R help - Apr 2003 - Hmisc interaction behavior

Dear R-helpers,

Can someone explain to me why the function interaction() from the Hmisc library
results in numeric?

test1 <- c("A","B","C")
test2 <- c("D","E","F")

is.numeric(interaction(test1,test2))

[1] TRUE

I had problems with this side effect in a different function.

thanks,

Remko Duursma

On Sat, 19 Apr 2003 20:42:29 -0700
Remko Duursma <den.duurs at lycos.com> wrote:
> Dear R-helpers,
> 
> Can someone explain to me why the function interaction() from the Hmisc
library results in numeric?
> 
> test1 <- c("A","B","C")
> test2 <- c("D","E","F")
> 
> is.numeric(interaction(test1,test2))
> 
> [1] TRUE
> 
> I had problems with this side effect in a different function.
> 
> thanks,
> 
> Remko Duursma
> 
interaction in Hmisc returns a factor just as the default interaction.  Why that
results tests TRUE for is.numeric unlike the default interaction is not clear.  
The returned objects tests TRUE for is.factor as it should [I seldom use
is.numeric but I use is.factor a lot].

The object returned by Hmisc's interaction is created by saying

levels(levs) <- labs
oldClass(levs) <- 'factor'

for reasons of compatibility with S-Plus 6.  For R I define oldClass<- as 
function (x, value) 
{
    class(x) <- value
    x
}

I should change interaction to use factor( ) instead but I'm curious to know
why this problem occurs.

---
Frank E Harrell Jr              Prof. of Biostatistics & Statistics
Div. of Biostatistics & Epidem. Dept. of Health Evaluation Sciences
U. Virginia School of Medicine  http://hesweb1.med.virginia.edu/biostat