c++ - Element at index in a std::set?

Question

I've stumbled upon this problem: I can't seem to select the item at the index' position in a normal std::set. Is this a bug in STD?

Below a simple example:

#include <iostream>
#include <set>

int main()
{
    std::set<int> my_set;
    my_set.insert(0x4A);
    my_set.insert(0x4F);
    my_set.insert(0x4B);
    my_set.insert(0x45);

    for (std::set<int>::iterator it=my_set.begin(); it!=my_set.end(); ++it)
        std::cout << ' ' << char(*it);  // ups the ordering

    //int x = my_set[0]; // this causes a crash!
}

Anything I can do to fix the issue?

Answer 1

It doesn't cause a crash, it just doesn't compile. set doesn't have access by index.

You can get the nth element like this:

std::set<int>::iterator it = my_set.begin();
std::advance(it, n);
int x = *it;

Assuming my_set.size() > n, of course. You should be aware that this operation takes time approximately proportional to n. In C++11 there's a nicer way of writing it:

int x = *std::next(my_set.begin(), n);

Again, you have to know that n is in bounds first.