[Rd] Dropping unused levels of a factor that has "NA" as a level

[Peter Dalgaard]

"J. Hosking" <jh910 at juno.com> writes:

> Is this a bug?
> 
>    > f1 <- factor(c("a", NA), levels = c("a", "NA") )
>    > f2 <- f1[, drop = TRUE]
>    > f2
>    [1] a    <NA>
>    Levels: a <NA>
> 
> I would have expected f2 to have only one level, "a".  It seems
> to me that the code in [.factor does not follow the advice in
> help("factor") on how to set factor codes to be missing when
> "NA" is a level of the factor.


Something odd is going on, that's for sure...

The problem is also there with factor(f1). And the logic in
as.character.factor seems to be at the root of it:

> as.character.factor
function (x, ...)
{
    cx <- levels(x)[x]
    if ("NA" %in% levels(x))
        cx[is.na(x)] <- "<NA>"
    cx
}
 
This looks like something from before we had character NA values. I
wonder if it is a mistake or there could actually be a reason to
keep it. 

-- 
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