Transposing matrices in Java

Question

I think I am missing something fundamental here about Java, I am not sure why my code below do not work, my steps:

0. the input is a 2x2 matrix,
1. copy the original matrix,
2. loop through rows then column,
3. assign original matrix's column values to the rows of the transpose matrix.

static void transpose(int[][] matrix) {
    int[][] temp = matrix.clone();
    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 2; j++) {
            matrix[i][j] = temp[j][i];
        }
    }
}

Answer 1

The problem is use the function clone() because that will create a new matrix sharing only row arrays.

Using the same code style, that works:

int[][] temp = new int[2][2];
for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 2; j++) {
        temp[i][j] = matrix[j][i];
    }
}

Note that, as temp is an empty matrix, the comparsion will be temp = matrix .

So this works for a simple 2x2 matrix, but if you need other dimensions, instead of

int[][] temp = new int[2][2];

Use

int[][] temp = new int[matrix[0].length][matrix.length];

By the way, update the reference memory of the matrix is not a good practice. I think your method should return temp matrix.

An entire update, a better method could be this one. Is almost the same you have but it can accept diferent lengths.

public static int[][] transpose(int [][] matrix){
    int[][] temp = new int[matrix[0].length][matrix.length];
    for (int i = 0; i < matrix.length; i++) {
        for (int j = 0; j < matrix[i].length; j++) {
            temp[j][i] = matrix[i][j];
        }
    }
    return temp;
}

Answer 2

You can use IntStream instead of for loop :

public static void main(String[] args) {
    int[][] m1 = {
            {1, 2, 3},
            {4, 5, 6},
            {7, 8, 9}};

    int[][] m2 = transpose(m1);

    Arrays.stream(m2).map(Arrays::toString).forEach(System.out::println);
    // [1, 4, 7]
    // [2, 5, 8]
    // [3, 6, 9]
}

static int[][] transpose(int[][] matrix) {
    return IntStream.range(0, matrix.length).mapToObj(i ->
            IntStream.range(0, matrix[i].length).map(j ->
                    matrix[j][i]).toArray())
            .toArray(int[][]::new);
}