map vs unordered_map in 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; 
  
} 

#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;
}