[SciPy-user] calculating factorial

Stefan van der Walt stefan at sun.ac.za
Sat Nov 19 04:13:34 CST 2005

I couldn't find "factorial", "combination", "permutation" etc. in
SciPy.  Luckily, they're simple -- only problem is that my
implementation doesn't work :)

See the code listing below.  I thought that the two methods for
calculating factorials should work equally well.  I get an integer
overflow, no matter what typecode I use.

I realise that this behaviour might have changed with the new
scipy-base, so I'd appreciate if someone could test it and see.

from scipy import prod, arange, __version__

print "Scipy version", __version__

def factorial1(n):
    f = 1
    while (n >= 2):
	f, n = f * n, n - 1
    return f

def factorial2(n):
    return prod(arange(2, n+1, typecode='u'))

print "\nFactorial method 1", factorial1(100)
print "\nFactorial method 2", factorial2(100)

Scipy version 0.3.3_309.4626

Factorial method 1 93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000

Factorial method 2
Traceback (most recent call last):
  File "./fact.py", line 17, in ?
    print "\nFactorial method 2", factorial2(100)
  File "./fact.py", line 14, in factorial2
    return prod(arange(2, n+1, typecode='u'))
  File "/home/stefan/lib/python/scipy_base/function_base.py", line 208, in prod
    return _no_axis_is_all(multiply.reduce, m, axis)
  File "/home/stefan/lib/python/scipy_base/function_base.py", line 164, in _no_axis_is_all
    r = function(m, axis)
ArithmeticError: Integer overflow in multiply.


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