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/2start()->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

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