TheAlgorithms/Python · error · ValueError

Length must be a positive integer.

Error message

Length must be a positive integer.

What it means

Raised by hexagonal_numbers() in maths/series/hexagonal_numbers.py when length is not an int or is <= 0. The function generates the first `length` hexagonal numbers via a list comprehension over range(length), so a negative, zero, or non-integer length is meaningless. Note the check order: `length <= 0 or not isinstance(length, int)` — a float like 5.0 is also rejected even though it equals a valid int.

Source

Thrown at maths/series/hexagonal_numbers.py:36

def hexagonal_numbers(length: int) -> list[int]:
    """
    :param len: max number of elements
    :type len: int
    :return: Hexagonal numbers as a list

    Tests:
    >>> hexagonal_numbers(10)
    [0, 1, 6, 15, 28, 45, 66, 91, 120, 153]
    >>> hexagonal_numbers(5)
    [0, 1, 6, 15, 28]
    >>> hexagonal_numbers(0)
    Traceback (most recent call last):
      ...
    ValueError: Length must be a positive integer.
    """

    if length <= 0 or not isinstance(length, int):
        raise ValueError("Length must be a positive integer.")
    return [n * (2 * n - 1) for n in range(length)]


if __name__ == "__main__":
    print(hexagonal_numbers(length=5))
    print(hexagonal_numbers(length=10))

View on GitHub (pinned to f5988cc097)

Solutions

1. Coerce to int when the value is whole: hexagonal_numbers(int(n))
2. Clamp or validate the computed length: length = max(1, int(length))
3. Reject non-integer input at the UI/API boundary with a clear message

Example fix

# before
hexagonal_numbers(n / 2)  # float -> ValueError

# after
hexagonal_numbers(int(n / 2))  # or n // 2

length = int(length) if isinstance(length, float) and length.is_integer() else length
if isinstance(length, int) and length > 0:
    print(hexagonal_numbers(length))

def is_positive_int(value: object) -> bool:
    return isinstance(value, int) and not isinstance(value, bool) and value > 0

When it happens

Trigger: Calling hexagonal_numbers(0), hexagonal_numbers(-3), or hexagonal_numbers(5.0). Booleans pass (isinstance(True, int) is True) but length=False raises because False <= 0.

Common situations: User-supplied count parsed from a string without int() conversion; a computed length that went negative after subtraction; division results (e.g. n/2 in Python 3 yields float) passed directly.

Related errors

• Input list must be a non empty list
• Input list must be a non empty list
• {num}: Invalid input, please enter a positive integer.
• max_set_length must be non-negative
• Number {n} must instead be a positive integer

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/aced6bc80503458e. Report an issue: GitHub.