C# shift two dimension array fast method

Question

I have a 2D string array in C# and I need to shift that array to left in one dimension how can I do that in efficient way

I dont want use nested for and i want an algurithm in O(n) not O(n²)

for (int i = 50; i < 300; i++)
{
    for (int j = 0; j < 300; j++)
    {
        numbers[i-50, j] = numbers[i, j];
    }
 }

What do you mean "shift that array to the left?" What have you tried so far? Do you have something that works, but isn't performant? — Matt Grande, Sep 27 '11 at 19:27
FYI the code you have posted is `O(n)` (n is the number of elements in the array) — Justin, Sep 28 '11 at 09:29

score 2 · Answer 1 · answered Sep 27 '11 at 19:37

2

The most efficient way would be to not shift it at all, but instead change how you access the array. For example, keep an offset that tells you where in the dimension the first column is.

answered Sep 27 '11 at 19:37

Guffa

687,336
108
737
1,005

score 2 · Accepted Answer · answered Sep 27 '11 at 19:43

If you want to shift large amounts of data around quickly, use Array.Copy rather than a loop that copies individual characters.

If you swap to a byte array and use Array.Copy or Buffer.BlockCopy you will probably improve the performance a bit more (but if you have to convert to/from character arrays you may lose everything you've gained).

(edit: Now that you've posted example code): If you use references to the array rows then you may be able to shift the references rather than having to move the data itself. Any you can still shift the references using Array.Copy)

But if you change your approach so you don't need to shift the data, you'll gain considerably better performance - not doing the work at all if you can avoid it is always faster! Chances are you can wrap the data in an accessor layer that keeps track of how much the data has been shifted and modifies your indexes to return the data you are after. (This will slightly slow down access to the data, but saves you shifting the data, so may result in a net win - depending on how much you access relative to how much you shift)

C# shift two dimension array fast method

2 Answers2