SICP問題2.2
線分の構成子と選択子
(define (make-segment start-point end-point) (cons start-point end-point)) (define (start-segment segment) (car segment)) (define (end-segment segment) (cdr segment))
点の構成子と選択子
(define (make-point x y) (cons x y)) (define (x-point point) (car point)) (define (y-point point) (cdr point))
中間点を求める手続き
(define (midpoint-segment segment) (make-point (average (x-point (start-segment segment)) (x-point (end-segment segment))) (average (y-point (start-segment segment)) (y-point (end-segment segment)))))