[R] levels of factor when subsetting the factor

Peter Dalgaard p.dalgaard at biostat.ku.dk
Tue Sep 12 17:45:07 CEST 2006

"Afshartous, David" <afshart at exchange.sba.miami.edu> writes:

> All,
> When I take a subset of a factor the reduced factor still maintains all
> the original levels of the factor when say forming the key in a plot.
> The data is correct, but the variable still "remembers" the original
> levels.  See below for reproducible code.  Does anyone know how to fix
> this?
> cheers,
> dave
> fact = as.factor(c(rep("A", 3),rep("B", 3), rep("C", 3)))
> new.fact = fact[1:6]
> > new.fact
> [1] A A A B B B
> Levels: A B C    ## should only show A B

Just use

> factor(new.fact)
[1] A A A B B B
Levels: A B


> fact[1:6, drop=T]
[1] A A A B B B
Levels: A B

And, no, it is not a bug. The fact that a subsample happens to consist
only of males does not turn gender into a one-level factor... (Apart
from the philosophy, it makes a real difference in tabulation.) 

   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907

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