How Can I Sort an std::map by Value in C ?

Sorting an std::map by Value

Question: How can I effortlessly sort an std::map based on its values?

Answer:

Traditional Approach (Prior to C 11)

Prior to the introduction of C 11, sorting a map by values required an intermediary step of converting the std::map into a std::pair. This involved using a custom sorting function and manually specifying the comparison parameters.

C 11 Approach

With the advent of C 11, a cleaner solution emerged. The following code snippet demonstrates how:

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;
}

This function effectively flips the key-value pairs within the map, allowing you to iterate over the sorted values.

Generic Associative Source (C 11 and Above)

For more flexibility, a generic version of the flipping function can be created, allowing it to work with any associative container:

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 function takes any associative container class as input, making it convenient for use with both std::map and std::unordered_map.

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