How to fix this python program?

import math

def p(n):
    return 393000*((288200/393000)^n * math.exp(-(288200/393000)))/math.factorial(n)

print p(3)

When I run it, I get the following error message:

Traceback (most recent call last):
  File "poisson.py", line 6, in <module>
    print p(3)
  File "poisson.py", line 4, in p
    return 393000*((288200/393000)^n * math.exp(-(288200/开发者_运维问答393000)))/math.factoria
l(n)
TypeError: unsupported operand type(s) for ^: 'int' and 'float'

Replace ^ with ** in

(288200/393000)^n

Bear in mind that

288200/393000

Returns 0

Maybe you should try using decimal numbers:

import math

def p(n):
    a = 393000.0 # <-- notice the .0 
    b = 288200.0
    c = b / a
    return a * ( c**n * math.exp(-c) )/ math.factorial(n)

print p(3)

Returns:

12406.890756

Is the ^ supposed to mean exponentiation? If so, use ** instead.

You can also use math.pow:

>>> import math
>>> math.pow(3,2)
9.0

Though actually it looks like maybe this isn't the best idea, since math.pow is more for C extension compatibility, and doesn't handle all the cases that ** does:

>>> 2**3000
1230231922161117176931558813276752514640713895736833715766118029160058800614672948775360067838593459582429649254051804908512884180898236823585082482065348331234959350355845017413023320111360666922624728239756880416434478315693675013413090757208690376793296658810662941824493488451726505303712916005346747908623702673480919353936813105736620402352744776903840477883651100322409301983488363802930540482487909763484098253940728685132044408863734754271212592471778643949486688511721051561970432780747454823776808464180697103083861812184348565522740195796682622205511845512080552010310050255801589349645928001133745474220715013683413907542779063759833876101354235184245096670042160720629411581502371248008430447184842098610320580417992206662247328722122088513643683907670360209162653670641130936997002170500675501374723998766005827579300723253474890612250135171889174899079911291512399773872178519018229989376L

vs.

>>> import math
>>> math.pow(2, 3000)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: math range error

see http://mail.python.org/pipermail/python-list/2003-November/236169.html for a little more detail

EDIT: In response to your question as to why it returns 0.0, that's because you are raising 0 to a power - you are using / for division, which by default is integer division and will truncate. use from __future__ import division to get floating point divison.