How Can I Sort an std::map by Value Instead of Key?

Sorting an std::map by Value

Sorting an std::map by value rather than key can be more complex than sorting by key. Here's an in-depth analysis of the issue and a solution:

Solution 1: Using a Multimap

To sort an std::map by value, we can create a multimap with the values as keys and the keys as values. This allows us to sort by the values, as in the following example:

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;
}

By iterating through the original map and inserting the flipped pairs into the multimap, we essentially invert the key-value relationship and can then sort by the new keys (the original values).

Solution 2: Generic Associative Source (C 11 Required)

For alternative associative containers (e.g., std::unordered_map), we can use the following generic solution:

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 overload works for both std::map and std::unordered_map by using variadic templates to handle different associative container types.

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