/*jinjin 0905209*/
#include <stdio.h>
#include <stdlib.h>

int main()
{
    int grades[20];
    int index;
    int value;
    int i=0;
    double sum=0;
    double average;
    i=0;
    do
    {
        printf("Enter student number: ");
        scanf("%d", &index);
        if (index >= 0 && index < 20)
        {
            printf("Enter grade for student number %d:", index);
            scanf("%d", &value);
            grades[index] = value;
        }
        else if (index==-1)
            break;
        else
        {
            printf("You entered an invalid student number\n");
        }
        i++;
        sum+=grades[index];
    }
    while(i<20);

    average=sum/i;
    printf("the average grade is %4.2f",average);
    return 0;
}
相同的学号，不同的成绩；怎么改可以覆盖啊？

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int grades[20];
    int index;
    int value;
    int i=0;
    double sum=0;
    double average;
    i=0;
     memset(grades,0,20*sizeof(int));//数组初始化为0
    do
    {
        printf("Enter student number: ");
        scanf("%d", &index);

        if (index >= 0 && index < 20)
        {
            printf("Enter grade for student number %d:", index);
            scanf("%d", &value);
            if(grades[index]== 0){
               grades[index] = value;
               i++;//拿到这来，因为未修改人数才加一
               }
            else {
                 sum -=   grades[index];
                  grades[index] = value;      
            }
        sum+=grades[index];//index正常才计算sum
        }
        else if (index==-1)
            break;
        else
        {
            printf("You entered an invalid student number\n");
        }
        
    }
    while(i<20);

    average=sum/i;
    printf("the average grade is %4.2f",average);
    system("pause");
    return 0;
}

大哥好像，运行了不太对诶。。数字很大啊，如果有相同的就是-1.#j   =.=

啥意思？你要是刚上来就输-1会发生除零错误，结果自然是-1.#J

没有一上来就输-1啊
我正常地输，结果就是很大的数字
我输入重复的学号，结果就带井号

/*jinjin 0905209*/
#include <stdio.h>
#include <stdlib.h>
#define N0 3        //调试好程序再改成20！！！
int main()
{
    int grades[N0]={0};            //数组初始化为0
    int index;
    int value;
    int i=0;
    double sum=0;
    double average;
    for(i=0;i<N0;i++)grades[i]=-1;        //-1表示此学生还没有被赋值。
    i=0;
    do
    {
        printf("Enter student number: ");
        scanf("%d", &index);
        if (index >= 0 && index < N0)
        {
            printf("Enter grade for student number %d:", index);
            scanf("%d", &value);
            if(grades[index] ==-1)
            {
                grades[index] = value;
                i++;
                sum+=grades[index];
            }
            else
            {
                sum-=grades[index]+value;
                grades[index] = value; //相同的学号，不同的成绩；这里可以覆盖。
            }
        }
        else if (index==-1)
            break;
        else
        {
            printf("You entered an invalid student number\n");
        }
    }
    while(i<N0);

    if(i>0)
    {
        average=sum/i;
        printf("the average grade is %4.2f\n",average);
    }
    else
        printf("input error.\n");
    return 0;
}

DEV下一切正常……

=。=
那个memset那一行报错，如果我把那一行删了，就不行了

很奇怪你的为什么不可以。那一行是用来初始化数组元素为零的，你在声明时改成int grades[20] = {0};
试试？

成功啦，万分感谢，不过还不是很明白，还要琢磨琢磨