Python/maths/double_factorial_recursive.py

32 lines
989 B
Python

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