dict

19962009 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 (==).

DATA TYPES


dictionary()
  as returned by new/0

append(Key, Value, Dict1) -> Dict2 Append a value to keys in a dictionary Key = Value = term() Dict1 = Dict2 = 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, Dict1) -> Dict2 Append new values to keys in a dictionary ValList = [Value] Key = Value = term() Dict1 = Dict2 = 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, Dict1) -> Dict2 Erase a key from a dictionary Key = term() Dict1 = Dict2 = dictionary()

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

fetch(Key, Dict) -> Value Look-up values in a dictionary Key = Value = term() Dict = 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.

fetch_keys(Dict) -> Keys Return all keys in a dictionary Dict = dictionary() Keys = [term()]

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

filter(Pred, Dict1) -> Dict2 Choose elements which satisfy a predicate Pred = fun(Key, Value) -> bool() Key = Value = term() Dict1 = Dict2 = dictionary()

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

find(Key, Dict) -> {ok, Value} | error Search for a key in a dictionary Key = Value = term() Dict = 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, Dict) -> Acc1 Fold a function over a dictionary Fun = fun(Key, Value, AccIn) -> AccOut Key = Value = term() Acc0 = Acc1 = AccIn = AccOut = term() Dict = 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 list is empty. The evaluation order is undefined.

from_list(List) -> Dict Convert a list of pairs to a dictionary List = [{Key, Value}] Dict = dictionary()

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

is_key(Key, Dict) -> bool() Test if a key is in a dictionary Key = term() Dict = dictionary()

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

map(Fun, Dict1) -> Dict2 Map a function over a dictionary Fun = fun(Key, Value1) -> Value2 Key = Value1 = Value2 = term() Dict1 = Dict2 = dictionary()

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

merge(Fun, Dict1, Dict2) -> Dict3 Merge two dictionaries Fun = fun(Key, Value1, Value2) -> Value Key = Value1 = Value2 = Value3 = term() Dict1 = Dict2 = Dict3 = dictionary()

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() -> dictionary() Create a dictionary

This function creates a new dictionary.

size(Dict) -> int() Return the number of elements in a dictionary Dict = dictionary()

Returns the number of elements in a Dict.

store(Key, Value, Dict1) -> Dict2 Store a value in a dictionary Key = Value = term() Dict1 = Dict2 = 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.

to_list(Dict) -> List Convert a dictionary to a list of pairs Dict = dictionary() List = [{Key, Value}]

This function converts the dictionary to a list representation.

update(Key, Fun, Dict1) -> Dict2 Update a value in a dictionary Key = term() Fun = fun(Value1) -> Value2 Value1 = Value2 = term() Dict1 = Dict2 = 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(Key, Fun, Initial, Dict1) -> Dict2 Update a value in a dictionary Key = Initial = term() Fun = fun(Value1) -> Value2 Value1 = Value2 = term() Dict1 = Dict2 = 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).

update_counter(Key, Increment, Dict1) -> Dict2 Increment a value in a dictionary Key = term() Increment = number() Dict1 = Dict2 = 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.