Adding deprecation warning

HISTORY.md

@@ -1,5 +1,9 @@
 ### mlpack ?.?.?
 ###### ????-??-??
+
+  * The DecisionStump class has been marked deprecated use DecisionTree
+    class with `NoRecursion=true` or use `ID3DecisionStump` (#2099).
+
   * Add Model() to the FFN class to access individual layers (#2043).
 
   * Update documentation for pip and conda installation packages (#2044).

src/mlpack/methods/decision_stump/decision_stump_main.cpp

@@ -125,6 +125,10 @@ static void mlpackMain()
 
   ReportIgnoredParam({{ "test", false }}, "predictions");
 
+  Log::Warn << "The Class DecisionStump is deprecated and will be removed in "
+            << "mlpack 4.0.0. Please use DecisionTree class with maximum_depth "
+            << "option set to 1 (that will produce a stump)." <<std::endl;
+
   // We must either load a model, or train a new stump.
   DSModel* model;
   if (CLI::HasParam("training"))

src/mlpack/methods/decision_tree/decision_tree.hpp

@@ -589,6 +589,9 @@ using DecisionStump = DecisionTree<FitnessFunction,
                                    ElemType,
                                    false>;
 
+/**
+ * Convenience typedef for decision tree (Three level decision trees).
+ */
 typedef DecisionTree<InformationGain,
                      BestBinaryNumericSplit,
                      AllCategoricalSplit,

src/mlpack/methods/decision_tree/decision_tree_impl.hpp

@@ -118,10 +118,8 @@ DecisionTree<FitnessFunction,
     const double minimumGainSplit,
     const size_t maximumDepth,
     DimensionSelectionType dimensionSelector,
-    const std::enable_if_t<
-        arma::is_arma_type<
-        typename std::remove_reference<
-        WeightsType>::type>::value>*)
+    const std::enable_if_t<arma::is_arma_type<
+        typename std::remove_reference<WeightsType>::type>::value>*)
 {
   using TrueMatType = typename std::decay<MatType>::type;
   using TrueLabelsType = typename std::decay<LabelsType>::type;
@@ -163,10 +161,8 @@ DecisionTree<FitnessFunction,
     WeightsType weights,
     const size_t minimumLeafSize,
     const double minimumGainSplit,
-    const std::enable_if_t<
-        arma::is_arma_type<
-        typename std::remove_reference<
-        WeightsType>::type>::value>*):
+    const std::enable_if_t<arma::is_arma_type<
+        typename std::remove_reference<WeightsType>::type>::value>*):
         NumericAuxiliarySplitInfo(other),
         CategoricalAuxiliarySplitInfo(other)
 {