; ======================================================================
;
; 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 05/30/05
; modified:
; (保持内容完整不变前提下，可以任意转载)
; ======================================================================
;; SICP No.2.12
;; 唯一的注意事项是符号，
;; 比如当c小于0时，make-center-percent定义中make-interval中两个操作数的先后
;; 以及如何保证percent函数的输出始终是正的。
(define (make-center-percent c p)
(if (or (< p 0) (> p 1))
(error "Percentage x should satisfy 0 <= x <= 1")
(let ((a (* c (+ 1.0 p)))
(b (* c (- 1.0 p))))
(make-interval (min a b) (max a b)))))
(define (center i)
(/ (+ (lower-bound i) (upper-bound i)) 2.0))
(define (percent i)
(let ((l (lower-bound i))
(u (upper-bound i)))
(/ (- u l) (abs (+ u l)))))
;; Test-it:
;; Welcome to MzScheme version 209, Copyright (c) 2004 PLT Scheme, Inc.
;; ;; .............
;;
;; > (make-center-percent -20.0 0.1)
;; (-22.0 . -18.0)
;; > (define v (make-center-percent -20.0 0.1))
;; > (center v)
;; -20.0
;; > (percent v)
;; 0.1
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