On 17/12/2010 11:40 AM, Thiem Alrik wrote:
Dear mailing list,Why does the following code produce numerical results for x.pos.l, but NaNs for x.neg.l? x.pos<- function(tau.e, tau.c){ tau.e + ((-tau.e^3 + 3*tau.e^2*tau.c - 3*tau.e*tau.c^2 + tau.c^3)^(1/3))/(2*2^(1/3)) } (x.pos.l<- x.pos(1, 2)) x.neg<- function(tau.c, tau.i){ tau.i + ((tau.c^3 - 3*tau.c^2*tau.i + 3*tau.c*tau.i^2 - tau.i^3)^(1/3))/(2*2^(1/3)) } (x.neg.l<- x.neg(2, 3))
The expression you're taking a cube root of tau.c^3 - 3*tau.c^2*tau.i + 3*tau.c*tau.i^2 - tau.i^3 is negative. You can't raise negative numbers to fractional powers in R. (This has been discussed lots of times on the list; use Google if you don't understand why it has to be that way.) What you should do is write a cuberoot function, something like cuberoot<- function(x) ifelse(x< 0, -(-x)^(1/3), x^(1/3)) and use that instead. 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.
