From: Stefan Monnier <monnier@iro.umontreal.ca>
Cc: Emacs-Devel <emacs-devel@gnu.org>
Subject: Re: Elisp manual, node "Comparison of Numbers"
Date: Mon, 29 May 2006 15:23:00 -0400 [thread overview]
Message-ID: <jwvfyiseixt.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <MEEKKIABFKKDFJMPIOEBEENDDBAA.drew.adams@oracle.com> (Drew Adams's message of "Mon, 29 May 2006 06:20:20 -0700")
> 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))))
There is no generally good solution to this problem. Depending on the
specific problem, one approach will work while that approach will fail
miserably in another context.
Welcome to the wonderful world of floating point and numerical analysis.
I don't think the Elisp manual should try to tackle these kinds of problems,
especially given how rarely floats are used in Emacs.
Stefan
prev parent reply other threads:[~2006-05-29 19:23 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
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 [this message]
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