Copy every other element using standard algorithms (downsampling)

say I have a std::vector with N elements. I would like to copy every n-th element of it to a new vector, or average up to that element then copy it (downsample the original vector). So I want to do this

std::vector<double> vec(N);
long n = 4;
std::vector<double> ds(N/n);
for(long i = 0; i < ds.size(); i+=n)
{
    ds[i] = vec[i*n];
}

or

for(long i = 0; i < ds.size(); i+=n)
{
    double tmp = 0;    
    for(long j = 0; j < n; j++)
    {
        tmp += vec[i*n+j];
    }
    ds[i] = tmp/static_cast<double>(n);
}

Is there a way to do this using the standard algorithms of C++? Like using std::copy with binary functions? I have billions of elements that I want to treat this way, and I want this to be as fast as possible.

PS: I would prefer not to use external libraries such as boost.