method for calculating term frequencies

fixes #28

README.md

@@ -116,7 +116,6 @@ Entire lists of tokens can be added for a particular address in one go using `ad
 
 ```
 
-
 You can **prune** a HTDM to just `n` levels with the method `prune(n)`.
 
 You can iterate over the document-term counts at the leaves of the HTDM with the method `leaf_entries()` (this returns a generator yielding `(document_address, term, count)` tuples). This is effectively a traditional TDM (the document IDs will still reflect the hierarchy but the aggregate counts aren't present).
@@ -133,6 +132,23 @@ You can deep copy an HTDM with `copy()`. You can also pass a prefix to `copy()`
 
 You can save out an HTDM with `save()` which takes a `filename` and optional `field_sep` (defaulting to tab) and `prefix` (if you just want to save out a subtree).
 
+### Calculations
+
+You can get a term frequency with `tf(term)` or `tf(term, address)`.
+
+```python
+>>> c = termdoc.HTDM()
+>>> c.increment_count("1", "foo")
+>>> c.increment_count("1", "bar", 3)
+>>> c.increment_count("2", "foo", 3)
+>>> c.increment_count("2", "bar")
+>>> c.tf("foo")
+0.5
+>>> c.tf("foo", "2")
+0.75
+
+```
+
 ### Duplicates Policy
 
 You can optionally pass in a `duplicates` setting to the constructor indicating the policy you want to follow if a term-document count is updated more than once.

termdoc/htdm.py

@@ -97,3 +97,6 @@ def copy(self, prefix=None):
         for document, term, count in self.leaf_entries(prefix):
             new.increment_count(document, term, count)
         return new
+
+    def tf(self, term, address=""):
+        return self.get_counts(address)[term] / self.get_counts(address).total()

tests.py

@@ -347,5 +347,18 @@ def test_add(self):
         self.assertEqual(c.get_counts("1")["bar"], 2)
 
 
+class Test4(unittest.TestCase):
+    def test_term_frequency(self):
+        import termdoc
+
+        c = termdoc.HTDM()
+        c.increment_count("1", "foo")
+        c.increment_count("1", "bar", 3)
+        c.increment_count("2", "foo", 3)
+        c.increment_count("2", "bar")
+        self.assertEqual(c.tf("foo"), 0.5)
+        self.assertEqual(c.tf("foo", "2"), 0.75)
+
+
 if __name__ == "__main__":
     unittest.main()