[FIXED] Flatmapping a 2D array by column rather than row in Java

Issue

Given the following two dimensional array

int[][] arr = {{1, 2}, {3, 4}, {5, 6}};

How can I flatten it column-by-column using the Java 8 Stream API? I want to get:

int[] result = {1, 3, 5, 2, 4, 6};

I tried doing a simple flatMap, but this flattens row-by-row and results in the wrong order:

// result is { 1, 2, 3, 4, 5, 6 }
int[] result = Arrays.stream(arr)
    .flatMapToInt(Arrays::stream)
    .toArray();

I considered transposing the array first so that I can use the above snippet, but creating an intermediate, transposed copy of the array seems unnecessary. How can I flatmap by column directly?

It can be assumed that all the nested arrays are of same length.

Solution

You can stream the inner indexes and flatMap to each outer array:

IntStream.range(0, arr[0].length)
        .flatMap(i -> Arrays.stream(arr).mapToInt(a -> a[i]))
        .toArray()

Answered By - shmosel
Answer Checked By - Gilberto Lyons (JavaFixing Admin)