Why does the order of the loops affect performance when iterating over a 2D array?

Question

Below are two programs that are almost identical except that I switched the i and j variables around. They both run in different amounts of time. C

Answer 1

The reason is cache-local data access. In the second program you're scanning linearly through memory which benefits from caching and prefetching. Your first program's memory usage pattern is far more spread out and therefore has worse cache behavior.