maps模块¶

find/2

结构:

find(Key, Map) -> {ok, Value} | error
Types
Key = term()
Map = map()
Value = term()

说明:

返回Map中key为Key对应的value

实例:

> Map = #{"hi" => 42},
  Key = "hi",
  maps:find(Key,Map).
{ok,42}

fold/3

注: OTP 17.0增加
fold(Fun, Init, MapOrIter) -> Acc
类型:
Fun = fun((Key, Value, AccIn) -> AccOut)
Init = term()
Acc = AccOut
AccIn = Init | AccOut
MapOrIter = #{Key => Value} | iterator(Key, Value)

实例:

Map = #{<<"25">> => <<"11111">>,
          <<"code">> => <<"scene_data_v2">>,
          <<"t">> => 1582010130302,
          <<"value">> => <<"Gordon">>
          }.
Fun = fun(Key, Value, AccIn) ->
              io:format("Key=~p, Value=~p~n", [Key, Value]),
              [{Key, Value}] ++ AccIn
           end.
Init = [].
maps:fold(Fun, Init, Map).
[{<<"value">>,<<"Gordon">>},
 {<<"t">>,1582010130302},
 {<<"code">>,<<"scene_data_v2">>},
 {<<"25">>,<<"11111">>}]

from_list/1

from_list(List) -> Map
类型:
List = [{Key, Value}]
Key = Value = term()
Map = map()

实例:

erl> List = [{"a",ignored},{1337,"value two"},{42,value_three},{"a",1}].
erl> maps:from_list(List).
#{42 => value_three,1337 => "value two","a" => 1}

get/2/3

get(Key, Map) -> Value
类型
Key = term()
Map = map()
Value = term()

说明:

如原Map中含有Key, 返回Map中key为Key对应的value
如原Map中不含有Key, {badkey,Key}
Map不是map类型, {badmap,Map}

get/3:

如原Map中不含有Key, 则返回默认值

实例:

> Key = 1337,
> Map = #{42 => value_two,1337 => "value one","a" => 1},
> maps:get(Key, Map).
"value one"

> Map = #{ key1 => val1, key2 => val2 }.
#{key1 => val1,key2 => val2}
> maps:get(key1, Map, "Default value").
val1
> maps:get(key3, Map, "Default value").
"Default value"

merge/2

结构:

merge(Map1, Map2) -> Map3
类型:
Map1 = Map2 = Map3 = map()

说明:

Merges two maps into a single map Map3.
If two keys exist in both maps, the value in Map1 is superseded by the value in Map2.

The call fails with a {badmap,Map} exception if Map1 or Map2 is not a map.

实例:

> Map1 = #{a => "value_one", b => "value_two"},
  Map2 = #{a => 1, c => 2},
  maps:merge(Map1,Map2).
#{a => 1,b => "value_two",c => 2}

put/3

结构:

put(Key, Value, Map1) -> Map2
Types
Key = Value = term()
Map1 = Map2 = map()

说明:

往map中插入一条记录{Key, Value},如原Map1中含有Key,则替换此值

实例:

> Map = #{"a" => 1}.
#{"a" => 1}
> maps:put("a", 42, Map).
#{"a" => 42}
> maps:put("b", 1337, Map).
#{"a" => 1,"b" => 1337}

remove/2

结构:

remove(Key, Map1) -> Map2
Types
Key = term()
Map1 = Map2 = map()

说明:

移除此Key对应的记录

实例:

> Map = #{"a" => 1}.
#{"a" => 1}
> maps:remove("a",Map).
#{}
> maps:remove("b",Map).
#{"a" => 1}