sicp习题试解 (2.10)

; ======================================================================

;

; 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 04/25/05

; modified:

; (保持内容完整不变前提下，可以任意转载)

; ======================================================================

;; SICP No.2.10

(define (div-interval x y)

(let ((y-l (lower-bound y))

(y-u (upper-bound y)))

(if (< (* y-l y-u) 0)

(error "divisor interval expands zero")

(mul-interval x

(make-interval

(/ 1.0 y-u)

(/ 1.0 y-l))))))

;; Test-it:

;; Welcome to MzScheme version 209, Copyright (c) 2004 PLT Scheme, Inc.

;; ;; .............

;; > test1

;; (3 . 5)

;; > test3

;; (-5 . 3)

;; > (div-interval test1 test3)

;; divisor interval expands zero

;; > (div-interval test3 test1)

;; (-1.6666666666666665 . 1.0)