第9章 文件保存和读取

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

struct Student {          // 学生结构体：学号、姓名、分数
    int id;
    char name[32];
    int score;
};

// 把 students 数组的前 count 条写入文件
int save_students(const char *filename, struct Student students[], int count)
{
    FILE *fp = fopen(filename, "w");   // "w" 表示写模式，文件不存在会创建
    if (fp == NULL) {
        return -1;                     // 打开失败
    }

    for (int i = 0; i < count; i++) {
        fprintf(fp, "%d %s %d\n",      // 格式和 printf 一样，只是输出到文件
                students[i].id,
                students[i].name,
                students[i].score);
    }

    fclose(fp);                        // 写完必须关
    return 0;
}

// 从文件读取学生记录，填入 students 数组，返回读到的条数
int load_students(const char *filename, struct Student students[], int max_count)
{
    FILE *fp = fopen(filename, "r");   // "r" 表示读模式
    if (fp == NULL) {
        return 0;                      // 文件不存在，返回 0 条
    }

    int count = 0;
    while (count < max_count &&
           fscanf(fp, "%d %31s %d",    // %31s 防止姓名写越界
                  &students[count].id,
                  students[count].name,
                  &students[count].score) == 3) {  // 成功读到 3 个字段才算一条
        count++;
    }

    fclose(fp);
    return count;
}

int main(void)
{
    struct Student students[100];   // 学生数组，最多100条记录
    int count = 0;                // 当前实际记录数

    // 先尝试从文件加载
    count = load_students("students.txt", students, 100);
    if (count > 0) {
        printf("Loaded %d records from file:\n", count);
    } else {
        printf("File not found, starting fresh.\n");
    }

    // 显示已有记录
    for (int i = 0; i < count; i++) {
        printf("  %d %s %d\n", students[i].id, students[i].name, students[i].score);
    }

    // 手动添加两条记录。写入数组前，先确认还有空位。
    if (count < 100) {
        students[count].id = 1001;
        snprintf(students[count].name, sizeof(students[count].name), "%s", "Alice");
        students[count].score = 86;
        count++;
    } else {
        printf("Table full, skipping Alice.\n");
    }

    if (count < 100) {
        students[count].id = 1002;
        snprintf(students[count].name, sizeof(students[count].name), "%s", "Bob");
        students[count].score = 91;
        count++;
    } else {
        printf("Table full, skipping Bob.\n");
    }

    // 保存到文件
    if (save_students("students.txt", students, count) == 0) {
        printf("Saved %d records to students.txt\n", count);
    } else {
        printf("Save failed!\n");
    }

    return 0;
}

FILE *fp = fopen("students.txt", "w");   // 以写模式打开文件，返回文件指针fp

fprintf(fp, "%d %s %d\n", students[i].id, students[i].name, students[i].score);   // 将格式化数据写入文件

fscanf(fp, "%d %31s %d", &students[count].id, students[count].name, &students[count].score)   // 从文件按格式读取数据

fclose(fp);   // 关闭文件，将缓冲区数据刷入磁盘

FILE *fp = fopen("data.txt", "r");
fscanf(fp, "%d", &x);    // 如果文件不存在，fp 是 NULL，这里会崩溃

FILE *fp = fopen("data.txt", "r");
if (fp == NULL) {
    printf("File not found\n");
    return -1;
}

FILE *fp = fopen("out.txt", "w");
fprintf(fp, "hello\n");
// 忘了 fclose，数据可能还在缓冲区，没写到磁盘

fscanf(fp, "%d %s %d", &id, name, &score);  // 不检查返回值

fscanf(fp, "%s", name);         // 如果文件里有一个很长的字符串，会越界写
fscanf(fp, "%31s", name);       // 对：限制最多 31 个字符

fopen("C:\\data\\students.txt", "r");   // Windows 路径要用双反斜杠
fopen("students.txt", "r");             // 或者只写文件名，放在当前目录

while (1) {
    printf("Enter ID (0 to stop): ");
    scanf("%d", &id);
    if (id == 0) break;
    // 读入姓名和分数，加入数组
}
save_students("students.txt", students, count);