How to rearrange elements in a 2D array?

Question

C++ / C#

How to rearrange elements in a 2D array up to a specific element? Without using std!
Example:
We have a two-dimensional array:
0 1 2
5 8 3
4 4 2
We want to get for n = 8 (counting from 0):
2 4 4
3 8 5
2 1 0 - completely inverted
If n=3 (counting from 0), then we change only the first four elements of the array, i.e. at the output:
5 2 1
0 8 3
4 4 2 - elements from 0 to 3 are swapped (zero to 3, 1 to 2)

Answer 1

If the array is on the stack, then this method will do:

const int n  = 5;
    int arr[3][3] = {0, 1, 2, 5, 8, 3, 4, 4, 2};
    int *ptr = reinterpret_cast<int *>(arr);

    for(int i = 0, c = (n + 1) / 2; i < c; ++i) {
        int tmp = ptr[i];
        ptr[i] = ptr[n - i];
        ptr[n - i] = tmp;
    }