attrassign: Create new-style "assign" attribute

Description

The "assign" attribute on model matrices describes which columns come from which terms in the model formula. It has two versions. R uses the original version, but the alternate version found in S-plus is sometimes useful.

Usage

# S3 method for default
attrassign(object, tt,...)
# S3 method for lm
attrassign(object,...)

Value

A list with names corresponding to the term names and elements that are vectors indicating which columns come from which terms

Arguments

object: model matrix or linear model object
tt: terms object
...: ignored

Details

For instance consider the following


    survreg(Surv(time, status) ~ age + sex + factor(ph.ecog), lung)

R gives the compact for for assign, a vector (0, 1, 2, 3, 3, 3); which can be read as ``the first column of the X matrix (intercept) goes with none of the terms, the second column of X goes with term 1 of the model equation, the third column of X with term 2, and columns 4-6 with term 3''.

The alternate (S-Plus default) form is a list


       $(Intercept)     1
       $age             2
       $sex             3
       $factor(ph.ecog) 4 5 6

Examples

Run this code

formula <- Surv(time,status)~factor(ph.ecog)
tt <- terms(formula)
mf <- model.frame(tt,data=lung)
mm <- model.matrix(tt,mf)
## a few rows of data
mm[1:3,]
## old-style assign attribute
attr(mm,"assign")
## alternate style assign attribute
attrassign(mm,tt)