[R] Creating a Model Matrix - keeping NAs

[Prof Brian D Ripley]

On Mon, 13 Aug 2001, Rachel Cunliffe wrote:

> I am wanting to create a model matrix and keep the NAs.
>
> stratmat <- model.matrix(myformula,mydata)
>
> Is there any way to do this?  model.matrix doesn't have na.action as a
> parameter.  Elsewhere I have made use of na.keep <- function(x){x}.

Yes, but it uses options("na.action"), soyou could set that.
Better, call model.frame then model.matrix, something like

stratmat <- model.matrix(myformula, model.frame(myformula,mydata,
na.action=function(x)x))

?model.matrix does tell you the second argument should be the result of
model.frame, which is a pretty strong hint.

>
> Many thanks,
>
> Rachel Cunliffe
>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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