How to find the index of current object in range-based for loop?

Question

Assume I have the following code:

vector<int> list;
for(auto& elem:list) {
    int i = elem;
}

Can I find the position of elem in the vector without maintaining a separate iterator?

Answer 1

Yes you can, it just take some massaging ;)

The trick is to use composition: instead of iterating over the container directly, you "zip" it with an index along the way.

Specialized zipper code:

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

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

    private:
        size_t _pos;
        inner_iterator _it;
    };

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

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

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

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

And using it:

#include <iostream>
#include <iterator>
#include <limits>
#include <vector>

// Zipper code here

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";
    }
}

You can see it at ideone, though it lacks the for-range loop support so it's less pretty.

EDIT:

Just remembered that I should check Boost.Range more often. Unfortunately no zip range, but I did found a pearl: boost::adaptors::indexed. However it requires access to the iterator to pull of the index. Shame :x

Otherwise with the counting_range and a generic zip I am sure it could be possible to do something interesting...

In the ideal world I would imagine:

int main() {
    std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9};

    for (auto tuple: zip(iota(0), v)) {
        std::cout << tuple.at<0>() << ": " << tuple.at<1>() << "\n";
    }
}

With zip automatically creating a view as a range of tuples of references and iota(0) simply creating a "false" range that starts from 0 and just counts toward infinity (or well, the maximum of its type...).

Answer 2

jrok is right : range-based for loops are not designed for that purpose.

However, in your case it is possible to compute it using pointer arithmetic since vector stores its elements contiguously (*)

vector<int> list;
for(auto& elem:list) { 
    int i = elem;
    int pos = &elem-&list[0]; // pos contains the position in the vector 

    // also a &-operator overload proof alternative (thanks to ildjarn) :
    // int pos = addressof(elem)-addressof(list[0]); 

}

But this is clearly a bad practice since it obfuscates the code & makes it more fragile (it easily breaks if someone changes the container type, overload the & operator or replace 'auto&' by 'auto'. good luck to debug that!)

NOTE: Contiguity is guaranteed for vector in C++03, and array and string in C++11 standard.

Answer 3

No, you can't (at least, not without effort). If you need the position of an element, you shouldn't use range-based for. Remember that it's just a convenience tool for the most common case: execute some code for each element. In the less-common circumstances where you need the position of the element, you have to use the less-convenient regular for loop.

Answer 4

Based on the answer from @Matthieu there is a very elegant solution using the mentioned boost::adaptors::indexed:

std::vector<std::string> strings{10, "Hello"};
int main(){
    strings[5] = "World";
    for(auto const& el: strings| boost::adaptors::indexed(0))
      std::cout << el.index() << ": " << el.value() << std::endl;
}

You can try it

This works pretty much like the "ideal world solution" mentioned, has pretty syntax and is concise. Note that the type of el in this case is something like boost::foobar<const std::string&, int>, so it handles the reference there and no copying is performed. It is even incredibly efficient: https://godbolt.org/g/e4LMnJ (The code is equivalent to keeping an own counter variable which is as good as it gets)

For completeness the alternatives:

size_t i = 0;
for(auto const& el: strings) {
  std::cout << i << ": " << el << std::endl;
  ++i;
}

Or using the contiguous property of a vector:

for(auto const& el: strings) {
  size_t i = &el - &strings.front();
  std::cout << i << ": " << el << std::endl;
}

The first generates the same code as the boost adapter version (optimal) and the last is 1 instruction longer: https://godbolt.org/g/nEG8f9

Note: If you only want to know, if you have the last element you can use:

for(auto const& el: strings) {
  bool isLast = &el == &strings.back();
  std::cout << isLast << ": " << el << std::endl;
}

This works for every standard container but auto&/auto const& must be used (same as above) but that is recommended anyway. Depending on the input this might also be pretty fast (especially when the compiler knows the size of your vector)

Replace the &foo by std::addressof(foo) to be on the safe side for generic code.

Answer 5

If you have a compiler with C++14 support you can do it in a functional style:

#include <iostream>
#include <string>
#include <vector>
#include <functional>

template<typename T>
void for_enum(T& container, std::function<void(int, typename T::value_type&)> op)
{
    int idx = 0;
    for(auto& value : container)
        op(idx++, value);
}

int main()
{
    std::vector<std::string> sv {"hi", "there"};
    for_enum(sv, [](auto i, auto v) {
        std::cout << i << " " << v << std::endl;
    });
}

Works with clang 3.4 and gcc 4.9 (not with 4.8); for both need to set -std=c++1y. The reason you need c++14 is because of the auto parameters in the lambda function.

Answer 6

If you insist on using range based for, and to know index, it is pretty trivial to maintain index as shown below. I do not think there is a cleaner / simpler solution for range based for loops. But really why not use a standard for(;;)? That probably would make your intent and code the clearest.

vector<int> list;
int idx = 0;
for(auto& elem:list) {
    int i = elem;
    //TODO whatever made you want the idx
    ++idx;
}

Answer 7

There is a surprisingly simple way to do this

vector<int> list;
for(auto& elem:list) {
    int i = (&elem-&*(list.begin()));
}

where i will be your required index.

This takes advantage of the fact that C++ vectors are always contiguous.

Answer 8

Here's a quite beautiful solution using c++20:

#include <array>
#include <iostream>
#include <ranges>

template<typename T>
struct EnumeratedElement {
    std::size_t index;
    T& element;
};

auto enumerate(std::ranges::range auto& range) 
    -> std::ranges::view auto 
{
    return range | std::views::transform(
        [i = std::size_t{}](auto& element) mutable {
            return EnumeratedElement{i++, element};
        }
    );
}

auto main() -> int {
    auto const elements = std::array{3, 1, 4, 1, 5, 9, 2};
    for (auto const [index, element] : enumerate(elements)) {
        std::cout << "Element " << index << ": " << element << '\n';
    }
}

The major features used here are c++20 ranges, c++20 concepts, c++11 mutable lambdas, c++14 lambda capture initializers, and c++17 structured bindings. Refer to cppreference.com for information on any of these topics.

Note that element in the structured binding is in fact a reference and not a copy of the element (not that it matters here). This is because any qualifiers around the auto only affect a temporary object that the fields are extracted from, and not the fields themselves.

The generated code is identical to the code generated by this (at least by gcc 10.2):

#include <array>
#include <iostream>
#include <ranges>

auto main() -> int {
    auto const elements = std::array{3, 1, 4, 1, 5, 9, 2};
    for (auto index = std::size_t{}; auto& element : elements) {
        std::cout << "Element " << index << ": " << element << '\n';
        index++;
    }
}

Proof: https://godbolt.org/z/a5bfxz

Answer 9

I read from your comments that one reason you want to know the index is to know if the element is the first/last in the sequence. If so, you can do

for(auto& elem:list) {
//  loop code ...
    if(&elem == &*std::begin(list)){ ... special code for first element ... }
    if(&elem == &*std::prev(std::end(list))){ ... special code for last element ... }
//  if(&elem == &*std::rbegin(list)){... (C++14 only) special code for last element ...}
//  loop code ... 
}

EDIT: For example, this prints a container skipping a separator in the last element. Works for most containers I can imagine (including arrays), (online demo http://coliru.stacked-crooked.com/a/9bdce059abd87f91):

#include <iostream>
#include <vector>
#include <list>
#include <set>
using namespace std;

template<class Container>
void print(Container const& c){
  for(auto& x:c){
    std::cout << x; 
    if(&x != &*std::prev(std::end(c))) std::cout << ", "; // special code for last element
  }
  std::cout << std::endl;
}

int main() {
  std::vector<double> v{1.,2.,3.};
  print(v); // prints 1,2,3
  std::list<double> l{1.,2.,3.};
  print(l); // prints 1,2,3
  std::initializer_list<double> i{1.,2.,3.};
  print(i); // prints 1,2,3
  std::set<double> s{1.,2.,3.};
  print(s); // print 1,2,3
  double a[3] = {1.,2.,3.}; // works for C-arrays as well
  print(a); // print 1,2,3
}

Answer 10

Tobias Widlund wrote a nice MIT licensed Python style header only enumerate (C++17 though):

GitHub

Blog Post

Really nice to use:

std::vector<int> my_vector {1,3,3,7};

for(auto [i, my_element] : en::enumerate(my_vector))
{
    // do stuff
}

Answer 11

Here's a macro-based solution that probably beats most others on simplicity, compile time, and code generation quality:

#include <iostream>

#define fori(i, ...) if(size_t i = -1) for(__VA_ARGS__) if(i++, true)

int main() {
    fori(i, auto const & x : {"hello", "world", "!"}) {
        std::cout << i << " " << x << std::endl;
    }
}

Result:

$ g++ -o enumerate enumerate.cpp -std=c++11 && ./enumerate 
0 hello
1 world
2 !

Answer 12

If you want to avoid having to write an auxiliary function while having the index variable local to the loop, you can use a lambda with a mutable variable.:

int main() {
    std::vector<char> values = {'a', 'b', 'c'};
    std::for_each(begin(values), end(values), [i = size_t{}] (auto x) mutable {
        std::cout << i << ' ' << x << '\n';
        ++i;
    });
}

Answer 13

You can use std::views::enumerate (C++23) to get the index of elements.

#include <iostream>
#include <ranges>
#include <vector>
 
int main()
{
    const auto letters = std::vector{'A', 'B', 'C', 'D', 'E'};
 
    for (const auto& [index, letter] : std::views::enumerate(letters))
    {
        std::cout << '(' << index << ':' << letter << ") ";
    }
}

Check the C++ 23 compiler support page to see if your compiler supports this feature.