Add XGBoosterGetNumFeature (#5856)

- add GetNumFeature to Learner
- add XGBoosterGetNumFeature to C API
- update c-api-demo accordingly

demo/c-api/c-api-demo.c

@@ -60,6 +60,10 @@ int main(int argc, char** argv) {
     printf("%s\n", eval_result);
   }
 
+  bst_ulong num_feature = 0;
+  safe_xgboost(XGBoosterGetNumFeature(booster, &num_feature));
+  printf("num_feature: %llu\n", num_feature);
+
   // predict
   bst_ulong out_len = 0;
   const float* out_result = NULL;

include/xgboost/c_api.h

@@ -563,6 +563,14 @@ XGB_DLL int XGBoosterSetParam(BoosterHandle handle,
                               const char *name,
                               const char *value);
 
+/*!
+ * \brief get number of features
+ * \param out number of features
+ * \return 0 when success, -1 when failure happens
+ */
+XGB_DLL int XGBoosterGetNumFeature(BoosterHandle handle,
+                                   bst_ulong *out);
+
 /*!
  * \brief update the model in one round using dtrain
  * \param handle handle

include/xgboost/learner.h

@@ -158,6 +158,12 @@ class Learner : public Model, public Configurable, public rabit::Serializable {
    */
   virtual void SetParam(const std::string& key, const std::string& value) = 0;
 
+  /*!
+   * \brief Get the number of features of the booster.
+   * \return number of features
+   */
+  virtual uint32_t GetNumFeature() = 0;
+
   /*!
    * \brief Set additional attribute to the Booster.
    *

src/c_api/c_api.cc

@@ -395,6 +395,14 @@ XGB_DLL int XGBoosterSetParam(BoosterHandle handle,
   API_END();
 }
 
+XGB_DLL int XGBoosterGetNumFeature(BoosterHandle handle,
+                                   xgboost::bst_ulong *out) {
+  API_BEGIN();
+  CHECK_HANDLE();
+  *out = static_cast<Learner*>(handle)->GetNumFeature();
+  API_END();
+}
+
 XGB_DLL int XGBoosterLoadJsonConfig(BoosterHandle handle, char const* json_parameters) {
   API_BEGIN();
   CHECK_HANDLE();

src/learner.cc

@@ -384,6 +384,10 @@ class LearnerConfiguration : public Learner {
     }
   }
 
+  uint32_t GetNumFeature() override {
+    return learner_model_param_.num_feature;
+  }
+
   void SetAttr(const std::string& key, const std::string& value) override {
     attributes_[key] = value;
     mparam_.contain_extra_attrs = 1;

tests/cpp/c_api/test_c_api.cc

@@ -112,9 +112,10 @@ TEST(CAPI, ConfigIO) {
 
 TEST(CAPI, JsonModelIO) {
   size_t constexpr kRows = 10;
+  size_t constexpr kCols = 10;
   dmlc::TemporaryDirectory tempdir;
 
-  auto p_dmat = RandomDataGenerator(kRows, 10, 0).GenerateDMatrix();
+  auto p_dmat = RandomDataGenerator(kRows, kCols, 0).GenerateDMatrix();
   std::vector<std::shared_ptr<DMatrix>> mat {p_dmat};
   std::vector<bst_float> labels(kRows);
   for (size_t i = 0; i < labels.size(); ++i) {
@@ -131,6 +132,10 @@ TEST(CAPI, JsonModelIO) {
   XGBoosterSaveModel(handle, modelfile_0.c_str());
   XGBoosterLoadModel(handle, modelfile_0.c_str());
 
+  bst_ulong num_feature {0};
+  ASSERT_EQ(XGBoosterGetNumFeature(handle, &num_feature), 0);
+  ASSERT_EQ(num_feature, kCols);
+
   std::string modelfile_1 = tempdir.path + "/model_1.json";
   XGBoosterSaveModel(handle, modelfile_1.c_str());