试图在Python中复制小C代码

Question

我正在尝试在python中复制一些C代码。

这是C代码。

#include <math.h>
double AttackerSuccessProbability(double q, int z)
{
 double p = 1.0 - q;
 double lambda = z * (q / p);
 double sum = 1.0;
 int i, k;
 for (k = 0; k <= z; k++)
 {
  double poisson = exp(-lambda);
  for (i = 1; i <= k; i++)
    poisson *= lambda / i;
    sum -= poisson * (1 - pow(q / p, z - k));
 }
 return sum;
}

C代码只是对此等式进行建模，取自Bitcoin's whitepaper

这是我在python中的尝试。

def BitcoinAttackerSuccessProbablity(q, z):
    # probablity of honest finding the next node is 1 - probablity the attacker will find the next
    p = 1 - q
    # lambda is equal to the number of blocks times by the division of attacker and honest finding the next block
    lam = z * (q / p)
    sum = 1.0
    i = 1
    k = 0

    while k <= z:
        poisson = math.exp(-1 * lam)
        while i <= k:
            poisson = poisson * (lam / i)
            i = i + 1
        sum = sum - (poisson * (1 - ((q / p) ** (z - k))))
        k = k + 1

    print(sum)


for x in range(0,11):
    BitcoinAttackerSuccessProbablity(0.1, x)

C代码的结果是。 （q和z是输入，而P是输出。）

q=0.1    

z=0 P=1.0000000
z=1 P=0.2045873
z=2 P=0.0509779
z=3 P=0.0131722
z=4 P=0.0034552
z=5 P=0.0009137
z=6 P=0.0002428
z=7 P=0.0000647
z=8 P=0.0000173
z=9 P=0.0000046
z=10 P=0.0000012

我试图通过转换C代码在python中准确地复制这些结果。我的前3个结果（z = 0，-3）是正确的，但是，其余结果是不正确的。当我将while循环更改为C中的等效循环时，只有前两个结果正确。

我的Python代码的结果

1.0
0.20458727394278242
0.05097789283933862
-0.057596282822218514
-0.1508215598462347
-0.22737700216279746
-0.28610012088884856
-0.3272432217416562
-0.3519591169464781
-0.36186779773540745
-0.35878211836275675

我认为在语言之间处理循环的方法很简单。

非常感谢任何帮助

编辑：

这是第一次尝试

def BitcoinAttackerSuccessProbablity(q, z):
# probablity of honest finding the next node is 1 - probablity the attacker will find the next
p = 1 - q
#lambda is equal to the number of blocks times by the division of attacker and honest finding the next block
lam = z * (q/p)
sum = 1.0


for k in range(0, z):
    poisson = math.exp(-1*lam)
    for i in range(1, k):
        poisson = poisson * (lam/i)
    sum = sum - (poisson * (1 - ((q/p)**(z-k))))

print(sum)

Answer 1

您不会在i循环之前将while i <= k重新初始化为零。

在Python中编写这个更惯用（更简短！）的方法是使用generator expression和sum()函数来执行求和，以及使用{{1}函数而不是计算循环中的阶乘。使用这种方法，代码成为原始数学公式的非常直接的翻译：

math.factorial()

请注意，范围设置为def BitcoinAttackerSuccessProbablity(q, z): # probablity of honest finding the next node is 1 - probablity the attacker # will find the next p = 1 - q # lambda is equal to the number of blocks times by the division of attacker # and honest finding the next block lam = z * (q / p) return 1 - sum( (lam ** k) * math.exp(-lam) / math.factorial(k) * (1 - (q / p) ** (z - k)) for k in range(0, z+1) ) 而不是range(0, z+1)，因为Python范围不包含停止值。