On Fri, 4 Nov 2005, Thomas Lumley wrote: > On Fri, 4 Nov 2005, Uwe Ligges wrote: > >> Dr Carbon wrote: >> >>> At the risk of being beaten about the face and body, can somebody explain >>> why the middle example: log2(2^3); floor(log2(2^3)) is different than >>> examples 1 and 3? >> >> >> Because >> >>> log2(2^3) - 3 >> [1] -4.440892e-16 >> > > > This is a less satisfactory answer than usual, because both 2^3 and > log(2^3) are integers and thus exactly representable in the R numeric > type. You could reasonably expect log(8) to be exactly 2, just as sqrt(4) > is exactly 2. > > The problem is that we compute all logarithms via the natural log, and > this introduces the problem of limited precision.
Well, we _did_ (and as others have noted, most machines manage to get log(8) to be exactly 2). R-devel now uses log2 for this, so > log2(2^29) - 29 [1] 0 -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ 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
