From ripley@stats.ox.ac.uk  Fri Jun  1 15:37:23 2001
From: ripley@stats.ox.ac.uk (Prof Brian Ripley)
Date: Fri, 1 Jun 2001 15:37:23 +0100 (BST)
Subject: [Rd] formulas in variables in modeling functions
In-Reply-To: <OFC33AA95D.63C7CD15-ON85256A5E.004E396D@rtp.epa.gov>
Message-ID: <Pine.LNX.4.31.0106011531410.24368-100000@gannet.stats>

On Fri, 1 Jun 2001 Setzer.Woodrow@epamail.epa.gov wrote:

> I first noticed this in gnls, but it also happens in lm, so I suspect the
> following phenomenon is universal in modeling functions.  Modify the
> example from the lm help:
>
> model <- as.formula(weight ~ group)
> lm(model)
>
> The result will be printed:
>
> > lm(model)
>
> Call:
> lm(formula = model)
> ^^^^^^^^^^^^^^^^^
> Coefficients:
> (Intercept)     groupTrt
>       5.032       -0.371
>
> This is because in lm, (where the return value is z) z$call is set to cl,
> and cl <- match.call().
>
> My question is:  would it break things to add the line
>
> cl[["formula"]] <- formula
> after
> cl <- match.call()
> ?

It would.  Think about how update() works.

What's wrong with

lm(weight ~ group)

or if you need it

eval(substitute(lm(model), model=model))

?  There is a difference between capturing the formula or using a
reference to a formula.

-- 
Brian D. Ripley,                  ripley@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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[Rd] formulas in variables in modeling functions
Prof Brian Ripley