kNN with Boost::Geometry

main.cpp

#include <functional>
#include <iostream>
#include <random>

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point.hpp>
#include <boost/geometry/index/rtree.hpp>


namespace bg = boost::geometry;
namespace bgi = boost::geometry::index;

typedef bg::model::point<float, 2, bg::cs::cartesian> point;
typedef bg::model::box<point> box;
typedef std::pair<point, unsigned> value;

int main(int argc, char *argv[])
{
    bgi::rtree< value, bgi::quadratic<16> > rtree;

    // create some values
    std::default_random_engine generator;
    std::uniform_real_distribution<float> distribution(0, 10);
    auto next_rand = std::bind ( distribution, generator );
    float x, y;
    for ( unsigned i = 0 ; i < 100000 ; ++i )
    {
        x = next_rand();
        y = next_rand();
        point p = point(x, y);
        rtree.insert(std::make_pair(p, i));
    }

    // search for nearest neighbours
    std::vector<value> returned_values;
    point sought = point(5, 5);
    box bounding_box(point(5 - 2, 5 - 2), point(5 + 2, 5 + 2));
    rtree.query(
                bgi::within(bounding_box) &&
                bgi::satisfies([&](value const& v) {return bg::distance(v.first, sought) < 2;}),
                std::back_inserter(returned_values));

    // print returned values
    std::cout << returned_values.size() << std::endl;
    //value to_print_out;
    //for (size_t i = 0; i < returned_values.size(); i++) {
        //to_print_out = returned_values[i];
        //float x = to_print_out.first.get<0>();
        //float y = to_print_out.first.get<1>();
        //std::cout << "Select point: " << to_print_out.second << std::endl;
        //std::cout << "x: " << x << ", y: " << y << std::endl;
    //}

    return 0;
}

That's a nice example how one can find points within a circle combining the predicates!

But to be perfectly clear. The example doesn't present the kNN (k Nearest Neighbors) query but a spatial query. The kNN query returns some number of values nearest to the sought point. The spatial query returns ALL points for which some spatial predicate is met. Like in this case, all values within a circle centered at point(5, 5) are returned.