TheAlgorithms/Python · error · ValueError

Either arr or size must be specified

Error message

Either arr or size must be specified

What it means

Raised by FenwickTree.__init__ when neither arr nor size is given. The constructor supports two modes — initialize from an array (self.init(arr)) or allocate an all-zero tree of a given size — and `FenwickTree()` with both arguments None falls into the else branch and raises ValueError. It mirrors the API of competitive-programming Fenwick implementations where an explicit extent is required.

Source

Thrown at data_structures/binary_tree/fenwick_tree.py:26

    More info: https://en.wikipedia.org/wiki/Fenwick_tree
    """

    def __init__(self, arr: list[int] | None = None, size: int | None = None) -> None:
        """
        Constructor for the Fenwick tree

        Parameters:
            arr (list): list of elements to initialize the tree with (optional)
            size (int): size of the Fenwick tree (if arr is None)
        """

        if arr is None and size is not None:
            self.size = size
            self.tree = [0] * size
        elif arr is not None:
            self.init(arr)
        else:
            raise ValueError("Either arr or size must be specified")

    def init(self, arr: list[int]) -> None:
        """
        Initialize the Fenwick tree with arr in O(N)

        Parameters:
            arr (list): list of elements to initialize the tree with

        Returns:
            None

        >>> a = [1, 2, 3, 4, 5]
        >>> f1 = FenwickTree(a)
        >>> f2 = FenwickTree(size=len(a))
        >>> for index, value in enumerate(a):
        ...     f2.add(index, value)
        >>> f1.tree == f2.tree
        True

View on GitHub (pinned to f5988cc097)

Solutions

1. Pass a size when you plan to add points incrementally: FenwickTree(size=n)
2. Pass the full array when values are known up front: FenwickTree(arr=values)
3. In wrappers, give one parameter a real default (e.g. size: int = 10**5) instead of None so both-None cannot reach the constructor

Example fix

# before
ft = FenwickTree()  # ValueError

# after
ft = FenwickTree(size=n)
# or
ft = FenwickTree(arr=[0] * n)

if arr is None and size is None:
    size = 1  # or raise your own descriptive error
ft = FenwickTree(arr=arr, size=size)

try:
    ft = FenwickTree(arr, size)
except ValueError:
    raise ValueError('FenwickTree needs either an array or an explicit size') from None

When it happens

Trigger: FenwickTree() with no arguments; passing arr=None, size=None explicitly (e.g. forwarding optional kwargs that ended up both None); constructing inside a wrapper whose defaults swallow both parameters.

Common situations: Wrapper/config layer that makes both parameters optional; refactoring that renamed arr to values and callers now pass nothing; interactive scripts that skip the size prompt.

Related errors

• Invalid input: {edge} is the wrong length.
• Invalid input: {edge} must have length 2.
• Incorrect input: {vertex} already exists in this graph.
• Polynomial degree must be non-negative
• The number of coefficients should be equal to the degree + 1

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/3e90cee56f8513a4. Report an issue: GitHub.