lisp - 尝试返回当前列表中偶数位置的值列表

Question

这里的目标是将一个列表传递给函数，并让它返回一个列表，其中包含前一个列表中偶数位置的值。

例如。返回偶数 '(1 2 3) => (2) , 返回偶数 '() => ()

我终于创建了这个函数，但是它只在我输入 0-3 个值时才起作用。当我输入 0 或 1 个值时，它不会返回“()”，而是返回“NIL”。

我怎样才能让它与超过 3 个值一起工作，我怎样才能返回它以便它以列表的形式打印出来，并用括号括起来？

(defun return-evens (lst)
    (let ((return-list ()) (i 1))
    (loop for x in lst
        while (numberp x)
            do (if (= (mod i 2) 0)
                    (setf return-list (append return-list x))
                    ()
                    )
            do (setq i (1+ i))
            finally (return  return-list))))

Answer 1

对您的代码的一些反馈。

第一个大问题:缩进不正确。

(defun return-evens (lst)          ; in Lisp you can call it list instead of lst
                                   ; return is not a useful prefix.
                                   ; Every function returns something

    (let ((return-list ()) (i 1))  ; LOOP can return lists and iterate.
                                   ; No need for this at all.

    (loop for x in lst

        while (numberp x)   ; why this test? Are there non-numbers in the list?

            do (if (= (mod i 2) 0)   ; Lisp has a function EVENP
                                     ; but generally it is not necessary

                    (setf return-list (append return-list x))
                            ; never append to the end. Never ever.
                            ; Let LOOP collect a list, don't do it this way

                    ()
                    )

            do (setq i (1+ i))  ; iterating manually is not necessary
                                ; that's what LOOP is for

            finally (return  return-list))))
                    ; if you would let LOOP do the work, no RETURN necessary

一个更简单的解决方案:

(defun elements-at-even-positions (list)
  (loop for element in list
        for even-position = nil then (not even-position)
        when even-position collect element))