编写一个递归递减输入列表的函数,跟踪它正在查看的元素的位置,并发出与cons
匹配的索引。这真是微不足道的;我认为这是你被定为功课的问题?
; 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))
测试这与GNU狡诈或使用MzScheme什么:
(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!