[Rd] assigning objects from .C/.Call

[Prof Brian Ripley]

On Wed, 25 Apr 2001, David James wrote:

> Hi,
>
> On Tue, 24 Apr 2001, Prof Brian D Ripley wrote:
>
> > Date: Tue, 24 Apr 2001 20:24:35 +0100 (BST)
> > From:
> > To: David James <dj@research.bell-labs.com>
> > cc: <r-devel@stat.math.ethz.ch>
> > Subject: Re: [Rd] assigning objects from .C/.Call
> > MIME-Version: 1.0
> >
> > On Tue, 24 Apr 2001, David James wrote:
> >
> > >
> > > Hi,
> > >
> > > In section 4.6.7 of "Writing R Extensions" (Version 1.2.1, 2001-01-15)
> > > the C function defineVar() is described as  "... the equivalent
> > > of assign(symbol, value, envir = rho, inherits = FALSE) ..."
> > >
> > > I interpreted the above (wrongly) as meaning that defineVar() makes
> > > a copy of its "value" argument into the object whose name is specified
> > > in "symbol" in the "rho" environment (that's how assign() works at the
> > > R/S level).  However, this is not exactly how defineVar() appears to work.
> > > In particular, it appears to assign the object in the environment "rho",
> > > but further changes in the C code to the object are reflected in the
> > > "assigned" object in rho (i.e., no actual copy of the object is made).
> > >
> > > I can see that the implemented behavior is quite useful, but it's not
> > > exactly equivalent to assign().  Perhaps the documentation should be more
> > > explicit and add a sentence to warn users that the data part of the
> > > object is shared? (The sentence may need to be more precise, though.)
> >
> > I'm happy to do so, but can someone provide the more precise sentence,
> > please.  When exactly does R make a copy?
>
> Using Peter Delgaard's suggestion (and John Chambers', privately)
> I suggest the following modified paragraph (edited from R-exts.texi):
>
>
> @code{defineVar}
> creates a new binding or changes the value of an existing binding in the
> specified environment frame; in this respect, it is analogous to
> @code{assign(symbol, value, envir = rho, inherits = FALSE)},
> but unlike \code{assign}, @code{defineVar} does not make a
> copy of the object (you can assign a copy of the object in the
> environment frame @code{rho} using
> @code{defineVar(symbol, duplicate(value), rho)}).

Thanks, will add (a variant of).

The real issue is that assign() allows `value' to be an expression, and so
uses the value of evaluating the expression, not that of a variable.

Brian

-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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