Prolog Path Search Pointers

Question

I need to check if a path is valid, true or false. It's given like this:

?-path(a,b,[(a,c),(c,d),(d,b)]).
true

In the list part, how do I access the a or c in (a,c)? Is it like a string"(a,c)"?

And in general how would one solve this type of path finding? Sample code/pseudo is appreciated. Is there a way to make it interpret the tuples () in the list as predicates?

Answer 1

The (a, c) is a compound term, you can access it in a predicate like this:

 my_predicate((A, B)) :-
    print(A),
    print(B).

Answer 2

It's been a while but off the top of my head you'll start with:

path(S, G, [(P, Q) | R]) :- ......

With S meaning start, G meaning goal, P and Q being connected nodes in your graph and R being the rest of your graph.

Answer 3

You have several questions in there...

Is it like a string"(a,c)"?

What do you mean by "like"? Do they unify? Nope.

?- "(a, c)" = (a, c).

No

In the list part, how do I access the a or c in (a,c)?

?- L = [(a, c) | _], L = [(A, C) | _].

L = [ (a, c)|_G184],
A = a,
C = c

Is there a way to make it interpret the tuples () in the list as predicates?

Maybe using call/N, but why would you want to do that?

Answer 4

I'll give you an example from when I was a 2nd year student:

% Representation [[a,b],[b,a],[b,c]]:
%
%          a <--> b -->c
%

% Does aexists a path beetween X and Y?
% Example:  path(c,b,[[a,b],[b,c],[d,e],[c,d],[b,e],[e,c],[e,f],[a,a]]). No
%           path(c,f,[[a,b],[b,c],[d,e],[c,d],[b,e],[e,c],[e,f],[a,a]]). Yes

path(X,Y,G):-pathAux(X,Y,G,[]).
pathAux(X,Y,G,_T):-member([X,Y],G).
pathAux(X,Y,G,T) :-member([X,Z],G),not(member([X,Z],T)),
                 append([[X,Z]],T,Tt),pathAux(Z,Y,G,Tt).

I used [a,b] instead of (a,b); but It's the same.