#include <cmath>
    double log( double num );

The function log() returns the natural (base e) logarithm of num. There's a domain error if num is negative, a range error if num is zero. In order to calculate the logarithm of x to an arbitrary base b, you can use:

    double answer = log(x) / log(b);

C++ also provides the following overloaded forms:

    #include <cmath>
    float log( float num ); // same as logf() in C99
    long double log( long double num ); // same as logl() in C99

Related Topics: exp, log10, pow, sqrt