Question

I’ve written the following recursive function to assign a parameter to the greater of two numbers:

max(A,B,X) :-   
    A >= B,
    X is A.

max(A,B,X) :-
    max(B,A,X).

This works as expected but as a newcomer to Prolog, I’m hesitant to believe this is the most efficient solution. Is there a better way of doing this?

Additionally, pressing ‘;’ after calling the function repeats it again. Is this correct behaviour?

Thank you in advance.

Answer 1

That seems convoluted to me. Try something like

max(A,B,A) :- A >  B .
max(A,B,B) :- A =< B .

Though you might want to put some type checking in.

[edited to remove the need for a cut, and provide better support for pre-instantiated 3rd parameter.]

Another option, of course, might be something like this:

max(A,B,M) :- A > B -> M=A ; M=B .

This will throw an error is A or B is unbound, while the first example will work regardless of how it is invoked (so long as 2 arguments are instantiated). For instance, invoking it as

max(A,3,5).

yields X=5.