R-beta: formula() and model formulae

Mike Meyer mikem at stat.cmu.edu
Tue May 6 17:21:39 CEST 1997

Several bugs (no solutions, yet).  These might be well known.

1) If one does, e.g.,  mymod <- lm(y ~ x); formula(mymod)
then one does not get back the formula (one gets, Error: invalid formula) 

2) if x is of mode numeric, then the model formula
	mymod <- lm(y ~ x + x^2)
is not processed as S would do it.    The model is fit ignoring the x^2 term, 
however mymod$call includes the x^2 term.  This seems to be a bug (or maybe 
feature) in applying model formulae operators to numeric quantities.  I expect 
(from experience with S) that x^2 will be interpreted as a math operator.  
Whatever the right thing to do is, it needs to be documented.

Regards,  --Mike

Mike Meyer, Department of Statistics, Carnegie Mellon University

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