dict

19962014 Ericsson AB. All Rights Reserved. The contents of this file are subject to the Erlang Public License, Version 1.1, (the "License"); you may not use this file except in compliance with the License. You should have received a copy of the Erlang Public License along with this software. If not, it can be retrieved online at http://www.erlang.org/. Software distributed under the License is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License for the specific language governing rights and limitations under the License. dict Robert Virding 1997-01-15 B

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 (==).

Dictionary as returned by new/0.

Append a value to keys in a dictionary

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

Append new values to keys in a 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 a key from a dictionary

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

Look-up values in a dictionary

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.

Return all keys in a dictionary

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

Choose elements which satisfy a predicate

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

Search for a key in a 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 a function over a dictionary

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.

Convert a list of pairs to a dictionary

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

Test if a key is in a dictionary

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

Map a function over a dictionary

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

Merge two dictionaries

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.

Create a dictionary

This function creates a new dictionary.

Return the number of elements in a dictionary

Returns the number of elements in a Dict.

Return true if the dictionary is empty

Returns true if Dict has no elements, false otherwise.

Store a value in a dictionary

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

Convert a dictionary to a list of pairs

This function converts the dictionary to a list representation.

Update a value in a dictionary

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 a value in a dictionary

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).

Increment a value in a 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 = 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.