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From: Joe Corneli <jcorneli@math.utexas.edu>
Subject: Re: sharing list structure
Date: Thu, 24 Mar 2005 18:27:28 -0600	[thread overview]
Message-ID: <E1DEcfg-0002gI-00@lab45.ma.utexas.edu> (raw)
In-Reply-To: <6dbd4d00050324161731ef0f51@mail.gmail.com> (message from Denis Bueno on Thu, 24 Mar 2005 19:17:15 -0500)


   > 
   > So I guess what I want is an "implicit pointer" to A.
   > 
   > Looking at the box diagrams in the manual, it seemed to me that
   > everything would be taken care of if I used "setcdr" to build the list
   > B.  But that didn't quite work:
   > 
   > (progn
   >   (setq A '(1 2 3))
   >   (setq B (list 'foo))
   >   (setcdr B A)
   >   (setq A (append A (list 4)))
   >   B)
   > ;=> (foo 1 2 3)

   The `append' doesn't alter the structure of the list A.

     (defvar *foo* (list 1 2 3))
     (append *foo* (list 4 5 6))
     *foo* => (1 2 3)

   Hence, the result of append doesn't alter A's structure.


Note the `setq' above, which make it look an awful lot like the
structure of A *is* being modified.  I mean, it comes out as a
different list --

(progn
  (setq A '(1 2 3))
  (setq B (list 'foo))
  (setcdr B A)
  (setq A (append A (list 4)))
  A)
;=> (1 2 3 4)

   > But is this the only way to go?  If it was possible, I would like to
   > set things up so that I could do anything I wanted to do to A, and
   > have B simply reflect that value at the end.

   You can do "anything you want" with A, as long as any function you run
   on A destructively modifies A. If it doesn't, then there's no way for
   B to reflect the change.

   So, just restrict yourself to destructive operations on A - like
   setcdr, setcar, etc. - and you'll be set. Just note that A will always
   have to be the "tail" part of B.

OK, I think I've got the idea now.  But still, I'm surprised that `setq'
is not among the list of "destructive functions".  What's that about?

  reply	other threads:[~2005-03-25  0:27 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2005-03-24 23:48 sharing list structure Joe Corneli
2005-03-25  0:17 ` Denis Bueno
2005-03-25  0:27   ` Joe Corneli [this message]
2005-03-25  0:35     ` Denis Bueno
     [not found]   ` <mailman.176.1111711460.28103.help-gnu-emacs@gnu.org>
2005-03-25  1:21     ` Thien-Thi Nguyen
2005-03-26 23:58     ` Stefan Monnier
     [not found] <mailman.173.1111709565.28103.help-gnu-emacs@gnu.org>
2005-03-25  1:01 ` rgb
2005-03-25  1:42   ` Joe Corneli
     [not found]   ` <mailman.182.1111716692.28103.help-gnu-emacs@gnu.org>
2005-03-25  5:44     ` rgb
2005-03-25  5:49       ` rgb
2005-03-25  6:08       ` Joe Corneli

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