[R] cube root of a negative number

[Prof Brian Ripley]

In this particular case it is part of the C99 stanadrd (7.12.7.4) for
the 'pow' function R uses.

On Wed, 27 Oct 2010, Berwin A Turlach wrote:

> G'day Gregory,
>
> On Tue, 26 Oct 2010 19:05:03 -0400
> Gregory Ryslik <rsaber at comcast.net> wrote:
>
>> Hi,
>>
>> This might be me missing something painfully obvious but why does the
>> cube root of the following produce an NaN?
>>
>>> (-4)^(1/3)
>> [1] NaN
>
> 1/3 is not exactly representable as a binary number.  My guess is that
> the number that is closest to 1/3 and representable cannot be used as
> the exponent for negative numbers, hence the NaN.
>
> Essentially, don't expect finite precision arithmetic to behave like
> infinite precision arithmetic, it just doesn't.  The resources
> mentioned in FAQ 7.31 can probably shed more light on this issue.
>
> Cheers,
>
> 	Berwin
>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
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