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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>1996</year><year>2014</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
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.
</legalnotice>
<title>dict</title>
<prepared>Robert Virding</prepared>
<docno></docno>
<date>1997-01-15</date>
<rev>B</rev>
</header>
<module>dict</module>
<modulesummary>Key-Value Dictionary</modulesummary>
<description>
<p><c>Dict</c> implements a <c>Key</c> - <c>Value</c> dictionary.
The representation of a dictionary is not defined.</p>
<p>This module provides exactly the same interface as the module
<c>orddict</c>. One difference is that while this module
considers two keys as different if they do not match (<c>=:=</c>),
<c>orddict</c> considers two keys as different if and only if
they do not compare equal (<c>==</c>).</p>
</description>
<datatypes>
<datatype>
<name name="dict" n_vars="2"/>
<desc><p>Dictionary as returned by <c>new/0</c>.</p></desc>
</datatype>
<datatype>
<name name="dict" n_vars="0"/>
</datatype>
</datatypes>
<funcs>
<func>
<name name="append" arity="3"/>
<fsummary>Append a value to keys in a dictionary</fsummary>
<desc>
<p>This function appends a new <c><anno>Value</anno></c> to the current list
of values associated with <c><anno>Key</anno></c>.</p>
</desc>
</func>
<func>
<name name="append_list" arity="3"/>
<fsummary>Append new values to keys in a dictionary</fsummary>
<desc>
<p>This function appends a list of values <c><anno>ValList</anno></c> to
the current list of values associated with <c><anno>Key</anno></c>. An
exception is generated if the initial value associated with
<c><anno>Key</anno></c> is not a list of values.</p>
</desc>
</func>
<func>
<name name="erase" arity="2"/>
<fsummary>Erase a key from a dictionary</fsummary>
<desc>
<p>This function erases all items with a given key from a
dictionary.</p>
</desc>
</func>
<func>
<name name="fetch" arity="2"/>
<fsummary>Look-up values in a dictionary</fsummary>
<desc>
<p>This function returns the value associated with <c><anno>Key</anno></c>
in the dictionary <c><anno>Dict</anno></c>. <c>fetch</c> assumes that
the <c><anno>Key</anno></c> is present in the dictionary and an exception
is generated if <c><anno>Key</anno></c> is not in the dictionary.</p>
</desc>
</func>
<func>
<name name="fetch_keys" arity="1"/>
<fsummary>Return all keys in a dictionary</fsummary>
<desc>
<p>This function returns a list of all keys in the dictionary.</p>
</desc>
</func>
<func>
<name name="filter" arity="2"/>
<fsummary>Choose elements which satisfy a predicate</fsummary>
<desc>
<p><c><anno>Dict2</anno></c> is a dictionary of all keys and values in
<c><anno>Dict1</anno></c> for which <c><anno>Pred</anno>(<anno>Key</anno>, <anno>Value</anno>)</c> is <c>true</c>.</p>
</desc>
</func>
<func>
<name name="find" arity="2"/>
<fsummary>Search for a key in a dictionary</fsummary>
<desc>
<p>This function searches for a key in a dictionary. Returns
<c>{ok, <anno>Value</anno>}</c> where <c><anno>Value</anno></c> is the value associated
with <c><anno>Key</anno></c>, or <c>error</c> if the key is not present in
the dictionary.</p>
</desc>
</func>
<func>
<name name="fold" arity="3"/>
<fsummary>Fold a function over a dictionary</fsummary>
<desc>
<p>Calls <c><anno>Fun</anno></c> on successive keys and values of
<c><anno>Dict</anno></c> together with an extra argument <c>Acc</c>
(short for accumulator). <c><anno>Fun</anno></c> must return a new
accumulator which is passed to the next call. <c><anno>Acc0</anno></c> is
returned if the dict is empty. The evaluation order is
undefined.</p>
</desc>
</func>
<func>
<name name="from_list" arity="1"/>
<fsummary>Convert a list of pairs to a dictionary</fsummary>
<desc>
<p>This function converts the <c><anno>Key</anno></c> - <c><anno>Value</anno></c> list
<c><anno>List</anno></c> to a dictionary.</p>
</desc>
</func>
<func>
<name name="is_key" arity="2"/>
<fsummary>Test if a key is in a dictionary</fsummary>
<desc>
<p>This function tests if <c><anno>Key</anno></c> is contained in
the dictionary <c><anno>Dict</anno></c>.</p>
</desc>
</func>
<func>
<name name="map" arity="2"/>
<fsummary>Map a function over a dictionary</fsummary>
<desc>
<p><c>map</c> calls <c><anno>Fun</anno></c> on successive keys and values
of <c><anno>Dict1</anno></c> to return a new value for each key.
The evaluation order is undefined.</p>
</desc>
</func>
<func>
<name name="merge" arity="3"/>
<fsummary>Merge two dictionaries</fsummary>
<desc>
<p><c>merge</c> merges two dictionaries, <c><anno>Dict1</anno></c> and
<c><anno>Dict2</anno></c>, to create a new dictionary. All the <c><anno>Key</anno></c>
- <c><anno>Value</anno></c> pairs from both dictionaries are included in
the new dictionary. If a key occurs in both dictionaries then
<c><anno>Fun</anno></c> is called with the key and both values to return a
new value. <c>merge</c> could be defined as:</p>
<code type="none">
merge(Fun, D1, D2) ->
fold(fun (K, V1, D) ->
update(K, fun (V2) -> Fun(K, V1, V2) end, V1, D)
end, D2, D1).</code>
<p>but is faster.</p>
</desc>
</func>
<func>
<name name="new" arity="0"/>
<fsummary>Create a dictionary</fsummary>
<desc>
<p>This function creates a new dictionary.</p>
</desc>
</func>
<func>
<name name="size" arity="1"/>
<fsummary>Return the number of elements in a dictionary</fsummary>
<desc>
<p>Returns the number of elements in a <c><anno>Dict</anno></c>.</p>
</desc>
</func>
<func>
<name name="is_empty" arity="1"/>
<fsummary>Return true if the dictionary is empty</fsummary>
<desc>
<p>Returns <c>true</c> if <c><anno>Dict</anno></c> has no elements, <c>false</c> otherwise.</p>
</desc>
</func>
<func>
<name name="store" arity="3"/>
<fsummary>Store a value in a dictionary</fsummary>
<desc>
<p>This function stores a <c><anno>Key</anno></c> - <c><anno>Value</anno></c> pair in a
dictionary. If the <c><anno>Key</anno></c> already exists in <c><anno>Dict1</anno></c>,
the associated value is replaced by <c><anno>Value</anno></c>.</p>
</desc>
</func>
<func>
<name name="to_list" arity="1"/>
<fsummary>Convert a dictionary to a list of pairs</fsummary>
<desc>
<p>This function converts the dictionary to a list
representation.</p>
</desc>
</func>
<func>
<name name="update" arity="3"/>
<fsummary>Update a value in a dictionary</fsummary>
<desc>
<p>Update a value in a dictionary by calling <c><anno>Fun</anno></c> on
the value to get a new value. An exception is generated if
<c><anno>Key</anno></c> is not present in the dictionary.</p>
</desc>
</func>
<func>
<name name="update" arity="4"/>
<fsummary>Update a value in a dictionary</fsummary>
<desc>
<p>Update a value in a dictionary by calling <c><anno>Fun</anno></c> on
the value to get a new value. If <c><anno>Key</anno></c> is not present
in the dictionary then <c><anno>Initial</anno></c> will be stored as
the first value. For example <c>append/3</c> could be defined
as:</p>
<code type="none">
append(Key, Val, D) ->
update(Key, fun (Old) -> Old ++ [Val] end, [Val], D).</code>
</desc>
</func>
<func>
<name name="update_counter" arity="3"/>
<fsummary>Increment a value in a dictionary</fsummary>
<desc>
<p>Add <c><anno>Increment</anno></c> to the value associated with <c><anno>Key</anno></c>
and store this value. If <c><anno>Key</anno></c> is not present in
the dictionary then <c><anno>Increment</anno></c> will be stored as
the first value.</p>
<p>This could be defined as:</p>
<code type="none">
update_counter(Key, Incr, D) ->
update(Key, fun (Old) -> Old + Incr end, Incr, D).</code>
<p>but is faster.</p>
</desc>
</func>
</funcs>
<section>
<title>Notes</title>
<p>The functions <c>append</c> and <c>append_list</c> are included
so we can store keyed values in a list <em>accumulator</em>. For
example:</p>
<pre>
> 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] </pre>
<p>This saves the trouble of first fetching a keyed value,
appending a new value to the list of stored values, and storing
the result.
</p>
<p>The function <c>fetch</c> should be used if the key is known to
be in the dictionary, otherwise <c>find</c>.</p>
</section>
<section>
<title>See Also</title>
<p><seealso marker="gb_trees">gb_trees(3)</seealso>,
<seealso marker="orddict">orddict(3)</seealso></p>
</section>
</erlref>
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