Skip to content
Related Articles

Related Articles

Improve Article

Double Hashing

Double hashing is a collision resolving technique in Open Addressed Hash tables. Double hashing uses the idea of applying a second hash function to key when a collision occurs.

Double hashing can be done using :
(hash1(key) + i * hash2(key)) % TABLE_SIZE
Here hash1() and hash2() are hash functions and TABLE_SIZE
is size of hash table.
(We repeat by increasing i when collision occurs)

First hash function is typically hash1(key) = key % TABLE_SIZE

A popular second hash function is : hash2(key) = PRIME – (key % PRIME) where PRIME is a prime smaller than the TABLE_SIZE.

A good second Hash function is:



  • It must never evaluate to zero
  • Must make sure that all cells can be probed




// CPP program to implement double hashing
#include <bits/stdc++.h>
using namespace std;
  
// Hash table size
#define TABLE_SIZE 13
  
// Used in second hash function.
#define PRIME 7
  
class DoubleHash {
    // Pointer to an array containing buckets
    int* hashTable;
    int curr_size;
  
public:
    // function to check if hash table is full
    bool isFull()
    {
  
        // if hash size reaches maximum size
        return (curr_size == TABLE_SIZE);
    }
  
    // function to calculate first hash
    int hash1(int key)
    {
        return (key % TABLE_SIZE);
    }
  
    // function to calculate second hash
    int hash2(int key)
    {
        return (PRIME - (key % PRIME));
    }
  
    DoubleHash()
    {
        hashTable = new int[TABLE_SIZE];
        curr_size = 0;
        for (int i = 0; i < TABLE_SIZE; i++)
            hashTable[i] = -1;
    }
  
    // function to insert key into hash table
    void insertHash(int key)
    {
        // if hash table is full
        if (isFull())
            return;
  
        // get index from first hash
        int index = hash1(key);
  
        // if collision occurs
        if (hashTable[index] != -1) {
            // get index2 from second hash
            int index2 = hash2(key);
            int i = 1;
            while (1) {
                // get newIndex
                int newIndex = (index + i * index2) % TABLE_SIZE;
  
                // if no collision occurs, store
                // the key
                if (hashTable[newIndex] == -1) {
                    hashTable[newIndex] = key;
                    break;
                }
                i++;
            }
        }
  
        // if no collision occurs
        else
            hashTable[index] = key;
        curr_size++;
    }
  
    // function to search key in hash table
    void search(int key)
    {
        int index1 = hash1(key);
        int index2 = hash2(key);
        int i = 0;
        while (hashTable[(index1 + i * index2) % TABLE_SIZE] != key) {
            if (hashTable[(index1 + i * index2) % TABLE_SIZE] == -1) {
                cout << key << " does not exist" << endl;
                return;
            }
            i++;
        }
        cout << key << " found" << endl;
    }
  
    // function to display the hash table
    void displayHash()
    {
        for (int i = 0; i < TABLE_SIZE; i++) {
            if (hashTable[i] != -1)
                cout << i << " --> "
                     << hashTable[i] << endl;
            else
                cout << i << endl;
        }
    }
};
  
// Driver's code
int main()
{
    int a[] = { 19, 27, 36, 10, 64 };
    int n = sizeof(a) / sizeof(a[0]);
  
    // inserting keys into hash table
    DoubleHash h;
    for (int i = 0; i < n; i++) {
        h.insertHash(a[i]);
    }
    // searching some keys
    h.search(36); // This key is present in hash table
    h.search(100); // This key does not exist in hash table
    // display the hash Table
    h.displayHash();
    return 0;
}
Output:
36 found
100 does not exist
0
1 --> 27
2
3
4
5 --> 10
6 --> 19
7
8
9
10 --> 36
11
12 --> 64

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.  To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.

In case you wish to attend live classes with experts, please refer DSA Live Classes for Working Professionals and Competitive Programming Live for Students.




My Personal Notes arrow_drop_up
Recommended Articles
Page :