How to Get the Index of an Element in a C Range-Based For Loop?

How to Determine an Object's Index in a Range-Based for Loop

In the context of range-based for loops, it is often desirable to access the index of the current element within the container. Consider the following code:

vector<int> list;
for (auto& elem : list) {
    int i = elem;
}

In this scenario, you may want to know the position of elem in the list vector without maintaining a separate iterator.

Zipper-Based Approach

To achieve this, a "zipper" approach can be employed. Instead of iterating directly over the container, you can "zip" it with an index along the way.

template <typename T>
struct iterator_extractor {
    typedef typename T::iterator type;
};

template <typename T>
struct iterator_extractor<T const&> {
    typedef typename T::const_iterator type;
};

template <typename T>
class Indexer {
public:
    class iterator {
    public:
        typedef std::pair<size_t, typename iterator_extractor<T>::type::reference> reference;

        iterator(typename iterator_extractor<T>::type it) : _pos(0), _it(it) {}

        reference operator*() const { return reference(_pos, *_it); }

        iterator& operator++() { ++_pos; ++_it; return *this; }
        iterator operator++(int) { iterator tmp(*this); ++*this; return tmp; }

        bool operator==(const iterator& it) const { return _it == it._it; }
        bool operator!=(const iterator& it) const { return !(*this == it); }

    private:
        size_t _pos;
        typename iterator_extractor<T>::type _it;
    };

    Indexer(T& t) : _container(t) {}

    iterator begin() const { return iterator(_container.begin()); }
    iterator end() const { return iterator(_container.end()); }

private:
    T& _container;
};

template <typename T>
Indexer<T>& index(T& t) { return Indexer<T>(t); }

This code creates an Indexer class that provides an iterator which combines the element and its index in a pair. By utilizing this Indexer, you can access the index and element simultaneously.

Example Usage

The following code demonstrates how to use the zipper approach:

#include <iostream>
#include <vector>

using namespace std;

int main() {
    vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9};

    for (auto p : index(v)) {
        cout << p.first << ": " << p.second << "\n";
    }

    return 0;
}

This code will print the index and element values in the following format:

0: 1
1: 2
2: 3
3: 4
4: 5
5: 6
6: 7
7: 8
8: 9

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