text: (This article was first published on Rcpp Gallery, and kindly contributed to R-bloggers)
  A StackOverflow question asked how convert from
  arma::umat to
  arma::mat. The former is format used for
  find and other logical indexing.
  For the particular example at hand, a call to the
  conv_to converter provided the solution. We rewrite the answer here using the newer format offered by Rcpp attributes and its
  sourceCpp() function.
  #include <RcppArmadillo.h> // [[Rcpp::depends(RcppArmadillo)]] using namespace Rcpp ; // [[Rcpp::export]] arma::mat matrixSubset(arma::mat M) { // logical conditionL where is transpose larger? arma::umat a = trans(M) > M; arma::mat N = arma::conv_to<arma::mat>::from(a); return N; }
  M <- matrix(1:9, 3, 3) M
  [,1] [,2] [,3] [1,] 1 4 7 [2,] 2 5 8 [3,] 3 6 9
  matrixSubset(M)
  [,1] [,2] [,3] [1,] 0 0 0 [2,] 1 0 0 [3,] 1 1 0
  This generalizes to other uses, and the vector or matrix of unsigned ints can be used inside the
  elem() member function. Here were we return all values of
  M * M' that are greater or equal to 100.
  // [[Rcpp::export]] arma::vec matrixSubset2(arma::mat M) { arma::mat Z = M * M.t(); arma::vec v = Z.elem( arma::find( Z >= 100 ) ); return v; }
  The result:
  matrixSubset2(M)
  [,1] [1,] 108 [2,] 108 [3,] 126...