[Rcpp-devel] can one modify array in R memory from C++ without copying it?
Chris, i set a tiny matrix mm<-matrix(1:10, nr=2), cut your code pasted it into R after i run myfun(mm), i still have exactly the same matrix when i print mm as before, not a doubled matrix The function myfun did return a doubled matrix, but original matrix mm stayed the same, so it still appears that original R memory was not changed from C++ I maybe missing something very obvious, i don't understand how it's possible that the same code seems to give you a different result, just reporting what i personally observe when i go exactly through the motions you suggested
On Wed, Nov 2, 2011 at 9:53 PM, Christian Gunning <xian at unm.edu> wrote:
On Wed, Nov 2, 2011 at 4:00 AM, <rcpp-devel-request at r-forge.wu-wien.ac.at> wrote:
1. in my previous post i used exactly that same constructor you are talking about as you can see from the code i posted
Sorry about that, completely missed it... Using what's below, this should work fine.
3. i am not 100% sure but this toy test that i did seem to indicate to me that a copy is done regardless by NumericalMatrix. I need to see the code for NumericMatrix to be absolutely sure, but i cannot explain things otherwise
This is a little tricky, I almost missed it myself (again).
From the Quickref:
"If R/C++ types match, use pointer to x."
Use a non-integer input to your function and see if that works. Take
a look at this example:
src <- '
Rcpp::NumericMatrix r_m(mem);
int nn = r_m.nrow() * r_m.ncol();
// int nn = r_m.size();
// above doesnt work with NumericMatrix, .size() is ambiguous??
for (int ii =0; ii < nn; ii++) {
r_m[ii] = r_m[ii] + r_m[ii];
}
return r_m;
'
require(inline)
require(RcppArmadillo)
myfun = cxxfunction(signature(mem='numeric'), src, plugin='Rcpp')
ll <<- mm <<- matrix((1:10)/10, nrow=2)
llint <<- mmint <<- matrix(1:10, nrow=2)
myfun(mm)
## should be false, since myfun modifies mm directly?
print(all.equal(mm, ll))
print(all.equal(mmint, llint))
hth,
-Christian
--
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