Define data block inputs conditionally

Is there a way where one might conditionally define parts of the data block, such that a “test set” could be supplied to automatically generate predictions for the “test set” from a “training set”?

Granted I am fully aware that it’s probably better from a bayesian perspective to use all of your data to fit a model, but often the first question I get when presenting results is “did you validate this on a test set?”. So, out of curiosity I was wondering if it would be feasible to do allow my stan models to have an optional “test set”, but if I wanted to fit a model to all of my data I could use the same model without having to have two separate versions of whatever regression model I happen to be fitting.

This code does not work, since the data block doesn’t seem to allow conditional statements, but I think it should demonstrate what I have in mind. Are there any workarounds or “hacks” that might make such a model feasible? :

data {
  int N; //the number of observations
  int P; //the number of columns in the model matrix
  real y[N]; //the response
  matrix[N,P] X; //the model matrix
  int test_set; //switch to indicate whether a test set is provided
  if (test_set)
      int Ntest;
      vector[Ntest] yTest;
      matrix[Ntest,P] Xtest;
parameters {
  real Intercept;
  vector[P] beta; //the regression parameters
  real<lower=0> sigma;
  Intercept ~ normal(0, 1);
  beta ~ student_t(3, 0, 2);
  sigma ~ student_t(3, 0, 10);
  y ~ normal(Intercept + X*beta, sigma);
generated quantities{
   vector[N] ySim;
     for(i in 1:N){
           ySim[i] = normal_rng(X[i] * beta + Intercept, sigma);
  if (test_set)
     vector[Ntest] yTest;
      for(i in 1:Ntest){
           yTest[i] = normal_rng(Xtest[i] * beta + Intercept, sigma);

You cannot have if clauses in the data block, but you can define their sizes to be zero. So you can do things like

data {
  int<lower = 0> N;
  int<lower = 0> P;
  vector[N] y;
  matrix[N, P] X;
  int<lower = 0> Ntest; 
  vector[Ntest] yTest;
  matrix[Ntest, p] Xtest;

If you don’t want to utilize the last three, just set Ntest to zero, yTest to be a size zero vector, and Xtest to be a matrix with no rows. You can also use the ternary operator if necessary to size things conditionally.

1 Like

Works like a charm, thank you!