Generate all possible pairs from A [ ] and store sums of all possible pairs in aux [ ]
int k = 0 ; for ( i = 0 ; i < n - 1 ; i ++ ) { for ( j = i + 1 ; j < n ; j ++ ) { aux [ k ] . sum = arr [ i ] + arr [ j ] ; aux [ k ] . first = i ; aux [ k ] . sec = j ; k ++ ; } }