Most modern computer languages (except Fortran) use row-major order, in which elements of a matrix row are adjacent in memory: A[i][j] is stored as:

         [j]

[i]

0	1	2	...	n
0	1	2	...	n
0	1	2	...	n

To get the best performance, j should be the index of the inner loop, so that j will vary fastest and accesses will be adjacent:

  for (i=0; i<1000; i++)
    for (j=0; j<1000; j++)
      sum = sum + arr[i][j];

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