[R] How do I coerce numeric factor columns of data frame to vector?

[Na Li]

On 9 Sep 2003, Michael A. Miller outgrape:

> > > > > >  "Thomas" == Thomas W Blackwell <tblackw at umich.edu> writes:
>  
> >  Michael - Because these columns are factors to begin with,
> >  using as.numeric() alone will have unexpected results.  See
> >  the section "Warning:" in help("factor").
>  
>  Ah, thanks for pointing that out.  When I've used as.numeric on
>  factors, it is usually because I've explicitly made a numeric
>  variable a factor.  I can see where something that I think aught
>  to be numeric would be interpreted as a factor, say due to an
>  error in formatting an input file.  If I do something like this:
>  
>  x <- some numeric array
>  f <- factor(x)
>  y <- as.numeric(x)
>  
>  is there a possibility of y not equalling x?  If x really is
>  numeric that is.

> x <- c(1, 3, 1)
> y <- factor (x)
> y
[1] 1 3 1
Levels: 1 3
> as.numeric (y)
[1] 1 2 1

I don't know a good way to convert it back however, except 

> as.numeric (levels (y)[as.numeric (y)])
[1] 1 3 1

which is a bit awkward.

Michael