SICP 問題 2.2

(define (make-segment start end)
  (cons start end))

(define (start-segment seg)
  (car seg))
(define (end-segment seg)
  (cdr seg))

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

(define (x-point point)
  (car point))
(define (y-point point)
  (cdr point))

(define (midpoint-segment seg)
  (let ((s-seg (start-segment seg))
        (e-seg (end-segment seg)))
    (make-point (/ (+ (x-point s-seg)
                      (x-point e-seg))
                   2)
                (/ (+ (y-point s-seg)
                      (y-point e-seg))
                   2))))

　
　　　
　　　
印字用手続きで確かめてみる.

(define (print-point p)
  (newline)
  (display "(")
  (display (x-point p))
  (display ",")
  (display (y-point p))
  (display ")"))

gosh> (define my-segment (make-segment '(0 . 0) '(2 . 2)))
my-segment
gosh> (cdr my-segment)
(2 . 2)
gosh> (car my-segment)
(0 . 0)
gosh> (print-point my-segment)

((0 . 0),(2 . 2))#<undef>
gosh> (print-point (midpoint-segment my-segment))

(1,1)#<undef>