This can be easily done by:

for (i = 0; i < 1<<N; i++) {
   ...
}

It will give us: 000, 001, 010, 011, 100, 101, 110, 111 in binary if N = 3

What if I want to loop in ascending order of the number of each subset's elements? For example, when N = 3, the order can be 000, 001, 010, 100, 011, 101, 110, 111 or 000, 001, 010, 100, 011, 110, 101, 111. Anyone knows the best way to do this?