R: Help Process the ‘specials’ Argument of the ‘terms’ Function.

untangle.specials {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)

[Package survival version 2.34 Index]