Draw a single sample from a Dirichlet

Base R does not provide an rdirichlet(). We use the gamma-normalisation method also used by the Stan model: draw an independent Gamma(alpha_i, 1) per bin and rescale by the segment sum. Bins with alpha == 0 stay at zero so structural zeros (e.g. the t = 0 generation-time bin) are preserved.

Usage

rdirichlet(alpha)

Arguments

alpha

A non-negative numeric vector of concentration parameters.

Value

A numeric vector the same length as alpha, summing to 1 over the positive-alpha entries.

References

Stan discourse, "Ragged array of simplexes", https://discourse.mc-stan.org/t/ragged-array-of-simplexes/1382/21.