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 may have explicitly stated that you prefer not to use Boost, but any non-Boost solution would really be implementing exactly this sort of thing anyway, so I'll show you how I would do it in Boost. Ultimately, I think you're better off writing a simple loop.

Downsampling uses strided

boost::copy(
        input | strided(2),
        std::back_inserter(output));

Downsampling average additionally uses transformed, though this solution is non-generic and specifically relies upon vector being contiguous:

boost::copy(
        input | strided(2) | transformed([](auto& x){
                return std::accumulate(&x, &x + 2, 0) / 2.;
            }),
        std::back_inserter(output));

Of course that has issues if the input isn't an exact multiple of the stride length, so it'd probably be better to do something like:

auto downsample_avg = [](auto& input, int n){
    return input | strided(n) | transformed([&,n](auto& x){
        auto begin = &x;
        auto end = begin + std::min(n, &input.back() - begin + 1);
        return std::accumulate(begin, end, 0.0) / (end - begin);
    });
};

boost::copy(
    downsample_avg(input, 2),
    std::back_inserter(output));