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46 lines
1.2 KiB
Python
46 lines
1.2 KiB
Python
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"""
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Doctest example from the official Python documentation.
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https://docs.python.org/3/library/doctest.html
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"""
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def factorial(n):
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"""Return the factorial of n, an exact integer >= 0.
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>>> [factorial(n) for n in range(6)]
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[1, 1, 2, 6, 24, 120]
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>>> factorial(30)
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265252859812191058636308480000000
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>>> factorial(-1)
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Traceback (most recent call last):
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...
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ValueError: n must be >= 0
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Factorials of floats are OK, but the float must be an exact integer:
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>>> factorial(30.1)
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Traceback (most recent call last):
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...
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ValueError: n must be exact integer
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>>> factorial(30.0)
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265252859812191058636308480000000
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It must also not be ridiculously large:
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>>> factorial(1e100)
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Traceback (most recent call last):
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...
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OverflowError: n too large
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"""
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import math
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if not n >= 0:
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raise ValueError("n must be >= 0")
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if math.floor(n) != n:
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raise ValueError("n must be exact integer")
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if n+1 == n: # catch a value like 1e300
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raise OverflowError("n too large")
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result = 1
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factor = 2
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while factor <= n:
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result *= factor
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factor += 1
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return result
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