[Rd] factor S4 class is NA when as.character method exists

[peter dalgaard]

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
> 
> 	[[alternative HTML version deleted]]
> 
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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