CODEWITHSUNDEEP
prolog
hmdb
hmdb

Reputation: 323

Prolog search list

In the code below, I am trying to search through a list and print "match" if there's a match in the list and print "no match" if there's no match. My code prints "match" if there's a match but, doesn't print "no match" instead returns false if there's no match. How can I change my code to print "no match" if there's no match in the list? thanks

    on(Item,[Item|_], X) :- X is 1.
    on(Item,[_|Tail],X):- on(Item,Tail,X).

    check(X,Y) :- on(X,Y,Z), (Z == 1, write('match');write('no match')).

Upvotes: 2

Views: 3380

Answers (1)

Xymostech
Xymostech

Reputation: 9850

You need to add a clause that catches the case when on reaches the end of the list:

on(_, [], X) :- X is 0.

However, in order to make that work, you need to put a cut operator, !, into the matching on clause, so that it doesn't continue and find the empty list matching clause also:

on(Item,[Item|_], X) :- X is 1, !.

Also, it looks like your printing code isn't doing what you want it to do, I think it should look something like:

(Z == 1 -> write('match'); write('no match')).

(note the -> in there). I'm not positive that's correct, but I had to change it to make it work correctly on mine.

Finally, to make your code look a little bit prettier, you can put the X is 0 and X is 1 stuff into the predicate itself, so it becomes:

on(_, [], 0).
on(Item, [Item|_], 1) :- !.

Upvotes: 3

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