Copy every other element using standard algorithms (downsampling)

Question

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

Answer 1

You could write your own generic algorithms inspired from the design principles used in .

For the copy of every n elements:

template
out_it copy_every_n( in_it b, in_it e, out_it r, size_t n) {
    for (size_t i=distance(b,e)/n; i--; advance (b,n)) 
        *r++ = *b;
    return r;
}

Example of use:

vector v {1,2,3,4,5,6,7,8,9,10};
vector z(v.size()/3); 
copy_every_n(v.begin(), v.end(), z.begin(), 3);     

For averaging the elements n by n, you can use:

template
out_it average_every_n( in_it b, in_it e, out_it r, size_t n) {
    typename out_it::value_type tmp=0;
    for (size_t cnt=0; b!=e; b++)  {
        tmp+=*b;
        if (++cnt==n) {
            cnt=0; 
            *r++=tmp/n;
            tmp=0;
        }
    }
    return r;
}

Example of use:

vector w(v.size()/3); 
average_every_n(v.begin(), v.end(), w.begin(), 3);  

The advantage over your inital loops, is that this will work not only on vectors, but on any container providing the begin() and end() iterator. And it avoids overheads that I pointed out in my other answer.