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 :
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| 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
// 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
// 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 20 : 100 5 : 10 3 : 5
