筛法求素数

 1 using System;
 2 public class PrimeFilter{
 3     public static void Main( string [] args ){
 4         int N = 100;
 5         bool [] a = new bool[N+1];
 6         for( int i=2; i<=N; i++ ) a[i]=true;
 7   
 8         for( int i=2; i<N; i++ )
 9         {
10             if(a[i]) for( int j=i*2; j<=N; j+=i )
11                 a[j]=false;
12         }
13   
14         for( int i=2; i<=N; i++) 
15             if( a[i] ) Console.Write( i + " " );
16     }
17 }