[Rcpp-devel] Multiplication of ComplexVector?

[Romain Francois]

Le 17/08/10 03:24, Dirk Eddelbuettel a écrit :
>
> On 16 August 2010 at 19:43, Dirk Eddelbuettel wrote:
> | Hi Christian,
> |
> | Thanks for your interest in Rcpp, and for posting here.
> |
> | On 16 August 2010 at 17:19, Christian Gunning wrote:
> | | Dear list,
> | |
> | | I'm trying to use the ComplexVector class, and I'm having difficulty
> | | with multiplication. I sugar multiplication, element-by-element
> | | multiplication (no * operator for either), and using temporary
> | | Rcomplex variables (don't really understand what I'm doing here).
> |
> | Well, NumericVector et al aren't really made for all possible math ops done
> | on vectors, real or complex. They are first and foremost storage types. But
> | for the math we do have Armadillo and RcppArmadillo. So in the short run, I
> | would suggest the following two-step procedure:
> |
> |    i)   have a good look at Armadillo (http://arma.sf.net) and use the cx_mat
> |         or cx_vec types for mulitplication; write a small test program to see
> |         that everything works
> |
> |    ii)  use RcppArmadillo to from R via Rcpp / RcppArmadillo to this
> |         functionality
> |
> | | I was able to successfully import get("*") as an Rcpp::Function and do
> | | the multiplication through R. Is this the best option for now?
> |
> | I am sure we can do better than this but I am not sure all pieces are in
> | place just yet.
>
> Looks like I wasn't all that far off.  This builds:
>
>    library(inline)
>
>    code<- '
>        ComplexVector y1(i), y2(ii);
>        //arma::cx_vec a1(y1.begin(), y1.size(), false);
>        //arma::cx_vec a2(y2.begin(), y2.size(), false);
>        arma::cx_vec a1(y1.size());
>        arma::cx_vec a2(y2.size());
>        arma::cx_vec a3 = a1 * a2;
>        arma::cx_vec a4 = a1 + a2;
>        List z = List::create(a1, a2, a3, a4);
>        return z;
>        '
>
>    fun<- cxxfunction(signature(i="compex", ii="comples"), code, plugin="RcppArmadillo")
>
> The only trouble is that nobody has written the corresponding 'glue' code to
> make
>
>        arma::cx_vec a1(y1.begin(), y1.size(), false);

cx_mat holds an array of std::complex<double>, not an array of Rcomplex. 
so the compiler is unhappy.

However, Rcomplex and std::complex<double> have the same layout in 
memory, so you are one reinterpret_cast away :


require( inline )

code<- '
     ComplexVector y1(i), y2(ii);
     arma::cx_vec a1( reinterpret_cast< std::complex<double>* 
 >(y1.begin()), y1.size(), false);
     arma::cx_vec a2( reinterpret_cast< std::complex<double>* 
 >(y2.begin()), y2.size(), false);
     arma::cx_vec a3 = a1 % a2; // element-wise multiplication in armadillo
     arma::cx_vec a4 = a1 + a2;
     List z = List::create(a1, a2, a3, a4);
     return z;
     '
fun<- cxxfunction(signature(i="complex", ii="complex"), code, 
plugin="RcppArmadillo")

fun( 1:10*(1+1i), (1:10)*2+2i )

> happen: create an Armadillo complex vector from an Rcpp::ComplexVector.  We
> can init by scalar size, what you'd need to insert for now is a simply (and
> very pedestrian) copy-loop.
>
> Hth, Dirk

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Romain Francois
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