TheAlgorithms/C-Sharp · error

Adjacency matrix must be square.

Error message

Adjacency matrix must be square.

What it means

ColorGraph requires a square 2D boolean matrix (n x n) where rows and columns both index vertices. If GetLength(0) != GetLength(1), the input cannot represent an adjacency matrix, so ArgumentException is thrown with parameter name adjacencyMatrix.

Solutions

1. Rebuild the matrix as square with dimension equal to the number of vertices.
2. Validate GetLength(0) == GetLength(1) at the call site before invoking.
3. Fix the export/import code that produced a non-square matrix.

Example fix

// before
var matrix = new bool[vertices, edges]; // wrong
var colors = solver.ColorGraph(matrix, 3);
// after
var matrix = new bool[vertices, vertices];
var colors = solver.ColorGraph(matrix, 3);

if (matrix is null || matrix.GetLength(0) != matrix.GetLength(1)) throw new ArgumentException("Adjacency matrix must be square.");

static bool IsSquare(bool[,] m) => m.GetLength(0) == m.GetLength(1);

try { colors = solver.ColorGraph(matrix, k); }
catch (ArgumentException ex) { Console.Error.WriteLine(ex.Message); }

When it happens

Trigger: Passing a rectangular matrix such as new bool[3, 4], or a matrix built with mismatched vertex/edge counts.

Common situations: Loading a matrix from CSV where rows have unequal column counts; building the matrix with (vertices, edges) dimensions by mistake; padding errors after deserialization.

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

• Number of samples and labels must match.
• Feature count mismatch.
• The value for some n_i is smaller than or equal to 1.
• The GCD of n_ = and n_ = equals and thus these values…
• Coins array must contain coin 1

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/cd83518009d3884e. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Problems/GraphColoring/GraphColoringSolver.cs:78

    /// </para>
    /// <para>
    /// <b>Example:</b> For a triangle graph (3 vertices, all connected), at least 3 colors
    /// are required. Calling this method with <c>numColors = 2</c> will throw an exception,
    /// while <c>numColors = 3</c> will return a valid coloring such as <c>[0, 1, 2]</c>.
    /// </para>
    /// </remarks>
    public int[] ColorGraph(bool[,] adjacencyMatrix, int numColors)
    {
        if (adjacencyMatrix is null)
        {
            throw new ArgumentNullException(nameof(adjacencyMatrix));
        }

        var numVertices = adjacencyMatrix.GetLength(0);

        if (numVertices != adjacencyMatrix.GetLength(1))
        {
            throw new ArgumentException("Adjacency matrix must be square.", nameof(adjacencyMatrix));
        }

        if (numColors <= 0)
        {
            throw new ArgumentException("Number of colors must be positive.", nameof(numColors));
        }

        // Handle empty graph
        if (numVertices == 0)
        {
            return Array.Empty<int>();
        }

        var colors = new int[numVertices];

        // Initialize all vertices as uncolored (-1)
        Array.Fill(colors, -1);

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