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Re: Elisp manual, node "Comparison of Numbers"
From: |
David Kastrup |
Subject: |
Re: Elisp manual, node "Comparison of Numbers" |
Date: |
Mon, 29 May 2006 16:08:45 +0200 |
User-agent: |
Gnus/5.11 (Gnus v5.11) Emacs/22.0.50 (gnu/linux) |
Lennart Borgman <address@hidden> writes:
> Kim F. Storm wrote:
>> 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!
>>
> 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
- 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, 2006/05/29
- Re: Elisp manual, node "Comparison of Numbers", Lennart Borgman, 2006/05/29
- Re: Elisp manual, node "Comparison of Numbers",
David Kastrup <=
- 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