sicp每日一题[2.49]

Exercise 2.49

Use segments->painter to define the following primitive painters:

a. The painter that draws the outline of the designated frame.
b. The painter that draws an “X” by connecting opposite corners of the frame.
c. The painter that draws a diamond shape by connecting the midpoints of the sides of the frame.
d. The wave painter.

---

这道题目我本来感觉不是很难，我按照题目要求把 frame 的3个点坐标取出来，然后用 make-segment 连成线，但是做了半天总是不行，
上网搜了一下其他人的答案，发现他们都是直接传的 segment-list，而不是传 frame，而且之前定义的 make-vect, make-segment 也都用不了。。
必须先导入 sicp-pict，然后再用它自带的 vect 和 segment 来创建向量和线段。完整代码如下：

(#%require sicp-pict); a 
(define outline (segments->painter (list (segment (vect 0.0 0.0) (vect 0.0 1.0)) (segment (vect 0.0 0.0) (vect 1.0 0.0)) (segment (vect 0.0 1.0) (vect 1.0 1.0)) (segment (vect 1.0 0.0) (vect 1.0 1.0))))); b 
(define x-painter (segments->painter (list (segment (vect 0.0 0.0) (vect 1.0 1.0)) (segment (vect 0.0 1.0) (vect 1.0 0.0))))) ; c
(define diamond (segments->painter (list (segment (vect 0.0 0.5) (vect 0.5 1.0)) (segment (vect 0.5 1.0) (vect 1.0 0.5)) (segment (vect 1.0 0.5) (vect 0.5 0.0)) (segment (vect 0.5 0.0) (vect 0.0 0.5))))) ; d
(define wave (segments->painter (list (segment (vect .25 0) (vect .35 .5)) (segment (vect .35 .5) (vect .3 .6)) (segment (vect .3 .6) (vect .15 .4)) (segment (vect .15 .4) (vect 0 .65)) (segment (vect 0 .65) (vect 0 .85)) (segment (vect 0 .85) (vect .15 .6)) (segment (vect .15 .6) (vect .3 .65)) (segment (vect .3 .65) (vect .4 .65)) (segment (vect .4 .65) (vect .35 .85)) (segment (vect .35 .85) (vect .4 1)) (segment (vect .4 1) (vect .6 1)) (segment (vect .6 1) (vect .65 .85)) (segment (vect .65 .85) (vect .6 .65)) (segment (vect .6 .65) (vect .75 .65)) (segment (vect .75 .65) (vect 1 .35)) (segment (vect 1 .35) (vect 1 .15)) (segment (vect 1 .15) (vect .6 .45)) (segment (vect .6 .45) (vect .75 0)) (segment (vect .75 0) (vect .6 0)) (segment (vect .6 0) (vect .5 .3)) (segment (vect .5 .3) (vect .4 0)) (segment (vect .4 0) (vect .25 0)) )))(paint outline)
(paint x-painter)
(paint diamond)
(paint wave)

效果如下：