Question

I have a simple predicate in prolog that when MaxScore >0, it gives yes, otherwise is no. So if MaxScore is 0, then obviously the result is No. But this doesn’t happen. Look closer at this:

Or corresponding code:

aiPlay(PlayerNum, Board, Pos, BotType) :-
    aiTryAll(Board, PlayerNum, ScoreList, 0),
    scoreListGetPos(ScoreList, 1, Pos, MaxScore),
    write('MaxScore: '), write(MaxScore), nl,
    MaxScore > 0.

And corresponding output:

| ? myQuery(Pos).
MaxScore: 0
MaxScore: 0
MaxScore: 1
Pos = 6 ?
yes
| ?

I am getting insane with this. Anyone have a clue about what is happening?

Answer 1

There isn’t really a problem; consider this code:

foo(0).
foo(0).
foo(1).

bar:-
    foo(X),
    write(X), nl,
    X > 0.

Prolog will first unify X with 0 (from the first foo(0)), will print the output and then fail since 0 > 0 is false. Then prolog will backtrack and pick the second foo(0); again the same. Finally, X will be unified with 1 and bar will return true.

Naturall, side-effects like printing cannot be reversed/backtracked, so in the terminal you will see:

0
0
1
true

How you solve it depends on what you want to do; if you want to print just Max Core = 1 then you can do the printing after you check if MaxScore>0. Generally though, it’s better to do the printing (if it’s required) at the very end, and not in a predicate deep in the program.

Oh, by the way, it’s better to post the actual code and the output instead of a screenshot; it makes it easier to copy-paste-debug your code. Also, it is advised to give a runnable example: half of the predicates you use in aiPlay/4 are undefined so I’m just guessing that the problem is what I mentioned above :p