CODEWITHSUNDEEP
arrays2doutput
user1800967
user1800967

Reputation: 955

2-dimensional array not being updated as expected

I'm running the following code on a 4x4 array:

private static final int DIMENSION = 4, MAXIMUM = 99;

public static int[][] floydAlgorithm(int[][] oldMatrix)
{
    int[][] newMatrix = new int[oldMatrix.length][oldMatrix[0].length];

    // nested for loops that update the new matrix with the minimum between the
    // old array entry and the sum of two elements along the same row and
    //column of the old array entry
    for (int x = 0; x < oldMatrix.length; x++)
    {
        for (int y = 0; y < oldMatrix[0].length; y++)
        {
            for (int i = 0; i < DIMENSION; i++)
                newMatrix[x][y] = Math.min(oldMatrix[x][y], 
                        oldMatrix[x][i] + oldMatrix[i][y]);
        }
    }

    return newMatrix;
}

public static void fillFloydMatrix(int[][] matrix) // fills 2d array with values
{
    int [] allInts = {0, 7, 5, MAXIMUM, MAXIMUM, 0, 7, 6,
            MAXIMUM, MAXIMUM, 0, MAXIMUM, 4, 1, 11, 0};

    for (int x = 0; x < matrix.length; x++)
        for (int y = 0; y < matrix[0].length; y++)
            matrix[x][y] = allInts[DIMENSION * x + y];
}

public static void displayMatrix(int[][] matrix)
{
    for (int x = 0; x < matrix.length; x++)
    {
        for (int y = 0; y < matrix[0].length; y++)
            System.out.print(matrix[x][y] + " ");

        System.out.println();
    }
}

public static void main(String[] args)
{
    int[][] floydMatrix = new int[DIMENSION][DIMENSION];

    fillFloydMatrix(floydMatrix);

    displayMatrix(floydMatrix);

    floydMatrix = floydAlgorithm(floydMatrix);

    System.out.println();

    displayMatrix(floydMatrix);
}

For some reason, this code outputs the two arrays below, the first of which prints as expected but the second does not:

0, 7, 5, 99

99, 0, 7, 6

99, 99, 0, 99

4, 1, 11, 0

*

0, 7, 5, 99

10, 0, 7, 6

99, 99, 0, 99

4, 1, 11, 0

The 99 on the leftmost column, 2nd row, correctly updates to 10 ([1][3] + [3][0], 6 + 4) but the upper rightmost 99 does not update to 13 ([0][1] + [1][3], 7 + 6) and the 11 at the bottom does not update to 8 ([1][2] + [3][1], 7 + 1). I've tried to find out why but ultimately, I have had no luck in spotting what the issue is. Thus I ask for help in finding the reason why? Thank you so much for your help and time in advance and of course, I will continue to look into why this behavior might be but additional eyes can't hurt.

Upvotes: 0

Views: 48

Answers (1)

Andrei Nicusan
Andrei Nicusan

Reputation: 4623

Change this

newMatrix[x][y] = Math.min(oldMatrix[x][y], 
                    oldMatrix[x][i] + oldMatrix[i][y]);

with this:

newMatrix[x][y] = Math.min(newMatrix[x][y], 
                    oldMatrix[x][i] + oldMatrix[i][y]);

You will also need to initialize your new matrix by filling with Integer.MAX_VALUE. See Arrays.fill .

Upvotes: 1

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