[R] Intercepts in linear models.

[Gabor Grothendieck]

You can use update to modify a formula:

   fo <- y ~ x
   fo <- update(fo, . ~ . -1)

willl remove the intercept.  See ?update.formula

On 3/23/06, Rolf Turner <rolf at math.unb.ca> wrote:
> A colleague asked me if there is a way to specify with a
> ***variable*** (say ``cflag'') whether there is an intercept in a
> linear model.
>
> She had in mind something like
>
>        lm(y ~ x - cflag)
>
> where cflag could be 0 or 1; if it's 0 an intercept should
> be fitted, if it's 1 then no intercept.
>
> This doesn't work ``of course''.  The cflag just gets treated
> as another predictor and because it is of the wrong length
> an error is generated.
>
> The best I could come up with was
>
>        lm(as.formula(paste("y ~ x -",cflag)))
>
> Which is kludgy.  It (of course!) also be done using an
> if statement.
>
> Is there a sexier way?
>
>                                cheers,
>
>                                        Rolf Turner
>
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