code: public int getUnpairedClusteringIndex() {
      for(int index = 0; index < clusterIds.length; index++) {
        if(clusterIds[index] == UNCLUSTERED) {
          return index;
        }
      }
      return -1;
    }
docs: Returns the index of the first clustering having an unpaired cluster, or -1
  no unpaired cluster exists.
  
  @return clustering id or -1