Question

In C++ I am trying to implement a log base 2 hash table utilizing the shift function. So, the code determines the position of where the value will be stored using log2. But am having issues using the shift part as everything just wants to pile on to zero.

//ChainingHash - log2
#include<iostream>
#include<vector>
#include<iterator>
#include<string>
#include<cmath>

using namespace std;

#define BUCKET 10 //no. of buckets

class Hash
{
   vector<int>*table; //ptr containing buckets
  
   public:
       Hash();
       void insert(int);
       //void append();
       void search(int);
       void remove(int);
       void print();
       int code(int);
};

Hash::Hash()
{
   table = new vector<int>[BUCKET];
}

void Hash::insert(int key)
{
   int index = code(key);
   table[index].push_back(key);
}

void Hash::search(int key)
{
   int index = code(key);
  
   vector<int> :: iterator i;
   for(i=table[index].begin(); i!=table[index].end(); i++)
   {
       if(*i == key)
       {
           cout << key << " was found.\n";
           return;
       }
   }
   cout << key << " was not found in the list.\n";
}

void Hash::remove(int key)
{
   int index = code(key);
  
   vector<int> :: iterator i;
   for(i=table[index].begin(); i!=table[index].end(); i++)
   {
       if(*i == key)
       {
           table[index].erase(i);
           return;
       }
   }
   cout << "Could not find value " << key << " to delete.\n";
}

void Hash::print()
{
   for (int i=0; i<BUCKET; i++)
   {
       cout << i;
       for(auto x : table[i])
           cout << " --> " << x;
       cout << endl;
   }
}

int Hash::code(int key) //if key = 31
{
   int index = pow(key,2); //961
   int numBits = ceil(log2(index)+1); //11
   int R = ceil(log2(BUCKET)); //4
  
   int lBits = ceil(((pow(numBits,2))-R)/2); //59
   int eBits = index >> lBits; // 961 >> 59, this equals 0??
   eBits = eBits &(0xFFFFFFFF >> (32-R)); //Does not shift, still 0
  
   return eBits%BUCKET; //is zero bc eBits is 0
}

int main()
{
   int choice, val=0;
  
   Hash h;
  
   do
   {
       cout << " -- MENU -- \n";
       cout << "1 - insert\t2 - search\n3 - delete\t4 - print\n\n0 - quit\n";
       cin >> choice;
      
       switch(choice)
       {
           case 1: //insert
           {  
               while(val>=0)
               {
                   cout << "Enter a number greater than -1 to insert (to exit insert -1): ";
                   cin >> val;
                  
                   if(val!=-1)   h.insert(val);
               }
               break;
           }
           case 2: //search
           {
               cout << "Enter a number you wish to search for: ";
               cin >> val;
              
               h.search(val);
               break;
           }
           case 3: //delete
           {
               cout << "Ender the number you wish to delete: ";
               cin >> val;
              
               h.remove(val);
               break;
           }
           case 4: //print  
               cout << "All inserted Values:\n";
               h.print();
               break;
           case 0:
               break;
           default:
               cout << "Invalid Response, try again.\n";
               break;
       }
   }while(choice !=0);
}

I need the code to not insert every value at zero. Thank you.

Answer 1

I have modified the code function so that correctly shifts bits.

So, value 0 and 1 should hash to bucket 0

Value 2 and 3 should hash to bucket 2

Value 4,5,6,7 should hash to bucket 3

Value 222 as log2(222) = 7.79441586635 should hash to bucket 8.

The modified code function is as follows:

 int Hash::code(int key) {   int count = 0;  bool perfectSquare = true;      while (key > 0)      {               if (key & 1)                {                       perfectSquare = false; /* If the number is perfectly divisible by 2, this code will never reach. */             }               key = key >> 1; /* this divides a number by 2 */          count++;        }       if (perfectSquare)      {               count--;        }       return count % BUCKET; }

A sample output screen looks like this.

The entire code is a follows:

//ChainingHash - log2 #include<iostream> #include<vector> #include<iterator> #include<string> #include<cmath> using namespace std; #define BUCKET 10 //no. of buckets class Hash {   vector<int>*table; //ptr containing buckets public:       Hash();         void insert(int);       //void append();        void search(int);       void remove(int);       void print();   int code(int); }; Hash::Hash() {        table = new vector<int>[BUCKET]; } void Hash::insert(int key) {   int index = code(key);  table[index].push_back(key); } void Hash::search(int key) {     int index = code(key);  vector<int> ::iterator i;         for (i = table[index].begin(); i != table[index].end(); i++)    {               if (*i == key)          {                       cout << key << " was found.\n";                     return;                 }       }       cout << key << " was not found in the list.\n"; } void Hash::remove(int key) {      int index = code(key);  vector<int> ::iterator i;         for (i = table[index].begin(); i != table[index].end(); i++)    {               if (*i == key)          {                       table[index].erase(i);                  return;                 }       }       cout << "Could not find value " << key << " to delete.\n"; } void Hash::print() {     for (int i = 0; i < BUCKET; i++)     {               cout << i;                for (auto x : table[i])                         cout << " --> " << x;            cout << endl;     } } int Hash::code(int key) {   int count = 0;  bool perfectSquare = true;      while (key > 0)      {               if (key & 1)                {                       perfectSquare = false; /* If the number is perfectly divisible by 2, this code will never reach. */             }               key = key >> 1; /* this divides a number by 2 */          count++;        }       if (perfectSquare)      {               count--;        }       return count % BUCKET; } int main() {   int choice, val = 0;    Hash h;         do      {               cout << " -- MENU -- \n";                 cout << "1 - insert\t2 - search\n3 - delete\t4 - print\n\n0 - quit\n";            cin >> choice;            switch (choice)                 {               case 1: //insert                {                       while (val >= 0)                     {                               cout << "Enter a number greater than -1 to insert (to exit insert -1): ";                                 cin >> val;                               if (val != -1) h.insert(val);                   }                       break;          }               case 2: //search                {                       cout << "Enter a number you wish to search for: ";                        cin >> val;                       h.search(val);                  break;          }               case 3: //delete                {                       cout << "Ender the number you wish to delete: ";                  cin >> val;                       h.remove(val);                  break;          }               case 4: //print                         cout << "All inserted Values:\n";                         h.print();                      break;          case 0:                         break;          default:                        cout << "Invalid Response, try again.\n";                         break;          }       } while (choice != 0); }