Armadillo sparse matrix get the index of an element

#include <iostream>#include <armadillo>using namespace std;using namespace arma;typedef sp_fmat::row_iterator it1_t;int main(int argc, char** argv){int dim = 10;sp_fmat a(dim,dim);sp_fmat b(dim,dim);for(size_t i = 0; i < dim; i++)  a(i,i) = i*8;a(2,3) = 3;b = b -a;cout<<b<<endl;cout<<a<<endl;for(it1_t it1 = a.begin_row(1); it1 != a.end_row(2);it1++)  cout<<*it1<<" "<<it1.row()<<" "<<it1.col()<<endl;cout<<b.n_rows<<" "<<b.n_cols<<endl; return 0;}