Sorting 2D array of integers by column

Question

I need to build a method in Java where the input is a 2D array of integers and get as a result a 2D array of integers where each element makes reference to a position of an elem

Answer 1

Sorting the indexes of matrix elements by columns is sorting the indexes of elements by the rows of a transposed matrix:

int m = 5;
int n = 6;
int[][] arr1 = new int[][]{
        {124, 188, 24, 254, 339, 3},
        {0, 7, 77, 145, 159, 1},
        {206, 340, 280, 523, 433, 5},
        {310, 265, 151, 411, 398, 4},
        {24, 104, 0, 183, 198, 2}};

int[][] arr2 = IntStream
        // iterate over the indices
        // of the rows of the array
        .range(0, n)
        .mapToObj(i -> IntStream
                // iterate over the
                // indices of the columns
                .range(0, m)
                .boxed()
                // sort indices of the elements of the
                // columns by its values in the array
                .sorted(Comparator.comparingInt(j -> arr1[j][i]))
                .mapToInt(Integer::intValue)
                // sorted column of indices is
                // a row in the new array
                .toArray())
        // return sorted array of indices
        .toArray(int[][]::new);

// transpose the array of indices
int[][] arr3 = new int[m][n];
IntStream.range(0, m).forEach(i ->
        IntStream.range(0, n).forEach(j ->
                arr3[i][j] = arr2[j][i]));

// output
Arrays.stream(arr3).map(Arrays::toString).forEach(System.out::println);

Output:

[1, 1, 4, 1, 1, 1]
[4, 4, 0, 4, 4, 4]
[0, 0, 1, 0, 0, 0]
[2, 3, 3, 3, 3, 3]
[3, 2, 2, 2, 2, 2]