OverflowError:long int太大，无法在python中转换为float

问题描述

我尝试如下计算python中的泊松分布:

I tried to calculate poisson distribution in python as below:

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

idx的范围是0

但是我得到了OverflowError: long int too large to convert to float

我试图将出发地转换为float，但没有结果.

I tried to convert depart to float but no results.

推荐答案

工厂迅速发展壮大

>>> math.factorial(170)
7257415615307998967396728211129263114716991681296451376543577798900561843401706157852350749242617459511490991237838520776666022565442753025328900773207510902400430280058295603966612599658257104398558294257568966313439612262571094946806711205568880457193340212661452800000000000000000000000000000000000000000L

注意L； 170的阶乘仍然可以转换为浮点数:

Note the L; the factorial of 170 is still convertable to a float:

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

但是下一个阶乘太大:

>>> 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模块;计算会比较慢，但是Decimal()类可以处理以下大小的阶乘:

You could use the decimal module; calculations will be slower, but the Decimal() class can handle factorials this size:

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

您必须在整个过程中使用Decimal()值:

You'll have to use Decimal() values throughout:

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)

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