boost::geometry: nearest neighbors using a circle

Question

I am using the Rtree implementation of boost::geometry to store (lots of) 2D points. Now I need to do distance-based nearest neigbors queries.

However, the manual on

Answer 1

The last example in the documented "User-defined queries" shows how to use a lambda in the predicate. This lambda can bind other variables in the scope, for instance, the point whose neighbors you are looking for.

Here is a small example. The example looks for points that are closer to (5, 5) than 2 units, for a collection of points that lie on the straight y = x. It should be easy to modify in order to check first if the sought point is in the R-tree, or get it directly out of the R-tree.

#include 

#include 
#include 
#include 


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

typedef bg::model::point point;
typedef std::pair value;

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

    // create some values
    for ( unsigned i = 0 ; i < 10 ; ++i )
    {
        point p = point(i, i);
        rtree.insert(std::make_pair(p, i));
    }

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

    // print returned values
    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;
}

Compile and run with Boost installed via Macports on OS X:

$ c++ -std=c++11 -I/opt/local/include -L/opt/local/lib main.cpp -o geom && ./geom
Select point: 4
x: 4, y: 4
Select point: 5
x: 5, y: 5
Select point: 6
x: 6, y: 6