adding some extra comments

main.py

@@ -99,20 +99,31 @@ def voc_ap(rec, prec):
   prec.insert(0, 0.0) # insert 0.0 at begining of list
   prec.append(0.0) # insert 0.0 at end of list
   mpre = prec[:]
+  """
+   This part makes the precision monotonically decreasing
+    (goes from the end to the beginning)
+  """
   # matlab indexes start in 1 but python in 0, so I have to do:
   #   range(start=(len(mpre) - 2), end=0, step=-1)
   # also the python function range excludes the end, resulting in:
   #   range(start=(len(mpre) - 2), end=-1, step=-1)
   for i in range(len(mpre)-2, -1, -1):
     mpre[i] = max(mpre[i], mpre[i+1])
+  """
+   This part creates a list of indexes where the recall changes
+  """
   # matlab: i=find(mrec(2:end)~=mrec(1:end-1))+1;
-  ind = []
-  for ind_1 in range(1, len(mrec)):
-    if mrec[ind_1] != mrec[ind_1-1]:
-      ind.append(ind_1) # if it was matlab would be ind_1 + 1
+  i_list = []
+  for i in range(1, len(mrec)):
+    if mrec[i] != mrec[i-1]:
+      i_list.append(i) # if it was matlab would be i + 1
+  """
+   The Average Precision (AP) is the area under the curve
+    (numerical integration)
+  """
   # matlab: ap=sum((mrec(i)-mrec(i-1)).*mpre(i));
   ap = 0.0
-  for i in ind:
+  for i in i_list:
     ap += ((mrec[i]-mrec[i-1])*mpre[i])
   return ap, mrec, mpre