From: storm@cua.dk (Kim F. Storm)
Cc: Drew Adams <drew.adams@oracle.com>, Emacs-Devel <emacs-devel@gnu.org>
Subject: Re: Elisp manual, node "Comparison of Numbers"
Date: Mon, 29 May 2006 15:42:00 +0200 [thread overview]
Message-ID: <m31wudq76f.fsf@kfs-l.imdomain.dk> (raw)
In-Reply-To: <85fyitc5w4.fsf@lola.goethe.zz> (David Kastrup's message of "Mon, 29 May 2006 15:33:31 +0200")
David Kastrup <dak@gnu.org> writes:
> "Drew Adams" <drew.adams@oracle.com> writes:
>
>> I blindly got bit by this one. The Elisp manual gives this as an example of
>> how to test near equality of floating-point numbers:
>>
>> (defvar fuzz-factor 1.0e-6)
>> (defun approx-equal (x y)
>> (or (and (= x 0) (= y 0))
>> (< (/ (abs (- x y))
>> (max (abs x) (abs y)))
>> fuzz-factor)))
>>
>> When either x or y is 0.0, but not both, this gives nil no matter how close
>> the other number is to zero. I think this is more like what is needed:
>>
>> (defun approx-equal (x y &optional fuzz)
>> (setq fuzz (or fuzz 1.0e-8))
>> (cond ((= x 0.0) (< y fuzz))
>> ((= y 0.0) (< x fuzz))
>> (t (< (/ (abs (- x y)) (max (abs x) (abs y))) fuzz))))
>
> The problem here is that fuzz is a _relative_ measure of equality, and
> you employ it as an absolute measure here. I don't think it a good
> idea at all that 1e-12 and 0.995e-12 are considered different, while
> 1e-8 and 0.0 are considered equal.
Agree!
And:
(approx-equal 0.0 -1.0e10) => t
--
Kim F. Storm <storm@cua.dk> http://www.cua.dk
next prev parent reply other threads:[~2006-05-29 13:42 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-05-29 13:20 Elisp manual, node "Comparison of Numbers" Drew Adams
2006-05-29 13:33 ` David Kastrup
2006-05-29 13:42 ` Kim F. Storm [this message]
2006-05-29 13:54 ` Lennart Borgman
2006-05-29 14:08 ` David Kastrup
2006-05-29 14:18 ` Lennart Borgman
2006-05-29 14:23 ` David Kastrup
2006-05-29 14:40 ` Lennart Borgman
2006-05-29 21:28 ` Richard Stallman
2006-05-29 13:40 ` Lennart Borgman
2006-05-29 18:14 ` Drew Adams
2006-05-30 3:46 ` Richard Stallman
2006-05-29 19:23 ` Stefan Monnier
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