factor S4 class is NA when as.character method exists

Here is code that does make factor() work on a new
class like yours.  It uses Sv3 methods.

   >  setClass("foo", contains="numeric")

   [1] "foo"

   >  as.character.foo<- function(x) paste("x=",x at .Data,sep="")
   >  unique.foo<- function(x, ...) structure(NextMethod("unique"), class=class(x))
   >  someFoo<- new("foo", c(11, 13, 11, 13, 12))
   >  str(factor(someFoo))

    Factor w/ 3 levels "x=11","x=12",..: 1 3 1 3 2

It would be nice to have a list of methods that one
needs to define for a new class in order to make it
do the "basic" things you expect.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

-----Original Message-----
From: r-devel-bounces at r-project.org [mailto:r-devel-bounces at r-project.org] On Behalf Of Dan Murphy
Sent: Monday, January 23, 2012 10:31 PM
To: peter dalgaard
Cc: r-devel at r-project.org
Subject: Re: [Rd] factor S4 class is NA when as.character method exists

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 at 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 at .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

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--
Peter Dalgaard, Professor
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com

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