#include <iostream>
using namespace std;

int main()

{   char mingling;
     i=1;
    while(i==1)
    {   cout<<"*********************";
        cout<<"计算机程序"<<'\n';
        cout<<"a是加法运算"<<'\n';
        cout<<"b是减法运算"<<'\n';
        cout<<"c是退出程序"<<'\n';

        cout<<"请你输入命令:";

          cin>>mingling;   

      switch (mingling)
   
        
    case 'a':
        cout<<"计算加法\n";
        cout<<"请输入第一个数:";
        cin>>a;
        cout<<"请输入第二个数:";
        cin>>b;
        cout<<"他们的合为:"<<a+b;
          break;
    case 'b':
        cout<<"计算减法\n";
        cout<<"请输入第一个数:";
        cin>>a;
        cout<<"请输入第二个数:";
        cin>>b;
        cout<<"他们的差为:"<<a+b;
        break;
      
    
    case 'c':
        cout<<"程序退出";
        break;
    default;
        cout<<"你输入的指令不合法\n";
        break;
    }

}

最后少了 return 0;

不对2楼.我加上了 还是编译错误

你那些变量怎么都不定义，拿来一个就用，不出错都怪了

#include <iostream>
using namespace std;

void main()

    int a,b,c,i;

{   char mingling;
     i=1;
    while(i==1)
    {   cout<<"*********************";
        cout<<"计算机程序"<<'\n';
        cout<<"a是加法运算"<<'\n';
        cout<<"b是减法运算"<<'\n';
        cout<<"c是退出程序"<<'\n';

        cout<<"请你输入命令:";

          cin>>mingling;   

      switch (mingling)
   
        
    case 'a':
        cout<<"计算加法\n";
        cout<<"请输入第一个数:";
        cin>>a;
        cout<<"请输入第二个数:";
        cin>>b;
        cout<<"他们的合为:"<<a+b;
          break;
    case 'b':
        cout<<"计算减法\n";
        cout<<"请输入第一个数:";
        cin>>a;
        cout<<"请输入第二个数:";
        cin>>b;
        cout<<"他们的差为:"<<a+b;
        break;
      
   
    case 'c':
        cout<<"程序退出";
        break;
    default;
        cout<<"你输入的指令不合法\n";
        break;
    }

}


我定义了 可没复制上来.这样还是不行  你受累在给看下

#include <iostream>
using namespace std;

int main()
{   char mingling;
     int a,b,c;
    while(1)
    {   cout<<endl;
        cout<<"*********************";
        cout<<"计算机程序"<<'\n';
        cout<<"a是加法运算"<<'\n';
        cout<<"b是减法运算"<<'\n';
        cout<<"c是退出程序"<<'\n';

        cout<<"请你输入命令:";

          cin>>mingling;   

      switch (mingling){
    case 'a':
        cout<<"计算加法\n";
        cout<<"请输入第一个数:";
        cin>>a;
        cout<<"请输入第二个数:";
        cin>>b;
        cout<<"他们的合为:"<<a+b;
          break;
    case 'b':
        cout<<"计算减法\n";
        cout<<"请输入第一个数:";
        cin>>a;
        cout<<"请输入第二个数:";
        cin>>b;
        cout<<"他们的差为:"<<a-b;
        break;
      
   
    case 'c':
        cout<<"程序退出";
        break;
    default:
        cout<<"你输入的指令不合法\n";
        break;
        }
    }
    getchar();
    return 0;
}
用dev-c++编译运行

输入C后不能退出程序！

缺个“{”在switch语句

错误太多，楼主应该自己好好看看书

#include <iostream>
using namespace std;

void main()

{
    int a,b,i;
    char mingling;

     i=1;
    while(i==1)
    {   cout<<"*********************";
        cout<<"计算机程序"<<'\n';
        cout<<"a是加法运算"<<'\n';
        cout<<"b是减法运算"<<'\n';
        cout<<"c是退出程序"<<'\n';

        cout<<"请你输入命令:";

          cin>>mingling;   

          switch (mingling)   
          {
            case 'a':
                cout<<"计算加法\n";
                cout<<"请输入第一个数:";
                cin>>a;
                cout<<"请输入第二个数:";
                cin>>b;
                cout<<"他们的合为:"<<a+b;
                break;
            case 'b':
                cout<<"计算减法\n";
                cout<<"请输入第一个数:";
                cin>>a;
                cout<<"请输入第二个数:";
                cin>>b;
                cout<<"他们的差为:"<<a+b;
                break;
            case 'c':
                cout<<"程序退出";
                break;
            default:
                cout<<"你输入的指令不合法\n";
                break;
        }
    }
}