Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!udel!princeton!phoenix!eliot
From: eliot@phoenix.Princeton.EDU (Eliot Handelman)
Newsgroups: comp.lang.lisp
Subject: Re: novice question
Message-ID: <3315@idunno.Princeton.EDU>
Date: 13 Oct 90 04:05:04 GMT
References: <90285.160549JEFHC@CUNYVM.BITNET>
Sender: news@idunno.Princeton.EDU
Organization: Shitson University, New Crapsey
Lines: 29

In article <90285.160549JEFHC@CUNYVM.BITNET> JEFHC@CUNYVM writes:
;I realize that this is a very elementary question, but after
;banging my head against the wall for several days I still
;can't find an answer.
;
;I am trying to write a procedure that will produce the
;mirror image of a list,
;      (mirror '((a b)(c(d e))))  gives ((( e d)c)(a b)
;                 but
;(defun mirror (l)
;   (cond((null l)nil)
;        ((atom l)l)
;        (cons (mirror(rest l))(mirror(first l))))))
;                      yields
;((NIL (NIL (NIL . E) .D) . C (NIL . B) .A)

Try this, which slightly generalizes the trick of consing up the
reversed list in an accumulator:

(defun mirror (list)
  (labels ((mirror-1 (list accumulator)
	       (cond ((endp list) accumulator)
		     ((atom (car list))
		      (mirror-1 (cdr list)
				(cons (car list) accumulator)))
		     (t (mirror-1 (cdr list)
				  (cons (mirror-1 (car list) nil)
					accumulator))))))
    (mirror-1 list nil)))