C++: how can I test if a number is power of ten?

Question

I want to test if a number double x is an integer power of 10. I could perhaps use cmath\'s log10 and then test if x == (int) x?

<

Answer 1

bool power_of_ten(double x) {
   if(x < 1.0 || x > 10E15) {
      warning("IEEE754 doubles can only precisely represent powers "
              "of ten between 1 and 10E15, answer will be approximate.");
   }
   double exponent;
   // power of ten if log10 of absolute value has no fractional part
   return !modf(log10(fabs(x)), &exponent);
}