Question

This question applies to both std::set and std::unsorted_set.

I have an iterator to an element in a set. I’d like to use the iterator to get an “index” for the element based on its location in the set.

For example, the indices for my set would be as follows:

int index = 0;

for(MySetType::iterator begin = mySet.begin(); begin != mySet.end(); begin++)
{
    cout << "The index for this element is " << index;
    index++;
}

I have tried doing arithmetic using iterators but it doesn’t work:

int index = mySetIterator - mySet.begin();

Is there any way to use the iterator to get an index value like this based on its location in the set?

Answer 1

Use STL distance, namely std::distance(set.begin(), mySetIterator)

Please note that:

Returns the number of elements between first and last. The behavior
is undefined if last is not reachable from first by (possibly
repeatedly) incrementing first.

Remark : Complexity is linear;

However, if InputIt additionally meets the requirements of
LegacyRandomAccessIterator, complexity is constant.