Sorting std::map using value

The Solution to Sorting std::map using value is


Even though correct answers have already been posted, I thought I'd add a demo of how you can do this cleanly:

template<typename A, typename B>
std::pair<B,A> flip_pair(const std::pair<A,B> &p)
{
    return std::pair<B,A>(p.second, p.first);
}

template<typename A, typename B>
std::multimap<B,A> flip_map(const std::map<A,B> &src)
{
    std::multimap<B,A> dst;
    std::transform(src.begin(), src.end(), std::inserter(dst, dst.begin()), 
                   flip_pair<A,B>);
    return dst;
}

int main(void)
{
    std::map<int, double> src;

    ...    

    std::multimap<double, int> dst = flip_map(src);
    // dst is now sorted by what used to be the value in src!
}

Generic Associative Source (requires C++11)

If you're using an alternate to std::map for the source associative container (such as std::unordered_map), you could code a separate overload, but in the end the action is still the same, so a generalized associative container using variadic templates can be used for either mapping construct:

// flips an associative container of A,B pairs to B,A pairs
template<typename A, typename B, template<class,class,class...> class M, class... Args>
std::multimap<B,A> flip_map(const M<A,B,Args...> &src)
{
    std::multimap<B,A> dst;
    std::transform(src.begin(), src.end(),
                   std::inserter(dst, dst.begin()),
                   flip_pair<A,B>);
    return dst;
}

This will work for both std::map and std::unordered_map as the source of the flip.

~ Answered on 2011-02-20 11:43:17


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