Process the
specials
Argument of the Terms Function
DESCRIPTION:
Given a terms structure and a desired special name, this returns an index
appropriate for subscripting the terms structure and another appropriate for
the data frame.
This function is used by the survival functions.
USAGE:
untangle.specials(tt, special, order=1)
REQUIRED ARGUMENTS:
- tt
-
a
terms
object.
- special
-
the name of a special function, presumably used in the terms object.
OPTIONAL ARGUMENTS:
- order
-
the order of the desired terms.
If set to 2, interactions with the special function will be included.
VALUE:
a list with two components:
- vars
-
a vector of variable names, as would be found in the data frame, of the
specials.
- terms
-
a numeric vector, suitable for subscripting the terms structure, that indexes
the terms in the expanded model formula which involve the special.
EXAMPLES:
# This is code from within the coxph program, m is the data frame and TT
# the terms structure for the formula. We wish to exclude first order
# strata() terms from the final X matrix, and also to extract these terms
# to a second data frame
TT <- terms(formula, specials='strata')
temp <- untangle.specials(TT, 'strata')
if(length(temp$vars)) {
X <- model.matrix( TT[-temp$terms], m)
m.strat <- m[temp$vars]
}
else X <- model.matrix(TT, m)