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 | of keys(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] = 500;
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 : 500 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] = 500;
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 3 : 500 5 : 10
Let us see the differences in a tabular form -:
map | unordered_map | |
1. | map is define in #include <map> header file | unordered_map is defined in #include <unordered_map> header file |
2. | It is implemented by red-black tree. | It is implemented using hash table. |
3. | It is slow. | It is fast. |
4. | Time complexity for operations is O(log N) | Time complexity for operations is O(1) |
5. | map is used to store elements as key,value pairs in order sorted by key. | unordered_map is used to store elements as key,value pairs in non-sorted order. |