The code for factorial was given earlier:
(define (fact n)
(if (= n 0)
1
(* n (fact (- n 1))) ) )
This is fine provided that it is given a good argument n.
But what happens if n is negative: (fact -3) ?
Rule: Test for a half-space rather than just for a point. It costs no more and can prevent errors.
Rule: Don't assume good input. Test it.
(define (factorial n)
(if (<= n 0)
1
(* n (factorial (- n 1))) ) )