added helper method

include/tadah/models/descriptors/d_base.h

@@ -26,6 +26,10 @@ public:
   int verbose;
   virtual ~D_Base();
 
+  // return position of an argument given to TYPExB for a given key
+  // if key is not used, returns -1
+  int get_arg_pos(const std::string &key) const;
+
   /** \brief Return dimension of the descriptor.
   */
   virtual size_t size() { return s; };

src/d_base.cpp

@@ -91,3 +91,10 @@ void D_Base::set_fcut(Cut_Base* cut, bool manage_memory) {
   fcut = cut;
   this->manage_memory = manage_memory;
 }
+int D_Base::get_arg_pos(const std::string &key) const {
+  auto it = std::find(keys.begin(), keys.end(), key);
+
+  if (it == keys.end()) return -1;
+
+  return std::distance(keys.begin(), it)+(nparams-keys.size());
+}