练习 2.47

题目要求我们为两个不同的 make-frame 实现提供适当的选择函数。

实现一

定义：

;;; 47-frame-represent-using-list.scm

(define (make-frame origin edge1 edge2)
    (list origin edge1 edge2))

(define (origin-frame f)
    (car f))

(define (edge1-frame f)
    (cadr f))

(define (edge2-frame f)
    (caddr f))

测试：

1 ]=> (load "47-frame-represent-using-list.scm")

;Loading "47-frame-represent-using-list.scm"... done
;Value: edge2-frame

1 ]=> (load "46-vect-represent.scm")                    ; 还需要练习 46 的向量表示

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

1 ]=> (define f (make-frame (make-vect 0.0 0.0)         ; origin
                            (make-vect 0.3 0.3)         ; edge1
                            (make-vect 0.6 0.6)))       ; edge2

;Value: f

1 ]=> (origin-frame f)

;Value 11: (0. 0.)

1 ]=> (edge1-frame f)

;Value 12: (.3 .3)

1 ]=> (edge2-frame f)

;Value 13: (.6 .6)

实现二

定义：

;;; 47-frame-represent-using-cons.scm

(define (make-frame origin edge1 edge2)
    (cons origin
          (cons edge1 edge2)))

(define (origin-frame f)
    (car f))

(define (edge1-frame f)
    (cadr f))

(define (edge2-frame f)
    (cddr f))

测试：

1 ]=> (load "47-frame-represent-using-cons.scm")

;Loading "47-frame-represent-using-cons.scm"... done
;Value: edge2-frame

1 ]=> (load "46-vect-represent.scm")                        ; 还需要练习 46 的向量表示

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

1 ]=> (define f (make-frame (make-vect 0.0 0.0)             ; origin
                            (make-vect 0.3 0.3)             ; edge1
                            (make-vect 0.6 0.6)))           ; edge2

;Value: f

1 ]=> (origin-frame f)

;Value 11: (0. 0.)

1 ]=> (edge1-frame f)

;Value 12: (.3 .3)

1 ]=> (edge2-frame f)

;Value 13: (.6 .6)

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