2.2.rkt

#lang planet neil/sicp
;;点
(define(make-point n d)
  (cons n d))
(define(x-point point)
  (car point))
(define(y-point point)
  (cdr point))
;;线段
(define(make-segment start-point end-point)
  (cons start-point end-point))
(define (start-segment seg)
  (car seg))
(define (end-segment seg)
  (cdr seg))
;;中点
(define (midpoint-segment seg)
  (let((start(start-segment seg))
       (end(end-segment seg)))
    (make-point(average(x-point start)
                       (x-point end))
               (average(y-point start)
                       (y-point end)))))
(define(average x y)
  (/(+ x y)2))
;;打印
(define (print-point p)
    (newline)
    (display "(")
    (display (x-point p))
    (display ",")
    (display (y-point p))
    (display ")"))
;;点（1,3）（4,3）的中点
(define start (make-point 1.0 3.0))
(define end (make-point 4.0 3.0))
(define seg (make-segment start end))
(define mid (midpoint-segment seg))
(print-point mid)