[Rd] bug or correct behaviour ?

[Alex Nu]

 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 ?
 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
>