TheAlgorithms/Python · error · ValueError

n must not be negative

Error message

n must not be negative

What it means

Raised by capture_area() in physics/basic_orbital_capture.py when the capture radius passed in is negative. The function computes sigma = pi * r^2, the effective cross-sectional capture area; a negative radius has no physical meaning. The guard sits between the docstring and the pi*r**2 computation.

Source

Thrown at backtracking/all_combinations.py:57

    >>> generate_all_combinations(n=5, k=4)
    [[1, 2, 3, 4], [1, 2, 3, 5], [1, 2, 4, 5], [1, 3, 4, 5], [2, 3, 4, 5]]
    >>> generate_all_combinations(n=3, k=3)
    [[1, 2, 3]]
    >>> generate_all_combinations(n=3, k=1)
    [[1], [2], [3]]
    >>> generate_all_combinations(n=1, k=0)
    [[]]
    >>> generate_all_combinations(n=1, k=1)
    [[1]]
    >>> from itertools import combinations
    >>> all(generate_all_combinations(n, k) == combination_lists(n, k)
    ...     for n in range(1, 6) for k in range(1, 6))
    True
    """
    if k < 0:
        raise ValueError("k must not be negative")
    if n < 0:
        raise ValueError("n must not be negative")

    result: list[list[int]] = []
    create_all_state(1, n, k, [], result)
    return result


def create_all_state(
    increment: int,
    total_number: int,
    level: int,
    current_list: list[int],
    total_list: list[list[int]],
) -> None:
    """
    Helper function to recursively build all combinations.

    >>> create_all_state(1, 4, 2, [], result := [])
    >>> result

View on GitHub (pinned to f5988cc097)

Solutions

1. Validate that the radius is >= 0 before calling capture_area().
2. If the radius comes from capture_radii(), inspect why it went negative — that function itself already guards mass/radius, so the sign flip happened in your code.
3. Use abs() only if the negative sign is provably a sign-convention artifact, not bad data.

Example fix

# before
area = capture_area(some_radius)

# after
if some_radius < 0:
    raise ValueError(f"radius must be >= 0, got {some_radius}")
area = capture_area(some_radius)

if capture_radius < 0:
    raise ValueError(f"capture_radius must be >= 0, got {capture_radius}")
sigma = capture_area(capture_radius)

def is_non_negative(x: object) -> bool:
    return isinstance(x, (int, float)) and not isinstance(x, bool) and x >= 0

try:
    sigma = capture_area(r)
except ValueError:
    sigma = capture_area(abs(r))  # only when sign is a proven artifact

When it happens

Trigger: Calling capture_area(-1); passing the unvalidated result of another computation that produced a negative radius; chaining capture_radii output through a subtraction that flipped sign.

Common situations: Reusing a radius variable that was negated elsewhere, or passing an error code / sentinel negative number straight into the function without checking.

Related errors

• Expected a_coeffs to have {self.order + 1} elements for {sel
• Depth cannot be less than 0
• Invalid velocity. Should be a positive number.
• All input parameters must be positive
• Invalid inputs. Enter positive value.

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/11d659bf201d2468. Report an issue: GitHub.