[Rd] bug or correct behaviour ?

[Prof Brian Ripley]

On Fri, 11 Jun 2004, Alex Nu wrote:

> 
>  Dr. Ripley,
> 
>  Thanks for your answer, I guest the key is that
>  in my code R can't know tmp is being modified.
>  
>  I couldn't find a duplicate function for the
>  R environment. Is there any ?

No.  You should be doing this in your C/C++ code.

>  So I could avoid doing tmp.last <-tmp*1.0
>   
>  Thanks
> 
>  Alex
>  
> 
> 
> --- Prof Brian Ripley <ripley at stats.ox.ac.uk> wrote:
> > Without seeing your code for the .Call parts it is
> > impossible for us to
> > know, but as .Call does not duplicate its arguments
> > (unlike .C), it is
> > possible for it to change both tmp and a copy of
> > tmp, tmp.last.
> > 
> > So it seems likely there is a bug in your C code if
> > that is not your 
> > intention.  The normal way to use .Call is to return
> > something, and to 
> > duplicate any arguments which will be changed
> > (possibly only provided 
> > NAMED(tmp) is true as otherwise there are no named
> > copies around).
> > 
> > In other words, it would be normal to have
> > 
> > tmp <- .Call("Replace", tmp, ...)
> > 
> > and call duplicate() inside the C code for Replace.
> > 
> > On Thu, 10 Jun 2004, Alex Nu wrote:
> > 
> > > 
> > >  This is the general outline of my code::
> > >  
> > > main(argc,argv){
> > >    ...
> > >    Rf_initEmbeddedR(argc,argv);
> > >    ...
> > >    Test_tryEval("source(test.r)");
> > >    ...
> > > }
> > 
> > Not sure of the relevance of this.
> > 
> > > ############# 
> > > # test.r
> > > #############
> > > ...
> > > dyn.load("toload.so")
> > > 
> > > tmp <-matrix(data=1,nrow=narray*2,ncol=nclust)
> > > 
> > > .Call("Init",tmp,...)
> > > while(...) {
> > >    criteria <-feval(tmp)
> > >    if (criteria < criteria.min) 
> > >         tmp.last <- tmp
> > 
> > That makes a potential copy. The actual copying
> > occurs if tmp is changed
> > and R is aware of it.  This is the purpose of the
> > 'named' element of a
> > SEXP, but I don't think that is documented anywhere.
> > 
> > >    else  
> > >        tmp <- tmp.last
> > >    ...
> > >    .Call("replace",tmp,...)
> > > }
> > > ####################################
> > > 
> > > 
> > >  When I try to recover tmp 
> > > 
> > >       tmp <- tmp.last
> > >  
> > >  I got the modified value of tmp.
> > >  It means that tmp.last is modified
> > >  when I modified tmp in the C funciont replace.
> > >  
> > >  The program seems to work fine if I change 
> > >  to this::
> > >             
> > >   tmp.last <- tmp*1.0
> > >   
> > >   tmp <- tmp.last*1.0
> > 
> > Those do definitely make new objects.
> > 
> > -- 
> > 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)
> > 1 South Parks Road,                     +44 1865
> > 272866 (PA)
> > Oxford OX1 3TG, UK                Fax:  +44 1865
> > 272595
> >
> 
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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)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595