C++ switch语句

switch(expression){    
case value1:    
 //要执行的代码；   
 break;  
case value2:    
 //要执行的代码；   
 break;  
......    
    
default:     
 //如果所有情况都不匹配，则要执行的代码；  
 break;  
}  

int main () {  
       int num;  
       cout<<"Enter a number to check grade:";    
       cin>>num;  
        switch (num)    
          {    
              case 10: cout<<"It is 10"; break;    
              case 20: cout<<"It is 20"; break;    
              case 30: cout<<"It is 30"; break;    
              default: cout<<"Not 10, 20 or 30"; break;    
          }    
    }