From: Nordlöw <per.nordlow@gmail.com>
To: help-gnu-emacs@gnu.org
Subject: Re: Hash-table elements
Date: Thu, 1 Oct 2009 07:52:28 -0700 (PDT) [thread overview]
Message-ID: <5230b46a-f95e-4497-a6e7-5c7d71d30b87@l2g2000yqd.googlegroups.com> (raw)
In-Reply-To: 7c7hvhki0b.fsf@pbourguignon.lefevre.anevia.com
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?
/Nordlöw
next prev parent reply other threads:[~2009-10-01 14:52 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 [this message]
2009-10-01 15:18 ` Pascal J. Bourguignon
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