# Stacking Vectors Into Array Within Stan File

**URL:** <https://discourse.mc-stan.org/t/stacking-vectors-into-array-within-stan-file/20005>\
**Category:** Modeling\
**Tags:** performance, specification, rstan\
**Created:** [December 31, 2020, 10:04pm UTC](https://discourse.mc-stan.org/t/stacking-vectors-into-array-within-stan-file/20005 "2020-12-31T22:04:36Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Lortiz42](https://avatars.discourse-cdn.com/v4/letter/l/46a35a/32.png) [@Lortiz42](https://discourse.mc-stan.org/u/Lortiz42)\
**Post date:** [December 31, 2020, 10:04pm UTC](https://discourse.mc-stan.org/t/stacking-vectors-into-array-within-stan-file/20005/1 "2020-12-31T22:04:36Z")

</div>

I have the stan model below implementing a multivariate normal model (the parameters are scaling variance and regression constants) with a custom log likelihood.  
I am working on implementing this with a separable covariance (i.e Sigma can be factored in A \otimes B).  
As part of this I need to take the residual vector Y-XB (here called mu\_minus\_y) and stack it into a matrix with column dimension equal to the dimension of B and row dimension equal to the dimension of A.  
I am looking at the rstan documentation to see if there is a built in function I can use for this but have been unable to find one.  
Does anyone happen to know a function I can use for this?  
If not, will I need to build this as a c++ function or can I build a custom function using other stan built ins?  
Thank you

* * *

```stan
functions{
real new_multiNormal_lpdf(vector y, vector mu, matrix sigma,int n){
vector[n] mu_minus_y_;
vector[n] quadratic_form_1_;  
real quadratic_form_;
real log_det_; 
real log_lik_;
real lprob_new_normal_;
mu_minus_y_=y-mu;
quadratic_form_1_= mdivide_left_spd(sigma,mu_minus_y_);
quadratic_form_=mu_minus_y_'*quadratic_form_1_;
//
log_det_=log_determinant(sigma);
//lprob_new_normal_=multi_normal_lpdf(y|mu,sigma);
lprob_new_normal_= -.5*log_det_-.5*quadratic_form_ -log(2*pi());
return lprob_new_normal_;  
}  
}
data {
  int<lower = 1> n;
  int<lower = 1> p;
  matrix[n, p] X;
  matrix[n,n] sigma;
  vector[p] beta_0;
  matrix[p,p] sigma_0;
  //vector[n] mu;
  vector[n] y; 
} 
parameters {
  vector[p] beta;
  real<lower = 0> rho;
   
}
model {
  beta ~ multi_normal(beta_0, sigma_0);
  rho ~ inv_gamma(2, 2);
  //y ~ multi_normal(X*beta, rho*sigma);
  target+=new_multiNormal_lpdf(y|X*beta,rho*sigma,n);
}

```

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<div class="post-metadata">

**Author:** ![andrjohns](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/andrjohns/32/15297_2.png) [@andrjohns](https://discourse.mc-stan.org/u/andrjohns)\
**Post date:** [January 1, 2021, 4:58am UTC](https://discourse.mc-stan.org/t/stacking-vectors-into-array-within-stan-file/20005/2 "2021-01-01T04:58:18Z")

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Hi Lucas,

To clarify, you have a vector of size `R*C` and you want to transform it to a matrix with `R` rows and `C` columns, is that right? If so, you can use the `to_matrix` function, covered in the manual here: [https://mc-stan.org/docs/2\_25/functions-reference/mixed-operations.html](https://mc-stan.org/docs/2_25/functions-reference/mixed-operations.html)

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<div class="post-metadata">

**Author:** ![Lortiz42](https://avatars.discourse-cdn.com/v4/letter/l/46a35a/32.png) [@Lortiz42](https://discourse.mc-stan.org/u/Lortiz42)\
**Post date:** [January 3, 2021, 1:05am UTC](https://discourse.mc-stan.org/t/stacking-vectors-into-array-within-stan-file/20005/3 "2021-01-03T01:05:34Z")

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Hi yes that is correct,  
thank you this exactly what I was looking for!