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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2000</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>orddict</title>
    <prepared>Robert Virding</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date>2007-04-16</date>
    <rev>B</rev>
    <file>orddict.xml</file>
  </header>
  <module>orddict</module>
  <modulesummary>Key-value dictionary as ordered list.</modulesummary>
  <description>
    <p>This module provides a <c>Key</c>-<c>Value</c> dictionary.
      An <c>orddict</c> is a representation of a dictionary, where a
      list of pairs is used to store the keys and values. The list is
      ordered after the keys in the <em>Erlang term order</em>.</p>

    <p>This module provides the same interface as the
      <seealso marker="dict"><c>dict(3)</c></seealso> module
      but with a defined representation. One difference is
      that while <c>dict</c> considers two keys as different if they
      do not match (<c>=:=</c>), this module considers two keys as
      different if and only if they do not compare equal (<c>==</c>).</p>
  </description>

  <datatypes>
    <datatype>
      <name name="orddict" n_vars="2"/>
      <desc><p>Dictionary as returned by
        <seealso marker="#new/0"><c>new/0</c></seealso>.</p></desc>
    </datatype>
    <datatype>
      <name name="orddict" n_vars="0"/>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="append" arity="3"/>
      <fsummary>Append a value to keys in a dictionary.</fsummary>
      <desc>
        <p>Appends a new <c><anno>Value</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>
        <p>See also section <seealso marker="#notes">Notes</seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="append_list" arity="3"/>
      <fsummary>Append new values to keys in a dictionary.</fsummary>
      <desc>
        <p>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>
        <p>See also section <seealso marker="#notes">Notes</seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="erase" arity="2"/>
      <fsummary>Erase a key from a dictionary.</fsummary>
      <desc>
        <p>Erases all items with a specified key from a dictionary.</p>
      </desc>
    </func>

    <func>
      <name name="fetch" arity="2"/>
      <fsummary>Look up values in a dictionary.</fsummary>
      <desc>
        <p>Returns the value associated with <c><anno>Key</anno></c>
          in dictionary <c><anno>Orddict</anno></c>. This function assumes that
          the <c><anno>Key</anno></c> is present in the dictionary. An exception
          is generated if <c><anno>Key</anno></c> is not in the dictionary.</p>
        <p>See also section <seealso marker="#notes">Notes</seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="fetch_keys" arity="1"/>
      <fsummary>Return all keys in a dictionary.</fsummary>
      <desc>
        <p>Returns a list of all keys in a dictionary.</p>
      </desc>
    </func>

    <func>
      <name name="take" arity="2" since="OTP 20.0"/>
      <fsummary>Return value and new dictionary without element with this value.</fsummary>
      <desc>
        <p>This function returns value from dictionary and new dictionary without this value.
            Returns <c>error</c> if the key is not present in the dictionary.</p>
      </desc>
    </func>

    <func>
      <name name="filter" arity="2"/>
      <fsummary>Select elements that satisfy a predicate.</fsummary>
      <desc>
        <p><c><anno>Orddict2</anno></c> is a dictionary of all keys and values
          in <c><anno>Orddict1</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>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>
        <p>See also section <seealso marker="#notes">Notes</seealso>.</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>Orddict</anno></c> together with an extra argument <c>Acc</c>
          (short for accumulator). <c><anno>Fun</anno></c> must return a new
          accumulator that is passed to the next call. <c><anno>Acc0</anno></c>
          is returned if the list is empty.</p>
      </desc>
    </func>

    <func>
      <name name="from_list" arity="1"/>
      <fsummary>Convert a list of pairs to a dictionary.</fsummary>
      <desc>
        <p>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_empty" arity="1" since="OTP 17.0"/>
      <fsummary>Return true if the dictionary is empty.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Orddict</anno></c> has no elements,
          otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="is_key" arity="2"/>
      <fsummary>Test if a key is in a dictionary.</fsummary>
      <desc>
        <p>Tests if <c><anno>Key</anno></c> is contained in
          dictionary <c><anno>Orddict</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="map" arity="2"/>
      <fsummary>Map a function over a dictionary.</fsummary>
      <desc>
        <p>Calls <c><anno>Fun</anno></c> on successive keys and values of
          <c><anno>Orddict1</anno></c> tvo return a new value for each key.</p>
      </desc>
    </func>

    <func>
      <name name="merge" arity="3"/>
      <fsummary>Merge two dictionaries.</fsummary>
      <desc>
        <p>Merges two dictionaries, <c><anno>Orddict1</anno></c> and
          <c><anno>Orddict2</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, <c><anno>Fun</anno></c> is called with the key and
          both values to return a new value.
	  <c>merge/3</c> can be defined as follows, but is faster:</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>
      </desc>
    </func>

    <func>
      <name name="new" arity="0"/>
      <fsummary>Create a dictionary.</fsummary>
      <desc>
        <p>Creates a new dictionary.</p>
      </desc>
    </func>

    <func>
      <name name="size" arity="1"/>
      <fsummary>Return the number of elements in an ordered dictionary.
      </fsummary>
      <desc>
        <p>Returns the number of elements in an <c><anno>Orddict</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="store" arity="3"/>
      <fsummary>Store a value in a dictionary.</fsummary>
      <desc>
        <p>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>Orddict1</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>Converts a dictionary to a list representation.</p>
      </desc>
    </func>

    <func>
      <name name="update" arity="3"/>
      <fsummary>Update a value in a dictionary.</fsummary>
      <desc>
        <p>Updates 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>Updates 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, <c><anno>Initial</anno></c> is stored as
          the first value. For example, <c>append/3</c> can be defined
          as follows:</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>Adds <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, <c><anno>Increment</anno></c> is stored as
          the first value.</p>
        <p>This can be defined as follows, but is faster:</p>
        <code type="none">
update_counter(Key, Incr, D) ->
    update(Key, fun (Old) -> Old + Incr end, Incr, D).</code>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Notes</title>
    <marker id="notes"/>
    <p>Functions <c>append/3</c> and <c>append_list/3</c> are included
      so that keyed values can be stored in a list <em>accumulator</em>, for
      example:</p>
    <pre>
> D0 = orddict:new(),
  D1 = orddict:store(files, [], D0),
  D2 = orddict:append(files, f1, D1),
  D3 = orddict:append(files, f2, D2),
  D4 = orddict:append(files, f3, D3),
  orddict: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>Function <c>fetch/2</c> is to be used if the key is known to
      be in the dictionary, otherwise function <c>find/2</c>.</p>
  </section>

  <section>
    <title>See Also</title>
    <p><seealso marker="dict"><c>dict(3)</c></seealso>,
      <seealso marker="gb_trees"><c>gb_trees(3)</c></seealso></p>
  </section>
</erlref>