iterator for the last element of a vector

Question

I need to get an iterator to the last element in a std::vector.

As known end() method returns an iterator referring to the past-the-end element in the vector container. Can I implement what I need by using end() - 1?

As far as I understand I can't use back() method, since it returns a reference.

See also [Iterating C++ vector from the end to the begin](http://stackoverflow.com/q/3610933/96780). — Daniel Daranas, Sep 10 '13 at 14:44

score 10 · Answer 1 · answered Sep 10 '13 at 14:44

10

Given that you know/have checked that the vector is not empty and C++11 is an option, you can use std::prev:

auto it = std::prev( your_vector.end() ); // or .cend() if a const_iterator is OK

Also note that using rbegin() returns a reverse iterator, which is different from a normal iterator.

answered Sep 10 '13 at 14:44

Daniel Frey

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Very helpful - I just discovered the issue with rbegin() returning a reverse iterator, when I tried assigning the result to a forward iterator. Thanks – Coder_Dan May 05 '17 at 12:09

score 9 · Accepted Answer · answered Sep 10 '13 at 14:40

9

The easiest is probably your_vec.rbegin().

answered Sep 10 '13 at 14:40

Jerry Coffin

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the iterators' type don't match. – athos Jun 15 '17 at 02:54
@athos: Doesn't match...what? He simply asked for an iterator to the last element, and `rbegin()` provides that. If you want some non-reverse iterator to the last element, well, perhaps you should ask a question about that; it's clearly a different question than the OP asked here though. – Jerry Coffin Jun 15 '17 at 02:57

Manu343726 · Answer 3 · 2013-09-10T15:10:28.247

3

Thats the rbegin() iterator. For example:

int main()
{
    std::vector<int> v = {0,1,2,3};

    for(auto& it = v.rbegin() ; it != v.rend() ; ++it)
        std::cout << *it << " ";
}

The output is:

3 2 1 0

edited Sep 10 '13 at 15:10

answered Sep 10 '13 at 14:45

Manu343726

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LOL I was reading the documentation I linked about [`std::rbegin()`](http://en.cppreference.com/w/cpp/iterator/rbegin) and I just noticed that feature is ***since C++14***. – Manu343726 Sep 10 '13 at 15:04
I always use `std::begin()`, so using `std::rbegin()` was a natural reflex action, even if it doesn't exist yet :) – Manu343726 Sep 10 '13 at 15:05

score 0 · Answer 4 · edited Sep 11 '13 at 12:17

0

You can use reverse_iterators. vec.rbegin() will give you a reverse_iterator pointing at the last element in the vector. More about it: http://en.cppreference.com/w/cpp/iterator/reverse_iterator

edited Sep 11 '13 at 12:17

answered Sep 10 '13 at 14:43

Rontogiannis Aristofanis

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score 0 · Answer 5 · answered Sep 10 '13 at 14:50

What do you need the iterator for? If it is for iterating from back to front, you can use the reverse iterators (rbegin, rend). If it is a function which expects an iterator (for example, because you want it to iterate through all but the last element), then you can use end() - 1 on a vector. This will only work on random access iterators, however; for other types, you will need std::prev (if you've got C++11) or the equivalent from your toolkit (pre C++11):

template <typename BidirectionalIterator>
BidirectionalIterator
prev( BidirectionalIterator it )
{
    -- it;
    return it;
}

(If you don't have it already, and aren't using C++11, add it to your toolkit.)

iterator for the last element of a vector

5 Answers5