如何按第二个元素对地图进行排序？

Question

I have a map of map map<int, map<char,int> > m; and i want to sort it by the 2nd element of inner map ie, by the int value of map<char, int> in descending order

i used this to sort 2nd element of a map but here i want to sort 2nd element of inner map in descending order

    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<string, int> mymap;
    // …

    vector<pair<string, int> > mapcopy(mymap.begin(), mymap.end());
    sort(mapcopy.begin(), mapcopy.end(), less_second<string, int>());

edit: for example my map has the following values:

{3 {A 1}
{D 2}},
{6 {C 2}},
{9 {A 1}
{B 2}},
{12 {B 3}
{C 1}}

then it should be sorted as per 2nd element of inner map in descending order

{12 {B 3}
{C 1}},
{6 {C 2}},
{3 {A 1}
{D 2}},
{9 {A 1}
{B 2}}

Answer 1

This copies the elements of a map<int, map<char, int>> into into a vector<pair<int, map<char, int>>> and sorts the vector by the value of the first element in the map

#include <numeric>
#include <iostream>
#include <map>
#include <string>
#include <vector>
using std::cout;
using std::pair;
using std::string;
using std::vector;
using namespace std::literals::string_literals;

auto f1(const std::map<char, int> &m) {
    return std::accumulate(
        std::next(std::begin(m)),
        std::end(m),
        std::begin(m)->first + ": "s + std::to_string(std::begin(m)->second),
        [](const auto &lhs, const auto &rhs) {
            return lhs + ", "s + rhs.first + ": "s + std::to_string(rhs.second);
        });
}

auto &operator<<(std::ostream &os, const std::vector<std::pair<int, std::map<char, int>>> &m) {
    return os << "{\n"
       << std::accumulate(
        std::next(std::begin(m)),
        std::end(m),
        std::to_string(std::begin(m)->first) + ": { "s + f1(std::begin(m)->second) + " }"s,
        [](const auto &lhs, const auto &rhs) {
            return lhs + ",\n"s + std::to_string(rhs.first) + ": { "s + f1(rhs.second) + " }"s;
        })
       << "\n}\n";
}

int main() {
    std::map<int, std::map<char, int>> mymap{ {3, {{'A', 1}, {'D', 2}}}, {6, {{'C', 2}}}, {9, {{'A', 1}, {'B', 2}}}, {12, {{'B', 3}, {'C', 1}}} };

    std::vector<std::pair<int, std::map<char, int>>> mapcopy(mymap.begin(), mymap.end());
    sort(mapcopy.begin(), mapcopy.end(), [](const auto &lhs, const auto &rhs){
        return lhs.second.cbegin()->second > rhs.second.cbegin()->second;
    });

    std::cout << mapcopy;
}

Output:

{
12: { B: 3, C: 1 },
6: { C: 2 },
3: { A: 1, D: 2 },
9: { A: 1, B: 2 }
}