TheAlgorithms/C-Sharp · error · InvalidOperationException

Heap malformed

Error message

Heap malformed

What it means

FibonacciHeap.Cut(x, y) throws InvalidOperationException "Heap malformed" when MinItem is null. Cutting a node out of the root/child lists requires the heap to still have a minimum node; a null MinItem means internal structure is inconsistent.

Solutions

1. Ensure DecreaseKey is only called on nodes in a non-empty live heap.
2. Do not call the protected Cut/CascadingCut from subclass code on an empty heap.
3. Rebuild the node-heap association: create a fresh node and Push it.

Example fix

// before
heap.DecreaseKey(detachedNode, k); // triggers Cut on empty heap
// after
var fresh = new FHeapNode<T>(k);
heap.Push(fresh);

if (heap.Count > 0) { heap.DecreaseKey(node, k); } // Cut is only reached via DecreaseKey/CascadingCut

bool InLiveHeap<T>(FibonacciHeap<T> heap, FHeapNode<T> node) => heap.Count > 0;

try { heap.DecreaseKey(node, k); } catch (InvalidOperationException ex) when (ex.Message == "Heap malformed") { /* rebuild heap */ }

When it happens

Trigger: Cut invoked (via DecreaseKey or CascadingCut) while the heap is empty — only reachable through inconsistent node/heap state or misuse of the protected API.

Common situations: Subclassing and calling Cut directly, or reusing nodes from a heap that was fully popped into a fresh heap.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

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

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/edc1b5e11ba955d9. Report an issue: GitHub.

Appendix: source

Thrown at DataStructures/Heap/FibonacciHeap/FibonacciHeap.cs:263

            CascadingCut(y);
        }

        if (x.Key.CompareTo(MinItem.Key) < 0)
        {
            MinItem = x;
        }
    }

    /// <summary>
    ///     Remove x from the child list of y.
    /// </summary>
    /// <param name="x">A child of y we just decreased the value of.</param>
    /// <param name="y">The now former parent of x.</param>
    protected void Cut(FHeapNode<T> x, FHeapNode<T> y)
    {
        if (MinItem == null)
        {
            throw new InvalidOperationException("Heap malformed");
        }

        if (y.Degree == 1)
        {
            y.Child = null;
            MinItem.AddRight(x);
        }
        else if (y.Degree > 1)
        {
            x.Remove();
        }
        else
        {
            throw new InvalidOperationException("Heap malformed");
        }

        y.Degree--;
        x.Mark = false;

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