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recursionprologfactorial

prolog factorial recursion giving expression as output

I've written a prlog recursive factorial clause which is:

factorial(X,Y):-
      (X>1)
   -> factorial(X-1,X*Y)
   ;  write(Y).

The problem is, for any valid call[for example, factorial(5,1). ], it is giving an expression rather than a value[(5-1-1-1)* ((5-1-1)* ((5-1)* (5*1)))]. How can I get a value rather than an expression.

Solution

  • The comment by @lurker is a bit simplistic. Comparison operators do evaluate expressions. So, your code could be made to work:

    factorial(X,Y):- X>1 -> factorial(X-1,F), Y=X*F ; Y=1.

?- factorial(5,X),F is X.
X = 5*((5-1)*((5-1-1)*((5-1-1-1)*1))),
F = 120.