path: root/lib/stdlib/doc/src/maps.xml

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  <header>
    <copyright>
      <year>2013</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>maps</title>
    <prepared>Björn-Egil Dahlberg</prepared>
    <docno>1</docno>
    <date>2014-02-28</date>
    <rev>A</rev>
  </header>
  <module>maps</module>
  <modulesummary>Maps processing functions.</modulesummary>
  <description>
    <p>This module contains functions for maps processing.</p>
  </description>

  <datatypes>
    <datatype>
      <name name="iterator"/>
      <desc>
        <p>An iterator representing the key value associations in a map.</p>
        <p>Created using <seealso marker="#iterator-1"><c>maps:iterator/1</c></seealso>.</p>
        <p>Consumed by <seealso marker="#next-1"><c>maps:next/1</c></seealso>,
          <seealso marker="#filter-2"><c>maps:filter/2</c></seealso>,
          <seealso marker="#fold-3"><c>maps:fold/3</c></seealso> and
          <seealso marker="#map-2"><c>maps:map/2</c></seealso>.</p>
      </desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="filter" arity="2"/>
      <fsummary>Select pairs that satisfy a predicate.</fsummary>
      <desc>
        <p>Returns a map <c><anno>Map</anno></c> for which predicate
          <c><anno>Pred</anno></c> holds true in <c><anno>MapOrIter</anno></c>.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>MapOrIter</anno></c> is not a map or valid iterator,
          or with <c>badarg</c> if <c><anno>Pred</anno></c> is not a
          function of arity 2.</p>
        <p><em>Example:</em></p>
        <code type="none">
> M = #{a => 2, b => 3, c=> 4, "a" => 1, "b" => 2, "c" => 4},
  Pred = fun(K,V) -> is_atom(K) andalso (V rem 2) =:= 0 end,
  maps:filter(Pred,M).
#{a => 2,c => 4}</code>
      </desc>
    </func>

    <func>
      <name name="find" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns a tuple <c>{ok, Value}</c>, where <c><anno>Value</anno></c>
          is the value associated with <c><anno>Key</anno></c>, or <c>error</c>
          if no value is associated with <c><anno>Key</anno></c> in
          <c><anno>Map</anno></c>.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>Map</anno></c> is not a map.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{"hi" => 42},
  Key = "hi",
  maps:find(Key,Map).
{ok,42}</code>
      </desc>
    </func>

    <func>
      <name name="fold" arity="3"/>
      <fsummary></fsummary>
      <desc>
        <p>Calls <c>F(K, V, AccIn)</c> for every <c><anno>K</anno></c> to value
          <c><anno>V</anno></c> association in <c><anno>MapOrIter</anno></c> in
          any order. Function <c>fun F/3</c> must return a new
          accumulator, which is passed to the next successive call.
          This function returns the final value of the accumulator. The initial
          accumulator value <c><anno>Init</anno></c> is returned if the map is
          empty.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>MapOrIter</anno></c> is not a map or valid iterator,
          or with <c>badarg</c> if <c><anno>Fun</anno></c> is not a
          function of arity 3.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Fun = fun(K,V,AccIn) when is_list(K) -> AccIn + V end,
  Map = #{"k1" => 1, "k2" => 2, "k3" => 3},
  maps:fold(Fun,0,Map).
6</code>
      </desc>
    </func>

    <func>
      <name name="from_list" arity="1"/>
      <fsummary></fsummary>
      <desc>
        <p>Takes a list of key-value tuples elements and builds a map. The
          associations can be in any order, and both keys and values in the
          association can be of any term. If the same key appears more than
          once, the latter (right-most) value is used and the previous values
          are ignored.</p>
        <p><em>Example:</em></p>
        <code type="none">
> List = [{"a",ignored},{1337,"value two"},{42,value_three},{"a",1}],
  maps:from_list(List).
#{42 => value_three,1337 => "value two","a" => 1}</code>
      </desc>
    </func>

    <func>
      <name name="get" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns value <c><anno>Value</anno></c> associated with
          <c><anno>Key</anno></c> if <c><anno>Map</anno></c> contains
          <c><anno>Key</anno></c>.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>Map</anno></c> is not a map, or with a <c>{badkey,Key}</c>
          exception if no value is associated with <c><anno>Key</anno></c>.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Key = 1337,
  Map = #{42 => value_two,1337 => "value one","a" => 1},
  maps:get(Key,Map).
"value one"</code>
      </desc>
    </func>

    <func>
      <name name="get" arity="3"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns value <c><anno>Value</anno></c> associated with
          <c><anno>Key</anno></c> if <c><anno>Map</anno></c> contains
          <c><anno>Key</anno></c>. If no value is associated with
          <c><anno>Key</anno></c>, <c><anno>Default</anno></c> is returned.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>Map</anno></c> is not a map.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{ key1 => val1, key2 => val2 }.
#{key1 => val1,key2 => val2}
> maps:get(key1, Map, "Default value").
val1
> maps:get(key3, Map, "Default value").
"Default value"</code>
      </desc>
    </func>

    <func>
      <name name="is_key" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns <c>true</c> if map <c><anno>Map</anno></c> contains
          <c><anno>Key</anno></c> and returns <c>false</c> if it does not
          contain the <c><anno>Key</anno></c>.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>Map</anno></c> is not a map.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{"42" => value}.
#{"42" => value}
> maps:is_key("42",Map).
true
> maps:is_key(value,Map).
false</code>
      </desc>
    </func>

    <func>
      <name name="iterator" arity="1"/>
      <fsummary>Create a map iterator.</fsummary>
      <desc>
        <p>Returns a map iterator <c><anno>Iterator</anno></c> that can
          be used by <seealso marker="#next-1"><c>maps:next/1</c></seealso>
          to traverse the key-value associations in a map. When iterating
          over a map, the memory usage is guaranteed to be bounded no matter
          the size of the map.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>Map</anno></c> is not a map.</p>
        <p><em>Example:</em></p>
        <code type="none">
> M = #{ a => 1, b => 2 }.
#{a => 1,b => 2}
> I = maps:iterator(M).
[{a,1},{b,2}]
> {K1, V1, I2} = maps:next(I).
{a,1,[{b,2}]}
> {K2, V2, I3} = maps:next(I2).
{b,2,[]}
> maps:next(I3).
none</code>
      </desc>
    </func>

    <func>
      <name name="keys" arity="1"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns a complete list of keys, in any order, which resides
          within <c><anno>Map</anno></c>.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>Map</anno></c> is not a map.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{42 => value_three,1337 => "value two","a" => 1},
  maps:keys(Map).
[42,1337,"a"]</code>
      </desc>
    </func>

    <func>
      <name name="map" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Produces a new map <c><anno>Map</anno></c> by calling function
          <c>fun F(K, V1)</c> for every <c><anno>K</anno></c> to value
          <c><anno>V1</anno></c> association in <c><anno>MapOrIter</anno></c> in
          any order. Function <c>fun F/2</c> must return value
          <c><anno>V2</anno></c> to be associated with key <c><anno>K</anno></c>
          for the new map <c><anno>Map</anno></c>.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>MapOrIter</anno></c> is not a map or valid iterator,
          or with <c>badarg</c> if <c><anno>Fun</anno></c> is not a
          function of arity 2.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Fun = fun(K,V1) when is_list(K) -> V1*2 end,
  Map = #{"k1" => 1, "k2" => 2, "k3" => 3},
  maps:map(Fun,Map).
#{"k1" => 2,"k2" => 4,"k3" => 6}</code>
      </desc>
    </func>

    <func>
      <name name="merge" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Merges two maps into a single map <c><anno>Map3</anno></c>. If two
          keys exist in both maps, the value in <c><anno>Map1</anno></c> is
          superseded by the value in <c><anno>Map2</anno></c>.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>Map1</anno></c> or <c><anno>Map2</anno></c> is not a map.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map1 = #{a => "value_one", b => "value_two"},
  Map2 = #{a => 1, c => 2},
  maps:merge(Map1,Map2).
#{a => 1,b => "value_two",c => 2}</code>
      </desc>
    </func>

    <func>
      <name name="new" arity="0"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns a new empty map.</p>
        <p><em>Example:</em></p>
        <code type="none">
> maps:new().
#{}</code>
      </desc>
    </func>

    <func>
      <name name="next" arity="1"/>
      <fsummary>Get the next key and value from an iterator.</fsummary>
      <desc>
        <p>Returns the next key-value association in
          <c><anno>Iterator</anno></c> and a new iterator for the
          remaining associations in the iterator.
        </p>
        <p>
          If there are no more associations in the iterator,
          <c>none</c> is returned.
        </p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{a => 1, b => 2, c => 3}.
#{a => 1,b => 2,c => 3}
> Iter = maps:iterator(Map).
[{a,1},{b,2},{c,3}]
> {_, _, Iter1} = maps:next(Iter).
{a,1,[{b,2},{c,3}]}
> {_, _, Iter2} = maps:next(Iter1).
{b,2,[{c,3}]}
> {_, _, Iter3} = maps:next(Iter2).
{c,3,[]}
> maps:next(Iter3).
none</code>
      </desc>
    </func>

    <func>
      <name name="put" arity="3"/>
      <fsummary></fsummary>
      <desc>
        <p>Associates <c><anno>Key</anno></c> with value
          <c><anno>Value</anno></c> and inserts the association into map
          <c>Map2</c>. If key <c><anno>Key</anno></c> already exists in map
          <c><anno>Map1</anno></c>, the old associated value is replaced by
          value <c><anno>Value</anno></c>. The function returns a new map
          <c><anno>Map2</anno></c> containing the new association and the old
          associations in <c><anno>Map1</anno></c>.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>Map1</anno></c> is not a map.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{"a" => 1}.
#{"a" => 1}
> maps:put("a", 42, Map).
#{"a" => 42}
> maps:put("b", 1337, Map).
#{"a" => 1,"b" => 1337}</code>
      </desc>
    </func>

    <func>
      <name name="remove" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Removes the <c><anno>Key</anno></c>, if it exists, and its
          associated value from <c><anno>Map1</anno></c> and returns a new map
          <c><anno>Map2</anno></c> without key <c><anno>Key</anno></c>.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>Map1</anno></c> is not a map.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{"a" => 1}.
#{"a" => 1}
> maps:remove("a",Map).
#{}
> maps:remove("b",Map).
#{"a" => 1}</code>
      </desc>
    </func>

    <func>
      <name name="size" arity="1"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns the number of key-value associations in
          <c><anno>Map</anno></c>. This operation occurs in constant time.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{42 => value_two,1337 => "value one","a" => 1},
  maps:size(Map).
3</code>
      </desc>
    </func>

    <func>
      <name name="take" arity="2"/>
      <fsummary></fsummary>
      <desc>
	<p>The function removes the <c><anno>Key</anno></c>, if it
	exists, and its associated value from <c><anno>Map1</anno></c>
	and returns a tuple with the removed <c><anno>Value</anno></c>
	and the new map <c><anno>Map2</anno></c> without key
	<c><anno>Key</anno></c>.  If the key does not exist
	<c>error</c> is returned.
	</p>
	<p>The call will fail with a <c>{badmap,Map}</c> exception if
	<c><anno>Map1</anno></c> is not a map.
	</p>
	<p>Example:</p>
	<code type="none">
> Map = #{"a" => "hello", "b" => "world"}.
#{"a" => "hello", "b" => "world"}
> maps:take("a",Map).
{"hello",#{"b" => "world"}}
> maps:take("does not exist",Map).
error</code>
      </desc>
    </func>

    <func>
      <name name="to_list" arity="1"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns a list of pairs representing the key-value associations of
          <c><anno>Map</anno></c>, where the pairs
          <c>[{K1,V1}, ..., {Kn,Vn}]</c> are returned in arbitrary order.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>Map</anno></c> is not a map.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{42 => value_three,1337 => "value two","a" => 1},
  maps:to_list(Map).
[{42,value_three},{1337,"value two"},{"a",1}]</code>
      </desc>
    </func>

    <func>
      <name name="update" arity="3"/>
      <fsummary></fsummary>
      <desc>
        <p>If <c><anno>Key</anno></c> exists in <c><anno>Map1</anno></c>, the
          old associated value is replaced by value <c><anno>Value</anno></c>.
          The function returns a new map <c><anno>Map2</anno></c> containing
          the new associated value.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>Map1</anno></c> is not a map, or with a <c>{badkey,Key}</c>
          exception if no value is associated with <c><anno>Key</anno></c>.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{"a" => 1}.
#{"a" => 1}
> maps:update("a", 42, Map).
#{"a" => 42}</code>
      </desc>
    </func>

    <func>
      <name name="update_with" arity="3"/>
      <fsummary></fsummary>
      <desc>
        <p>Update a value in a <c><anno>Map1</anno></c> associated
        with <c><anno>Key</anno></c> by calling
        <c><anno>Fun</anno></c> on the old value to get a new
        value. An exception <c>{badkey,<anno>Key</anno>}</c> is
        generated if <c><anno>Key</anno></c> is not present in the
        map.</p>
        <p>Example:</p>
        <code type="none">
> Map = #{"counter" => 1},
  Fun = fun(V) -> V + 1 end,
  maps:update_with("counter",Fun,Map).
#{"counter" => 2}</code>
      </desc>
    </func>

    <func>
      <name name="update_with" arity="4"/>
      <fsummary></fsummary>
      <desc>
        <p>Update a value in a <c><anno>Map1</anno></c> associated
        with <c><anno>Key</anno></c> by calling
        <c><anno>Fun</anno></c> on the old value to get a new value.
        If <c><anno>Key</anno></c> is not present in
        <c><anno>Map1</anno></c> then <c><anno>Init</anno></c> will be
        associated with <c><anno>Key</anno></c>.
        </p>
        <p>Example:</p>
        <code type="none">
> Map = #{"counter" => 1},
  Fun = fun(V) -> V + 1 end,
  maps:update_with("new counter",Fun,42,Map).
#{"counter" => 1,"new counter" => 42}</code>
            </desc>
        </func>

    <func>
      <name name="values" arity="1"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns a complete list of values, in arbitrary order, contained in
          map <c>Map</c>.</p>
        <p>The call fails with a <c>{badmap,Map}</c> exception if
          <c><anno>Map</anno></c> is not a map.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{42 => value_three,1337 => "value two","a" => 1},
  maps:values(Map).
[value_three,"value two",1]</code>
      </desc>
    </func>

    <func>
      <name name="with" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns a new map <c><anno>Map2</anno></c> with the keys <c>K1</c>
          through <c>Kn</c> and their associated values from map
          <c><anno>Map1</anno></c>. Any key in <c><anno>Ks</anno></c> that does
          not exist in <c><anno>Map1</anno></c> is ignored.</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{42 => value_three,1337 => "value two","a" => 1},
  Ks = ["a",42,"other key"],
  maps:with(Ks,Map).
#{42 => value_three,"a" => 1}</code>
      </desc>
    </func>

    <func>
      <name name="without" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns a new map <c><anno>Map2</anno></c> without keys <c>K1</c>
          through <c>Kn</c> and their associated values from map
          <c><anno>Map1</anno></c>. Any key in <c><anno>Ks</anno></c> that does
          not exist in <c><anno>Map1</anno></c> is ignored</p>
        <p><em>Example:</em></p>
        <code type="none">
> Map = #{42 => value_three,1337 => "value two","a" => 1},
  Ks = ["a",42,"other key"],
  maps:without(Ks,Map).
#{1337 => "value two"}</code>
      </desc>
    </func>
  </funcs>
</erlref>