[R] dropping factor levels in subset

[Marc Schwartz]

>-----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 1:35 AM
>To: Marc Schwartz
>Cc: r-help at stat.math.ethz.ch; 'Nick Bond'
>Subject: RE: [R] dropping factor levels in subset
>
>
>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.

SNIP

>

Prof. Ripley,

Thank you for pointing this out.  I checked both ?factor and ?"[" and
note that this behavior is now documented.

A question:  How long (roughly) has this been present in R for
factors? 

I ask because I had a vague recollection this morning, after seeing
your reply, of an exchange between Frank Harrell and others regarding
just such a 'feature' in R some time ago.  It turns out to have been
back in January of 2002 based upon my search of the r-help archive
this morning
(http://maths.newcastle.edu.au/~rking/R/help/01c/3809.html). In this
exchange, Frank suggested using just such an approach (ie. "x <-
x[,drop=T]") for factor objects, whereas Peter in that same thread
noted the use of 'x <- factor(x)' in his reply, which is what I tend
to use. If my re-read of the thread is correct, I believe that Frank
was also arguing in favor of a global options() setting regarding this
behavior.

A recent (May 2003) exchange between Duncan Murdoch and John Chambers
(http://maths.newcastle.edu.au/~rking/R/devel/03a/1003.html) would
suggest that such a feature was present for vectors, but perhaps
incompletely documented as you perhaps suggest, given Duncan's
question if my read of the exchange is correct.

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. 

Lastly, I note that references to "[" in the "White Book" include a
'drop' argument on pg 465 and in the "Green Book" on pg 340, which
would suggest that it has been around for some time, at least as a
high level method, though with no specific reference that I could note
for factor objects.

Regards and thanks,

Marc