c++ get std::vector<int> from std::vector<std::tuple<int, float>>

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i want to initialize a const std::vector<int> member variable in the initializer list of a constructor, given a std::vector<std::tuple<int, float>> constructor argument. The vector should contain all the first tuple items.

Is there a one-liner that extracts an std::vector<int> containing all the first tuple entries from an std::vector<std::tuple<int, float>> ?

stdtuple stdvector c++
4 Answers

With C++20 ranges:

struct X
{
    const std::vector<int> v;

    template <std::ranges::range R>
        requires std::convertible_to<std::ranges::range_value_t<R>, int>
    X(R r)
        : v{r.begin(), r.end()}
    {}

    X(const std::vector<std::tuple<int, float>>& vt)
        : X{vt | std::ranges::views::elements<0>}
    {}
};

With ranges-v3:

struct X
{
    const std::vector<int> v;

    X(const std::vector<std::tuple<int, float>>& vt)
        : v{vt | ranges::views::transform([] (auto t) {
                    return std::get<0>(t); })
               | ranges::to<std::vector>()}
    {}
};

And a Frankenstein's monster:

#include <ranges>
#include <range/v3/range/conversion.hpp>

struct X
{
    const std::vector<int> v;

    X(const std::vector<std::tuple<int, float>>& vt)
        : v{vt | std::ranges::views::elements<0>
               | ranges::to<std::vector>()}
    {}
};
user_28053050

Adding to @bolov's answer, let's talk about what you might have liked to do, but can't in a one-liner.

  • There's the to<>() function from ranges-v3 from @bolov 's answer - it materializes a range into an actual container (a range is lazily-evaluated, and when you create it you don't actually iterate over the elements). There is no reason it shouldn't be in the standard library, if you ask me - maybe they'll add it 2023?

  • You may be wondering - why do I need that to()? Can't I just initialize a vector by a range? And the answer is - sort of, sometimes, maybe. Consider this program:

    #include <vector>
#include <tuple>
#include <ranges>

void foo()
{
    using pair_type = std::tuple<int, double>;

    std::vector<pair_type> tvec {
         {12, 3.4}, {56, 7.8}, { 90, 91.2}
    };

    auto tv = std::ranges::transform_view(
        tvec, 
        [](const pair_type& p) { return std::get<0>(p);}
    );
    std::vector<int> vec1 { tv.begin(), tv.end() };

    std::vector<std::tuple<int>> vec2 { 
        std::ranges::transform_view{
            tvec,
            [](const pair_type& p) { return std::get<0>(p);} 
        }.begin(),
        std::ranges::transform_view{
            tvec, 
            [](const pair_type& p) { return std::get<0>(p);} 
        }.end()
    };
}

    The vec1 statement will compile just fine, but the vec2 statement will fail (GodBolt.org). This is surprising (to me anyway)!

  • You may also be wondering why you even need to go through ranges at all. Why isn't there a...

    template <typename Container, typename UnaryOperation>
auto transform(const Container& container, UnaryOperation op);

    which constructs the transformed container as its return value? That would allow you to write:

    std::vector<int> vec3 {
     transform(
         tvec, 
         [](const pair_type& p) { return std::get<0>(p); }
     ) 
}

    ... and Bob's your uncle. Well, we just don't have functions in the C++ standard library which take containers. It's either iterator pairs, which is the "classic" pre-C++20 standard library, or ranges.

    Now, the way I've declared the transform() function, it is actually tricky/impossible to implement generally, since you don't have a way of converting the type of a container to the same type of container but with a different element. So, instead, let's write something a little easier:

    template <
    typename T,
    template <typename> class Container,
    typename UnaryOperation
>
auto transform(const Container<T>& container, UnaryOperation op)
{
    auto tv = std::ranges::transform_view(container, op);
    using op_result = decltype(op(*container.begin()));
    return Container<op_result> { tv.begin(), tv.end() };
}

    and this works (GodBolt.org).

user_15930770

Not a one-liner to setup, but certainly one to use - you can write a function to do the conversion, and then the constructor can call that function when initializing the vector member, eg:

std::vector<int> convertVec(const std::vector<std::tuple<int, float>> &arg)
{
    std::vector<int> vec;
    vec.reserve(arg.size());
    std::transform(arg.begin(), arg.end(), std::back_inserter(vec),
        [](const std::tuple<int, float> &t){ return std::get<int>(t); }
    );
    return vec;
}

struct Test
{
    const std::vector<int> vec;

    Test(const std::vector<std::tuple<int, float>> &arg)
        : vec(convertVec(arg))
    {
    } 
};
user_658630

Yes, there is a one-liner if you use this library in Github.

The code goes like this.

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

#include <LazyExpression/LazyExpression.h>

using std::vector;
using std::tuple;
using std::ref;
using std::cout;

using LazyExpression::Expression;

int main()
{
    // Define test data
    vector<tuple<int, float>> vecTuple { {1,1.1}, {2,2.2}, {3, 3.3} };

    // Initialize int vector with the int part of the tuple (the one-liner :)
    vector<int> vecInt = Expression([](const tuple<int, float>& t) { return std::get<int>(t); }, ref(vecTuple))();

    // print the results
    for (int i : vecInt)
        cout << i << ' ';
    cout << "\n";
}
// Output:  1 2 3
user_111475550
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