打印列表的相邻重复项(方案)

问题描述

我正在尝试创建一个返回列表的相邻重复项的函数，例如(dups'(1 2 1 1 1 4 4)应该返回列表(1 4).

I'm trying to create a function that returns the adjacent duplicates of a list, for example (dups '(1 2 1 1 1 4 4) should return the list (1 4).

这是我到目前为止想出的代码:

This is the code I came up with so far:

(define (dups lst)
    (if (equal? (car lst)(car(cdr lst)))
        (cons(cdr lst) '())
        (dups(cdr lst))))

此函数不会返回所有相邻的重复项，而只会返回第一个相邻的重复项！如何修复它，使其返回列表的所有相邻重复项?

This function doesn't return all the adjacent duplicates, it only returns the first adjacent duplicates!How can I fix it so that it returns all the adjacent duplicates of a list?

谢谢.

推荐答案

一旦您的代码发现重复项，它将停止处理列表的其余部分:当if测试为true时，将产生(cons (cdr lst) '()).无论是否找到重复项，它都应该仍在调用dups来处理列表的其余部分.

Once your code finds a duplicate, it stops processing the rest of the list: when the if test is true, it yields (cons (cdr lst) '()). Whether or not it finds a duplicate, it should still be calling dups to process the rest of the list.

此外:如果您的列表没有重复项，则会遇到麻烦.

Also: if your list has no duplicates, it it going to run into trouble.

这是比发布的其他解决方案更简单的解决方案:

Here's a simpler solution than the others posted:

(define (dups lst)
    (if (< (length lst) 2)
        ; No room for duplicates
        '()
        ; Check for duplicate at start
        (if (equal? (car lst) (cadr lst))
            ; Starts w/ a duplicate
            (if (or (null? (cddr lst)) ; end of list
                    (not (equal? (car lst) (caddr lst)))) ; non-matching symbol next
                ; End of run of duplicates; add to front of what we find next
                (cons (car lst) (dups (cdr lst)))
                ; Othersise keep looking
                (dups (cdr lst)))
            ; No duplicate at start; keep looking
            (dups (cdr lst)))))

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