The complete program is:

adjacent(edgware_road,paddington,district,1).
adjacent(paddington,bayswater,district,1).
adjacent(bayswater,nottinghill_gate,district,1).
adjacent(paddington,warwick_avenue,bakerloo,1).
adjacent(warwick_avenue,maida_vale,bakerloo,1).

next(X,Y,L):-adjacent(X,Y,L,_).
next(X,Y,L):-adjacent(Y,X,L,_).
direct_connect(X,Y,L,S,F):-
next(X,Z,L),
not(member(Z,S)),
direct_connect(Z,Y,L,[Z|S],F).
direct_connect(X,Y,L,S,[Y|S]):- next(X,Y,L).
one_change(X,Y,L,F):-
direct_connect(X,Z,L,[X],F1),
direct_connect(Z,Y,L2,[Z|F1],F),
L\=L2.
exist(X):-next(X,_,_).
member(X,[X|_]).
member(X,[_|T]):-member(X,T).
route(X,Y,F):-exist(X),exist(Y),
direct_connect(X,Y,_,[X],F),
write('Direct Connection'),nl,
revwrite(F).
route(X,Y,F):-exist(X),exist(Y),
one_change(X,Y,_,F),
write('One change required'),nl,
revwrite(F).
revwrite([X]):-write(X).
revwrite([H|T]):-revwrite(T), write('->'),write(H).

If you are using the downloaded version enter all of the clauses into the editor and compile the program.

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If you are using the online version enter all of the clauses into the lefthand panel.

Switch to the command line and enter:

route(nottinghill_gate,maida_vale,F).

Which produces the output (make sure you have table results ticked in the online version):

One change required
nottinghill_gate->bayswater->paddington->
paddington->warwick_avenue->maida_vale

Note the double occurrence of paddington is due to this being the change of line.

I admit that there is a bug that can cause time wasting inefficient journeys but you can fix it in no time at all.

I hope you enjoy using this route finder but if you get lost then my advice is to ask a policeman...

You can see that Prolog, and logic programming in general,require a big change in the way that you think about problems but it is often more effective than the apparently more direct procedural approach - something that is rarely stated but is very true.

If you try to extend the tube program you will almost certainly have many a frustrating hour or two as your brain retrains itself to think in terms of logical assertions, but you will have a lot of fun on the way to a working program.

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The Triumph Of Deep Learning

Robot cars - provably uncrashable?

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