How to Sort Vector of Points Based on a Y-Axis?

Question

I have a group of coordinates for example :

10,40; 9,27; 5,68; 7,55; 8,15;

How do I sort those coordinates without losing the correct X-Axis of the sorted Y-Axis.

From the example above I want to sort the coordinates so the correct output will be:

8,15; 9,27; 10,40; 7,55; 5,68.

Any suggestion will be greatly appreciated. Thank you.

Answer 1

Documentation for std::sort

#include "opencv2/core/core.hpp"
#include <algorithm>    // std::sort

// This defines a binary predicate that, 
// taking two values of the same type of those 
// contained in the list, returns true if the first 
// argument goes before the second argument
struct myclass {
    bool operator() (cv::Point pt1, cv::Point pt2) { return (pt1.y < pt2.y);}
} myobject;

int main () {
    // input data
    std::vector<cv::Point> pts(5);
    pts[0] = Point(10,40);
    pts[1] = Point(9,27);
    pts[2] = Point(5,68);
    pts[3] = Point(7,55);
    pts[4] = Point(8,15);

    // sort vector using myobject as comparator
    std::sort(pts.begin(), pts.end(), myobject);
}

Answer 2

You need to specify how exactly you are storing your group of coordinates.

The easiest way is to store them as a new struct you create and apply a basic bubble sort algorithm over the top of it, using the Y value as your sort parameter. Then when you "swap" the position of the structs, the X & Y stay together.

struct Vector {
  float x;
  float y;
};

Answer 3

You could create a class that maps a coordinate, and if you are using STL as your Vector, you can use the sort method to sort your whole vector based on the Y coordinate.

Here and here are similar questions from stack.