are expected to always be present. Structs, defined via defstruct/1, are one example of such "static maps", where the keys can also be checked during compile time.

Maps can be pattern matched on. When a map is on the left-hand side of a pattern match, it will match if the map on the right-hand side contains the keys on the left-hand side and their values match the ones on the left-hand side. This means that an empty map matches every map.

iex> %{} = %{foo: "bar"}
%{foo: "bar"}
iex> %{a: a} = %{:a => 1, "b" => 2, [:c, :e, :e] => 3}
iex> a
1

But this will raise a MatchError exception:

%{:c => 3} = %{:a => 1, 2 => :b}

Variables can be used as map keys both when writing map literals as well as when matching:

iex> n = 1
1
iex> %{n => :one}
%{1 => :one}
iex> %{^n => :one} = %{1 => :one, 2 => :two, 3 => :three}
%{1 => :one, 2 => :two, 3 => :three}

Maps also support a specific update syntax to update the value stored under existing atom keys:

iex> map = %{one: 1, two: 2}
iex> %{map | one: "one"}
%{one: "one", two: 2}

When a key that does not exist in the map is updated a KeyError exception will be raised:

%{map | three: 3}

The functions in this module that need to find a specific key work in logarithmic time. This means that the time it takes to find keys grows as the map grows, but it's not directly proportional to the map size. In comparison to finding an element in a list, it performs better because lists have a linear time complexity. Some functions, such as keys/1 and values/1, run in linear time because they need to get to every element in the map.

Maps also implement the Enumerable protocol, so many functions to work with maps are found in the Enum module. Additionally, the following functions for maps are found in Kernel:

Link to this section Summary

Functions

Deletes the entry in map for a specific key.

Drops the given keys from map.

Checks if two maps are equal.

Fetches the value for a specific key in the given map.

Fetches the value for a specific key in the given map, erroring out if map doesn't contain key.

Converts a struct to map.

Gets the value for a specific key in map.

Gets the value from key and updates it, all in one pass.

Gets the value from key and updates it, all in one pass. Raises if there is no key.

Gets the value for a specific key in map.

Returns whether the given key exists in the given map.

Returns all keys from map.

Merges two maps into one.

Merges two maps into one, resolving conflicts through the given fun.

Returns a new empty map.

Creates a map from an enumerable.

Creates a map from an enumerable via the given transformation function.

Removes the value associated with key in map and returns the value and the updated map.

Removes the value associated with key in map and returns the value and the updated map, or it raises if key is not present.

Lazily returns and removes the value associated with key in map.

Puts the given value under key in map.

Puts the given value under key unless the entry key already exists in map.

Evaluates fun and puts the result under key in map unless key is already present.

Puts a value under key only if the key already exists in map.

Puts a value under key only if the key already exists in map.

Takes all entries corresponding to the given keys in map and extracts them into a separate map.

Returns a new map with all the key-value pairs in map where the key is in keys.

Converts map to a list.

Updates the key in map with the given function.

Updates key with the given function.

Returns all values from map.

Link to this section Types

@type key() :: any()
@type value() :: any()

Link to this section Functions

@spec delete(map(), key()) :: map()

Deletes the entry in map for a specific key.

If the key does not exist, returns map unchanged.

Inlined by the compiler.

Examples

iex> Map.delete(%{a: 1, b: 2}, :a)
%{b: 2}
iex> Map.delete(%{b: 2}, :a)
%{b: 2}
@spec drop(map(), [key()]) :: map()

Drops the given keys from map.

If keys contains keys that are not in map, they're simply ignored.

Examples

iex> Map.drop(%{a: 1, b: 2, c: 3}, [:b, :d])
%{a: 1, c: 3}
@spec equal?(map(), map()) :: boolean()

Checks if two maps are equal.

Two maps are considered to be equal if they contain the same keys and those keys contain the same values.

Note this function exists for completeness so the Map and Keyword modules provide similar APIs. In practice, developers often compare maps using ==/2 or ===/2 directly.

Examples

iex> Map.equal?(%{a: 1, b: 2}, %{b: 2, a: 1})
true
iex> Map.equal?(%{a: 1, b: 2}, %{b: 1, a: 2})
false

Comparison between keys and values is done with ===/3, which means integers are not equivalent to floats:

iex> Map.equal?(%{a: 1.0}, %{a: 1})
false
@spec fetch(map(), key()) :: {:ok, value()} | :error

Fetches the value for a specific key in the given map.

If map contains the given key then its value is returned in the shape of {:ok, value}. If map doesn't contain key, :error is returned.

Inlined by the compiler.

Examples

iex> Map.fetch(%{a: 1}, :a)
{:ok, 1}
iex> Map.fetch(%{a: 1}, :b)
:error
@spec fetch!(map(), key()) :: value()

Fetches the value for a specific key in the given map, erroring out if map doesn't contain key.

If map contains key, the corresponding value is returned. If map doesn't contain key, a KeyError exception is raised.

Inlined by the compiler.

Examples

iex> Map.fetch!(%{a: 1}, :a)
1
@spec from_struct(atom() | struct()) :: map()

Converts a struct to map.

It accepts the struct module or a struct itself and simply removes the __struct__ field from the given struct or from a new struct generated from the given module.

Example

defmodule User do
  defstruct [:name]
end

Map.from_struct(User)
#=> %{name: nil}

Map.from_struct(%User{name: "john"})
#=> %{name: "john"}
Link to this function

get(map, key, default \\ nil)

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@spec get(map(), key(), value()) :: value()

Gets the value for a specific key in map.

If key is present in map then its value value is returned. Otherwise, default is returned.

If default is not provided, nil is used.

Examples

iex> Map.get(%{}, :a)
nil
iex> Map.get(%{a: 1}, :a)
1
iex> Map.get(%{a: 1}, :b)
nil
iex> Map.get(%{a: 1}, :b, 3)
3
Link to this function

get_and_update(map, key, fun)

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@spec get_and_update(
  map(),
  key(),
  (value() | nil -> {current_value, new_value :: value()} | :pop)
) :: {current_value, new_map :: map()}
when current_value: value()

Gets the value from key and updates it, all in one pass.

fun is called with the current value under key in map (or nil if key is not present in map) and must return a two-element tuple: the current value (the retrieved value, which can be operated on before being returned) and the new value to be stored under key in the resulting new map. fun may also return :pop, which means the current value shall be removed from map and returned (making this function behave like Map.pop(map, key)).

The returned value is a two-element tuple with the current value returned by fun and a new map with the updated value under key.

Examples

iex> Map.get_and_update(%{a: 1}, :a, fn current_value ->
...>   {current_value, "new value!"}
...> end)
{1, %{a: "new value!"}}

iex> Map.get_and_update(%{a: 1}, :b, fn current_value ->
...>   {current_value, "new value!"}
...> end)
{nil, %{a: 1, b: "new value!"}}

iex> Map.get_and_update(%{a: 1}, :a, fn _ -> :pop end)
{1, %{}}

iex> Map.get_and_update(%{a: 1}, :b, fn _ -> :pop end)
{nil, %{a: 1}}
Link to this function

get_and_update!(map, key, fun)

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@spec get_and_update!(
  map(),
  key(),
  (value() | nil -> {current_value, new_value :: value()} | :pop)
) :: {current_value, map()}
when current_value: value()

Gets the value from key and updates it, all in one pass. Raises if there is no key.

Behaves exactly like get_and_update/3, but raises a KeyError exception if key is not present in map.

Examples

iex> Map.get_and_update!(%{a: 1}, :a, fn current_value ->
...>   {current_value, "new value!"}
...> end)
{1, %{a: "new value!"}}

iex> Map.get_and_update!(%{a: 1}, :b, fn current_value ->
...>   {current_value, "new value!"}
...> end)
** (KeyError) key :b not found in: %{a: 1}

iex> Map.get_and_update!(%{a: 1}, :a, fn _ ->
...>   :pop
...> end)
{1, %{}}
@spec get_lazy(map(), key(), (() -> value())) :: value()

Gets the value for a specific key in map.

If key is present in map then its value value is returned. Otherwise, fun is evaluated and its result is returned.

This is useful if the default value is very expensive to calculate or generally difficult to setup and teardown again.

Examples

iex> map = %{a: 1}
iex> fun = fn ->
...>   # some expensive operation here
...>   13
...> end
iex> Map.get_lazy(map, :a, fun)
1
iex> Map.get_lazy(map, :b, fun)
13
@spec has_key?(map(), key()) :: boolean()

Returns whether the given key exists in the given map.

Inlined by the compiler.

Examples

iex> Map.has_key?(%{a: 1}, :a)
true
iex> Map.has_key?(%{a: 1}, :b)
false
@spec keys(map()) :: [key()]

Returns all keys from map.

Inlined by the compiler.

Examples

iex> Map.keys(%{a: 1, b: 2})
[:a, :b]
@spec merge(map(), map()) :: map()

Merges two maps into one.

All keys in map2 will be added to map1, overriding any existing one (i.e., the keys in map2 "have precedence" over the ones in map1).

If you have a struct and you would like to merge a set of keys into the struct, do not use this function, as it would merge all keys on the right side into the struct, even if the key is not part of the struct. Instead, use Kernel.struct/2.

Inlined by the compiler.

Examples

iex> Map.merge(%{a: 1, b: 2}, %{a: 3, d: 4})
%{a: 3, b: 2, d: 4}
@spec merge(map(), map(), (key(), value(), value() -> value())) :: map()

Merges two maps into one, resolving conflicts through the given fun.

All keys in map2 will be added to map1. The given function will be invoked when there are duplicate keys; its arguments are key (the duplicate key), value1 (the value of key in map1), and value2 (the value of key in map2). The value returned by fun is used as the value under key in the resulting map.

Examples

iex> Map.merge(%{a: 1, b: 2}, %{a: 3, d: 4}, fn _k, v1, v2 ->
...>   v1 + v2
...> end)
%{a: 4, b: 2, d: 4}
@spec new() :: map()

Returns a new empty map.

Examples

iex> Map.new()
%{}
@spec new(Enumerable.t()) :: map()

Creates a map from an enumerable.

Duplicated keys are removed; the latest one prevails.

Examples

iex> Map.new([{:b, 1}, {:a, 2}])
%{a: 2, b: 1}
iex> Map.new(a: 1, a: 2, a: 3)
%{a: 3}
Link to this function

new(enumerable, transform)

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@spec new(Enumerable.t(), (term() -> {key(), value()})) :: map()

Creates a map from an enumerable via the given transformation function.

Duplicated keys are removed; the latest one prevails.

Examples

iex> Map.new([:a, :b], fn x -> {x, x} end)
%{a: :a, b: :b}
Link to this function

pop(map, key, default \\ nil)

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@spec pop(map(), key(), default) :: {value(), updated_map :: map()} | {default, map()}
when default: value()

Removes the value associated with key in map and returns the value and the updated map.

If key is present in map, it returns {value, updated_map} where value is the value of the key and updated_map is the result of removing key from map. If key is not present in map, {default, map} is returned.

Examples

iex> Map.pop(%{a: 1}, :a)
{1, %{}}
iex> Map.pop(%{a: 1}, :b)
{nil, %{a: 1}}
iex> Map.pop(%{a: 1}, :b, 3)
{3, %{a: 1}}
Link to this function

pop!(map, key)

View Source (since 1.10.0)
@spec pop!(map(), key()) :: {value(), updated_map :: map()}

Removes the value associated with key in map and returns the value and the updated map, or it raises if key is not present.

Behaves the same as pop/3 but raises if key is not present in map.

Examples

iex> Map.pop!(%{a: 1}, :a)
{1, %{}}
iex> Map.pop!(%{a: 1, b: 2}, :a)
{1, %{b: 2}}
iex> Map.pop!(%{a: 1}, :b)
** (KeyError) key :b not found in: %{a: 1}
@spec pop_lazy(map(), key(), (() -> value())) :: {value(), map()}

Lazily returns and removes the value associated with key in map.

If key is present in map, it returns {value, new_map} where value is the value of the key and new_map is the result of removing key from map. If key is not present in map, {fun_result, map} is returned, where fun_result is the result of applying fun.

This is useful if the default value is very expensive to calculate or generally difficult to setup and teardown again.

Examples

iex> map = %{a: 1}
iex> fun = fn ->
...>   # some expensive operation here
...>   13
...> end
iex> Map.pop_lazy(map, :a, fun)
{1, %{}}
iex> Map.pop_lazy(map, :b, fun)
{13, %{a: 1}}
@spec put(map(), key(), value()) :: map()

Puts the given value under key in map.

Inlined by the compiler.

Examples

iex> Map.put(%{a: 1}, :b, 2)
%{a: 1, b: 2}
iex> Map.put(%{a: 1, b: 2}, :a, 3)
%{a: 3, b: 2}
Link to this function

put_new(map, key, value)

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@spec put_new(map(), key(), value()) :: map()

Puts the given value under key unless the entry key already exists in map.

Examples

iex> Map.put_new(%{a: 1}, :b, 2)
%{a: 1, b: 2}
iex> Map.put_new(%{a: 1, b: 2}, :a, 3)
%{a: 1, b: 2}
Link to this function

put_new_lazy(map, key, fun)

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@spec put_new_lazy(map(), key(), (() -> value())) :: map()

Evaluates fun and puts the result under key in map unless key is already present.

This function is useful in case you want to compute the value to put under key only if key is not already present, as for example, when the value is expensive to calculate or generally difficult to setup and teardown again.

Examples

iex> map = %{a: 1}
iex> fun = fn ->
...>   # some expensive operation here
...>   3
...> end
iex> Map.put_new_lazy(map, :a, fun)
%{a: 1}
iex> Map.put_new_lazy(map, :b, fun)
%{a: 1, b: 3}
Link to this function

replace(map, key, value)

View Source (since 1.11.0)
@spec replace(map(), key(), value()) :: map()

Puts a value under key only if the key already exists in map.

Examples

iex> Map.replace(%{a: 1, b: 2}, :a, 3)
%{a: 3, b: 2}

iex> Map.replace(%{a: 1}, :b, 2)
%{a: 1}
Link to this function

replace!(map, key, value)

View Source (since 1.5.0)
@spec replace!(map(), key(), value()) :: map()

Puts a value under key only if the key already exists in map.

If key is not present in map, a KeyError exception is raised.

Inlined by the compiler.

Examples

iex> Map.replace!(%{a: 1, b: 2}, :a, 3)
%{a: 3, b: 2}

iex> Map.replace!(%{a: 1}, :b, 2)
** (KeyError) key :b not found in: %{a: 1}
@spec split(map(), [key()]) :: {map(), map()}

Takes all entries corresponding to the given keys in map and extracts them into a separate map.

Returns a tuple with the new map and the old map with removed keys.

Keys for which there are no entries in map are ignored.

Examples

iex> Map.split(%{a: 1, b: 2, c: 3}, [:a, :c, :e])
{%{a: 1, c: 3}, %{b: 2}}
@spec take(map(), [key()]) :: map()

Returns a new map with all the key-value pairs in map where the key is in keys.

If keys contains keys that are not in map, they're simply ignored.

Examples

iex> Map.take(%{a: 1, b: 2, c: 3}, [:a, :c, :e])
%{a: 1, c: 3}
@spec to_list(map()) :: [{term(), term()}]

Converts map to a list.

Each key-value pair in the map is converted to a two-element tuple {key, value} in the resulting list.

Inlined by the compiler.

Examples

iex> Map.to_list(%{a: 1})
[a: 1]
iex> Map.to_list(%{1 => 2})
[{1, 2}]
Link to this function

update(map, key, default, fun)

View Source
@spec update(
  map(),
  key(),
  default :: value(),
  (existing_value :: value() -> new_value :: value())
) :: map()

Updates the key in map with the given function.

If key is present in map then the existing value is passed to fun and its result is used as the updated value of key. If key is not present in map, default is inserted as the value of key. The default value will not be passed through the update function.

Examples

iex> Map.update(%{a: 1}, :a, 13, fn existing_value -> existing_value * 2 end)
%{a: 2}
iex> Map.update(%{a: 1}, :b, 11, fn existing_value -> existing_value * 2 end)
%{a: 1, b: 11}
@spec update!(map(), key(), (existing_value :: value() -> new_value :: value())) ::
  map()

Updates key with the given function.

If key is present in map then the existing value is passed to fun and its result is used as the updated value of key. If key is not present in map, a KeyError exception is raised.

Examples

iex> Map.update!(%{a: 1}, :a, &(&1 * 2))
%{a: 2}

iex> Map.update!(%{a: 1}, :b, &(&1 * 2))
** (KeyError) key :b not found in: %{a: 1}
@spec values(map()) :: [value()]

Returns all values from map.

Inlined by the compiler.

Examples

iex> Map.values(%{a: 1, b: 2})
[1, 2]