How to iterate over two sets at the same time?

Question

I have two sets:

std::set<int> one;
std::set<int> two;

Each set contains indices of some objects - I need to deactivate the indices only in one, and activate the indices only in two. The indices that are in both will remain as is.

If these were sorted vectors, I would do something like:

int c1 = 0; int c2 = 0;
while(true){
    if(one[c1] < two[c2]){
       one[c1].deactivate();
       c1++;
    }else if(one[c1]==two[c2]){
       c1++; c2++;
    }else{
       two[c2].activate();
       c2++;
    }
    if(c1 == one.size() || c2 == two.size()){
        break;
    }
}

while(c1<one.size()){
   one[c1].deactivate();
   c1++;
}
while(c2<two.size()){
   two[c2].activate();
   c2++;
}

Example:

one = {1,3,5,6,7,8,10}
two = {2,4,6,8,10,12}
Before running the algorithm:
Active: 1,3,5,6,7,8,20
After running the algorithm:
Active: 2,4,6,8,10,12

But as these are sets, I'm not sure how to iterate over them this way. How do I accomplish the same thing ?

There's not "good" way to do it in c++11. In c++23 exist [`std::ranges::views::zip_view`](https://en.cppreference.com/w/cpp/ranges/zip_view). — Sugar, Jan 12 '23 at 09:48
Forget about `while(true)` when you just want to iterate over a container (put the running condition in it instead) — Fareanor, Jan 12 '23 at 09:50
The code you have above will translate pretty directly into code using set iterators (or vector iterators for that matter). — john, Jan 12 '23 at 10:06
I suppose `two[c2].activate();` is some hypothetical syntax, but it should be `two.activate(c2)` , because `int` has no member functions at all — 463035818_is_not_an_ai, Jan 12 '23 at 10:23
Is the starting list of active indices actually 1, 3, 5, 6, 7, 8, **10**? — Bob__, Jan 12 '23 at 10:37
This particular problem is probably solved better with set operations [set_difference](https://en.cppreference.com/w/cpp/algorithm/set_difference) and [set_intersection](https://en.cppreference.com/w/cpp/algorithm/set_intersection). — j6t, Jan 12 '23 at 11:15

score 3 · Accepted Answer · answered Jan 12 '23 at 09:58

How to iterate over two sets at the same time?

You can use iterators for this:

auto it1 = one.begin();
auto it2 = two.begin();
while (it1 != one.end() && it2 != two.end()) {
    int i1 = *it1;
    int i2 = *it2;
    
    // Do something with indexes

    it1++;
    it2++;
}

How to iterate over two sets at the same time?

1 Answers1