It could be done like this: > a <- if (TRUE) . ~ . + freeny.x[,4] else . ~ . > fo <- update(freey.y ~ freeny.x[, -4], a) > fo freey.y ~ freeny.x[, -4] + freeny.x[, 4]
On Mon, Mar 3, 2008 at 6:24 PM, Rolf Turner <[EMAIL PROTECTED]> wrote: > > 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 > > 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. > > > ###################################################################### > Attention:\ This e-mail message is privileged and confid...{{dropped:9}} > > > ______________________________________________ > 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 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.
