TheAlgorithms/Python · error · ValueError

Capacity cannot be negative

Error message

Capacity cannot be negative

What it means

Thrown by fractional_cover() (greedy fractional knapsack/cover) when capacity is negative. A negative capacity is physically meaningless — no item can be taken — and silently returning 0.0 would hide the caller's bug, so the function raises ValueError. Note capacity == 0 is legal and returns 0.0.

Source

Thrown at greedy_methods/fractional_cover_problem.py:79

    >>> fractional_cover(items=[], capacity=50)
    0.0

    >>> fractional_cover(items=[Item(10, 60)], capacity=5)
    30.0

    >>> fractional_cover(items=[Item(10, 60)], capacity=1)
    6.0

    >>> fractional_cover(items=[Item(10, 60)], capacity=0)
    0.0

    >>> fractional_cover(items=[Item(10, 60)], capacity=-1)
    Traceback (most recent call last):
        ...
    ValueError: Capacity cannot be negative
    """
    if capacity < 0:
        raise ValueError("Capacity cannot be negative")

    total_value = 0.0
    remaining_capacity = capacity

    # Sort the items by their value-to-weight ratio in descending order
    for item in sorted(items, key=attrgetter("ratio"), reverse=True):
        if remaining_capacity == 0:
            break

        weight_taken = min(item.weight, remaining_capacity)
        total_value += weight_taken * item.ratio
        remaining_capacity -= weight_taken

    return total_value


if __name__ == "__main__":
    import doctest

View on GitHub (pinned to f5988cc097)

Solutions

1. Pass a non-negative capacity; clamp at the source: max(0, remaining_budget).
2. Replace -1 sentinels for 'unset capacity' with None and branch before calling.
3. If capacity legitimately reaches 0, keep the call — it is valid and yields 0.0.

Example fix

# before
fractional_cover(items, capacity=budget - spent)  # may be negative

# after
remaining = budget - spent
if remaining < 0:
    raise ValueError(f"overspent budget: {remaining}")
fractional_cover(items, capacity=remaining)

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

def is_valid_capacity(capacity: float) -> bool:
    return isinstance(capacity, (int, float)) and capacity >= 0

try:
    value = fractional_cover(items, capacity)
except ValueError as e:
    if "Capacity" in str(e):
        value = 0.0  # treat exhausted budget as empty cover
    else:
        raise

When it happens

Trigger: Calling fractional_cover(items, capacity=-1), or passing a capacity computed as a difference (budget - used) that has gone negative, or unpacking a negative value from user input / a config file.

Common situations: Budget already consumed before the call (remaining capacity goes below zero), sign errors when converting units (e.g. passing -kg), or a default of -1 used as a 'not set' sentinel leaking into the algorithm.

Related errors

• The length of profit and weight must be same.
• max_weight must greater than zero.
• Profit can not be negative.
• Weight can not be negative.
• number must be positive

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/3ddb27ad461707a2. Report an issue: GitHub.