Question

I am asking for help in basic Prolog, a language whose paradigms are difficult for me to grasp. I am very familiar with other languages (C++, Lisp, Java, Assembly, etc.) but am a complete novice with Prolog.

What needs to be solved – in basic English: given 2 parameters, find a corresponding number in a 2D Array.

The problem is a brainlessly easy puzzle on the web which asks you to pick a number, choose that number’s color and then pick the house that contains your given number. The puzzle is set up such that there is only one number for each corresponding color/house combination.

What is currently in place:

function guess(Color, Houses) :-
<--Need what goes here -->

green(1, 15, 23, 24).
pink(2, 6, 10, 18).
etc...

houseA(2, 4, 7, 14).
etc...

The code must match the colors and houses to pick out the correct number. So for example, given "?- guess(pink, houseA)" should return "Your number is 2."

I have been writing down ideas on how to implement this in prolog and none of them get me any further. I do not know how I would implement if/else statements to check which color I should search, or how to check which numbers would correspond between house and color, or even how to “return” values!

It seems to me that I am missing a key point or…way of thinking about the language.

Any help would be appreciated. Thank you!

Answer 1

To return a value, you need another parameter in your predicate (not function btw). This parameter will be a free variable and you will bind it to the result.

In prolog specifications, such parameters are noted -Parameter while already bound parameters are noted +Parameter and parameters that can be both bound and free are noted ?Parameter. So here for you you could have a comment such as :

% guess/3 (specify the arity of your predicate)
% guess(+Color, +House, -Result) (give info about your parameters)
% guess finds a color shared by Color and House and binds it to Result.

Then, predicates such as pink, houseA and so on are not that great to find numbers from. You can turn them into more adapted predicates by having numbers stored in a list :

green([1, 15, 23, 24]).
pink([2, 6, 10, 18]).
houseA([2, 4, 7, 14]).

When we got that list, we can write :

guess(Color, House, Result) :-
    call(Color, Pool1),

If you call guess(pink, houseA), that will call your predicate pink with argument Pool1, prolog will try to match Pool1 and [2, 6, 10, 18] so Pool1 is going to be bound to this list precisely.

    call(House, Pool2),

Same with houseA and Pool2.

    member(Result, Pool1),

Now we tell prolog that we want our Result to be a member of Pool1

    member(Result, Pool2),

And a member of Pool2.

    write('Your number is '),
    write(Result),
    write(.),
    nl.

Finally we display the message.

Prolog will display result as R = x; false below that, if you don’t want the ; false part, you can add a “cut” :

    nl.

=>

    nl,
    !.

The !/0 predicate (cut) tells prolog not to backtrack to try to find other solutions. There it would have backtracked on member explaining why prolog returned a choice with only one result and false.

Hope it helps. If you got problems to understand some parts please say so and I’ll give infos.