std :: map按值大小排序（设置 <int> ）

Question

Using C++, I have a map with an int key and a set value. I'd like to sort the map for output based on the size of the set (ie, value.size()). I know the map automatically sorts itself based on the key. Is there some way to make the map sort by number of elements in its value? Also, if I flip the map, as some posts suggest, and the key becomes set, how does the map determine the sort order? Any help understanding this would be much appreciated.

mymap<int, set<int> >;
/* code to fill map */
/* How to sort by mymap[node].second().size() ??? */

Answer 1

You can't change the sort order of an existing map .

Illustrating one way to create an index into the map in the form of a sorted vector ...

std::vector<std::pair<int, int>> size_key;

for (auto& x : mymap)
    size_key.emplace_back(x.second.size(), x.first);
std::sort(std::begin(size_index), std::end(size_index));

// work's done above - just display the results to illustrate access...

for (auto& sk : size_key)
{
    std::cout << "with size " << sk.first
              << ", value " << mymap[sk.second].first << '\n';
    for (auto& n : mymap[sk.second].second)
         std::cout << "  " << n << '\n'
}

Also, if I flip the map, as some posts suggest...

Not a good idea: just "flipping" key and value won't help without also changing the sort comparitor, you'd need a multimap if set sizes could be repeated, and slower and more wasteful of memory than building an index as above (unless you can throw away the original mymap once the re-ordered multimap is constructed).

Answer 2

You just want to output the map in some weird sorted order.

You should not want to modify the order of keys inside a std::map (that does not make any sense, since the fixed order -ie the key compare function - is the second template parameter of std::map ).

So get the std::vector of keys from the std::map :

std::vector<int> keyvec;
keyvec.reserve(mymap.size());
for (auto&x : mymap)
  keyvec.push_back(x.first);

then sort (using std::stable_sort to keep keys of same size in original order) it according to your criteria

std::stable_sort(keyvec.begin(), keyvec.end(),
                 [&mymap,=](int k1, int k2) 
                 { return mymap[k1].size() < mymap[k2].size(); });

and at last display your map using

for (auto k: keyvec)
   std::cout << "[" << k << "]:" << mymap[k] << std::endl;