R help - Mar 2008 - How to include an externally defined NULL value in lm

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!!

>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!!
>
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