[C++ STL] unordered_map (hash table) example

stl_unordered_map_example.cpp

#include <iostream>
#include <string>
#include <unordered_map>

// how to make a new datatype in hashtable
struct X{int i,j,k;};

namespace std {
    template <>
        class hash<X>{
        public :
        size_t operator()(const X &x ) const{
            return hash<int>()(x.i) ^ hash<int>()(x.j) ^ hash<int>()(x.k);
        }
    };
}

void std_unorderedmap_test(){
	// 01. initialization & list all elements in it
	std::unordered_map<std::string, std::string> mymap;
	mymap = {{"USA","D.C"}, {"KO", "Seoul"}, {"Japan", "Tokyo"}};

	std::cout << "mymap contains:";
	for (auto it = mymap.begin(); it != mymap.end(); it++){
		std::cout << " " << it->first << ":" << it->second;		
		// it->first = key, it->second = value
	}
	std::cout << std::endl;
	
	// 02. how to use it in general way
	std::unordered_map<int, int> ht;
	ht[1] = 10;
	ht[2] = 20;
	std::cout << ht[1] << std::endl;
	std::cout << ht.find(2)->second << std::endl;
	if (ht.find(3) == ht.end())
		std::cout << "3 doesn't have the key in ht" << std::endl;
}