Concatenating two std::vectors

Question

How do I concatenate two std::vectors?

Answer 1

If you want to be able to concatenate vectors concisely, you could overload the += operator.

template <typename T>
std::vector<T>& operator +=(std::vector<T>& vector1, const std::vector<T>& vector2) {
    vector1.insert(vector1.end(), vector2.begin(), vector2.end());
    return vector1;
}

Then you can call it like this:

vector1 += vector2;

Answer 2

Concatenate two std::vector-s with for loop in one std::vector.

    std::vector <int> v1 {1, 2, 3}; //declare vector1
    std::vector <int> v2 {4, 5}; //declare vector2
    std::vector <int> suma; //declare vector suma

    for(int i = 0; i < v1.size(); i++) //for loop 1
    {
         suma.push_back(v1[i]);
    }

    for(int i = 0; i< v2.size(); i++) //for loop 2
    {
         suma.push_back(v2[i]);
    }

    for(int i = 0; i < suma.size(); i++) //for loop 3-output
    {
         std::cout << suma[i];
    }

Answer 3

Try, create two vectors and add second vector to first vector, code:

std::vector<int> v1{1,2,3};
std::vector<int> v2{4,5};

for(int i = 0; i<v2.size();i++)
{
     v1.push_back(v2[i]);
}

v1:1,2,3.

Description:

While i int not v2 size, push back element , index i in v1 vector.

Answer 4

vector1.insert( vector1.end(), vector2.begin(), vector2.end() );

Answer 5

With range v3, you may have a lazy concatenation:

ranges::view::concat(v1, v2)

Demo.

Answer 6

If your goal is simply to iterate over the range of values for read-only purposes, an alternative is to wrap both vectors around a proxy (O(1)) instead of copying them (O(n)), so they are promptly seen as a single, contiguous one.

std::vector<int> A{ 1, 2, 3, 4, 5};
std::vector<int> B{ 10, 20, 30 };

VecProxy<int> AB(A, B);  // ----> O(1)!

for (size_t i = 0; i < AB.size(); i++)
    std::cout << AB[i] << " ";  // ----> 1 2 3 4 5 10 20 30

Refer to https://stackoverflow.com/a/55838758/2379625 for more details, including the 'VecProxy' implementation as well as pros & cons.