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

On Tue, Nov 1, 2011 at 9:11 PM,

.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

A man, a plan, a cat, a ham, a yak, a yam, a hat, a canal ? Panama!