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<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><marker id="type-dict">dict()</marker></name>
<desc><p>Dictionary as returned by <c>new/0</c>.</p></desc>
</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 list 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>
