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nth root
3 messages · Martin Biuw, Stavros Macrakis, David Winsemius
Matlab's Nthroot calculates the real nth root. For positive a, you
can use a^(1/b); for negative a, b must be odd for the result to be
real, and you can use -abs(a)^(1/b). So you could write:
nthroot <- function(a,b) ifelse( b %% 2 == 1 | a >= 0,
sign(a)*abs(a)^(1/b), NaN)
This is a so-called Vectorized function, i.e. f(A,B) == c(
f(A[1],B[1]), f(A[2],B[2]), ...) for vectors A, B of equal length.
This does *not* handle the case where b is the inverse of an integer
(e.g. nthroot(-1,1/3) ==? (-1)^3), since in general you cannot count
on 1/(1/int) being an integer.
If on the other hand you want the complex principal value of the nth
root, you can use (a+0i)^(1/b).
-s
On Thu, Mar 19, 2009 at 11:21 AM, Martin Biuw <martin.biuw at npolar.no> wrote:
Hi, Is there a function in R to calculate the nth root, similar to the MATLAB function NTHROOT()? Thanks, Martin Biuw ? ? ? ?[[alternative HTML version deleted]]
______________________________________________ R-help at r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
x^(1/n) # ? exp(log(x)/n) # ? Either of those could be rolled into a function with the desired name:
On Mar 19, 2009, at 11:21 AM, Martin Biuw wrote:
Hi, Is there a function in R to calculate the nth root, similar to the MATLAB function NTHROOT()? Thanks, Martin Biuw
David Winsemius, MD Heritage Laboratories West Hartford, CT