---
name: "maps"
section: "3erl"
platform: "linux"
package: "erlang-manpages"
version: "1:25.2.3+dfsg-1+deb12u4"
release: "Debian 12 (Bookworm)"
platforms: ["linux"]
url: "https://new.carta.tech/maps"
---

# maps(3erl)

Maps processing functions.

## NAME

maps - Maps processing functions.

## DESCRIPTION

This module contains functions for maps processing. The Efficiency Guide contains a chapter that describes how to use maps efficiently.

## DATA TYPES

```
iterator(Key, Value)
```

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
```

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}
```

```
filtermap(Fun, MapOrIter) -> Map
```

Types:

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

Returns a map *Map* that is the result of calling *Fun(Key, Value1)* for every *Key* to value *Value1* association in *MapOrIter* in any order.

If *Fun(Key, Value1)* returns *true*, the association is copied to the result map. If it returns *false*, the association is not copied. If it returns *{true, NewValue}*, the value for *Key* is replaced with *NewValue*at this position is replaced in the result 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,V) when is_atom(K) -> {true, V*2}; (_,V) -> (V rem 2) =:= 0 end,
  Map = #{k1 => 1, "k2" => 2, "k3" => 3},
  maps:filtermap(Fun,Map).
#{k1 => 2,"k2" => 2}
```

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

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
```

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
```

```
foreach(Fun, MapOrIter) -> ok
```

Types:

Fun = fun((Key, Value) -\> term())\
 MapOrIter = #{Key =\> Value} \| iterator(Key, Value)\

Calls *fun F(Key, Value)* for every *Key* to value *Value* association in *MapOrIter* in any order.

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.

```
from_keys(Keys, Value) -> Map
```

Types:

Keys = list()\
 Value = term()\
 Map = map()\

Takes a list of keys and a value and builds a map where all keys point to the same value. The key can be in any order, and keys and value can be of any term.

*Example:*

```
> Keys = ["a", "b", "c"], maps:from_keys(Keys, ok).
#{"a" => ok,"b" => ok,"c" => ok}
```

```
from_list(List) -> Map
```

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
```

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
```

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"
```

```
groups_from_list(Fun, List) -> MapOut
```

Types:

Fun = fun((Elem :: T) -\> Selected)\
 MapOut = #{Selected =\> List}\
 Selected = term()\
 List = \[T\]\
 T = term()\

The result is a map where each key is given by Fun and each value is a list of elements. The order of elements within each list is preserved from the list.

*Examples:*

```
> maps:groups_from_list(fun(X) -> X rem 2 end, [1,2,3]).
#{0 => [2], 1 => [1, 3]}
> maps:groups_from_list(fun erlang:length/1, ["ant", "buffalo", "cat", "dingo"]).
#{3 => ["ant", "cat"], 5 => ["dingo"], 7 => ["buffalo"]}
```

```
groups_from_list(Fun, ValueFun, List) -> MapOut
```

Types:

Fun = fun((Elem :: T) -\> Key)\
 ValueFun = fun((Elem :: T) -\> ValOut)\
 MapOut = #{Key := ListOut}\
 Key = ValOut = term()\
 List = \[T\]\
 ListOut = \[ValOut\]\
 T = term()\

The result is a map where each key is given by Fun and each value is a list of elements given by the ValueFun. The order of elements within each list is preserved from the list.

*Examples:*

```
> maps:groups_from_list(fun(X) -> X rem 2 end, fun(X) -> X*X end, [1,2,3]).
#{0 => [4], 1 => [1, 9]}
> maps:groups_from_list(fun erlang:length/1, fun lists:reverse/1, ["ant", "buffalo", "cat", "dingo"]).
#{3 => ["tna","tac"],5 => ["ognid"],7 => ["olaffub"]}
```

```
intersect(Map1, Map2) -> Map3
```

Types:

Map1 = #{Key =\> term()}\
 Map2 = #{term() =\> Value2}\
 Map3 = #{Key =\> Value2}\

Intersects two maps into a single map *Map3*. If a key exists 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:intersect(Map1,Map2).
#{a => 1}
```

```
intersect_with(Combiner, Map1, Map2) -> Map3
```

Types:

Map1 = #{Key =\> Value1}\
 Map2 = #{term() =\> Value2}\
 Combiner = fun((Key, Value1, Value2) -\> CombineResult)\
 Map3 = #{Key =\> CombineResult}\

Intersects two maps into a single map *Map3*. If a key exists in both maps, the value in *Map1* is combined with the value in *Map2* by the *Combiner* fun. When *Combiner* is applied the key that exists in both maps is the first parameter, the value from *Map1* is the second parameter, and the value from *Map2* is the third parameter.

The call fails with a *{badmap,Map}* exception if *Map1* or *Map2* is not a map. The call fails with a *badarg* exception if *Combiner* is not a fun that takes three arguments.

*Example:*

```
> Map1 = #{a => "value_one", b => "value_two"},
  Map2 = #{a => 1, c => 2},
  maps:intersect_with(fun(_Key, Value1, Value2) -> {Value1, Value2} end, Map1, Map2).
#{a => {"value_one",1}}
```

```
is_key(Key, Map) -> boolean()
```

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
```

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
```

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
```

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
```

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}
```

```
merge_with(Combiner, Map1, Map2) -> Map3
```

Types:

Map1 = #{Key1 =\> Value1}\
 Map2 = #{Key2 =\> Value2}\
 Combiner = fun((Key1, Value1, Value2) -\> CombineResult)\
 Map3 = #{Key1 =\> CombineResult, Key1 =\> Value1, Key2 =\> Value2}\

Merges two maps into a single map *Map3*. If a key exists in both maps, the value in *Map1* is combined with the value in *Map2* by the *Combiner* fun. When *Combiner* is applied the key that exists in both maps is the first parameter, the value from *Map1* is the second parameter, and the value from *Map2* is the third parameter.

The call fails with a *{badmap,Map}* exception if *Map1* or *Map2* is not a map. The call fails with a *badarg* exception if *Combiner* is not a fun that takes three arguments.

*Example:*

```
> Map1 = #{a => "value_one", b => "value_two"},
  Map2 = #{a => 1, c => 2},
  maps:merge_with(fun(_Key, Value1, Value2) -> {Value1, Value2} end, Map1, Map2).
#{a => {"value_one",1},b => "value_two",c => 2}
```

```
new() -> Map
```

Types:

Map = #{}\

Returns a new empty map.

*Example:*

```
> maps:new().
#{}
```

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

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
```

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
```

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
```

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
```

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}]
```

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
```

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
```

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
```

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
```

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
```

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
```

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"}
```