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         reason   3     reasonable   1     reasonably   1     reassemble   1    reassembled   2      recognize  92         record  48        records   7           recs 

I need to get the values in order, but the kicker is that the keys need to be in alphabetical order after the values are in order. How can I do this?

Answer 1

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

You can use std::vector<std::pair<K,V>> then sort it using std::sort followed by std::stable_sort:

std::vector<std::pair<K,V>> 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.

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@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<K,V> const & a, std::pair<K,V> 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<V,K> instead of std::pair<K,V> 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<std::pair<V,K>> items; //... std::sort(items.begin(), items.end()); 

That should work great.