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.
J. R. M. Hosking
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