[Rd] lazy evaluation and DUP=F

[Thomas Lumley]

On Thu, 22 Mar 2001, Paul Gilbert wrote:

> I am having some difficulty understanding the implication of lazy
> evaluation mixed with DUP=F in a .Fortran call.  In qr.qty from base
> DUP is not used as an argument so defaults to T. I am calling qr.qty
> with a very large array and would like to set DUP=F in the .Fortran
> call so that qr.qty would be defined as copied below. Is there some
> risk that a variable used as the argument in the original calling
> environment would be modified, or is there some other reason that this
> should not be done?

I am not sure that DUP=FALSE can't change objects in the original calling
environment (this is why help(.Fortran) says that there is a risk). It's
not precisely lazy evaluation that causes the problem: if the lazy
evaluation really worked under the hood the way it appears to work there
would be no problem. The problem is that R actually calls by
reference until an object is modified. It should be fairly easy to tell by
experiment whether this can happen

	-thomas

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