Relational operators use in prolog for variables

Question

I am writing the following code and is giving perfect results.

edge(s,a,300).
edge(s,d,20).
edge(a,d,400).
edge(a,b,1500).
edge(b,c,9).
edge(b,e,200).
edge(c,d,2000).
edge(c,g,12).
edge(d,e,3).
edge(e,f,400).
edge(f,g,800).

connected(X,Y,D) :- edge(X,Y,D) ; edge(Y,X,D).

path(A,B,D,Path) :- 
    travel(A,B,D,[A],Q),
    reverse(Q,Path).

travel(A,B,D,P,[B|P]) :-
    connected(A,B,D).

travel(A,B,D,Visited,Path) :-
    connected(A,X,D1),
    X \== B,
    \+member(X,Visited),
    D2 is D - D1,
    travel(X,B,D2,[X|Visited],Path).

Here if I query like

| ?- path(s,e,23,P).
P = [s,d,e] ? ;
no
| ?-

I do get the correct response. But no I wish to get the results for a D<50, say. How to do?

Answer 1

You should let Prolog compute the distance D as well as the Path:

travel(A,B,D,Visited,Path) :-
    connected(A,X,D1),
    X \== B,
    \+member(X,Visited),
    travel(X,B,D2,[X|Visited],Path),
    D is D2 + D1.

Then you can query it

?- path(Start, Stop, D, P), D < 50.

and will get (on backtracking) all paths with D < 50.

?- path(s,e,X,P),X<50.
X = 23,
P = [s, d, e] ;
false.