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<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>2013</year><year>2014</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
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights 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="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>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>.
If no value is associated with <c><anno>Key</anno></c> then the call will
fail with an exception.
</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="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>.
The function will fail with an 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>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>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>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>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>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. If <c><anno>Key</anno></c> does not exist in <c><anno>Map1</anno></c> an exception is
generated.
</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>M</c>.
</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="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>
