Get index in C++11 foreach loop

Question

Is there a convenient way to get the index of the current container entry in a C++11 foreach loop, like enumerate in python:

for idx, obj in enumerate(container):
    pass

I could imagine an iterator that can also return the index or similar.

Of course I could have a counter, but often iterators don't give guarantees of the order they iterate over a container.

Answer 1

A good implementation of the feature you are requested can be found here:

https://github.com/ignatz/pythonic

The idea behind is, that you build a wrapper struct with a custom iterator that does the counting. Below is a very minimal exemplary implementation to illustrate the idea:

// Distributed under the terms of the GPLv2 or newer

#include <iostream>
#include <vector>
#include <tuple>

// Wrapper class
template <typename T>
class enumerate_impl
{
public:
    // The return value of the operator* of the iterator, this
    // is what you will get inside of the for loop
    struct item
    {
        size_t index;
        typename T::value_type & item;
    };
    typedef item value_type;

    // Custom iterator with minimal interface
    struct iterator
    {
        iterator(typename T::iterator _it, size_t counter=0) :
            it(_it), counter(counter)
        {}

        iterator operator++()
        {
            return iterator(++it, ++counter);
        }

        bool operator!=(iterator other)
        {
            return it != other.it;
        }

        typename T::iterator::value_type item()
        {
            return *it;
        }

        value_type operator*()
        {
            return value_type{counter, *it};
        }

        size_t index()
        {
            return counter;
        }

    private:
        typename T::iterator it;
        size_t counter;
    };

    enumerate_impl(T & t) : container(t) {}

    iterator begin()
    {
        return iterator(container.begin());
    }

    iterator end()
    {
        return iterator(container.end());
    }

private:
    T & container;
};

// A templated free function allows you to create the wrapper class
// conveniently 
template <typename T>
enumerate_impl<T> enumerate(T & t)
{
    return enumerate_impl<T>(t);
}



int main()
{
    std::vector<int> data = {523, 1, 3};
    for (auto x : enumerate(data))
    {
        std::cout << x.index << ": " << x.item << std::endl;
    }
}

Answer 2

What about a simple solution like:

int counter=0;
for (auto &val: container)
{
    makeStuff(val, counter);

    counter++;
}

You could make a bit more "difficult" to add code after the counter by adding a scope:

int counter=0;
for (auto &val: container)
{{
    makeStuff(val, counter); 
}counter++;}

As @graham.reeds pointed, normal for loop is also a solution, that could be as fast:

int counter=0;
for (auto it=container.begin(); it!=container.end(); ++it, ++counter)
{
    makeStuff(val, counter);
}

And finally, a alternative way using algorithm:

int counter = 0;
std::for_each(container.begin(), container.end(), [&counter](int &val){ 
    makeStuff(val, counter++);
});

Note: the order between range loop and normal loop is guaranteed by the standard 6.5.4. Meaning the counter is able to be coherent with the position in the container.

Answer 3

If you have access to Boost its range adaptors can be used like this:

#include <boost/range/adaptor/indexed.hpp>
using namespace boost::adaptors;
 
for (auto const& elem : container | indexed(0))
{
    std::cout << elem.index() << " - " << elem.value() << '\n';
}

Source (where there are also other examples)

Answer 4

C++17 and structured bindings makes this look OK - certainly better than some ugly mutable lambda with a local [i = 0](Element&) mutable or whatever I've done before admitting that probably not everything should be shoehorned into for_each() et al. - and than other solutions that require a counter with scope outside the for loop.

for (auto [it, end, i] = std::tuple{container.cbegin(), container.cend(), 0};
     it != end; ++it, ++i)
{
      // something that needs both `it` and `i`ndex
}

You could make this generic, if you use this pattern often enough:

template <typename Container>
auto
its_and_idx(Container&& container)
{
    using std::begin, std::end;
    return std::tuple{begin(container), end(container), 0};
}

// ...

for (auto [it, end, i] = its_and_idx(foo); it != end; ++it, ++i)
{
    // something
}

---

C++ Standard proposal P2164 proposes to add views::enumerate, which would provide a view of a range giving both reference-to-element and index-of-element to a user iterating it.

We propose a view enumerate whose value type is a struct with 2 members index and value representing respectively the position and value of the elements in the adapted range.

[ . . .]

This feature exists in some form in Python, Rust, Go (backed into the language), and in many C++ libraries: ranges-v3, folly, boost::ranges (indexed).

The existence of this feature or lack thereof is the subject of recurring stackoverflow questions.

Hey, look! We're famous.

Answer 5

If you need the index then a traditional for works perfectly well.

for (int idx=0; idx<num; ++idx)
{
// do stuff
}