Help Process the `specials' Argument of the `terms' Function.

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.

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")
#> $strata
#> [1] 4
#> 
## main effects 
untangle.specials(tms, "strata")
#> $vars
#> [1] "strata(id)"
#> 
#> $tvar
#> [1] 3
#> 
#> $terms
#> [1] 3
#> 
## and interactions
untangle.specials(tms, "strata", order=1:2)
#> $vars
#> [1] "strata(id)"
#> 
#> $tvar
#> [1] 3
#> 
#> $terms
#> [1] 3 4
#>