C++: Iterate over a Vector in Reverse Order – (backward direction)

In this article, we will discuss three different ways to iterate or loop over a C++ Vector in reverse direction.

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C++: Iterate over a vector in reverse order using Indexing

Suppose we have a vector of size N, then we can iterate over the vector in reverse order by traversing from index position N-1 to 0. For example,

#include<iostream>
#include<vector>
using namespace std;

int main()
{
    // Create a vector of integers
    vector<int> vec_of_num{1, 3, 4, 7, 8, 9};

    // Iterate over a vector in backward direction using
    // indexing
    for(int i = vec_of_num.size() - 1; i >= 0; i--)
    {
        std::cout<<vec_of_num[i]<<", ";
    }

    return 0;
}

Output:

9, 8, 7, 4, 3, 1,

The benefit of this approach is that you will also be aware of indexes of elements while iteration.

C++: Iterate over a vector in reverse order using Reverse Iterator

A reverse iterator is a kind of iterator, that travels in backwards direction.
It means when we increment a reverse_iterator, it goes to the previous element in container. So, to iterate over a vector in reverse direction, we can use the reverse_iterator to iterate from end to start.

vector provides two functions which returns a reverse_iterator i.e.

• vector::rbegin() –> Returns a reverse iterator that points to the last element of vector
• vector::rend() –> Returns a reverse iterator that points to the virtual element before the start of vector.

Now we can use a while loop to traverse from end to start in a vector using reverse_iterator returned by the functions rbegin() and rend(). For example,

#include<iostream>
#include<vector>
using namespace std;

int main()
{
    // Create a vector of integers
    vector<int> vec_of_num{1, 3, 4, 7, 8, 9};

    // Iterate over a vector in backward direction using
    // reverse iterators
    std::vector<int>::reverse_iterator it = vec_of_num.rbegin();
    while (it != vec_of_num.rend())
    {
        std::cout<<*it<<", ";
        it++;
    }

    return 0;
}

Output:

9, 8, 7, 4, 3, 1,

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C++: Iterate over a vector in reverse order in single line

In the previous example, we used a while loop to iterate over a vector using reverse_iterator. But we can do same in a single line using STL Algorithm for_each().

Using STL Algorithm for_each(start, end, callback), we can iterate over all elements of a vector in a single line. It accepts three arguments i.e.

• Start Iterator -> Iterator pointing to the start of a range
• End Iterator -> Iterator pointing to the End of a range
• Callback Function -> A function that needs to be applied to all elements in the range from start to end.

for_each() iterates over all the elements between start & end iterator and apply the given callback on each element.

If we pass the reverse iterators returned by rbegin() and rend() to for_each(), then it will iterate over the elements of vector in reverse direction and apply a lambda function on each item of the vector. Inside the lambda function we can perform any operation like printing the elements or any other operation. For example,

#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;

int main()
{
    // Create a vector of integers
    vector<int> vec_of_num{1, 3, 4, 7, 8, 9};

    // Iterate over a vector in backward direction using
    // reverse iterators, for_each() and Lambda function
    std::for_each(  vec_of_num.rbegin(), 
                    vec_of_num.rend(),
                    [](const auto & elem){
                        std::cout<<elem<<", "; 
                    });

    return 0;
}

Output:

9, 8, 7, 4, 3, 1,

We iterated over all elements of a vector in reverse direction.

Summary

We learned about three different ways to iterate or loop over a vector in reverse direction.