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

using equal_range:

std::vector results;
for(auto i = begin(v); i != end(v);)
{
    auto r = std::equal_range(i, end(v), *i);
    results.emplace_back( std::distance(r.first, r.second) );
    i = r.second;
}

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SSCCE:

#include 
#include 
#include 
#include 

int main()
{
    std::vector v{1.0, 2.0, 1.0, 2.0, 1.0, 3.0};
    std::sort(begin(v), end(v));

    std::vector results;
    for(auto i = begin(v); i != end(v);)
    {
        auto r = std::equal_range(i, end(v), *i);
        results.emplace_back( std::distance(r.first, r.second) );
        i = r.second;
    }

    for(auto const& e : results) std::cout << e << "; ";
}