有偿，问题不难 求超人解答

有偿！不难 通过文件读写实现用户注册，要识别用户id是否存在
现在只能识别第一行（第一个）用户

```c
#include
#include
#include

typedef struct UserData
{
    char userID[20];
    char userName[20];
    char userSex;
    char userNumber[20];
    char userPass[20];
    char userEmail[30];
    char userAddress[50];
    float usermoney;
}userdata;

void Regist()
{
    userdata a={0},b={0};
    char temp[20];
    char tempid[20];
    int q=0;
    int t=0;
    char tempmoney[50];
    char* tmoney;
    int n=0;
    FILE *pf;
    userdata local[20];
    pf=fopen("userdata.txt","r");
    if(pf==NULL)
    {
        printf("打开注册文件失败\n");
        return;
    }

    while(!feof(pf))
    {
        fscanf(pf,"%s %s %s %s %s %s %s %f",local[q].userID,local[q].userName,local[q].userSex,local[q].userNumber,local[q].userPass,local[q].userEmail,local[q].userAddress,&local[q].usermoney);
        q++;
    }

    printf("欢迎注册超市用户账号！( ´∀`)\n");
    printf("请输入你的用户ID:\n");
    scanf("%s",tempid);
    for ( t = 0 ; t < q ; t++)
    {
        if(strcmp(tempid,local[t].userID)==0)
        {
            printf("该ID已被用户注册，请输入新的用户ID");
            scanf("%s",tempid);
        }
        else
        {
            strcpy(a.userID,tempid);
            printf("您的ID为：%s",a.userID);
            break;
        }

    }
    



    printf("请输入姓名：\n");
    scanf("%s",a.userName);
    printf("请输入性别(f(女)/m(男))：\n");
    scanf("%c",&a.userSex);
    do
    {
        if(a.userSex != 'f' && a.userSex != 'm')
        {
            printf("性别输入错误！请重新输入！\n");
            scanf("%c",&a.userSex);
        }
    }while(a.userSex != 'f' && a.userSex != 'm');
    getchar();
    printf("请输入电话号码：\n");
    scanf("%s",a.userNumber);
    printf("请输入密码\n");
    scanf("%s",a.userPass);
    printf("请确认密码\n");
    scanf("%s",temp);
    do
    {
        if(strcmp(a.userPass,temp)==0)
        {

           printf("账号注册成功，请输入邮箱\n"); 
           fclose(pf);
           break;
        }
        else
        {
            printf("两次密码不匹配！请重新输入！\n");
            scanf("%s",a.userPass);
            printf("请确认密码\n");
            scanf("%s",temp);
        }
    }while(1);
    getchar();
    scanf("%s",a.userEmail);

    
    printf("请输入地址\n");
    scanf("%s",a.userAddress);
    printf("请输入您的余额：");
    scanf("%f",&a.usermoney);

    pf = fopen("userdata.txt","a");
    fwrite(&a, sizeof(struct UserData), 1, pf);
    fclose(pf);
    return;
}

 
  
int main()
{
    int input=-1; 
    do
    {
        printf("\t\t\t----------------------------------\n");
        printf("\t\t\t|           1.登录               |\n");
        printf("\t\t\t|           2.注册               |\n");
        printf("\t\t\t|           0.退出               |\n");
        printf("\t\t\t----------------------------------\n");
        printf("请选择您想进行的操作\n");
        scanf("%d",&input);
        switch(input)
        {
            case 2:Regist();break;
            case 0:puts("退出成功"); return 0;
        }
    }while(1);
 
 return 0;
}

```

这是我以前写的，和你要求的功能相同的代码，有对识别用户id是否存在，同时密码输入里有对输入密码(六位[数字+字符])位数的检查及数字与字符位数的要求，供参考：

#include <ctype.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
typedef struct The_users
{
    char id[100];
    char password[20];
    char phone[12];
    char mailbox[100];
    struct The_users* next;
}users;
void add_users(char* s = NULL)
{
    FILE* fp;
    errno_t err;
    int count = 0, i = 0, flg = 0, flag = 0;
    char  name_id[100], password[20], phone[12], mailbox[100];
 
    if ((err = fopen_s(&fp, "users.txt", "a")) != 0)
        printf("文件打开失败!\n");
    else {
        if (s == NULL){
            printf("新建用户名:");
            scanf_s("%s", name_id, (int)sizeof(name_id));
        }
        else
            strcpy(name_id, s);
        do {
            printf("请输入密码(六位[数字+字符]):");
            scanf_s("%s", password, (int)sizeof(password));
            count = strlen(password);
            for (i = 0, flg = 0, flag = 0; i < count; i++)
                if (isdigit(password[i]))
                    flg++;
                else
                    flag++;
        } while (count != 6 || flg == 0 || flag == 0);
        printf("请输入电话:");
        scanf_s("%s", phone, (int)sizeof(phone));
        printf("请输入邮箱:");
        scanf_s("%s", mailbox, (int)sizeof(mailbox));
        fprintf(fp, "%s %s %s %s\n", name_id, password, phone, mailbox);
        fclose(fp);
        printf("恭喜,您的账号注册成功！\n");
    }
}
void registers()
{
    char  name_id[100], password[20], phone[12], mailbox[100];
    users* head, * tail, * p;
    FILE* fp;
    errno_t err;
    printf("欢迎来到注册界面！\n");
    if ((err = fopen_s(&fp, "users.txt", "r")) != 0){
        add_users();
    }
    else {
        head = NULL, tail = NULL, p = NULL;
        while (1)
        {
            if (fscanf(fp, "%s %s %s %s\n", name_id, password, phone, mailbox) != 4) break;
            p = (users*)malloc(sizeof(struct The_users));
            p->next = NULL;
            strcpy(p->id, name_id); strcpy(p->password, password);
            strcpy(p->phone, phone); strcpy(p->mailbox, mailbox);
            if (head == NULL)
                head = p;
            else
                tail->next = p;
            tail = p;
        }
        fclose(fp);
        do {
            printf("新建用户名:");
            scanf("%s", name_id);
            tail = head;
            while (tail != NULL && strcmp(tail->id, name_id) != 0)
            {
                tail = tail->next;
            }
            if (tail == NULL)
                add_users(name_id);
            else
                printf("该用户名已经被注册,请重新输入!\n");
        } while (tail != NULL);
    }
    return;
}
int login()
{
    char  name_id[100], password[20], phone[12], mailbox[100];
    users* head, * tail, * p;
    errno_t err;
    FILE* fp;
    int i = 0, flg = 0;
    if ((err = fopen_s(&fp, "users.txt", "r")) != 0){
        printf("无注册用户，请先注册用户。\n");
    }
    else {
        printf("欢迎来到登录界面\n");
        head = NULL, tail = NULL, p = NULL;
        while (1)
        {
            if (fscanf(fp, "%s %s %s %s\n", name_id, password, phone, mailbox) != 4) break;
            p = (users*)malloc(sizeof(struct The_users));
            p->next = NULL;
            strcpy(p->id, name_id);  strcpy(p->password, password);
            strcpy(p->phone, phone); strcpy(p->mailbox, mailbox);
            if (head == NULL)
                head = p;
            else
                tail->next = p;
            tail = p;
        }
        fclose(fp);
        printf("请输入用户名:");
        scanf_s("%s", name_id, (int)sizeof(name_id));
        tail = head;
        while (tail != NULL && strcmp(tail->id, name_id) != 0)
        {
            tail = tail->next;
        }
        if (tail != NULL)
        {
            do {
                printf("密 码:");
                scanf_s("%s", password, (int)sizeof(password));
                if (strcmp(tail->password, password) == 0)
                {
                    printf("登录成功！\n");
                    return 1;
                }
                else {
                    i++;
                    if (i != 3)
                        printf("输入密码错误，还有%d次机会，请重新输入!\n", 3 - i);
                    else {
                        printf("输入密码错误,3次机会已用完，退出登录!\n");
                        break;
                    }
                }
            } while (strcmp(tail->password, password) != 0);
        }
        else
            printf("该用户名不存在！\n");
    }
    return 0;
}
int main()
{
    int n;
    while (1)
    {
        printf("***********菜单*************\n");
        printf("**   1：登录              **\n");
        printf("**   2：注册              **\n");
        printf("**   3：退出              **\n");
        printf("***************************\n");
        printf("请输入1-3编号：");
        scanf_s("%d", &n);
        if (n == 2)
            registers();
        else if (n == 3)
            break;
        else if (n == 1)
            login();
    }
    return 0;
}
 

printf("请输入姓名：\n");
scanf("%s",a.userName);
printf("请输入性别(f(女)/m(男))：\n");
scanf("%c",&a.userSex);
这种写法都是有问题的，性别根本没有输入，直接接收输入姓名后的换行符了
需要在姓名输入完后，增加一个getchar();接收到换行符才行

你这个差的有点远，一个函数咋能完成你说那么多功能呢。你可以参考这篇文章
https://blog.csdn.net/weixin_64811333/article/details/126108201

问题有点多啊，
A。 结构体的初始化不是你那样的；
B。读完文件就close 留着干嘛？
C。第一个for 循环，
如果存在了，你会再次scanf，进入下次循环，但是，你要知道再次进循环，之前的就没有再比较了，因为 t++了；
D。 do while循环，改成while ，里面的if 可以不用

可以看看我写的这个

#include<stdio.h>
#include<windows.h>
#define Max 25
#define obj_num 5
struct Stu
{
    char Name[25];     //姓名
    int Number;        //学号
    int Language_score;//语文成绩
    int Math_score;    //数学成绩
    int English_score; //英语成绩
    int PE_score;      //体育成绩
    int C_score;       //C语言成绩
    int Sum_score;     //总成绩
};
void input(struct Stu Class[],int j)
{
    printf("输入第%d位同学姓名:",j+1);
    scanf("%s",Class[j].Name);
    getchar();
    printf("输入第%d位同学学号:",j+1);
    scanf("%d",&Class[j].Number);
    printf("输入第%d位同学语文成绩:",j+1);
    scanf("%d",&Class[j].Language_score);
    printf("输入第%d位同学数学成绩:",j+1);
    scanf("%d",&Class[j].Math_score);
    printf("输入第%d位同学英语成绩:",j+1);
    scanf("%d",&Class[j].English_score);
    printf("输入第%d位同学体育成绩:",j+1);
    scanf("%d",&Class[j].PE_score);
    printf("输入第%d位同学C语言成绩:",j+1);
    scanf("%d",&Class[j].C_score);
    Class[j].Sum_score=Class[j].Language_score+Class[j].Math_score+Class[j].English_score+Class[j].PE_score+Class[j].C_score;
}
void output(struct Stu Class[],int n)
{
    printf("---------------------------------------------\n");
    printf("姓名  学号  语文  数学  英语  体育  C语言  总分");
    printf("\n");
    int j;
    for(j=0; j<n; j++)
    {
        printf("%5s",Class[j].Name);
        printf("%5d",Class[j].Number);
        printf("%6d",Class[j].Language_score);
        printf("%6d",Class[j].Math_score);
        printf("%6d",Class[j].English_score);
        printf("%6d",Class[j].PE_score);
        printf("%6d",Class[j].C_score);
        printf("%7d",Class[j].Sum_score);
        printf("\n");
    }
    printf("---------------------------------------------\n");
}
void outputOne(struct Stu Class[],int j)
{
    printf("姓名  学号  语文  数学  英语  体育  C语言  总分");
    printf("\n");
    printf("%5s",Class[j].Name);
    printf("%5d",Class[j].Number);
    printf("%6d",Class[j].Language_score);
    printf("%6d",Class[j].Math_score);
    printf("%6d",Class[j].English_score);
    printf("%6d",Class[j].PE_score);
    printf("%6d",Class[j].C_score);
    printf("%7d",Class[j].Sum_score);
    printf("\n");
}
void Sort(struct Stu Class[],int n)
{
    int j,k;
    struct Stu t;
    for(j=0; j<n-1; j++)
    {
        for(k=0; k<n-j-1; k++)
        {
            if(Class[k].Sum_score<Class[k+1].Sum_score)
            {
                t=Class[k];
                Class[k]=Class[k+1];
                Class[k+1]=t;
            }
        }
    }
}
int search(int number,int n,struct Stu Class[])
{
    int i,p=0;
    for(i=0; i<n; i++)
    {
        if(Class[i].Number==number)
        {
            p=i;
            return p;
        }
    }
    return -1;
}
void modify(int number,int n,struct Stu Class[])
{
    int index=search(number,n,Class);
    if(index==-1)
    {
        printf("找不到该学号\n");
    }
    else
    {
        printf("学号%d的学生是第%d位\n",number,index+1);
        input(Class,index);
        printf("修改成功\n");
        output(Class,n);
    }
}
void del(int number,int n,struct Stu Class[])
{
    int index=search(number,n,Class);
    if(index==-1)
    {
        printf("找不到该学号\n");
    }
    else
    {
        printf("学号%d的学生是第%d位\n",number,index);
        int i;
        for(i=index; i<n-1; i++)
        {
            Class[i]= Class[i+1];
        }
        printf("删除成功\n");
        output(Class,n-1);
    }
}
double calave(int n,struct Stu Class[],double ave[])
{
    int Language_score_sum=0;
    int Math_score_sum=0;
    int English_score_sum=0;
    int PE_score_sum=0;
    int C_score_sum=0;
    for(int i=0; i<n; i++)
    {
        Language_score_sum+=Class[i].Language_score;
        Math_score_sum+=Class[i].Math_score;
        English_score_sum+=Class[i].English_score;
        PE_score_sum+=Class[i].PE_score;
        C_score_sum+=Class[i].C_score;
    }
    ave[0]=(double)Language_score_sum/n;
    ave[1]=(double)Math_score_sum/n;
    ave[2]=(double)English_score_sum/n;
    ave[3]=(double)PE_score_sum/n;
    ave[4]=(double)C_score_sum/n;
}
void save(int n,struct Stu Class[])
{
    FILE *fp;
    fp = fopen("score_data.txt","w");
    if(fp == NULL)
    {
        printf("error!");
        return;
    }
    else
    {
        for(int j=0; j<n; j++)
        {
            fprintf(fp,"%s %d %d %d %d %d %d %d\n",Class[j].Name,Class[j].Number,Class[j].Language_score,Class[j].Math_score,Class[j].English_score,Class[j].PE_score,Class[j].C_score,Class[j].Sum_score);
        }
        printf("文件写入成功\n");
    }
    fclose(fp);
}
int read(int n,struct Stu Class[])
{
    FILE *fp;
    fp = fopen("score_data.txt","r");
    int count=0;
    if(fp == NULL)
    {
        printf("error!");
        return 0;
    }
    else
    {
        for(int j=0; j<n; j++)
        {
            int flag=fscanf(fp,"%s %d %d %d %d %d %d %d\n",Class[j].Name,&Class[j].Number,&Class[j].Language_score,&Class[j].Math_score,&Class[j].English_score,&Class[j].PE_score,&Class[j].C_score,&Class[j].Sum_score);
            if(flag!=-1) count++;
        }
        printf("文件读取成功\n");
    }
    fclose(fp);
    return count;
}
void menu(struct Stu Class[])
{
    int n=0,count;
    int option=1;
    while(option<=9&&option>=1)
    {
        printf("------------------------\n");
        printf("------学生成绩管理------\n");
        printf("------1.输入成绩  ------\n");
        printf("------2.输出成绩  ------\n");
        printf("------3.按总成绩排序----\n");
        printf("------4.按学号查询成绩--\n");
        printf("------5.修改信息  ------\n");
        printf("------6.删除学生  ------\n");
        printf("------7.计算各科平均分--\n");
        printf("------8.文件读入  ------\n");
        printf("------9.保存至文件------\n");
        printf("------other.退出  ------\n");
        printf("------------------------\n");
        printf("输入选项：");
        scanf("%d",&option);
        switch(option)
        {
            case 1:
            {
                system("cls");
                printf("输入学生的个数(不大于%d):",Max-n);
                scanf("%d",&count);
                while(count--)
                {
                    input(Class,n++);
                }
                output(Class,n);
                break;
            }
            case 2:
            {
                system("cls");
                output(Class,n);
                break;
            }
            case 3:
            {
                system("cls");
                Sort(Class,n);
                output(Class,n);
                break;
            }
            case 4:
            {
                system("cls");
                int number;
                printf("输入要查找的学号：");
                scanf("%d",&number);
                int index=search(number,n,Class);
                if(index==-1)
                {
                    printf("查找失败\n");
                }
                else
                {
                    printf("查找成功\n");
                    outputOne(Class,index);
                }
                break;
            }
            case 5:
            {
                system("cls");
                output(Class,n);
                int number;
                printf("输入要修改成绩的学生对应的学号：");
                scanf("%d",&number);
                modify(number,n,Class);
                output(Class,n);
                break;
            }
            case 6:
            {
                system("cls");
                output(Class,n);
                int number;
                printf("输入要删除的学号：");
                scanf("%d",&number);
                del(number,n,Class);
                n--;
                system("cls");
                output(Class,n);
                break;
            }
            case 7:
            {
                system("cls");
                output(Class,n);
                double ave[obj_num];
                calave(n,Class,ave);
                char str[]="平均分";
                printf("\n");
                printf("%5s",str);
                printf("     ");
                printf("%5.2lf",ave[0]);
                printf("%6.2lf",ave[1]);
                printf("%6.2lf",ave[2]);
                printf("%6.2lf",ave[3]);
                printf("%6.2lf",ave[4]);
                printf("\n");
                break;
            }
            case 8:
            {
                system("cls");
                n-=n; 
                n+=read(Max,Class);
                break;
            }
            case 9:
            {
                system("cls");
                save(n,Class);
                break;
            }
            default :
            {
                break;
            }
        }
    }
}
int main()
{
    struct Stu Class [Max];
    menu(Class);
    return 0;
}