c入门第二十篇: 学生成绩管理系统优化(指针入门之动态内存管理，如何支持更多的学生？)

前言

前面提到了数组最大的问题，就是不支持动态的扩缩容，它是静态内存分配的，一旦分配完成，其容量是固定的。为了支持学生的动态增长，才引入链表。

其实除了链表，还有一种解决该问题的方式，就是动态数组——动态申请内存的数组。

正文

在之前学生成绩管理系统中，我们定义了一个存储学生信息的结构，该结构初始化大小为MAX_STUDENTS，导致系统最大支持的学生数为MAX_STUDENTS。

#define MAX_STUDENTS 100
Student students[MAX_STUDENTS]; // 学生数组

可以将它改为动态内存申请的形式(指针的典型用法)。

Student *stu_sys_init(int size)
{
    Student *stu_sys;
    stu_sys = malloc(size * sizeof(Student));
    if (stu_sys == NULL) {
        printf("student system malloc failed!\n");
    }

    return stu_sys;
}

其中核心处理如下，当存储学生的数组大小不够时，重新申请2倍内存，存放学生数据，释放原有内存。

    if(student_count >= g_max_student) {
        old_max_student = g_max_student;
        g_max_student = g_max_student << 1;
        stu_reinit = stu_sys_init(g_max_student);
        if (stu_reinit == NULL) {
            printf("Database is full!\n");
            return;
        }
        memcpy(stu_reinit, students, old_max_student * sizeof(Student));
        free(students);
        students = stu_reinit;
    }

所以整个系统可以重新实现为:

#include 
#include 
#include 
#include  // for access() function

#define MAX_STUDENTS 100
#define MAX_NAME_LEN 50
#define STUDENT_SYSTEM "student_system"
#define TRUE 1
#define FALSE 0

typedef struct {
    int id; // 学号
    char name[MAX_NAME_LEN]; // 姓名
    float score; // 成绩
} Student;

int student_count = 0; // 学生数量

Student *students; // 学生数组指针
int g_max_student = MAX_STUDENTS;

Student *stu_sys_init(int num)
{
    Student *stu_sys;
    stu_sys = malloc(num * sizeof(Student));
    if (stu_sys == NULL) {
        printf("student system malloc failed!\n");
    }

    return stu_sys;
}

int write_student_info(Student *s)
{
    FILE *fp = fopen(STUDENT_SYSTEM, "a");
    if (fp == NULL) {
        printf("fopen student_system failed!\n");
        return 1;
    }

    fprintf(fp, "%-4d %-10s %-.2f\n", s->id, s->name, s->score);

    fclose(fp);
    return 0;
}

int check_if_student_exsit(int id)
{
    int i;

    for(i = 0; i < student_count; i++) {
        if(students[i].id == id) {
            return 1;
        }
    }

    return 0;
}

void update_student_info(Student s, int need_write)
{
    Student *stu_reinit;
    int old_max_student;

    if(student_count >= g_max_student) {
        old_max_student = g_max_student;
        g_max_student = g_max_student << 1;
        stu_reinit = stu_sys_init(g_max_student);
        if (stu_reinit == NULL) {
            printf("Database is full!\n");
            return;
        }
        memcpy(stu_reinit, students, old_max_student * sizeof(Student));
        free(students);
        students = stu_reinit;
    }

    if (!check_if_student_exsit(s.id)) {
        students[student_count++] = s;
        if (need_write) {
            printf("Student added successfully, all student: %d!\n", student_count);
            write_student_info(&s);
        }
    } else {
        printf("student has in db, do nothing!\n");
    }
}

void add_student()
{
    Student s;

    printf("Enter student ID: ");
    scanf("%d", &s.id);
    printf("Enter student name: ");
    scanf("%s", s.name);
    s.score = 0.0; // 初始成绩设置为0

    update_student_info(s, TRUE);
}

void print_title()
{
    printf("%-4s %-10s %-5s\n", "ID", "Name", "Score");
}

void display_all_students()
{
    int i;

    printf("-------- All students info --------\n");
    if (student_count == 0) {
        printf("No students!\n");
    } else {
        print_title();
        for(i = 0; i < student_count; i++) {
            printf("%-4d %-10s %-.2f\n", students[i].id, students[i].name, students[i].score);
        }
    }
    printf("--------      End       -----------\n");
}

void find_student_by_id()
{
    int id, i;

    printf("Enter student ID to search: ");
    scanf("%d", &id);

    for(i = 0; i < student_count; i++) {
        if(students[i].id == id) {
            print_title();
            printf("%-4d %-10s %-.2f\n", students[i].id, students[i].name, students[i].score);
            return;
        }
    }

    printf("Student with ID %d not found!\n", id);
}

void find_student_by_name()
{
    int i, is_find = 0;
    char name[MAX_NAME_LEN];

    printf("Enter student name to search: ");
    scanf("%s", name);

    for(i = 0; i < student_count; i++) {
        if(strcmp(students[i].name, name) == 0) {
            print_title();
            printf("%-4d %-10s %-.2f\n", students[i].id, students[i].name, students[i].score);
            is_find = 1;
        }
    }

    if (is_find == 0) {
        printf("Student with name %s not found!\n", name);
    }
}

void add_score()
{
    int id, i;
    float score;

    printf("Enter student ID: ");
    scanf("%d", &id);
    printf("Enter student score: ");
    scanf("%f", &score);

    for(i = 0; i < student_count; i++) {
        if(students[i].id == id) {
            students[i].score = score;
            printf("Score added successfully!\n");
            return;
        }
    }
    printf("Student with ID %d not found!\n", id);
}

void display_average_score()
{
    float total = 0.0;
    int i;

    for(i = 0; i < student_count; i++) {
        total += students[i].score;
    }

    printf("Average score of all students: %.2f\n", total / student_count);
}

int init_student_info()
{
    if(access(STUDENT_SYSTEM, F_OK) != 0) { // 文件不存在
        return 0;
    }

    FILE *fp = fopen(STUDENT_SYSTEM, "r");
    if (fp == NULL) {
        printf("fopen student_system failed!\n");
        return 1;
    }

#define BUF_SIZE 1024
    char buf[BUF_SIZE];
    int i = 0;
    Student s;
    while(fgets(buf, BUF_SIZE - 1, fp) != NULL) {
        sscanf(buf, "%d %s %f\n", &s.id, s.name, &s.score);
        update_student_info(s, FALSE);
    }

    fclose(fp);
    return 0;

}

int main()
{
    int choice;
    int ret;

    students = stu_sys_init(MAX_STUDENTS);
    if (students == NULL) {
        printf("student system init failed, exit!\n");
        return -1;
    }

    ret = init_student_info();
    if (ret) {
        printf("init_student_info failed!\n");
        return 1;
    }
    display_all_students();

    do {
        printf("\nStudent Score Management System\n");
        printf("1. Add Student\n");
        printf("2. Display All Students\n");
        printf("3. Find Student by ID\n");
        printf("4. Find Student by Name\n");
        printf("5. Add Score\n");
        printf("6. Display Average Score\n");
        printf("7. Exit\n");
        printf("Enter your choice: ");
        scanf("%d", &choice);

        switch(choice) {
            case 1:
                add_student();
                break;
            case 2:
                display_all_students();
                break;
            case 3:
                find_student_by_id();
                break;
            case 4:
                find_student_by_name();
                break;
            case 5:
                add_score();
                break;
            case 6:
                display_average_score();
                break;
            case 7:
                printf("Exiting...\n");
                break;
            default:
                printf("Invalid choice!\n");
        }
    } while(choice != 7);

    return 0;
}

总结

使用指针来动态管理内存的时候，需要注意释放内存，否则容易引起内存泄漏，导致系统内存被消耗完，最后可能出现oom的问题。