Get multiple solutions in SWI-Prolog

6
votes

I'm beginner in SWI-Prolog (but have some experience in Borland Prolog), and I've faced with a strange behavior for the following test code:

test(10).
test(1).

It is expected for query ?-test(A) to get 2 solutions, something like A = 10; A = 1. However, only A = 10 is produced. I don't use the cut here. Maybe backtracking is off by default in SWI-Prolog?

Thanks in advance

2
prologprolog-toplevel
when A=10 returned pres ; to get another solutions and so on... – coder
Coder, many thanks for reply, and sorry for stupid question. I didn't expect that it is necessary to press Spacebar or semicolon after each solution, and I pressed Return. – Spectorsky

2 Answers

9
votes

Sorry, the answer is very simple (see SWI-Prolog doc):

The user can type the semi-colon (;) or spacebar, if (s)he wants another solution. Use the return key if you do not want to see the more answers. Prolog completes the output with a full stop (.) if the user uses the return key or Prolog knows there are no more answers. If Prolog cannot find (more) answers, it writes false.

1
votes

bagof/3 is probably what you're looking for.

?- bagof(X, test(X), Xs).

where Xs is a list of all matching results.
Know that anonymous variables don't work how you might expect with bagof. In the following example:

test(1,odd).
test(2,even).
test(3,odd).
test(4,even).

bagof(X, test(X,_), Xs) will only give values of X where the second term is uniform; in this case only the even numbers. If you want to return all matching value you need to do something like

?- bagof(X, A^test(X,A), Xs).