Line # 96 in merf.merf.py might be better when modifying "len(indices_i)" to "sum(indices_i)"

[CCChen-Menggg]

Hello,
Thanks for your great work on merf!
When I debug merf, I found that there is one line that does not work in any case:

63   def predict(self, X: np.ndarray, Z: np.ndarray, clusters: pd.Series):
           ...
          for cluster_id in self.cluster_counts.index:
                indices_i = clusters == cluster_id

               # If cluster doesn't exist in test data that's ok. Just move on.
96           if len(indices_i) == 0:  < ------------------ might revise to: if sum(indices_i) == 0
                    continue

               # If cluster does exist, apply the correction.
                ...

I marked Line # 96 and suggested changing "len()" to "sum()", otw, this if will never run in any case because indices_i is a pd.Series has the same shape with the input cluster.

And if possible, I would like to ask another question about the random effect matrix Z.

In the given example notebook, I noticed that when considering one variance as a random effect, the Z is not simply composed of one column but also has another column of ones. Why are the ones necessary?

Thanks in advance!
meng