sicp习题试解 (2.2)

王朝other·作者佚名 2006-01-09

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

;

; 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 04/18/05

; modified:

; (保持内容完整不变前提下，可以任意转载)

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

;; SICP No.2.2

(define (make-segment s e) (cons s e))

(define (start-segment x) (car x))

(define (end-segment x) (cdr x))

(define (make-point x y) (cons x y))

(define (x-point p) (car p))

(define (y-point p) (cdr p))

(define (midpoint-segment x)

(define (avr f a b)

(/ (+ (f a) (f b)) 2))

(let ((s (start-segment x))

(e (end-segment x)))

(make-point (avr x-point s e)

(avr y-point s e))))

;; Test-it:

;; > (let ((test-seg (make-segment (make-point -10 20)

;; (make-point 80 -40))))

;; (midpoint-segment test-seg))

;; (35 . -10)

;;

;; 为了简便，没有使用原文中提供的print-point函数，

;; 而直接使用了MzScheme Interpreter返回的原始值，即一个pair