Question

我收到了以下事件：项目（C，X，G，P），（其中C是产品编号，X＆＃39;的名称，G它的价格，P它的费用）。
当我直接在prolog控制台上使用命令 item(n3001,_,_,P) 时，我会得到答案 G = 1.25 X = 100但是当我写出等式p3(C)-: item(C,_,_,P).时，我会查阅我得到的文字yes。
我澄清的问题是，有一次我得到了P我想要的价值，而另一次我得到它是真是假？

Answer 1

There are no return values in Prolog and p3/1 does not constitute a function but a relation. Your definition

p3(C) :-
   item(C,_,_,P).

reads: If item(C,_,_,P) succeeds then p3(C) succeeds as well. For the sake of argument, let's assume that your code includes the following fact:

item(n3001,100,1.25,1).

If you query

?- p3(n3001).

Prolog unifies C in the head of your rule with n3001 and then tries your goal item(C,_,_,P) which succeeds. Hence the rule succeeds and Prolog tells you:

   ?- p3(n3001).
yes

If you want to know the price corresponding to n3001 you have to to define a rule where P appears in the head of the rule as well, e.g.:

p3(C,P) :-
   item(C,_,_,P).

If you query that you'll get to see the value of P corresponding to n3001:

   ?- p3(n3001,P).
P = 1

If you query item/4 directly P appears in the arguments and therefore you get to see a substitution for it that satisfies your query:

   ?- item(n3001,_,_,P).
P = 1

Prolog程序返回yes而不是value

1 个答案: