orddict(3)

MODULE
orddict - Key-Value Dictionary as Ordered List

DESCRIPTION

Orddict implements a Key - Value dictionary. An orddict is a representation of a dictionary, where a list of pairs is used to store the keys and values. The list is ordered after the keys.

This module provides exactly the same interface as the module dict but with a defined representation. One difference is that while dict considers two keys as different if they do not match (=:=), this module considers two keys as different if and only if they do not compare equal (==).

DATA TYPES
ordered_dictionary()
  as returned by new/0

EXPORTS

append(Key, Value, Orddict1) -> Orddict2

    Types
        Key = Value = term()
        Orddict1 = Orddict2 = ordered_dictionary()

    This function appends a new Value 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.

append_list(Key, ValList, Orddict1) -> Orddict2

    Types
        ValList = [Value]
        Key = Value = term()
        Orddict1 = Orddict2 = ordered_dictionary()

    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, Orddict1) -> Orddict2

    Types
        Key = term()
        Orddict1 = Orddict2 = ordered_dictionary()

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

fetch(Key, Orddict) -> Value

    Types
        Key = Value = term()
        Orddict = ordered_dictionary()

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

fetch_keys(Orddict) -> Keys

    Types
        Orddict = ordered_dictionary()
        Keys = [term()]

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

filter(Pred, Orddict1) -> Orddict2

    Types
        Pred = fun(Key, Value) -> bool()
        Key = Value = term()
        Orddict1 = Orddict2 = ordered_dictionary()

    Orddict2 is a dictionary of all keys and values in Orddict1 for which Pred(Key, Value) is true.

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

    Types
        Key = Value = term()
        Orddict = ordered_dictionary()

    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, Orddict) -> Acc1

    Types
        Fun = fun(Key, Value, AccIn) -> AccOut
        Key = Value = term()
        Acc0 = Acc1 = AccIn = AccOut = term()
        Orddict = ordered_dictionary()

    Calls Fun on successive keys and values of Orddict 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) -> Orddict

    Types
        List = [{Key, Value}]
        Orddict = ordered_dictionary()

    This function converts the key/value list List to a dictionary.

is_key(Key, Orddict) -> bool()

    Types
        Key = term()
        Orddict = ordered_dictionary()

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

map(Fun, Orddict1) -> Orddict2

    Types
        Fun = fun(Key, Value1) -> Value2
        Key = Value1 = Value2 = term()
        Orddict1 = Orddict2 = ordered_dictionary()

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

merge(Fun, Orddict1, Orddict2) -> Orddict3

    Types
        Fun = fun(Key, Value1, Value2) -> Value
        Key = Value1 = Value2 = Value3 = term()
        Orddict1 = Orddict2 = Orddict3 = ordered_dictionary()

    merge merges two dictionaries, Orddict1 and Orddict2, 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() -> ordered_dictionary()

    This function creates a new dictionary.

store(Key, Value, Orddict1) -> Orddict2

    Types
        Key = Value = term()
        Orddict1 = Orddict2 = ordered_dictionary()

    This function stores a Key - Value pair in a dictionary. If the Key already exists in Orddict1, the associated value is replaced by Value.

to_list(Orddict) -> List

    Types
        Orddict = ordered_dictionary()
        List = [{Key, Value}]

    This function converts the dictionary to a list representation.

update(Key, Fun, Orddict1) -> Orddict2

    Types
        Key = term()
        Fun = fun(Value1) -> Value2
        Value1 = Value2 = term()
        Orddict1 = Orddict2 = ordered_dictionary()

    Update the 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, Orddict1) -> Orddict2

    Types
        Key = Initial = term()
        Fun = fun(Value1) -> Value2
        Value1 = Value2 = term()
        Orddict1 = Orddict2 = ordered_dictionary()

    Update the 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, Orddict1) -> Orddict2

    Types
        Key = term()
        Increment = number()
        Orddict1 = Orddict2 = ordered_dictionary()

    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 = orddict:new(),
  D1 = orddict:store(files, [], D0),
  D2 = orddict:append(files, f1, D1),
  D3 = orddict:append(files, f2, D2),
  D4 = orddict:append(files, f3, D3),
  orddict: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

dict(3), gb_trees(3)

AUTHOR
Robert Virding - support@erlang.ericsson.se