How can I sort a std::map first by value, then by key?

Question

I need to sort a std::map by value, then by key. The map contains data like the following:

  1  realistically
  8         really
  4         rea         


        

Answer 1

std::map will sort its elements by keys. It doesn't care about the values when sorting.

You can use std::vector> then sort it using std::sort followed by std::stable_sort:

std::vector> items;

//fill items

//sort by value using std::sort
std::sort(items.begin(), items.end(), value_comparer);

//sort by key using std::stable_sort
std::stable_sort(items.begin(), items.end(), key_comparer);

The first sort should use std::sort since it is nlog(n), and then use std::stable_sort which is n(log(n))^2 in the worst case.

Note that while std::sort is chosen for performance reason, std::stable_sort is needed for correct ordering, as you want the order-by-value to be preserved.

---

@gsf noted in the comment, you could use only std::sort if you choose a comparer which compares values first, and IF they're equal, sort the keys.

auto cmp = [](std::pair const & a, std::pair const & b) 
{ 
     return a.second != b.second?  a.second < b.second : a.first < b.first;
};
std::sort(items.begin(), items.end(), cmp);

That should be efficient.

But wait, there is a better approach: store std::pair instead of std::pair and then you don't need any comparer at all — the standard comparer for std::pair would be enough, as it compares first (which is V) first then second which is K:

std::vector> items;
//...
std::sort(items.begin(), items.end());

That should work great.