cpython/Demo/scripts/fact.py

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#! /usr/local/python
# Factorize numbers.
# The algorithm is not efficient, but easy to understand.
# If there are large factors, it will take forever to find them,
# because we try all odd numbers between 3 and sqrt(n)...
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import sys
from math import sqrt
error = 'fact.error' # exception
def fact(n):
if n < 1: raise error # fact() argument should be >= 1
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if n == 1: return [] # special case
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res = []
# Treat even factors special, so we can use i = i+2 later
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while n%2 == 0:
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res.append(2)
n = n/2
# Try odd numbers up to sqrt(n)
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limit = sqrt(float(n+1))
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i = 3
while i <= limit:
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if n%i == 0:
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res.append(i)
n = n/i
limit = sqrt(n+1)
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else:
i = i+2
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if n != 1:
res.append(n)
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return res
def main():
if len(sys.argv) > 1:
for arg in sys.argv[1:]:
n = eval(arg)
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print n, fact(n)
else:
try:
while 1:
n = input()
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print n, fact(n)
except EOFError:
pass
main()