# ベータ分布をPythonで書く

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DBDAの波に乗り遅れてしまった感あるので最近一人でちまちまと読んでいる。DBDAはRを例にして進めているけどPythonで書いた。式の気持ちの説明などは教科書やWeb上に情報があると思うので割愛。

## コード

beta.py

```#!/usr/bin/python
#-*- coding:utf8 -*-
"""
DBDA FIGURE 5.1
"""
import math
import numpy as np
from pylab import *

def beta(theta, a, b):
"""
beta distribution

theta^{alpha-1} * (1-theta)^{beta-1}
------------------------------------
B(alpha, beta)

B(alpha, beta)
gamma(a) * gamma(b)
= -------------------
gamma(a + b)
"""
B = math.gamma(a) * math.gamma(b) / math.gamma(a + b)
return (theta ** (a - 1)) * ((1 - theta) ** (b - 1)) / B

if __name__ == '__main__':
thetas = np.arange(0, 1, 0.01)
params = [(0.5, 0.5), (1., 0.5), (2., 0.5), (3., 0.5), (4., 0.5),
(0.5, 1.), (1., 1.), (2., 1.), (3., 1.), (4., 1.),
(0.5, 2.), (1., 2.), (2., 2.), (3., 2.), (4., 2.),
(0.5, 3.), (1., 3.), (2., 3.), (3., 3.), (4., 3.),
(0.5, 4.), (1., 4.), (3., 4.), (3., 4.), (4., 4.)
]

axisnum = 0
for (a, b) in params:
axisnum += 1
subplot(5, 5, axisnum)
betas = [beta(theta, a, b) for theta in thetas]
plot(thetas, betas)
xlim(0, 1.)
ylim(0, 3.)
text(0.5, 2.7, 'a=%.1f b=%.1f' % (a, b),
horizontalalignment='center', verticalalignment='center')
xlabel('theta')
ylabel('p(theta)')
show()
```