I found this part in the documentation of 'factor'.
'factor(x, exclude=NULL)' applied to a factor is a no-operation
unless there are unused levels: in that case, a factor with the
reduced level set is returned. If 'exclude' is used it should
also be a factor with the same level set as 'x' or a set of codes
for the levels to be excluded.
Regarding the last sentence, this is the actual behavior.
> x <- factor(c("a","b"), levels=c("a","b"))
> x
[1] a b
Levels: a b
> factor(x, exclude=factor("a", levels=c("a","b")))
[1] a b
Levels: a b
> factor(x, exclude=1L)
[1] a b
Levels: a b
I expect "a" to be removed from levels.
> sessionInfo()
R version 2.15.2 (2012-10-26)
Platform: i386-w64-mingw32/i386 (32-bit)
locale:
[1] LC_COLLATE=English_United States.1252
[2] LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United States.1252
attached base packages:
[1] stats graphics grDevices utils datasets methods base
loaded via a namespace (and not attached):
[1] tools_2.15.2
The results are the same in R 2.5.1.
In R 2.5.1, if function 'match' did not apply 'as.character' to factor (and
used internal code of factor instead), it would work to set 'exclude' as in the
above quotation of the documentation. In the example above, "a" would be
removed from levels.
One cause of the trouble is this code in the definition of function 'factor',
in R 2.15.2 or in R 2.5.1.
exclude <- as.vector(exclude, typeof(x))
What is the intent actually?
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