[R] dropping factor levels in subset

[Prof Brian Ripley]

A more transparent solution is

old.factor[1:3, drop = TRUE]

That has worked for a long time, but apparently not been documented in R
until 1.7.1 (docs added a couple of hours before release). So you could do
(probably, since there are some bugs prior to 1.8.0)

crb[] <- lapply(crb, function(x) x[drop=TRUE])

to remove the unused levels on all factors in the data frame.

On Thu, 26 Jun 2003, Marc Schwartz wrote:

> >-----Original Message-----
> >From: r-help-bounces at stat.math.ethz.ch 
> >[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Nick Bond
> >Sent: Thursday, June 26, 2003 10:08 PM
> >To: r-help at stat.math.ethz.ch
> >Subject: [R] dropping factor levels in subset
> >
> >
> >Dear all,
> >I've taken a subset of data from a data frame using
> >
> >crb<-subset(all.raw, creek %in% c("CR") & year %in% 
> >c(2000,2001) & substrate
> >%in% ("b"))
> >
> >this works fine, except that all of the original factor levels are
> >maintained. This results in NA's for these empty levels when I 
> >try to do
> >summaries based on factors using by(). Is there a simple way 
> >to drop the
> >factor levels that are no longer represented. I've used na.omit on
> the
> >results from by, but then I have to deal with the attr setting, which
> >catches me too. Probably a silly question, but I've done a search and
> >couldn't find anything.  Can someone help me please.
> >Regards
> >Nick
> 
> See ?factor for additional information, but a quick example where
> using factor(old.factor) will return the factor with unused levels
> dropped.
> 
> # Create a factor
> > old.factor <- factor(c("One", "Two", "Three", "Four"))
> > old.factor
> [1] One   Two   Three Four 
> Levels: Four One Three Two
> 
> # Create a subset of three noting that all four
> # levels are retained
> > new.factor <- old.factor[1:3]
> > new.factor
> [1] One   Two   Three
> Levels: Four One Three Two
> 
> # Drop unused level
> > new.factor2 <- factor(new.factor)
> > new.factor2
> [1] One   Two   Three
> Levels: One Three Two
> 
> 
> HTH,
> 
> Marc Schwartz
> 
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> 

-- 
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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595