To take it one step further: > x <- as.complex(-4) > cx <- x^(1/3) > > r <- complex(modulus = Mod(cx), argument = Arg(cx)*c(1,3,5)) > r [1] 0.793701+1.37473i -1.587401+0.00000i 0.793701-1.37473i > r^3 [1] -4+0i -4+0i -4+0i >
So when you ask for "the" cube root of -4, R has a choice of three possible answers it can give you. It is no surprise that this does not work when working in the real domain, except "by fluke" with something like > -4^(1/3) [1] -1.587401 > where the precedence of the operators is not what you might expect. Now that could be considered a bug, since apparently > -4^(1/2) [1] -2 which comes as rather a surprise! Bill. -----Original Message----- From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On Behalf Of Kjetil Halvorsen Sent: Wednesday, 27 October 2010 9:17 AM To: Gregory Ryslik Cc: r-help Help Subject: Re: [R] cube root of a negative number Look at this: > x <- as.complex(-4) > x [1] -4+0i > x^(1/3) [1] 0.793701+1.37473i > (-4)^(1/3) [1] NaN It seems that R gives you the principal root, which is complex, and not the real root. Kjetil On Tue, Oct 26, 2010 at 8:05 PM, Gregory Ryslik <rsa...@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 >> > > As we can see: > >> (-1.587401)^3 > [1] -4 > > Thanks! > > Greg > ______________________________________________ > R-help@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. > ______________________________________________ R-help@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. ______________________________________________ R-help@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.
