It also works for raising a number to a negative integer: > (-3)^(-3) [1] -0.03703704 >
On Sun, Jul 19, 2009 at 6:23 PM, Rolf Turner<r.tur...@auckland.ac.nz> wrote: > > On 20/07/2009, at 9:13 AM, jim holtman wrote: > >> If the power that a number is being raised to is integer, then is does >> evaluate honoring the unary minus. >> >>> (-2) ^ 5 #integer power >> >> [1] -32 >>> >>> (-2) ^ 5.1 >> >> [1] NaN > > <snip> > > I was vaguely aware of this ... but it now triggers in my mind the > question of how the ^ function decides when the exponent is an integer. > > A bit of experimentation seems to indicate that, e.g., (-2)^x ``works'' > if (and only if?) round(x)==x returns TRUE. > > Note that (-2)^x may NOT ``work'' in some cases were all.equal(x,round(x)) > returns TRUE. > > Young players should also be aware of the following trap. It > can happen that n + epsilon ``is an integer'' according to my > rule, but m + epsilon is NOT an integer according to this rule. > Where m and n are both integers. > > E.g.: > >> eps <- 0.4e-15 >> x <- 5+eps >> x==round(x) > [1] TRUE >> y <- 3+eps >> y==round(y) > [1] FALSE > > This is of course due to the exigencies of how n and m are represented > in floating point arithmetic. Not too deep once you're aware of the > problem, but it can still be a ``gotcha'' if one is not alert. > > (Be alert. The world needs more lerts!) > > cheers, > > Rolf Turner > > P. S. Perhaps young players should be reminded at this point that > is.integer() is > no help here. This function tells you about the ***storage mode*** of its > argument. > Only. > > R. T. > > > ###################################################################### > Attention:This e-mail message is privileged and confidential. If you are not > theintended recipient please delete the message and notify the sender.Any > views or opinions presented are solely those of the author. > > This e-mail has been scanned and cleared by > MailMarshalwww.marshalsoftware.com > ###################################################################### > -- Jim Holtman Cincinnati, OH +1 513 646 9390 What is the problem that you are trying to solve? ______________________________________________ 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.
