test for a formulae

On Fri, 2 Jun 2000, Paulo J. Ribeiro Jr wrote:

I'd like to check within a whether an argument is a formulae of the type
~X or similar.
What's the right way to do so?

I'm using: is.language(obj) but not sure this is right

That allows lots of other things.

Formulae have class "formula", so inherits(obj, "formula") looks best.

And if you want to ensure that it is ~X+Z+W rather than Y~X+Z+W you can
use
 inherits(obj,"formula") && (length(obj)==2)


	-thomas

Thomas Lumley
Assistant Professor, Biostatistics
University of Washington, Seattle

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