[計算機程序的構造和解釋學習筆記]-1.牛頓法求平方根

書中的scheme實現如下：

(define (square x)
    (* x x))
(define (abs x)
    (if (< x 0) (- x) x))
(define (average x y)
    (/ (+ x y) 2))
(define (improve guess x)
    (average guess (/ x guess)))
(define (good-enough? guess x)
    (< (abs (- (square guess) x)) 0.001))
(define (sqrt-iter guess x)
    (if (good-enough? guess x)
        guess
        (sqrt-iter (improve guess x) x)))
(define (sqrt x)
    (sqrt-iter 1.0 x))

common lisp 的實現如下：

(defun my-square (x)
  (* x x))
(defun my-abs (x)
  (if (< x 0)
      (- x)
      x))
(defun my-average (x y)
  (/ (+ x y) 2.0))
(defun my-improve (guess x)
  (my-average guess (/ x guess)))
(defun good-enough-p (guess x)
  (< (my-abs (- (my-square guess) x)) 0.001))
(defun sqrt-iter (guess x)
  (if (good-enough-p guess x)
      guess
      (sqrt-iter (my-improve guess x) x)))
(defun my-sqrt (x)
  (sqrt-iter 1.0 x))