問題解答

問 2.2

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

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

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

(define (make-segment p1 p2)
  (cons p1 p2))

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

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

(define (midpoint-segment segment)
  (make-point (/ (+ (x-point (start-segment segment))
                    (x-point (end-segment segment))) 2)
              (/ (+ (y-point (start-segment segment))
                    (y-point (end-segment segment))) 2)))

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

(define p1 (make-point 0.0 0.0))
(define p2 (make-point 1.0 1.0))
(define s (make-segment p1 p2))
(define m (midpoint-segment s))
(print-point m)

これは手を動かすだけなので簡単だった。