[R] Creating subsets with factors

[Peter Dalgaard BSA]

Sven Garbade <garbade at psy.uni-muenchen.de> writes:

> Hi all,
> 
> I don't understand the following output. I've created a data subset from
> a data frame by
> 
> > p1.sub <- subset(p1.dat, vp!="p1")
> 
> this is ok. But 
> 
> > attach(p1.sub)
> > vp
>  [1] p1ab p1ab p1ab p1ab p1ab p1br p1br p1br p1br p1br p1kf p1kf p1kf
> p1kf p1kf
> [16] p1mg p1mg p1mg p1mg p1mg p1mw p1mw p1mw p1mw p1mw
> Levels:  p1 p1ab p1br p1kf p1mg p1mw 
> 
> shows me that the factor vp has 6 levels instead of 5? 5 should be the
> correct number of levels, because p1 isn't in the data subset.

Nope. Factors can have levels that are not present in the data set.
There are good reasons for this. For instance you cannot c(f1,f2) if
f1 and f2 are factors with different level sets. 

If you want to reduce the levels to those present in the factor, use  

p1.sub$vp <- factor(p1.sub$vp)

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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