i_use_the_internet i_use_the_internet - 3 months ago 110
C++ Question

c++ Sorting 2D Points clockwise

i wrote a program to arrange points on a graph in clockwise manner from 12 o'clock such that, a vector containing these points is sorted in that order. I am using atan2 to get the angle from 12 o'clock and then making adjustments based on the quadrant. i am trying to figure out where the bug is coming from as it is not ordering them correctly. So given 4 random points like those in the photo, it should order then in the containing vector as P1,P2,P3,P4order
Here is my code:

//sort_points.cpp
#include <iostream>
#include <math.h>
#include <algorithm>
#include <vector>

using namespace std;

class Point
{
public:
double x;
double y;
Point(double xx, double yy) : x(xx), y(yy) {}
~Point();
inline friend ostream& operator<<(ostream& output, const Point& point)
{
output << "[" << point.x << ", " << point.y <<"]";
return output;
}
};


Point::~Point() {;}


/* get quadrant from 12 o'clock*/
int get_quadrant (const Point& p)
{
int result = 4; //origin

if (p.x > 0 && p.y > 0)
return 1;
else if(p.x < 0 && p.y > 0)
return 2;
else if(p.x < 0 && p.y < 0)
return 3;
//else 4th quadrant
return result;
}

double get_clockwise_angle(const Point& p)
{
double angle = 0.0;
int quadrant = get_quadrant(p);

/*making sure the quadrants are correct*/
cout << "Point: " << p << " is on the " << quadrant << " quadrant" << endl;

/*add the appropriate pi/2 value based on the quadrant. (one of 0, pi/2, pi, 3pi/2)*/
switch(quadrant)
{
case 1:
angle = atan2(p.x,p.y) * 180/M_PI;
break;
case 2:
angle = atan2(p.y, p.x)* 180/M_PI;
angle += M_PI/2;
break;
case 3:
angle = atan2(p.x,p.y)* 180/M_PI;
angle += M_PI;
break;
case 4:
angle = atan2(p.y, p.x)* 180/M_PI;
angle += 3*M_PI/2;
break;
}
return angle;
}
bool compare_points(const Point& a, const Point& b)
{
return (get_clockwise_angle(a) < get_clockwise_angle(b));
}
int main(int argc, char const *argv[])
{
std::vector <Point> points;
points.push_back( Point( 1, 3 ) );
points.push_back( Point( 2, 1 ) );
points.push_back( Point( -3, 2 ) );
points.push_back( Point( -1, -1 ) );

cout << "\nBefore sorting" << endl;
for (int i = 0; i < points.size(); ++i)
{
cout << points.at(i) << endl;
}

std::sort(points.begin(), points.end(),compare_points);

cout << "\nAfter sorting" << endl;
for (int i = 0; i < points.size(); ++i)
{
cout << points.at(i) << endl;
}
return 0;
}

Answer

You don't need the adjustment. atan2 will give you the angle from positive direction of x axis, counterclockwise in range of -PI to PI.

Firstly, to make the starting point positive direction of y axis, let me give parameter to atan2 as if negative direction of y axis is positive direction of x axis and positive direction of x axis is positive direction is y axis.

Then, this will make the angle counterclockwise, so negate the angle in order to reverse the order.

double get_clockwise_angle(const Point& p)
{   
    double angle = 0.0;
    int quadrant = get_quadrant(p);

    /*making sure the quadrants are correct*/
    cout << "Point: " << p << " is on the " << quadrant << " quadrant" << endl;

    /*calculate angle and return it*/
    angle = -atan2(p.x,-p.y);
    return angle;
}