TheAlgorithms/Python · error · ValueError

Input value of [number={number}] must be > 0

Error message

Input value of [number={number}] must be > 0

What it means

Raised by catalan_number() when number is an int but < 1. The first Catalan number is C(1) = 1 (this implementation's convention — it returns 1 for number=1), so 0 and negatives have no value under its iterative product formula and are rejected. The type check has already passed by this point, so this error means 'right type, out-of-range value'.

Source

Thrown at maths/special_numbers/catalan_number.py:39

        ...
    ValueError: Input value of [number=0] must be > 0
    >>> catalan(-1)
    Traceback (most recent call last):
        ...
    ValueError: Input value of [number=-1] must be > 0
    >>> catalan(5.0)
    Traceback (most recent call last):
        ...
    TypeError: Input value of [number=5.0] must be an integer
    """

    if not isinstance(number, int):
        msg = f"Input value of [number={number}] must be an integer"
        raise TypeError(msg)

    if number < 1:
        msg = f"Input value of [number={number}] must be > 0"
        raise ValueError(msg)

    current_number = 1

    for i in range(1, number):
        current_number *= 4 * i - 2
        current_number //= i + 1

    return current_number


if __name__ == "__main__":
    import doctest

    doctest.testmod()

View on GitHub (pinned to f5988cc097)

Solutions

1. Shift 0-based indices to the 1-based convention this function expects: catalan_number(i + 1)
2. Guard the boundary: catalan_number(n) if n >= 1 else 1 (if your math needs C(0)=1)
3. Start loops at 1: range(1, n + 1) instead of range(n)

Example fix

# before
for i in range(n):
    values.append(catalan_number(i))  # i=0 -> ValueError

# after
for i in range(1, n + 1):
    values.append(catalan_number(i))

if number >= 1:
    c = catalan_number(number)
else:
    c = 1  # if your math assumes C(0) = 1, handle it yourself

def is_valid_catalan_index(value: object) -> bool:
    return isinstance(value, int) and not isinstance(value, bool) and value >= 1

When it happens

Trigger: Calling catalan_number(0) or catalan_number(-1). Typical when an index computed as a difference or a parsed default lands on 0. Note that the standard math convention C(0) = 1 is NOT supported here — callers porting formulas that use 0-based Catalan indices must shift by one.

Common situations: Porting code that assumes C(0)=1 and calling with 0; loop bounds starting at 0 (for i in range(0, n): catalan_number(i)); user input of 0 treated as valid.

Related errors

• max_set_length must be non-negative
• Input list must be a non empty list
• Input series is not valid, valid series - [1, 2/3, 2]
• Input list must be a non empty list
• Input series cannot have 0 as an element

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/336927da202f73d3. Report an issue: GitHub.