Elixir v1.3.4 Map View Source
A set of functions for working with maps.
Maps are key-value stores where keys can be any value and
are compared using the match operator (===
). Maps can be
created with the %{}
special form defined in the
Kernel.SpecialForms
module.
Link to this section Summary
Functions
Deletes the entries 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
and returns it in a tuple
Fetches the value for specific key
Converts a struct
to map
Gets the value for a specific key
Gets the value from key
and updates it, all in one pass
Gets the value from key
and updates it. Raises if there is no key
Gets the value for a specific key
Returns whether a given key
exists in the given map
Returns all keys from map
Merges two maps into one
Merges two maps into one
Returns a new empty map
Creates a map from an enumerable
Creates a map from an enumerable
via the transformation function
Returns and removes the value associated with key
in map
Lazily returns and removes the value associated with key
in map
Puts the given value
under key
Puts the given value
under key
unless the entry key
already exists
Evaluates fun
and puts the result under key
in map unless key
is already present
Takes all entries corresponding to the given keys
and extracts them into a
separate map
Takes all entries corresponding to the given keys and returns them in a new map
Converts map
to a list
Updates the key
in map
with the given function
Updates the key
with the given function
Returns all values from map
Link to this section Types
Link to this section Functions
Deletes the entries in map
for a specific key
.
If the key
does not exist, returns map
unchanged.
Examples
iex> Map.delete(%{a: 1, b: 2}, :a)
%{b: 2}
iex> Map.delete(%{b: 2}, :a)
%{b: 2}
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.
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
Fetches the value for a specific key
and returns it in a tuple.
If the key
does not exist, returns :error
.
Examples
iex> Map.fetch(%{a: 1}, :a)
{:ok, 1}
iex> Map.fetch(%{a: 1}, :b)
:error
Fetches the value for specific key
.
If key
does not exist, a KeyError
is raised.
Examples
iex> Map.fetch!(%{a: 1}, :a)
1
iex> Map.fetch!(%{a: 1}, :b)
** (KeyError) key :b not found in: %{a: 1}
Converts a struct
to map.
It accepts the struct module or a struct itself and
simply removes the __struct__
field from the struct.
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
.
If key
does not exist, return the default value
(nil
if no default value).
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
Gets the value from key
and updates it, all in one pass.
This fun
argument receives the value of key
(or nil
if key
is not present) and must return a two-element tuple: the “get” value
(the retrieved value, which can be operated on before being returned)
and the new value to be stored under key
. The fun
may also
return :pop
, implying the current value shall be removed
from map
and returned.
The returned value is a tuple with the “get” 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, %{b: "new value!", a: 1}}
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}}
Gets the value from key
and updates it. Raises if there is no key
.
This fun
argument receives the value of key
and must return a
two-element tuple: the “get” value (the retrieved value, which can be
operated on before being returned) and the new value to be stored under
key
.
The returned value is a tuple with the “get” 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)
** (KeyError) key :b not found
iex> Map.get_and_update!(%{a: 1}, :a, fn _ ->
...> :pop
...> end)
{1, %{}}
Gets the value for a specific key
.
If key
does not exist, lazily evaluates fun
and returns its result.
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
Returns whether a given key
exists in the given map
.
Examples
iex> Map.has_key?(%{a: 1}, :a)
true
iex> Map.has_key?(%{a: 1}, :b)
false
Merges two maps into one.
All keys in map2
will be added to map1
, overriding any existing one.
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
.
Examples
iex> Map.merge(%{a: 1, b: 2}, %{a: 3, d: 4})
%{a: 3, b: 2, d: 4}
Merges two maps into one.
All keys in map2
will be added to map1
. The given function will
be invoked with the key, value1 and value2 to solve conflicts.
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}
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}
Creates a map from an enumerable
via the 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}
Returns and removes the value associated with key
in map
.
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}}
Lazily returns and removes the value associated with key
in map
.
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
.
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.
Examples
iex> Map.put_new(%{a: 1}, :b, 2)
%{b: 2, a: 1}
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 is useful if the 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
...> 3
...> end
iex> Map.put_new_lazy(map, :a, fun)
%{a: 1}
iex> Map.put_new_lazy(map, :b, fun)
%{a: 1, b: 3}
Takes all entries corresponding to the given keys
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}}
Takes all entries corresponding to the given keys and returns them in a new map.
Examples
iex> Map.take(%{a: 1, b: 2, c: 3}, [:a, :c, :e])
%{a: 1, c: 3}
Converts map
to a list.
Examples
iex> Map.to_list(%{a: 1})
[a: 1]
iex> Map.to_list(%{1 => 2})
[{1, 2}]
Updates the key
in map
with the given function.
If the key
does not exist, inserts the given initial
value.
Examples
iex> Map.update(%{a: 1}, :a, 13, &(&1 * 2))
%{a: 2}
iex> Map.update(%{a: 1}, :b, 11, &(&1 * 2))
%{a: 1, b: 11}
Updates the key
with the given function.
If the key
does not exist, raises KeyError
.
Examples
iex> Map.update!(%{a: 1}, :a, &(&1 * 2))
%{a: 2}
iex> Map.update!(%{a: 1}, :b, &(&1 * 2))
** (KeyError) key :b not found