Efficient way to find frequencies of each unique value in the std::vector

Question

Given a vector std::vector v, we can find unique elements efficiently by:

std::vector uv(v.begin(), v.end());
std::         


        

Answer 1

After you sort, before you erase:

std::vector freq_uv;
freq_uv.push_back(0);
auto prev = uv[0];        // you should ensure !uv.empty() if previous code did not already ensure it.
for (auto const & x : uv)
{
    if (prev != x)
    {
        freq_uv.push_back(0);
        prev = x;
    }
    ++freq_uv.back();
}

Note that, while I generally like to count occurences with a map, as Yakk is doing, in this case I think it is doing a lot of unnecessary work as we already know the vector is sorted.

Another possibility is to use a std::map (not unordered), instead of sorting. This will get your frequencies first. Then, since the map is ordered, you can just create the sorted, unique vector, and the frequency vector directly from the map.

// uv not yet created
std::map freq_map;
for (auto const & x : v)
    ++freq_map[x];
std::vector uv;
std::vector freq_uv;
for (auto const & p : freq_map)
{
    uv.push_back(p.first);
    freq_uv.push_back(p.second);
}