[Rcpp-devel] Integer matrices in Rcpp and Armadillo

[Simon Zehnder]

Dear Dirk, Kevin and Romain,

this has been a very interesting discussion to me and made the things clear. Thank you very much for your contributions!

Best

Simon

On Jun 8, 2013, at 10:19 PM, Dirk Eddelbuettel <edd at debian.org> wrote:

> 
> On 8 June 2013 at 12:48, Kevin Ushey wrote:
> | I'm pretty the sure the Rcpp / RcppArmadillo way is to reuse memory whenever
> | the types match between the as'ed containers, and copy if type coercion makes
> | sense. Ie, an R integer vector --> an Rcpp IntegerVector would reuse the same
> | memory, while an R integer vector --> an Rcpp NumericVector would require a
> | copy and a coercion from integer to numeric. Similar things will apply between
> | R and RcppArmadillo. Just remember that with R, numeric <-> double, integer <->
> | signed int.
> | 
> | A simple example to illustrate (also shows how you can discover these things
> | yourself):
> | 
> | ---
> | 
> | #include <Rcpp.h>
> | 
> | // [[Rcpp::export]]
> | Rcpp::NumericVector test_numeric(Rcpp::NumericVector x) {
> |   
> |   x = x + 1;
> |   return x;
> |   
> | }
> | 
> | // [[Rcpp::export]]
> | Rcpp::IntegerVector test_integer(Rcpp::IntegerVector x) {
> |   x = x + 1;
> |   return x;
> | }
> | 
> | /*** R
> | x <- c(1, 2, 3)
> | 
> | test_numeric(x) ## returns c(2, 3, 4)
> | print(x) ## x is now c(2, 3, 4)
> | 
> | test_integer(x) ## returns c(3, 4, 5)
> | print(x) ## x is still c(2, 3, 4)
> | */
> | 
> | ---
> 
> This is a known case and documented; I use something similar in every
> (longer) talk about Rcpp when we motivate clone() as well. The long and short
> of it is that a change in _type_ is equivalent to a copy and hence use of
> clone(). 
> 
> | Since this thread has become rather long, I suggest generating your own minimal
> | examples whenever you need to discover / be certain whether you're reusing
> | memory or forcing a copy.
> 
> Yes, please. 
> 
> And editing out unused material from a repost is polite too.
> 
> Dirk
> 
> -- 
> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com