lisp do loop factorial code

Question

(defun fact (n)
    (do
       ((i 1 (+ 1 i))
        (prod 1 (* i prod)))
       ((equal i n) prod)))

I have done the code above and when i try, fact(4), it give me ans is 6. I am not sure what is going wrong. Can anyone help me?

Answer 1

Change to

(defun fact (n)
    (do
       ((i 1 (+ 1 i))
        (prod 1 (* i prod)))
       ((equal i (+ n 1)) prod)))

Basically, you were doing one iteration less than necessary.