# maps

## Maps Processing Functions

This module contains functions for maps processing.

### filter(Pred, Map1) -> Map2

• `Pred = fun((Key, Value) -> boolean())`
• `Key = Value = term()`
• `Map1 = Map2 = #{}`

Returns a map `Map2` for which predicate `Pred` holds true in `Map1`.

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

• `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`.

The call will fail 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, 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`.

The call will fail 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

• `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`.

The call will fail 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()

• `Key = term()`
• `Map = #{}`

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

The call will fail 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```

### keys(Map) -> Keys

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

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

The call will fail 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, 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`.

The call will fail 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

• `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`.

The call will fail 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

• `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`.

The call will fail 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

• `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.

The call will fail 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

• `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.

The call will fail 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}```

### values(Map) -> Values

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

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

The call will fail 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

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

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` are 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

• `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"}```