STDLIB

Reference Manual

Version 3.11

Table of Contents

maps

Module

maps

Module Summary

Maps processing functions.

Since

Module maps was introduced in OTP 17.0.

Description

This module contains functions for maps processing.

Data Types

An iterator representing the associations in a map with keys of type Key and values of type Value.

Created using maps:iterator/1.

Consumed by maps:next/1, maps:filter/2, maps:fold/3 and maps:map/2.

iterator() = iterator(term(), term())

Exports

filter(Pred, MapOrIter) -> Map
OTP 18.0

Types

Pred = fun((Key, Value) -> boolean())
MapOrIter = #{Key => Value} | iterator(Key, Value)
Map = #{Key => Value}

Returns a map Map for which predicate Pred holds true in MapOrIter.

The call fails with a {badmap,Map} exception if MapOrIter is not a map or valid iterator, or with badarg if Pred is not a function of arity 2.

Example:

> M = #{a => 2, b => 3, c=> 4, "a" => 1, "b" => 2, "c" => 4},
  Pred = fun(K,V) -> is_atom(K) andalso (V rem 2) =:= 0 end,
  maps:filter(Pred,M).
#{a => 2,c => 4}

find(Key, Map) -> {ok, Value} | error
OTP 17.0

Types

Map = #{Key => Value, term() => term()}

Returns a tuple {ok, Value}, where Value is the value associated with Key, or error if no value is associated with Key in Map.

The call fails with a {badmap,Map} exception if Map is not a map.

Example:

> Map = #{"hi" => 42},
  Key = "hi",
  maps:find(Key,Map).
{ok,42}

fold(Fun, Init, MapOrIter) -> Acc
OTP 17.0

Types

Fun = fun((Key, Value, AccIn) -> AccOut)
Init = term()
Acc = AccOut
AccIn = Init | AccOut
MapOrIter = #{Key => Value} | iterator(Key, Value)

Calls F(Key, Value, AccIn) for every Key to value Value association in MapOrIter in any order. Function fun F/3 must return a new accumulator, which is passed to the next successive call. This function returns the final value of the accumulator. The initial accumulator value Init is returned if the map is empty.

The call fails with a {badmap,Map} exception if MapOrIter is not a map or valid iterator, or with badarg if Fun is not a function of arity 3.

Example:

> Fun = fun(K,V,AccIn) when is_list(K) -> AccIn + V end,
  Map = #{"k1" => 1, "k2" => 2, "k3" => 3},
  maps:fold(Fun,0,Map).
6

from_list(List) -> Map
OTP 17.0

Types

List = [{Key, Value}]
Key = Value = term()
Map = map()

Takes a list of key-value tuples elements and builds a map. The associations can be in any order, and both keys and values in the association can be of any term. If the same key appears more than once, the latter (right-most) value is used and the previous values are ignored.

Example:

> List = [{"a",ignored},{1337,"value two"},{42,value_three},{"a",1}],
  maps:from_list(List).
#{42 => value_three,1337 => "value two","a" => 1}

get(Key, Map) -> Value
OTP 17.0

Types

Key = term()
Map = map()
Value = term()

Returns value Value associated with Key if Map contains Key.

The call fails with a {badmap,Map} exception if Map is not a map, or with a {badkey,Key} exception if no value is associated with Key.

Example:

> Key = 1337,
  Map = #{42 => value_two,1337 => "value one","a" => 1},
  maps:get(Key,Map).
"value one"

get(Key, Map, Default) -> Value | Default
OTP 17.1

Types

Map = #{Key => Value, term() => term()}

Returns value Value associated with Key if Map contains Key. If no value is associated with Key, Default is returned.

The call fails with a {badmap,Map} exception if Map is not a map.

Example:

> Map = #{ key1 => val1, key2 => val2 }.
#{key1 => val1,key2 => val2}
> maps:get(key1, Map, "Default value").
val1
> maps:get(key3, Map, "Default value").
"Default value"

is_key(Key, Map) -> boolean()
OTP 17.0

Types

Key = term()
Map = map()

Returns true if map Map contains Key and returns false if it does not contain the Key.

The call fails with a {badmap,Map} exception if Map is not a map.

Example:

> Map = #{"42" => value}.
#{"42" => value}
> maps:is_key("42",Map).
true
> maps:is_key(value,Map).
false

iterator(Map) -> Iterator
OTP 21.0

Types

Map = #{Key => Value}
Iterator = iterator(Key, Value)

Returns a map iterator Iterator that can be used by maps:next/1 to traverse the key-value associations in a map. When iterating over a map, the memory usage is guaranteed to be bounded no matter the size of the map.

The call fails with a {badmap,Map} exception if Map is not a map.

Example:

> M = #{ a => 1, b => 2 }.
#{a => 1,b => 2}
> I = maps:iterator(M), ok.
ok
> {K1, V1, I2} = maps:next(I), {K1, V1}.
{a,1}
> {K2, V2, I3} = maps:next(I2),{K2, V2}.
{b,2}
> maps:next(I3).
none

keys(Map) -> Keys
OTP 17.0

Types

Map = #{Key => term()}
Keys = [Key]

Returns a complete list of keys, in any order, which resides within Map.

The call fails with a {badmap,Map} exception if Map is not a map.

Example:

> Map = #{42 => value_three,1337 => "value two","a" => 1},
  maps:keys(Map).
[42,1337,"a"]

map(Fun, MapOrIter) -> Map
OTP 17.0

Types

Fun = fun((Key, Value1) -> Value2)
MapOrIter = #{Key => Value1} | iterator(Key, Value1)
Map = #{Key => Value2}

Produces a new map Map by calling function fun F(Key, Value1) for every Key to value Value1 association in MapOrIter in any order. Function fun Fun/2 must return value Value2 to be associated with key Key for the new map Map.

The call fails with a {badmap,Map} exception if MapOrIter is not a map or valid iterator, or with badarg if Fun is not a function of arity 2.

Example:

> Fun = fun(K,V1) when is_list(K) -> V1*2 end,
  Map = #{"k1" => 1, "k2" => 2, "k3" => 3},
  maps:map(Fun,Map).
#{"k1" => 2,"k2" => 4,"k3" => 6}

merge(Map1, Map2) -> Map3
OTP 17.0

Types

Map1 = Map2 = Map3 = map()

Merges two maps into a single map Map3. If two keys exist in both maps, the value in Map1 is superseded by the value in Map2.

The call fails with a {badmap,Map} exception if Map1 or Map2 is not a map.

Example:

> Map1 = #{a => "value_one", b => "value_two"},
  Map2 = #{a => 1, c => 2},
  maps:merge(Map1,Map2).
#{a => 1,b => "value_two",c => 2}

new() -> Map
OTP 17.0

Types

Map = #{}

Returns a new empty map.

Example:

> maps:new().
#{}

next(Iterator) -> {Key, Value, NextIterator} | none
OTP 21.0

Types

Iterator = NextIterator = iterator(Key, Value)

Returns the next key-value association in Iterator and a new iterator for the remaining associations in the iterator.

If there are no more associations in the iterator, none is returned.

Example:

> Map = #{a => 1, b => 2, c => 3}.
#{a => 1,b => 2,c => 3}
> I = maps:iterator(Map), ok.
ok
> {K1, V1, I1} = maps:next(I), {K1, V1}.
{a,1}
> {K2, V2, I2} = maps:next(I1), {K2, V2}.
{b,2}
> {K3, V3, I3} = maps:next(I2), {K3, V3}.
{c,3}
> maps:next(I3).
none

put(Key, Value, Map1) -> Map2
OTP 17.0

Types

Key = Value = term()
Map1 = Map2 = map()

Associates Key with value Value and inserts the association into map Map2. If key Key already exists in map Map1, the old associated value is replaced by value Value. The function returns a new map Map2 containing the new association and the old associations in Map1.

The call fails with a {badmap,Map} exception if Map1 is not a map.

Example:

> Map = #{"a" => 1}.
#{"a" => 1}
> maps:put("a", 42, Map).
#{"a" => 42}
> maps:put("b", 1337, Map).
#{"a" => 1,"b" => 1337}

remove(Key, Map1) -> Map2
OTP 17.0

Types

Key = term()
Map1 = Map2 = map()

Removes the Key, if it exists, and its associated value from Map1 and returns a new map Map2 without key Key.

The call fails with a {badmap,Map} exception if Map1 is not a map.

Example:

> Map = #{"a" => 1}.
#{"a" => 1}
> maps:remove("a",Map).
#{}
> maps:remove("b",Map).
#{"a" => 1}

size(Map) -> integer() >= 0
OTP 17.0

Types

Map = map()

Returns the number of key-value associations in Map. This operation occurs in constant time.

Example:

> Map = #{42 => value_two,1337 => "value one","a" => 1},
  maps:size(Map).
3

take(Key, Map1) -> {Value, Map2} | error
OTP 19.0

Types

Map1 = #{Key => Value, term() => term()}
Map2 = #{term() => term()}

The function removes the Key, if it exists, and its associated value from Map1 and returns a tuple with the removed Value and the new map Map2 without key Key. If the key does not exist error is returned.

The call will fail with a {badmap,Map} exception if Map1 is not a map.

Example:

> Map = #{"a" => "hello", "b" => "world"}.
#{"a" => "hello", "b" => "world"}
> maps:take("a",Map).
{"hello",#{"b" => "world"}}
> maps:take("does not exist",Map).
error

to_list(Map) -> [{Key, Value}]
OTP 17.0

Types

Map = #{Key => Value}

Returns a list of pairs representing the key-value associations of Map, where the pairs [{K1,V1}, ..., {Kn,Vn}] are returned in arbitrary order.

The call fails with a {badmap,Map} exception if Map is not a map.

Example:

> Map = #{42 => value_three,1337 => "value two","a" => 1},
  maps:to_list(Map).
[{42,value_three},{1337,"value two"},{"a",1}]

update(Key, Value, Map1) -> Map2
OTP 17.0

Types

Map1 = #{Key := term(), term() => term()}
Map2 = #{Key := Value, term() => term()}

If Key exists in Map1, the old associated value is replaced by value Value. The function returns a new map Map2 containing the new associated value.

The call fails with a {badmap,Map} exception if Map1 is not a map, or with a {badkey,Key} exception if no value is associated with Key.

Example:

> Map = #{"a" => 1}.
#{"a" => 1}
> maps:update("a", 42, Map).
#{"a" => 42}

update_with(Key, Fun, Map1) -> Map2
OTP 19.0

Types

Map1 = #{Key := Value1, term() => term()}
Map2 = #{Key := Value2, term() => term()}
Fun = fun((Value1) -> Value2)

Update a value in a Map1 associated with Key by calling Fun on the old value to get a new value. An exception {badkey,Key} is generated if Key is not present in the map.

Example:

> Map = #{"counter" => 1},
  Fun = fun(V) -> V + 1 end,
  maps:update_with("counter",Fun,Map).
#{"counter" => 2}

update_with(Key, Fun, Init, Map1) -> Map2
OTP 19.0

Types

Map1 = #{Key => Value1, term() => term()}
Map2 = #{Key := Value2 | Init, term() => term()}
Fun = fun((Value1) -> Value2)

Update a value in a Map1 associated with Key by calling Fun on the old value to get a new value. If Key is not present in Map1 then Init will be associated with Key.

Example:

> Map = #{"counter" => 1},
  Fun = fun(V) -> V + 1 end,
  maps:update_with("new counter",Fun,42,Map).
#{"counter" => 1,"new counter" => 42}

values(Map) -> Values
OTP 17.0

Types

Map = #{term() => Value}
Values = [Value]

Returns a complete list of values, in arbitrary order, contained in map Map.

The call fails with a {badmap,Map} exception if Map is not a map.

Example:

> Map = #{42 => value_three,1337 => "value two","a" => 1},
  maps:values(Map).
[value_three,"value two",1]

with(Ks, Map1) -> Map2
OTP 17.3

Types

Ks = [K]
Map1 = #{K => V, term() => term()}
Map2 = #{K => V}

Returns a new map Map2 with the keys K1 through Kn and their associated values from map Map1. Any key in Ks that does not exist in Map1 is ignored.

Example:

> Map = #{42 => value_three,1337 => "value two","a" => 1},
  Ks = ["a",42,"other key"],
  maps:with(Ks,Map).
#{42 => value_three,"a" => 1}

without(Ks, Map1) -> Map2
OTP 17.0

Types

Ks = [K]
Map1 = Map2 = map()
K = term()

Returns a new map Map2 without keys K1 through Kn and their associated values from map Map1. Any key in Ks that does not exist in Map1 is ignored

Example:

> Map = #{42 => value_three,1337 => "value two","a" => 1},
  Ks = ["a",42,"other key"],
  maps:without(Ks,Map).
#{1337 => "value two"}