c++ Sort Vector based on distance to external Point

Question

I was wondering if there is a nice way to sort a vector based on some external value. For instance, I have a vector of k nearest neighbors to a point. I want to sort this vector based on their distance to the query point. The query point is not included in the results, and std::sort allows you to define a comparison function for two elements in the vector (rather than each element vs a fixed point). Is there any pre-built method to do this sort of sort? Or would I need to build my own custom sorting algorithm? Thanks

Answer 1

If you have some class Point and the point you want to sort against is

Point p

Also assume that points is defined as

std::vector<Point> points;

and you had some function dist that took 2 Point and returned the distance (e.g. Euclidean distance) like

double dist(const Point& lhs, const Point& rhs)
{
    // compute Euclidean distance or whatever
}

Then you can use std::sort with a lambda function

std::sort(begin(points),
          end(points),
          [p](const Point& lhs, const Point& rhs){ return dist(p, lhs) < dist(p, rhs); });

Edit
If you do not have C++11 access, you need to define a functor

struct DistanceFunc
{
    DistanceFunc(const Point& _p) : p(_p) {}

    bool operator()(const Point& lhs, const Point& rhs) const
    {
        return dist(p, lhs) < dist(p, rhs);
    }

private:
    Point p;
};

Then you can sort in a similar way

std::sort(points.begin(), points.end(), DistanceFunc(p));