TheAlgorithms/C-Sharp · error · ArgumentException

x is not from the heap

Error message

x is not from the heap

What it means

FibonacciHeap.DecreaseKey(x, k) throws ArgumentException with "x is not from the heap" when MinItem is null, i.e. the heap is empty so node x cannot belong to it. The check names the node argument but really detects an empty heap.

Solutions

1. Check heap.Count > 0 / MinItem != null before DecreaseKey().
2. Keep the node reference tied to the same live heap instance.
3. Catch ArgumentException if node membership is uncertain.

Example fix

// before
heap.DecreaseKey(node, newVal);
// after
if (heap.Count > 0)
{
    heap.DecreaseKey(node, newVal);
}

if (heap.Count > 0) { heap.DecreaseKey(node, k); }

try { heap.DecreaseKey(node, k); } catch (ArgumentException) { /* node not in heap */ }

When it happens

Trigger: Calling DecreaseKey on an empty heap (MinItem == null), or with a node from another/already-emptied heap.

Common situations: Decreasing a key after the heap was fully popped, or passing a node captured from a heap instance that was since unioned or cleared.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• not in heap!
• Heap is empty!
• The heap is empty
• x has no value
• Value cannot be increased

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/04a939d0a4e24e19. Report an issue: GitHub.

Appendix: source

Thrown at DataStructures/Heap/FibonacciHeap/FibonacciHeap.cs:227

            throw new InvalidOperationException("The heap is empty");
        }

        return MinItem.Key;
    }

    /// <summary>
    ///     Reduce the key of x to be k.
    /// </summary>
    /// <remarks>
    ///     k must be less than x.Key, increasing the key of an item is not supported.
    /// </remarks>
    /// <param name="x">The item you want to reduce in value.</param>
    /// <param name="k">The new value for the item.</param>
    public void DecreaseKey(FHeapNode<T> x, T k)
    {
        if (MinItem == null)
        {
            throw new ArgumentException($"{nameof(x)} is not from the heap");
        }

        if (x.Key == null)
        {
            throw new ArgumentException("x has no value");
        }

        if (k.CompareTo(x.Key) > 0)
        {
            throw new InvalidOperationException("Value cannot be increased");
        }

        x.Key = k;
        var y = x.Parent;
        if (y != null && x.Key.CompareTo(y.Key) < 0)
        {
            Cut(x, y);
            CascadingCut(y);

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