TheAlgorithms/C-Sharp · error · ItemNotFoundException

Exception of type…

Error message

Exception of type 'Utilities.Exceptions.ItemNotFoundException' was thrown.

What it means

FastSearcher.FindIndex throws ItemNotFoundException when the item cannot be located: either the array is empty or the item lies outside the sorted array's [first, last] range, so binary search cannot contain it. It is the library's typed 'not found' signal.

Solutions

1. Check array.Length > 0 and that item is within [array[0], array[^1]] before calling.
2. Catch ItemNotFoundException and treat it as 'absent' rather than an exceptional case.
3. If data may be unsorted, sort it first or use a linear search.

Example fix

// before
int idx = searcher.FindIndex(span, key);
// after
if (span.Length == 0 || key < span[0] || key > span[^1]) return -1;
int idx = searcher.FindIndex(span, key);

bool searchable = array.Length > 0 && item >= array[0] && item <= array[^1];

static bool CanContain(Span<int> array, int item) => array.Length > 0 && item >= array[0] && item <= array[^1];

try { int idx = searcher.FindIndex(array, item); }
catch (ItemNotFoundException) { return -1; } // item absent

When it happens

Trigger: Calling FindIndex on an empty Span<int>, or searching for a value smaller than array[0] or larger than array[^1] in a sorted span.

Common situations: Searching for a key absent from a lookup table; passing unsorted or partially sorted data; stale cached min/max assumptions after data changed; searching empty results from an upstream query.

Understand the failure class

Background: Record Not Found Errors: "not found", RecordNotFound, and "was not found" — what they mean and how to fix them — this error's family across 28 libraries.

Related errors

• Exception of type…
• is not a positive integer
• is not invertible in Z/ Z.
• Adjacency matrix must be square!
• Adjacency matrix must be symmetric!

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/8c9b3130278a791d. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Search/FastSearcher.cs:27

///     average case: O(log log n),
///     best case: O(1).
///     Note: This algorithm is recursive and the array has to be sorted beforehand.
/// </summary>
public class FastSearcher
{
    /// <summary>
    ///     Finds index of first item in array that satisfies specified term
    ///     throws ItemNotFoundException if the item couldn't be found.
    /// </summary>
    /// <param name="array">Span of sorted numbers which will be used to find the item.</param>
    /// <param name="item">Term to check against.</param>
    /// <returns>Index of first item that satisfies term.</returns>
    /// <exception cref="ItemNotFoundException"> Gets thrown when the given item couldn't be found in the array.</exception>
    public int FindIndex(Span<int> array, int item)
    {
        if (array.Length == 0)
        {
            throw new ItemNotFoundException();
        }

        if (item < array[0] || item > array[^1])
        {
            throw new ItemNotFoundException();
        }

        if (array[0] == array[^1])
        {
            return item == array[0] ? 0 : throw new ItemNotFoundException();
        }

        var (left, right) = ComputeIndices(array, item);
        var (from, to) = SelectSegment(array, left, right, item);

        return from + FindIndex(array.Slice(from, to - from + 1), item);
    }

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