TheAlgorithms/Python · error · ValueError

All elements in candidates must be non-negative

Error message

All elements in candidates must be non-negative

What it means

Raised by casimir_force() in physics/casimir_effect.py when the force argument is negative (and exactly one argument was 0). The Casimir force magnitude in this formula is treated as a non-negative quantity; a negative magnitude is rejected. Note force is checked first, so a negative force raises even if area or distance is also negative.

Source

Thrown at backtracking/combination_sum.py:60

    """
    >>> combination_sum([2, 3, 5], 8)
    [[2, 2, 2, 2], [2, 3, 3], [3, 5]]
    >>> combination_sum([2, 3, 6, 7], 7)
    [[2, 2, 3], [7]]
    >>> combination_sum([-8, 2.3, 0], 1)
    Traceback (most recent call last):
        ...
    ValueError: All elements in candidates must be non-negative
    >>> combination_sum([], 1)
    Traceback (most recent call last):
        ...
    ValueError: Candidates list should not be empty
    """
    if not candidates:
        raise ValueError("Candidates list should not be empty")

    if any(x < 0 for x in candidates):
        raise ValueError("All elements in candidates must be non-negative")

    path = []  # type: list[int]
    answer = []  # type: list[int]
    backtrack(candidates, path, answer, target, 0)
    return answer


def main() -> None:
    print(combination_sum([-8, 2.3, 0], 1))


if __name__ == "__main__":
    import doctest

    doctest.testmod()
    main()

View on GitHub (pinned to f5988cc097)

Solutions

1. Pass the magnitude: use abs(force) if the sign only encodes attraction direction.
2. Check force >= 0 before the call if the value is user-supplied.
3. Wrap in try/except ValueError to surface a clear message for bad imports.

Example fix

# before
casimir_force(force=measured_force, area=0, distance=d)  # -912e-12

# after
casimir_force(force=abs(measured_force), area=0, distance=d)

if force is not None and force < 0:
    force = abs(force)  # magnitude API; sign encodes attraction elsewhere
casimir_force(force=force, area=a, distance=d)

try:
    casimir_force(force=f, area=0, distance=d)
except ValueError as e:
    if "force" in str(e):
        casimir_force(force=abs(f), area=0, distance=d)
    else:
        raise

When it happens

Trigger: casimir_force(force=-912e-12, area=0, distance=0.09374); passing a signed force measurement where the sign convention was 'attractive = negative' into a magnitude-based API.

Common situations: Physics datasets often encode the attractive Casimir force as negative; feeding such measurements directly into this magnitude-based solver without taking abs() triggers the error.

Related errors

• Open Knight Tour cannot be performed on a board of size {n}
• Expected a_coeffs to have {self.order + 1} elements for {sel
• Expected b_coeffs to have {self.order + 1} elements for {sel
• k must not be negative
• n must not be negative

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/6705ca18cdc67724. Report an issue: GitHub.