Implementing own Hash Table with Open Addressing Linear Probing

18 Aug, 2021
Prerequisite – Hashing Introduction, Implementing our Own Hash Table with Separate Chaining in JavaIn Open Addressing, all elements are stored in the hash table itself. So at any point, size of table must be greater than or equal to total number of keys (Note that we can increase table size by copying old data if needed). 
Insert(k) – Keep probing until an empty slot is found. Once an empty slot is found, insert k.
Search(k) – Keep probing until slot’s key doesn’t become equal to k or an empty slot is reached.
Delete(k) – Delete operation is interesting. If we simply delete a key, then search may fail. So slots of deleted keys are marked specially as “deleted”.
Here, to mark a node deleted we have used dummy node with key and value -1. Insert can insert an item in a deleted slot, but search doesn’t stop at a deleted slot.The entire process ensures that for any key, we get an integer position within the size of the Hash Table to insert the corresponding value. So the process is simple, user gives a (key, value) pair set as input and based on the value generated by hash function an index is generated to where the value corresponding to the particular key is stored. So whenever we need to fetch a value corresponding to a key that is just O(1). 
Code – 
CPP
#include <bits/stdc++.h>using namespace std;  // template for generic typetemplate <typename K, typename V>  // Hashnode classclass HashNode {public:    V value;    K key;      // Constructor of hashnode    HashNode(K key, V value)    {        this->value = value;        this->key = key;    }};  // template for generic typetemplate <typename K, typename V>  // Our own Hashmap classclass HashMap {    // hash element array    HashNode<K, V>** arr;    int capacity;    // current size    int size;    // dummy node    HashNode<K, V>* dummy;  public:    HashMap()    {        // Initial capacity of hash array        capacity = 20;        size = 0;        arr = new HashNode<K, V>*[capacity];          // Initialise all elements of array as NULL        for (int i = 0; i < capacity; i++)            arr[i] = NULL;          // dummy node with value and key -1        dummy = new HashNode<K, V>(-1, -1);    }    // This implements hash function to find index    // for a key    int hashCode(K key)    {        return key % capacity;    }      // Function to add key value pair    void insertNode(K key, V value)    {        HashNode<K, V>* temp = new HashNode<K, V>(key, value);          // Apply hash function to find index for given key        int hashIndex = hashCode(key);          // find next free space        while (arr[hashIndex] != NULL               && arr[hashIndex]->key != key               && arr[hashIndex]->key != -1) {            hashIndex++;            hashIndex %= capacity;        }          // if new node to be inserted        // increase the current size        if (arr[hashIndex] == NULL            || arr[hashIndex]->key == -1)            size++;        arr[hashIndex] = temp;    }      // Function to delete a key value pair    V deleteNode(int key)    {        // Apply hash function        // to find index for given key        int hashIndex = hashCode(key);          // finding the node with given key        while (arr[hashIndex] != NULL) {            // if node found            if (arr[hashIndex]->key == key) {                HashNode<K, V>* temp = arr[hashIndex];                  // Insert dummy node here for further use                arr[hashIndex] = dummy;                  // Reduce size                size--;                return temp->value;            }            hashIndex++;            hashIndex %= capacity;        }          // If not found return null        return NULL;    }      // Function to search the value for a given key    V get(int key)    {        // Apply hash function to find index for given key        int hashIndex = hashCode(key);        int counter = 0;          // finding the node with given key        while (arr[hashIndex] != NULL) { // int counter =0; // BUG!              if (counter++ > capacity) // to avoid infinite loop                return NULL;              // if node found return its value            if (arr[hashIndex]->key == key)                return arr[hashIndex]->value;            hashIndex++;            hashIndex %= capacity;        }          // If not found return null        return NULL;    }      // Return current size    int sizeofMap()    {        return size;    }      // Return true if size is 0    bool isEmpty()    {        return size == 0;    }      // Function to display the stored key value pairs    void display()    {        for (int i = 0; i < capacity; i++) {            if (arr[i] != NULL && arr[i]->key != -1)                cout << "key = " << arr[i]->key                     << "  value = "                     << arr[i]->value << endl;        }    }};  // Driver method to test map classint main(){    HashMap<int, int>* h = new HashMap<int, int>;    h->insertNode(1, 1);    h->insertNode(2, 2);    h->insertNode(2, 3);    h->display();    cout << h->sizeofMap() << endl;    cout << h->deleteNode(2) << endl;    cout << h->sizeofMap() << endl;    cout << h->isEmpty() << endl;    cout << h->get(2);      return 0;}
Output –  
key = 1 value = 1
key = 2 value = 3
2
3
1
0
0
This article is contributed by Chhavi. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. 
howdyganesh
Juande Manjon
C++
Hash
Hash
CPP
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.