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

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

<erlref>
	<header>
		<copyright>
			<year>2013</year><year>2014</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>
	<funcs>

		<func>
			<name name="filter" arity="2"/>
			<fsummary>Choose pairs which satisfy a predicate</fsummary>
			<desc>
				<p>
                    Returns a map <c><anno>Map2</anno></c> for which predicate
                    <c><anno>Pred</anno></c> holds true in <c><anno>Map1</anno></c>.
				</p>
				<p>
                    The call will fail with a <c>{badmap,Map}</c> exception if
                    <c><anno>Map1</anno></c> is not a map or with <c>badarg</c> if
                    <c><anno>Pred</anno></c> is not a function of arity 2.
				</p>
				<p>Example:</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 will fail with a <c>{badmap,Map}</c> exception if <c><anno>Map</anno></c> is not a map.
				</p>
				<p>Example:</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>Map</anno></c> in
					arbitrary order. The function <c>fun F/3</c> must return a new accumulator
					which is passed to the next successive call. <c>maps:fold/3</c> 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>Example:</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>
					The function takes a list of key-value tuples elements and builds a
					map. The associations may be in any order and both keys and values in the
					association may be of any term. If the same key appears more than once,
					the latter (rightmost) value is used and the previous values are ignored.
				</p>
				<p>Example:</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 the 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 will fail 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>Example:</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 the 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> then returns <c><anno>Default</anno></c>.
				</p>
				<p>
					The call will fail with a <c>{badmap,Map}</c> exception if <c><anno>Map</anno></c> is not a map.

				</p>
				<p>Example:</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 will fail with a <c>{badmap,Map}</c> exception if <c><anno>Map</anno></c> is not a map.
				</p>
				<p>Example:</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="keys" arity="1"/>
			<fsummary></fsummary>
			<desc>
				<p>
					Returns a complete list of keys, in arbitrary order, which resides within <c><anno>Map</anno></c>.
				</p>
				<p>
					The call will fail with a <c>{badmap,Map}</c> exception if <c><anno>Map</anno></c> is not a map.
				</p>
				<p>Example:</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>
					The function produces a new map <c><anno>Map2</anno></c> by calling the 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>Map1</anno></c> in arbitrary order.
					The function <c>fun F/2</c> must return the value <c><anno>V2</anno></c> to be associated with key <c><anno>K</anno></c> for
					the new map <c><anno>Map2</anno></c>.
				</p>
				<p>Example:</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 exists in both maps the
					value in <c><anno>Map1</anno></c> will be superseded by the value in <c><anno>Map2</anno></c>.
				</p>
				<p>
					The call will fail 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>Example:</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>Example:</p>
				<code type="none">
> maps:new().
#{}</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 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" => 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>
					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 new map <c><anno>Map2</anno></c> without key <c><anno>Key</anno></c>.
				</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" => 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>
					The function returns the number of key-value associations in the <c><anno>Map</anno></c>.
					This operation happens in constant time.
				</p>
				<p>Example:</p>
				<code type="none">
> Map = #{42 => value_two,1337 => "value one","a" => 1},
  maps:size(Map).
3</code>
			</desc>
		</func>

		<func>
			<name name="to_list" arity="1"/>
			<fsummary></fsummary>
			<desc>
				<p>
					The fuction 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 will fail with a <c>{badmap,Map}</c> exception if <c><anno>Map</anno></c> is not a map.
				</p>
				<p>Example:</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 will fail 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>Example:</p>
				<code type="none">
> Map = #{"a" => 1}.
#{"a" => 1}
> maps:update("a", 42, Map).
#{"a" => 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 will fail with a <c>{badmap,Map}</c> exception if <c><anno>Map</anno></c> is not a map.
				</p>
				<p>Example:</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> are ignored.
				</p>
				<p>Example:</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 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> are ignored.
				</p>
				<p>Example:</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>