列表中不常见的输出元素

Question

我创建了一个函数，它应该返回两个列表没有共同点的元素。目前，他们正在准确地输出传递给它的内容。有关如何解决此问题的任何建议？

(define (findDifference lst1 lst2)
    (if (null? lst1) lst2
    (cons (car lst1) (findDifference (cdr lst1) lst2))))

(findDifference '(2 3 4 (2 3) 2 (4 5)) '(2 4 (4 5))

电流输出：(2 3 4 (2 3) 2 (4 5) 2 4 (4 5)) 所需输出：(3 (2 3))

Answer 1

您要的是两个列表中的symmetric difference。试试这个：

(define (diff list1 list2)
  (union (complement list1 list2)
         (complement list2 list1)))

使用以下帮助程序：

(define (union list1 list2)
  (cond ((null? list1) list2)
        ((member (car list1) list2) (union (cdr list1) list2))
        (else (cons (car list1) (union (cdr list1) list2)))))

(define (complement list1 list2)
  (cond ((null? list1) '())
        ((member (car list1) list2) (complement (cdr list1) list2))
        (else (cons (car list1) (complement (cdr list1) list2)))))

另外请注意，如果您使用的是 Racket，您可以简单地使用内置的set-symmetric-difference 过程来获得相同的效果。例如：

(diff '(2 3 4 (2 3) 2 (4 5)) '(2 4 (4 5)))
=> '(3 (2 3))

Answer 2

因为它看起来像家庭作业，我不想破坏乐趣，这里是蛮力算法，省略了一些。如果你真的被卡住了，我会给你完整的源代码。

(define (sym-diff xs ys)
  ;; Since we have the helper function we can determine all the elements that are in the first list, 
  ;; but not in the second list.
  ;; Then we can pass this intermediate result to the second call to sym-diff-helper. 
  ;;This will return us all the elements that are in the second list but not the first.
  (let ((in-first-not-second ...))
    (sym-diff-helper ys xs in-first-not-second)))

;; This function will return all the elements from the first list that are not in the second list!
(define (sym-diff-helper xs ys acc)
  (cond
    ;; If the first list is empty we have checked it.
    (...
     acc)
    ;; If the first list is not empty yet, check if the first element 
    ;; is in the second list.
    ;; If so, discard it and continue with the rest of the list.
    ((member ... ...)
     (sym-diff-helper ... ... ...)
    ;; If the first element of the first list is not in the second list, 
    ;; add it to the accumulator and continue with the rest of the list.
    (else
     (sym-diff-helper ... ... ...)))

(sym-diff-helper '(1 2 3) '(2 3 4) '())
;; == (1)
(sym-diff-helper '(1 2 (3 4) 5) '(2 3 4) '())  
;; == (5 (3 4) 1)


(sym-diff '(2 3 4 (2 3) 2 (4 5)) '(2 4 (4 5)))
;; == ((2 3) 3)

请注意，我选择使用member。还有一些其他的搜索功能，但它们不太适合这种情况。因此，我把它留在那里。有关搜索功能的更多信息可以在这里找到：http://docs.racket-lang.org/reference/pairs.html#%28part..List.Searching%29