Python's enumerate for C++

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?

To be honest I'm not sure why would you choose the second snippet over the first. — freakish, Nov 29 '18 at 15:16
@freakish I can see it. Counter variables are a bit ew :) But to bring in a 3rd party library for it doesn't seem worthwhile. — Lightness Races in Orbit, Nov 29 '18 at 15:17
@freakish you never wanted that? What if you have a range based for loop that only in some branch needs the index? — 463035818_is_not_an_ai, Nov 29 '18 at 15:17
Only a couple of days old: http://reedbeta.com/blog/python-like-enumerate-in-cpp17/ — lubgr, Nov 29 '18 at 15:17
@freakish: Another nice aspect is that one can use the index in an `std::transform` with this. — Martin Ueding, Nov 29 '18 at 15:19
@MartinUeding that's a nice example. Although I'm referring to the concrete case you've posted, i.e. the for loop. — freakish, Nov 30 '18 at 09:24
[std::views::enumerate](https://en.cppreference.com/w/cpp/ranges/enumerate_view) is a C++23 solution. — Steve Ward, Aug 09 '23 at 01:45

Lightness Races in Orbit · Accepted Answer · 2018-11-29T15:18:15.573

17

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.

edited Nov 29 '18 at 15:18

answered Nov 29 '18 at 15:15

Lightness Races in Orbit

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FWIW, this was the top Google result for `boost enumerate`.... – Lightness Races in Orbit Nov 29 '18 at 15:15
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According to the documentation you can omit the index to start from zero. – Quentin Nov 29 '18 at 15:16
@LightnessRacesinOrbit: I searched for “C++ enumerate” but did not think about adding “boost” to the search terms. Thanks for the hint! – Martin Ueding Nov 29 '18 at 15:18
In C++11 you can use an initializer list, `boost::assign` is no longer necessary here. – Maxim Egorushkin Nov 29 '18 at 15:21
3

@MaximEgorushkin Indeed, hence "now-redundant" - however I chose to simply reproduce the Boost example verbatim. Readers are of course free to adapt it (lol) as they see fit. – Lightness Races in Orbit Nov 29 '18 at 15:22

lubgr · Answer 2 · 2018-11-29T15:32:00.333

3

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";;

edited Nov 29 '18 at 15:32

answered Nov 29 '18 at 15:23

lubgr

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Which C++ standard is this? It does not look like C++11, right? – Martin Ueding Nov 29 '18 at 15:24
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@MartinUeding C++17, with a library being considered for inclusion in `std` for C++20 – Caleth Nov 29 '18 at 15:25
1

This updated snippet compiles, tested with `clang++ -std=c++11`. – lubgr Nov 29 '18 at 15:32

Maxim Egorushkin · Answer 3 · 2018-11-29T15:23:12.170

2

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();
}

edited Nov 29 '18 at 15:23

answered Nov 29 '18 at 15:18

Maxim Egorushkin

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Unfortunately a bit risky -- forgot the `&`? container doesn't have proper reference semantics (`vector`)? Your `index` is madness and the subtraction has UB. But if you know what you're doing this is certainly a way to get the index if the iteration itself doesn't provide it. – Lightness Races in Orbit Nov 29 '18 at 15:21
@LightnessRacesinOrbit Added a note about `operator&`. – Maxim Egorushkin Nov 29 '18 at 15:23
Hadn't even thought of that. `std::addressof` can be your friend there. But now this is getting verbose. – Lightness Races in Orbit Nov 29 '18 at 15:23
@LightnessRacesinOrbit True, but I'd like to keep it simple. – Maxim Egorushkin Nov 29 '18 at 15:25

Vittorio Romeo · Answer 4 · 2018-11-29T15:35:10.893

1

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

edited Nov 29 '18 at 15:35

answered Nov 29 '18 at 15:27

Vittorio Romeo

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Generic lambdas are since C++14 only. Still, this isn't bad :) – Lightness Races in Orbit Nov 29 '18 at 15:28
The Haskell part of me really likes this! Unfortunately the code that I currently work on is a high performance production code, where we have to be grateful for using even C++11. I'd like to keep this answer, perhaps we can use it in five years. – Martin Ueding Nov 29 '18 at 15:29
3

@MartinUeding: you can use this in C++11, just replace `auto` in the lambda with the concrete type of the object. – Vittorio Romeo Nov 29 '18 at 15:29
@MartinUeding: added C++11-compliant version in the answer. – Vittorio Romeo Nov 29 '18 at 15:34

Python's enumerate for C++

4 Answers4