How to swap rows and columns of a 2d array?

Question

I\'m trying to write a method for \'transpose\' a two-dimensional array of integers, in which the rows and columns of the original matrix are exchanged.

However, I have n

Answer 1

The transpose of a matrix

int m = 4;
int n = 5;

// original matrix
int[][] arr1 = {
        {11, 12, 13, 14, 15},
        {16, 17, 18, 19, 20},
        {21, 22, 23, 24, 25},
        {26, 27, 28, 29, 30}};

// transposed matrix
int[][] arr2 = new int[n][m];

// swap rows and columns
IntStream.range(0, n).forEach(i ->
        IntStream.range(0, m).forEach(j ->
                arr2[i][j] = arr1[j][i]));

---

// output to the markdown table
String matrices = Stream.of(arr1, arr2)
        .map(arr -> Arrays.stream(arr).map(Arrays::toString)
                .collect(Collectors.joining("<br>")))
        .collect(Collectors.joining("</pre> | <pre>"));

System.out.println("| original matrix | transposed matrix |");
System.out.println("|---|---|");
System.out.println("| <pre>" + matrices + "</pre> |");

original matrix	transposed matrix
[11, 12, 13, 14, 15] [16, 17, 18, 19, 20] [21, 22, 23, 24, 25] [26, 27, 28, 29, 30]	[11, 16, 21, 26] [12, 17, 22, 27] [13, 18, 23, 28] [14, 19, 24, 29] [15, 20, 25, 30]

---

^{See also: Printing a snake pattern using an array}

Answer 2

You could iterate over the rows and columns and assign each element [i,j] to the transposed [j,i]:

/**
 * Transposses a matrix.
 * Assumption: mat is a non-empty matrix. i.e.:
 * 1. mat != null
 * 2. mat.length > 0
 * 3. For every i, mat[i].length are equal and mat[i].length > 0
 */
public static int[][] transpose(int[][] mat) {
    int[][] result = new int[mat[0].length][mat.length];
    for (int i = 0; i < mat.length; ++i) {
        for (int j = 0; j < mat[0].length; ++j) {
            result[j][i] = mat[i][j];
        }
    }
    return result;
}