Re: [R] How to include an externally defined NULL value in lm

From: Rolf Turner <r.turner_at_auckland.ac.nz>
Date: Tue, 04 Mar 2008 12:24:12 +1300

Matthieu wants to manipulate the *formula* --- i.e. control what predictors go into the formula --- rather than to add something to one of the predictors (or not, as the case maybe).

I.e. what is wanted is that if ``condition'' is TRUE then the formula should be

        freeny.y ~ freeny.x[,-4] + freeny.x[,4]

and otherwise it should be

        freeny.y ~ freeny.x[,-4]

The subtlety is that `` y ~ x + NULL'' gives the model ``y = beta_0 + beta_1 * x + E''
Whereas ``y ~ x + a'' gives the model ``y = beta_0 + beta_1 * x + beta_2 * a + E''.
Irrespective of what the object a happens to be --- or even of whether it exists.
(This is a bit of an oversimplification I think, but close enuff.)

Then when lm() tries to fit y = beta_0 + beta_1 * x + beta_2 * a + E it gets all in a dither when a is NULL, because the model does not then make sense.

I'm afraid I don't know how to get around the problem using the NULL concept.
Wiser heads than mine may be able to suggest a way.

There are however different approaches to the problem, ***not*** using NULL.

E.g.

        fff <- if(connie) as.formula(freeny.y ~ freeny.x[,-4] + freeny.x[, 4]) else as.formula(freeny.y ~ freeny.x[,-4])

        fit <- lm(fff)

or (more sensibly?)

        fff <- if(connie) as.formula(freeny.y ~ freeny.x) else as.formula (freeny.y ~ freeny.x[,-4])

        fit <- lm(fff)

More sensibly still:

	fff <- if(connie) y ~ . else y ~ . - market.potential
	fit <- lm(fff,data=freeny)

Where ``connie'' is some condition (with value either TRUE or FALSE).

Formulae can also be constructed as text strings (e.g. using ``paste'') and then converted
to actual formulae using as.formula().

HTH.         cheers,

                Rolf Turner

On 4/03/2008, at 11:34 AM, <markleeds_at_verizon.net> wrote:

>> From: Matthieu Stigler <stigler3@etu.unige.ch>
>> Date: 2008/03/03 Mon PM 04:07:09 CST
>> To: r-help_at_r-project.org
>> Subject: [R] How to include an externally defined NULL value in lm
>
> maybe you should a<-0 unless there's special
> behavior of NULL that's unknownst to me ?
>
>
>> Hello!
>>
>> I would love to be able to include an external variable to a lm
>> call, I
>> mean something:
>>
>> if(TRUE)
>> a<-freeny.x[,4]
>> else
>> a<-NULL
>>
>> lm(freeny.y~freeny.x[,-4] +a)
>>
>> but it does not work with a<-NULL, whereas
>>
>> lm(freeny.y~freeny.x[,-4] +NULL)
>>
>> I don't understand why and did not find an answer in the
>> manuals... do
>> you see it? Any idea?
>>
>> Thanks!!
>>
>> ______________________________________________
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>> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
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