# dict

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Module

dict

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Module Summary

Key-value dictionary.

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Description

This module provides a Key-Value dictionary. The representation of a dictionary is not defined.

This module provides the same interface as the orddict(3) module. One difference is that while this module considers two keys as different if they do not match (=:=), orddict considers two keys as different if and only if they do not compare equal (==).

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Data Types

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dict(Key, Value)

Dictionary as returned by new/0.

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dict() = dict(term(), term())

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Exports

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append(Key, Value, Dict1) -> Dict2

### Types

Dict1 = Dict2 = dict(Key, Value)

Appends a new Value to the current list of values associated with Key.

See also section Notes.

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append_list(Key, ValList, Dict1) -> Dict2

### Types

Dict1 = Dict2 = dict(Key, Value)
ValList = [Value]

Appends a list of values ValList to the current list of values associated with Key. An exception is generated if the initial value associated with Key is not a list of values.

See also section Notes.

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erase(Key, Dict1) -> Dict2

### Types

Dict1 = Dict2 = dict(Key, Value)

Erases all items with a given key from a dictionary.

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fetch(Key, Dict) -> Value

### Types

Dict = dict(Key, Value)

Returns the value associated with Key in dictionary Dict. This function assumes that Key is present in dictionary Dict, and an exception is generated if Key is not in the dictionary.

See also section Notes.

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fetch_keys(Dict) -> Keys

### Types

Dict = dict(Key, Value :: term())
Keys = [Key]

Returns a list of all keys in dictionary Dict.

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take(Key, Dict) -> {Value, Dict1} | error
OTP 20.0

### Types

Dict = Dict1 = dict(Key, Value)
Key = Value = term()

This function returns value from dictionary and a new dictionary without this value. Returns error if the key is not present in the dictionary.

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filter(Pred, Dict1) -> Dict2

### Types

Pred = fun((Key, Value) -> boolean())
Dict1 = Dict2 = dict(Key, Value)

Dict2 is a dictionary of all keys and values in Dict1 for which Pred(Key, Value) is true.

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

### Types

Dict = dict(Key, Value)

Searches for a key in dictionary Dict. Returns {ok, Value}, where Value is the value associated with Key, or error if the key is not present in the dictionary.

See also section Notes.

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fold(Fun, Acc0, Dict) -> Acc1

### Types

Fun = fun((Key, Value, AccIn) -> AccOut)
Dict = dict(Key, Value)
Acc0 = Acc1 = AccIn = AccOut = Acc

Calls Fun on successive keys and values of dictionary Dict together with an extra argument Acc (short for accumulator). Fun must return a new accumulator that is passed to the next call. Acc0 is returned if the dictionary is empty. The evaluation order is undefined.

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from_list(List) -> Dict

### Types

Dict = dict(Key, Value)
List = [{Key, Value}]

Converts the Key-Value list List to dictionary Dict.

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is_empty(Dict) -> boolean()
OTP 17.0

### Types

Dict = dict()

Returns true if dictionary Dict has no elements, otherwise false.

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is_key(Key, Dict) -> boolean()

### Types

Dict = dict(Key, Value :: term())

Tests if Key is contained in dictionary Dict.

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map(Fun, Dict1) -> Dict2

### Types

Fun = fun((Key, Value1) -> Value2)
Dict1 = dict(Key, Value1)
Dict2 = dict(Key, Value2)

Calls Fun on successive keys and values of dictionary Dict1 to return a new value for each key. The evaluation order is undefined.

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merge(Fun, Dict1, Dict2) -> Dict3

### Types

Fun = fun((Key, Value1, Value2) -> Value)
Dict1 = dict(Key, Value1)
Dict2 = dict(Key, Value2)
Dict3 = dict(Key, Value)

Merges two dictionaries, Dict1 and Dict2, to create a new dictionary. All the Key-Value pairs from both dictionaries are included in the new dictionary. If a key occurs in both dictionaries, Fun is called with the key and both values to return a new value. merge can be defined as follows, but is faster:

    merge(Fun, D1, D2) ->
        fold(fun (K, V1, D) ->
                     update(K, fun (V2) -> Fun(K, V1, V2) end, V1, D)
             end, D2, D1).

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new() -> dict()

Creates a new dictionary.

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size(Dict) -> integer() >= 0

### Types

Dict = dict()

Returns the number of elements in dictionary Dict.

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store(Key, Value, Dict1) -> Dict2

### Types

Dict1 = Dict2 = dict(Key, Value)

Stores a Key-Value pair in dictionary Dict2. If Key already exists in Dict1, the associated value is replaced by Value.

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to_list(Dict) -> List

### Types

Dict = dict(Key, Value)
List = [{Key, Value}]

Converts dictionary Dict to a list representation.

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update(Key, Fun, Dict1) -> Dict2

### Types

Dict1 = Dict2 = dict(Key, Value)
Fun = fun((Value1 :: Value) -> Value2 :: Value)

Updates a value in a dictionary by calling Fun on the value to get a new value. An exception is generated if Key is not present in the dictionary.

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update(Key, Fun, Initial, Dict1) -> Dict2

### Types

Dict1 = Dict2 = dict(Key, Value)
Fun = fun((Value1 :: Value) -> Value2 :: Value)
Initial = Value

Updates a value in a dictionary by calling Fun on the value to get a new value. If Key is not present in the dictionary, Initial is stored as the first value. For example, append/3 can be defined as:

    append(Key, Val, D) ->
        update(Key, fun (Old) -> Old ++ [Val] end, [Val], D).

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update_counter(Key, Increment, Dict1) -> Dict2

### Types

Dict1 = Dict2 = dict(Key, Value)
Increment = number()

Adds Increment to the value associated with Key and stores this value. If Key is not present in the dictionary, Increment is stored as the first value.

This can be defined as follows, but is faster:

    update_counter(Key, Incr, D) ->
        update(Key, fun (Old) -> Old + Incr end, Incr, D).

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Notes

Functions append and append_list are included so that keyed values can be stored in a list accumulator, for example:

    > D0 = dict:new(),
      D1 = dict:store(files, [], D0),
      D2 = dict:append(files, f1, D1),
      D3 = dict:append(files, f2, D2),
      D4 = dict:append(files, f3, D3),
      dict:fetch(files, D4).
    [f1,f2,f3]

This saves the trouble of first fetching a keyed value, appending a new value to the list of stored values, and storing the result.

Function fetch is to be used if the key is known to be in the dictionary, otherwise function find.

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See Also

gb_trees(3), orddict(3)