From macrakis at alum.mit.edu  Sat Jun 20 14:44:21 2009
From: macrakis at alum.mit.edu (Stavros Macrakis)
Date: Sat, 20 Jun 2009 08:44:21 -0400
Subject: [Rd] Floating point precision / guard digits? (PR#13771)
In-Reply-To: <20090619113511.336752832009@mail.pubhealth.ku.dk>
References: <20090619113511.336752832009@mail.pubhealth.ku.dk>
Message-ID: <8b356f880906200544v5e47505cq33fda38801c9110e@mail.gmail.com>

a) this is not a bug, so this is the wrong list

b) 'underflow' does not mean what you think it means.

c) guard digits and sticky bits do improve rounding behavior, but
floating point will always remain approximate.

d) if it is important to your application to perform exact arithmetic
on rational numbers (and I suspect it is not), you might want to use
that instead of floating-point. But even if implemented in R, most R
calculations cannot use it.

You may want to study up on floating-point arithmetic some more, though.

         -s

On 6/19/09, rbugs09 at kreil.org <rbugs09 at kreil.org> wrote:
> Full_Name: D Kreil
> Version: 2.8.1 and 2.9.0
> OS: Debian Linux
> Submission from: (NULL) (141.244.140.179)
>
>
> Group: Accuracy
>
> I understand that most floating point numbers are approximated due to their
> binary storage. On the other hand, I thought that modern math CPUs used
> guard
> digits to protect against trivial underflows. Not true?
>
> # integers, no problem
>> 1+1+1==3
> [1] TRUE
> # binary floating point approximation underflows
>> .1+.1+.1==.3
> [1] FALSE
>> .1+.1+.1==.3
> [1] FALSE
> # binary floating point exact for certain numbers
>> .1+.1==.2
> [1] TRUE
>
> I know that safe code should not test for quality of floats. Still, is R
> underutilizing the power of the underlying hardware?
>
> Grateful for comments,
> David.
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>



[Rd] Floating point precision / guard digits? (PR#13771)
Stavros Macrakis