#include<stdio.h>

int main(void)
{
  int  num1, demon1,num2,demon2,result_num,result_demon,b;
  int a;

  printf("Enter first fraction: ");
  scanf("%d/%d",&num1,&demon1);

  printf("Enter second fraction: ");
  scanf("%d/%d",&num2,&demon2);
  printf("'+'=1\n");
  printf("'-'=2\n");
  printf("'*'=3\n");
  printf("'/'=4\n");
  printf("Enter a digital: ");
  scanf("%d",&a);

  switch(a)
    {
  case 1 : result_num=num1*demon2+num2*demon1;
            result_demon=demon1*demon2;
            printf("The sum is %d/%d\n",result_num,result_demon);
            break;
  case 2: result_num=num1*demon2-num2*demon1;
            result_demon=demon1*demon2;
            printf("The sum is %d/%d\n",result_num,result_demon);
            break;
  case 3 : result_num=num1*num2;
            result_demon=demon1*demon2;
            printf("The sum is %d/%d\n",result_num,result_demon);
            break;
  case 4: result_num=num1*demon2;
            result_demon=num2*demon1;
            printf("The sum is %d/%d\n",result_num,result_demon);
            break;
       default: printf("ERROR!\n");
               break;
     }

      return 0；
}

int a;                      + - * / 为字符型的  而不是INT型
scanf("%d",&a);         

来看看

你看看是这个样子吗？
#include<stdio.h>

int main(void)
{
    int  num1, demon1,num2,demon2,result_num,result_demon;
    char a;
   
    printf("Enter first fraction: ");
    scanf("%d/%d",&num1,&demon1);
   
    printf("Enter second fraction: ");
    scanf("%d/%d",&num2,&demon2);
   
    printf("Enter a character(+,-,*,/): ");
    getchar();
    scanf("%c",&a);
   
    switch(a)
    {
    case '+': result_num=num1*demon2+num2*demon1;
        result_demon=demon1*demon2;
        printf("The sum is %d/%d\n",result_num,result_demon);
        break;
    case '-': result_num=num1*demon2-num2*demon1;
        result_demon=demon1*demon2;
        printf("The sum is %d/%d\n",result_num,result_demon);
        break;
    case '*': result_num=num1*num2;
        result_demon=demon1*demon2;
        printf("The sum is %d/%d\n",result_num,result_demon);
        break;
    case '/': result_num=num1*demon2;
        result_demon=num2*demon1;
        printf("The sum is %d/%d\n",result_num,result_demon);
        break;
    default: printf("ERROR!\n");
        break;
    }
   
    return 0;
}

...你的a定义的是int型,5楼那个是char型,OK?
getchar是char型独有的输入语句,它的执行效率比scanf("%c",&a);要高很多,因为scanf是一个函数,必须要调用它,而getchar不是函数.

请问一下哦```

``为什么 %d/%d  ???  %d再输出%d 吗`？

 为什么不能直接是%d%d呢`？

#include<Stdio.h>
main()
{
  int  num1, demon1,num2,demon2,result_num,result_demon;
  char a;

  printf("Enter first fraction: ");
  scanf("%d/%d",&num1,&demon1);

  printf("Enter second fraction: ");
  scanf("%d/%d",&num2,&demon2);

  printf("Enter a character(+,-,*,/): ");
  scanf("%s",&a);

  switch(a)
    {
  case '+': result_num=num1*demon2+num2*demon1;
            result_demon=demon1*demon2;
            printf("The sum is %d/%d\n",result_num,result_demon);
            break;
  case '-': result_num=num1*demon2-num2*demon1;
            result_demon=demon1*demon2;
            printf("The sum is %d/%d\n",result_num,result_demon);
            break;
  case '*': result_num=num1*num2;
            result_demon=demon1*demon2;
            printf("The sum is %d/%d\n",result_num,result_demon);
            break;
  case '/': result_num=num1*demon2;
            result_demon=num2*demon1;
            printf("The sum is %d/%d\n",result_num,result_demon);
            break;
       default: printf("ERROR!\n");
               break;
     }
}

有2个数所以用 %d/%d 用 / 把2个数分开。。。看的明白。。要是直接 %d%d 就分不出来了，比如输出 2 和 63 用 %d/%d 输出是 2/63 要是直接用 %d%d 输出的话变成 263 理解就有歧义了。你可以理解成 2/63 也可以理解成 26/3 .......

[ 本帖最后由 kinwyb 于 2010-5-22 15:12 编辑 ]