[Rd] lm(rnorm(1000)~rnorm(1000)) kills rw1000 (PR#476)

[Thomas Lumley]

On Tue, 7 Mar 2000, Thomas Lumley wrote:

> On Tue, 7 Mar 2000, Prof Brian D Ripley wrote:
> 
> > On Tue, 7 Mar 2000 cberry@tajo.ucsd.edu wrote:
> > 
> > > It does not happen on rw0901 or rw0651 (? version number ?) on my HOME
> > > PC. Nor on R 0.90.1 under Solaris
> > 
> > It does happen on R 1.0.0 on Solaris and on rw1000 on a real PC.
> 
> Something strange is happening where we ensure that the response does not
> appear on the rhs of the formula.  
> 
> At lines 1643ff of model.c we count how many columns of the design matrix
> are needed for each model term.  This count is not done if the term is the
> response, which leaves the count uninitialised.
> 
> We then use this uninitialised count at 1673ff to construct the assign
> attribute.

Incidentally, this command doesn't do what the user probably thinks it
does:  lm thinks that the rnorm(1000) on the lhs is the same as the one on
the rhs and drops it from the model, fitting only an intercept.

I think this is unavoidable, since model.matrix can't be expected to know
whether an arbitrary function is deterministic or random.

The work-around is
	a<-rnorm(1000)
	b<-rnorm(1000)
	lm(a~b)


	-thomas


Thomas Lumley
Assistant Professor, Biostatistics
University of Washington, Seattle


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