[Scipy-svn] r4897 - in trunk/scipy/stats: . tests

[scipy-svn at scipy.org]

Author: stefan
Date: 2008-11-01 11:04:04 -0500 (Sat, 01 Nov 2008)
New Revision: 4897

Modified:
   trunk/scipy/stats/distributions.py
   trunk/scipy/stats/tests/test_distributions.py
Log:
Fix a sign in `entropy` [patch by Herman Engelbrecht].

Modified: trunk/scipy/stats/distributions.py
===================================================================
--- trunk/scipy/stats/distributions.py	2008-11-01 15:17:33 UTC (rev 4896)
+++ trunk/scipy/stats/distributions.py	2008-11-01 16:04:04 UTC (rev 4897)
@@ -3180,10 +3180,10 @@
     """S = entropy(pk,qk=None)
 
     calculate the entropy of a distribution given the p_k values
-    S = -sum(pk * log(pk),axis=0)
+    S = -sum(pk * log(pk), axis=0)
 
     If qk is not None, then compute a relative entropy
-    S = -sum(pk * log(pk / qk),axis=0)
+    S = sum(pk * log(pk / qk), axis=0)
 
     Routine will normalize pk and qk if they don't sum to 1
     """
@@ -3200,7 +3200,7 @@
         #   too, the relative entropy is infinite.
         if any(take(pk,nonzero(qk==0.0),axis=0)!=0.0, 0):
             return inf
-        vec = where (pk == 0, 0.0, pk*log(pk / qk))
+        vec = where (pk == 0, 0.0, -pk*log(pk / qk))
     return -sum(vec,axis=0)
 
 

Modified: trunk/scipy/stats/tests/test_distributions.py
===================================================================
--- trunk/scipy/stats/tests/test_distributions.py	2008-11-01 15:17:33 UTC (rev 4896)
+++ trunk/scipy/stats/tests/test_distributions.py	2008-11-01 16:04:04 UTC (rev 4897)
@@ -224,5 +224,15 @@
         if stats.bernoulli.__doc__ is not None:
             self.failUnless("bernoulli" in stats.bernoulli.__doc__.lower())
 
+class TestEntropy(TestCase):
+    def test_entropy_positive(self):
+        """See ticket #497"""
+        pk = [0.5,0.2,0.3]
+        qk = [0.1,0.25,0.65]
+        eself = stats.entropy(pk,pk)
+        edouble = stats.entropy(pk,qk)
+        assert(0.0 == eself)
+        assert(edouble >= 0.0)
+
 if __name__ == "__main__":
     run_module_suite()