Re: [R] weird behavior with the 3rd root....
This comes up from time to time. The problem is that one needs complex numbers to address taking the third root: there are three cube roots for any nonzero number (real or complex). To wit: (-0.084121928394+0i)^(1/3) [1] 0.2190818+0.3794609i (-0.084121928394-0i)^(1/3) [1] 0.2190818+0.3794609i (-0.084121928394+1e-100i)^(1/3) [1] 0.2190818+0.3794609i (-0.084121928394-1e-100i)^(1/3) [1] 0.2190818-0.3794609i Note the first two are identical but the second two differ. Anyone care to start discussing signed zero again? [you probably want the *real* cube root, in which case it is best to take minus the unique real cube root of the absolute value: -(0.084121928394)^(1/3) [1] -0.4381637 (which is what you did, of course!)] HTH rksh Juan Manuel Barreneche wrote: Well, this is what i got... -0.084121928394^(1/3) [1] -0.438163696867656 (-0.084121928394)^(1/3) [1] NaN and i don't have a clue of why this happens or how to avoid it, any suggestions? thank you, Juan __ 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. -- Robin K. S. Hankin Senior Research Associate Cambridge Centre for Climate Change Mitigation Research (4CMR) Department of Land Economy University of Cambridge [EMAIL PROTECTED] 01223-764877 __ 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] weird behavior with the 3rd root....
Well, this is what i got... -0.084121928394^(1/3) [1] -0.438163696867656 (-0.084121928394)^(1/3) [1] NaN and i don't have a clue of why this happens or how to avoid it, any suggestions? thank you, Juan __ 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.
Re: [R] weird behavior with the 3rd root....
'^' has higher precedence than '-', i.e. your first line is equivalent to - ( 0.08xyz. ^(1/3) ) Gabor On Sun, Oct 26, 2008 at 9:05 PM, Juan Manuel Barreneche [EMAIL PROTECTED] wrote: Well, this is what i got... -0.084121928394^(1/3) [1] -0.438163696867656 (-0.084121928394)^(1/3) [1] NaN and i don't have a clue of why this happens or how to avoid it, any suggestions? thank you, Juan __ 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. -- Gabor Csardi [EMAIL PROTECTED] UNIL DGM __ 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.
Re: [R] weird behavior with the 3rd root....
On 26/10/2008 4:05 PM, Juan Manuel Barreneche wrote: Well, this is what i got... -0.084121928394^(1/3) [1] -0.438163696867656 (-0.084121928394)^(1/3) [1] NaN and i don't have a clue of why this happens or how to avoid it, any suggestions? R won't raise negative numbers to fractional powers, because it uses exp(log(x)*y) for x^y. (If y is a whole number it will work.) Your first case is -(x^y), your second is (-x)^y, so that's why you got the different answers. To avoid it, don't try to take fractional powers of negative numbers. Duncan Murdoch __ 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.
