Dear Thomas and Hadley, I'd propose the following: If the sets of levels of all arguments are the same, then c.factor() would return a factor with the common set of levels; if the sets of levels differ, then, as Hadley suggests, the level-set of the result would be the union of sets of levels of the arguments, but a warning would be issued.
Best, John > -----Original Message----- > From: r-devel-boun...@r-project.org [mailto:r-devel-boun...@r-project.org] On > Behalf Of Thomas Lumley > Sent: February-04-10 12:07 PM > To: Hadley Wickham > Cc: r-devel@r-project.org > Subject: Re: [Rd] Why is there no c.factor? > > On Thu, 4 Feb 2010, Hadley Wickham wrote: > > > Hi all, > > > > Is there are reason that there is no c.factor method? Analogous to > > c.Date, I'd expect something like the following to be useful: > > > > c.factor <- function(...) { > > factors <- list(...) > > levels <- unique(unlist(lapply(factors, levels))) > > char <- unlist(lapply(factors, as.character)) > > > > factor(char, levels = levels) > > } > > > > c(factor("a"), factor("b"), factor(c("c", "b","a")), factor("d")) > > # [1] a b c b a d > > # Levels: a b c d > > > > It's well established that different people have different views on what > factors should do, but this doesn't match mine. I think of factors as > enumerated data types where the factor levels already specify all the valid > values for the factor, so I wouldn't want to be able to combine two factors > with different sets of levels. > > For example: > A <- factor("orange",levels=c("orange","yellow","red","purple")) > B <- factor("orange", levels=c("orange","apple","mango", "banananana")) > > On the other hand, I think the current behaviour, which reduces them to > numbers, is just wrong. > > > -thomas > > Thomas Lumley Assoc. Professor, Biostatistics > tlum...@u.washington.edu University of Washington, Seattle > > ______________________________________________ > 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
