练习 2.48

其实在 练习 2.2 我们已经做过类似的练习了，做法是一样的：

;;; 48-segment-represent.scm

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

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

(define (end-segment s)
    (cadr s))

测试：

1 ]=> (load "48-segment-represent.scm")

;Loading "48-segment-represent.scm"... done
;Value: end-segment

1 ]=> (load "46-vect-represent.scm")

;Loading "46-vect-represent.scm"... done
;Value: ycor-vect

1 ]=> (define seg (make-segment (make-vect 0.0 0.0)
                                (make-vect 1.0 1.0)))

;Value: seg

1 ]=> (start-segment seg)

;Value 11: (0. 0.)

1 ]=> (end-segment seg)

;Value 12: (1. 1.)

讨论

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