# Seeing "Warning: the parameter ... has no priors" when I shouldn't

**URL:** <https://discourse.mc-stan.org/t/seeing-warning-the-parameter-has-no-priors-when-i-shouldnt/22839>\
**Category:** Modeling\
**Created:** [June 3, 2021, 1:51pm UTC](https://discourse.mc-stan.org/t/seeing-warning-the-parameter-has-no-priors-when-i-shouldnt/22839 "2021-06-03T13:51:53Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![defjaf](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/defjaf/32/413_2.png) [@defjaf](https://discourse.mc-stan.org/u/defjaf)\
**Post date:** [June 3, 2021, 1:51pm UTC](https://discourse.mc-stan.org/t/seeing-warning-the-parameter-has-no-priors-when-i-shouldnt/22839/1 "2021-06-03T13:51:53Z")

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The following model gives the (incorrect) warning “`The parameter x_m has no priors`” in pystan3. It seems to be due to the data `d_m` having dimension `2*ell+1`, whereas the parameter `x_m` has dimension `Nm`, even though that is actually defined to be the same quantity.

```stan
data {
  int ell; // number of data = Nm = 2*ell+1;
  vector[2] d_m[2*ell+1];  
}

transformed data {
  int Nm = 2*ell+1; // useful constant. Too bad I can't use it in d_m above.
}

parameters { 
  vector[2] x_m[Nm];
}

model {
  for (i in 1:Nm) {
      d_m[i] ~ normal(x_m[i], 1);
      x_m[i] ~ normal(0, 1);
  }
}

```

The problem goes away if I just make `Nm` into `data` and lose all references to the `ell` parameter. (Nb. the actual model is somewhat more complicated; this is an MWE.)

Not a big deal, but this feels like a bug, or at least an inaccurate warning.

Relatedly, is there any way to declare a constant which depends on the data, in such a way that it can actually be used in the `data` block so that we could replace `vector[2] d_m[2*ell+1]` with `vector[2] d_m[Nm]`?

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**Author:** ![jsocolar](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/jsocolar/32/2486_2.png) [@jsocolar](https://discourse.mc-stan.org/u/jsocolar)\
**Post date:** [June 3, 2021, 1:56pm UTC](https://discourse.mc-stan.org/t/seeing-warning-the-parameter-has-no-priors-when-i-shouldnt/22839/2 "2021-06-03T13:56:22Z")

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What interface to Stan are you using?

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**Author:** ![defjaf](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/defjaf/32/413_2.png) [@defjaf](https://discourse.mc-stan.org/u/defjaf)\
**Post date:** [June 3, 2021, 2:08pm UTC](https://discourse.mc-stan.org/t/seeing-warning-the-parameter-has-no-priors-when-i-shouldnt/22839/3 "2021-06-03T14:08:20Z")

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Sorry – pystan3, as now mentioned in the question.

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**Author:** ![ahartikainen](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/ahartikainen/32/321_2.png) [@ahartikainen](https://discourse.mc-stan.org/u/ahartikainen)\
**Post date:** [June 3, 2021, 2:20pm UTC](https://discourse.mc-stan.org/t/seeing-warning-the-parameter-has-no-priors-when-i-shouldnt/22839/4 "2021-06-03T14:20:33Z")

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That warning comes from `stanc3`.

Can you just declare `int Nm=3;`

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**Author:** ![defjaf](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/defjaf/32/413_2.png) [@defjaf](https://discourse.mc-stan.org/u/defjaf)\
**Post date:** [June 3, 2021, 2:25pm UTC](https://discourse.mc-stan.org/t/seeing-warning-the-parameter-has-no-priors-when-i-shouldnt/22839/5 "2021-06-03T14:25:47Z")

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So in my case the “natural” variable for the problem is `ell`, even though it results in arrays of size `2*ell+1`, which I also declare to be the value of the `transformed data`, `Nm`. Of course I can indeed just use it either by never actually referring to `Nm` or by never referring to `ell` itself (and instead giving `Nm` as data).

But it seems like there is no real reason why the compiler shouldn’t be able to create `transformed data`-like constants directly in the `data` block, at least if they are only used for purposes like array sizes, and in this case it could then keep track of the possibly-different array sizes which are actually the same.

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**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:** [June 3, 2021, 2:36pm UTC](https://discourse.mc-stan.org/t/seeing-warning-the-parameter-has-no-priors-when-i-shouldnt/22839/6 "2021-06-03T14:36:15Z")

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It’s because the two blocks are used for different purposes, the `data` block is for interfacing with externally provided data while the `transformed data` block is for manipulations of data. Because of this, the two blocks transpile to different C++ with different purposes.

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**Author:** ![defjaf](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/defjaf/32/413_2.png) [@defjaf](https://discourse.mc-stan.org/u/defjaf)\
**Post date:** [June 3, 2021, 2:45pm UTC](https://discourse.mc-stan.org/t/seeing-warning-the-parameter-has-no-priors-when-i-shouldnt/22839/7 "2021-06-03T14:45:10Z")

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Thanks, @andrjohns for that explanation. But I think my feature request still stands!

The `data` block can already have declarations that depend on the data — I am just requesting the ability to capture those calculations in a name. I don’t think allowing this expressiveness would break the internal code model of Stan (but of course I could be wrong!).

My code above is already a case where Stan is forcing _me_ to violate “DRY” – I need to repeat the expression `2*ell+1` at least once, and even then it gives me a warning unless I am willing to repeat it everywhere the current code uses `Nm` (which is more times in the more complicated production version). Alternately I can of course just provide `Nm` as data instead of `ell`, but this is moving the model a further step away from the way it is actually expressed “scientifically” (and I can certainly think of cases where various different functions of `ell` appear in other data dimensions). All of this can of course lead to bugs in my code (and it’s part of Stan’s job to make it harder to code programs with bugs!)

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**Author:** ![Funko\_Unko](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/funko_unko/32/10786_2.png) [@Funko\_Unko](https://discourse.mc-stan.org/u/Funko_Unko)\
**Post date:** [June 3, 2021, 3:07pm UTC](https://discourse.mc-stan.org/t/seeing-warning-the-parameter-has-no-priors-when-i-shouldnt/22839/8 "2021-06-03T15:07:53Z")

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I wholeheartedly agree with this feature request.