将向量元素复制到 std::array

Question

I have a vector of elements of type uint8 which I am copying into one std::array. After some operations again I need to copy the elements from std::array to a vector.Please find the code below

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#include <iostream>
#include <vector>
#include <array>
#include <cstdint>
#include <limits>
#include <algorithm>
std::array<std::uint32_t, 5> elem_arr ;
void copy_from_vector(std::vector<std::uint32_t> elem_vec)
{
    std::copy(elem_vec.begin(),elem_vec.end(),elem_arr.begin());
}
std::vector<std::uint32_t> copy_from_array()
{
    std::vector<std::uint32_t> ele_vec_copy {elem_arr.begin(),elem_arr.end()};
    return ele_vec_copy;
}
int main()
{

    std::vector<std::uint32_t> ele_vec {1,2,3};

    copy_from_vector(ele_vec);

    auto ele_vec_copy = copy_from_array();

    for(auto ele : ele_vec_copy)
        std::cout << ele << std::endl;

    return 0;
}

output:
======
1
2
3
0
0

Now the problem is my array size is 5 but vector having only 3 elements. when I copy the elements from the array to a new vector I am getting extra 2 zeros. How can I get the output as below

output:
======
1
2
3

Answer 1

You could copy by size of vector.

#include <iostream>
#include <vector>
#include <array>
#include <cstdint>
#include <limits>
#include <algorithm>

int main()
{
    std::array<std::uint32_t, 5> elem_arr ;
    std::vector<std::uint32_t> ele_vec {1,2,3};
    std::copy(ele_vec.begin(),ele_vec.end(),elem_arr.begin());
    std::vector<std::uint32_t> ele_vec_copy { elem_arr.begin() , elem_arr.begin() + ele_vec.size() };

    for(auto ele : ele_vec_copy)
        std::cout << ele << std::endl;

    return 0;
}

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After you edited the question

It is not possible to know how many elements used/set in std::array but this feature could be mimic by choosing a unique value which represents the element is null or not set like nullptr . But it comes with a constraint, you must not use the unique value at somewhere else in the array.

So the possible implementation is :

#include <iostream>
#include <vector>
#include <array>
#include <cstdint>
#include <limits>
#include <algorithm>
#include <limits>

std::array<std::uint32_t, 5> elem_arr;

void copy_from_vector(std::vector<std::uint32_t> elem_vec)
{
    std::copy(elem_vec.begin(),elem_vec.end(),elem_arr.begin());
}

std::vector<std::uint32_t> copy_from_array()
{
    std::vector<std::uint32_t> ele_vec_copy;
    std::copy_if( elem_arr.begin() , elem_arr.end() , 
                  std::back_inserter( ele_vec_copy ) , 
                  []( std::uint32_t val ) { 
                      return val != std::numeric_limits<std::uint32_t>::max(); 

                  } );
    return ele_vec_copy;
}

int main()
{

    elem_arr.fill( std::numeric_limits<std::uint32_t>::max() );
    std::vector<std::uint32_t> ele_vec {1,2,3};

    copy_from_vector(ele_vec);

    auto ele_vec_copy = copy_from_array();

    for(auto ele : ele_vec_copy)
        std::cout << ele << std::endl;

    return 0;
}

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Or store the elements as std::optional and don't copy null elements. Possible implementation is :

#include <iostream>
#include <vector>
#include <array>
#include <cstdint>
#include <limits>
#include <algorithm>
#include <limits>
#include <optional>

std::array<std::optional<std::uint32_t>, 5> elem_arr;

void copy_from_vector(std::vector<std::uint32_t> elem_vec)
{
    std::transform( elem_vec.begin() , elem_vec.end() , 
                    elem_arr.begin() ,
                    []( auto val ) { return std::optional<std::uint32_t>{ val }; }
                   );
}

std::vector<std::uint32_t> copy_from_array()
{
    std::vector<std::uint32_t> ele_vec_copy;
    std::vector<std::optional<std::uint32_t>> non_null_elements;
    std::copy_if( elem_arr.begin() , elem_arr.end() , 
                  std::back_inserter( non_null_elements ) , 
                  []( auto val ) { 
                      return val;
                  } );
    std::transform( non_null_elements.begin(), non_null_elements.end() , 
                    std::back_inserter( ele_vec_copy ) ,
                    []( auto val ) { return *val; } 
                  );
    return ele_vec_copy;
}

int main()
{
    std::vector<std::uint32_t> ele_vec {1,2,3};

    copy_from_vector(ele_vec);

    auto ele_vec_copy = copy_from_array();

    for(auto ele : ele_vec_copy)
        std::cout << ele << std::endl;

    return 0;
}

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