Right Brian, I don't see it as a limitation,
but an extension would be helpful. An option to
all.vars to produce a list of vectors containing
names for variables in each "term" would be
especially helpful. -Frank
P.S. Thanks for pointing out also that all.vars
is in S - I had missed that.
Prof Brian Ripley wrote:
>
> On Sat, 28 Jul 2001 fharrell@virginia.edu wrote:
>
> > Dear Jan,
> >
> > Thank you very much for your excellent description of the
> > problem and the self-contained test code. This is a
> > problem that I've been meaning to either document better
> > or solve for some time. The root of the problem is with
> > the builtin S-Plus terms.inner function:
> >
> > > attr(terms.inner(asthma ~ pol(age,kx) + smok),'variables')
> > expression(age, kx, smok)
> >
> > You can see that terms.inner inappropriately includes kx
> > as an independent variable as it does not know that
> > the first argument to pol is the special variable.
> > When a constant replaces kx, all is well.
> >
> > As I am relying on the C code called by terms.inner to
> > do the job, I don't have a ready solution. I would
> > be happy if someone comes up with a solution. The
> > all.vars function in the R language has the same
> > limitation:
> >
> > all.vars(asthma ~ pol(age,kx) + smok)
> > [1] "asthma" "age" "kx" "smok"
>
> Well,
>
> 1) all.vars comes from S, and works the same under R.
>
> 2) I don't think it is a limitation. all.vars is described as
>
> Description:
>
> Return a character vector containing all the names which occur in
> an expression or call.
>
> and kx is such a name. Indeed, there is no way to know from just the
> formula if it is length n-vector or a scalar. I don't know what pol()
> is, but guess it is from your library, in which case the interpretation
> may well depend on where (if anywhere) that library is in the search path.
>
> If you want special-purpose names in formulae (like strata) you need to
> extend the formula-handling code. Neither S4 nor R make that easy.
>
> Brian
>
> --
> Brian D. Ripley, ripley@stats.ox.ac.uk
> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
> University of Oxford, Tel: +44 1865 272861 (self)
> 1 South Parks Road, +44 1865 272860 (secr)
> Oxford OX1 3TG, UK Fax: +44 1865 272595
>
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