作者：未知 来源：月光软件站 加入时间：2005-5-13　月光软件站

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

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