arma::vec
dir_generator::sample(const arma::vec &alpha)
{
arma::vec x = arma::zeros<arma::vec>( alpha.n_elem );
for(int i = 0; i < alpha.n_elem; i++)
{
double runif = m_generator( ) / ( m_generator.max( ) - m_generator.min( ) + 1.0 );
x[ i ] = gamma_cdf_inv( runif, alpha[ i ], 1.0 );
}
return x / sum( x );
}
double beta_that_forces_requested_median(double alpha,double median)
/*
If Gamma(alpha,1) has median K
Then Gamma(alpha,beta) has median K/beta.
We want median = K/beta, so we put beta = K/median where
K = Median(Gamma(alpha,1))
*/
{
double k = gamma_cdf_inv(0.5,alpha,1.0);
double result = k / median;
{
/*
double check = gamma_cdf_inv(0.5,alpha,result);
printf("alpha=%g, goal_median=%g, chosen_beta=%g,actual_median=%g\n",
alpha,median,result,check
);
wait_for_key();
*/
}
return(result);
}
