使用 for/fold 检查嵌套列表中是否存在元素的正确方法是什么；Racket？

Question

所以，我正在尝试编写一个函数，它接受两个参数，一个嵌套的数字 L 列表和一个数字 n。这函数 return 如果数字 n 属于任何列表，则为真，否则为假。
例如，(sob35 '((1 2 3) (5 4) (6 7) (8 9 10 11)) 8) returns true 和 (sob35 '((1 2 3) (5 4) ( 6 7) (8 9 10 11)) 22) return 错误。

我用 rest 的成员函数测试了这个，看它是否会检查 6 是否是嵌套列表的成员，

 (member 6 ((rest  '((1 2 3) (5 4) (6 7) (8 9 10 11))))) it returned



application: not a procedure;
 expected a procedure that can be applied to arguments
  given: '((5 4) (6 7) (8 9 10 11))  what i had given it was indeed a list



 (define sob35
(lambda ( l1 l2 )
(for/list ([ L l1 ] [ N l2 ])
   (if ( member N (rest (L))) "True" "False"))))

我认为我的想法不正确，因为我认为如果 N 是 L 的成员，它应该 return true 否则它应该 return false，虽然当我运行它

(sob35 '((1 2 3) (5 4) (6 7) (8 9 10 11)) 8)
. . application: not a procedure;
 expected a procedure that can be applied to arguments
  given: '(1 2 3)

我不明白应用程序错误是在暗示什么；它应该包含一个列表

Answer 1

在 List of Lists 输入上使用 for/or，以实现所需...

(define two-sequences 
  (lambda (l1 l2) 
    (for/or ([N l1]) 
      (and (member l2 N) #t))))

测试

racket@> (define two-sequences (lambda (l1 l2) (for/or ([N l1]) (and (member l2 N) #t))))
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 6)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 16)
#f
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 8)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 10)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 3)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 12)
#f
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 1)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 2)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 3)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 4)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 5)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 6)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 7)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 8)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 9)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 10)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 11)
#t
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 12)
#f
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 13)
#f
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 14)
#f
racket@> (two-sequences '((1 2 3) (5 4) (6 7) (8 9 10 11)) 15)
#f
racket@>

这个怎么样...

(member 6 (rest  '((1 2 3) (5 4) (6 7) (8 9 10 11))))

在原来的调用中，就像...

(member 6 ( (rest  '((1 2 3) (5 4) (6 7) (8 9 10 11))) ))
        ;;^                                         ;; ^

在评估 (rest ...) 之后得到 function application 的处理。

(let ((result-of-evaluation-of-rest (rest  '((1 2 3) (5 4) (6 7) (8 9 10 11))))) 
  (member 6 (result-of-evaluation-of-rest)))

其中 result-of-evaluation-of-rest 不是一个函数，而是一个列表。而我们真正想要的是...

(let ((result-of-evaluation-of-rest (rest  '((1 2 3) (5 4) (6 7) (8 9 10 11))))) 
  (member 6 result-of-evaluation-of-rest))

Answer 2

您需要一个以嵌套方式工作的 member 函数。在 lisp 传统中，它将被称为 member* - 因为它是 member 的递归工作形式（用于平面列表）。

(define (member*? nested-list element (test equal?))
  (cond ((null? nested-list) #f)
        ((list? (car nested-list)) (or (member*? (car nested-list) element test)
                                       (member*? (cdr nested-list) element test)))
        ((test (car nested-list) element) #t)
        (else (member*? (cdr nested-list) element test))))

因为嵌套可以任意深但递归不是，可以使用 tail-recursive 函数。

(define (member*? nested-list element (test equal?) (acc #f))
  (cond ((null? nested-list) acc)
        ((list? (car nested-list)) (member*? (cdr nested-list) element test
                                             (or acc (member*? (car nested-list) element test))))
        ((test (car nested-list) element) #t)
        (else (member*? (cdr nested-list) element test acc))))

Answer 3

一个简单的“for 循环”答案可能特定于“列表列表”参数。
问题中引用的 Racket 的 for/fold 可以使用：

(define (member-of-sub-list? lols x)
  (for/fold ([result #f])
    ([ls lols])
    (or result (not (not (member x ls))))))
    
;(member-of-sub-list? '((1) (2 3 4)) 3)  => #t
;(member-of-sub-list? '((1) (2 3 4)) 99) => #f

但是，当学习Scheme或Racket时，系统地开发这样的功能是有帮助的。
首先，编写一个 stub 函数，带有 signature 和 purpose，以及 simple tests:

(define (member-of-sub-list? lolox x) ;; (ListOf ListOfX) X -> Boolean
  ;; produce whether x occurs in sub-lists of lolox
  #f)

(check-expect (member-of-sub-list? '(())  0) #f)
(check-expect (member-of-sub-list? '((0)) 0) #t)

然后使用示例和 standard template 填写函数，测试：

(define (member-of-sub-list? lolox x) ;; (ListOf ListOfX) X -> Boolean
  ;; produce whether x occurs in sub-lists of lolox
  (cond
    [(empty? lolox) #f ]
    [(member x (first lolox)) #t ]
    [else (member-of-sub-list? (rest lolox) x) ]))

(check-expect (member-of-sub-list? '((1) (2 3 4) ()) 0) #f)
(check-expect (member-of-sub-list? '((1) (2 0 4) ()) 0) #t)

（编写此解决方案比阅读和理解 for/fold 文档花费的时间更少）

使用 for/fold 检查嵌套列表中是否存在元素的正确方法是什么；Racket？

What is the correct way to use for/fold check if an element exists in nested list ;Racket?

list

for-loop

scheme

lisp

racket