map vs unordered_map in C++

Pre-requisite : std::map, std::unordered_map

When it comes to efficiency, there is a huge difference between maps and unordered maps.
We must know the internal working of both to decide which one is to be used.

Difference :

                  | 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

  • You need ordered data.
  • You would have to print/access the data (in sorted order).
  • You need predecessor/successor of elements.
  • See advantages of BST over Hash Table for more cases.
// CPP program to demonstrate use of std::map
#include <bits/stdc++.h>
  
int main()
{
    // Ordered map
    std::map<int, int> order;
  
    // Mapping values to keys
    order[5] = 10;
    order[3] = 5;
    order[20] = 100;
    order[1] = 1;
  
    // Iterating the map and printing ordered values
    for (auto i = order.begin(); i != order.end(); i++) {
        std::cout << i->first << " : " << i->second << '\n';
    }
}

Output :

1 : 1
3 : 5
5 : 10
20 : 100

Use std::unordered_map when

  • You need to keep count of some data (Example – strings) and no ordering is required.
  • You need single element access i.e. no traversal.
// CPP program to demonstrate use of 
// std::unordered_map
#include <bits/stdc++.h>
  
int main()
{
    // Unordered map
    std::unordered_map<int, int> order;
  
    // Mapping values to keys
    order[5] = 10;
    order[3] = 5;
    order[20] = 100;
    order[1] = 1;
  
    // Iterating the map and printing unordered values
    for (auto i = order.begin(); i != order.end(); i++) {
        std::cout << i->first << " : " << i->second << '\n';
    }
}

Output :

1 : 1
3 : 5
20 : 100
5 : 10


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