[R] cube root on array

[Marc Schwartz]

On Dec 12, 2009, at 9:31 AM, Rodrigo Tsai wrote:

> Dear R developers,
>
> is that right?
>
>> -27^(1/3)
> [1] -3


library(fortunes)

 > fortune("^")

Thomas Lumley: The precedence of ^ is higher than that of unary minus.
It may be surprising, [...]
Hervé Pagès: No, it's not surprising. At least to me... In the country
where I grew up, I've been teached that -x^2 means -(x^2) not (-x)^2.
    -- Thomas Lumley and Hervé Pagès (both explaining that operator
       precedence is working perfectly well)
       R-devel (January 2006)



Also see R FAQ 7.33:

http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-are-powers-of-negative-numbers-wrong_003f

Using the example in the FAQ:

 > as.list(quote(-27^(1/3)))
[[1]]
`-`

[[2]]
27^(1/3)


So what you see above is the consequence of operator precedence, thus:

 > (-27)^(1/3)
[1] NaN

which is what you are getting below for the first value in the vector.


>> c(-27,27)^(1/3)
> [1] NaN   3
>
> i'm using sign( c(-27,27) ) * abs( c(-27,27)) ^(1/3) ,
> thanks
>

That seems to be a reasonable approach and if memory serves, has been  
posted to the list previously.

HTH,

Marc Schwartz