indexed

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##### indexed

Syntax | Code
--- | ---
Pipe | `rng | boost::adaptors::indexed`
Function | `boost::adaptors::index(rng)`

- Returns: A range adapted to return both the element and the associated index. The returned range consists of iterators that have in addition to the usual iterator member functions an `index()` member function that returns the appropriate index for the element in the sequence corresponding with the iterator.
- Range Category: Single Pass Range
- Returned Range Category: The range category of `rng`

###### indexed example

    #include <boost/range/adaptor/indexed.hpp>
    #include <boost/range/algorithm/copy.hpp>
    #include <boost/assign.hpp>
    #include <algorithm>
    #include <iostream>
    #include <vector>

    template<class Iterator>
    void display_element_and_index(Iterator first, Iterator last)
    {
        for (Iterator it = first; it != last; ++it)
        {
            std::cout << "Element = " << *it << " Index = " << it.index() << std::endl;
        }
    }

    template<class SinglePassRange>
    void display_element_and_index(const SinglePassRange& rng)
    {
        display_element_and_index(boost::begin(rng), boost::end(rng));
    }

    template<class Iterator1, class Iterator2>
    void check_element_and_index(
            Iterator1 test_first,
            Iterator1 test_last,
            Iterator2 reference_first,
            Iterator2 reference_last)
    {
        BOOST_CHECK_EQUAL( std::distance(test_first, test_last),
                           std::distance(reference_first, reference_last) );

        int reference_index = 0;

        Iterator1 test_it = test_first;
        Iterator2 reference_it = reference_first;
        for (; test_it != test_last; ++test_it, ++reference_it, ++reference_index)
        {
            BOOST_CHECK_EQUAL( *test_it, *reference_it );
            BOOST_CHECK_EQUAL( test_it.index(), reference_index );
        }
    }

    int main(int argc, const char* argv[])
    {
        using namespace boost::assign;
        using namespace boost::adaptors;

        std::vector<int> input;
        input += 10,20,30,40,50,60,70,80,90;

        display_element_and_index( input | indexed(0) );

        return 0;
    }

This would produce the output:

    Element = 10 Index = 0
    Element = 20 Index = 1
    Element = 30 Index = 2
    Element = 40 Index = 3
    Element = 50 Index = 4
    Element = 60 Index = 5
    Element = 70 Index = 6
    Element = 80 Index = 7
    Element = 90 Index = 8