返回每个其他元素的列表的方案过程 [英] A scheme procedure that returns a list of every other element

问题描述

我在 Scheme 中实施这个程序时遇到了一些麻烦，尽管我认为我已经完成了 90%.不幸的是，我需要对此有点含糊，因为这是一项家庭作业.我想 (A B C D ) 返回 ( B D ) .但我收到一个错误，提示作为参数传递给安全车的对象 () 不是一对|"这是我的代码:

I'm having a bit of trouble implementing this program in Scheme, although I think I'm 90% of the way there. Unfortunately I need to be a little vague about it since this is a homework assignment. I want to (A B C D ) to return ( B D) . but i am getting an error that says The object (), passed as an argument to safe-car, is not a pair | " This is my code:

(DEFINE (other_el lis)
  (COND
   (( NULL? lis ) '())
   ((LIST? lis)
    (append (CADR lis) (other_el (CDR lis))))
   (ELSE (show " USAGE: (other_el [LIST])"))))

推荐答案

在我演示正确的代码之前，应该提到这段代码的一些小问题.

There are a number of minor issues with this code that should be mentioned before I demonstrate the proper code.

1. 不要将过程的名称大写，例如 cdr 和 Scheme 中的定义.
2. 不要手动显示错误消息.使用例外.
3. 你应该总是缩进你的代码.(看起来有人编辑了问题的代码以包含缩进)

无论如何，这是您正在寻找的功能:

Anyway, here is the function you are looking for:

(define (evens lst)
  (if (or (null? lst)             ; if the list is empty 
          (null? (cdr lst)))      ; or the list has a single element
      '()                         ; then return the empty list
      (cons (cadr lst)            ; otherwise `cons` the second element
            (evens (cddr lst))))) ; and recursively advance two elements

我在 DrRacket 5.3 中测试了该函数，并且 (evens '(A B C D)) 返回 '(B D)，如您所指定.如果您有任何麻烦，请告诉我.祝你作业顺利！

I tested the function in DrRacket 5.3 and (evens '(A B C D)) returns '(B D), as you specified. If you have any trouble, let me know. Good luck with your homework!

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