Thanks for all the points. Riding my bike back home I realised that the fractional power was a problem. I also forgot about the operator precedence: > b <- 48.3 > -53.1^b [1] -2.099256e+83 > (-53.1)^b [1] NaN
Pretty clear now. Back to my maths than... > version # sorry forgot this last time _ platform i386-pc-mingw32 version.string R version 2.5.1 (2007-06-27) Willem On 8/30/07, Duncan Murdoch <[EMAIL PROTECTED]> wrote: > On 8/30/2007 12:11 PM, Martin Becker wrote: > > 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 > >> > >> # seems fine > >> > > > > Well, this seems not to be what you intended to do, you didn't raise a > > negative value to a power, but you got the negative of a positive number > > raised to that power (operator precedence, -51^B is the same as -(51^B) > > and not the same as (-51)^B...). > > > > If you really want to raise a negative value to a fractional power, you > > may want to tell R to use complex numbers: > > > > B <- 47.73092 > > x <- complex(real=seq(-51,-49,length=10)) > > > > x^B > > > > [1] 2.117003e+81-2.387323e+81i 1.718701e+81-1.938163e+81i > > [3] 1.394063e+81-1.572071e+81i 1.129702e+81-1.273954e+81i > > [5] 9.146212e+80-1.031409e+81i 7.397943e+80-8.342587e+80i > > [7] 5.978186e+80-6.741541e+80i 4.826284e+80-5.442553e+80i > > [9] 3.892581e+80-4.389625e+80i 3.136461e+80-3.536955e+80i > > But watch out if you do this, because of the arbitrary choice of a root. > You get oddities like this: > > > x <- complex(real = -1) > > x > [1] -1+0i > > 1/x > [1] -1+0i > > x^(1/3) > [1] 0.5+0.8660254i > > (1/x)^(1/3) > [1] 0.5-0.8660254i > > i.e. even though x and 1/x are equal, the 1/3 powers of them are not. > > Duncan Murdoch > > P.S. I'm tempted to say, "But don't worry about it, the difference is > only imaginary", but I'll refrain. > ______________________________________________ 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.
