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  <header>
    <copyright>
      <year>1996</year><year>2011</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
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      Software distributed under the License is distributed on an "AS IS"
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      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>

  <section>
    <title>DATA TYPES</title>
    <code type="none">
dictionary()
  as returned by new/0</code>
  </section>
  <funcs>
    <func>
      <name>append(Key, Value, Dict1) -> Dict2</name>
      <fsummary>Append a value to keys in a dictionary</fsummary>
      <type>
        <v>Key = Value = term()</v>
        <v>Dict1 = Dict2 = dictionary()</v>
      </type>
      <desc>
        <p>This function appends a new <c>Value</c> to the current list
          of values associated with <c>Key</c>.  An exception is
          generated if the initial value associated with <c>Key</c> is
          not a list of values.</p>
      </desc>
    </func>
    <func>
      <name>append_list(Key, ValList, Dict1) -> Dict2</name>
      <fsummary>Append new values to keys in a dictionary</fsummary>
      <type>
        <v>ValList = [Value]</v>
        <v>Key = Value = term()</v>
        <v>Dict1 = Dict2 = dictionary()</v>
      </type>
      <desc>
        <p>This function appends a list of values <c>ValList</c> to
          the current list of values associated with <c>Key</c>. An
          exception is generated if the initial value associated with
          <c>Key</c> is not a list of values.</p>
      </desc>
    </func>
    <func>
      <name>erase(Key, Dict1) -> Dict2</name>
      <fsummary>Erase a key from a dictionary</fsummary>
      <type>
        <v>Key = term()</v>
        <v>Dict1 = Dict2 = dictionary()</v>
      </type>
      <desc>
        <p>This function erases all items with a given key from a
          dictionary.</p>
      </desc>
    </func>
    <func>
      <name>fetch(Key, Dict) -> Value</name>
      <fsummary>Look-up values in a dictionary</fsummary>
      <type>
        <v>Key = Value = term()</v>
        <v>Dict = dictionary()</v>
      </type>
      <desc>
        <p>This function returns the value associated with <c>Key</c>
          in the dictionary <c>Dict</c>. <c>fetch</c> assumes that
          the <c>Key</c> is present in the dictionary and an exception
          is generated if <c>Key</c> is not in the dictionary.</p>
      </desc>
    </func>
    <func>
      <name>fetch_keys(Dict) -> Keys</name>
      <fsummary>Return all keys in a dictionary</fsummary>
      <type>
        <v>Dict = dictionary()</v>
        <v>Keys = [term()]</v>
      </type>
      <desc>
        <p>This function returns a list of all keys in the dictionary.</p>
      </desc>
    </func>
    <func>
      <name>filter(Pred, Dict1) -> Dict2</name>
      <fsummary>Choose elements which satisfy a predicate</fsummary>
      <type>
        <v>Pred = fun(Key, Value) -> bool()</v>
        <v>&nbsp;Key = Value = term()</v>
        <v>Dict1 = Dict2 = dictionary()</v>
      </type>
      <desc>
        <p><c>Dict2</c> is a dictionary of all keys and values in
          <c>Dict1</c> for which <c>Pred(Key, Value)</c> is <c>true</c>.</p>
      </desc>
    </func>
    <func>
      <name>find(Key, Dict) -> {ok, Value} | error</name>
      <fsummary>Search for a key in a dictionary</fsummary>
      <type>
        <v>Key = Value = term()</v>
        <v>Dict = dictionary()</v>
      </type>
      <desc>
        <p>This function searches for a key in a dictionary. Returns
          <c>{ok, Value}</c> where <c>Value</c> is the value associated
          with <c>Key</c>, or <c>error</c> if the key is not present in
          the dictionary.</p>
      </desc>
    </func>
    <func>
      <name>fold(Fun, Acc0, Dict) -> Acc1</name>
      <fsummary>Fold a function over a dictionary</fsummary>
      <type>
        <v>Fun = fun(Key, Value, AccIn) -> AccOut</v>
        <v>Key = Value = term()</v>
        <v>Acc0 = Acc1 = AccIn = AccOut = term()</v>
        <v>Dict = dictionary()</v>
      </type>
      <desc>
        <p>Calls <c>Fun</c> on successive keys and values of
          <c>Dict</c> together with an extra argument <c>Acc</c>
          (short for accumulator). <c>Fun</c> must return a new
          accumulator which is passed to the next call. <c>Acc0</c> is
          returned if the list is empty. The evaluation order is
          undefined.</p>
      </desc>
    </func>
    <func>
      <name>from_list(List) -> Dict</name>
      <fsummary>Convert a list of pairs to a dictionary</fsummary>
      <type>
        <v>List = [{Key, Value}]</v>
        <v>Dict = dictionary()</v>
      </type>
      <desc>
        <p>This function converts the <c>Key</c> - <c>Value</c> list
          <c>List</c> to a dictionary.</p>
      </desc>
    </func>
    <func>
      <name>is_key(Key, Dict) -> bool()</name>
      <fsummary>Test if a key is in a dictionary</fsummary>
      <type>
        <v>Key = term()</v>
        <v>Dict = dictionary()</v>
      </type>
      <desc>
        <p>This function tests if <c>Key</c> is contained in
          the dictionary <c>Dict</c>.</p>
      </desc>
    </func>
    <func>
      <name>map(Fun, Dict1) -> Dict2</name>
      <fsummary>Map a function over a dictionary</fsummary>
      <type>
        <v>Fun = fun(Key, Value1) -> Value2</v>
        <v>&nbsp;Key = Value1 = Value2 = term()</v>
        <v>Dict1 = Dict2 = dictionary()</v>
      </type>
      <desc>
        <p><c>map</c> calls <c>Func</c> on successive keys and values
          of <c>Dict</c> to return a new value for each key.
          The evaluation order is undefined.</p>
      </desc>
    </func>
    <func>
      <name>merge(Fun, Dict1, Dict2) -> Dict3</name>
      <fsummary>Merge two dictionaries</fsummary>
      <type>
        <v>Fun = fun(Key, Value1, Value2) -> Value</v>
        <v>&nbsp;Key = Value1 = Value2 = Value3 = term()</v>
        <v>Dict1 = Dict2 = Dict3 = dictionary()</v>
      </type>
      <desc>
        <p><c>merge</c> merges two dictionaries, <c>Dict1</c> and
          <c>Dict2</c>, to create a new dictionary.  All the <c>Key</c>
          - <c>Value</c> pairs from both dictionaries are included in
          the new dictionary.  If a key occurs in both dictionaries then
          <c>Fun</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>new() -> dictionary()</name>
      <fsummary>Create a dictionary</fsummary>
      <desc>
        <p>This function creates a new dictionary.</p>
      </desc>
    </func>
    <func>
      <name>size(Dict) -> int()</name>
      <fsummary>Return the number of elements in a dictionary</fsummary>
      <type>
        <v>Dict = dictionary()</v>
      </type>
      <desc>
        <p>Returns the number of elements in a <c>Dict</c>.</p>
      </desc>
    </func>
    <func>
      <name>store(Key, Value, Dict1) -> Dict2</name>
      <fsummary>Store a value in a dictionary</fsummary>
      <type>
        <v>Key = Value = term()</v>
        <v>Dict1 = Dict2 = dictionary()</v>
      </type>
      <desc>
        <p>This function stores a <c>Key</c> - <c>Value</c> pair in a
          dictionary. If the <c>Key</c> already exists in <c>Dict1</c>,
          the associated value is replaced by <c>Value</c>.</p>
      </desc>
    </func>
    <func>
      <name>to_list(Dict) -> List</name>
      <fsummary>Convert a dictionary to a list of pairs</fsummary>
      <type>
        <v>Dict = dictionary()</v>
        <v>List = [{Key, Value}]</v>
      </type>
      <desc>
        <p>This function converts the dictionary to a list
          representation.</p>
      </desc>
    </func>
    <func>
      <name>update(Key, Fun, Dict1) -> Dict2</name>
      <fsummary>Update a value in a dictionary</fsummary>
      <type>
        <v>Key = term()</v>
        <v>Fun = fun(Value1) -> Value2</v>
        <v>&nbsp;Value1 = Value2 = term()</v>
        <v>Dict1 = Dict2 = dictionary()</v>
      </type>
      <desc>
        <p>Update a value in a dictionary by calling <c>Fun</c> on
          the value to get a new value.  An exception is generated if
          <c>Key</c> is not present in the dictionary.</p>
      </desc>
    </func>
    <func>
      <name>update(Key, Fun, Initial, Dict1) -> Dict2</name>
      <fsummary>Update a value in a dictionary</fsummary>
      <type>
        <v>Key = Initial = term()</v>
        <v>Fun = fun(Value1) -> Value2</v>
        <v>&nbsp;Value1 = Value2 = term()</v>
        <v>Dict1 = Dict2 = dictionary()</v>
      </type>
      <desc>
        <p>Update a value in a dictionary by calling <c>Fun</c> on
          the value to get a new value.  If <c>Key</c> is not present
          in the dictionary then <c>Initial</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>update_counter(Key, Increment, Dict1) -> Dict2</name>
      <fsummary>Increment a value in a dictionary</fsummary>
      <type>
        <v>Key = term()</v>
        <v>Increment = number()</v>
        <v>Dict1 = Dict2 = dictionary()</v>
      </type>
      <desc>
        <p>Add <c>Increment</c> to the value associated with <c>Key</c>
          and store this value.  If <c>Key</c> is not present in
          the dictionary then <c>Increment</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>

