按地图排序（Lexicographical Order）

Question

the output should be sorted in lexicographically sorted order of names and if two names are same then those are sorted in decreasing order of marks.

#include <iostream>
#include <map>
#include <tuple>

int main() {
    int t;
    std::cin >> t;
    while(t--) {
        int n;
        std::cin >> n;
        std::string name;
        int marks;
        std::map<std::pair<std::string, int>, int> hash;
        for(int i = 0; i < n; i++) {
            std::cin >> name >> marks;
            std::pair<std::string, int> p;
            p.first = name;
            p.second = marks;
            hash[p]++;
        }

        for(auto it = hash.begin(); it != hash.end(); ++it) {
            std::cout << (it->first).first << " " << (it->first).second << " "
                      << it->second << "\n";
        }
    }
    return 0;
}

Answer 1

If you want the entries of a map sorted in a particular order (and the default order is operator < , which doesn't do what you're asking), then you need to instantiate your map with a custom comparator.

struct myComp {
  bool operator()(const std::pair<std::string, int>& lhs, 
                  const std::pair<std::string, int>& rhs) const
      { /* your code here */ } 
};

std::map<std::pair<std::string, int>, int, myComp> m;

Your comparison object should impose a strict weak ordering on the values. That means that for any std::pair<std::string, int> a,b,c and myComp cmp :

• cmp(a, a) is false.
• if cmp(a, b) is true, then cmp(b, a) is false.
• if cmp(a, b) is true and cmp(b, c) is true, then cmp(a, c) is true.