How Can I Reverse a Range-Based For-Loop in C Using Boost.Range?

Reverse Range-Based For-Loops in C 11 with Boost.Range

In C , the range-based for-loop is a powerful tool for iterating over containers. However, by default, it iterates from the beginning of the container to the end. Is there a way to reverse this direction?

Reverse Iterators with the Range-Based For-Loop

One way to iterate over a container in reverse is to use the rbegin() and rend() member functions. However, this requires the use of explicit iterators and can become unwieldy for complex scenarios:

for (auto i = c.rbegin(); i != c.rend(); ++i) { ... }

Boost.Range's Reverse Adapter

Fortunately, Boost.Range provides an elegant solution with the boost::adaptors::reverse adapter. This adapter wraps a container and reverses the direction of its iterators. This allows for seamless integration with range-based for-loops:

for (auto& i : boost::adaptors::reverse(c)) { ... }

Usage Example

Here's an example of using boost::adaptors::reverse to iterate over a list in reverse order:

#include <list>
#include <iostream>
#include <boost/range/adaptor/reversed.hpp>

int main() {
    std::list<int> x{2, 3, 5, 7, 11, 13, 17, 19};
    for (auto i : boost::adaptors::reverse(x)) {
        std::cout << i << '\n';
    }
    for (auto i : x) {
        std::cout << i << '\n';
    }
    return 0;
}

Output:

19
17
13
11
7
5
3
2
2
3
5
7
11
13
17
19

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