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# factorial and combin functions needed in statistical computations
# Bruce Sherwood, Carnegie Mellon University, 2000
def factorial(x):
if x <= 0.:
if x == 0.: return 1.
else: raise 'Cannot take factorial of negative number'
fact = 1.
nn = 2.
while nn <= x:
fact = fact*nn
nn = nn+1.
if nn <> x+1: raise 'Argument of factorial must be an integer'
return fact
def combin(x, y):
# combin(x,y) = factorial(x)/[factorial(y)*factorial(x-y)]
z = x-y
num = 1.0
if y > z:
y,z = z,y
nn = int(z+1.)
while nn <= x:
num = num*nn
nn = nn+1.
if nn <> x+1: raise 'Illegal arguments for combin function'
return num/factorial(y)
if __name__ == '__main__':
print 'factorial(6) = 6! =', factorial(6)
print 'combin(6,2) = 6!/(2!(6-2)!) =', combin(6,2)
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