Question

Which of the following orderings of nested loops to iterate over a 2D array is more efficient in terms of time (cache performance)? Why?

int a[100][100];

for(i=0; i<100; i++)
{
   for(j=0; j<100; j++)
   {
       a[i][j] = 10;    
   }
}

or

for(i=0; i<100; i++)
{
   for(j=0; j<100; j++)
   {
      a[j][i] = 10;    
   }
}

Answer 1

The first method is slightly better, as the cells being assigned to lays next to each other.

First method:

[ ][ ][ ][ ][ ] ....
^1st assignment
   ^2nd assignment
[ ][ ][ ][ ][ ] ....
^101st assignment

Second method:

[ ][ ][ ][ ][ ] ....
^1st assignment
   ^101st assignment
[ ][ ][ ][ ][ ] ....
^2nd assignment