; ======================================================================
;
;          Structure and Interpretation of Computer Programs
;                  (trial answer to excercises)
;
;                 计算机程序的构造和解释(习题试解)
;
;                                             created: code17 02/24/05
;                                             modified:
; (保持内容完整不变前提下，可以任意转载)
; ======================================================================

;; SICP No.1.8

;; 只需要替换No.1.7中的improve函数算法即可
(define (improve guess x)
  (/ (+ (/ x
           (* guess guess))
        (* 2 guess))
     3))
(define (good-enough? oldguess guess)
  (< (abs (/ (- guess oldguess)
             guess))
     0.001))
(define (cube-iter oldguess guess x)
  (if (good-enough? oldguess guess)
      guess
      (cube-iter guess
                 (improve guess x)
                 x)))
(define (cube-root x)
  (cube-iter 0.0 1.0 x))


;; Test-it:
;; Welcome to MzScheme version 209, Copyright (c) 2004 PLT Scheme, Inc.
;; > (cube-root 0)
;; 4.9406564584125e-324
;; > (cube-root 125)
;; 5.000000000287929
;; > (cube-root 1000)
;; 10.000000145265767
;; > (cube-root -27)
;; -3.000000005383821
;; >