[R] Extracting formula from an lm object

[Prof Brian Ripley]

On Sun, 24 Aug 2008, Berwin A Turlach wrote:

> G'day Murray,
>
> On Sun, 24 Aug 2008 18:34:39 +1200
> Murray Jorgensen <maj at stats.waikato.ac.nz> wrote:
>
>> I want to extra the part of the formula not including the response
>> variable from an lm object. For example if the lm object ABx.lm was
>> created by the call
>>
>> ABx.lm <- lm( y ~ A + B + x, ...)
>>
>> Then ACx.lm is saved as part of a workspace.
>> I wish to extract   "~ A + B + x".  Later in my code I will fit
>> another linear model of the form z ~ A + B + x for some other
>> response variable z. I would be grateful for any suggestions of a
>> nice way to do this.
>
> AFAIK, a formula is essentially a list of two or three components.  The
> first component is "~".  The second is the LHS of the formula if there
> are three components; otherwise the RHS of the formula.  The third
> component, if it exists, is the RHS of the formula.
>
> So storing "~ A + B + x" and manipulating this part for different
> responses could turn out to be painful; you would have to insert the
> new LHS as the second component of the list.  I would suggest that it
> is easier to store the complete formula and just manipulate the LHS;
> see:

An ulternative is to use update.formula on the formula extracted, or even 
just use update() on the lm fit.  For the information in this posting 
(there could be more going on, like where to find 'z'),

update(ABX.lm, z ~ .)

should be all that is needed.

>
> R> library(MASS)
> R> fm <- lm(time~dist+climb, hills)
> R> formula(fm)
> time ~ dist + climb
> R> formula(fm)[[1]]
> `~`
> R> formula(fm)[[2]]
> time
> R> formula(fm)[[3]]
> dist + climb
> R> tt <- formula(fm)
> R> tt[[2]] <- NULL
> R> tt
> ~dist + climb
>
> R> tt <- formula(fm)
> R> class(tt[[2]])
> [1] "name"
> R> typeof(tt[[2]])
> [1] "symbol"
> R> tt[[2]] <- as.name("y")
> R> tt
> y ~ dist + climb
>
> R> tt <- formula(fm)
> R> tt[[2]] <- as.symbol("z")
> R> tt
> z ~ dist + climb
>
> HTH.
>
> Cheers,
>
> 	Berwin
>
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-- 
Brian D. Ripley,                  ripley at 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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595