fixed iterator bug when indexing causing the nan error during postprocessing

mlair/data_handler/iterator.py

@@ -55,7 +55,7 @@ class DataCollection(Iterable):
 
     def add(self, element):
         self._collection.append(element)
-        self._mapping[str(element)] = len(self._collection)
+        self._mapping[str(element)] = len(self._collection) - 1
 
     def _set_mapping(self):
         for i, e in enumerate(self._collection):

test/test_data_handler/test_iterator.py

@@ -52,6 +52,13 @@ class TestDataCollection:
         for e, i in enumerate(data_collection):
             assert i == e
 
+    def test_add(self):
+        data_collection = DataCollection()
+        data_collection.add("first_element")
+        assert len(data_collection) == 1
+        assert data_collection["first_element"] == "first_element"
+        assert data_collection[0] == "first_element"
+
 
 class DummyData: