尝试从方案中的列表中获取元素的索引

Question

所以我试图从列表中获取索引 ex:

(get-indices 'G (list 'A 'G 'T 'X 'I 'T 'G))

(2 7)

其中索引从 1 开始，所以 ' A 是索引一

我正在考虑使用辅助函数，它需要 elt lst 和索引 ex: (get-indices-helper el lst index)

我也在考虑可能使用 list-ref 并喜欢切换它以使其以获取索引的方式工作，但是我找不到它的实际方案定义。

Answer 1

编写一个函数，递归输入列表，跟踪它正在查看的元素的位置，并使用 cons 发出匹配索引。这实在是微不足道的事；我想这是一个你被设置为家庭作业的问题？

; Walk down the list given in haystack, returning a list of indices at which
; values equal? to needle appear.
(define (get-indices needle haystack)
  ; Loop along the haystack.
  (define (loop rest-of-haystack index)
    ; If the haystack is empty, return the empty list.
    (if (null? rest-of-haystack) '()
      ; Recurse to the next position in the list.
      (let ((rest-of-indices (loop (cdr rest-of-haystack) (+ index 1))))
        (if (equal? (car rest-of-haystack) needle)
          ; If haystack is here, emit the current index.
          (cons index rest-of-indices)
          ; Otherwise, return rest-of-indices.
          rest-of-indices))))
  ; Run the loop defined above, from the beginning of haystack, with
  ; the first element being assigned an index of 1.
  (loop haystack 1))

使用 GNU Guile 或 MzScheme 或其他东西对此进行测试：

(display (get-indices 'G (list 'A 'G 'T 'X 'I 'T 'G))) (newline)
(display (get-indices 1 (list 1 1 1 2 1 3))) (newline)

打印：

(2 7)
(1 2 3 5)

耶！