COMP1521 Day 1 - Final Review

📝 Problem Statement 📝 题目描述

Task: Write a C program that takes two arguments (filename and line number n), and deletes the n-th line from the file. Lines are numbered starting at 1. 任务：编写一个C程序，接收两个参数（文件名和行号n），从文件中删除第n行。行号从1开始计数。

# Example Usage# 使用示例
printf 'Hello\nhow\nare\nyou\n' > hello.txt
cat hello.txt
Hello
how
are
you

./22t3final_q6 hello.txt 3
cat hello.txt
Hello
how
you
            

⚠️ Key Requirements: ⚠️ 关键要求：

Use a temporary file (cannot read and write to the same file simultaneously) 使用临时文件（不能同时读写同一文件）
No output to stdout or stderr 不输出到 stdout 或 stderr
If file has fewer than n lines, do nothing 如果文件行数少于n，则不做任何操作
No error checking required 不需要错误检查

✅ Complete Solution ✅ 完整答案

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

int main(int argc, char *argv[]) {
    char *filename = argv[1];
    int n = atoi(argv[2]);

    FILE *input = fopen(filename, "r");
    FILE *output = fopen("temp_file.txt", "w");

    char line[10000];
    int line_number = 0;

    while (fgets(line, sizeof(line), input) != NULL) {
        line_number++;
        if (line_number != n) {
            fputs(line, output);
        }
    }

    fclose(input);
    fclose(output);

    if (line_number < n) {
        remove("temp_file.txt");
        return 0;
    }

    remove(filename);
    rename("temp_file.txt", filename);
    return 0;
}
            

🔄 Process Flow Diagram 🔄 处理流程图

1️⃣ Get filename and line number from command-line arguments 1️⃣ 从命令行参数获取文件名和行号

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2️⃣ Open original file for reading (fopen with "r" mode) 2️⃣ 打开原文件进行读取（fopen 使用 "r" 模式）

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3️⃣ Create temporary file (fopen with "w" mode) 3️⃣ 创建临时文件（fopen 使用 "w" 模式）

↓

4️⃣ Read lines one by one, skip line n, write others to temp file 4️⃣ 逐行读取，跳过第n行，其他行写入临时文件

↓

5️⃣ Close both files 5️⃣ 关闭两个文件

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6️⃣ Check if file had enough lines 6️⃣ 检查文件是否有足够的行数

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7️⃣ Replace original file with temp file (remove + rename) 7️⃣ 用临时文件替换原文件（remove + rename）

🔧 Essential Functions Reference 🔧 核心函数参考

Function函数	Purpose用途	Example示例
`fopen()`	Open file, returns FILE* 打开文件，返回 FILE*	`fopen("file.txt", "r")`
`fclose()`	Close file 关闭文件	`fclose(file)`
`fgets()`	Read one line (includes \n) 读取一行（包含 \n）	`fgets(buf, size, file)`
`fputs()`	Write string to file 写入字符串到文件	`fputs(str, file)`
`remove()`	Delete file 删除文件	`remove("file.txt")`
`rename()`	Rename/move file 重命名/移动文件	`rename("old", "new")`
`atoi()`	String to integer 字符串转整数	`atoi("123")`

📂 File Opening Modes 📂 文件打开模式

Mode模式	Meaning含义	File Doesn't Exist文件不存在	File Exists文件存在
`"r"`	Read 读取	Error 错误	Open file 打开文件
`"w"`	Write 写入	Create new 创建新文件	Clear content 清空内容
`"a"`	Append 追加	Create new 创建新文件	Append to end 追加到末尾
`"r+"`	Read+Write 读写	Error 错误	Open file 打开文件
`"w+"`	Write+Read 写读	Create new 创建新文件	Clear content 清空内容

⚠️ Common Mistakes to Avoid ⚠️ 常见错误

❌ Mistake 1: Not Using Temporary File ❌ 错误1：不使用临时文件

Problem: Trying to read and write to the same file simultaneously causes data corruption. 问题：尝试同时读写同一文件会导致数据损坏。

Solution: Always use a temporary file: read from original → write to temp → replace original. 解决方案：始终使用临时文件：从原文件读取 → 写入临时文件 → 替换原文件。

❌ Mistake 2: Wrong File Replacement Order ❌ 错误2：文件替换顺序错误

Problem: Calling rename() before remove() causes errors. 问题：在 remove() 之前调用 rename() 会导致错误。

Correct Order: 1) remove(filename) → 2) rename(temp_filename, filename) 正确顺序：1) remove(filename) → 2) rename(temp_filename, filename)

❌ Mistake 3: Forgetting to Check Line Count ❌ 错误3：忘记检查行数

Problem: If file has fewer than n lines, should do nothing (not replace file). 问题：如果文件行数少于n，应该什么都不做（不替换文件）。

Solution: Check if (line_number < n) and remove temp file if true. 解决方案：检查 if (line_number < n)，如果为真则删除临时文件。

❌ Mistake 4: Line Numbering Confusion ❌ 错误4：行号计数混淆

Problem: Lines start at 1 (not 0), but counter starts at 0. 问题：行号从1开始（不是0），但计数器从0开始。

Solution: Initialize line_number = 0, increment at start of loop: line_number++ 解决方案：初始化 line_number = 0，在循环开始时递增：line_number++

💡 Key Concepts Summary 💡 核心概念总结

🔑 Why Use Temporary Files? 🔑 为什么使用临时文件？

You cannot safely read from and write to the same file simultaneously. The solution is to read from the original file, write modified content to a temporary file, then replace the original with the temporary file. 不能安全地同时读写同一文件。解决方案是从原文件读取，将修改后的内容写入临时文件，然后用临时文件替换原文件。

🔑 File Modification Pattern 🔑 文件修改模式

// Standard pattern for modifying files// 修改文件的标准模式
fopen(filename, "r")          // Open original for reading// 打开原文件读取
fopen("temp.txt", "w")        // Create temp file for writing// 创建临时文件写入
while (fgets()) { ... }       // Process line by line// 逐行处理
fclose()                      // Close both files// 关闭两个文件
remove() + rename()           // Replace original// 替换原文件
                