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.
    */