[R] model.matrix()

[Prof Brian D Ripley]

On Mon, 30 Aug 1999, Thomas Yee wrote:

> Hi,
> 
> In R I get the following:
> 
> > x <- 1:9
> > f <- as.factor(1:9)
> > i <- model.matrix(~x+f)
> > 
> > attr(i, "assign")
>  [1] 0 1 2 2 2 2 2 2 2 2
> 
> whereas in S I get the following:
> 
> >  attr(i, "assign")
> $"(Intercept)":
> [1] 1
> 
> $x:
> [1] 2
> 
> $f:
> [1]  3  4  5  6  7  8  9 10
> 
> Has anybody written a model.matrix function to get the same
> result as S? It's just that S's output is what I want, viz.,
> 
> 1. the names "(Intercept)","x","f", and 
> 
> 2. the vector values.

Look in the function summary.aov, then.  You need to be very careful with
singular matrices, as R and S differ even more there.

Something like

my.model.matrix <- function(formula, data = sys.frame(sys.parent()), ...)
{
  mf <- model.frame(formula, data)
  terms <- attr(mf, "terms")
  tl <- attr(terms, "term.labels")
  intr <- attr(terms, "intercept")
  if(intr > 0) tl <- c("(Intercept)", tl)
  m <- model.matrix(formula, mf, ...)
  asgn <- attr(m, "assign")
  s <- split(seq(along=asgn), asgn)
  names(s) <- tl
  attr(m, "my.assign") <- s
  m
}

may do want you want.  Beware of what happens when you fit a singular
model, though,

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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