TheAlgorithms/C-Sharp · warning

No unvisited cities remain.

Error message

No unvisited cities remain.

What it means

SolveNearestNeighbor throws InvalidOperationException when no unvisited city can be found for the next hop (next == -1) before the tour covers all cities. Under normal input this is unreachable — it fires only if the matrix contains values the search cannot consume (e.g. NaN or infinity distances) so no candidate ever wins the minimum scan.

Solutions

1. Sanitize the matrix: replace NaN/Infinity with large finite values or handle disconnected pairs explicitly.
2. Validate all entries are finite doubles before calling.
3. If the graph is genuinely disconnected, use a TSP variant that supports it instead of the plain nearest-neighbor solver.

Example fix

// before
for (int i=0;i<n;i++) for (int j=0;j<n;j++) if (double.IsNaN(m[i,j])) m[i,j] = double.PositiveInfinity;
solver.SolveNearestNeighbor(m);
// after
for (int i=0;i<n;i++) for (int j=0;j<n;j++) if (!double.IsFinite(m[i,j])) m[i,j] = 1e9;
var r = solver.SolveNearestNeighbor(m);

for (int i = 0; i < m.GetLength(0); i++)
    for (int j = 0; j < m.GetLength(1); j++)
        if (!double.IsFinite(m[i, j])) throw new ArgumentException($"Non-finite distance at [{i},{j}]");

try { var r = TravelingSalesmanSolver.SolveNearestNeighbor(m, start); }
catch (InvalidOperationException ex) { logger.LogError(ex, "Solver could not progress; check for NaN/Infinity distances"); }

When it happens

Trigger: A square matrix containing double.NaN or double.PositiveInfinity entries such that every unvisited city is 'worse' than the sentinel, leaving next at -1 mid-tour.

Common situations: Missing/unknown pairwise distances represented as NaN; disconnected graph encoded as infinity; matrix built by formula producing NaN (0/0, sqrt of negative).

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.

Related errors

• Distance matrix must be square.
• At least two cities are required.
• Specified argument was out of the range of valid values…
• Detected the same node twice. Confusion how this could ever…
• Invalid parameter settings for Ascon Hash

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/36f36972a397a6e5. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Problems/TravelingSalesman/TravelingSalesmanSolver.cs:100

        visited[start] = true;
        double totalDistance = 0;
        int current = start;
        for (int step = 1; step < n; step++)
        {
            double minDist = double.MaxValue;
            int next = -1;
            for (int j = 0; j < n; j++)
            {
                if (!visited[j] && distanceMatrix[current, j] < minDist)
                {
                    minDist = distanceMatrix[current, j];
                    next = j;
                }
            }

            if (next == -1)
            {
                throw new InvalidOperationException("No unvisited cities remain.");
            }

            route.Add(next);
            visited[next] = true;
            totalDistance += minDist;
            current = next;
        }

        totalDistance += distanceMatrix[current, start];
        route.Add(start);
        return (route.ToArray(), totalDistance);
    }

    /// <summary>
    /// Generates all permutations of the input array.
    /// Used for brute-force TSP solution.
    /// </summary>
    private static IEnumerable<int[]> Permute(int[] arr)

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