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Re: Elisp manual, node "Comparison of Numbers"
From: |
Kim F. Storm |
Subject: |
Re: Elisp manual, node "Comparison of Numbers" |
Date: |
Mon, 29 May 2006 15:42:00 +0200 |
User-agent: |
Gnus/5.11 (Gnus v5.11) Emacs/22.0.50 (gnu/linux) |
David Kastrup <address@hidden> writes:
> "Drew Adams" <address@hidden> 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 <address@hidden> http://www.cua.dk
- Elisp manual, node "Comparison of Numbers", Drew Adams, 2006/05/29
- Re: Elisp manual, node "Comparison of Numbers", David Kastrup, 2006/05/29
- Re: Elisp manual, node "Comparison of Numbers",
Kim F. Storm <=
- Re: Elisp manual, node "Comparison of Numbers", Lennart Borgman, 2006/05/29
- Re: Elisp manual, node "Comparison of Numbers", David Kastrup, 2006/05/29
- Re: Elisp manual, node "Comparison of Numbers", Lennart Borgman, 2006/05/29
- Re: Elisp manual, node "Comparison of Numbers", David Kastrup, 2006/05/29
- Re: Elisp manual, node "Comparison of Numbers", Lennart Borgman, 2006/05/29
Re: Elisp manual, node "Comparison of Numbers", Richard Stallman, 2006/05/29
Re: Elisp manual, node "Comparison of Numbers", Lennart Borgman, 2006/05/29
Re: Elisp manual, node "Comparison of Numbers", Stefan Monnier, 2006/05/29