methods to delete an attribute and get names of available attributes

include/xgboost/c_api.h

@@ -433,16 +433,27 @@ XGB_DLL int XGBoosterGetAttr(BoosterHandle handle,
                              const char** out,
                              int *success);
 /*!
- * \brief Set string attribute.
+ * \brief Set or delete string attribute.
  *
  * \param handle handle
- * \param key The key of the symbol.
- * \param value The value to be saved.
+ * \param key The key of the attribute.
+ * \param value The value to be saved. 
+ *              If nullptr, the attribute would be deleted.
  * \return 0 when success, -1 when failure happens
  */
 XGB_DLL int XGBoosterSetAttr(BoosterHandle handle,
                              const char* key,
                              const char* value);
+/*!
+ * \brief Get the names of all attribute from Booster.
+ * \param handle handle
+ * \param len the argument to hold the output length
+ * \param out pointer to hold the output attribute stings
+ * \return 0 when success, -1 when failure happens
+ */
+XGB_DLL int XGBoosterGetAttrNames(BoosterHandle handle,
+                                  bst_ulong* out_len,
+                                  const char*** out);
 
 // --- Distributed training API----
 // NOTE: functions in rabit/c_api.h will be also available in libxgboost.so

include/xgboost/learner.h

@@ -121,9 +121,20 @@ class Learner : public rabit::Serializable {
    *  The property will be saved along the booster.
    * \param key The key of the attribute.
    * \param out The output value.
-   * \return Whether the key is contained in the attribute.
+   * \return Whether the key exists among booster's attributes.
    */
   virtual bool GetAttr(const std::string& key, std::string* out) const = 0;
+  /*!
+   * \brief Delete an attribute from the booster.
+   * \param key The key of the attribute.
+   * \return Whether the key was found among booster's attributes.
+   */
+  virtual bool DelAttr(const std::string& key) = 0;
+  /*!
+   * \brief Get a vector of attribute names from the booster.
+   * \return vector of attribute name strings.
+   */
+  virtual std::vector<std::string> GetAttrNames() const = 0;
   /*!
    * \return whether the model allow lazy checkpoint in rabit.
    */

src/c_api/c_api.cc

@@ -685,7 +685,28 @@ int XGBoosterSetAttr(BoosterHandle handle,
                      const char* value) {
   Booster* bst = static_cast<Booster*>(handle);
   API_BEGIN();
-  bst->learner()->SetAttr(key, value);
+  if (value == nullptr) {
+    bst->learner()->DelAttr(key);
+  } else {
+    bst->learner()->SetAttr(key, value);
+  }
+  API_END();
+}
+
+int XGBoosterGetAttrNames(BoosterHandle handle,
+                     bst_ulong* out_len,
+                     const char*** out) {
+  std::vector<std::string>& str_vecs = XGBAPIThreadLocalStore::Get()->ret_vec_str;
+  std::vector<const char*>& charp_vecs = XGBAPIThreadLocalStore::Get()->ret_vec_charp;
+  Booster *bst = static_cast<Booster*>(handle);
+  API_BEGIN();
+  str_vecs = bst->learner()->GetAttrNames();
+  charp_vecs.resize(str_vecs.size());
+  for (size_t i = 0; i < str_vecs.size(); ++i) {
+    charp_vecs[i] = str_vecs[i].c_str();
+  }
+  *out = dmlc::BeginPtr(charp_vecs);
+  *out_len = static_cast<bst_ulong>(charp_vecs.size());
   API_END();
 }
 

src/learner.cc

@@ -330,6 +330,22 @@ class LearnerImpl : public Learner {
     return true;
   }
 
+  bool DelAttr(const std::string& key) override {
+    auto it = attributes_.find(key);
+    if (it == attributes_.end()) return false;
+    attributes_.erase(it);
+    return true;
+  }
+
+  std::vector<std::string> GetAttrNames() const override {
+    std::vector<std::string> out;
+    out.reserve(attributes_.size());
+    for(auto &p: attributes_) {
+      out.push_back(p.first);
+    }
+    return out;
+  }
+
   std::pair<std::string, float> Evaluate(DMatrix* data, std::string metric) {
     if (metric == "auto") metric = obj_->DefaultEvalMetric();
     std::unique_ptr<Metric> ev(Metric::Create(metric.c_str()));