问题
I am messing around in language and need some help. I would like to create a swap function that swaps the first with the second. So if (swap '(a b c d e g)) should return (b a d c e g). I dont want to store any values doing it. Is there a function or way to do it in scheme? I have no idea if I would define a list like
(DEFINE list1 (LIST 'a 'b 'c 'd 'e ))
then not sure what to do
回答1:
The trick is to process two elements at a time, swapping them and advancing two elements in the recursion. This is what I mean:
(define (swap lst)
; if the list is empty or has a single element
(cond ((or (null? lst) (null? (cdr lst)))
; then return that list
lst)
; otherwise build a new list
(else
; by first adding the second element
(cons (cadr lst)
; and then adding the first element
(cons (car lst)
; finally, advance the recursion over two elements
(swap (cddr lst)))))))
I believe the sample output in the question is wrong, where does the f come from? For example the results I'd expect would be:
(swap '(a b c d e g))
=> '(b a d c g e)
(swap '(a b c d e))
=> '(b a d c e)
(swap '(a))
=> '(a)
(swap '())
=> '()
来源：https://stackoverflow.com/questions/22795186/would-like-to-swap-elements-in-programming