[英]Two Element List Scheme
I need to write a function that determines if the given list is a pair of elements. 我需要编写一个确定给定列表是否为一对元素的函数。 The program will simply respond #t if the list contains exactly two elements or #f if it does not, such that:
如果列表恰好包含两个元素,则程序将仅响应#t;否则,将仅响应#f,例如:
(zipper? '((a 1)(b 2))) => #t
and 和
(zipper? '((foo 100)(bar 2 3))) => #f
I'm still fairly new to Scheme so any help would be much appreciated! 我对Scheme还是很陌生,因此不胜感激! Thanks!
谢谢!
It isn't clear if the "correct" input for the procedure is an arbitrary list or a two-element list. 尚不清楚该过程的“正确”输入是任意列表还是两元素列表。 If it's strictly a two-element list, this will work:
如果严格来说是两个元素的列表,它将起作用:
(define (is-two-element-list? lst)
(and (list? lst)
(= (length lst) 2)))
(define (zipper? lst)
(and (is-two-element-list? lst)
(is-two-element-list? (first lst))
(is-two-element-list? (second lst))))
… And if it's an arbitrary-length list whose elements we want to check, this will work in Racket, using andmap
: …如果这是一个任意长度的列表,我们要检查其元素,则可以使用
andmap
在Racket中运行:
(define (zipper? lst)
(andmap is-two-element-list? lst))
If you are not using Racket, then this solution using every
will work in any interpreter with SRFIs: 如果您不使用Racket,那么使用
every
此解决方案将在具有SRFI的任何解释器中运行:
(require srfi/1)
(define (zipper? lst)
(every is-two-element-list? lst))
Either way, notice that the trick was defining the is-two-element-list?
无论哪种方式,请注意,诀窍是定义
is-two-element-list?
procedure, which verifies the two-element-list property, after that we can apply it as needed. 验证两个元素列表属性的过程,之后我们可以根据需要应用它。
Think of it this way. 这样想吧。 If the
zipper
list is '()
then the answer is #t
. 如果
zipper
列表为'()
则答案为#t
。 If the zipper
list is not '()
then if the first element is two elements and the rest is another zipper?
如果
zipper
列表不是'()
那么第一个元素是两个元素,其余元素是另一个zipper?
, then return #t
. ,然后返回
#t
。
(define (zipper? list)
(or (null? list)
(and (= 2 (length (car list)))
(zipper? (cdr list)))))
or maybe you mean: 也许你的意思是:
(define (zipper? list)
(or (not (pair? list))
(and (= 2 (length list))
(zipper? (list-ref list 0))
(zipper? (list-ref list 1)))))
every element, at any level, has two elements. 每个元素在任何级别都有两个元素。
> (zipper? '((a 1 2) '(b)))
#f
> (zipper? '(a b))
#t
> (zipper? '(((a (b b)) c) (1 2)))
#t
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