R-beta: formula() and model formulae

Bill Venables wvenable at attunga.stats.adelaide.edu.au
Fri May 9 08:30:44 CEST 1997

Ross Ihaka writes:

 > I agree that this is a problem (its certainly bitten me), however
 > there is problem in implementing things this way.  All the model
 > manipulations are carried out by the "terms" function which only sees
 > a formula and is blissfully unaware of whether variables are factors
 > or numeric.  It then has no basis for deciding between
 > 	a:a -> a	(factors)
 > 	x:x -> x^2	(variables)
 > I supose that we could make "terms" a bit more context aware, but
 > sometimes it's useful to use it for its purely symbolic effect.

I'm sorry to say it but this has to be a design flaw.  The
solution must be to leave a:a terms in at the symbolic stage and
not to remove them.  Some steps can only be done when it is known
which terms are factors, why not this one as well?

I can appreciate, though, that if this correction requires a
large re-design effort then it has to take low priority.  Pity.


Bill Venables, Head, Dept of Statistics,    Tel.: +61 8 8303 5418
University of Adelaide,                     Fax.: +61 8 8303 3696
South AUSTRALIA.     5005.   Email: Bill.Venables at adelaide.edu.au
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