Re: Hints on recursion

OK, so I may be breaking netiquette, posting when I've not been here a
day, but I had to throw this out.

I'm a newbie, my Lisp sucks. But I did come up with what i think might
be a valid solution, although I did use cons:

1. return true when the element appears in the first two places in the
list
2. return false if the list has fewer than 2 elements
3. delete any non-matching elements from the list and recurse

My version looks like this, but someone who actually knows what they're
doing could do it better, I'm sure:

(defun find-twice (a lst)
(cond ((and (eq a (first lst)) (eq a (first (rest lst)))) lst)
((or (eq (first lst) nil) (eq (rest lst) nil)) nil)
((not (eq (first lst) a)) (find-twice a (rest lst)))
(t (find-twice a ((first lst) . (cdr (rest lst)))))))

- mpeever

.

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