[Rd] 'droplevels' inappropriate change

[Martin Maechler]

>>>>> Suharto Anggono Suharto Anggono via R-devel <r-devel at r-project.org>
>>>>>     on Sat, 27 Aug 2016 03:17:32 +0000 writes:

    > In R devel r71157, 'droplevels' documentation, in "Arguments" section, says this about argument 'exclude'.
    > passed to factor(); factor levels which should be excluded from the result even if present.  Note that this was implicitly NA in R <= 3.3.1 which did drop NA levels even when present in x, contrary to the documentation.  The current default is compatible with x[ , drop=FALSE].

    > The part
    > x[ , drop=FALSE]
    > should be
    > x[ , drop=TRUE]

Yes, definitely, thank you!
a "typo" by me. .. fixed now.

    > Saying that 'exclude' is factor levels is not quite true for NA element. NA may be not an original level, but NA in 'exclude' affects the result.

    > For a factor 'x', factor(x, exclude = exclude) doesn't really work for excluding in general. See, for example, https://stat.ethz.ch/pipermail/r-help/2005-September/079336.html .
    > factor(factor(c("a","b","c")), exclude="c")

    > However, this excludes "2":
    > factor(factor(2:3), exclude=2)

    > Rather unexpectedly, this excludes NA:
    > factor(factor(c("a",NA), exclude=NULL), exclude="c")

    > For a factor 'x', factor(x, exclude = exclude) can only exclude integer-like or NA levels. An explanation is in https://stat.ethz.ch/pipermail/r-help/2011-April/276274.html .

Well, Peter Dalgaard (in that R-devel e-mail, a bit more than 5
years ago) is confirming the problem there,  and suggesting (as
you, right?) that actually   `factor()` is not behaving
correctly here.

And your persistence is finally getting close to convince me
that it is not just droplevels(), but  factor() itself which
needs care here.

Interestingly, the following patch *does* pass 'make check-all'
(after small change in tests/reg-tests-1b.R which is ok),
and leads to behavior which is much closer to the documentation,
notably for your two examples above would give what one would
expect.

(( If the R-Hub would support experiments with branches of R-devel 
   from R-core members,  I could just create such a branch and R Hub
   would run 'R CMD check <pkg>'  for thousands of CRAN packages
   and provide a web page with the *differences* in the package
   check results ... so we could see ... ))

I do agree that we should strongly consider such a change.

Martin


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