maps

適用于需要在運(yùn)行時(shí)改變數(shù)據(jù)結(jié)構(gòu)(record則不行)的場(chǎng)景，可以動(dòng)態(tài)增加key

數(shù)據(jù)量不宜過大，具體多大沒有實(shí)際數(shù)據(jù)，

maps from_list ?如果list表很長(zhǎng)，則相應(yīng)的耗時(shí)時(shí)間會(huì)很長(zhǎng)，此時(shí)最好用lists模塊。

由于map是動(dòng)態(tài)結(jié)構(gòu)，速度上必然無法匹敵record。

內(nèi)存大小介于tuple與list之間

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.

示例代碼

-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 模塊封裝了一些函數(shù)

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).

更多閱讀：

http://learnyousomeerlang.com/maps

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

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