From: David Winsemius > > On Mar 1, 2010, at 12:07 PM, Nicholas Lewin-Koh wrote: > > > Hi, > > consider the following > >> a<-gl(3,3,9) > >> a > > [1] 1 1 1 2 2 2 3 3 3 > > Levels: 1 2 3 > >> levels(a)<-3:1 > > That may look like the same re-ordered factor but you instead merely > re-labeled each level where the internal numbers that represent the > factor values stayed the same.. > > >> a > > [1] 3 3 3 2 2 2 1 1 1
Indeed this is one of the (few, I believe) traps of R, because: R> a [1] 3 3 3 2 2 2 1 1 1 Levels: 3 2 1 R> as.numeric(a) [1] 1 1 1 2 2 2 3 3 3 R> as.numeric(as.character(a)) [1] 3 3 3 2 2 2 1 1 1 Andy > > Levels: 3 2 1 > >> a<-gl(3,3,9) > >> factor(a,levels=3:1) > > That is the right way IMO to safely change the ordering of > the levels > without changing the "semantics" or the "meaning" of the > factor level > assignments. > > Try: > > levels(a) <- letters[4:6] > a > > [1] d d d e e e f f f > Levels: d e f > > a <- factor(a, levels=letters[1:3]) > > a > [1] <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> > Levels: a b c > > Using the second form sets any non-existent (in the new level > vector) > factor values to NA's, in this case all of them. It is better in my > mind to get assignments to NA than it would be to get assignments to > incorrect levels. > > > b <-factor(c(0,0,0,0, 1, 1)) > > b > [1] 0 0 0 0 1 1 > Levels: 0 1 > > levels(b) <-c(1,0) > > b > [1] 1 1 1 1 0 0 # No longer the same "meaning" > Levels: 1 0 > > b <-factor(c(0,0,0,0, 1, 1)) > > b<- factor(b, levels=c(1,0)) > > b > [1] 0 0 0 0 1 1 > Levels: 1 0 # Only the ordering has changed but the meaning is > the same > > > This is especially so when working with factors as components of > data.frames. > > > -- > David. > > > > > [1] 1 1 1 2 2 2 3 3 3 > > Levels: 3 2 1 > > It is probably something obvious I missed, but reading the > > documentation > > of factor, and levels I would have thought > > that both should produce the same output as > > factor(a,levels=3:1) > > [1] 1 1 1 2 2 2 3 3 3 > > Levels: 3 2 1 > > The closest I could find in a quick search was this > > http://tolstoy.newcastle.edu.au/R/e5/help/08/09/2503.html > > > > Thanks > > Nicholas > > > > sessionInfo() > > R version 2.10.1 Patched (2009-12-20 r50794) > > x86_64-unknown-linux-gnu > > > > locale: > > [1] LC_CTYPE=en_US.UTF-8 LC_NUMERIC=C > > [3] LC_TIME=en_US.UTF-8 LC_COLLATE=en_US.UTF-8 > > [5] LC_MONETARY=C LC_MESSAGES=en_US.UTF-8 > > [7] LC_PAPER=en_US.UTF-8 LC_NAME=C > > [9] LC_ADDRESS=C LC_TELEPHONE=C > > [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C > > attached base packages: > > [1] splines tcltk stats graphics grDevices utils > > datasets > > [8] methods base > > > > other attached packages: > > [1] mvtnorm_0.9-9 latticeExtra_0.6-9 RColorBrewer_1.0-2 > > lattice_0.18-3 > > [5] nlme_3.1-96 XML_2.6-0 gsubfn_0.5-0 > > proto_0.3-8 > > > > loaded via a namespace (and not attached): > > [1] grid_2.10.1 tools_2.10.1 > > > > ______________________________________________ > > 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. > > David Winsemius, MD > Heritage Laboratories > West Hartford, CT > > ______________________________________________ > 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. > Notice: This e-mail message, together with any attachme...{{dropped:10}} ______________________________________________ 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.
