Sum of the all of the outer product (for Rcpp)

sumouterprod.cpp

#include "RcppArmadillo.h"
// [[Rcpp::depends(RcppArmadillo)]]
using namespace Rcpp;
using namespace arma;

// [[Rcpp::export]]
double sumouterprod(const arma::field<arma::vec> & V){
  int K = V.n_rows;
  arma::vec tout = V(0);
  for(int j=1; j<K; j++){
    arma::vec V1 = V(j);
    tout = vectorise(tout * V1.t());
  }
  return sum(tout);
}