[R] Revert a factor to its numeric values

[Thomas Lumley]

On Wed, 18 Aug 2004, Göran Broström wrote:

> I'm trying a recommendation on the help page for 'factor':
>
> > x <- c(1, 2, 1, 2)
> > x <- factor(x, labels = c("one", "two"))
> > x
> [1] one two one two
> Levels: one two
> > as.numeric(levels(x))[x]
> [1] NA NA NA NA
> Warning message:
> NAs introduced by coercion
>

usually when people want to "revert a factor to its numeric values" they
mean that the labels are numbers and they want those numbers.  In that
case as.numeric(x) or unclass(x) are wrong because they give you the
underlying codes.  You, somewhat unusually, actually want the underlying
codes, which you get with unclass(x).

	-thomas