Python's enumerate for C++ [duplicate] - c++

Question

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How to find the index of current object in range-based for loop?
(12 answers)
Closed 4 years ago.
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";;

Answer 3

Another way that only works:
with references to elements, and
array-based containers, and
elements do not overload operator&
for(auto const &it : vector) {
size_t index = &it - vector.data();
}

Answer 4

Here's a version using an higher-order function. I like it because it's simple to implement and doesn't require you to know the nuances of structured bindings. It also doesn't require any extra dependency.
template <typename Container, typename F>
void enumerate(Container&& c, F&& f)
{
std::size_t i = 0;
for(auto&& x : std::forward<Container>(c))
{
f(i++, forward_like<Container>(x));
}
}
(Where forward_like moves x if Container is an rvalue.)
Usage:
enumerate(std::vector{'a', 'b', 'c'}, [](auto index, auto x)
{
std::cout << index << ": " << x << '\n';
});
Prints:
0: 'a'
1: 'b'
2: 'c'
live example on wandbox.org
C++11 compliant version: live example on wandbox.org