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December 27, 2016 15:25
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ガチャで複数の当たりを引く確率
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def combination(n, r) | |
factorial(n) / factorial(r) / factorial(n - r) | |
end | |
def permutation(n, r) | |
factorial(n) / factorial(n - r) | |
end | |
$factorial = { 0 => 1 } | |
def factorial(n) | |
$factorial[n] ||= (1..n).inject(:*) | |
end | |
if __FILE__ == $0 | |
p = 0.01 # 当選率 p のくじで | |
r = 5 # r 回以上の当たりを引く確率が | |
s = 0.9 # s 以上になるには | |
# 何回の試行が必要か? | |
x = 0.0 | |
(r..10000).each do |n| | |
x += p**r * (1 - p)**(n - r) * combination(n - 1, r - 1) | |
if x >= s | |
puts "#{n}: #{x}" | |
break | |
end | |
end | |
end |
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