View Source Map (Elixir v1.13.0)
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"]
nilTo 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 expectdatato be an atom representing a module and attempt to call the functionkey/0in 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
1But 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.
Returns a map containing only those pairs from map
for which fun returns a truthy value.
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.
Maps the function fun over all key-value pairs in map, returning a map
with all the values replaced with the result of the function.
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.
Returns map excluding the pairs from map for which fun returns a truthy value.
Map.reject/2 is faster than using map |> Enum.reject(fun) |> Enum.into(%{}),
as no intermediate list is being built.
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
Link to this section Functions
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}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}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})
falseComparison between keys and values is done with ===/3,
which means integers are not equivalent to floats:
iex> Map.equal?(%{a: 1.0}, %{a: 1})
falseFetches 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)
:errorFetches 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)
1Returns a map containing only those pairs from map
for which fun returns a truthy value.
fun receives the key and value of each of the
elements in the map as a key-value pair.
Map.filter/2 is faster than using map |> Enum.filter(fun) |> Enum.into(%{}),
as no intermediate list is being built.
See also reject/2 which discards all elements where the
function returns a truthy value.
Examples
iex> Map.filter(%{one: 1, two: 2, three: 3}, fn {_key, val} -> rem(val, 2) == 1 end)
%{one: 1, three: 3}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"}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@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}}@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, %{}}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)
13Returns 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)
falseReturns all keys from map.
Inlined by the compiler.
Examples
iex> Map.keys(%{a: 1, b: 2})
[:a, :b]Maps the function fun over all key-value pairs in map, returning a map
with all the values replaced with the result of the function.
Examples
iex> Map.map(%{1 => "joe", 2 => "mike", 3 => "robert"}, fn {_key, val} -> String.capitalize(val) end)
%{1 => "Joe", 2 => "Mike", 3 => "Robert"}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}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}@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}@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}}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}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}}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}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}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}Returns map excluding the pairs from map for which fun returns a truthy value.
Map.reject/2 is faster than using map |> Enum.reject(fun) |> Enum.into(%{}),
as no intermediate list is being built.
See also filter/2.
Examples
iex> Map.reject(%{one: 1, two: 2, three: 3}, fn {_key, val} -> rem(val, 2) == 1 end)
%{two: 2}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}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}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}}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}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}]@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}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}Returns all values from map.
Inlined by the compiler.
Examples
iex> Map.values(%{a: 1, b: 2})
[1, 2]