optimization - Fastest way to loop through a 2d array?

Question

I just stumbled upon this blog post. The author shows two code samples that loop through a rectangle and compute something (my guess is the computing code is just a placeholder). On one of the example, he scans the rectangle vertically, and on the other horizontally. He then says the second is fastest, and every programmer should know why. Now I must not be a programmer, because to me it looks exactly the same. Can anyone explain that one to me?

Thanks.

Answer 1

Cache coherence. When you scan horizontally, your data will be closer together in memory, so you will have less cache misses and thus performance will be faster. For a small enough rectangle, this won't matter.