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32 lines
989 B
Python
32 lines
989 B
Python
def double_factorial(n: int) -> int:
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"""
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Compute double factorial using recursive method.
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Recursion can be costly for large numbers.
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To learn about the theory behind this algorithm:
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https://en.wikipedia.org/wiki/Double_factorial
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>>> import math
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>>> all(double_factorial(i) == math.prod(range(i, 0, -2)) for i in range(20))
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True
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>>> double_factorial(0.1)
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Traceback (most recent call last):
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...
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ValueError: double_factorial() only accepts integral values
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>>> double_factorial(-1)
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Traceback (most recent call last):
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...
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ValueError: double_factorial() not defined for negative values
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"""
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if not isinstance(n, int):
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raise ValueError("double_factorial() only accepts integral values")
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if n < 0:
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raise ValueError("double_factorial() not defined for negative values")
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return 1 if n <= 1 else n * double_factorial(n - 2)
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if __name__ == "__main__":
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import doctest
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doctest.testmod()
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