What causes "Board size must be positive." in TheAlgorithms/C-Sharp?

Pass a positive board size (n >= 1).

Board size must be positive.

Error message

Board size must be positive.

What it means

This ArgumentException is the size guard at the top of OpenKnightTour.Tour (OpenKnightTour.cs:57). The knight's tour is searched on an n x n board, so a non-positive n would allocate nothing or an invalid board and no tour search is meaningful. It fires when Tour is called with n <= 0. Note the method can also throw later (same ArgumentException type) when no tour exists for small boards such as n = 2, 3, 4.

Solutions

Pass a positive board size (n >= 1).
Validate n > 0 at the call site and fail with a clearer user-facing message.
Fix the config/CLI parsing that yielded 0 or a negative size.

Example fix

// before
var n = int.Parse(args[0]); // may be <= 0
var board = tour.Tour(n);
// after
var n = int.Parse(args[0]);
if (n <= 0) throw new ArgumentException("Board size must be a positive integer.");
var board = tour.Tour(n);

Defensive patterns

Strategy: validation

Validate before calling

if (n <= 0) throw new ArgumentException("Board size must be a positive integer.", nameof(n));

Type guard

static bool IsValidBoardSize(int n) => n > 0;

Try / catch

try { board = tour.Tour(n); }
catch (ArgumentException ex) { Console.Error.WriteLine(ex.Message); }

Prevention

Validate CLI/config integers before use
Reject zero/negative sizes at input parsing
Use int.TryParse with explicit failure handling

When it happens

Trigger: Calling Tour(0), Tour(-3), or passing a board size from unparsed/default-zero config input.

Common situations: Missing command-line/config value defaulting to 0; integer parse failure swallowed and leaving 0; loop variable misuse in generated calls.

Related errors

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/e189d4fcdf611dc2. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Problems/KnightTour/OpenKnightTour.cs:57

    /// </returns>
    /// <exception cref="ArgumentException">
    /// Thrown when <paramref name="n"/> ≤ 0, or when no tour exists / is found for the given <paramref name="n"/>.
    /// </exception>
    /// <remarks>
    /// <para>
    /// This routine tries every square as a starting point. As soon as a complete tour is found,
    /// the filled board is returned. If no tour is found, an exception is thrown.
    /// </para>
    /// <para>
    /// <b>Performance:</b> Exponential in the worst case. For larger boards, consider adding
    /// Warnsdorff’s heuristic (choose next moves with the fewest onward moves) or a hybrid approach.
    /// </para>
    /// </remarks>
    public int[,] Tour(int n)
    {
        if (n <= 0)
        {
            throw new ArgumentException("Board size must be positive.", nameof(n));
        }

        var board = new int[n, n];

        // Try every square as a starting point.
        for (var r = 0; r < n; r++)
        {
            for (var c = 0; c < n; c++)
            {
                board[r, c] = 1; // first step
                if (KnightTourHelper(board, (r, c), 1))
                {
                    return board;
                }

                board[r, c] = 0; // backtrack and try next start
            }
        }

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