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 at 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!!