What is the correct way to iterate backward through an std::array or std::vector?

Question

So my basic idea for doing that was this kind of loop :

std::array<int,10> t{};
for (int k = t.size()-1; k >= 0; k--)
    cout << t[k] << " ";

The problem is that the .size() method returns an std::array::size_type, which, from what i've read, could be larger than an int.

The problem is that size_type is unsigned, so when i do this :

for (auto k = t.size()-1; k >= 0; k--)
    cout << t[k] << " ";

It just loops infinitely because k can't be negative. So how would you iterate an std::array or std::vector backwars using a size_type variable ?

Using iterators is a "correct" way: https://stackoverflow.com/questions/3610933/iterating-c-vector-from-the-end-to-the-begin — sshashank124, Dec 28 '19 at 14:31
Precisely, [this](https://stackoverflow.com/a/24851790/6205379) answer. — Timo, Dec 28 '19 at 14:32
I like the idea of a reverse range based for loop: [https://stackoverflow.com/a/54401349/487892](https://stackoverflow.com/a/54401349/487892) — drescherjm, Dec 28 '19 at 14:37

Sam Varshavchik · Accepted Answer · 2019-12-28T14:36:00.423

7

If for some reason iterators don't work, and you must use indexes for some reason:

for (auto k = t.size(); k > 0; )
{
    --k;

    // The rest of your loop
}

edited Dec 28 '19 at 14:36

answered Dec 28 '19 at 14:33

Sam Varshavchik

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florestan · Answer 2 · 2019-12-28T14:39:10.027

7

If you can use C++11 or higher, you could / should use iterators. Soething like this:

#include <vector>
#include <iostream>

int main() {
    std::vector<int> v = {1,2,3,4,5};
    for(auto it = v.rbegin(); it != v.rend(); ++it) {
        std::cout << *it << '\n';
    }

    return 0;
}

Live example here.

edited Dec 28 '19 at 14:39

answered Dec 28 '19 at 14:34

florestan

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2

You should note it's possible if C++ 11 and higher is used. – Anatolii Dec 28 '19 at 14:37
2

Couldn't i use (auto it = v.end(); it != v.begin(); i--) instead of using rbegin() and rend() ? – lairv Dec 28 '19 at 14:38
1

@Anatolii right, thanks. – florestan Dec 28 '19 at 14:39
1

@lairv only if `i--` is inside of the loop, – Anatolii Dec 28 '19 at 14:39
1

@lairv dereferencing `v.end()` invokes undefined behavior, and in such loop (`auto it = v.end(); it != v.begin();`), you would skip the element referenced by `v.begin()` – Algirdas Preidžius Dec 28 '19 at 14:40
@lariv: No, because you'd dereference the end iterator in the first iteration. – florestan Dec 28 '19 at 14:42
Oh ok i didn't really know where the begin() and end() iterator where pointing to, i found this site useful : https://en.cppreference.com/w/cpp/iterator/rend – lairv Dec 28 '19 at 14:43

What is the correct way to iterate backward through an std::array or std::vector?

2 Answers2