Hi,

I am wondering whether we could create an empty vector in Stan and add multiple elements to it based on some criteria (like creating a vector beta_selected = c() and assign elements to it in R). For example, in the below Stan code, I would like to construct a vector of parameters of length N (beta_selected) based on a matrix of parameters (beta) in my model. The selection criteria are as follows: We have a known matrix A matching the dimension of matrix beta, and elements in A are either 0 or 1. We loop through all the columns i and rows j of A, and if A[i,j] = 1 we attach the parameter beta[i,j] to beta_selected and if A[i,j] = 0 we do nothing to beta_selected. May I ask that how could I implement such processes in Stan?

```
data {
int<lower= 1> N; // N < 36
matrix[6, 6] A;
}
parameter {
matrix[6, 6] beta;
}
transformed parameter {
vector[N] beta_selected;
}
```

Thanks so much for your help!