How Can I Sort a std::map by Value, Not Key?

Sorting std::map by Value

Sorting a std::map by value requires an alternative approach as the standard sort() function only sorts elements by key. To achieve this, consider the following solution:

Flip Key and Value

Create a new multimap that flips the key and value pairs of the original map using the following function:

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;
}

Usage:

After creating the original map (e.g., std::map src), simply flip the map to sort by value:

std::multimap<double, int> dst = flip_map(src);

Generic Associative Source

This solution can be generalized to work with any associative container using variadic templates:

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 works for both std::map and std::unordered_map as the source of the flip.

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