# Summing elements from an array

**URL:** <https://discourse.mc-stan.org/t/summing-elements-from-an-array/31965>\
**Category:** Modeling\
**Created:** [July 3, 2023, 8:19pm UTC](https://discourse.mc-stan.org/t/summing-elements-from-an-array/31965 "2023-07-03T20:19:29Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![josh21](https://avatars.discourse-cdn.com/v4/letter/j/9f8e36/32.png) [@josh21](https://discourse.mc-stan.org/u/josh21)\
**Post date:** [July 3, 2023, 8:19pm UTC](https://discourse.mc-stan.org/t/summing-elements-from-an-array/31965/1 "2023-07-03T20:19:29Z")

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I am trying to do some vector arithmetic on an array but unsure of the general syntax that would achieve this. In a very simple form, consider

```stan
data {
vector[K] p;
}

parameters {
  array[K] vector[D] mu;
} 

```

As part of my model, I would like to construct the sum mu[k] \* p[k] which is a D dimensional vector. How do I do this as efficiently as possible?

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<div class="post-metadata">

**Author:** ![andre.pfeuffer](https://avatars.discourse-cdn.com/v4/letter/a/e480ec/32.png) [@andre.pfeuffer](https://discourse.mc-stan.org/u/andre.pfeuffer)\
**Post date:** [July 7, 2023, 8:06am UTC](https://discourse.mc-stan.org/t/summing-elements-from-an-array/31965/2 "2023-07-07T08:06:00Z")

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> [@josh21](#):
>
> mu[k] \* p[k]

Your dimensions don’t match. `mu[k]` has dimension D.  
I’d use a matrix for `mu` instead of an array of vectors.