# maps

## Maps Processing Functions

This module contains functions for maps processing.

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

• `Key = term()`
• `Map = #{}`
• `Value = 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`.

Example:

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

### fold(Fun, Init, Map) -> Acc

• `Fun = fun((K, V, AccIn) -> AccOut)`
• `Init = Acc = AccIn = AccOut = term()`
• `Map = #{}`
• `K = V = term()`

Calls `F(K, V, AccIn)` for every `K` to value `V` association in `Map` in arbitrary order. The function `fun F/3` must return a new accumulator which is passed to the next successive call. `maps:fold/3` returns the final value of the accumulator. The initial accumulator value `Init` is returned if the map is empty.

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

• `List = [{Key, Value}]`
• `Key = Value = term()`
• `Map = #{}`

The function takes a list of key-value tuples elements and builds a map. The associations may be in any order and both keys and values in the association may be of any term. If the same key appears more than once, the latter (rightmost) 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

• `Key = term()`
• `Map = #{}`
• `Value = term()`

Returns the value `Value` associated with `Key` if `Map` contains `Key`. If no value is associated with `Key` then the call will fail with an exception.

Example:

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

### get(Key, Map, Default) -> Value | Default

• `Key = term()`
• `Map = #{}`
• `Value = Default = term()`

Returns the value `Value` associated with `Key` if `Map` contains `Key`. If no value is associated with `Key` then returns `Default`.

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()

• `Key = term()`
• `Map = #{}`

Returns `true` if map `Map` contains `Key` and returns `false` if it does not contain the `Key`. The function will fail with an 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```

### keys(Map) -> Keys

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

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

Example:

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

### map(Fun, Map1) -> Map2

• `Fun = fun((K, V1) -> V2)`
• `Map1 = Map2 = #{}`
• `K = V1 = V2 = term()`

The function produces a new map `Map2` by calling the function `fun F(K, V1)` for every `K` to value `V1` association in `Map1` in arbitrary order. The function `fun F/2` must return the value `V2` to be associated with key `K` for the new map `Map2`.

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

• `Map1 = Map2 = Map3 = #{}`

Merges two maps into a single map `Map3`. If two keys exists in both maps the value in `Map1` will be superseded by the value in `Map2`.

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

• `Map = #{}`

Returns a new empty map.

Example:

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

### put(Key, Value, Map1) -> Map2

• `Key = Value = term()`
• `Map1 = Map2 = #{}`

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`.

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

• `Key = term()`
• `Map1 = Map2 = #{}`

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

Example:

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

### size(Map) -> integer() >= 0

• `Map = #{}`

The function returns the number of key-value associations in the `Map`. This operation happens in constant time.

Example:

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

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

• `Map = #{}`
• `Key = Value = term()`

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

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

• `Key = Value = term()`
• `Map1 = Map2 = #{}`

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. If `Key` does not exist in `Map1` an exception is generated.

Example:

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

### values(Map) -> Keys

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

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

Example:

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

### without(Ks, Map1) -> Map2

• `Ks = [K]`
• `Map1 = Map2 = #{}`
• `K = term()`

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

Example:

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