erlang编程指南 练习3-4

练习3-4    编写创建一个数据库模块db.erl，它能够模拟数据库 存储，检索和删除元素。

提示使用列表和元组作为你的主要数据结构，当测试程序时，请记住Erlang中变量是单次赋值。

接口：

db:new()       =>Db.

db:destroy(Db)     =>ok.

db:write(Key,Element,Db)  =>newDb.

db:read(Key,Db)  =>{ok,Element}|{error,instance}.

db:match(Element,Db) =>[key1,.......,keyN].

 

代码如下：

 

%% Author: liu.guangjun
%% Created: 2011-12-5
%% Description: TODO: Add description to db
-module(db).

%%
%% Include files
%%

%%
%% Exported Functions
%%
-export([new/0,write/3,delete/2,read/2,match/2]).
% -export([destroy/1]).
%%
%% Local Functions
%%
new()->[].
%destroy(Db)-> f(Db).
write(Key,Element,Db)->
	[{Key,Element}|Db].		  
		
%% read(Key,Db)->
%% 	case lists:keyfind(Key, 1, Db) of
%% 		{_,Element} ->{ok,Element};
%% 		false->{error,instance}
%%     end.

read(_,[])->{error,instance};
read(Key,[H1|T1])->
    case H1 of 
		{Key,Element} ->
			{ok,Element};
		_ ->  
			read(Key,T1)
  	end.
	


delete(Key,Db)->delete_element(Key,Db,[]).
delete_element(_,[],List2)->List2;
delete_element(Key,[H1|T1],T2)->
	case H1 of
		{Key,_} ->
	   		delete_element(Key,T1,T2);
 		_->
			delete_element(Key,T1,[H1|T2])
 	end.

match(Element,Keys)->match_element(Element,Keys,[]).
match_element(_,[],Keys)->Keys;
match_element(Element,[H|T],T2)->
	case H of
		{H2,Element} ->
			match_element(Element,T,[H2|T2]);
		_->
			match_element(Element,T,T2)
	end.