[R] factor documentation issue

[Geoff Russell]

There is a warning in the documentation for ?factor  (R version 2.3.0)
as follows:

" The interpretation of a factor depends on both the codes and the
  '"levels"' attribute.  Be careful only to compare factors with the
  same set of levels (in the same order).  In particular,
  'as.numeric' applied to a factor is meaningless, and may happen by
  implicit coercion.  To "revert" a factor 'f' to its original
  numeric values, 'as.numeric(levels(f))[f]' is recommended and
  slightly more efficient than 'as.numeric(as.character(f))'.


But as.numeric seems to work fine whereas as.numeric(levels(f))[f] doesn't
always do anything useful.

For example:

> f<-factor(1:3,labels=c("A","B","C"))
> f
[1] A B C
Levels: A B C
> as.numeric(f)
[1] 1 2 3
> as.numeric(levels(f))[f]
[1] NA NA NA
Warning message:
NAs introduced by coercion

And also,

> f<-factor(1:3,labels=c(1,5,6))
> f
[1] 1 5 6
Levels: 1 5 6
> as.numeric(f)
[1] 1 2 3
> as.numeric(levels(f))[f]
[1] 1 5 6

Is the documentation wrong, or is the code wrong, or have I missed
something?

Cheers,
Geoff Russell