Python/maths/double_factorial_iterative.py
jonabtc 4761fef1a5
Double factorial iterative (#4760)
* Adding the double factorial algorithm

* Adding the double factorial algorithm

Co-authored-by: Jonathan Ocles <jocles@chiang.ec>
2021-09-19 03:22:47 +02:00

34 lines
974 B
Python

def double_factorial(num: int) -> int:
"""
Compute double factorial using iterative method.
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(num, int):
raise ValueError("double_factorial() only accepts integral values")
if num < 0:
raise ValueError("double_factorial() not defined for negative values")
value = 1
for i in range(num, 0, -2):
value *= i
return value
if __name__ == "__main__":
import doctest
doctest.testmod()