R devel - Nov 2011 - a^b when a is large and b < 1 (64bit R on windows 7)

Hi,

I'm getting some strange behaviour when trying to use the power operator
(a^b) when a is large and b is less than one:

big <- .Machine$double.xmax
big
big^0.5
sqrt(big)
> big <- 1.797693134862315708384e+308
> big^0.5
[1] Inf
> sqrt(big)
[1] 1.340781e+154


I'm guessing that this behaviour is not expected, or am I missing
something about ^?

Cheers

Martyn


> sessionInfo()
R version 2.14.0 (2011-10-31)
Platform: x86_64-pc-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=English_United Kingdom.1252
[2] LC_CTYPE=English_United Kingdom.1252
[3] LC_MONETARY=English_United Kingdom.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United Kingdom.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

________________________________________________________________________
The Numerical Algorithms Group Ltd is a company registered in England
and Wales with company number 1249803. The registered office is:
Wilkinson House, Jordan Hill Road, Oxford OX2 8DR, United Kingdom.

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I can reproduce on the latest patched build, but this is specific for
the 64bit version of R. On a 32bit it works as expected:
> big <- 1.797693134862315708385e+308
> sqrt(big)
[1] 1.340781e+154
> big^0.5
[1] 1.340781e+154

The 64bit part might have something to do with it, and now move out of
the way for people that actually know what they're talking about...

Cheers
Joris


On Mon, Nov 21, 2011 at 3:41 PM, Martyn Byng <Martyn.Byng at nag.co.uk>
wrote:
> Hi,
>
> I'm getting some strange behaviour when trying to use the power
operator
> (a^b) when a is large and b is less than one:
>
> big <- .Machine$double.xmax
> big
> big^0.5
> sqrt(big)
>
>> big <- 1.797693134862315708384e+308
>> big^0.5
> [1] Inf
>> sqrt(big)
> [1] 1.340781e+154
>
>
> I'm guessing that this behaviour is not expected, or am I missing
> something about ^?
>
> Cheers
>
> Martyn
>
>
>
>> sessionInfo()
> R version 2.14.0 (2011-10-31)
> Platform: x86_64-pc-mingw32/x64 (64-bit)
>
> locale:
> [1] LC_COLLATE=English_United Kingdom.1252
> [2] LC_CTYPE=English_United Kingdom.1252
> [3] LC_MONETARY=English_United Kingdom.1252
> [4] LC_NUMERIC=C
> [5] LC_TIME=English_United Kingdom.1252
>
> attached base packages:
> [1] stats ? ? graphics ?grDevices utils ? ? datasets ?methods ? base
>
> ________________________________________________________________________
> The Numerical Algorithms Group Ltd is a company registered in England
> and Wales with company number 1249803. The registered office is:
> Wilkinson House, Jordan Hill Road, Oxford OX2 8DR, United Kingdom.
>
> This e-mail has been scanned for all viruses by Star. Th...{{dropped:4}}
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>


-- 
Joris Meys
Statistical consultant

Ghent University
Faculty of Bioscience Engineering
Department of Mathematical Modelling, Statistics and Bio-Informatics

tel : +32 9 264 59 87
Joris.Meys at Ugent.be
-------------------------------
Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php

On Mon, 2011-11-21 at 14:41 +0000, Martyn Byng wrote:
> I'm getting some strange behaviour when trying to use the power
> operator
> (a^b) when a is large and b is less than one:
> 
> big <- .Machine$double.xmax
> big
> big^0.5
> sqrt(big)
> 
> > big <- 1.797693134862315708384e+308
> > big^0.5
> [1] Inf
> > sqrt(big)
> [1] 1.340781e+154
> 
> 
> I'm guessing that this behaviour is not expected, or am I missing
> something about ^? 
On a recent Ubuntu 64bit install with R2.14.0 from the repositories, I
get:
> big <- .Machine$double.xmax
> big
[1] 1.797693e+308
> big^0.5
[1] 1.340781e+154
> sqrt(big)
[1] 1.340781e+154

so it does seem to be specific either to your environment.

-- 
Brian G. Peterson
http://braverock.com/brian/
Ph: 773-459-4973
IM: bgpbraverock