R help - Nov 2015 - Why does a custom function called is.numeric.factor break lattice?

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.

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.

Cheers,
Bert


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.
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

You need to read about S3 classes, and either make your custom function behave
the way that function needs to behave or use a different function name for your
custom function.

I think this is an example of the old saying that if it hurts when you slam your
head against the wall, then don't do that.
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On November 15, 2015 6:22:42 PM PST, 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.
>
>______________________________________________
>R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>https://stat.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide
>http://www.R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.

Hi,

Interesting, based upon my understanding of S3 methods, I would have 
assumed that my function would be applied to objects of class 
"numeric.factor". I was not aware of this multiple
"dispatch".

Thanks for pointing this out.


On 11/15/2015 9:42 PM, Bert Gunter wrote:
> 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.
>
> Cheers,
> Bert
>
>
> 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.
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.