erlang ets表

一、表遍历

通过ets:first/1获取表的第一个关键字，表中下一个关键字用ets:next/2得到，直到ets:next/2返回'$end_of_table'

当多几个进程并发访问ets表时，可以使用ets：safe_fixtable/2函数来保证，每个元素只被访问一次。

-module(traversal_ets).
-export([traversal_ets/1]).

traversal_ets(Tab) ->
    traversal_ets(Tab, ets:first(Tab), []).
traversal_ets(_,'$end_of_table',Acc) ->
    Acc;    
traversal_ets(Tab, Prv, Acc) ->
    traversal_ets(Tab, ets:next(Tab,Prv), [Prv|Acc]).

二、提取表信息，匹配match和match_object
24> ets:new(countries, [bag,named_table]).
countries
25> ets:insert(countries,{yves,france,cook}).
true
26> ets:insert(countries,{marcoitaly,cook}).
true
27> ets:insert(countries,{sean,ireland,bartender}).
true
28> ets:insert(countries,{chris,ireland,tester}).
true

1、match指定匹配输出

9> ets:match(countries,{'$1',ireland,'_'}).
[[sean],[chris]]
10> ets:match(countries,{'$1','$0',cook}).  
[[italy,marco],[france,yves]]
11> ets:match(countries,{'$2',ireland,'_'}).
[[sean],[chris]]
12> ets:match(countries,{'$2',cook,'_'}).   
[]
2、matc_object全部匹配模式元祖

13> ets:match_object(countries,{'_',ireland,'_'}).
[{sean,ireland,bartender},{chris,ireland,tester}]

3、match_delete 删除匹配输出
14> NewTab = ets:match_delete(countries,{'_',ireland,'_'}).
true

三、提取表信息筛选

ets:select

1、用匹配规约做参数

ets:select(countries,[{{'$1','$2','$3'},[{'/=','$3',cook}],[['$2','$1']]}]).

{'$1','$2','$3'} 模式

{'/=','$3',cook}保护元

['$2','$1'] 返回表达式

2、用ets:fun2ms返回匹配规约

30> MS = ets:fun2ms(fun({Name, Country,Job}) when Job /= cook ->
30> [Country,Name] end).
[{{'$1','$2','$3'},[{'/=','$3',cook}],[['$2','$1']]}]
31> ets:select(countries, MS).
[[ireland,sean],[ireland,chris]]