Generate zipf distribution in C++

Standard library and Gnu scientific library GSL, do not provide a way to get the probability mass function of the zipf distribution directly. By looking around from wiki and stackoverflow post, implemented a couple lines of code for zipf pmf:

#include 

 

// when one doesn't know the population size

// assume the population is infinity

// then the generalized harmonic number

// can be approximated as zeta function

double get_pmf_zipf(int rank, double skew) {

 // use zeta function, one can approximate the zipf pmf

 return pow(rank, -1*skew)/gsl_sf_zeta(skew);

}

 

// when one knows the population

// we can calculate the generalized harmonic number

double get_pmf_zipf(int rank, int N, double skew) {

 double base = 0;

 for (int k = 1; k <= N; k++) base += pow(k, -1*skew);

 return pow(rank, -1*skew)/base;

}