消除内括号进入空列表并且不消除使用缺点

Question

目标是消除所有内部括号.

(flatten'(a(bc)d))变为'(abcd)

这是我在Racket中的代码

; if slist is null, return empty
; otherwise, if it is a pair, recursively solve car and cdr and concat them
; if it is a symbol, return the symbol

(define flatten
  (lambda (slist)
    (cond
      [ (null? slist) '()]
      [ (pair? slist)  
        (cons ((flatten (car slist)) (flatten (cdr slist))))]
      [ (symbol? slist) slist])))

它在抱怨

procedure application: expected procedure, given: c; arguments were: ()

这意味着我正在尝试访问car和cdr列出一个空列表.

I did the trace:
> (flatten '(a (b c) d))
pair?-car-cdr
a
((b c) d)
symbol?
a
pair?-car-cdr
(b c)
(d)
pair?-car-cdr
b
(c)
symbol?
b
pair?-car-cdr
c
()
symbol?
c
(stops here)

跟踪代码很简单 - 一堆显示.

(define flatten
  (lambda (slist)
    (cond
      [ (null? slist) '()]
      [ (pair? slist) 
        (display 'pair?-car-cdr)
        (newline)
        (display (car slist))
        (newline)
        (display (cdr slist))
        (newline)
        (cons ((flatten (car slist)) (flatten (cdr slist))))]
      [ (symbol? slist) 
         (display 'symbol?)
         (newline)
         (display slist)
         (newline)
        slist])))

我不明白的是,第一个条件怎么(null? slist)没有赶上空名单？我有两个递归调用.如果它确实捕获了空列表,它将转到下一个递归,即列表{d}.

我的递归逻辑有什么问题？

---

更新版本

(define flatten
  (lambda (slist)
    (cond
      [ (null? slist) '()]
      [ (pair? slist)  
        (cons (flatten (car slist)) (flatten (cdr slist)))]
      [ (symbol? slist) slist])))

(display (equal? (flatten '(a (b a) b a c (a b) c (e f (b a)))) '(a b a b a c a b c e f b a)))
(newline)
(display (equal? (flatten '(a b c)) '(a b c)))
(newline)
(display (equal? (flatten '(a (b c))) '(a b c)))
(newline)
(display (equal? (flatten '((a)(b)(c) d)) '(a b c d)))
(newline)
(display (equal? (flatten '(a (b) ((c)) (((d))) ((((e (f g))))))) '(a b c d e f g )))
(newline)
(display (equal? (flatten '()) '()))
(newline)
(display (equal? (flatten '(a b () ())) '(a b)))
(newline)

正如Ross Larson所建议的那样,追加将使该计划有效.但是为了学习,如果有人有空闲时间,我的测试结果只显示传递的基本情况(第二个和空列表)

我想过编写一个调用的包装函数 (cons (flatten slist) empty)

Answer 1

问题是这个表达式:

((压扁(汽车滑行))...)

这意味着,应用任何(展平...)返回.但由于这会返回一个列表,因此应用程序失败.

将其更改为

(压扁(汽车滑行))