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SF bug 130030: Claim of bad betavariate algorithm.

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Tim Peters 2001-01-26 06:49:56 +00:00
parent 2786543b70
commit 85e2e4742d
2 changed files with 21 additions and 7 deletions
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25
Lib/random.py
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@ -461,14 +461,27 @@ class Random:
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 --------------------