Tania Oh wrote: > Dear Uwe, > thank you very much for this. > After reading your solution below, I searched the help pages for > data.frame, which, factor but I didn't see the option for "drop" in them.
In fact, ?factor links to ?[.factor whcih explains it. Uwe > I googled and found "drop" associated with the function subset. is this > the help page you were alluding to? > > > Sorry if I've missed something. > thanks so much in advance again. > tania > > On 14 Apr 2008, at 12:39, Uwe Ligges wrote: > >> >> >> Tania Oh wrote: >>> Dear all, >>> I used "which" to obtain a subset of values from my data.frame. >>> however, I find that there is a "trace" of the values I have >>> removed. Any suggestions would be greatly appreciate. >>> Below is my data: >>> d <- data.frame( val = 1:10, >>> group = sample(LETTERS[1:5], 10, repl=TRUE) ) >>> >d >>> val group >>> 1 1 B >>> 2 2 E >>> 3 3 B >>> 4 4 C >>> 5 5 A >>> 6 6 B >>> 7 7 A >>> 8 8 E >>> 9 9 E >>> 10 10 A >>> ## selecting everything that is not group "A" >>> d<-d[which(d$group !="A"),] >>> > d >>> val group >>> 1 1 B >>> 2 2 E >>> 3 3 B >>> 4 4 C >>> 6 6 B >>> 8 8 E >>> 9 9 E >>> > levels(d$group) >>> [1] "A" "B" "C" "E" >>> ## why is group A still reflected here? >> >> >> Because you have removed elements from a factor objects that has >> particular levels. You remove elements (=observations), but the factor >> still knows that all levels are possible (stired in attributes of the >> object). >> >> If you want to remove all levels without corresponding observations, >> use explicit drop=TRUE as the help page suggests, e.g.: >> >> >> d <- d[d$group != "A", ] >> d$group <- d$group[ , drop = TRUE] >> >> Uwe Ligges >> >> >> >>> Many thanks in advance, >>> tania >>> D.phil student >>> Department of Physiology, Anatomy and Genetics >>> Oxford University >>> ______________________________________________ >>> 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. > ______________________________________________ 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.
