How to get largest n values in c++ vector (Eigen)?

Question

I have a quite large Eigen matrix in c++, but now I want in each column only 6 values that are unequal 0. Herefore, I want to set all values to 0, except for the largest 6. I don't know how I can do that, can anyone help me?

Answer 1

//std
#include <iostream>
#include <vector>
#include <numeric>
//eigen
#include <Eigen/Dense>
 
using namespace std;
 
int main()
{
  Eigen::MatrixXf m(4,4);
  m <<  1, 2, 3, 4,
        5, 6, 7, 8,
        9,10,11,12,
       13,14,15,16;
       
  // Take the 2nd column
  auto col1 = m.col(1);

  // Generate a sequence of indices for each value in the column
  std::vector<size_t> idx(m.rows(), 0);
  std::iota(idx.begin(), idx.end(), 0);

  // Sort the indices according to their respective value
  std::sort(idx.begin(), idx.end(), [&col1](auto& lhv, auto& rhv){ return col1(lhv) < col1(rhv); });

  // Ignore the last 2 (so, the 2 biggest). Or 6 in your case.
  idx.resize(idx.size() - 2);

  // Set the rest to 0
  for(auto id: idx) {
      col1(id) = 0;
  }

  cout << m << endl; 
  // Output : 
  // 1  0  3  4
  // 5  0  7  8
  // 9  10 11 12
  // 13 14 15 16
}