c语言编写学生成绩管理系统，需要先登录，学生成绩已事先录入，可修改密码，可实现表示列出所有学生记录；“3”表示列出总分最高的学生；“4”表示按姓名查找学生记录；“5”表示按学号查找学生记录；“6”表示按平均分降序排序；“7”表示退出系统等功能的完整代码

好的，以下是一个简单的学生成绩管理系统的完整代码，包含登录、修改密码、查询和排序等功能：

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

#define MAX_STUDENT_NUM 100
#define MAX_NAME_LEN 20
#define MAX_PASSWORD_LEN 20

typedef struct student {
    char name[MAX_NAME_LEN];
    char id[MAX_NAME_LEN];
    int math;
    int english;
    int computer;
    int total;
    float average;
} Student;

void login(char* password);
void change_password(char* password);
void list_students(Student* students, int num);
void find_by_name(Student* students, int num);
void find_by_id(Student* students, int num);
void sort_by_average(Student* students, int num);

int main() {
    Student students[MAX_STUDENT_NUM];
    int num = 0;
    char password[MAX_PASSWORD_LEN];
    int choice;
    
    // 添加学生记录
    strcpy(students[num].name, "张三");
    strcpy(students[num].id, "001");
    students[num].math = 80;
    students[num].english = 90;
    students[num].computer = 85;
    students[num].total = students[num].math + students[num].english + students[num].computer;
    students[num].average = students[num].total / 3.0f;
    num++;
    
    strcpy(students[num].name, "李四");
    strcpy(students[num].id, "002");
    students[num].math = 75;
    students[num].english = 85;
    students[num].computer = 90;
    students[num].total = students[num].math + students[num].english + students[num].computer;
    students[num].average = students[num].total / 3.0f;
    num++;
    
    strcpy(students[num].name, "王五");
    strcpy(students[num].id, "003");
    students[num].math = 85;
    students[num].english = 80;
    students[num].computer = 95;
    students[num].total = students[num].math + students[num].english + students[num].computer;
    students[num].average = students[num].total / 3.0f;
    num++;
    
    login(password);
    do {
        printf("\n请选择操作：\n");
        printf("1. 修改密码\n");
        printf("2. 列出所有学生记录\n");
        printf("3. 列出总分最高的学生\n");
        printf("4. 按姓名查找学生记录\n");
        printf("5. 按学号查找学生记录\n");
        printf("6. 按平均分降序排序\n");
        printf("7. 退出系统\n");
        printf("请选择（1-7）：");
        scanf("%d", &choice);
        switch (choice) {
            case 1:
                change_password(password);
                break;
            case 2:
                list_students(students, num);
                break;
            case 3:
                printf("总分最高的学生是：\n");
                printf("姓名\t学号\t数学\t英语\t计算机\t总分\t平均分\n");
                int max_total = -1;
                int max_index = -1;
                for (int i = 0; i < num; i++) {
                    if (students[i].total > max_total) {
                        max_total = students[i].total;
                        max_index = i;
                    }
                }
                printf("%s\t%s\t%d\t%d\t%d\t%d\t%.2f\n", students[max_index].name, students[max_index].id, students[max_index].math, students[max_index].english, students[max_index].computer, students[max_index].total, students[max_index].average);
                break;
            case 4:
                find_by_name(students, num);
                break;
            case 5:
                find_by_id(students, num);
                break;
            case 6:
                sort_by_average(students, num);
                list_students(students, num);
                break;
            case 7:
                printf("谢谢使用！\n");
                break;
            default:
                printf("无效选择，请重新选择！\n");
                break;
        }
    } while (choice != 7);
    
    return 0;
}

// 登录
void login(char* password) {
    printf("请输入密码：");
    scanf("%s", password);
}

// 修改密码
void change_password(char* password) {
    char old_password[MAX_PASSWORD_LEN];
    char new_password[MAX_PASSWORD_LEN];
    printf("请输入旧密码：");
    scanf("%s", old_password);
    if (strcmp(old_password, password) == 0) {
        printf("请输入新密码：");
        scanf("%s", new_password);
        strcpy(password, new_password);
        printf("密码修改成功！\n");
    } else {
        printf("密码错误，无法修改！\n");
    }
}

// 列出所有学生记录
void list_students(Student* students, int num) {
    printf("姓名\t学号\t数学\t英语\t计算机\t总分\t平均分\n");
    for (int i = 0; i < num; i++) {
        printf("%s\t%s\t%d\t%d\t%d\t%d\t%.2f\n", students[i].name, students[i].id, students[i].math, students[i].english, students[i].computer, students[i].total, students[i].average);
    }
}

// 按姓名查找学生记录
void find_by_name(Student* students, int num) {
    char name[MAX_NAME_LEN];
    printf("请输入要查找的姓名：");
    scanf("%s", name);
    printf("姓名\t学号\t数学\t英语\t计算机\t总分\t平均分\n");
    int found = 0;
    for (int i = 0; i < num; i++) {
        if (strcmp(students[i].name, name) == 0) {
            printf("%s\t%s\t%d\t%d\t%d\t%d\t%.2f\n", students[i].name, students[i].id, students[i].math, students[i].english, students[i].computer, students[i].total, students[i].average);
            found = 1;
        }
    }
    if (!found) {
        printf("找不到该学生！\n");
    }
}

// 按学号查找学生记录
void find_by_id(Student* students, int num) {
    char id[MAX_NAME_LEN];
    printf("请输入要查找的学号：");
    scanf("%s", id);
    printf("姓名\t学号\t数学\t英语\t计算机\t总分\t平均分\n");
    int found = 0;
    for (int i = 0; i < num; i++) {
        if (strcmp(students[i].id, id) == 0) {
            printf("%s\t%s\t%d\t%d\t%d\t%d\t%.2f\n", students[i].name, students[i].id, students[i].math, students[i].english, students[i].computer, students[i].total, students[i].average);
            found = 1;
        }
    }
    if (!found) {
        printf("找不到该学生！\n");
    }
}

// 按平均分降序排序
void sort_by_average(Student* students, int num) {
    for (int i = 0; i < num - 1; i++) {
        for (int j = 0; j < num - 1 - i; j++) {
            if (students[j].average < students[j+1].average) {
                Student temp = students[j];
                students[j] = students[j+1];
                students[j+1] = temp;
            }
        }
    }
}

这是一个简单的实现，可以根据需要进行修改和优化。在实际开发中，还需要考虑输入校验、存储方式、界面设计等方面的问题。