R-beta: formula() and model formulae
Ross Ihaka
ihaka at stat.auckland.ac.nz
Fri May 9 00:42:30 CEST 1997
Bill Venables writes:
> There is an anomaly in the way : and ^ terms are handled in the
> sense that the logical and useful thing is obvious but does not
> happen. Let me give an example. Suppose a and b are factors, x
> and y are not.
>
> A term such as (a + b + x + y)^2 should be expanded out binomial
> fashion, coefficients stripped away and the remaining products
> treated as : products. Then S copes with terms like a:a, a:b and
> a:x fine, even x:y is handled by having it generate a column of
> xy-products, as it should.
>
> But a term such as x:x does not generate a column of x-squares,
> it is merely removed as it would be if it were a factor. This is
> a complete anomaly, and one that I don't think would be hard or
> dangerous for R to rectify. Indeed it would be very useful to
> generate a complete second degree regression in three variables
> using y ~ (1 + x1 + x2 + x3)^2. As it is now it generates linear
> and product terms only and omits the powers. Go figure.
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.
Ross
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