fig20_40.cpp

这是C＋＋编程经典的书上实例的源代码。 （英文名是《C＋＋ how to program》第3版）

// Fig. 20.40: fig20_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 <bitset>
#include <cmath>

int main()
{
   const int size = 1024;
   int i, value, counter;
   std::bitset< size > sieve;

   sieve.flip();

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

   for ( i = 2; i < finalBit; ++i ) 
      if ( sieve.test( i ) ) 
         for ( int j = 2 * i; j < size; j += i ) 
            sieve.reset( j );

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

   for ( i = 2, counter = 0; i < size; ++i )
      if ( sieve.test( i ) ) {
         cout << setw( 5 ) << i;

         if ( ++counter % 12 == 0 ) 
            cout << '\n';
      }            
   
   cout << endl;

   // get a value from the user to determine if it is prime
   cout << "\nEnter a value from 1 to 1023 (-1 to end): ";
   cin >> value;

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

   return 0;
}


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 * (C) Copyright 2000 by Deitel & Associates, Inc. and Prentice Hall.     *
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