From: Geoffrey B Milord
Subject: Some Questions.
Date: Sat, 24 Feb 1996 00:00:00 +0000
Message-ID: <4gm1d6$ho6@netnews.upenn.edu>

Can somebody please explain why 1) & 2) do not destructively alter the input
list and 3) does?
Thanks alot!!!!

1)
> (defun mypush (e l)
  (cond ((null l) (cons e ()))
	(t (setf l (cons (car l) (mypush e (cdr l)))))))
mypush
> (setq li '(a b c))
(a b c)
> (mypush 'd li)
(a b c d)
> li
(a b c)

2)
> (defun mypush (e l)
  (cond ((null l) (cons e ()))
	(t (setq l (cons (car l) (mypush e (cdr l)))))))
mypush
> (mypush 'd li)
(a b c d)
> li
(a b c)

3)
> (defun mypush (e l)
	(cond ((null (cdr l)) (rplacd l (cons e ())))
		(t (setq l (cons (car l) (mypush e (cdr l)))))))
mypush
> (mypush 'd li)
(a b c d)
> li
(a b c d)
>