CODEWITHSUNDEEP
prolog
Marcin
Marcin

Reputation: 430

Prolog ERROR: Undefined procedure, <= involved

I am working on a program in prolog, and am stuck with the following issue: I have defined the predicate neighbors(+,+,+,+,?) like this:

neighbors(X, Y, Height, Width, Neighbors):-
   Xup is X-1,
   Xdown is X+1,
   Yleft is Y-1,
   Yright is Y+1,
   findall((A,B,C),(
            between(Xup, Xdown, A),
            between(Yleft, Yright, B),
            A>=1,
            B>=1),
            Neighbors).

Now the query neighbors(5,5,5,5,X) works as expected, unifying X with a list of it's neighbors, namely 

X = [ (4, 4, _G2809), (4, 5, _G2800), (4, 6, _G2791), (5, 4, _G2782), (5, 5, _G2773), (5, 6, _G2764), (6, 4, _G2755), (6, ..., ...), (..., ...)] .

However a problem arises when I try to add the following lines to my findall Goal:

A<=Height,
B<=Width

the full predicate looks like this:

neighbors(X, Y, Height, Width, Neighbors):-
   Xup is X-1,
   Xdown is X+1,
   Yleft is Y-1,
   Yright is Y+1,
   findall((A,B,C),(
            between(Xup, Xdown, A),
            between(Yleft, Yright, B),
            A>=1,
            B>=1,
            A<=Height,
            B<=Width
            ),
            Neighbors).

Now the same query, neighbors(5,5,5,5,X). results in me getting the following error:

ERROR: Undefined procedure: neighbors/5
ERROR:     However, there are definitions for:
ERROR:         neighbor/2
ERROR:         neighbors/2
false.

What could be the reason? I imagine it's got to do with the way I'm comparing those variables, but since Width and Height are instantiated I thought that should work. Thanks.

Upvotes: 1

Views: 677

Answers (1)

Yasel
Yasel

Reputation: 3120

The problem is in your comparison operators. The syntax for the less than or equal to operator is =</2. So your goals should be:

...
A=<Height,
B=<Width
...

Upvotes: 1

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