在列表中的列表中查找列表的元素

问题描述：

我需要一个过程，它接受一个列表并检查一个元素是否是该列表的一部分，即使该列表包含列表也是如此.到目前为止，我已经写了这个:

I need a procedure which takes a list and checks if an element is part of that list even when the list contains lists. So far, I've written this:

(define (element-of-set? element set)
    (cond ((null? set) #f)
    ((eq? element (car set)) #t)
    (else (element-of-set? element (cdr set))))))

但是，如果您有一个如下所示的列表:

But, if you have a list that looks like this:

(a (a b b (c b) 3) 5 5 (e s) (s e s))

那么我写的 element-of-set? 不承认 3 是这个列表的一部分.我做错了什么?

then my written element-of-set? does not recognize that 3 is a part of this list. What am I doing wrong?

答

你只需要另一个子句来检查 car 是否是一个列表.将此添加到您的 cond(在 eq? 子句之后):

You just need another clause to check if the car is a list. Add this to your cond (after the eq? clause):

((list? (car set))
 (or (element-of-set? element (car set))
     (element-of-set? element (cdr set))))

这会在任何子列表上递归.

This recurses on any sublists.