#include<stdio.h>
main()
{
    int a,b,d;

    char ch;

    printf("please input a expression:\n");

    scanf("%d",&a);
    ch=getchar();
    scanf("%d",&b);


    switch('ch')

    {
    case'+':d=a+b;
        printf("%d+%d=%lf\n",a,b,d);
        break;
    case'-':d=a-b;
        printf("%d-%d=%lf\n",a,b,d);
        break;
    case'*':d=a*b;
        printf("%d*%d=%lf\n",a,b,d);
        break;
    case'/':
        {
        if (b==0)
            printf("Divisor is zero\n");
        else
        {
            d=a/b;
            printf("%d/%d=%lf\n",a,b,d);
        }
        }
        break;
    default:
        printf("input operator error!\n");
    }

}

#include<stdio.h>
 main()
 {
     int a,b;
     float d;
 
    char ch;
 
    printf("please input a expression:\n");
 
    scanf("%d%c%d",&a,&ch,&b);
     
    switch(ch)
 
    {
     case'+':d=a+b;
         printf("%d+%d=%lf\n",a,b,d);
         break;
     case'-':d=a-b;
         printf("%d-%d=%lf\n",a,b,d);
         break;
     case'*':d=a*b;
         printf("%d*%d=%lf\n",a,b,d);
         break;
     case'/':
         
         if (b==0)
             printf("Divisor is zero\n");
         else
        {
             d=a/b;
             printf("%d/%d=%lf\n",a,b,d);
         }
         
         break;
     default:
         printf("input operator error!\n");
     }
     
 
}

#include<stdio.h>
 main()
 {
     int a,b;
float d;//与下边的d对应
 
    char ch;
 
    printf("please input a expression:\n");
 
    scanf("%d",&a);
     ch=getchar();
     scanf("%d",&b);
 

    switch(ch)//变量不要用引号
 
    {
     case'+':d=a+b;
         printf("%d+%d=%lf\n",a,b,d);
         break;
     case'-':d=a-b;
         printf("%d-%d=%lf\n",a,b,d);
         break;
     case'*':d=a*b;
         printf("%d*%d=%lf\n",a,b,d);
         break;
     case'/':
         {
         if (b==0)
             printf("Divisor is zero\n");
         else
        {
             d=a/b;
             printf("%d/%d=%lf\n",a,b,d);
         }
         }
         break;
     default:
         printf("input operator error!\n");
     }
 
}

刚才我自己也找出错误了。谢谢哈
#include<stdio.h>
main()
{
int a,b; double d;

char ch;

printf("please input a expression:\n");

scanf("%d%c%d",&a,&ch,&b);



  switch(ch)

{
case'+':d=a+b;
printf("%d+%d=%.2lf\n",a,b,d);
break;
case'-':d=a-b;
printf("%d-%d=%.2lf\n",a,b,d);
break;
case'*':d=a*b;
printf("%d*%d=%.2lf\n",a,b,d);
break;
case'/':
{
if (b==0)
printf("Divisor is zero\n");
else
{
d=a/b;
printf("%d/%d=%.2lf\n",a,b,d);
}
}
break;
default:
printf("input operator error!\n");
}

}