Function hypergeometric_lpmf() considers the case n=N as invalid

Developers
1

Hi, I’ve been having trouble with a hypergeometric model, and I narrowed the problem down to this:

The function hypergeometric_lpmf(int n | int N, int a, int b) stops when n=N claiming that it should be n<N strictly. However, the case of sampling n=N successes in N trials should be valid.

Reproducible Steps:

library(rstan)
code <- "
  functions {
   real log_dhyper(int n, int N, int a, int b) {
      real lp;
      lp = hypergeometric_lpmf(n | N, a, b);
      return lp;
    }
  }
"
expose_stan_functions(stanc(model_code = code))
log_dhyper(10, 10, 50, 50)
#> Error in log_dhyper(10, 10, 50, 50): Exception: hypergeometric_lpmf: Draws parameter is 10, but must be greater than 10  (in 'unknown file name' at line 5)

Created on 2018-10-17 by the reprex package (v0.2.1)

I opened an issue in GH. But then I saw that you are using the forums to report possible bugs. I’m sorry, I don’t know which venue is more convenient for you.

2

GitHub is great for issues. We’d rather have people ask here first if they’re unsure about whether what they’re reporting is an issue or a question. We’ll fix this one—thanks.

3

I’m wondering if this bug was solved? I updated to rstan 2.19.2, but the reproducible steps still output an error as:

Error in log_dhyper(10, 10, 50, 50) :
Exception: hypergeometric_lpmf: Draws parameter is 10, but must be greater than 10 (in ‘unknown file name’ at line 5)

4

From the github activity, I gather that the issue is still open unfortunately.

1 Like
5

Thanks for replying! For anyone who has the same issue, the way I dealt with this is using lchoose().

hypergeometric_lpmf(n | N, a, b) is the same as lchoose(a, n) + lchoose(b, N-n) - lchoose(a+b, N)

2 Likes
6

This bug has been fixed, so a working version of hypergeometric_lpmf for n=N will appear in the next release.

5 Likes