TheAlgorithms/Java · error · IllegalArgumentException

Cost matrix must be rectangular with equal row lengths

Error message

Cost matrix must be rectangular with equal row lengths

What it means

HungarianAlgorithm.validate throws this IllegalArgumentException when any row is null or its length differs from the first row's length c. Unlike the flow algorithms, the Hungarian algorithm accepts rectangular (non-square) matrices but requires consistent column counts across all rows.

Source

Thrown at src/main/java/com/thealgorithms/graph/HungarianAlgorithm.java:141

            if (j >= 0 && j < cols) {
                assignment[i] = j;
                total += cost[i][j];
            }
        }
        return new Result(assignment, total);
    }

    private static void validate(int[][] cost) {
        if (cost == null || cost.length == 0) {
            throw new IllegalArgumentException("Cost matrix must not be null or empty");
        }
        int c = cost[0].length;
        if (c == 0) {
            throw new IllegalArgumentException("Cost matrix must have at least 1 column");
        }
        for (int i = 0; i < cost.length; i++) {
            if (cost[i] == null || cost[i].length != c) {
                throw new IllegalArgumentException("Cost matrix must be rectangular with equal row lengths");
            }
            for (int j = 0; j < c; j++) {
                if (cost[i][j] < 0) {
                    throw new IllegalArgumentException("Costs must be non-negative");
                }
            }
        }
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Pad all rows to the same column count (max across workers) before calling.
2. Ensure every row is non-null and of equal length.
3. Validate row lengths against the first row during construction.

Example fix

// before
int[][] cost = { {1,2}, {3} }; // ragged

// after
int[][] cost = { {1,2}, {3,0} }; // pad missing with large/zero cost

int cols = cost[0].length;
for (int[] row : cost) {
    if (row == null || row.length != cols) {
        throw new IllegalArgumentException("Ragged cost matrix");
    }
}

boolean isRectangular(int[][] cost) {
    if (cost == null || cost.length == 0) return false;
    int c = cost[0].length;
    for (int[] row : cost) if (row == null || row.length != c) return false;
    return true;
}

When it happens

Trigger: Passing a ragged array where rows have differing lengths, or a matrix with a null row.

Common situations: Appending tasks per worker without aligning column counts. A null row from incomplete initialization. Mixing data sources with different task counts.

Related errors

• Cost matrix must have at least 1 column
• Capacity matrix must be square
• Capacity matrix must be square
• Cost matrix must not be null or empty
• Costs must be non-negative

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/83b04f8f73c3fa09. Report an issue: GitHub.