Hi Wilhelm, I agree it's confusing to have a levels() function that does something so different from the levels argument of the factor function. Personally I use levels() only as an extractor, never to change levels. For that I use factor(), with the levels and labels arguments as needed.
Best, Ista On Thu, Feb 25, 2010 at 8:40 PM, Schwab,Wilhelm K <bsch...@anest.ufl.edu> wrote: > Phil, > > That works[*], but I still think there is a big problem given how easy it is > to do the wrong thing, and that searches lead to dangerous instructions. > Hopefully this will serve to keep others out of trouble, but so might an > immutable return value from levels(). > > [*] I have not yet done anything with selecting parts of the data frame. > Using a separate factor, I quickly hit trouble with size mismatches, though I > could probably work around them by recreating the factor after any such > change. Proceeding with caution... > > Bill > > --- > Wilhelm K. Schwab, Ph.D. > > > > -----Original Message----- > From: Phil Spector [mailto:spec...@stat.berkeley.edu] > Sent: Thursday, February 25, 2010 7:06 PM > To: Schwab,Wilhelm K > Subject: Re: [R] Ordering categories on a boxplot - a serious trap?? > > Wilhelm - > I don't know if this is correct for your problem because you didn't > provide a reproducible example, but perhaps you could try > > v$doseGroup = factor(v$doseGroup,levels=c("L", "M", "H")) > > instead of setting the levels directly. > > - Phil Spector > Statistical Computing Facility > Department of Statistics > UC Berkeley > spec...@stat.berkeley.edu > > On Thu, 25 Feb 2010, Schwab,Wilhelm K wrote: > >> Hello all, >> >> I think I probably did something stupid, and R's part was to allow me to do >> it. My goal was to control the order of factor levels appearing >> horizontally on a boxplot. Enter search engines and perhaps some creative >> stupidity on my part, and I came up with the following: >> >> v=read.table("factor-order.txt",header=TRUE); >> levels(v$doseGroup) = c("L", "M", "H"); >> boxplot(v$dose~v$doseGroup); >> >> >> A good way to see the trap is to evaluate: >> >> v=read.table("factor-order.txt",header=TRUE); >> par(mfrow=c(2,1)); >> boxplot(v$dose~v$doseGroup); >> levels(v$doseGroup) = c("L", "M", "H"); >> boxplot(v$dose~v$doseGroup); >> par(mfrow=c(1,1)); >> >> The above creates two plots, one correct with the factors in an inconvient >> order, and one that is WRONG. In the latter, the labels appear in the >> desired order, but the data does not "move with them." I did not discover >> the problem until I repeated the same type of plot with something that had a >> known relationship with the levels, and the result was clearly not correct. >> >> I *think* the problem is to assign to the return value of levels(). >> How did I think to do that? I'm not really sure, but please look at >> >> https://stat.ethz.ch/pipermail/r-help/2008-August/171884.html >> >> >> Perhaps it does not say to do exactly what I did, but it sure was easy to >> follow to the mistake, it appeared to do what I wanted, and the consequences >> of the mistake are ugly. Perhaps levels() should return something that is >> immutable?? If I am looking at this correctly, levels() is an accident >> waiting to happen. >> >> What should I have done? It seems: >> >> read data and order factor levels >> v=read.table("factor-order.txt",header=TRUE); >> group = factor(v$doseGroup,levels = c("L", "M", "H") ); >> boxplot(v$dose~group); >> >> >> One disappointment is that the above factor() call apparently needs to be >> repeated for any subset of v - I'm still trying to get my mind around that >> one. >> >> Can anyone confirm this? It strikes me as a trap that should be addressed >> so that an error results rather than a garbage graph. >> >> Bill >> >> >> --- >> Wilhelm K. Schwab, Ph.D. >> >> ______________________________________________ >> 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. > -- Ista Zahn Graduate student University of Rochester Department of Clinical and Social Psychology http://yourpsyche.org ______________________________________________ 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.
