Iterating over arma::mat and retrieving element locations

Looking at the armadillo source code, row_col_iterator provides row and column indices of each element. This works like the sparse matrix iterator, but doesn’t skip zeros. Adapting your code:

mat X(10,10,fill::randu);

mat::const_row_col_iterator it     = X.begin_row_col();
mat::const_row_col_iterator it_end = X.end_row_col();

for (; it != it_end; ++it) {
  cout << "val: " << (*it)    << endl;
  cout << "row: " << it.row() << endl;
  cout << "col: " << it.col() << endl;
}