[Rd] [bug] droplevels() also drop object attributes (comment…)

[Martin Maechler]

>>>>> Serge Bibauw <sbibauw at gmail.com>
>>>>>     on Mon, 15 May 2017 11:59:32 -0400 writes:

    > Hi,

    > Just reporting a small bug… not really a big deal, but I don’t think that is intended: droplevels() also drops all object’s attributes.

Yes.  The help page for droplevels (or the simple definition of
'droplevels.factor') clearly indicate that the method for
factors is really just a call to   factor(x, exclude = *)

and that _is_ quite an important base function whose semantic
should not be changed lightly. Still, let's continue :

Looking a bit, I see that the current behavior of factor() {and
hence droplevels} has been unchanged in this respect  for the
whole history of R, well, at least for more than 17 years (R 1.0.1, April 2000).

I'd agree there _is_ a bug, at least in the documentation which
does *not* mention that currently, all attributes are dropped but "names",
"levels" (and "class").

OTOH, factor() would only need a small change to make it
preserve all attributes (but "class" and "levels" which are set explicitly).

I'm sure this will break some checks in some packages.
Is it worth it?

e.g., our own R  QC checks currently check (the printing of) the
following (in tests/reg-tests-2.R ):

    > ## some tests of factor matrices
    > A <- factor(7:12)
    > dim(A) <- c(2, 3)
    > A
	 [,1] [,2] [,3]
    [1,] 7    9    11  
    [2,] 8    10   12  
    Levels: 7 8 9 10 11 12
    > str(A)
     factor [1:2, 1:3] 7 8 9 10 ...
     - attr(*, "levels")= chr [1:6] "7" "8" "9" "10" ...
    > A[, 1:2]
	 [,1] [,2]
    [1,] 7    9   
    [2,] 8    10  
    Levels: 7 8 9 10 11 12
    > A[, 1:2, drop=TRUE]
    [1] 7  8  9  10
    Levels: 7 8 9 10

with the proposed change to factor(),
the last call would change its result:

    > A[, 1:2, drop=TRUE]
	 [,1] [,2]
    [1,] 7    9   
    [2,] 8    10  
    Levels: 7 8 9 10

because 'drop=TRUE' calls factor(..) and that would also
preserve the "dim" attribute.
I would think that the changed behavior _is_ better, and is also
according to documentation, because the help page for
 [.factor
explains that 'drop = TRUE' drops levels, but _not_ that it
transforms a factor matrix into a factor (vector).


Martin


    > Example:

    >> > test <- c("hello", "something", "hi")
    >> > test <- factor(test)
    >> > comment(test) <- "this is a test"
    >> > attr(test, "description") <- "this is another test"
    >> > attributes(test)
    >> $levels
    >> [1] "hello"     "hi"        "something"
    >> 
    >> $class
    >> [1] "factor"
    >> 
    >> $comment
    >> [1] "this is a test"
    >> 
    >> $description
    >> [1] "this is another test"
    >> 
    >> > test <- droplevels(test)
    >> > attributes(test)
    >> $levels
    >> [1] "hello"     "hi"        "something"
    >> 
    >> $class
    >> [1] "factor"


    > Serge