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, <[EMAIL PROTECTED]> wrote:
>> From: Matthieu Stigler <[EMAIL PROTECTED]>
>> Date: 2008/03/03 Mon PM 04:07:09 CST
>> To: r-help@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!!
>>
>> ______________________________________________
>> R-help@r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-
>> guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help@r-project.org mailing list
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> PLEASE do read the posting guide http://www.R-project.org/posting-
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> and provide commented, minimal, self-contained, reproducible code.
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