CODEWITHSUNDEEP
prolog
salomepx
salomepx

Reputation: 83

Not expected building results

I have a little question about my code : (it can be interpreted as a grid for example) 

free(1,1).
free(1,3).
free(2,3).
free(2,1).
free(1,5).
free(5,6).
free(5,2).
free(5,4).

busy(5,1,white,pion).
busy(1,2,black,pion).
busy(1,4,black,pion).
busy(1,6,black,pion).
busy(5,3,white,pion).
busy(5,5,white,pion).

%all clauses for  move/6

move(X,Y, X2,Y2,white,pion) :-
   busy(X,Y,white,pion), free(X2,Y2), X2=X-1, Y2=Y-1.
move(X,Y, X2,Y2,white,pion) :-
   busy(X,Y,white,pion), free(X2,Y2), X2=X-1, Y2=Y+1.
move(X,Y, X2,Y2,black,pion) :-
   busy(X,Y,black,pion), free(X2,Y2), X2=X+1, Y2=Y-1.
move(X,Y, X2,Y2,black,pion) :-
   busy(X,Y,black,pion), free(X2,Y2), X2=X+1, Y2=Y+1.

When I execute this : 

move(1,2,X,Y,black,pion).

in SWI-Prolog it says : false. whereas it should says true and return two statements :

I do not understand why it doesn't work, could you please help me ?

Upvotes: 0

Views: 41

Answers (1)

David Tonhofer
David Tonhofer

Reputation: 15338

It is simply because you use = ("make sure that left-hand-side and right hand side unify") instead of is/2 (evaluate the arithmetic expression on the right-hand side and unify with the left-hand-side) or even better #= (constrain the arithmetic values to be the same, even if not all the variables can be resolved yet), here:

X2=X-1, Y2=Y-1.

This unifies X2 with the actual structure -(X,1). Which will fail, as X2 will be a value.

Use:

:- use_module(library(clpfd)).  % for #= predicate

move(X,Y, X2,Y2,white,pion) :-
   busy(X,Y,white,pion), free(X2,Y2), X2 #= X-1, Y2 #= Y-1.

Also, move the test forwards:

move(X,Y, X2,Y2,white,pion) :-
   busy(X,Y,white,pion), X2 #= X-1, Y2 #= Y-1, free(X2,Y2).

Upvotes: 1

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