R devel - Jul 2006 - .Call question

Writing R Ext says to treat R objects that are arguments to .Call as
read only (i.e. don't modify). 

I have a long list of lists that and I want to avoid the overhead of a
copy in my C code.  I would just like to modify some of the elements 
of list by replacing them with elements of exactly the same size/type.

below is an example of the essence of the problem.  This seems to work.
Is this dangerous or ok? (of course, I want to do something more
complicated
since I can do this in R, but I hope this illustrates my question).


SEXP modlist(SEXP list, SEXP ind, SEXP element){
   int index;
   index=INTEGER_VALUE(ind);
   SET_ELEMENT(list,index,element);
return(ind);
}

called in R as in 

lst=list("a","b","c")
element="d"
out=.Call("modlist",lst,1,element)

here the .Call is used just for its effect on lst.

thanks!

peter
 

................................
  Peter E. Rossi
 Joseph T. and Bernice S. Lewis Professor of Marketing and Statistics
 Editor, Quantitative Marketing and Economics
 Rm 353, Graduate School of Business, U of Chicago
 5807 S. Woodlawn Ave, Chicago IL 60637, USA
 Tel: (773) 702-7513   |   Fax: (773) 834-2081

On Wed, 26 Jul 2006, Rossi, Peter E. wrote:
>
> Writing R Ext says to treat R objects that are arguments to .Call as
> read only (i.e. don't modify).
>
> I have a long list of lists that and I want to avoid the overhead of a
> copy in my C code.  I would just like to modify some of the elements
> of list by replacing them with elements of exactly the same size/type.
>
> below is an example of the essence of the problem.  This seems to work.
> Is this dangerous or ok? (of course, I want to do something more
> complicated
> since I can do this in R, but I hope this illustrates my question).
It is dangerous.

If you do

lst1<-list("a","b","c")
lst2<-lst1
element<-"d"
.Call("modlist",lst1, 1, element)

then both lst1 and lst2 will change. The danger is just that you can 
break the call-by-value illusion that R maintains, in two ways. The first 
is that objects that have shared structure can be modified, the 
second is that you can create shared structure without R knowing: if you 
change "element" subsequently then lst1 and lst2 may change.

 	-thomas
>
> SEXP modlist(SEXP list, SEXP ind, SEXP element){
>   int index;
>   index=INTEGER_VALUE(ind);
>   SET_ELEMENT(list,index,element);
> return(ind);
> }
>
> called in R as in
>
> lst=list("a","b","c")
> element="d"
> out=.Call("modlist",lst,1,element)
>
> here the .Call is used just for its effect on lst.
>
> thanks!
>
> peter
>
>
> ................................
>  Peter E. Rossi
> Joseph T. and Bernice S. Lewis Professor of Marketing and Statistics
> Editor, Quantitative Marketing and Economics
> Rm 353, Graduate School of Business, U of Chicago
> 5807 S. Woodlawn Ave, Chicago IL 60637, USA
> Tel: (773) 702-7513   |   Fax: (773) 834-2081
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle