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

How about a code like this:

#include 
#define MAX 20
bool check_pow10(double num)
{
   char arr[MAX];
   sprintf(arr,"%lf",num);
   char* ptr = arr;
   bool isFirstOne = true;

   while (*ptr)
   {
     switch (*ptr++)
     {
       case '1':
                if (isFirstOne)
                   isFirstOne = false;
                else
                   return false;
                break;
       case '0':
                break;
       case '.':
                break;
       default:
                return false;
     }
   }

 return true;
}

int main()
{
  double number;
  scanf("%lf",&number);
  printf("isPower10: %s\n",check_pow10(number)?"yes":"no");
}

That would not work for negative powers of 10 though.
EDIT: works for negative powers also.