Herve, the answer is simple - it's as.character() - it has nothing to do with factor or table.
> as.character(x) [1] "3.66666666666667" "3.66666666666667" "3.66666666666666" "3.66666666666667" That's what you are passing to factor, so you get the corresponding results. Cheers, Simon On Feb 23, 2011, at 2:55 PM, Hervé Pagès wrote: > Hi, > > When 'x' is a vector of doubles, it's not clear how 'factor(x)' > compares its values in order to determine the levels. For example, > here all the values in 'x' are "conceptually" the same: > > x <- c(11/3, > 2/3 + 4/3 + 5/3, > 50 + 11/3 - 50, > 7.00001 - 1000003/300000) > > However, due to machine rounding errors, they are not strictly equal: > > > duplicated(x) > [1] FALSE FALSE FALSE FALSE > > unique(x) > [1] 3.666667 3.666667 3.666667 3.666667 > > but they are nearly equal: > > > all.equal(x, rep(11/3, 4)) > [1] TRUE > > Now factor(), and therefore table() (which seems to be using factor() > internally), have a different opinion: > > > factor(x) > [1] 3.66666666666667 3.66666666666667 3.66666666666666 3.66666666666667 > Levels: 3.66666666666666 3.66666666666667 > > > table(x) > x > 3.66666666666666 3.66666666666667 > 1 3 > > So factor() doesn't seem to be using "strict equality" or "near > equality" to determine the levels. What does it use? Sorry if I > missed it but I couldn't find any information about this in its > man page. > > Wouldn't it be better if factor() was consistent with either > duplicated() or all.equal() instead of introducing its own way > of comparing doubles that lies somewhere in between? > > Cheers, > H. > > > sessionInfo() > R version 2.12.0 (2010-10-15) > Platform: x86_64-unknown-linux-gnu (64-bit) > > locale: > [1] LC_CTYPE=en_US.utf8 LC_NUMERIC=C > [3] LC_TIME=en_US.utf8 LC_COLLATE=en_US.utf8 > [5] LC_MONETARY=C LC_MESSAGES=en_US.utf8 > [7] LC_PAPER=en_US.utf8 LC_NAME=C > [9] LC_ADDRESS=C LC_TELEPHONE=C > [11] LC_MEASUREMENT=en_US.utf8 LC_IDENTIFICATION=C > > attached base packages: > [1] stats graphics grDevices utils datasets methods base > > loaded via a namespace (and not attached): > [1] tools_2.12.0 > > -- > Hervé Pagès > > Program in Computational Biology > Division of Public Health Sciences > Fred Hutchinson Cancer Research Center > 1100 Fairview Ave. N, M2-B876 > P.O. Box 19024 > Seattle, WA 98109-1024 > > E-mail: hpa...@fhcrc.org > Phone: (206) 667-5791 > Fax: (206) 667-1319 > > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel > > ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
