[R] Basic question on concatenating factors

[udi cohen]

Hi all,

I hope it's not too trivial for the list - I'm trying to concatenate
two factor arrays, and obtain the following:

> f1<-factor(c("a","a","b"))
> f1
[1] a a b
Levels: a b
> f2<-factor(c("b","b","a"))
> f2
[1] b b a
Levels: a b
> c(f1,f2)
[1] 1 1 2 2 2 1

Instead of getting:

[1] a a b b b a
Levels: a b

a related question is: how do I add a level which does not exists yet
in a factored vector, so I'll be able to add later these values,
without getting:

In `[<-.factor`(`*tmp*`, 2, value = "c") :
  invalid factor level, NAs generated

Thanks,

EC