From: David Kastrup <dak@gnu.org>
Cc: Emacs-Devel <emacs-devel@gnu.org>,
Drew Adams <drew.adams@oracle.com>, "Kim F. Storm" <storm@cua.dk>
Subject: Re: Elisp manual, node "Comparison of Numbers"
Date: Mon, 29 May 2006 16:08:45 +0200 [thread overview]
Message-ID: <85bqtgditu.fsf@lola.goethe.zz> (raw)
In-Reply-To: <447AFCFD.6030405@student.lu.se> (Lennart Borgman's message of "Mon, 29 May 2006 15:54:05 +0200")
Lennart Borgman <lennart.borgman.073@student.lu.se> writes:
> Kim F. Storm wrote:
>> 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!
>>
> Hm. It is the relative distance from 0.
It is merely the distance from 0, not the "relative distance".
--
David Kastrup, Kriemhildstr. 15, 44793 Bochum
next prev parent reply other threads:[~2006-05-29 14:08 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
2006-05-29 13:54 ` Lennart Borgman
2006-05-29 14:08 ` David Kastrup [this message]
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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