[R] Why does a custom function called is.numeric.factor break lattice?

Bert Gunter bgunter.4567 at gmail.com
Mon Nov 16 03:42:44 CET 2015

Think about it.

I shall assume that you are familiar with S3 methods.  What do you
think would happen when xyplot code calls is.numeric() on a factor
object expecting it to call the is.numeric primitive but, instead,
finding a factor method defined, calls that? Note that your factor
method has no arguments, but the is.numeric() primitive does. Hence
when the code calls the primitive on the factor object, the error you
saw is thrown.

I would say that this is a weakness of the informal S3 "class" system,
although you probably should not have been surprised that is.numeric
is called on factors as the "x" argument, so you were inviting trouble
by defining a factor method that overrides this behavior.
Nevertheless, I would argue that one cannot know in general when this
occurs for other S3 classes, and that therefore allowing methods for
is.numeric() is dangerous.

Of course, full qualification in the original xyplot code
(base::is.numeric() rather than is.numeric() ) would avoid such
things, but that's a drag.

Contrary opinions and corrections to any flawed understanding on my
part are welcome, of course.


Bert Gunter

"Data is not information. Information is not knowledge. And knowledge
is certainly not wisdom."
   -- Clifford Stoll

On Sun, Nov 15, 2015 at 6:22 PM, sbihorel
<Sebastien.Bihorel at cognigencorp.com> wrote:
> Hi,
> Pretty much everything is in the title of the post. An example is below.
> library(lattice)
> data <-
> data.frame(x=rep(1:10,8),y=rnorm(80),trt=factor(rep(1:4,each=20)),groups=rep(1:8,each=10))
> xyplot <- xyplot(y~x|trt,data,groups=groups)
> is.numeric.factor <- function(){
>   print('hello world')
> }
> xyplot <- xyplot(y~x|trt,data,groups=groups)
> Thanks for shedding some light on this.
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