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From: raja@copper.ucs.indiana.edu (Raja Sooriamurthi)
Newsgroups: comp.lang.lisp
Subject: Re: novice question
Message-ID: <raja.655772214@copper>
Date: 12 Oct 90 22:56:54 GMT
References: <90285.160549JEFHC@CUNYVM.BITNET>
Sender: news@iuvax.cs.indiana.edu
Lines: 34

JEFHC@CUNYVM writes:

>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)

What you need is a function known as 'snoc' which is similar to
cons except that (as its name indicates) it works backwards.

So, (snoc '(a b c) 'd) => (a b c d)
You could write it as

(defun snoc (l x) (append l (list x)))

Mirror then becomes,

(defun mirror (l)
  (cond ((null l) nil)
        ((listp l) (snoc (mirror (rest l))
                         (mirror (first l))))
        (t (snoc (mirror (rest l)) (first l)))))

[I might have the syntax mixed up (listp - ?) as I usually program in
Scheme, but you get the idea I presume]

- Raja
raja@cs.indiana.edu