double log10(double);

base-10 logarithm of a number

#include <iostream> /* including standard library */
#include <cmath> /* including math library */
 
using namespace std;
int main ( void )
{
    double x;
    x = log10(42000);
 
    //cout.setf(ios::fixed, ios::floatfield);
    cout.precision(8);
 
    cout << "The log10 of 42000 is " << x << endl;
    return 0;
}

   The log10 of 42000 is 4.6232493