Chapter 3 exercise Exercise 3.3

[kagankorkmaz]

Are you sure about the solution? It looks like OUT OF LOCAL STACK will be occur.

[Victordmz]

This one works. The ! is a cut, meaning once a solution is found, Prolog does not have to backtrack (and thus does not have to find any alternative routes). The trick was to use another functor for the other direction, called travelBetweenReverseDirection, so that Prolog cannot go back to the initial direction using travelBetween.

directTrain(forbach,saarbruecken).
directTrain(freyming,forbach).
directTrain(fahlquemont,stAvold).
directTrain(stAvold,forbach).
directTrain(saarbruecken,dudweiler).
directTrain(metz,fahlquemont).
directTrain(nancy,metz).

travelBetween(X,Y) :- directTrain(X,Y),!.
travelBetween(X,Y) :-
	directTrain(X,Z),
	travelBetween(Z,Y),
	!.
travelBetween(X,Y) :- travelBetweenReverseDirection(Y,X),!.
travelBetweenReverseDirection(X,Y) :- directTrain(X,Y).
travelBetweenReverseDirection(X,Y) :-
	directTrain(X,Z),
	travelBetweenReverseDirection(Z,Y).