[R] Problems with levels of factors

[Uli Flenker; Raum 704]

On Thu, 10 Aug 2000, Prof Brian D Ripley wrote:

> On Thu, 10 Aug 2000, Uli Flenker; Raum 704 wrote:
> 
> > Dear R-helpers,
> > 
> > I frequently run into problems when I modify elements of factors (R 1.0.0,
> > Linux 2.0.25).  For example, after splitting a data frame accordinng to
> 
> That's already two versions old, three come next Tuesday.
> 
> > whatever criterion, it might well happen that not all levels of a factor
> > are present in all new data frames. However, R doesn't seem to care about. 
> > When performing some kind of analysis seperately on the new frames, each
> > time the unreduced number of levels is present.
> > 
> > That can be very problematic, as a lot of methods tend to crash because no
> > data are found for the "lost" level. 
> > 
> > Neither explicit setting of "data=data.frame.xyz" nor any straightforward
> > use of "detach()" or "attach()" nor
> > "levels(problem.factor)<-c("reduced","levels", ...)" did help.
> > 
> > In the latter case I obtain 
> > 
> > 	Error in levels<-.factor(*tmp*, value = c("A", "B")) : 
> > 		number of levels differs
> > 
> > no matter whether the relevant data.frame is attached to the search list
> > or not, no matter whether the number of levels is appropriate in that data
> > frame and no matter whether an "extra" factor of the name exists or not.
> > 
> > I also tried to modify the attributes of the problematic factors
> > explicitly without succes.
> > 
> > The persistance of levels of factors once established seems to be so
> > robust that I suspect its a feature. As far as I remember no one
> > compalained about this properties so far. Am I missing something obvious?
> > Are there any suggestions for workarounds?
> 
> It's definitely a feature. What methods crash? (I hope you mean gave an
> error, not crashed R.)

Sorry for that! I think I have to choose words more carefully. When
writing "crash" I meant "stop of evaluation without result".

To mention two things: kruskal.test from package ctest gives "NA" for p
when one level has no data and qda from the MASS library stops because of
too small groups.

Sorry for the second time: These are of course no method functions! I had
in mind "statistical methods". This MUST cause confusion admittedly!

> The levels of a factor are the set of possible values, not the set of
> achieved values.  Methods should cope with that (possibly with warnings).
> 
> 
> The best way to drop levels, BTW, is
> 
> problem.factor <- problem.factor[,drop=TRUE]

... and sorry for the third time. I could have found out by myself reading
VR99 more carefully.

Thank you very much!

> -- 
> Brian D. Ripley,                  ripley at stats.ox.ac.uk
> Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
> University of Oxford,             Tel:  +44 1865 272861 (self)
> 1 South Parks Road,                     +44 1865 272860 (secr)
> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
> 
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        Uli Flenker
        Institut fuer Biochemie
        Deutsche Sporthochschule Koeln
        Carl-Diem-Weg 6

        50933 Koeln

        Tel.: D/0221/4982-493
                                  -494

 


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