Sorting std::map using value

by

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.

More Related Contents:

  1. A std::map that keep track of the order of insertion?
  2. What is the fastest way to change a key of an element inside std::map
  3. Why Can’t I store references in a `std::map` in C++?
  4. map with incomplete value type
  5. Converting std::__cxx11::string to std::string
  6. Why is std::map implemented as a red-black tree?
  7. Flattening iterator
  8. declaring a priority_queue in c++ with a custom comparator
  9. Is the order of iterating through std::map known (and guaranteed by the standard)?
  10. What can and can’t I specialize in the std namespace?
  11. What is the default constructor for C++ pointer?
  12. How to convert std::chrono::time_point to calendar datetime string with fractional seconds?
  13. C++ Loop through Map
  14. Is the C++ std::set thread-safe?
  15. How to choose between map and unordered_map?
  16. Super high performance C/C++ hash map (table, dictionary) [closed]
  17. std::bind overload resolution
  18. C++ Add months to chrono::system_clock::time_point
  19. Passing rvalues through std::bind
  20. error C2065: ‘cout’ : undeclared identifier
  21. Using std::map where V has no usable default constructor
  22. How can I make the map::find operation case insensitive?
  23. std::remove_const with const references
  24. Syntax for using std::greater when calling std::sort in C++
  25. Can a declaration affect the std namespace?
  26. Find mapped value of map
  27. Hashtable in C++?
  28. What’s the time complexity of iterating through a std::set/std::map?
  29. How to make sure that std::random_shuffle always produces a different result?
  30. C++ “error: passing ‘const std::map’ as ‘this’ argument of …”

Leave a Comment