Thank you for your reply, Peter. But that didn't work either. Continuing
the example:

setGeneric("unique")
setMethod("unique", "foo",  function(x,  incomparables = FALSE, ...){
    y <- callNextMethod(x = getDataPart(x),  incomparables = incomparables,
...)
    new("foo", y)
    })

> unique(bar)
An object of class "foo"
[1] 12
> factor(bar)
[1] <NA>
Levels: 12

Indeed I had tried stepping through the 'factor' call, but perhaps in an
unsophisticated manner -- I had copied the body of 'factor' to a local
version of the function:

myfactor <- function (x = character(), levels, labels = levels, exclude =
NA,
    ordered = is.ordered(x))
{
    if (is.null(x)) ...
etc.

And 'myfactor' worked as desired:

> myfactor(bar)
[1] x= 12
Levels: x= 12

I hypothesized that there might be a deeper interaction of an S4
'as.character' method with base::factor, but, having exhausted my woeful
lack of expertise, I decided to write my original email.

Thanks for your consideration.

Dan

On Mon, Jan 23, 2012 at 8:25 AM, peter dalgaard <pdalgd@gmail.com> wrote:

>
> On Jan 23, 2012, at 16:07 , Dan Murphy wrote:
>
> > Hello,
> >
> > 'factor' returns <NA> for my S4 object when the class is given an
> > "as.character" method. Here is a minimal example:
> >
> >> setClass("foo", contains="numeric")
> >> bar <- new("foo", 12)
> >> factor(bar)
> > [1] 12
> > Levels: 12
> >> setMethod("as.character", "foo", function(x) paste("x=", x@.Data))
> > [1] "as.character"
> >> as.character(bar)
> > [1] "x= 12"
> >> factor(bar)
> > [1] <NA>
> > Levels: 12
> >
> > I would like to 'aggregate' by my S4 objects, but 'factor' seems to be
> > getting in the way. Is there an 'as.character' implementation that works
> > better for S4 classes? I searched help.search("factor S4 class") and
> > help.search("factor S4 as.character") without success.
>
> Single-stepping the factor call would have shown you that the real problem
> is that you don't have a unique() method for your class:
>
> > unique(bar)
> [1] 12
>
> i.e., you are getting the default numeric method, which returns a numeric
> vector, so the levels become as.character(unique(bar)) which is c("12") and
> doesn't match any of the values of as.character(bar).
>
> So, either provide a unique() method, or use factor(as.character(bar)).
>
> >
> > Thank you.
> >
> > Dan Murphy
> >
> >       [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-devel@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
>
> --
> Peter Dalgaard, Professor
> Center for Statistics, Copenhagen Business School
> Solbjerg Plads 3, 2000 Frederiksberg, Denmark
> Phone: (+45)38153501
> Email: pd.mes@cbs.dk  Priv: PDalgd@gmail.com
>
>

	[[alternative HTML version deleted]]

