On Wed, 21 Mar 2007, Dave Ewart wrote: > On Wednesday, 21.03.2007 at 14:07 +0800, Berwin A Turlach wrote: > >>> All help/suggestions/appreciated! >> >> The calculations are done in floating point arithmetic, not integer >> arithmetic. From the help page on `choose' one might already guess >> so much, but reading "R-2.4.1/src/nmath/choose.c" definitely confirms >> this fact. >> >> Thus, your problem is covered by FAQ 7.31: >> >> http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f > > Thanks Berwin. > > OK, I understand the problem now. I was wondering if it something along > those lines, but initially dismissed that as a possibility since I > expected 'choose' to be an integer calculation.
It is not, because it would be far too easy to get integer overflow. > However, the underlying problem that gave rise to the difficulty was as > follows. A colleague wishes to create a matrix, where one of the > dimensions of the matrix is the result of the 'choose' function, i.e. > > mycols<-choose(11,6) > a_matrix<-matrix(0,nrow=11,ncol=mycols) > > Clearly, 'ncol' casts mycols as as integer. In this case, a_matrix has > only 461 columns, not 462. > > What's the best way to make this work as required? Use round(mycols). I think we ought to do this internally: we do when using computations via beta/gamma functions. -- 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 ______________________________________________ [email protected] 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.
