# C++ Program to use above below primitive to test whether two lines intersect

C++Server Side ProgrammingProgramming

Here is a C++ program to use above below primitive to test whether two lines intersect. It can be used to test whether a line intersects a line segment. It does if and only if one endpoint of the segment is to the left of the line and the other is to the right.

## Algorithm

Begin
For generating equation of the first line, generate random numbers for coefficient of x and y by using rand at every time of compilation.
For generating equation of the second line, generate random numbers for coefficient of x and y by using rand at every time of compilation.
Find the segment of line 1 as Y1.
if (Y1 < 0)
Find the segment of line 2
if (Y2 >= 0)
print they are intersecting.
else if (Y2 < 0)
print they are not intersecting.
else if (Y1 >0)
Find the segment of line 2
if (Y2 <= 0)
print they are intersecting.
else if (Y2 >0)
print they are not intersecting.
End.

## Example Code

Live Demo

#include<time.h>
#include<stdlib.h>
#include<iostream>
#include<math.h>

using namespace std;
const int L = 2;
const int H= 20;
int main(int argc, char **argv) {
time_t s;
time(&s);
srand((unsigned int) s);

int x1, x2, y1, y2;
x1 = rand() % (H - L+ 1) + L;
x2 = rand() % (H - L+ 1) + L;
y1 = rand() % (H- L+ 1) + L;
y2 = rand() % (H - L + 1) + L;

cout << "The Equation of the 1st line is : (" << (y2 - y1) << ")x+(" << (x1 - x2) << ")y+(" << (x2 * y1 - x1 * y2) << ") = 0\n";

int p1, p2, q1, q2;
p1 = rand() % (H- L+ 1) + L;
p2 = rand() % (H- L + 1) + L;
q1 = rand() % (H - L + 1) + L;
q2 = rand() % (H - L + 1) + L;

cout << "The Equation of the 2nd line is : (" << (q2 - q1) << ")x+(" << (p1 - p2) << ")y+(" << (p2 * q1 - p1 * q2) << ") = 0\n";

int Y1 = (y2 - y1) * p1 + (x1 - x2) * q1 + (x2 * y1 - x1 * y2); //Y1 segment
if (Y1 < 0) {
int Y2 = (y2 - y1) * p2 + (x1 - x2) * q2 + (x2 * y1 - x1 * y2); //Y2 segment
if (Y2 >= 0)
cout << "Lines are intersecting";
else if (Y2 < 0)
cout << "Lines are not intersecting";
} else if (Y1 >0) {
int Y2 = (y2 - y1) * p2 + (x1 - x2) * q2 + (x2 * y1 - x1 * y2);
if (Y2 <= 0)
cout << "Lines are intersecting";
else if (Y2 >0)
cout << "Lines are not intersecting";
} else
cout << "The point lies on the line";
return 0;
}

## Output

The Equation of the 1st line is : (-3)x+(2)y+(1) = 0
The Equation of the 2nd line is : (-5)x+(-5)y+(130) = 0
Lines are intersecting

The Equation of the 1st line is : (-1)x+(7)y+(-15) = 0
The Equation of the 2nd line is : (-4)x+(4)y+(-8) = 0
Lines are not intersecting