Hi,

I am a little stuck trying to vectorise something.

I am modelling the probability of players winning points in a tennis match. Originally, I had the following setup:

```
int s_id[num_matches]; // server ids
vector[num_players] s; // serving ability
```

I could then pick out the ones specific to a match using:

Now, I want to make the model time-dependent. This is the new setup:

```
int s_id[num_matches]; // server ids
int period[num_matches]; // the current period
matrix[num_periods, num_players] s; // serving ability
```

I would like to vectorise the new statement. I tried:

but it returns a matrix. I really only want to pick out the *vector* corresponding to the indices (i, j), where i is given by one integer array and j by another. Is there a way to do this in vectorised form?

Thanks a lot,

Martin