[R] Intercepts in linear models.

Rolf Turner rolf at math.unb.ca
Thu Mar 23 17:43:53 CET 2006

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?


					Rolf Turner

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