// Fig. 23.40: Fig23_40.cpp                                
// Using a bitset to demonstrate the Sieve of Eratosthenes.
#include <iostream>
using std::cin;
using std::cout;
using std::endl;

#include <iomanip>
using std::setw;

#include <cmath> 
using std::sqrt; // sqrt prototype

#include <bitset> // bitset class definition

int main()
{
   const int SIZE = 1024;
   int value;
   std::bitset< SIZE > sieve; // create bitset of 1024 bits
   sieve.flip(); // flip all bits in bitset sieve
   sieve.reset( 0 ); // reset first bit (number 0)         
   sieve.reset( 1 ); // reset second bit (number 1)        

   // perform Sieve of Eratosthenes
   int finalBit = sqrt( static_cast< double > ( sieve.size() ) ) + 1;

   // determine all prime numbers from 2 to 1024
   for ( int i = 2; i < finalBit; i++ )
   {
      if ( sieve.test( i ) ) // bit i is on
      {
         for ( int j = 2 * i; j < SIZE; j += i ) 
            sieve.reset( j ); // set bit j off
      } // end if
   } // end for

   cout << "The prime numbers in the range 2 to 1023 are:\n";

   // display prime numbers in range 2-1023
   for ( int k = 2, counter = 1; k < SIZE; k++ )
   {
      if ( sieve.test( k ) ) // bit k is on
      {
         cout << setw( 5 ) << k;

         if ( counter++ % 12 == 0 ) // counter is a multiple of 12
            cout << '\n';
      } // end if          
   } // end for    
   
   cout << endl;

   // get value from user 
   cout << "\nEnter a value from 2 to 1023 (-1 to end): ";
   cin >> value;

   // determine whether user input is prime
   while ( value != -1 ) 
   {
      if ( sieve[ value ] ) // prime number
         cout << value << " is a prime number\n";
      else // not a prime number
         cout << value << " is not a prime number\n";
      
      cout << "\nEnter a value from 2 to 1023 (-1 to end): ";
      cin >> value;
   } // end while

   return 0;
} // end main

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 * (C) Copyright 1992-2005 by Deitel & Associates, Inc. and               *
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