# [R] Advice on parsing formulae

Heather Turner Heather.Turner at warwick.ac.uk
Wed Dec 15 11:41:02 CET 2004

```I think this will do what you want:

# Need this function to remove spaces from term labels later on
> removeSpace <-  function(string) gsub("[[:space:]]", "", string)

# Specify which terms are in a "tvar" group
# (could remove spaces separately)
> tvar <- unname(sapply(c("x:A", "z", "B", "poly(v,3)"), removeSpace))

# Use terms to get term labels from formula
> formula <- Y ~ 1 + x:A + z + u + B + poly(v,3)
> term.labels <- unname(sapply(attr(terms(formula), "term.labels"), removeSpace))
> tvar
 "x:A"       "z"         "B"         "poly(v,3)"
> term.labels
 "z"         "u"         "B"         "poly(v,3)" "x:A"

# Get assign variable for parameters
# (You would use first two lines, but I don't have data so defined assign variable myself)
> #X <- model.matrix(formula)
> #pAssign <- attr(X, "assign")
> pAssign <- c(0,1,2,3,4,4,4,5,5)

# Define "tvarAssign"
> tvarAssign <- match(pAssign, sort(match(tvar, term.labels)))
> tvarAssign[is.na(tvarAssign)] <- 0
> tvarAssign
 0 1 0 2 3 3 3 4 4

HTH

Heather

Mrs H Turner
Research Assistant
Dept. of Statistics
University of Warwick

>>> "Claus Dethlefsen" <dethlef at math.aau.dk> 12/13/04 04:10pm >>>
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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