Python's enumerate for C++ [duplicate]

Question

In Python there is enumerate which takes a sequence/iterator and yields pairs of an integer index and the value itself. In C++ I occasionally find myself writing

for (size_t i = 0; i != vector.size(); ++i) {
    auto const &elem = vector[i];
    // ...

Similar to Python I would like to write

for (auto const &it : enumerate(vector)) {
    // it.first is the index (size_t)
    // it.second is the element (T const&)

Does such an enumerate exist in either the STL or a common library like Boost?

Answer 1

Yes, this is what Boost's adapators::indexed does.

Their example (which also uses the now-redundant Boost.Assign for terse container initialisation) follows:

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


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;

    for (const auto& element : input | indexed(0))
    {
        std::cout << "Element = " << element.value()
                  << " Index = " << element.index()
                  << std::endl;
    }

    return 0;
}

Nothing in the standard library, though it's not hard to write.

Answer 2

Here is an example using range-v3. A bit more verbose than a handcrafted solution, but it's nice IMHO how you can assemble such a range from existing views.

#include <range/v3/view/indices.hpp>
#include <range/v3/view/zip.hpp>

using namespace ranges;

std::vector<int> vec{42, 43, 44};

for (const auto& idxAndValue : view::zip(view::indices, vec))
   std::cout << ideAndValue.first << " : " << idxAndValue.second << "\n";;