[R] Advice on parsing formulae
Chuck Berry
cberry at tajo.ucsd.edu
Tue Dec 14 20:04:15 CET 2004
Claus Dethlefsen <dethlef <at> math.aau.dk> writes:
>
> Dear list
>
> I would like to be able to group terms in a formula using a function that I
> will call tvar(), eg. the formula
>
> Y ~ 1 + tvar(x:A) + tvar(z) + u + tvar(B) + tvar(poly(v,3))
>
> where x,u and v are numeric and A and B are factors - binary, say.
>
> As output, I want the model.matrix as if tvar had not been there at all. In
> addition, I would like to have information on the grouping, as a vector as
> long as ncol( model.matrix ) with zeros corresponding to terms outside tvar
> and with an index grouping the terms inside each tvar(). In the (sick)
Since you want to single out terms with 'tvar' in them, something like
tvar.terms <- terms( your.formula.above, specials = "tvar" )
will likely be what you want. You'll want to study
?terms
?terms.object
and then
print( tvar.terms )
And likely 'deparse' will help, too.
> example:
[rest deleted]
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