Find exponent and mantissa of a double to the base 10

Question

I need to break numerical values of type double into: exponent and mantissa. I found that I can do this using the frexp() function from math.h. However, this function assumes the base to be 2.

Is there some way by which I may find the exponent and mantissa to the base 10 -- such that both the returned mantissa and exponent are of the type integer.

#include <cstdio>
#include <cmath>

int main()
{
   int e;
   const double x = 1024;
   const double fraction = frexp(x, &e);
   std::printf("x = %.2lf = %.2lf * 2^%d\n", x, fraction, e);
}

The compiler which I have is: gcc (Ubuntu/Linaro 4.6.4-6ubuntu2) 4.6.4

Answer 1

Basic logarithms will help. The idea is to use std::log10 to get the exponent and then divide the original number by 10^exp to get the mantissa.

double frexp10(double arg, int * exp)
{
   *exp = (arg == 0) ? 0 : 1 + (int)std::floor(std::log10(std::fabs(arg) ) );
   return arg * std::pow(10 , -(*exp));    
}