From: "Pascal J. Bourguignon" <pjb@informatimago.com>
To: help-gnu-emacs@gnu.org
Subject: Re: Hash tables - how to look up values?
Date: Wed, 04 Jul 2012 15:59:27 +0200 [thread overview]
Message-ID: <87d34bfwzk.fsf@kuiper.lan.informatimago.com> (raw)
In-Reply-To: mailman.4023.1341409554.855.help-gnu-emacs@gnu.org
Thorsten Jolitz <tjolitz@googlemail.com> writes:
> Hi List,
> I would need a function that does the opposite of
>
> ,--------------------
> | (gethash key table)
> `--------------------
>
> i.e. something like
>
> ,-------------------
> | (getkey val table)
> `-------------------
>
> that returns the corresponding key of a known (string) value in a hash
> table (similar to 'rassoc' for alists).
>
> I could not find such a function - does it exist already?
rassoc returns the first hit, because lists, even association lists are
ordered. Hash tables are not ordered. We can write something similar,
returning the first hit, but there's no guarantee the first will always
be the same.
(require 'cl)
(defun* getkey (val table &key (test (function eql)) (default nil))
;; (hash-table-test table) is for keys, not for values…
(maphash (lambda (k v)
(when (funcall test val v) (return-from getkey k)))
table)
default)
(let ((h (make-hash-table)))
(setf (gethash :one h) "un"
(gethash :two h) "deux"
(gethash :a h) "un")
(getkey "un" h :test (function string=)))
--> :one ; or :a, who knows?
I'd rather write:
(defun* getkeys (val table &key (test (function eql)))
;; (hash-table-test table) is for keys, not for values…
(let ((keys '()))
(maphash (lambda (k v)
(when (funcall test val v) (push k keys)))
table)
keys))
(let ((h (make-hash-table)))
(setf (gethash :one h) "un"
(gethash :two h) "deux"
(gethash :a h) "un")
(getkeys "un" h :test (function string=)))
--> (:a :one)
--
__Pascal Bourguignon__ http://www.informatimago.com/
A bad day in () is better than a good day in {}.
next parent reply other threads:[~2012-07-04 13:59 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.4023.1341409554.855.help-gnu-emacs@gnu.org>
2012-07-04 13:59 ` Pascal J. Bourguignon [this message]
2012-07-04 14:21 ` Hash tables - how to look up values? Thorsten Jolitz
2012-07-04 13:48 Thorsten Jolitz
2012-07-04 14:09 ` Teemu Likonen
2012-07-04 14:09 ` Bastien
2012-07-04 14:31 ` Thorsten Jolitz
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=87d34bfwzk.fsf@kuiper.lan.informatimago.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.