java multi-dimensional array transposing

I saw that all of the answers create a new resultant matrix. This is simple: matrix[i][j] = matrix[j][i]; However, you can also do this in-place, in case of square matrix.

// Transpose, where m == n
for(int i = 0; i < m; i++) {
  for(int j = i+1; j < n; j++) {
    int temp = matrix[i][j];
    matrix[i][j] = matrix[j][i];
    matrix[j][i] = temp;
  }
}

This is better for larger matrices, where creating a new resultant matrix is wasteful in terms of memory. If its not square, you can create a new one with NxM dimensions and do the out of place method. Note: for in-place, take care of j = i+1; Its not 0.

try this:

@Test
    public void transpose() {

        final int[][] original = new int[][] { { 1, 2, 3, 4 }, { 5, 6, 7, 8 }, { 9, 10, 11, 12 } };
        for (int i = 0; i < original.length; i++) {
            for (int j = 0; j < original[i].length; j++) {
                System.out.print(original[i][j] + " ");
            }
            System.out.print("\n");
        }
        System.out.print("\n\n matrix transpose:\n");
        // transpose
        if (original.length > 0) {
            for (int i = 0; i < original[0].length; i++) {
                for (int j = 0; j < original.length; j++) {
                    System.out.print(original[j][i] + " ");
                }
                System.out.print("\n");
            }
        }
    }

output:

1 2 3 4 
5 6 7 8 
9 10 11 12 


 matrix transpose:
1 5 9 
2 6 10 
3 7 11 
4 8 12 

I´m just digging this thread up because i did not find a working solution in the answers, therefore i will post one to help anyone who searches for one:

public int[][] transpose (int[][] array) {
  if (array == null || array.length == 0)//empty or unset array, nothing do to here
    return array;

  int width = array.length;
  int height = array[0].length;

  int[][] array_new = new int[height][width];

  for (int x = 0; x < width; x++) {
    for (int y = 0; y < height; y++) {
      array_new[y][x] = array[x][y];
    }
  }
  return array_new;
}

you should call it for example via:

int[][] a = new int[][] {{1,2,3,4},{5,6,7,8}};
for (int i = 0; i < a.length; i++) {
  System.out.print("[");
  for (int y = 0; y < a[0].length; y++) {
    System.out.print(a[i][y] + ",");
  }
  System.out.print("]\n");
}

a = transpose(a); // call
System.out.println("");

for (int i = 0; i < a.length; i++) {
  System.out.print("[");
  for (int y = 0; y < a[0].length; y++) {
    System.out.print(a[i][y] + ",");
  }
  System.out.print("]\n");
} 

which will as expected output:

[1,2,3,4,]
[5,6,7,8,]

[1,5,]
[2,6,]
[3,7,]
[4,8,]