Eigen fill up Sparse RowMajor matrix with triplets

Question

I am trying to fill up a Sparse RowMajor matrix. Following the guide I was using the triplets method:

Eigen::SparseMatrix<double, Eigen::RowMajor> data_matrix(rows, cols);
....

void get_data(const char *dir_name, std::vector<T> tripletList, Eigen::SparseMatrix<double, Eigen::RowMajor> data_matrix) {
uint64_t row_iter = 0;

for (std::string file_n : sorted_files) {
   ...
   if (words.find(word_freq[0]) != words.end())
       tripletList.push_back(T(row_iter, words[word_freq[0]], std::stoi(word_freq[1])));
   }

   row_iter++;

}

data_matrix.setFromTriplets(tripletList.begin(), tripletList.end());

However, this approach generates an empty matrix. I couldn't find examples of filling RowMajor matrices with the triplet list method, is it not possible?

Answer 1

Works for me, here is a selfcontained example:

#include <iostream>
#include <Eigen/SparseCore>
#include <vector>
using namespace Eigen;
using namespace std;

int main()
{
  int m = 3, n = 7;
  SparseMatrix<double, RowMajor> M(m,n);
  typedef Triplet<double,int> T;
  vector<T> entries;
  for(int k=1; k<=9;++k)
    entries.push_back( T(internal::random<int>(0,m-1), internal::random<int>(0,n-1), k) );
  M.setFromTriplets(entries.begin(), entries.end());
  cout << MatrixXd(M) << "\n";
}

that produces:

 1  0  0  8  4  0  0
 0  3  0  6  0  0  0
16  0  0  2  0  0  5

EDIT:

So the problem is in the structure of your code, I see that get_data gets the triplet list and the sparse matrix by value, whereas they are modified by this function, so you most likely want to pass them by reference.