R does not "know" that 1/3 is 1/3. It is represented internally as
0.33333...3, so certain mathematical facts such as the existence of
real roots of fractional integer powers are opaque to R (since it is
not a symbolic algebra system.)
Try seaarching for cube roots on R-search (for example):
http://finzi.psych.upenn.edu/Rhelp08/2009-July/205006.html # Further
comments by Ted Harding
> complex(real=-6.108576e-05)^(1/3)
[1] 0.01969171+0.03410703i'
So the safest way to get the real cube root (as opposed the complex
roots) is to use:
sign(tmp)*abs(tmp)^1/3
> sign(tmp)*abs(tmp)^(1/3)
[1] -0.03938341 0.03478442 0.03285672 0.08950802 -0.11696726
On Nov 16, 2009, at 5:24 AM, carol white wrote:
Hi,
I want to apply ^ operator to a vector but it is applied to some of
the elements correctly and to some others, it generates NaN. Why is
it not able to calculate -6.108576e-05^(1/3) even though it exists?
tmp
[1] -6.108576e-05 4.208762e-05 3.547092e-05 7.171101e-04
-1.600269e-03
tmp^(1/3)
[1] NaN 0.03478442 0.03285672 0.08950802 NaN
-6.108576e-05^(1/3)
[1] -0.03938341
--
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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