#include"stdio.h"
#include"math.h"
#include"string.h"
#include"stdlib.h"
FILE *fp;
struct information
{
    long num;
    char name[20];
    char major[20];
    float Math;
    float Chinese;
    float English;
}stu[30];
void main()
{
    int a,i,j=1;
    if((fp=fopen("D:\\work\\test.txt","a+"))==NULL)
    {
        printf("cannot open this file\n");
        exit(0);
    }
    for(i=0; i<30;i++)//把文件中的内容储存到结构体中
        {
            fread(&stu[i],sizeof(struct information),1,fp);
        }
    fclose(fp);
    printf("1.录入学生信息\n");
    printf("2.输出学生信息\n");
    scanf("%d",&a);
    switch(a)

    {
    case 1:
        for(i=0;i<30;i++)
        {
            if((fp=fopen("D:\\work\\test.txt","a"))==NULL)
            {
                printf("cannot open this file\n");
                exit(0);
            }
            printf("--------------------------------\n");
            printf("请输入第%d位的学号:",j);
            scanf("%ld",&stu[i].num);
            printf("--------------------------------\n");
            printf("请输入第%d位的姓名:",j);
            scanf("%s",stu[i].name);
            printf("--------------------------------\n");
            printf("请输入第%d位的专业:",j);
            scanf("%s",stu[i].major);
            printf("--------------------------------\n");
            printf("请输入第%d位的数学成绩:",j);
            scanf("%f",&stu[i].Math);
            printf("--------------------------------\n");
            printf("请输入第%d位的语文成绩:",j);
            scanf("%f",&stu[i].Chinese);
            printf("--------------------------------\n");
            printf("请输入第%d位的英语成绩:",j);
            scanf("%f",&stu[i].English);
            printf("--------------------------------\n");
            j++;
            fwrite(&stu[i],sizeof(struct information),1,fp);
            fclose(fp);
        }
        break;
    case 2:
            printf("\tN0.\tname\tmajor\tMath\tChinese\tEnglish\n");
            for(i=0,j=1;i<30;i++)
            {
                printf("-------------------------------------------------------\n");
                printf("%d\t",j);
                printf("%d",stu[i].num);
                printf("\t%s",stu[i].name);
                printf("\t%s",stu[i].major);
                printf("\t%.1f",stu[i].Math);
                printf("\t%.1f",stu[i].Chinese);
                printf("\t%.1f",stu[i].English);
                printf("\n");
                j++;
            }
            fclose(fp);
            break;
    }
}



用文件的方法录入信息后，查询信息是从第26个开始信息就丢失了，这是什么问题？？？

哥们你牛，测试竟超过26组数据，真有耐心啊，我一般测两组三组过了就OK了。
在楼主的代码中，连续打开了31次文件，虽然是追加打开，感觉没必要吧

那个打开文件确实多余了，这是一个作业的一部分，必须是30个元素。做好了测试，这里就出现数据丢失