From: pjb@informatimago.com (Pascal J. Bourguignon)
To: help-gnu-emacs@gnu.org
Subject: Re: Hash-table elements
Date: Thu, 01 Oct 2009 17:18:48 +0200 [thread overview]
Message-ID: <7cpr97gn1j.fsf@pbourguignon.lefevre.anevia.com> (raw)
In-Reply-To: 5230b46a-f95e-4497-a6e7-5c7d71d30b87@l2g2000yqd.googlegroups.com
Nordlöw <per.nordlow@gmail.com> writes:
> On Sep 29, 3:17 pm, p...@informatimago.com (Pascal J. Bourguignon)
> wrote:
>> Nordlöw <per.nord...@gmail.com> writes:
>> > Can hash-tables contain references other "global" structures?
>>
>> > I have a hash-table that maps filenames to their metadata,file-scan-
>> > hits/misses.
>> > I don't want to pull-change-push the whole meta-data value whenever I
>> > change parts of these metadata/hits.
>>
>> > Is this possible somehow?
>>
>> Yes. Only do not modify the keys! You wouldn't be able to retrieve
>> them, since changing the state of a key would probably change its hash
>> value, and therefore the bucket where the hash-table stored it.
>> But you can do whatever you want on the value.
>>
>> (require 'cl)
>> (let ((h (make-hash-table)))
>>
>> (let ((value (list 1 2 3))
>> (key :my-list))
>>
>> (setf (gethash key h) value)
>> (setf (car value) 0
>> (cdr value) 'z)
>> (gethash key h)))
>> --> (0 . z)
>>
>> --
>> __Pascal Bourguignon__
>
> So with the use of setf() we realize the reference pattern in Emacs-
> Lisp as we use pointers in C?
>
> So the value of (gethash key h) is actually a reference to "value" and
> when "value" goes out of scope the interpreter knows that it is
> referenced from another structure and does not delete its contents?
>
> I have understood it correctly?
Yes.
However, some data types are immutable. (Out of blue, I can think
only of numbers in emacs lisp that are immutable). For these
immutable data types, since you cannot modify them, the system could
make copy of them, instead of using references.
--
__Pascal Bourguignon__
prev parent reply other threads:[~2009-10-01 15:18 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-09-29 9:52 Hash-table elements Nordlöw
2009-09-29 13:17 ` Pascal J. Bourguignon
2009-10-01 14:52 ` Nordlöw
2009-10-01 15:18 ` Pascal J. Bourguignon [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=7cpr97gn1j.fsf@pbourguignon.lefevre.anevia.com \
--to=pjb@informatimago.com \
--cc=help-gnu-emacs@gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this external index
https://git.savannah.gnu.org/cgit/emacs.git
https://git.savannah.gnu.org/cgit/emacs/org-mode.git
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.