Question

I m stuck on a simple problem in prolog. Let s consider the program

worker(bill).
worker(smitt).
worker(fred).
worker(dany).
worker(john).
car(bmw).
car(mazda).
car(audi).
owner(fred,mazda).
owner(dany,bmw).
owner(john,audi).

I need to add one more predicate no_car(X),that will be true if the worker X has no cars,i.e,if we input a query ?:- no_car(X). the prolog should answer

X=smitt,
X=bill,
yes

做了些什么

   hascar(X):-owner(X,_).
   nocar(X):- worker(X),not hascar(X).

But this approach does not work because anonimous variables are avaliable only for queries. So,i m really stuck on this. I know there are "NOT EXISTS" words in SQL which allow to express this logic in a query,but is there something similar to them in prolog?

Answer 1

以下是我的工作成果:

no_car(W):-
   worker(W),
   + owner(W, _).

现在,这接近你们。某件事,请can当然使用_,但不限于查询。通常使用<代码>*进行否定,not 这里我有一 error错!

EDIT:

Ah! 在我的发言中,尽管注明日期,但必须使用<条码>而不是(scar(X),使程序发挥作用,因此无需使用第1条作为术语,而不必使用经营者。但该手册还称,not 改为+。

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