How can I get the strictly lower value than upper bound in a std::set?

Question

Say I have a set

s={1 5 10}

Now if I run a loop for [0,2] and everytime check for the strictly lower than upper bound of the set than how can I handle lower value than s.begin() ?See the code for further clarification-

    set<int>s;
    s={1,5,10};
    for(auto i:s)
    {
        cout<<i<<"\n\n";
        for(int j=0;j<3;++j)
        {
            auto it=s.upper_bound(j);
            cout<<"For upper_bound of "<<j<<" ->"<<*it<<"~~~";
            it--;
            cout<<"For strictly lower than upper_bound of "<<j<<" ->"<<*it<<"\n";
        }
        cout<<endl;
    }

Here For strictly lower than upper_bound of 0 -> 3.Here one possible solution may be always checks for greater or equal value than s.begin().Is there any other better way to do that?

Answer 1

You can return a std::optional if (it == s.begin()) and accordingly print some suitable default such as none.

auto strictly_lower = [&] () -> std::optional<int> {
                if(it == s.begin()) {
                    return std::nullopt;
                }
                it--;
                return std::optional{*it};
            }();
            
std::cout<<"For strictly lower than upper_bound of "<<j<<" ->"<< (strictly_lower.has_value() ? std::to_string(*strictly_lower) : "none") <<"\n";

Code Link