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?

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Answer 1

Depending on the platform your code needs to run on the log might be very expensive.

Since the amount of numbers that are 10^n (where n is natural) is very small, it might be faster to just use a hardcoded lookup table.

(Ugly pseudo code follows:)

bool isPowerOfTen( int16 x )
{
  if( x == 10       // n=1
    || x == 100     // n=2
    || x == 1000    // n=3
    || x == 10000 ) // n=4
  return true;

  return false;
}

This covers the whole int16 range and if that is all you need might be a lot faster. (Depending on the platform.)