for(i = 0; i < n; i++)
{
  for(j = 0; j < m; j++)
  {
    /* Accessing a[j][i] is inefficient because we are not accessing  array sequentially in memory */
  }
}


/* After Interchanging */
for(j = 0; j < m; j++)
{
  for(i = 0; i < n; i++)
  {
    /* Accessing a[j][i] is efficient now*/
  }
}