organization in the src structure, modification of README

src/builtin_structures/dicts/defaultdict_example.py

@@ -0,0 +1,27 @@
+#!/usr/bin/python3
+# mari von steinkirch @2013
+# steinkirch at gmail
+
+from collections import defaultdict
+
+def  defaultdict_example():
+    ''' show some examples for defaultdicts ''' 
+    pairs = {('a', 1), ('b',2), ('c',3)}   
+    
+    d1 = {}
+    for key, value in pairs:
+        if key not in d1:
+            d1[key] = []
+        d1[key].append(value)
+    print(d1)
+
+    d2 = defaultdict(list)
+    for key, value in pairs:
+        d2[key].append(value)
+    print(d2)
+
+
+if __name__ == '__main__':
+    defaultdict_example()
+
+