[Rcpp-devel] Pass matrix by copy

[Kevin Ushey]

This is by design; we pass objects by reference for efficiency. Use
`Rcpp::clone()` to explicitly duplicate objects.

On Tue, Apr 14, 2015 at 10:15 AM, <soeren.vogel at posteo.ch> wrote:

> Hello,
>
> I have just noticed that my small Rcpp function changes the original
> matrix on the R side -- why and how to pass by copy?
>
> Thanks
> Sören
>
> <source c++>
> #include <Rcpp.h>
> using namespace Rcpp;
>
> // [[Rcpp::export]]
> NumericMatrix test_min( NumericMatrix M ) {
>         int n = M.nrow();
>         for ( int i = 0; i < n; ++i ) {
>                 for ( int j = 0; j < n; ++j ) {
>                         M(i, j) = M(i, j) * 2.0;
>                 }
>         }
>         return M;
> }
> </source>
>
> <source R>
> library( 'Rcpp' )
> mat <- matrix(runif(4), nrow=2)
> sourceCpp('min.cpp')
> test_min(mat) # mat * 2
> test_min(mat) # mat * 2 * 2
> test_min(mat) #...
> mat
> </source>
>
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