C++ Program to Implement Hash Tables with Double Hashing

A hash table is a data structure which is used to store key-value pairs. Hash function is used by hash table to compute an index into an array in which an element will be inserted or searched.

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

This is a C++ program to Implement Hash Tables chaining with double hashing.

Algorithm

For search a key:

Begin
   Declare Function SearchKey(int k, HashTable *ht)
      int hashVal= HashFunc1(k, ht->s)
      int stepSize= HashFunc2(k, ht->s)
      while (ht->t[hashVal].info != Emp and
         ht->t[hashVal].e != k)
            hashVal = hashVal + stepSize
            hashVal = hashVal % ht->s
      return hashVal
End

For insert:

Begin.
   Declare Function Insert(int k, HashTable *ht)
      int pos = SearchKey(k, ht)
      if (ht->t[pos].info != Legi )
         ht->t[pos].info = Legi
         ht->t[pos].e = k
End

For Display:

Begin
   Declare function display(HashTable *ht)
      for (int i = 0; i < ht->s; i++)
         int v= ht->t[i].e;
         if (!v)
            Print "Position: "
               Print the position of the pointer
            Print " Element: Null"
         else
            Print "Position: "
               Print the position of the pointer
            Print " Element: "
               Print the element
End.

For rehash function:

Begin
   Declare function Rehash(HashTable *ht)
      int s = ht->s
      HashTableEntry *t= ht->t
      ht = initiateTable(2*s)
      for (int i = 0; i < s; i++)
         if (t[i].info == Legi)
            Insert(t[i].e, ht)
      free(t)
      return ht
End.

Example Code

 Live Demo

#include <iostream>
#include <cstdlib>
#define T_S 5
using namespace std;
enum EntryType {Legi, Emp};
struct HashTableEntry {
   int e;
   enum EntryType info;
};
struct HashTable {
   int s;
   HashTableEntry *t;
};
int HashFunc1(int k, int s) {
   return k % s;
}
int HashFunc2(int k, int s) {
   return (k * s - 1) % s;
}
HashTable *initiateTable(int s) {
   HashTable *ht;
   if (s < T_S) {
      cout<<"Table Size is Too Small"<<endl;
      return NULL;
   }
   ht= new HashTable;
   if (ht == NULL) {
      cout<<"Out of Space"<<endl;
      return NULL;
   }
   ht->s = s;
   ht->t = new HashTableEntry[ht->s];
   if (ht->t== NULL) {
      cout<<"Table Size is Too Small"<<endl;
      return NULL;
   }
   for (int i = 0; i < ht->s; i++) {
      ht->t[i].info = Emp;
      ht->t[i].e=NULL;
   }
   return ht;
}
int SearchKey(int k, HashTable *ht) {
   int hashVal= HashFunc1(k, ht->s);
   int stepSize= HashFunc2(k, ht->s);
   while (ht->t[hashVal].info != Emp &&
      ht->t[hashVal].e != k) {
         hashVal = hashVal + stepSize;
         hashVal = hashVal % ht->s;
      }
      return hashVal;
}
void Insert(int k, HashTable *ht) {
   int pos = SearchKey(k, ht);
   if (ht->t[pos].info != Legi ) {
      ht->t[pos].info = Legi;
      ht->t[pos].e = k;
   }
}
void display(HashTable *ht) {
   for (int i = 0; i < ht->s; i++) {
      int v= ht->t[i].e;
      if (!v)
         cout<<"Position: "<<i + 1<<" Element: Null"<<endl;
      else
         cout<<"Position: "<<i + 1<<" Element: "<<v<<endl;
   }
}
HashTable *Rehash(HashTable *ht) {
   int s = ht->s;
   HashTableEntry *t= ht->t;
   ht = initiateTable(2*s);
   for (int i = 0; i < s; i++) {
      if (t[i].info == Legi)
         Insert(t[i].e, ht);
   }
   free(t);
   return ht;
}
int main() {
   int v, s, pos, i = 1;
   int c;
   HashTable *ht;
   while(1) {
      cout<<"1.Initialize size of the table"<<endl;
      cout<<"2.Insert element into the table"<<endl;
      cout<<"3.Display Hash Table"<<endl;
      cout<<"4.Rehash Hash Table"<<endl;
      cout<<"5.Exit"<<endl;
      cout<<"Enter your choice: ";
      cin>>c;
      switch(c) {
         case 1:
            cout<<"Enter size of the Hash Table: ";
            cin>>s;
            ht = initiateTable(s);
         break;
         case 2:
            if (i > ht->s) {
               cout<<"Table is Full, Rehash the table"<<endl;
               continue;
            }
            cout<<"Enter element to be inserted: ";
            cin>>v;
            Insert(v, ht);
            i++;
         break;
         case 3:
            display(ht);
         break;
         case 4:
            ht= Rehash(ht);
         break;
         case 5:
            exit(1);
         default:
         cout<<"\nEnter correct option\n";
      }
   }
   return 0;
}

Output

1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 1
Enter size of the Hash Table: 4
Table Size is Too Small
Enter your choice: 1
Enter size of the Hash Table: 10
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Enter element to be inserted: 1
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Enter element to be inserted: 3
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Enter element to be inserted: 4
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Enter element to be inserted: 5
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Enter element to be inserted: 6
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Enter element to be inserted: 7
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Enter element to be inserted: 8
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice:
9
Enter correct option
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Enter element to be inserted: 9
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Enter element to be inserted: 10
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Enter element to be inserted: 11
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Table is Full, Rehash the table
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 12
Enter correct option
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Table is Full, Rehash the table
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Table is Full, Rehash the table
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 3
Position: 1 Element: 10
Position: 2 Element: 1
Position: 3 Element: 11
Position: 4 Element: 3
Position: 5 Element: 4
Position: 6 Element: 5
Position: 7 Element: 6
Position: 8 Element: 7
Position: 9 Element: 8
Position: 10 Element: 9
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 4
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 3
Position: 1 Element: Null
Position: 2 Element: 1
Position: 3 Element: Null
Position: 4 Element: 3
Position: 5 Element: 4
Position: 6 Element: 5
Position: 7 Element: 6
Position: 8 Element: 7
Position: 9 Element: 8
Position: 10 Element: 9
Position: 11 Element: 10
Position: 12 Element: 11
Position: 13 Element: Null
Position: 14 Element: Null
Position: 15 Element: Null
Position: 16 Element: Null
Position: 17 Element: Null
Position: 18 Element: Null
Position: 19 Element: Null
Position: 20 Element: Null
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 2
Enter element to be inserted: 20
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 3
Position: 1 Element: 20
Position: 2 Element: 1
Position: 3 Element: Null
Position: 4 Element: 3
Position: 5 Element: 4
Position: 6 Element: 5
Position: 7 Element: 6
Position: 8 Element: 7
Position: 9 Element: 8
Position: 10 Element: 9
Position: 11 Element: 10
Position: 12 Element: 11
Position: 13 Element: Null
Position: 14 Element: Null
Position: 15 Element: Null
Position: 16 Element: Null
Position: 17 Element: Null
Position: 18 Element: Null
Position: 19 Element: Null
Position: 20 Element: Null
1.Initialize size of the table
2.Insert element into the table
3.Display Hash Table
4.Rehash Hash Table
5.Exit
Enter your choice: 5