# Select a random integer index in Stan

**URL:** <https://discourse.mc-stan.org/t/select-a-random-integer-index-in-stan/17536>\
**Category:** General\
**Tags:** rstan\
**Created:** [August 19, 2020, 12:46am UTC](https://discourse.mc-stan.org/t/select-a-random-integer-index-in-stan/17536 "2020-08-19T00:46:03Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![srtaheri](https://avatars.discourse-cdn.com/v4/letter/s/c57346/32.png) [@srtaheri](https://discourse.mc-stan.org/u/srtaheri)\
**Post date:** [August 19, 2020, 12:46am UTC](https://discourse.mc-stan.org/t/select-a-random-integer-index-in-stan/17536/1 "2020-08-19T00:46:04Z")

</div>

I want to generate a random integer between 1 to 3000. For that, first I generate a random value from (0,1) called`unif`, and then take the ceiling of `unif*3000`. To generate a uniform value I found the function `uniform_rng` where I defined in the generated quantities block, but I cant get the ceiling part of the code to work. In addition to that, I’m not sure if defining `unif` and ceiling of `unif*3000` inside generated quantities block is a good idea. It didn’t work inside the transformed parameters block and produced an error. Here is a small and simplified example of my code:

```
stan_model_hmc_whc2 <- "
data {
  int D;
  matrix[D, 1] u[3000];
  matrix[1, L] mu[3000];
}
parameters {
  matrix[1 1] eta;
}
transformed parameters {
// empty
  }
}
model {
  for (j in 1:L){
    target += normal_lpdf(eta[j,] | 0, 10);
  }
  for (i in 1:D){
    target += normal_lpdf(u[i, idx] | mu[1,idx], 1); // likelihood
  }
}
generated quantities{
  real unif = uniform_rng(0,1);
  idx = ceil(unif * 3000); //this line produces error
}
"

```

Can anybody help me to on how to define a random integer between 0 and 3000?

---

<div class="post-metadata">

**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:** [August 19, 2020, 8:34am UTC](https://discourse.mc-stan.org/t/select-a-random-integer-index-in-stan/17536/2 "2020-08-19T08:34:08Z")

</div>

Would this work?

So get idx and then loop from 1 to 3000, use continue to skip until correct n is found, Then break.

```
generated quantities{
  real unif = uniform_rng(0,1);
  real idx = unif * 3000;
  for (n in 1:3000) {
    if (n < idx) { continue }
    ...
    break
  }
}

```

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<div class="post-metadata">

**Author:** ![srtaheri](https://avatars.discourse-cdn.com/v4/letter/s/c57346/32.png) [@srtaheri](https://discourse.mc-stan.org/u/srtaheri)\
**Post date:** [August 19, 2020, 2:22pm UTC](https://discourse.mc-stan.org/t/select-a-random-integer-index-in-stan/17536/3 "2020-08-19T14:22:52Z")

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Thank you. This works. Do you know why I can’t use `real unif = uniform_rng(0,1);` inside the transformed parameters block? It seems like `uniform_rng` only works inside the generated quantities block.

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<div class="post-metadata">

**Author:** ![spinkney](https://avatars.discourse-cdn.com/v4/letter/s/dec6dc/32.png) [@spinkney](https://discourse.mc-stan.org/u/spinkney)\
**Post date:** [August 19, 2020, 2:37pm UTC](https://discourse.mc-stan.org/t/select-a-random-integer-index-in-stan/17536/4 "2020-08-19T14:37:53Z")

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all `_rng` functions only work in `transformed data` and `generated quantities` blocks. I’m guessing the reasons they don’t allow it in `transformed parameters` and `model` blocks is that these produce discontinuities which cannot be handled by Stan.

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<div class="post-metadata">

**Author:** ![srtaheri](https://avatars.discourse-cdn.com/v4/letter/s/c57346/32.png) [@srtaheri](https://discourse.mc-stan.org/u/srtaheri)\
**Post date:** [August 19, 2020, 2:59pm UTC](https://discourse.mc-stan.org/t/select-a-random-integer-index-in-stan/17536/5 "2020-08-19T14:59:37Z")

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I see. so if I use it like this:

```
transformed data {
  real unif = uniform_rng(0,1);
  real idx = ceil(unif * 3000);
}

```

Is there a way to convert indx to an integer instead of using the for loop that you suggested? Are we allowed to have for loops inside transformed data block?

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<div class="post-metadata">

**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:** [August 19, 2020, 3:21pm UTC](https://discourse.mc-stan.org/t/select-a-random-integer-index-in-stan/17536/6 "2020-08-19T15:21:47Z")

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Why do you want to have random locations in transformed parameters?

You could provide random array with data (transformed data?)

---

<div class="post-metadata">

**Author:** ![srtaheri](https://avatars.discourse-cdn.com/v4/letter/s/c57346/32.png) [@srtaheri](https://discourse.mc-stan.org/u/srtaheri)\
**Post date:** [August 19, 2020, 3:24pm UTC](https://discourse.mc-stan.org/t/select-a-random-integer-index-in-stan/17536/7 "2020-08-19T15:24:46Z")

</div>

yes you are right. It makes sense to have it in transformed data, not the transformed parameters. I ended up writting it like this:

```
transformed data {
  real unif = uniform_rng(0,1);
  real id = ceil(unif * 3000);
  int idx;
  for (n in 1:3000) {
    if (n == id) {
      idx = n;
      break;
    }
  }
}

```

If there is a better way to convert real to int please let me know. I used the for loop trick that you suggested and it worked for me.