fixes

include/LightGBM/dataset.h

@@ -252,8 +252,15 @@ class Metadata {
   * \return number of different position IDs
   */
   inline size_t num_position_ids() const {
-      size_t max = *std::max_element(positions_.begin(), positions_.end());
-      return max + 1;
+      if (!positions_.empty()) {
+        size_t max = *std::max_element(positions_.begin(), positions_.end());
+        return max + 1;
+      }
+      else
+      {
+        return 0;
+      }
+
   }
 
   /*!

src/objective/rank_objective.hpp

@@ -45,7 +45,6 @@ class RankingObjective : public ObjectiveFunction {
     // get position ids
     // get number of different position ids
     num_position_ids_ = static_cast<data_size_t>(metadata.num_position_ids());
-    Log::Info("Number of different position ids: %d", num_position_ids_);
     // get boundaries
     query_boundaries_ = metadata.query_boundaries();
     if (query_boundaries_ == nullptr) {