TheAlgorithms/C-Sharp · error · ArgumentException

The path collection should not be empty.

Error message

The path collection should not be empty.

What it means

FindScapegoatInPath walks a stack representing the insertion path and finds the first (deepest) ancestor that is unbalanced relative to the alpha weight condition. It requires a non-empty path; an empty stack means there is no path to inspect, so it throws ArgumentException naming `path`.

Solutions

1. Check path.Count > 0 before calling FindScapegoatInPath.
2. Only invoke rebalancing after a successful Insert that actually pushed the path.
3. Push the newly inserted node's ancestors onto the stack before searching for the scapegoat.

Example fix

// before
tree.FindScapegoatInPath(path);
// after
if (path.Count == 0) return;
tree.FindScapegoatInPath(path);

if (path == null || path.Count == 0) return; // or handle empty-tree case

static bool HasPath<TKey>(Stack<Node<TKey>> path) => path != null && path.Count > 0;

try { var (parent, scapegoat) = tree.FindScapegoatInPath(path); } catch (ArgumentException) { /* empty path: nothing to balance */ }

When it happens

Trigger: Calling FindScapegoatInPath with an empty Stack<Node<TKey>>, e.g. after fully draining the path or calling it before any nodes were pushed (such as on an empty tree).

Common situations: Calling rebalancing logic on an empty tree, popping the path stack in caller code before passing it, or reusing an already-consumed path stack.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• The parameter's value is invalid.
• The alpha parameter's value should be in 0.5..1.0 range.
• Invalid parameter settings for Ascon Hash
• Cash flows list cannot be empty
• Array is empty.

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/7e9ae629c5c073ed. Report an issue: GitHub.

Appendix: source

Thrown at DataStructures/ScapegoatTree/ScapegoatTree.cs:251

    /// <param name="value">New alpha value.</param>
    public void Tune(double value)
    {
        CheckAlpha(value);
        Alpha = value;
    }

    /// <summary>
    /// Searches for a scapegoat node in provided stack.
    /// </summary>
    /// <param name="path">Stack instance with nodes, starting with root node.</param>
    /// <returns>Scapegoat node with its parent node. Parent can be null if scapegoat node is root node.</returns>
    /// <exception cref="ArgumentException">Thrown if path stack is empty.</exception>
    /// <exception cref="InvalidOperationException">Thrown if scapegoat wasn't found.</exception>
    public (Node<TKey>? Parent, Node<TKey> Scapegoat) FindScapegoatInPath(Stack<Node<TKey>> path)
    {
        if (path.Count == 0)
        {
            throw new ArgumentException("The path collection should not be empty.", nameof(path));
        }

        var depth = 1;

        while (path.TryPop(out var next))
        {
            if (depth > next.GetAlphaHeight(Alpha))
            {
                return path.TryPop(out var parent) ? (parent, next) : (null, next);
            }

            depth++;
        }

        throw new InvalidOperationException("Scapegoat node wasn't found. The tree should be unbalanced.");
    }

    private static void CheckAlpha(double alpha)

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