I made k-nearest neighbor program based on Boost.Accumulators plz see attached file. usage ublas::vector<double> v1(3),v2(3),v3(3), v0(3); v1(0)=3; v1(1)=1; v1(2)=2; v2(0)=1; v2(1)=20; v2(2)=0; v3(0)=1; v3(1)=1; v3(2)=3; v0=ublas::zero_vector<double>(3); struct norm_2_diff_functor { double operator ()(const boost::numeric::ublas::vector<double> &e1 , const boost::numeric::ublas::vector<double> &e2) { return(boost::numeric::ublas::norm_2(e1-e2) ); } }; accumulators::accumulator_set< ublas::vector<double> , accumulators::stats< accumulators::tag::k_nearest_neighbor > > acc( accumulators::k_nearest_neighbor_distance = norm_2_diff_functor() ,accumulators::sample =v0 ); acc(v1); acc(v2); acc(v3); typedef ublas::vector<double> float_type; typedef double distance_type; typedef std::vector<float_type> array_type; typedef boost::iterator_range<typename array_type::iterator> array_range_type; typedef std::pair<distance_type, typename array_type::iterator> dist_it_type; typedef std::vector< dist_it_type > dist_it_array_type; typedef boost::iterator_range<typename dist_it_array_type::iterator> dist_it_array_range_type; typedef std::pair< dist_it_array_range_type , array_range_type > result_type; result_type result; result =accumulators::k_nearest_neighbor( acc ,accumulators::k_nearest_neighbor_k=1 ,accumulators::k_nearest_neighbor_target=v0 ); std::cout << result.first[0].first << " " << *result.first[0].second << std::endl; result =accumulators::k_nearest_neighbor( acc ,accumulators::k_nearest_neighbor_k=3 ,accumulators::k_nearest_neighbor_target=v0 ); std::cout << result.first[0].first << " " << result.first[1].first << " " << result.first[2].first;