# [R] cube root

Tsjerk Wassenaar tsjerkw at gmail.com
Sun Apr 17 09:46:34 CEST 2011

```Hey :)

In addition, although (-2)^3 = -8, (-8)^(1/3) != -2, mathematically. A
fractional power of a negative number is complex. And this can be
obtained properly in R:

> (-8+0i)^(1/3)
[1] 1+1.732051i

Cheers,

Tsjerk

>> This is some interesting:
>>
>>> -8^(1/3)
>>
>> [1] -2
>>
>>> x=(-8:8)
>>
>>> y=x^(1/3)
>>
>>> y
>>
>>  [1]      NaN      NaN      NaN      NaN      NaN      NaN      NaN
>> NaN
>> 0.000000 1.000000
>>
>> [11] 1.259921 1.442250 1.587401 1.709976 1.817121 1.912931 2.000000
>> So, can anybody explain this?! (Why is x[1]^(1/3)=y[1]=NaN, but
>> -8^(1/3)=-2?)
>>
>
> Operator precedence.
>
> See R FAQ 7.33 Why are powers of negative numbers wrong?
>
>
> Berend
>
>
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--
Tsjerk A. Wassenaar, Ph.D.

post-doctoral researcher
Molecular Dynamics Group
* Groningen Institute for Biomolecular Research and Biotechnology
* Zernike Institute for Advanced Materials
University of Groningen
The Netherlands

```