[R] Advice on parsing formulae
Claus Dethlefsen
dethlef at math.aau.dk
Mon Dec 13 17:10:29 CET 2004
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)
example:
> model.matrix(Y ~ 1 + tvar(x:A) + tvar(z) + u + tvar(B) + tvar(poly(v,3)))
(Intercept) z u B2 poly(v, 3)1 poly(v, 3)2 poly(v, 3)3 x:A1
x:A2
1 1 -1.55 -1.03 0 0.160 -0.350 -0.281 0.66
0.00
2 1 -1.08 0.55 0 -0.164 -0.211 0.340 0.91
0.00
3 1 0.29 -0.26 0 -0.236 -0.073 0.311 -1.93
0.00
4 1 -1.11 0.96 0 0.222 -0.285 -0.385 -0.23
0.00
5 1 0.43 -0.76 1 -0.434 0.515 -0.532 0.22
0.00
I would like the vector
c(0,1,0,2,3,3,3,4,4)
pointing to the tvar-grouped terms.
Thus what I would like, looks a bit like the 'assign' attribute of the
model.matrix() output. I have not figured out a way of doing this in a nice
way and would like some help, please.
I hope somebody can help me (or point the manual-pages I should read),
Best,
Claus Dethlefsen
----------------------------------------------------------------------------
---
Assistant Professor, Claus Dethlefsen, Ph.D.
mailto:dethlef at math.auc.dk, http://www.math.auc.dk/~dethlef
Dpt. of Mathematical Sciences, Aalborg University
Fr. Bajers Vej 7G, 9220 Aalborg East
Denmark
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