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

This line the culprit :

x[j][i]=i+j;

The second version uses continuous memory thus will be substantially faster.

I tried with

x[50000][50000];

and the time of execution is 13s for version1 versus 0.6s for version2.