How to perform basic operations in rstan?

Interfaces RStan
4

When you call counts[i] %/% 10 you’re still not using a valid signature, unfortunately the %/% operator is not vectorised, so you’ll have to loop over each integer in counts[i]:

model {
  target += dirichlet_lpdf(theta | alpha);

  for(i in 1:N) {
    int counts_normalised[G];
    for(g in 1:G) {
      counts_normalised[g] = counts[i, g] %/% 10;
    }
    target += multinomial_lpmf(counts_normalised | theta);
  }
}

Note that I’ve also moved the dirichlet_lpdf outside of the loop, as including it within the loop is essentially weighting the posterior N times.

Just to also quickly clarify, the key thing here is that T is constant throughout all iterations (in this model at least), so there is no benefit to performing the division in the transformed parameters/parameters blocks.

If this more of a dummy model, and you’re intending to construct T using something declared in the parameters block, then you’re likely to run into a different set of issues. Taking a continuous parameter and discretising it to return an integer would result in discontinuities in the respective parameters, and result in seriously poor sampling performance