untangle.specials          package:survival          R Documentation

Help 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.

Usage:

     untangle.specials(tt, special, order=1)

Arguments:

      tt: a 'terms' object. 

 special: the name of a special function, presumably used in the terms
          object. 

   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:

     formula<-Surv(tt,ss)~x+z*strata(id)
     tms<-terms(formula,specials="strata")
     ## the specials attribute
     attr(tms,"specials")
     ## main effects 
     untangle.specials(tms,"strata")
     ## and interactions
     untangle.specials(tms,"strata",order=1:2)

