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?
next prev parent 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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