On 8/30/2007 11:08 AM, willem vervoort wrote: > Dear all, > I am struggling to understand this. > > What happens when you raise a negative value to a power and the result > is a very large number? > > B > [1] 47.73092 > >> -51^B > [1] -3.190824e+81
You should be using parentheses. You evaluated -(51^B), not (-51)^B. The latter gives NaN. > > # seems fine > # now this: >> x <- seq(-51,-49,length=100) > >> x^B > [1] NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN > <snip> >> is.numeric(x^B) > [1] TRUE >> is.real(x^B) > [1] TRUE >> is.infinite(x^B) > [1] FALSE FALSE FALSE FALSE FALSE > > I am lost, I checked the R mailing help, but could not find anything > directly. I loaded package Brobdingnag and tried: > as.brob(x^B) > [1] NAexp(NaN) NAexp(NaN) NAexp(NaN) NAexp(NaN) NAexp(NaN) >> as.brob(x)^B > [1] NAexp(187.67) NAexp(187.65) NAexp(187.63) NAexp(187.61) > > I guess I must be misunderstanding something fundamental. Two things: operator precedence (the ^ has higher precedence than the unary minus), and the mathematical definition of raising something to a fractional power. The approach R takes to the latter is to define x^B to be exp(B * ln(x)), and ln(x) is undefined for negative x. Duncan Murdoch ______________________________________________ R-help@stat.math.ethz.ch 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.
