On Fri, 21 Mar 2008, John Lande wrote: > dear all, > > I report a problem very simple, that I does non know how to handle. > > look at the following code: > >> a = rep(16.256, 5) >> sum(a[1:5]^2) - (sum(a[1:5])^2/5) > [1] 2.273737e-13 > > as you can see i retrieve a non 0 value, when i am expected to. what can I > do?
Use mean() when you want a mean: > sum(a[1:5]^2) - 5*mean(a[1:5])^2 [1] 0 Your number is not exactly representable so some rounding error would be acceptable, but there are more accurate ways to do this, e.g. sum((a-mean(a))^2) or make use of var(), which is written by someone knowledgable about these things. > >> sessionInfo() > R version 2.6.2 (2008-02-08) > i386-pc-mingw32 > > locale: > LC_COLLATE=Italian_Italy.1252;LC_CTYPE=Italian_Italy.1252;LC_MONETARY=Italian_Italy.1252;LC_NUMERIC=C;LC_TIME=Italian_Italy.1252 > > attached base packages: > [1] stats graphics grDevices utils datasets methods base > > loaded via a namespace (and not attached): > [1] rcompgen_0.1-17 > > [[alternative HTML version deleted]] > > ______________________________________________ > 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. > -- 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@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.
