unordered_map c++

                  | map             | unordered_map
---------------------------------------------------------
Ordering        | increasing  order   | no ordering
                | (by default)        |

Implementation  | Self balancing BST  | Hash Table
                | like Red-Black Tree |  

search time     | log(n)              | O(1) -> Average 
                |                     | O(n) -> Worst Case

Insertion time  | log(n) + Rebalance  | Same as search
                      
Deletion time   | log(n) + Rebalance  | Same as search


::-> Use std::map when
1. You need ordered data.
2. You would have to print/access the data (in sorted order).
3. You need predecessor/successor of elements.
  
::-> Use std::unordered_map when
1. You need to keep count of some data (Example – strings) and no ordering is required.
2. You need single element access i.e. no traversal.

// C++ program to demonstrate functionality of unordered_map 
#include <iostream> 
#include <unordered_map> 
using namespace std; 
  
int main() 
{ 
    // Declaring umap to be of <string, int> type 
    // key will be of string type and mapped value will 
    // be of double type 
    unordered_map<string, int> umap; 
  
    // inserting values by using [] operator 
    umap["GeeksforGeeks"] = 10; 
    umap["Practice"] = 20; 
    umap["Contribute"] = 30; 
  
    // Traversing an unordered map 
    for (auto x : umap) 
      cout << x.first << " " << x.second << endl; 
  
} 

unordered_map<string, int> umap;

for (auto x : umap) 
	cout << x.first << " " << x.second << endl;

unordered_map is an associated container that stores elements formed by combination of key value and a mapped value. 
The key value is used to uniquely identify the element and mapped value is the content associated with the key. 
Both key and value can be of any type predefined or user-defined.

unordered_map.find(key);

#include <bits/stdc++.h>
#include <iostream>
#include <map>
#include <unordered_map>

using namespace std;

int main() {
  	map<char, int> M; //based on balanced binary tree takes O(logn) access time
	unordered_map<char, int> U; //uses hashing and accessing elements takes O(1)
	//U.add(key,value);
  	//U.erase(key,value);
  	
  	//map each letter to their occurance
  	string s = "Sumant Tirkey";
  	for (char c : s) {
  		M[c]++;
	  }
	for (char c : s){
		U[c]++;
	}
  
  return 0;
}