Copy part of 3-d array to 2-d array

Question

Let's say I have a multidimensional array:

var arr = new double[2, 5, 5]
{
    {
        { 1, 1, 1, 1, 1 },
        { 1, 1, 1, 1, 1 },
        { 1, 1, 1, 1, 1 },
        { 1, 1, 1, 1, 1 },
        { 1, 1, 1, 1, 1 }
    },
    {
        { 1, 1, 1, 1, 1 },
        { 1, 2, 2, 2, 1 },
        { 1, 2, 2, 2, 1 },
        { 1, 2, 2, 2, 1 },
        { 1, 1, 1, 1, 1 }
    },
};

I want to copy 3 by 3 part of that array starting from index [1,1,1] till index [1,3,3] (all 2 values).

What is the most efficient way of doing so ? So far, I do it with a loop:

var arr2 = new int[3, 3];

int x_start = 1;
int y_start = 1;

for (int i = 0; i < arr2.GetLength(0); i++)
{
    for (int j = 0; j < arr2.GetLength(1); j++)
    {
        arr2[i, j] = arr[1, x_start + i, y_start + j];
    }
}

But I wonder if there is a more efficient way of doing it ?

That’s the most efficient way to do it. You are using fast loops and only loop over those indexes you are interested in. — poke
– poke, Commented Aug 6, 2018 at 22:46

Jared Merritt · Accepted Answer · 2018-08-06 23:10:22Z

1

poke already made the same point in their comment, but this is essentially the best way of doing this.

That’s the most efficient way to do it. You are using fast loops and only loop over those indexes you are interested in. - poke

You could possibly cache the two GetLength() calls in an integer, but I doubt that'd make any meaningful difference in performance.

answered Aug 6, 2018 at 23:10

Jared Merritt

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