How Can I Get the Index of the Current Element in a C Range-Based For Loop?

Finding the Index of the Current Object in a Range-Based For Loop

In a range-based for loop, iterating through a container provides access to each element directly without requiring an explicit iterator. However, there may arise situations where it becomes necessary to determine the index of the current element in the loop without relying on a separate iterator. This question explores a method to achieve this using composition and a specialized zipper technique.

Zipper Technique

The key to this approach lies in "zipping" the container with an index along the way. A zipper class is designed to create an iterator-like object that provides access to both the index and the value of the element in the container. This zipper effectively serves as a wrapper around the original container's iterator.

Implementation

The provided C code demonstrates this technique:

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

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

    private:
        size_t _pos;
        inner_iterator _it;
    };

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

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

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

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

This code defines a Indexer class that creates an indexed iterator for a given container T. The iterator class provides a reference to a pair containing the index and the value of the element. The index function returns an Indexer object that can be used in the range-based for loop to access both the index and the value.

Usage

To use this technique, simply zip the vector with a range of integers using the index function:

#include <iostream>
#include <iterator>
#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 indexed p variable iterates over the elements of the vector v, providing access to both the index and the value. You can then easily access these values within the loop.

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