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# dict

### MODULE

dict

### MODULE SUMMARY

Key-Value Dictionary

### DESCRIPTION

Dict implements a Key - Value dictionary. The representation of a dictionary is not defined.

This module provides exactly the same interface as the module orddict. 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 (==).

### DATA TYPES

dict()

Dictionary as returned by new/0.

### EXPORTS

append(Key, Value, Dict1) -> Dict2

Types:

Key = Value = term()
Dict1 = Dict2 = dict()

This function appends a new Value to the current list of values associated with Key.

append_list(Key, ValList, Dict1) -> Dict2

Types:

Key = term()
ValList = [Value :: term()]
Dict1 = Dict2 = dict()

This function 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.

erase(Key, Dict1) -> Dict2

Types:

Key = term()
Dict1 = Dict2 = dict()

This function erases all items with a given key from a dictionary.

fetch(Key, Dict) -> Value

Types:

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

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

fetch_keys(Dict) -> Keys

Types:

Dict = dict()
Keys = [term()]

This function returns a list of all keys in the dictionary.

filter(Pred, Dict1) -> Dict2

Types:

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

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

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

Types:

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

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

fold(Fun, Acc0, Dict) -> Acc1

Types:

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

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

from_list(List) -> Dict

Types:

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

This function converts the Key - Value list List to a dictionary.

is_key(Key, Dict) -> boolean()

Types:

Key = term()
Dict = dict()

This function tests if Key is contained in the dictionary Dict.

map(Fun, Dict1) -> Dict2

Types:

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

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

merge(Fun, Dict1, Dict2) -> Dict3

Types:

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

merge 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 then Fun is called with the key and both values to return a new value. merge could be defined as:

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

but is faster.

new() -> dict()

This function creates a new dictionary.

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

Types:

Dict = dict()

Returns the number of elements in a Dict.

store(Key, Value, Dict1) -> Dict2

Types:

Key = Value = term()
Dict1 = Dict2 = dict()

This function stores a Key - Value pair in a dictionary. If the Key already exists in Dict1, the associated value is replaced by Value.

to_list(Dict) -> List

Types:

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

This function converts the dictionary to a list representation.

update(Key, Fun, Dict1) -> Dict2

Types:

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

Update 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.

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

Types:

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

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

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

update_counter(Key, Increment, Dict1) -> Dict2

Types:

Key = term()
Increment = number()
Dict1 = Dict2 = dict()

Add Increment to the value associated with Key and store this value. If Key is not present in the dictionary then Increment will be stored as the first value.

This could be defined as:

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

but is faster.

### Notes

The functions append and append_list are included so we can store keyed values 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.

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

### See Also

gb_trees(3), orddict(3)