Concatenating two std::vectors

Question

How do I concatenate two std::vectors?

Answer 1

To be honest, you could fast concatenate two vectors by copy elements from two vectors into the other one or just only append one of two vectors!. It depends on your aim.

Method 1: Assign new vector with its size is the sum of two original vectors' size.

vector<int> concat_vector = vector<int>();
concat_vector.setcapacity(vector_A.size() + vector_B.size());
// Loop for copy elements in two vectors into concat_vector

Method 2: Append vector A by adding/inserting elements of vector B.

// Loop for insert elements of vector_B into vector_A with insert() 
function: vector_A.insert(vector_A .end(), vector_B.cbegin(), vector_B.cend());

Answer 2

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

Answer 3

vector<int> v1 = {1, 2, 3, 4, 5};
vector<int> v2 = {11, 12, 13, 14, 15};
copy(v2.begin(), v2.end(), back_inserter(v1));

Answer 4

This solution might be a bit complicated, but boost-range has also some other nice things to offer.

#include <iostream>
#include <vector>
#include <boost/range/algorithm/copy.hpp>

int main(int, char**) {
    std::vector<int> a = { 1,2,3 };
    std::vector<int> b = { 4,5,6 };
    boost::copy(b, std::back_inserter(a));
    for (auto& iter : a) {
        std::cout << iter << " ";
    }
    return EXIT_SUCCESS;
}

Often ones intention is to combine vector a and b just iterate over it doing some operation. In this case, there is the ridiculous simple join function.

#include <iostream>
#include <vector>
#include <boost/range/join.hpp>
#include <boost/range/algorithm/copy.hpp>

int main(int, char**) {
    std::vector<int> a = { 1,2,3 };
    std::vector<int> b = { 4,5,6 };
    std::vector<int> c = { 7,8,9 };
    // Just creates an iterator
    for (auto& iter : boost::join(a, boost::join(b, c))) {
        std::cout << iter << " ";
    }
    std::cout << "\n";
    // Can also be used to create a copy
    std::vector<int> d;
    boost::copy(boost::join(a, boost::join(b, c)), std::back_inserter(d));
    for (auto& iter : d) {
        std::cout << iter << " ";
    }
    return EXIT_SUCCESS;
}

For large vectors this might be an advantage, as there is no copying. It can be also used for copying an generalizes easily to more than one container.

For some reason there is nothing like boost::join(a,b,c), which could be reasonable.

Answer 5

If what you're looking for is a way to append a vector to another after creation, vector::insert is your best bet, as has been answered several times, for example:

vector<int> first = {13};
const vector<int> second = {42};

first.insert(first.end(), second.cbegin(), second.cend());

Sadly there's no way to construct a const vector<int>, as above you must construct and then insert.

---

If what you're actually looking for is a container to hold the concatenation of these two vector<int>s, there may be something better available to you, if:

1. Your vector contains primitives
2. Your contained primitives are of size 32-bit or smaller
3. You want a const container

If the above are all true, I'd suggest using the basic_string who's char_type matches the size of the primitive contained in your vector. You should include a static_assert in your code to validate these sizes stay consistent:

static_assert(sizeof(char32_t) == sizeof(int));

With this holding true you can just do:

const u32string concatenation = u32string(first.cbegin(), first.cend()) + u32string(second.cbegin(), second.cend());

For more information on the differences between string and vector you can look here: https://stackoverflow.com/a/35558008/2642059

For a live example of this code you can look here: http://ideone.com/7Iww3I

Answer 6

I prefer one that is already mentioned:

a.insert(a.end(), b.begin(), b.end());

But if you use C++11, there is one more generic way:

a.insert(std::end(a), std::begin(b), std::end(b));

---

Also, not part of a question, but it is advisable to use reserve before appending for better performance. And if you are concatenating vector with itself, without reserving it fails, so you always should reserve.

---

So basically what you need:

template <typename T>
void Append(std::vector<T>& a, const std::vector<T>& b)
{
    a.reserve(a.size() + b.size());
    a.insert(a.end(), b.begin(), b.end());
}