Prolog factorial recursion

Question

I\'m having trouble understanding the following factorial program

fact1(0,Result) :-
    Result is 1.
fact1(N,Result) :-
    N > 0,
    N1 is N-1,
    fac         


        

Answer 1

Base case is declared. The conditions that N must be positive and multiply with previous term.

 factorial(0, 1).
 factorial(N, F) :- 
       N > 0, 
       Prev is N -1, 
       factorial(Prev, R), 
       F is R * N.

To run:

factorial(-1,X).