Array indexing in STAN or a where statement?

[Jason Tilley]

I'm trying to calculate some parameters in my code. In this case I would like the mean of a parameter when it belongs to a particular group. Here's an example:

for (l in 1:G) // for each group

  group_mu[g] <- mean(mu[group==g])); 

Is there a way to do this in STAN? 

Thanks

[Jonah Gabry]

Even with the new indexing functionality it's not possible to do R-style logical subverting like that. I think the best way to do it is to pass in integer arrays of the start and end positions of the group and use mu[start[g]:end[g]].

That assumes the values are ordered by group. If they're not then you can pass in an array of integer arrays containing the indices corresponding to each group and then use mu[idx[g]], where idx[g] is the array of indices for group g.

Hope that helps,

Jonah

[Jonah Gabry]

That should say logical subsetting not subverting

[Jason Tilley]

Thanks, that is very helpful.