/*output the student score in A,B,C,D,E class*/
#include<stdio.h>
#include<stdlib.h>
int main()
{
        int score;
        char ch;
   test:{
        printf("please input the score: ");
        scanf("%d",&score);

        if(score>=90)
                ch='A';
        else if(score>=80&&score<=89)
                ch='B';
        else if(score>=70&&score<=79)
                ch='C';
        else if(score>=60&&score<=69)
                ch='D';
        else
                ch='E';

        printf("this student score is : %c class\n",ch);
     test2:  {
        printf("continue the test , Y/N ? : ");
        char judge='Y';
        scanf("%c",&judge);
        if(judge=='Y'||judge=='y')
                goto test;
        else if(judge=='N'||judge=='n')
                exit(0);
        else
                printf("Error :wrong input!!!!!!!! \n");
                goto test2;
       }
}

return 0;
}

输入23后，运行结果
please input the score: 23
this student score is : E class
continue the test , Y/N ? : Error :wrong input!!!!!!!!
continue the test , Y/N ? :
 
请大家分析一下为什么会出现continue the test , Y/N ? : Error :wrong input!!!!!!!!

因为你输入数字后。。。要按回车。。所以judge的第一次读的值就是回车。。。在scanf后面再加个scanf("%c",&judge);，或者getchar (judge);就行了。。。

谢谢，运行正确，但是再请教一下，为什么在按下回车的时候还是要再scanf一次？这样的情况在什么条件下才会发生？