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How to use unordered_map efficiently in C++
• Difficulty Level : Expert
• Last Updated : 28 Jul, 2020

Pre-requisite: unordered_set,  unordered_map

C++ provides std::unordered_set and std::unordered_map to be used as a hash set and hash map respectively. They perform insertion/deletion/access in constant average time.

1. However, the worst-case complexity is O(n2).
2. The reason is that the unordered_map store’s key-value pair by taking the modulo of input value by a prime number and then stores it in a hash table.
3. When the input data is big and input values are multiples of this prime number a lot of collisions take place and may cause the complexity of O(n2).
4. Depending on the compiler the prime number maybe 107897 or 126271.

Example 1: If we insert multiples of the above two prime numbers and compute execution time. One of the prime numbers takes a much longer time than the other.

## C++

 `// C++ program to determine worst case``// time complexity of an unordered_map`` ` `#include ``using` `namespace` `std;``using` `namespace` `std::chrono;``int` `N = 55000;``int` `prime1 = 107897;``int` `prime2 = 126271;`` ` `void` `insert(``int` `prime)``{`` ` `    ``// Starting the clock``    ``auto` `start``        ``= high_resolution_clock::now();`` ` `    ``unordered_map<``int``, ``int``> umap;`` ` `    ``// Inserting multiples of prime``    ``// number as key in the map``    ``for` `(``int` `i = 1; i <= N; i++)``        ``umap[i * prime] = i;`` ` `    ``// Stopping the clock``    ``auto` `stop``        ``= high_resolution_clock::now();`` ` `    ``// Typecasting the time to``    ``// milliseconds``    ``auto` `duration``        ``= duration_cast(``            ``stop - start);`` ` `    ``// Time in seconds``    ``cout << ``"for "` `<< prime << ``" : "``         ``<< duration.count() / 1000.0``         ``<< ``" seconds "``         ``<< endl;``}`` ` `// Driver code``int` `main()``{``    ``// Function call for prime 1``    ``insert(prime1);`` ` `    ``// Function call for prime 2``    ``insert(prime2);``}`
Output:
```for 107897 : 2.261 seconds
for 126271 : 0.024 seconds
```

Clearly, for one of the prime numbers, the time complexity is O(n2).

The standard inbuilt hash function on which unordered_map works is similar to this:

## C++

 `struct` `hash {``    ``size_t` `operator()(uint64_t x)``        ``const` `{ ``return` `x; }``};`

The above function can produce numerous collisions. The keys inserted in HashMap are not evenly distributed, and after inserting numerous prime multiples, further insertion causes the hash function to reallocate all previous keys to new slots hence making it slow. So, the idea is that we have to randomize the hash function.
The idea is to use a method so that the keys in our hashmap are evenly distributed. This will prevent collisions to take place. For this, we use Fibonacci numbers. The golden ratio related to the Fibonacci sequence (Phi = 1.618) has a property that it can subdivide any range evenly without looping back to the starting position.

We can create our own simple hash function. Below is the hash function:

## C++

 `struct` `modified_hash {``    ``static` `uint64_t splitmix64(uint64_t x)``    ``{`` ` `        ``// 0x9e3779b97f4a7c15,``        ``// 0xbf58476d1ce4e5b9,``        ``// 0x94d049bb133111eb are numbers``        ``// that are obtained by dividing``        ``// high powers of two with Phi``        ``// (1.6180..) In this way the``        ``// value of x is modified``        ``// to evenly distribute``        ``// keys in hash table``        ``x += 0x9e3779b97f4a7c15;``        ``x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;``        ``x = (x ^ (x >> 27)) * 0x94d049bb133111eb;``        ``return` `x ^ (x >> 31);``    ``}`` ` `    ``int` `operator()(uint64_t x) ``const``    ``{``        ``static` `const` `uint64_t random``            ``= steady_clock::now()``                  ``.time_since_epoch()``                  ``.count();`` ` `        ``// The above line generates a``        ``// random number using``        ``// high precision clock``        ``return` `splitmix64(`` ` `            ``// It returns final hash value``            ``x + random);``    ``}``};`

Basically, the above hashing function generates random hash values to store keys. To know more about this please refer to this article Fibonacci hashing.

Example 2: Using the above hashing function, the program runs very quickly.

## C++

 `// C++ program to determine worst case``// time complexity of an unordered_map``// using modified hash function`` ` `#include ``using` `namespace` `std;``using` `namespace` `std::chrono;`` ` `struct` `modified_hash {`` ` `    ``static` `uint64_t splitmix64(uint64_t x)``    ``{``        ``x += 0x9e3779b97f4a7c15;``        ``x = (x ^ (x >> 30))``            ``* 0xbf58476d1ce4e5b9;``        ``x = (x ^ (x >> 27))``            ``* 0x94d049bb133111eb;``        ``return` `x ^ (x >> 31);``    ``}`` ` `    ``int` `operator()(uint64_t x) ``const``    ``{``        ``static` `const` `uint64_t random``            ``= steady_clock::now()``                  ``.time_since_epoch()``                  ``.count();``        ``return` `splitmix64(x + random);``    ``}``};`` ` `int` `N = 55000;``int` `prime1 = 107897;``int` `prime2 = 126271;`` ` `// Function to insert in the hashMap``void` `insert(``int` `prime)``{``    ``auto` `start = high_resolution_clock::now();`` ` `    ``// Third argument in initialisation``    ``// of unordered_map ensures that``    ``// the map uses the hash function``    ``unordered_map<``int``, ``int``, modified_hash>``        ``umap;`` ` `    ``// Inserting multiples of prime``    ``// number as key in the map``    ``for` `(``int` `i = 1; i <= N; i++)``        ``umap[i * prime] = i;`` ` `    ``auto` `stop``        ``= high_resolution_clock::now();`` ` `    ``auto` `duration``        ``= duration_cast(``            ``stop - start);`` ` `    ``cout << ``"for "` `<< prime << ``" : "``         ``<< duration.count() / 1000.0``         ``<< ``" seconds "``         ``<< endl;``}`` ` `// Driver Code``int` `main()``{``    ``// Function call for prime 1``    ``insert(prime1);`` ` `    ``// Function call for prime 2``    ``insert(prime2);``}`
Output:
```for 107897 : 0.025 seconds
for 126271 : 0.024 seconds
```
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