#include <stdio.h>
int main()

{  
    struct Week
    {
       char Mon;
       char Tue;
       char Wed;
       char Thu;
       char Fri;
       char Sat;
       char Sun;
    };
    struct Week_1;
    struct Week*p;
    p=&stu_1;
      stu_1.Mon="Monday";  //给stu_1赋值
      stu_1.Tue="Tuesday";
      stu_1.Wed="Wednesday";
      stu_1.Thu="Thursday";
      stu_1.Fri="Friday";
      stu_1.Sat="Saturday";
      stu_1.Sun="Sunday"; //同上
  int i;
  scanf("%d"&i);
  switch(i)
  {
  case 1:printf("%s",(*p).Mon);break;
  case 2:printf("%s",(*p).Tue);break;
  case 3:printf("%s",(*p).Wed);break;
  case 4:printf("%s",(*p).Thu);break;
  case 5:printf("%s",(*p).Fri);break;
  case 6:printf("%s",(*p).Sat);break;
  case 7:printf("%s",(*p).Sun);break;
  }

return 0;
  }

&stu_1这个东西还没定义就在使用了吧。再有，那些成员是char型，只能接受单个字符输入。想接受字符串的话要用char型数组。程序的逻辑建议改进一下，成员变量名已经周日到周六每个日子各占用一个了，哪还有赋值的意义。

#include <stdio.h>
int main()

{  
    struct Week
    {
       char Mon[7];   //char型数组
       char Tue[8];
       char Wed[10];
       char Thu[9];
       char Fri[7];
       char Sat[9];
       char Sun[7];
    };
    struct Week stu_1;   //定义stu_1
    struct Week*p;
    p=&stu_1;
      stu_1.Mon="Monday";  //为什么赋值没意义啊？看不懂。
      stu_1.Tue="Tuesday";
      stu_1.Wed="Wednesday";
      stu_1.Thu="Thursday";
      stu_1.Fri="Friday";
      stu_1.Sat="Saturday";
      stu_1.Sun="Sunday"; //同上
  int i;
  scanf("%d",&i);
  switch(i)
  {
  case 1:printf("%s",(*p).Mon);break;
  case 2:printf("%s",(*p).Tue);break;
  case 3:printf("%s",(*p).Wed);break;
  case 4:printf("%s",(*p).Thu);break;
  case 5:printf("%s",(*p).Fri);break;
  case 6:printf("%s",(*p).Sat);break;
  case 7:printf("%s",(*p).Sun);break;
  }

return 0;
  }

#include <stdio.h>
#include <string.h>  //应用strcpy函数时需要头文件<string.h>
int main()

{  
    struct Week
    {
       char Mon[7];   //char型数组,不用数组只能存储一个字母
       char Tue[8];
       char Wed[10];
       char Thu[9];
       char Fri[7];
       char Sat[9];
       char Sun[7];
    };
    struct Week stu_1;   //定义stu_1
    struct Week*p;
    p=&stu_1;
          strcpy(stu_1.Mon,"Monday");   //用字符串复制函数给stu_1.Mon赋值
          strcpy(stu_1.Tue,"Tuesday ");
          strcpy(stu_1.Wed,"Wednesday");
          strcpy(stu_1.Thu,"Thursday");
          strcpy(stu_1.Fri,"Friday");
          strcpy(stu_1.Sat,"Saturday");
          strcpy(stu_1.Sun,"Sunday");//同上
  int i;
  printf("输入阿拉伯数字显示对应星期几的英文\n请输入阿拉伯数字回车键结束：\n");
  scanf("%d",&i);        //switch作为数字选择输出
  switch(i)
  {
  case 1:printf("对应星期为：%s\n",(*p).Mon);break;
  case 2:printf("对应星期为：%s\n",(*p).Tue);break;
  case 3:printf("对应星期为：%s\n",(*p).Wed);break;
  case 4:printf("对应星期为：%s\n",(*p).Thu);break;
  case 5:printf("对应星期为：%s\n",(*p).Fri);break;
  case 6:printf("对应星期为：%s\n",(*p).Sat);break;
  case 7:printf("对应星期为：%s\n",(*p).Sun);break;
  }

return 0;
  }
谢谢，突然发现了字符串赋值要用复制赋值的，谢谢你。