How can I get the index of an element while iterating through a vector using a range-based for loop in C ?

Index Extraction using Range-Based For Loops

In C programming, a range-based for loop offers a concise way to iterate over elements of a container. However, sometimes it becomes necessary to determine the index of the current element during the iteration without relying on an additional iterator.

Problem:

Given a vector of values and a range-based for loop as shown below:

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

Is there a way to find the position of elem within the vector without maintaining a separate iterator?

Solution:

Yes, it is possible to extract the index while iterating through a range-based for loop. This technique involves a combination of templated structures and iterators:

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 {
        typedef typename iterator_extractor<T>::type inner_iterator;
        typedef typename std::iterator_traits<inner_iterator>::reference inner_reference;
    public:
        typedef std::pair<size_t, inner_reference&> reference;

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

    private:
        size_t _pos;
        inner_iterator _it;
    };

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

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

private:
    T& _container;
}; // class Indexer

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

This code snippet provides the ability to iterate over a container while capturing the index and the value simultaneously. The Indexer class wraps the container and provides a custom iterator that yields pairs of indices and element references.

Example Usage:

To use the Indexer class in a range-based for loop, you can do the following:

#include <iostream>
#include <limits>
#include <vector>

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

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

In this example, the loop will print each element of the vector along with its index:

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

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