dict

Key-Value Dictionary

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 (==).

Types

dict(Key, Value)

Dictionary as returned by new/0.

dict()

dict() is equivalent to dict(term(), term()).

Functions

append(Key, Value, Dict1) -> Dict2

  • Dict1 = Dict2 = dict(Key, Value)

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

append_list(Key, ValList, Dict1) -> Dict2

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

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

  • Dict1 = Dict2 = dict(Key, Value)

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

fetch(Key, Dict) -> Value

  • Dict = dict(Key, Value)

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

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

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

filter(Pred, Dict1) -> Dict2

  • 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.

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

  • Dict = dict(Key, Value)

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

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

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 dict is empty. The evaluation order is undefined.

from_list(List) -> Dict

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

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

is_key(Key, Dict) -> boolean()

  • Dict = dict(Key, Value :: term())

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

map(Fun, Dict1) -> Dict2

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

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

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

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

  • Dict = dict()

Returns the number of elements in a Dict.

is_empty(Dict) -> boolean()

  • Dict = dict()

Returns true if Dict has no elements, false otherwise.

store(Key, Value, Dict1) -> Dict2

  • Dict1 = Dict2 = dict(Key, Value)

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

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

This function converts the dictionary to a list representation.

update(Key, Fun, Dict1) -> Dict2

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

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

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

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

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

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)