R-beta: formula() and model formulae

[Thomas Lumley]

On 6 May 1997, Peter Dalgaard BSA wrote:
> Mike Meyer <mikem at stat.cmu.edu> writes:
> > 
> > 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) 
> 
> Yep. Seems that we need a
> 
> formula.lm<-function(x)formula(x$terms)
> 

We also need to fix formula.default. At the moment it only looks for
x$formula.  Other standard places to keep a formula are x$call$formula and
x$terms.  How about 

formula.default<-function (x)
{
        if (!is.null(x$formula))
                return(eval(x$formula))
        if (!is.null(x$call$formula))
                return(eval(x$call$formula))
        if (!is.null(x$terms))
                return(x$terms)
        switch(typeof(x), NULL = structure(NULL, class = "formula"),
                character = formula(eval(parse(text = x)[[1]])),
                call = eval(x), stop("invalid formula"))
}

One disdvantage to extracting the formula from $terms instead of
$call$formula is that in S a terms object is not a formula. On the other
hand it doesn't really matter as long as people use the formula()
function.


Thomas Lumley
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