Comparing floating point number to zero

Question

The C++ FAQ lite \"[29.17] Why doesn\'t my floating-point comparison work?\" recommends this equality test:

#include   /* for std::abs(double) *         


        

Answer 1

You can use std::nextafter with a fixed factor of the epsilon of a value like the following:

bool isNearlyEqual(double a, double b)
{
  int factor = /* a fixed factor of epsilon */;

  double min_a = a - (a - std::nextafter(a, std::numeric_limits::lowest())) * factor;
  double max_a = a + (std::nextafter(a, std::numeric_limits::max()) - a) * factor;

  return min_a <= b && max_a >= b;
}