Question

I am trying to figure out a way to check for a undefined value of a slope in which case it would be vertical. I have tried using NULL but that doesn’t seem to work.

double Point::Slope(Point &p2)
{
   double slop = 0;
   slop = (y - p2.y) / (x - p2.x);

   if (slop == NULL)
   {
      slop = 10e100;
   }

   return slop;   
}

Answer 1

If you mean nan (‘not a number’) with “undefined”, you should avoid computing one in the first place, i.e. by checking that the denominator of a ‘/’ operation is not zero. Second, you can always check for nan by

#include <cmath>
bool std::isnan(x);   // since C++11
bool isnan(x);        // pre C++11, from the C math library, defined as macro

see the man pages, or cppreference.