[R] Caution on the use of model.matrix.

[Dimitris Rizopoulos]

I think you could bit that using (inside your function) something like 
this

old.o <- options(na.action = na.fail)
# old.o <- options(na.action = na.pass)
on.exit(old.o)


I hope it helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/336899
Fax: +32/16/337015
Web: http://www.med.kuleuven.ac.be/biostat/
     http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Rolf Turner" <rolf at math.unb.ca>
To: <r-help at stat.math.ethz.ch>
Sent: Thursday, June 02, 2005 4:14 PM
Subject: [R] Caution on the use of model.matrix.


>I have just been bitten by a quirk in the behaviour of model.matrix.
> I used model.matrix inside a function, and passed to it a formula
> that was built elsewhere.
>
> The formula was of the form ``y ~ x + w + z''.  Now, model.matrix
> cheerfully accepts formulae of this form, although it only
> ***needs*** the right hand side, i.e. ``~ x + w + z'' --- the ``y''
> can be dropped (but in general needn't be).
>
> The quirk by which I was bitten was that if the y column of the data
> frame being used contains missing values, then the corresponding 
> rows
> are dropped (silently) and the resulting design matrix has rows
> corresponding only to the non-missing values of y.  This was not the
> desired behaviour in my application.
>
> Might I respectfully suggest to R Core that a WARNING be added to 
> the
> help for model.matrix to the effect that
>
> model.matrix(y~x + w + z,XXX)
> and
> model.matrix(~x + w + z,XXX)
>
> give DIFFERENT results if the column ``y'' of the data frame XXX
> contains missing values?
>
> cheers,
>
> Rolf Turner
> rolf at math.unb.ca
>
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