c++ - Evaluating the intersection point with a vertical line involved -

here function returning intersection point between 2 lines defined points p0, p1 , p2, p3:

#include "std_lib_facilities.h" #include <iostream>  struct point{     point(int xx, int yy): x(xx), y(yy) { } int x; int y; };  // find intersection of 2 lines point intersectpoint(point& p0, point& p1, point& p2, point& p3){     // line formed p0p1     double dx1 = p1.x - p0.x;     double dy1 = p1.y - p0.y;     double m1 = dy1 / dx1;     double c1 = p1.y - m1 * p1.x;     cout <<"m1: "<< m1 <<'\n';      // line formed p2p3     double dx2 = p3.x - p2.x;     double dy2 = p3.y - p2.y;     double m2 = dy2 / dx2;     double c2 = p3.y - m1 * p3.x;     cout <<"m2: "<< m2 <<'\n';     // find intersection point     double epsilon = 1e-6;     // (-1,-1) represents no intersection      if (abs(m1 - m2) < epsilon) return point(-1,-1);     else{        double interx = (c2 - c1) / (m1 - m2);        double intery = m1 * interx + c1;        cout <<"(interx, intery): "<< interx <<", "<< intery <<'\n';        return point(interx, intery);    } } //------------------------------------------------------------------------------------------------ int main(){      point verticalstart(100, 100);     point verticalend(100,300);      point horizontalstart(50,200);     point horizontalend(150,200);      point intersection = intersectpoint(horizontalstart ,horizontalend, verticalstart, verticalend);     cout <<"returned value: \n";      cout << "( "<< intersection.x <<", "<< intersection.y <<" )\n";       getchar();     return 0; } 

in case of intersection between vertical , horizontal output:

m1: 0 m2: 1.#inf (interx, intery): -0, 200 returned value: ( 0, 200 ) 

question:

how handle case, function returns valid points of intersection?

note: code compiled on mvs2010.

i suggest not use slope-intercept representation, i.e.: y = a*x + b vector-parametric representation:

pa = (p1-p0)*t + p0 pb = (p3-p2)*u + p2 

in way lines x = k have representation.

solve system variables t , u (2 variables, 2 equations, 2 coordinates x , y) pa = pb (the intersection point) , back-substitute in 1 of these equations intersection point.