Convert std::array<std::array<T,N>> into std::vector<T> in modern C++

Question

I have a buch of triangles represented by a vector of 3d vectors ( std::vector<Vector3> ) and indices, represented in the following way

using int3 = std::array<int, 3>;
std::vector<int3> indices;

I need to convert the indices into a std::vector<size_t> format to interact with a library.

I know how to do this using raw loops, but I would like to here about how to do it using algorithms. I tried something like

std::vector<size_t> indices_converted;
std::transform(std::begin(indices), std::end(indices), std::back_inserter(indices_converted), [](const auto &v) {
});

but is not enough, since I can't insert three values at once into the indices_converted vector.

So, what are your ideas?

Answer 1

One-liner using latest version of range-v3 library:

#include <array>
#include <vector>

#include <range/v3/view/join.hpp>
#include <range/v3/range/conversion.hpp>

// ...

std::vector<std::array<int, 3>> v = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
const auto result = ranges::views::join(v) | ranges::to<std::vector>();

On godbolt

Answer 2

You can use either the range-based for loop or the standard algorithm std::for_each .

For example

#include <iostream>
#include <array>
#include <vector>
#include <iterator>

int main() 
{
    using int3 = std::array<int, 3>;
    std::vector<int3> indices = 
    {
        { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 }
    };

    std::vector<size_t> indices_converted;
    indices_converted.reserve( 3 * indices.size() );

    for ( const auto &item : indices )
    {
        indices_converted.insert( std::end( indices_converted ), std::begin( item ), std::end( item ) );
    }

    for ( const auto &item : indices_converted ) std::cout << item << ' ';
    std::cout << '\n';

    return 0;
}

or

#include <iostream>
#include <array>
#include <vector>
#include <iterator>
#include <algorithm>

int main() 
{
    using int3 = std::array<int, 3>;
    std::vector<int3> indices = 
    {
        { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 }
    };

    std::vector<size_t> indices_converted;
    indices_converted.reserve( 3 * indices.size() );

    std::for_each( std::begin( indices ), std::end( indices ),
                   [&indices_converted]( const auto &item )
                   {
                    indices_converted.insert( std::end( indices_converted ), std::begin( item ), std::end( item ) );        
                   } );

    for ( const auto &item : indices_converted ) std::cout << item << ' ';
    std::cout << '\n';

    return 0;
}

In the both cases the output is

1 2 3 4 5 6 7 8 9