CODEWITHSUNDEEP
prolog
bring112
bring112

Reputation: 63

Prolog gives fail during searching for solution, when there is one

Hi guys :) I have encountered problem during programming in prolog. I have partial order defined as facts, and I have defined maximal and greatest element as predicates. We can think of partial order like greater or equal, so le(6,7) is something like 6 <= 7.

le(6,7).
le(4,4).
le(6,6).
le(5,6).
le(5,5).
le(4,5).
le(4,4).

maximal(X) :-
    not((le(X,Z) , X\=Z)).
greatest(X) :-
    not(le(X,_)).
minimal(X) :-
    not((le(Z,X) , X\=Z)).
smallest(X) :-
    not(le(_,X)).

When I have typed query like maximal(7) the prolog output is true, and when I ask prolog to find the solution like maximal(X) it gives mi fail. I am beginner prolog programmer, so sorry if the question is too trivial, but I couldn't find the solution on my own.

Upvotes: 2

Views: 477

Answers (1)

Evgeny
Evgeny

Reputation: 3274

Prologs uses pre-defined steps to find the answer and negation is always tricky.

To make a long story short, to make the code easier to reason about most logic should be positive and negation applied to expressions where values of all variables are known.

Here is how you could make the code work.

le(6,7).
le(4,4).
le(6,6).
le(5,6).
le(5,5).
le(4,5).
le(4,4).

is_my_number(X) :- le(X, _).
is_my_number(X) :- le(_, X).

equal_or_not_le(X, Y) :- X = Y.
equal_or_not_le(X, Y) :- not(le(X, Y)).

maximal(X) :- is_my_number(X), forall(is_my_number(Y), equal_or_not_le(X, Y)).

Then query:

?- maximal(X).
X = 7 .

Good luck!

Upvotes: 2

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