Question

I have std::set<std::pair<float,float>> which represents points on map ( 2d , x and y value) and I have one point with values x1 and y1. How to sort set in ascending order by distance from point ( x1,y1) ?

Answer 1

std::set is an ordered container, and ordering happens upon insertion, depending on a sorting criteria which can be specified with a second template argument. So use a set with a predicate which returns true or false based on the distance to the reference point.

struct DistanceCompare
{
  DistanceCompare(const std::pair<float,float>& point) : point_(point) {} 
  bool operator()(const std::pair<float,float>& lhs, 
                  const std::pair<float,float>& rhs) const
  {
    return distance2(lhs) < distance2(rhs);
  };

 private:
  float distance2(const std::pair<float,float>& point) const
  {
    // calculate distance squared between point and point_
    const float x = point.first  - point_.first;
    const float y = point.second - point_.second;
    return x*x + y*y;

  }
  std::pair<float, float> point_;
};

....
std::pair<float,float> refPoint = ....;
DistanceCompare comp(refPoint);
std::set<std::pair<float, float>, DistanceCompare> pointSet(comp);

It is enough to compare the distance squared, thus avoiding calls to std::sqrt.