SF bug 130030: Claim of bad betavariate algorithm.
diff --git a/Lib/random.py b/Lib/random.py
index d098d39..a22449c 100644
--- a/Lib/random.py
+++ b/Lib/random.py
@@ -461,14 +461,27 @@
return mu + z*sigma
## -------------------- beta --------------------
+## See
+## http://sourceforge.net/bugs/?func=detailbug&bug_id=130030&group_id=5470
+## for Ivan Frohne's insightful analysis of why the original implementation:
+##
+## def betavariate(self, alpha, beta):
+## # Discrete Event Simulation in C, pp 87-88.
+##
+## y = self.expovariate(alpha)
+## z = self.expovariate(1.0/beta)
+## return z/(y+z)
+##
+## was dead wrong, and how it probably got that way.
def betavariate(self, alpha, beta):
-
- # Discrete Event Simulation in C, pp 87-88.
-
- y = self.expovariate(alpha)
- z = self.expovariate(1.0/beta)
- return z/(y+z)
+ # This version due to Janne Sinkkonen, and matches all the std
+ # texts (e.g., Knuth Vol 2 Ed 3 pg 134 "the beta distribution").
+ y = self.gammavariate(alpha, 1.)
+ if y == 0:
+ return 0.0
+ else:
+ return y / (y + self.gammavariate(beta, 1.))
## -------------------- Pareto --------------------