Question

Basically you have two ways for doing this:

for (int x = 0; x < UPPER_X; x++)     for (int y = 0; y < UPPER_Y; y++)     {                 arr1[x, y] = get_value();         arr2[y, x] = get_value();     } 

The only difference is what variable to change in the inner loop: first or second. I heard that the results differ from language to language.

What is right order for .NET?

Answer 1

The reason one is faster than the other has to do with the processor’s cache, and the way the data is laid out in memory.

There are two normal ways to store the two-dimensional data is in one-dimensional address space, Either you can store all of the data for the first row, then the second row, and so on (aka row major order), or you can do it by columns (aka column major order). Below is what the memory locations would be for a 3×3 array for both of these options.

Rows:

1 2 3 4 5 6 7 8 9 

Columns:

1 4 7 2 5 8 3 6 9 

When you access one memory location, an entire cache line (which can be between 8 and 512 bytes, according to Wikipedia) is loaded into the cache. So if you access the next memory location it will already be in the cache. Thus, it can be much faster to access memory sequentially than to jump around in the address space. Thus, with large two-dimensional arrays, there can be a significant speed difference between choosing rows or columns as your inner loop variable.