fractorial.lisp

;; loop
(defun frac (n)
  (apply #'*
         (loop for i from 1 to n
               collect i)))
;; dotimes 
;; (dotimes (x 5) ...) => 0, 1, 2, 3, 4
(defun frac (n)
  (let (fin)
    (dotimes (x n)
      (push (1+ x) fin))
    (apply #'* fin)
    ))

;; do
;; (do ((VAR INIT [STEP])...)
;;     (END-TEST [RESULT...])
;;   BODY...)
(defun frac (x)
  (do ((i 2 (1+ i)) (fin 1))              ;定義變數
      ((> i x) fin) ;end-test和整個do的輸出結果。test一旦成立就立刻輸出fin，不再eval本體
    (setq fin (* fin i))))                 ;本體

;; while
(defun frac (x)
  (let ((fin x))
    (while (> x 1)
      (setq x (1- x))
      (setq fin (* fin x)))
    fin))

;; recursive
(defun frac (n)
  (if (<= n 1)
      1
    (* n (frac (1- n)))))