Takes a vector of counts for 1:n collections, and returns a Dirichlet random variable generated by adding the prior to each collection value, and simulating an alpha from a gamma distribution with this shape parameter.
dirch_from_counts(C, lambda)
a vector giving counts of categories
priors for the categories
The categories are labeled in C from 1 up to n. n is the length of lambda
,
which is a vector of priors. Note that all elements of lambda
must be strictly greater than 0.