python - 溢出错误 : long int too large to convert to float in python

Question

我尝试在 python 中计算泊松分布如下:

p = math.pow(3,idx)
depart = math.exp(-3) * p 
depart = depart / math.factorial(idx)

idx 范围为 0

但我得到 OverflowError: long int too large to convert to float

我尝试将离开转换为 float 但没有结果。

Answer 1

因子变大真的很快:

>>> math.factorial(170)
7257415615307998967396728211129263114716991681296451376543577798900561843401706157852350749242617459511490991237838520776666022565442753025328900773207510902400430280058295603966612599658257104398558294257568966313439612262571094946806711205568880457193340212661452800000000000000000000000000000000000000000L

注意L； 170 的阶乘仍然可以转换为 float :

>>> float(math.factorial(170))
7.257415615307999e+306

但是下一个阶乘太大了:

>>> float(math.factorial(171))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: long int too large to convert to float

您可以使用 decimal module ;计算会更慢，但 Decimal() 类可以处理这种大小的阶乘:

>>> from decimal import Decimal
>>> Decimal(math.factorial(171))
Decimal('1241018070217667823424840524103103992616605577501693185388951803611996075221691752992751978120487585576464959501670387052809889858690710767331242032218484364310473577889968548278290754541561964852153468318044293239598173696899657235903947616152278558180061176365108428800000000000000000000000000000000000000000')

您必须始终使用 Decimal() 值:

from decimal import *

with localcontext() as ctx:
    ctx.prec = 32  # desired precision
    p = ctx.power(3, idx)
    depart = ctx.exp(-3) * p 
    depart /= math.factorial(idx)