[R] why doesn't table() have a data=argument?

[Marc Schwartz]

on 02/20/2009 09:11 AM Michael Friendly wrote:
> In an Rweave tutorial written for possibly naive R users, I felt it
> necessary to explain why
> table() had to be used inside with(), whereas other tools like xtabs()
> had a data= argument.
> 
> with() is quite nice for such cases, but it seems an unnecessary thing
> to learn right off.
> Before I turn this question into a request for R-devel, is there any
> inherent reason why it might be
> hard to add a data= argument to table()?  I've looked at the code, but
> am not enlightened
> on this question.
> 
> \emph{Example}: Convert the \code{Arthritis} data in case form to a
> 3-way table of
> \code{Treatment} $\times$ \code{Sex} $\times$ \code{Improved}.%
> \footnote{
> Unfortunately, \codefun{table} does not allow a \code{data} argument to
> provide
> an environment in which the table variables are to be found.  In the
> examples in \secref{sec:table} I used \code{attach(mydata)} for this
> purpose,
> but \codefun{attach} leaves the variables in the global environment,
> while \codefun{with} just evaluates the \codefun{table} expression in a
> temporary environment of the data.
> }
> <<convert-ex2,results=verbatim>>=
> Art.tab <-with(Arthritis, table(Treatment, Sex, Improved))
> str(Art.tab)
> ftable(Art.tab)
> @


xtabs and other functions (such as modeling and plot functions) that
include a 'data' argument have a formula based argument as the means by
which you indicate the columns of the data frame to be included.

These are then passed to model.frame() internally to create the data
frame to be used subsequently by the function. Since model.frame() is
evaluated within the environment of the data frame indicated by the
'data' argument if present, it is needed there.

It's always dangerous to say always, but in my experience, functions
that have a 'data' argument fit the above profile.

If table() had a formula method, then having a 'data' argument would
make sense.

HTH,

Marc Schwartz