// distance between two points using a function
// and scaling to avoid overflow
//
#include <stdio.h>
#include <math.h>

// Pythagorean formula, sqrt( a*a + b*b);
//
double pythag( double a, double b)
{
  double c, scale;

  a = fabs(a);  b = fabs(b);

  // a and b (the function arguments) are local variables here,
  // same as c and scale, except a and b are initialized by the caller

  if( a > b) scale = a; else scale = b;

  // or use conditional operator (see section 3.3):
  //
  // scale = (a > b) ? a : b;

  if( scale == 0) return 0; // avoid divide by zero

  a /= scale;  b /= scale;

  c = sqrt( a*a + b*b);  c *= scale;
 
  return c;
}

int main(void)
{
   double x1=1e200, y1=5e200, x2=4e200, y2=7e200, distance;

   distance = pythag( x2-x1, y2-y1);

   printf("The distance between the two points is %g\n",distance);

   return 0;
}

/* output:

The distance between the two points is 3.60555e+200

*/