How to append and prepend 2D arrays in Java without Array copy assuming the column sizes are the same?

Question

I've tried following this one How do you append two 2D array in java properly? removing all of the array copys but something is wrong. I also tried another guide but that only worked if the rows were the same.

public int [][] appendMatrix(int[][]matrix, int [][] matrix2)
    {

        this.matrix = new int[matrix.length + matrix2.length][matrix[0].length];
        for(int i = 0; i < matrix.length; i++)
        {
            for(int j = 0; j < matrix[i].length; j++)
            {
                this.matrix[i][j] = matrix[i][j];
            }
            for(int j = matrix.length; j < matrix.length + matrix2.length; j++)
            {
                this.matrix[i][j]= matrix2[i-matrix.length][j];
            }   
        }
        return this.matrix;**

Answer 1

The important thing to consider is when we go from the final row in our first matrix, we want to keep that value so we can use it to add the first through n'th row of our second matrix to our resulting matrix, without losing track.

Example output: output as an array and visualized as a matrix

package matrix;

// I know you don't want to use imports, this is simply for testing purposes.
import java.util.Arrays;

public class MatrixAddition
{
    public static void main(String[] args)
    {
        int[][] matrix1 =
        {
                { 1, 2, 3 },
                { 4, 5, 6 },
                { 7, 8, 9 },
                { 10, 11, 12 } };
        int[][] matrix2 =
        {
                { 1, 1, 1 },
                { 2, 3, 4 } };


        System.out.println("Appending the two matrices results in: ");
        System.out.println(Arrays.deepToString(twoDMatrixAppend(matrix1, matrix2)));
        printMatrix(twoDMatrixAppend(matrix1, matrix2));

        System.out.println("\nPrepending the two matrices results in: ");
        System.out.println(Arrays.deepToString(twoDMatrixPrepend(matrix1, matrix2)));
        printMatrix(twoDMatrixPrepend(matrix1, matrix2));
    }


    private static int[][] twoDMatrixAppend(int[][] matrix1, int[][] matrix2)
    {
        if (matrix1[0].length != matrix2[0].length)
        {
            return null; // Or throw new incompatible matrices exception
        }

        int resultingRowLength = matrix1.length + matrix2.length; // The new length of the resulting matrix

        int[][] result = new int[resultingRowLength][matrix1[0].length];

        int currentRow, col, matrixTwoRowStart;
        for (currentRow = 0; currentRow < matrix1.length; currentRow++)
        {
            for (col = 0; col < matrix1[0].length; col++)
            {
                result[currentRow][col] = matrix1[currentRow][col];
            }
        }

        for (matrixTwoRowStart = 0; matrixTwoRowStart < matrix2.length; matrixTwoRowStart++, currentRow++)
        {
            for (col = 0; col < matrix2[0].length; col++)
            {
                result[currentRow][col] = matrix2[matrixTwoRowStart][col];
            }
        }

        return result;
    }

    private static int[][] twoDMatrixPrepend(int[][] matrix1, int[][] matrix2)
    {
        return twoDMatrixAppend(matrix2, matrix1);
    }

    private static void printMatrix(int[][] arr)
    {
        System.out.println();
        int row, col;
        for (row = 0; row < arr.length; row++)
        {
            for (col = 0; col < arr[0].length; col++)
            {
                System.out.print(String.format("%4d", arr[row][col]));
            }
            System.out.println();
        }
    }

}