On Fri, 20 Feb 2009, 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.
I think the simple rule is: If there is a "formula", there is (typically)
also a "data" argument, otherwise not.
There are probably exceptions but
So the question should probably be: Why is there xtabs() with interface
xtabs(~ x + y, data = mydata)
instead of a "formula" method for table() that would allow
table(~ x + y, data = mydata)
Well, table() is not generic, to some extent probably due to historical
reasons. But maybe it would be worth considering a change to this?
Z
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)
@
--
Michael Friendly Email: friendly AT yorku DOT ca Professor, Psychology
Dept.
York University Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street http://www.math.yorku.ca/SCS/friendly.html
Toronto, ONT M3J 1P3 CANADA
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