# Parser doesn't recognise array\[N\] x;

**URL:** <https://discourse.mc-stan.org/t/parser-doesnt-recognise-array-n-x/26191>\
**Category:** RStan\
**Tags:** specification\
**Created:** [February 2, 2022, 7:32am UTC](https://discourse.mc-stan.org/t/parser-doesnt-recognise-array-n-x/26191 "2022-02-02T07:32:50Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![benskov](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/benskov/32/7580_2.png) [@benskov](https://discourse.mc-stan.org/u/benskov)\
**Post date:** [February 2, 2022, 7:32am UTC](https://discourse.mc-stan.org/t/parser-doesnt-recognise-array-n-x/26191/1 "2022-02-02T07:32:50Z")

</div>

Hi all,

This morning I encountered an odd problem. When trying to run the GP example straight out of the manual ([10.3 Fitting a Gaussian process | Stan User’s Guide](https://mc-stan.org/docs/2_28/stan-users-guide/fit-gp.html#)), I run into trouble with the compiler.

Model:

```stan
data {
  int<lower=1> N;
  array[N] real x;
  vector[N] y;
}
transformed data {
  vector[N] mu = rep_vector(0, N);
}
parameters {
  real<lower=0> rho;
  real<lower=0> alpha;
  real<lower=0> sigma;
}
model {
  matrix[N, N] L_K;
  matrix[N, N] K = cov_exp_quad(x, alpha, rho);
  real sq_sigma = square(sigma);

  // diagonal elements
  for (n in 1:N) {
    K[n, n] = K[n, n] + sq_sigma;
  }

  L_K = cholesky_decompose(K);

  rho ~ inv_gamma(5, 5);
  alpha ~ std_normal();
  sigma ~ std_normal();

  y ~ multi_normal_cholesky(mu, L_K) T[0, 1];
}

```

The error:

```r
> stan("models/gp", chains = 1, iter = 100)
SYNTAX ERROR, MESSAGE(S) FROM PARSER:
 error in 'modelfd8a6dfc59a6_gp_truncated_normal' at line 4, column 2
  -------------------------------------------------
     2: int<lower=1> N;
     3: vector[N] y;
     4: array[N] real x;
         ^
     5: }
  -------------------------------------------------

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 '}' to close variable declarations>

```

In the script editor pane, the word `array` shows as black text (unlike i.a. `vector`) suggesting that it’s not recognised:  
 ![image](https://canada1.discourse-cdn.com/flex030/uploads/mc_stan/original/2X/4/4334da2d5766634b8d847de7e1e412a686f4ef97.png)

I just followed the steps in the RStan manual to reinstall RStan and it works (and also clearly the compiler is called):

```r
> sessionInfo("rstan")
R version 4.1.2 (2021-11-01)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Catalina 10.15.7

Matrix products: default
BLAS: /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/4.1/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
character(0)

other attached packages:
[1] rstan_2.21.3

loaded via a namespace (and not attached):
 [1] Rcpp_1.0.8 lubridate_1.8.0 tidyr_1.1.4 prettyunits_1.1.1   
 [5] ps_1.6.0 assertthat_0.2.1 digest_0.6.29 utf8_1.2.2          
 [9] R6_2.5.1 cellranger_1.1.0 backports_1.4.1 reprex_2.0.1        
[13] stats4_4.1.2 evaluate_0.14 httr_1.4.2 ggplot2_3.3.5       
[17] pillar_1.7.0 utils_4.1.2 rlang_1.0.0 readxl_1.3.1        
[21] rstudioapi_0.13 callr_3.7.0 rmarkdown_2.11 stringr_1.4.0       
[25] readr_2.1.2 loo_2.4.1 munsell_0.5.0 broom_0.7.12        
[29] compiler_4.1.2 modelr_0.1.8 xfun_0.29 pkgconfig_2.0.3     
[33] stats_4.1.2 pkgbuild_1.3.1 htmltools_0.5.2 tidyselect_1.1.1    
[37] tibble_3.1.6 gridExtra_2.3 codetools_0.2-18 matrixStats_0.61.0  
[41] grDevices_4.1.2 fansi_1.0.2 crayon_1.4.2 dplyr_1.0.7         
[45] tzdb_0.2.0 dbplyr_2.1.1 withr_2.4.3 grid_4.1.2          
[49] jsonlite_1.7.3 gtable_0.3.0 lifecycle_1.0.1 DBI_1.1.2           
[53] magrittr_2.0.2 datasets_4.1.2 StanHeaders_2.21.0-7 scales_1.1.1        
[57] RcppParallel_5.1.5 stringi_1.7.6 cli_3.1.1 fs_1.5.2            
[61] tidyverse_1.3.1 xml2_1.3.3 ellipsis_0.3.2 graphics_4.1.2      
[65] generics_0.1.2 vctrs_0.3.8 base_4.1.2 tools_4.1.2         
[69] forcats_0.5.1 glue_1.6.1 purrr_0.3.4 hms_1.1.1           
[73] processx_3.5.2 parallel_4.1.2 fastmap_1.1.0 yaml_2.2.2          
[77] inline_0.3.19 colorspace_2.0-2 rvest_1.0.2 knitr_1.37          
[81] haven_2.4.3 patchwork_1.1.1 methods_4.1.2   

```

Does anyone have a good idea as to why this happens? I have a nasty feeling that I’m missing something obvious.

Best,  
Ben

---

<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:** [February 2, 2022, 7:37am UTC](https://discourse.mc-stan.org/t/parser-doesnt-recognise-array-n-x/26191/2 "2022-02-02T07:37:35Z")

</div>

This because the Stan version included in RStan is a little out of date, and does not yet support the new syntax. You can either install the preview of the next rstan version using:

```r
remove.packages(c("StanHeaders", "rstan"))
install.packages("StanHeaders", repos = c("https://mc-stan.org/r-packages/", getOption("repos")))
install.packages("rstan", repos = c("https://mc-stan.org/r-packages/", getOption("repos")))

```

Alternatively, you can just use the old array syntax if you don’t want to upgrade:

```stan
real x[N];

```

---

<div class="post-metadata">

**Author:** ![rok\_cesnovar](https://avatars.discourse-cdn.com/v4/letter/r/7bcc69/32.png) [@rok\_cesnovar](https://discourse.mc-stan.org/u/rok_cesnovar)\
**Post date:** [February 2, 2022, 8:36am UTC](https://discourse.mc-stan.org/t/parser-doesnt-recognise-array-n-x/26191/3 "2022-02-02T08:36:01Z")

</div>

The third option is to look at the older version of the Users guide (for 2.21): [10.3 Fitting a Gaussian Process | Stan User’s Guide](https://mc-stan.org/docs/2_21/stan-users-guide/fit-gp-section.html)

---

<div class="post-metadata">

**Author:** ![jonah](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/jonah/32/200_2.png) [@jonah](https://discourse.mc-stan.org/u/jonah)\
**Post date:** [February 3, 2022, 9:07pm UTC](https://discourse.mc-stan.org/t/parser-doesnt-recognise-array-n-x/26191/4 "2022-02-03T21:07:43Z")

</div>

Another option is to use CmdStanR to run Stan from R instead of RStan. That would give you access to the latest syntax.

---

<div class="post-metadata">

**Author:** ![benskov](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/benskov/32/7580_2.png) [@benskov](https://discourse.mc-stan.org/u/benskov)\
**Post date:** [February 5, 2022, 4:25pm UTC](https://discourse.mc-stan.org/t/parser-doesnt-recognise-array-n-x/26191/5 "2022-02-05T16:25:36Z")

</div>

Thanks a bunch, guys! As I said, something obvious and all good fixes. Will just stick with the old syntax for now as I’m in the early trial-and-error phase anyway. Cheers!