如何修复我的代码，避免在球拍中使用map时返回重复的对？

Question

此函数应返回L的传递闭包。例如：

(Transitive-Closure'((a b) (b c) (a c))) ---> '((a b) (b c) (a c))
(Transitive-Closure'((a a) (b b) (c c))) ---> '((a a) (b b) (c c))
(Transitive-Closure'((a b) (b a)))  ---> '((a b) (b a) (a a) (b b)))
(Transitive-Closure'((a b) (b a) (a a)))---> '((a b) (b a) (a a) (b b))
(Transitive-Closure'((a b) (b a) (a a) (b b)))---> '((a b) (b a) (a a) (b b))
(Transitive-Closure'())---> '()

这是我在球拍中拥有的：

(define (Transitive-Closure L)
  (apply append
  ; Iterate over each pair (a b) in L,
  (map (lambda (x)
            ;Iterate over each pair (c d) in L,
            (map (lambda (y)
                      (let ([a (car x)]
                            [b (cadr x)]               
                            [c (car y)]
                            [d (cadr y)])
                        ;if b equal to c, and (a d) does not exist in L, it will add (a d) to L . Otherwise, return L.
                        (if  (and (eq? b c) (not (member (list a d) L)))
                             (list a d)
                             (append x))))L)) L)))

我的代码只有在不可传递的情况下才能工作。如何修改代码以避免在传递时返回重复的对？

例如，我的输出：

;This is wrong. It should return '((a b)(b c)(a c)) 
(Transitive-Closure '((a b)(b c)(a c))) ---> '((a b) (a b) (a b) (b c) (b c) (b c) (a c) (a c) (a c))
; This is right.
(Transitive-Closure '((a b)(b a)))---> '((a b) (a a) (b b) (b a))

Answer 1

这不是一个map问题，这是一个foldr问题，因为您没有在适当的位置修改列表，而是将列表压缩为一项。该项目恰好是一个list，但重要的是，该列表可以小于map可以返回的内容。此外，您还可以根据累加器中是否已经存在该对来检查是否应该在另一条if语句中添加。

如果顺序无关紧要(我假设，你只是想要集合，如果不是这样，请让我知道)

(define (Transitive-Closure L)
  ; Iterate over each pair (a b) in L,
  (foldr (lambda (x transitive-pairs)
            ;Iterate over each pair (c d) in L,
            (foldr (lambda (y sofar)
                      (let ([a (car x)]
                            [b (cadr x)]               
                            [c (car y)]
                            [d (cadr y)])
                        ;if b equal to c, and (a d) does not exist in L, it will add (a d) to L . Otherwise, return L.
                        (cond [(and (eq? b c) (not (member (list a d) L))) (cons (list a d) sofar)]
                              [(not (member x sofar)) (cons x sofar)]
                              [else sofar])))
                               transitive-pairs L)) '() L))

(check-expect (Transitive-Closure '((a b) (b c) (a c))) '((a b) (b c) (a c)))
(check-expect (Transitive-Closure '((a a) (b b) (c c))) '((a a) (b b) (c c)))
(check-expect (Transitive-Closure '((a b) (b a))) '((a b) (a a) (b b) (b a)))
(check-expect (Transitive-Closure '((a b) (b a) (a a))) '((a b) (b b) (b a) (a a)))
(check-expect (Transitive-Closure '((a b) (b a) (a a) (b b))) '((a b) (b a) (a a) (b b)))