# AR(1) Syntax Error Parse

**URL:** <https://discourse.mc-stan.org/t/ar-1-syntax-error-parse/22328>\
**Category:** Modeling\
**Tags:** rstan\
**Created:** [May 5, 2021, 7:07pm UTC](https://discourse.mc-stan.org/t/ar-1-syntax-error-parse/22328 "2021-05-05T19:07:25Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![hillco97](https://avatars.discourse-cdn.com/v4/letter/h/ea666f/32.png) [@hillco97](https://discourse.mc-stan.org/u/hillco97)\
**Post date:** [May 5, 2021, 7:07pm UTC](https://discourse.mc-stan.org/t/ar-1-syntax-error-parse/22328/1 "2021-05-05T19:07:25Z")

</div>

I am trying to do a simple AR(1) model, however, I keep getting an error that reads:  
SYNTAX ERROR, MESSAGE(S) FROM PARSER:  
Dimension mismatch in assignment; variable name = y\_rep, type = real; right-hand side type = real[].  
Illegal statement beginning with non-void expression parsed as  
y\_rep[n]  
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

I have posted the code below.

```stan
AR1<-"
data {
  int<lower=0> N;
  int<lower=0> k;
  vector[N] g;
  matrix[N,k] x;
}
parameters {
  real alpha;
  real beta;
  real <lower=0> sigma;
}
model {
  for (n in 2: N)
    g ~ normal (alpha + x[n-1]*beta, sigma);
}
generated quantities {
  int<lower=0, upper=1> mean_gt;
  int<lower=0, upper=1> sd_gt;
  int<lower=0, upper=1> max_gt;
  vector[N] log_lik;
  vector[N] y_rep;
  for (n in 2:N){
  log_lik[n]= normal_lpdf(g | alpha + x[n-1]*beta,sigma);
}    
  for (n in 2:N) {
  y_rep[n]= normal_rng(alpha + x[n-1]*beta, sigma) ;
  mean_gt= mean(y_rep) > mean(g); // p-value for mean
  sd_gt = sd(y_rep) > sd(g); // p-value for sd
  max_gt = max(y_rep) > max(g); // p-value for max
}
}
"

g<- log(data$sale/data$xbar)
N=nrow(data)
x=model.matrix(~g)
N=nrow(data)
k=ncol(x)
dat <- list(N=nrow(data),k=k, g=g, x=x)
****
```

---

<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:** [May 6, 2021, 7:02am UTC](https://discourse.mc-stan.org/t/ar-1-syntax-error-parse/22328/2 "2021-05-06T07:02:36Z")

</div>

Hi Connor,

The issue here is that `x` is an N\text{x}K matrix, implying N observations and K covariates, but you’re only specifying a single regression coefficient \beta (i.e., `real beta`). This implies that you want every covariate to have the same regression coefficient, which I’m guessing you don’t want.

In this case, you just need to specify that you want K regression coefficients:

```
vector[k] beta;

```

In the case where you did want to have every covariate use the same regression coefficient, you just need to sum the multiplication of `x` and `beta`:

```stan
model {
  ...
    g ~ normal (alpha + sum(x[n-1]*beta), sigma);
}
generated quantities {
  ...
  log_lik[n]= normal_lpdf(g | alpha + sum(x[n-1]*beta),sigma);
  ... 
  y_rep[n]= normal_rng(alpha + sum(x[n-1]*beta), sigma) ;
  ...
}

```