R help - Dec 2010 - Passing parameter to a function

I am trying to pass a couple of variable names to a xtabs formula:
> tab <- function(x,y){
    xtabs(time~x+y, data=D)
}

But when I run:
> tab(A,B)
I get:

Error in eval(expr, envir, enclos) : object "A" not found

I am quite sure that there is some easy way out, but I have tried with different
combinations of deparse(), substitute(), eval(), etc without success, can
someone help?

Thanks,
Luca

Luca Meyer
www.lucameyer.com
IBM SPSS Statistics release 19.0.0
R version 2.12.1 (2010-12-16)
Mac OS X 10.6.5 (10H574) - kernel Darwin 10.5.0

On 20/12/2010 1:13 PM, Luca Meyer wrote:
> I am trying to pass a couple of variable names to a xtabs formula:
>
> >  tab<- function(x,y){
>      xtabs(time~x+y, data=D)
> }
>
> But when I run:
>
> >  tab(A,B)
>
> I get:
>
> Error in eval(expr, envir, enclos) : object "A" not found
>
> I am quite sure that there is some easy way out, but I have tried with
different combinations of deparse(), substitute(), eval(), etc without success,
can someone help?
I assume that A and B are columns in D?  If so, you could use

tab(D$A, D$B)

to get what you want.  If you really want tab(A,B) to work, you'll need 
to do messy work with substitute, e.g. in the tab function, something like

fla <- substitute(time ~ x + y, list(x = substitute(x), y = substitute(y))
xtabs(fla, data=D)

Duncan Murdoch