erlang的map基本使用

maps

　　适用于需要在运行时改变数据结构（record则不行）的场景，可以动态增加key

数据量不宜过大，具体多大没有实际数据，

maps from_list  如果list表很长，则相应的耗时时间会很长，此时最好用lists模块。

由于map是动态结构，速度上必然无法匹敌record。

内存大小介于tuple与list之间

lists:sort([1,#{}, {}, []]).

[1,{},#{},[]]

Operations	Records	Maps	Dict
Immutable	✓	✓	✓
Keys of any type		✓	✓
Usable with maps/folds		✓	✓
Content opaque to other modules	✓
Has a module to use it		✓	✓
Supports pattern matching	✓	✓
All keys known at compile-time	✓
Can merge with other instance		✓	✓
Testing for presence of a key		✓	✓
Extract value by key	✓	✓	✓
Per-key Dialyzer type-checking	✓	*
Conversion from/to lists		✓	✓
Per-element default values	✓
Standalone data type at runtime		✓
Fast direct-index access	✓

* The EEP recommends making this possible for keys known at compile-time, but has no ETA on when or if this will happen.

示例代码

-module (test).
-compile([export_all]).

% maps:
% find/2         fold/3         from_list/1    get/2          get/3
% is_key/2       keys/1         map/2          merge/2        module_info/0
% module_info/1  new/0          put/3          remove/2       size/1
% to_list/1      update/3       values/1       with/2         without/2

start() ->
    A = #{k1 => 1, k2 => 2, k3 => 3},
    #{k1 := K1} = A,
    A1 = A#{ k1 := K1 + 1}.

%
make() ->
    % maps: new | from_list
    M1 = maps:new(),
    % insert new one
    M2 = M1#{ k1 => 1},
    % update
    M3 = M2#{ k1 := 2},

    % maps 模块封装了一些函数
    maps:put(k2, 2, M3).

%  匹配
match(#{k1 := K1}) -> io:format("~p ~n", [K1]);
match(#{nothing := N}) -> io:format("~p ~n", [N]).

raw() ->
    [ {I, I}|| I <- lists:seq(1, 10000000)].

get() ->
    L = raw(),
    lists:keyfind(999, 1, L).

get2() ->
    M = maps:from_list(raw()),

    maps:get(999, M).

更多阅读：

　　http://learnyousomeerlang.com/maps

http://erlang.org/doc/man/maps.html