# Adding normal\_rng and normal\_lpdf to matrix specification

**URL:** <https://discourse.mc-stan.org/t/adding-normal-rng-and-normal-lpdf-to-matrix-specification/25907>\
**Category:** Modeling\
**Created:** [January 8, 2022, 3:42pm UTC](https://discourse.mc-stan.org/t/adding-normal-rng-and-normal-lpdf-to-matrix-specification/25907 "2022-01-08T15:42:12Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![dkaplan](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/dkaplan/32/13835_2.png) [@dkaplan](https://discourse.mc-stan.org/u/dkaplan)\
**Post date:** [January 8, 2022, 3:42pm UTC](https://discourse.mc-stan.org/t/adding-normal-rng-and-normal-lpdf-to-matrix-specification/25907/1 "2022-01-08T15:42:12Z")

</div>

Greetings all,

I’m trying to get information to calculate PPCs and LOO from a horseshoe regression using matrix notation for the model. Here is the code

```
modelString = "
data {
  int<lower=1> n; // Number of data
  int<lower=1> p; // Number of covariates
  matrix[n,p] X;
  real readscore[n];
}

parameters {
  vector[p] beta;
  vector<lower=0>[p] lambda;
  real<lower=0> tau;
  real alpha;
  real<lower=0> sigma;
}

model {
  beta ~ normal(0, tau * lambda);
  tau ~ cauchy(0, 1);
  lambda ~ cauchy(0, 1);
  alpha ~ normal(0, 1);
  sigma ~ cauchy(0, 1);
  
  readscore ~ normal(alpha + X * beta, sigma);
}

// For posterior predictive checking and loo cross-validation
generated quantities {
  vector[n] readscore_rep;
  vector[n] log_lik;
  for (i in 1:n) {
    readscore_rep[i] = normal_rng(alpha + X * beta, sigma);
    log_lik[i] = normal_lpdf(readscore[i] | alpha + X * beta, sigma);
  }
}

"

```

I’m getting this error message which I presume is a conflict in the assignment, but I am not sure what is needed to get this to run.

```
SYNTAX ERROR, MESSAGE(S) FROM PARSER:
Dimension mismatch in assignment; variable name = readscore_rep, type = real; right-hand side type = real[].
Illegal statement beginning with non-void expression parsed as
  readscore_rep[i]
Not a legal assignment, sampling, or function statement. Note that
  * Assignment statements only allow variables (with optional indexes) on the left;
  * Sampling statements allow arbitrary value-denoting expressions on the left.
  * Functions used as statements must be declared to have void returns

 error in 'model10676ac612dc_50bc7d8a3e141c2f4fdff6b380d8232a' at line 32, column 4
  -------------------------------------------------
    30: vector[n] log_lik;
    31: for (i in 1:n) {
    32: readscore_rep[i] = normal_rng(alpha + X * beta, sigma);
           ^
    33: 
  -------------------------------------------------

PARSER EXPECTED: "}"
Error in stanc(file = file, model_code = model_code, model_name = model_name, : 
  failed to parse Stan model '50bc7d8a3e141c2f4fdff6b380d8232a' due to the above error.

```

As always, thank you for your time an patience.

---

<div class="post-metadata">

**Author:** ![nhuurre](https://avatars.discourse-cdn.com/v4/letter/n/ad7895/32.png) [@nhuurre](https://discourse.mc-stan.org/u/nhuurre)\
**Post date:** [January 8, 2022, 4:18pm UTC](https://discourse.mc-stan.org/t/adding-normal-rng-and-normal-lpdf-to-matrix-specification/25907/2 "2022-01-08T16:18:59Z")

</div>

> [@dkaplan](#):
>
> I’m getting this error message which I presume is a conflict in the assignment, but I am not sure what is needed to get this to run.

The error says

```no-highlight
Dimension mismatch in assignment; variable name = readscore_rep, type = real; right-hand side type = real[].

```

The `normal_rng()` expression is vectorized and generates an array of values but this is inside a loop so you’re trying to assign that array into a single element of `readscore_rep`. You can move it out of the loop and have

```stan
readscore_rep = normal_rng(alpha + X * beta, sigma);

```

or select a single row of `X` so that the `normal_rng` generates single value

```stan
for (i in 1:n) {
  readscore_rep[i] = normal_rng(alpha + X[i,:] * beta, sigma);
  log_lik[i] = normal_lpdf(readscore[i] | alpha + X[i,:] * beta, sigma);
}

```

---

<div class="post-metadata">

**Author:** ![dkaplan](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/dkaplan/32/13835_2.png) [@dkaplan](https://discourse.mc-stan.org/u/dkaplan)\
**Post date:** [January 8, 2022, 5:25pm UTC](https://discourse.mc-stan.org/t/adding-normal-rng-and-normal-lpdf-to-matrix-specification/25907/3 "2022-01-08T17:25:45Z")

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Niko,

Thanks. The second approach works. The first approach is giving me the following

```
SYNTAX ERROR, MESSAGE(S) FROM PARSER:
Dimension mismatch in assignment; variable name = readscore_rep, type = vector; right-hand side type = real[].
Illegal statement beginning with non-void expression parsed as
  readscore_rep
Not a legal assignment, sampling, or function statement. Note that
  * Assignment statements only allow variables (with optional indexes) on the left;
  * Sampling statements allow arbitrary value-denoting expressions on the left.
  * Functions used as statements must be declared to have void returns

 error in 'model2108373c6251_86e53254a4d33155e368c3d5d9fb9f68' at line 31, column 2
  -------------------------------------------------
    29: vector[n] readscore_rep;
    30: vector[n] log_lik;
    31: readscore_rep = normal_rng(alpha + X * beta, sigma);
         ^
    32: log_lik = normal_lpdf(readscore| alpha + X * beta, sigma);
  -------------------------------------------------

PARSER EXPECTED: <one of the following:
  a variable declaration, beginning with type
      (int, real, vector, row_vector, matrix, unit_vector,
       simplex, ordered, positive_ordered,
       corr_matrix, cov_matrix,
       cholesky_corr, cholesky_cov
  or a <statement>
  or '}' to close variable declarations and definitions>
Error in stanc(file = file, model_code = model_code, model_name = model_name, : 
  failed to parse Stan model '86e53254a4d33155e368c3d5d9fb9f68' due to the above error.
> 

```

Still, things seem to be working. Thanks again.

---

<div class="post-metadata">

**Author:** ![nhuurre](https://avatars.discourse-cdn.com/v4/letter/n/ad7895/32.png) [@nhuurre](https://discourse.mc-stan.org/u/nhuurre)\
**Post date:** [January 8, 2022, 5:28pm UTC](https://discourse.mc-stan.org/t/adding-normal-rng-and-normal-lpdf-to-matrix-specification/25907/4 "2022-01-08T17:28:04Z")

</div>

> [@dkaplan](#):
>
> The first approach is giving me the following

Ooh, I forgot the “vectorized” `normal_rng` outputs an array and not a vector. You could convert that with `to_vector()` but the second approach is probably better anyway.