How can I copy a vector to the end of another vector?

Question

I have been working on what I thought would just be a simple function for quite a while now. We are expected to use push_back(), but I haven't been able to get it to work.

Here's what I have right now:

void append(const vector<int>& v1, vector<int> v2)
{
    for (int x : v1)
        v2.push_back(x);
}

void println(const vector<int>& v)
{
    for (int x : v)
        cout << x << ' ';
    cout << endl;
}

int main()
{
    vector <int> v1(3, 15);
    vector <int> v2(3, 25);
    append(v1, v2);
    println(v1);
    println(v2);
}

You are passing v2 to append by value. Pass it by reference. — Pedro Boechat, Oct 08 '20 at 17:33

score 5 · Accepted Answer · answered Oct 08 '20 at 17:05

v2 in void append(const vector<int>& v1, vector<int> v2) is a copy of what is passed, so modifying it won't affect what is passed.

You should use reference (add &) to have the function modify what is passed.

void append(const vector<int>& v1, vector<int>& v2) // add & before v2
{
    for (int x : v1)
        v2.push_back(x);
}

If you are not expected to use push_back, std::copy is useful.

#include <algorithm>

void append(const vector<int>& v1, vector<int>& v2)
{
    std::copy(v1.begin(), v1.end(), std::back_inserter(v2));
}

In this case `v2.insert(v2.end(), v1.begin(), v1.end())` may be better than `std::copy` — Justin, Oct 08 '20 at 17:24

How can I copy a vector to the end of another vector?

1 Answers1