eigen/doc/examples/Tutorial_ReductionsVisitorsBroadcasting_broadcast_1nn.cpp~

#include <iostream>
#include <Eigen/Dense>

using namespace std;
using namespace Eigen;

int main()
{
  Eigen::MatrixXf m(2,4);
  Eigen::VectorXf v(2);
  
  m << 1, 2, 6, 9,
       3, 1, 7, 2;
       
  v << 2,
       3;

  MatrixXf::Index index;
  // find nearest neighbour
  (m.colwise() - v).colwise().squaredNorm().minCoeff(&index);

  cout << "Nearest neighbour is column " << index << ":" << endl;
  cout << m.col(index) << endl;
}