summing a vector of changing lengths

[EN Pete]

Hi All-

I have a model where I want to sum components within a vector that changes length based on on state s. For example, in the below code I want to sum gamma_rt for region r year t within state s. For every state the number of regions changes, so I include a region index region_s which gives which regions are included in state s. nregions_s is the total number of regions in state s which changes by state. 

I can show the whole model if helpful, but this is really the only line that needs help. 

When I try to run this, I get the error related to vector indexes within a function and that it requires a scalar. 

Can anyone help me on how I would execute this in Nimble?

sum(gamma_rt[regions_s[s,2:nregions_s[s]],t])

[Chris Paciorek]

I think you'll need to write a short nimbleFunction to do the summation. Something like this:

```

summation <- nimbleFunction(

  run = function(gamma = double(1), regions = double(1), nregions = double(0)) {

     returnType(double(0))

     return(sum(gamma[regions[2:nregions]]))

}

```

and in your model:

```

sum(gamma_rt[1:M, t], regions_s[s,1:P], nregions_s[s])

```

where M and P are what they need to be to indicate the extent of the relevant dimensions.

Let me know if that doesn't do it and we can iterate as needed.

-chris

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[Daniel Turek]

One comment, in the model code, I think Chris means you'd use the (custom-written) "summation" function, rather than regular old "sum".

So, the model code part would be a bit more like:

some_model_variable[s,t] <- summation(gamma_rt[1:M, t], regions_s[s,1:P], nregions_s[s])

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[Chris Paciorek]

Yes, thanks for noticing my omission, Daniel. One needs to use `summation` in the model.

-Chris