sicp每日一题[2.45]

news2025/1/13 13:41:15

Exercise2.45

$r i g h t - s pl i t$ and $u p - s pl i t$ can be expressed as instances of a general splitting operation. Define a procedure $s pl i t$ with the property that evaluating

(define right-split-2 (split beside below))
(define up-split-2 (split below beside))

produces procedures $r i g h t - s pl i t$ and $u p - s pl i t$ with the same behaviors as the ones already defined.

这道题本身没什么难度，但是我再次遇到了环境问题。。之前在台式电脑上装的sicp包，用同样的步骤在笔记本上怎么都不行，卸载重装了好几次，最后参考这篇文章终于找到了解决办法。
方法其实也不复杂，随便打开一个窗口，输入 #lang planet neil/sicp，然后点击右上角的 run 等待安装完毕重启 DrRacket，再依次点击 Language -> Choose Language，就可以选择看到 SICP(PLaneT 1.18) 了。但是这么简单的方法我用了2个多小时才找到。。
回到这道题，只要注意到 $s pl i t$ 的返回值也是一个函数，且它的两个参数分别为 $p ain t er$ 和 $n$ ，至于实现，仿照原来的 $r i g h t - s pl i t$ 或 $u p - s pl i t$ 即可。

(define wave einstein)

(define (right-split-1 painter n)
  (if (= n 0)
      painter
      (let ((smaller (right-split-1 painter (- n 1))))
        (beside painter (below smaller smaller)))))

(define (up-split-1 painter n)
  (if (= n 0)
      painter
      (let ((smaller (up-split-1 painter (- n 1))))
        (below painter (beside smaller smaller)))))

(define (split op1 op2)
  (define (iter painter n)
    (if (= n 0)
        painter
        (let ((smaller (iter painter (- n 1))))
          (op1 painter (op2 smaller smaller)))))
  iter)

(define right-split-2 (split beside below))
(define up-split-2 (split below beside))


(paint (right-split-1 wave 1))
(paint (right-split-2 wave 1))

(paint (up-split-1 wave 1))
(paint (up-split-2 wave 1))