计算Python的条件概率

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计算Python的条件概率

我试图计算一种具有层次结构树的结果的概率

图中最上面的是计算机A，接下来是计算机B和C，最后是计算机BD、BE、CD、CE。我试图找到的概率是，如果计算机A感染了病毒，B或C会感染病毒的概率。如果B或C感染了病毒，BD、BE、CD、CE会感染病毒的概率是多少。

我想运行100次试验来得到答案。我是第一次在python上进行概率运算。然而，这是目前为止我写出的代码：

import random, time
#prob that computers will get virus
CompA = 0.50
CompB = .25 
CompC = .25
CompBD = .125
CompBE= .125
CompCD= .125
CompCE= .125
def generate():
    x = random.random()
    if x =< CompA: #Computer A has virus
       prob_compa= sum(generate() for i in range(100)) #prob that Comp A has virus  in a 100 rounds
       print (prob_compa/100 + 'percent chance of getting virus')
        try:
            if CompB<.125:
                 prob_compa sum(generate() for i in range(100)) #prob that Comp B has virus  in a 100 rounds
                print (prob_compa/100 + 'percent chance of getting virus')
                 elif CompB<.125:
                 prob_compa= sum(generate() for i in range(100)) #prob that Comp C is sick  in a 100 rounds
       print (prob_compa/100 + 'percent chance of getting virus')
      #I continue this method for the rest of the tree

我有没有更好、更简单地获得结果的方法？random.uniform？

admin 更改状态以发布 2023年5月21日
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2 答案

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非常棒的代码来自mabe02，或许值得在核心函数中增加一个非常小的改进，以避免混淆/未来的错误：

def check_probability(computer_name, n_repetitions):
    prob_comp, repetitions = 0, 0
    p_computer = virus_probabilities[computer_name]
    while repetitions < n_repetitions:
     x = random.random()
     if x <= p_computer:
          prob_comp += 1
     repetitions += 1
    print ("{0} % changes of getting virus on {1}".format(round(prob_comp/n_repetitions, 2), computer_name))

这样做实际上会使概率更接近开始时的概率，随着n_repetitions越来越大，这是预期的。

虽然有关条件概率更具体的信息，您应该一定要查看这篇文章：Naive Bayes分类的简单解释

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据我理解，您想要达到的目标是：

#python_test2.py
import random, time
virus_probabilities= { "CompA" : 0.50, "CompB" : .25, "CompC" : .25, "CompBD" : .125,
                   "CompBE" : .125, "CompCD" : .125, "CompCE" : .125}
def check_probability(computer_name, n_repetitions = 100):
    prob_comp, repetitions = 0, 0
    p_computer = virus_probabilities[computer_name]
    while repetitions < n_repetitions:
     x = random.random()
     if x <= p_computer:
          prob_comp += 1
     repetitions += 1
    print ("{0} % chance of getting virus on {1}".format(round(prob_comp/100.0, 2), computer_name))
for key in virus_probabilities:
     check_probability(key, 1000)

当我从控制台运行文件时，我得到：

mabe@ubuntu:~/Desktop $ python test_2.py
2.49 % chance of getting virus on CompB
2.6 % chance of getting virus on CompC
5.07 % chance of getting virus on CompA
1.38 % chance of getting virus on CompBE
1.16 % chance of getting virus on CompBD
1.18 % chance of getting virus on CompCD
1.36 % chance of getting virus on CompCE

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