Compare double to zero using epsilon

Question

Today, I was looking through some C++ code (written by somebody else) and found this section:

double someValue = ...
if (someValue <  std::numeric_limits&         


        

Answer 1

The difference between X and the next value of X varies according to X.
epsilon() is only the difference between 1 and the next value of 1.
The difference between 0 and the next value of 0 is not epsilon().

Instead you can use std::nextafter to compare a double value with 0 as the following:

bool same(double a, double b)
{
  return std::nextafter(a, std::numeric_limits::lowest()) <= b
    && std::nextafter(a, std::numeric_limits::max()) >= b;
}

double someValue = ...
if (same (someValue, 0.0)) {
  someValue = 0.0;
}