[Rcpp-devel] can one modify array in R memory from C++ without copying it?

[Christian Gunning]

On Tue, Nov 1, 2011 at 9:11 PM,
<rcpp-devel-request at r-forge.wu-wien.ac.at> wrote:
>
>> .Call("modify", m)
>> m
>     [,1] [,2] [,3] [,4] [,5]
> [1,]    1    3    5    7    9
> [2,]    2    4    6    8   10
>
>
> It didn't segfault, but the memory in R process didn't change as you see,
> although the matrix in armadillo code doubled. Which means that
> NumericMatrix copied memory from R process and passed a pointer to
> armadillo.

3 quick points:

1:  For the NumericMatrix->arma, you can use an advanced constructor
to get the behavior that you desire.
http://arma.sourceforge.net/docs.html#Mat -- you'll want copy_aux_mem
= false.

2:  Are you actually doing matrix math?  If you're just doing simple
element-by-element arithmetic, you might get just as good performance
with a simple loop or iterator.  You might try this first to
understand the R/C++ process, and *then* move to using Armadillo :)

3:  For completeness, note that "Rcpp::NumericMatrix r_m(clone(mem));"
*forces* a copy, thus restoring R's "no side-effects" semantics.

-Christian



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