output warning when only set max_depth (#838)

* output warning when only set `max_depth`

* Remove check of max_depth

max_depth can take both positive and negative value

include/LightGBM/config.h

@@ -20,6 +20,7 @@ const std::string kDefaultTreeLearnerType = "serial";
 const std::string kDefaultDevice = "cpu";
 const std::string kDefaultBoostingType = "gbdt";
 const std::string kDefaultObjectiveType = "regression";
+const int kDefaultNumLeaves = 31;
 
 /*!
 * \brief The interface for Config
@@ -202,7 +203,7 @@ public:
   double lambda_l2 = 0.0f;
   double min_gain_to_split = 0.0f;
   // should > 1
-  int num_leaves = 31;
+  int num_leaves = kDefaultNumLeaves;
   int feature_fraction_seed = 2;
   double feature_fraction = 1.0f;
   // max cache size(unit:MB) for historical histogram. < 0 means no limit

src/io/config.cpp

@@ -237,6 +237,14 @@ void OverallConfig::CheckParamConflict() {
       boosting_config.tree_config.histogram_pool_size = -1;
     }
   }
+  // Check max_depth and num_leaves
+  if (boosting_config.tree_config.max_depth > 0) {
+    int full_num_leaves = std::pow(2, boosting_config.tree_config.max_depth);
+    if (full_num_leaves > boosting_config.tree_config.num_leaves 
+        && boosting_config.tree_config.num_leaves == kDefaultNumLeaves) {
+      Log::Warning("Accuarcy may be bad since you didn't set num_leaves.");
+    }
+  }
 }
 
 void IOConfig::Set(const std::unordered_map<std::string, std::string>& params) {