TheAlgorithms/Python · error · ValueError

double_factorial_iterative() not defined for negative values

Error message

double_factorial_iterative() not defined for negative values

What it means

Raised by double_factorial_iterative() in maths/double_factorial.py when num is a negative integer. range(num, 0, -2) with negative num is empty, which would silently return 1 — a wrong answer for an undefined input — so the function explicitly rejects negatives with ValueError instead.

Source

Thrown at maths/double_factorial.py:50

    To learn about the theory behind this algorithm:
    https://en.wikipedia.org/wiki/Double_factorial

    >>> from math import prod
    >>> all(double_factorial_iterative(i) == prod(range(i, 0, -2)) for i in range(20))
    True
    >>> double_factorial_iterative(0.1)
    Traceback (most recent call last):
        ...
    ValueError: double_factorial_iterative() only accepts integral values
    >>> double_factorial_iterative(-1)
    Traceback (most recent call last):
        ...
    ValueError: double_factorial_iterative() not defined for negative values
    """
    if not isinstance(num, int):
        raise ValueError("double_factorial_iterative() only accepts integral values")
    if num < 0:
        raise ValueError("double_factorial_iterative() 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()

View on GitHub (pinned to f5988cc097)

Solutions

Check num >= 0 before calling and raise a domain-specific error in your own code.
Repair the arithmetic that produced the negative input.
Do not 'fix' it by defaulting to 1 — that is exactly the silent wrong answer this guard exists to prevent.

Example fix

# before
double_factorial_iterative(n - 3)  # ValueError when n < 3

# after
m = n - 3
if m < 0:
    raise ValueError(f'invalid argument {m}: must be >= 0')
double_factorial_iterative(m)

Defensive patterns

Strategy: validation

Validate before calling

if num < 0:
    raise ValueError(f'double factorial undefined for negative {num}')

Try / catch

try:
    r = double_factorial_iterative(num)
except ValueError as e:
    if 'negative values' in str(e):
        raise ValueError(f'upstream produced negative num={num}') from e
    raise

Prevention

Never default negative inputs to 1 — the guard exists to stop that silent wrong answer.
Bound-check loop-derived values before factorial-family calls.

When it happens

Trigger: Calling double_factorial_iterative(-1) or any negative int. The guard is num < 0, evaluated after the isinstance check.

Common situations: Negative loop bounds from off-by-one arithmetic; user-supplied numbers with a minus sign; subtracting sizes (len(a) - len(b)) that can go negative.

Related errors

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/e291a59897239c6c. Report an issue: GitHub.