#include <stdio.h>
void main( )
{
    char sign;
    int x,y;
    printf("输入x 运算符 y：");
    scanf("%d%c%d", &x, &sign, &y);
    { if(sign='*')            
     printf("%d * %d = %d\n",x,y,x*y);
      else if(sign='/')
     printf("%d / %d = %d\n",x,y,x/y);
     else if(sign='%')
        printf("%d Mod %d = %d\n",x,y,x%y);
    else
        printf("运算符输入错误！\n");        }
}

#include <stdio.h>
void main( )
{
    char sign;
    int x,y;
    printf("输入x 运算符 y：");
    scanf("%d%c%d", &x, &sign, &y);

    if(sign=='*')           
     printf("%d * %d = %d\n",x,y,x*y);
      else if(sign=='/')
     printf("%d / %d = %d\n",x,y,x/y);
     else if(sign=='%')
        printf("%d Mod %d = %d\n",x,y,x%y);
    else
        printf("运算符输入错误！\n");
}

以后要比较一个变量和一个常量是否相等  请用if(5 == a)

在c当中=是赋值，而==才是相等！

哦哦 谢谢啊

嗯嗯 知道了 谢谢

#include <stdio.h>
void main( )
{
    char sign;
    int x,y;
    printf("input x & y\n");
    scanf("%d%c%d", &x, &sign, &y);

    switch(sign)
        {
          case '+':printf("%d + %d = %d\n",x,y,x+y);break;
          case '-':printf("%d - %d = %d\n",x,y,x-y);break;
          case '*':printf("%d * %d = %d\n",x,y,x*y);break;
          case '/':printf("%d / %d = %d\n",x,y,x/y);break;
          case '%':printf("%d Mod %d = %d\n",x,y,x%y);break;
          default :printf("input is worng!");break;

         }
         getch();
}

改写一下，顺道也练练手，楼主请参考一下，我个人觉得既然是在学语法了，SWITCH用这里肯定更好些：
   

还有字符类型的不用加地址符号吧&；
还有==是判断哦，=是赋值