sieve.cpp

corejava的源程序内有好多的源代码

/**
   @version 1.20 1999-07-03
   author Cay Horstmann
*/

#ifndef AVOID_STANDARD_BITSET

#include <bitset>

#else

template<int N>
class bitset
{
public:
   bitset() : bits(new char[(N - 1) / 8 + 1]) {}

   bool test(int n)
   {  
      return (bits[n >> 3] & (1 << (n & 7))) != 0;
   }

   void set(int n)
   {  
      bits[n >> 3] |= 1 << (n & 7);
   }

   void reset(int n)
   {  
      bits[n >> 3] &= ~(1 << (n & 7));
   }

private:
   char* bits;
};

#endif

#include <iostream>
#include <ctime>

using namespace std;

int main()
{  
   const int N = 1000000;
   clock_t cstart = clock();

   bitset<N + 1> b;
   int count = 0;
   int i;
   for (i = 2; i <= N; i++)
      b.set(i);
   i = 2;
   while (i * i <= N)
   {  
      if (b.test(i))
      {  
         int k = 2 * i;
         while (k <= N)
         {  b.reset(k);
            k += i;
         }
      }
      i++;
   }
   for (i = 2; i <= N; i++)
   {  
      if (b.test(i))
      {
#ifdef PRINT
         cout << i << "\n";
#endif
         count++;
      }
   }

   clock_t cend = clock();
   double millis = 1000.0
      * (cend - cstart) / CLOCKS_PER_SEC;

   cout << count << " primes\n"
      << millis << " milliseconds\n";

   return 0;
}