Question

I have written the following code in SWI-Prolog:

:- dynamic state_a/1 .
 :- dynamic state_b/1 .
 :- dynamic state_c/1 .
 state_a([1,2,3,4,5,0]).
 state_b([0]).
 chop(LIST,HEAD,TAIL) :- LIST=[HEAD|TAIL].
 move_ab :- !,state_a(X),
            chop(X,Ha,Ba),
            Ha>0,
            state_b(Y),
            chop(Y,Hb,Bb),!,
            (Ha<Hb ; Hb =:= 0),
            asserta(state_a(Ba)),asserta(state_b([Ha|Y])),
            retract(state_a(X)), retract(state_b(Y));
            write('Wrong Move.Choose Another').

There are 2 OR(;) in my code. When I ask move_ab in Prolog for the first time all the conditions before the second OR are true so i get the answer true from Prolog.
But the second time I ask move_ab in Prolog I get just the answer false.
I don’t know how this can happen. Some of the conditions before the second OR are not true so Prolog should check the condition after the second OR and write me the message Wrong Move.Choose Another..
I tried using () to group the conditions but I still get the same message.
Any idea about what is going on? By the way I am a newb in Prolog, just started 2 days ago 🙂

Answer 1

The problem lies in the use of cut (!/0) before the conditions are properly tested. Cut removes choice points. Here it means that the choices are removed before you even test anything. So if the tests fail, everything fails !

BTW, manipulating the database is maybe not the best idea there. To represent states, you could use global variables as follows :

:- nb_setval(state_a, [1,2,3,4,5,0]).
:- nb_setval(state_b, [0]).

move_ab :-
    nb_getval(state_a, [Ha|Ta]),
    Ha > 0,
    nb_getval(state_b, [Hb|Tb]),
    (Ha < Hb ; Hb =:= 0),
    nb_setval(state_a, Ta),
    nb_setval(state_b, [Ha, Hb|Tb]),
    !
    ;
    write('Wrong Move.Choose Another').

A general advice given to beginners in prolog is to stay away from database manipulation if possible, as often problems are solvable without it. Here though it could be justified, but global variables will be faster and easier to manipulate.