Another option is pruneLevels() in library nlme. x <- factor( c( 0,1,2,1,2 ) )
x
[1] 0 1 2 1 2 Levels: 0 1 2
pruneLevels( x[-1] )
[1] 1 2 1 2 Levels: 1 2 -----Original Message----- From: Marc Schwartz [mailto:mschwartz at medanalytics.com] Sent: Saturday, June 28, 2003 2:31 AM To: 'Prof Brian Ripley' Cc: r-help at stat.math.ethz.ch; 'Nick Bond' Subject: RE: [R] dropping factor levels in subset
-----Original Message----- From: r-help-bounces at stat.math.ethz.ch [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Prof Brian Ripley Sent: Friday, June 27, 2003 12:11 PM To: Marc Schwartz Cc: r-help at stat.math.ethz.ch; 'Nick Bond' Subject: RE: [R] dropping factor levels in subset Re: [, drop=TRUE} for factors It's been in S-PLUS (but not S I believe) for a long time, probably since before 1994: it is in S+3.4, 1996 vintage. It appears to have been added to R around August 1998. Yes, Frank Harrell argues for the default to be true and I believe
his
Hmisc package overrides this. Although less unsafe than it used to be (a lot more consistency checking of factor levels has been added) it is still I believe undesirable. The argument `drop.unused.levels' to model.frame will usually do all that is required. (That's another thing that is very-little known.)
Thanks for the clarifications. SNIP
I now note that for factor objects, this is included in MASS 4 (pg 19), whereas it is a footnote in MASS 3 (pg 20) and I could not
find
it in MASS 1 (I don't have a copy of MASS 2 to review). It is also
a
footnote in S Programming (pg 14). Not sure if any
significance should
be attached to being a footnote versus being in the body of
the text. None.
OK. I initially had the impression that it may have been either chronologically associated with the addition of this method for factors or the greater emphasis on R in MASS 4, since it moved from a footnote to the body. I was wrong. Also, I realized a typo in the MASS 4 page number I had above, it should be 16. Regards, Marc ______________________________________________ R-help at stat.math.ethz.ch mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help