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# View Source maps (stdlib v6.0.1)

Maps processing functions.

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

# Summary

## Types

iterator()
iterator(Map, Order)

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

iterator_order()
iterator_order(Key)

Key-based iterator order option that can be one of `undefined` (default for `maps:iterator/1`), `ordered` (sorted in map-key order), `reversed`, or a custom sorting function.

## Functions

filter(Pred, MapOrIter)

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

filtermap(Fun, MapOrIter)

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.

find(Key, Map)

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

fold(Fun, Init, MapOrIter)

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.

foreach(Fun, MapOrIter)

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

from_keys(Keys, Value)

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.

from_list(List)

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.

get(Key, Map)

Returns value `Value` associated with `Key` if `Map` contains `Key`.

get(Key, Map, Default)

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

groups_from_list(KeyFun, List)

Partitions the given `List` into a map of groups.

groups_from_list(KeyFun, ValueFun, List)

Partitions the given `List` into a map of groups.

intersect(Map1, Map2)

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 If a key exists in both maps, the value in `Map1` is superseded by the value in `Map2`.

intersect_with(Combiner, Map1, Map2)

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.

is_key(Key, Map)

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

iterator(Map)

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.

iterator(Map, Order)

Returns a map iterator `Iterator` that can be used by `maps:next/1` to traverse the key-value associations in a map sorted by key using the given `Order`.

keys(Map)

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

map(Fun, MapOrIter)

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`.

merge(Map1, Map2)

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`.

merge_with(Combiner, Map1, Map2)

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.

new()

Returns a new empty map.

next(Iterator)

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

put(Key, Value, Map1)

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`.

remove(Key, Map1)

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

size(Map)

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

take(Key, Map1)

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.

to_list(MapOrIterator)

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

update(Key, Value, Map1)

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.

update_with(Key, Fun, Map1)

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.

update_with(Key, Fun, Init, Map1)

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`.

values(Map)

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

with(Ks, Map1)

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.

without(Ks, Map1)

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

# Types

Link to this type

# iterator()

View Source (since OTP 17.0)

    -type iterator() :: iterator(term(), term()).

Link to this opaque

# iterator(Map, Order)

View Source (since OTP 17.0)

    -opaque 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` or `maps:iterator/2`.

Consumed by:

- `maps:next/1`
- `maps:filter/2`
- `maps:filtermap/2`
- `maps:fold/3`
- `maps:foreach/2`
- `maps:map/2`
- `maps:to_list/1`

Link to this type

# iterator_order()

View Source (since OTP 17.0)

    -type iterator_order() :: iterator_order(term()).

Link to this type

# iterator_order(Key)

View Source (since OTP 17.0)

    -type iterator_order(Key) :: undefined | ordered | reversed | fun((A :: Key, B :: Key) -> boolean()).

Key-based iterator order option that can be one of `undefined` (default for `maps:iterator/1`), `ordered` (sorted in map-key order), `reversed`, or a custom sorting function.

Used by `maps:iterator/2`.

The Expressions section contains descriptions of how terms are ordered.

# Functions

Link to this function

# filter(Pred, MapOrIter)

View Source (since OTP 18.0)

    -spec filter(Pred, MapOrIter) -> Map
                    when
                        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}

Link to this function

# filtermap(Fun, MapOrIter)

View Source (since OTP 24.0)

    -spec filtermap(Fun, MapOrIter) -> Map
                       when
                           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` 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}

Link to this function

# find(Key, Map)

View Source (since OTP 17.0)

    -spec find(Key, Map) -> {ok, Value} | error when Map :: #{Key => Value, _ => _}.

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}

Link to this function

# fold(Fun, Init, MapOrIter)

View Source (since OTP 17.0)

    -spec fold(Fun, Init, MapOrIter) -> Acc
                  when
                      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

Link to this function

# foreach(Fun, MapOrIter)

View Source (since OTP 24.0)

    -spec foreach(Fun, MapOrIter) -> ok
                     when
                         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.

Link to this function

# from_keys(Keys, Value)

View Source (since OTP 24.0)

    -spec from_keys(Keys, Value) -> Map when 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}

Link to this function

# from_list(List)

View Source (since OTP 17.0)

    -spec from_list(List) -> Map when List :: [{Key, Value}], Key :: term(), 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}

Link to this function

# get(Key, Map)

View Source (since OTP 17.0)

    -spec get(Key, Map) -> Value when 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"

Link to this function

# get(Key, Map, Default)

View Source (since OTP 17.1)

    -spec get(Key, Map, Default) -> Value | Default when Map :: #{Key => Value, _ => _}.

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"

Link to this function

# groups_from_list(KeyFun, List)

View Source (since OTP 25.0)

    -spec groups_from_list(KeyFun, List) -> GroupsMap
                              when
                                  KeyFun :: fun((Elem) -> Key),
                                  GroupsMap :: #{Key => Group},
                                  Key :: term(),
                                  List :: [Elem],
                                  Group :: [Elem],
                                  Elem :: term().

Partitions the given `List` into a map of groups.

The result is a map where each key is given by `KeyFun` and each value is a list of elements from the given `List` for which `KeyFun` returned the same key.

The order of elements within each group list is preserved from the original list.

Examples:

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

Link to this function

# groups_from_list(KeyFun, ValueFun, List)

View Source (since OTP 25.0)

    -spec groups_from_list(KeyFun, ValueFun, List) -> GroupsMap
                              when
                                  KeyFun :: fun((Elem) -> Key),
                                  ValueFun :: fun((Elem) -> Value),
                                  GroupsMap :: #{Key := Group},
                                  Key :: term(),
                                  Value :: term(),
                                  List :: [Elem],
                                  Group :: [Value],
                                  Elem :: term().

Partitions the given `List` into a map of groups.

The result is a map where each key is given by `KeyFun` and each value is a list of elements from the given `List`, mapped via `ValueFun`, for which `KeyFun` returned the same key.

The order of elements within each group list is preserved from the original list.

Examples:

    > EvenOdd = fun(X) -> case X rem 2 of 0 -> even; 1 -> odd end end,
    > Square = fun(X) -> X * X end,
    > maps:groups_from_list(EvenOdd, Square, [1, 2, 3]).
    #{even => [4], odd => [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"]}

Link to this function

# intersect(Map1, Map2)

View Source (since OTP 24.0)

    -spec intersect(Map1, Map2) -> Map3
                       when 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}

Link to this function

# intersect_with(Combiner, Map1, Map2)

View Source (since OTP 24.0)

    -spec intersect_with(Combiner, Map1, Map2) -> Map3
                            when
                                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}}

Link to this function

# is_key(Key, Map)

View Source (since OTP 17.0)

    -spec is_key(Key, Map) -> boolean() when 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

Link to this function

# iterator(Map)

View Source (since OTP 21.0)

    -spec iterator(Map) -> Iterator when 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

Link to this function

# iterator(Map, Order)

View Source (since OTP 26.0)

    -spec iterator(Map, Order) -> Iterator
                      when
                          Map :: #{Key => Value},
                          Order :: iterator_order(Key),
                          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 sorted by key using the given `Order`.

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

Example (when `Order` is `ordered`):

    > M = #{ a => 1, b => 2 }.
    #{a => 1,b => 2}
    > OrdI = maps:iterator(M, ordered), ok.
    ok
    > {K1, V1, OrdI2} = maps:next(OrdI), {K1, V1}.
    {a,1}
    > {K2, V2, OrdI3} = maps:next(OrdI2),{K2, V2}.
    {b,2}
    > maps:next(OrdI3).
    none

Example (when `Order` is `reversed`):

    > M = #{ a => 1, b => 2 }.
    #{a => 1,b => 2}
    > RevI = maps:iterator(M, reversed), ok.
    ok
    > {K2, V2, RevI2} = maps:next(RevI), {K2, V2}.
    {b,2}
    > {K1, V1, RevI3} = maps:next(RevI2),{K1, V1}.
    {a,1}
    > maps:next(RevI3).
    none

Example (when `Order` is an arithmetic sorting function):

    > M = #{ -1 => a, -1.0 => b, 0 => c, 0.0 => d }.
    #{-1 => a,0 => c,-1.0 => b,0.0 => d}
    > ArithOrdI = maps:iterator(M, fun(A, B) -> A =< B end), ok.
    ok
    > maps:to_list(ArithOrdI).
    [{-1,a},{-1.0,b},{0,c},{0.0,d}]
    > ArithRevI = maps:iterator(M, fun(A, B) -> B < A end), ok.
    ok
    > maps:to_list(ArithRevI).
    [{0.0,d},{0,c},{-1.0,b},{-1,a}]

Link to this function

# keys(Map)

View Source (since OTP 17.0)

    -spec keys(Map) -> Keys when Map :: #{Key => _}, 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"]

Link to this function

# map(Fun, MapOrIter)

View Source (since OTP 17.0)

    -spec map(Fun, MapOrIter) -> Map
                 when
                     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}

Link to this function

# merge(Map1, Map2)

View Source (since OTP 17.0)

    -spec merge(Map1, Map2) -> Map3 when Map1 :: map(), Map2 :: map(), 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}

Link to this function

# merge_with(Combiner, Map1, Map2)

View Source (since OTP 24.0)

    -spec merge_with(Combiner, Map1, Map2) -> Map3
                        when
                            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}

Link to this function

# new()

View Source (since OTP 17.0)

    -spec new() -> Map when Map :: #{}.

Returns a new empty map.

Example:

    > maps:new().
    #{}

Link to this function

# next(Iterator)

View Source (since OTP 21.0)

    -spec next(Iterator) -> {Key, Value, NextIterator} | none
                  when Iterator :: iterator(Key, Value), 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

Link to this function

# put(Key, Value, Map1)

View Source (since OTP 17.0)

    -spec put(Key, Value, Map1) -> Map2 when Key :: term(), Value :: term(), Map1 :: map(), 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}

Link to this function

# remove(Key, Map1)

View Source (since OTP 17.0)

    -spec remove(Key, Map1) -> Map2 when Key :: term(), Map1 :: map(), 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}

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# size(Map)

View Source (since OTP 17.0)

    -spec size(Map) -> non_neg_integer() when 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

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# take(Key, Map1)

View Source (since OTP 19.0)

    -spec take(Key, Map1) -> {Value, Map2} | error when Map1 :: #{Key => Value, _ => _}, Map2 :: #{_ => _}.

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

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# to_list(MapOrIterator)

View Source (since OTP 17.0)

    -spec to_list(MapOrIterator) -> [{Key, Value}]
                     when MapOrIterator :: #{Key => Value} | iterator(Key, Value).

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

The call fails with a `{badmap,Map}` exception if `MapOrIterator` is not a map or an iterator obtained by a call to `iterator/1` or `iterator/2`.

Example:

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

Example (using `iterator/2`):

    > Map = #{ z => 1, y => 2, x => 3 }.
    #{x => 3,y => 2,z => 1}
    > maps:to_list(maps:iterator(Map, ordered)).
    [{x,3},{y,2},{z,1}]

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# update(Key, Value, Map1)

View Source (since OTP 17.0)

    -spec update(Key, Value, Map1) -> Map2 when Map1 :: #{Key := _, _ => _}, Map2 :: #{Key := Value, _ => _}.

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}

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# update_with(Key, Fun, Map1)

View Source (since OTP 19.0)

    -spec update_with(Key, Fun, Map1) -> Map2
                         when
                             Map1 :: #{Key := Value1, _ => _},
                             Map2 :: #{Key := Value2, _ => _},
                             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}

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# update_with(Key, Fun, Init, Map1)

View Source (since OTP 19.0)

    -spec update_with(Key, Fun, Init, Map1) -> Map2
                         when
                             Map1 :: #{Key => Value1, _ => _},
                             Map2 :: #{Key := Value2 | Init, _ => _},
                             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}

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# values(Map)

View Source (since OTP 17.0)

    -spec values(Map) -> Values when Map :: #{_ => 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]

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# with(Ks, Map1)

View Source (since OTP 17.3)

    -spec with(Ks, Map1) -> Map2 when Ks :: [K], Map1 :: #{K => V, _ => _}, 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}

Link to this function

# without(Ks, Map1)

View Source (since OTP 17.0)

    -spec without(Ks, Map1) -> Map2 when Ks :: [K], Map1 :: map(), 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"}