Powers lower than 1

General
1

For some reason, Stan outputs NA for powers lower than one.
This seems to be the case for both “^” and the pow() function.
I don’t know if the error happens only on my machine, but seems to persist in both RmdStan and CmdStan.
This happend when I tried to work with utility functions in stan and wrote something like
PERIODU[t] = (C[t]*1000/L[t])^(1-elasmu)-1)/(1-elasmu)-1;
I was confused as to why this was an NA as I have defined C[t], L[t] (vectors of size t) and elasmu (a constant) before. Turns out, elasmu was 1.45, giving a power that could not be processed. I have tested this using simpler expressions, such as 2^(0.45) and the error persists. I don’t know if that is a common experience or maybe a result of my coding errors.
I’d be grateful if anyone could share if that is the case on other machines too.

2

Hi Lukasz, welcome to the Stan forums! Powers less than 1 should work fine, so perhaps there’s something going on with your particular Stan program or installation. Before asking you to share a reproducible example so we can look at your Stan code and try running it, let’s just check a super simple example. What happens if you run the following Stan program for just a couple of iterations? It tests a fractional power and a negative power:

parameters {
  real x;
}
model {
  print("x = ", x, ", x^0.5 = ", x^0.5, ", x^-3 = ", x^-3);
  x ~ std_normal();
}

You should see something like this:

Chain 1 x = 1.40705, x^0.5 = 1.18619, x^-3 = 0.358985 
Chain 1 x = -2.02891, x^0.5 = nan, x^-3 = -0.119732 
Chain 1 x = 1.40705, x^0.5 = 1.18619, x^-3 = 0.358985

When x is positive there shouldn’t be any nan values, but for negative values of x you should get nan for x^0.5. What happens for you?

3

Hi Jonah,
Thanks for helping me on this.
It works fine indeed. Turns out, the problem was caused by my misunderstanding of how the sampling functions. Thanks for clarifying that!

1 Like
4

Great, glad you worked it out.

I also meant to mention in my previous reply that if I had used real<lower=0> x; then there wouldn’t have been any nans at all due to the positive constraint.