I see the logic now. I think that more sentences in the document would
be very helpful, however. What is written is very subtle.
I suggest the following small expansion for model.matrix.Rd:
\item{data}{a data frame. If the object has a \code{terms} attribute
then it is assumed to be the result of a call to \code{model.frame},
otherwise \code{model.frame} will be called first.}
I often forget that model.frames are not a class, but an "implied"
class based on the presence of a terms component. Many users, I
suspect, do not even have this starting knowledge.
Off to make changes to model.frame.coxph and model.matrix.coxph...
Thanks for the feeback.
Terry
On Wed, 2011-03-30 at 16:36 +0100, Prof Brian Ripley wrote:
> On Wed, 30 Mar 2011, Terry Therneau wrote:
>
> > I'm working on predict.survreg and am confused about xlevels.
> > The model.frame method has the argument, but none of the standard
> > methods (model.frame.lm, model.frame.glm) appear to make use of it.
>
> But I see this in predict.lm:
>
> m <- model.frame(Terms, newdata, na.action = na.action,
> xlev = object$xlevels)
>
> It is used to remap levels in newdata to those used in the fit.
>
> >
> > The documentation for model.matrix states:
> > xlev: to be used as argument of model.frame if data has no "terms"
> > attribute.
>
> Well, the code says
>
> if (is.null(attr(data, "terms")))
> data <- model.frame(object, data, xlev=xlev)
>
> > But the terms attribute has no xlevels information in it, so I find this
> > statement completely confusing. Any insight is appreciated.
>
> It means exactly what it says: a 'data' argument with a terms
> attribute is considered to be a model frame.
>
> >
> > Terry Therneau
> >
> > ______________________________________________
> > 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.
> >
>
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