在C ++中对地图排序

Question

I have a map with a number attached to an alphabet . The map is sorted by default using first value but I want to sort it by frequency ie second value and then print it. Please help me do that Edit:: There is nothing I can try here

Answer 1

You can't sort map directly according to its second value. But, it can be done manually. First, save the data of the map into a vector of pair , and then sort the vector according to your need.

But anyways here is a little code snippet to perform the above explained operation:

template <typename T1, typename T2> struct less_second {
    typedef pair<T1, T2> type;
    bool operator ()(type const& a, type const& b) const {
        return a.second < b.second;
    }
};

map<key_t, value_t> mymap; /* It is map you want to sort according to the second argument*/
/* ...
   ...
   ...  */

vector<pair<key_t, value_t> > mapcopy(mymap.begin(), mymap.end());
sort(mapcopy.begin(), mapcopy.end(), less_second<key_t, value_t>());

Now, mapcopy (it is a vector) has your desired output.