chapter 3: exercice 3.4

Author: omorel

chap3/db.erl

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+-module (db).
+-export ([new/0, 
+          destroy/1, 
+          write/3, 
+          delete/2, 
+          read/2, 
+          match/2, 
+          test_db/0]).
+
+
+new() -> 
+	[]. 
+
+destroy(_Db) -> 
+	ok. 
+	
+write(Key, Element, [{Key, _Value}|T]) -> 
+	[{Key, Element}|T]; 
+write(Key, Element, []) -> 
+	[{Key, Element}];
+write(Key, Element, [H|T]) -> 
+    [H|write(Key, Element, T)]. 
+	
+delete(Key, [{Key, _Value}|T]) -> 
+ 	T;
+delete(Key, [H|T]) -> 
+	[H|delete(Key, T)]; 
+delete(_Key, []) -> 
+	[]. 
+
+read(Key, [{Key, Element}|_T]) -> 
+	{ok, Element}; 
+read(_Key, []) ->
+	{error, instance}; 
+read(Key, [_H|T]) ->
+	read(Key, T).
+
+match(Element, Db) -> 
+	match_acc(Element, [], Db). 
+	
+match_acc(Element, Matches, [{Key, Element}|T]) -> 
+	match_acc(Element, [Key|Matches], T); 
+match_acc(_Element, Matches, []) ->
+	Matches; 
+match_acc(Element, Matches, [_H|T]) ->
+	match_acc(Element, Matches, T). 
+
+
+test_db() -> 
+	Db = new(), 
+	io:fwrite("Db = ~p~n", [Db]),
+	Db1 = write(francesco, london, Db),  
+	io:fwrite("Db1 = ~p~n", [Db1]),
+	Db2 = write(lelle, stockholm, Db1), 
+	io:fwrite("Db2 = ~p~n", [Db2]),
+	Read1 = read(francesco, Db2),
+	io:fwrite("Read = ~p~n", [Read1]), 
+	Db3 = write(joern, stockholm, Db2),
+	io:fwrite("Db3 = ~p~n", [Db3]),
+	Read2 = read(ola, Db3), 
+	io:fwrite("Read2 = ~p~n", [Read2]),
+	Match1 = match(stockholm, Db3), 
+	io:fwrite("Match1 = ~p~n", [Match1]),
+	Db4 = delete(lelle, Db3),
+	io:fwrite("Db4 = ~p~n", [Db4]), 
+	Match2 = match(stockholm, Db4),
+	io:fwrite("Match2 = ~p~n", [Match2]).