Welcome to this lesson, where we'll explore an essential technique in text data manipulation: reading files line-by-line using Rust! In many real-world scenarios, especially those involving large datasets, reading a file line-by-line is a must for efficiently handling logs, data streams, and more. By processing data in smaller chunks, we can save both memory and time, making our applications more robust and effective.

In this lesson, we'll dive into the foundations of opening files in Rust, leveraging buffers for efficient reading, and processing each line in a straightforward and idiomatic way. By the end, you'll be able to confidently open a file, iterate through its lines, and apply any processing logic you desire.

Before we read a file line-by-line, let's briefly revisit how to open files in Rust. We use the standard library's filesystem utilities, particularly:
• std::fs::File – To handle file creation and opening.
• The ? operator – To simplify error handling by propagating errors up the call stack.

Below is a simple example showcasing how to open a file for reading:

Here, File::open returns a Result type. By using ?, we either get a valid file handle or propagate the error without writing extra boilerplate code.

In Rust, one of the easiest ways to read a file line-by-line is through the BufRead::lines() method. When you call lines() on a buffered reader, it provides an iterator over the lines in that reader. Each iteration yields a Result<String, std::io::Error>. This means we need to handle the possibility of errors when reading each line.

When you get a line from the iterator, you typically use something like:

Because each item is a Result, you can gracefully handle problems that come up mid-read, or simply use the ? operator to propagate them.

Let's put the above knowledge to work and read a file line-by-line. We'll also remove surrounding whitespace in the process:

Explanation:
• We use Path to handle file paths.
• BufReader wraps the File to read data more efficiently.
• The lines() iterator yields each line in sequence, which we then trim and print.

Let's take it a step further by reading numbers from a file and adding them up. Suppose our file, numbers.txt, looks like this:

We can parse each line into an integer and accumulate a running total:

Key points:
• line.trim().parse::<i32>() attempts to convert the trimmed string into an i32.
• If the line fails to parse, we print a warning and skip that line.
• total_sum accumulates the result for each valid number.

In this lesson, you learned how to open files, buffer reads for efficient line-by-line processing, and parse strings into integers when necessary — all using Rust's standard library. Understanding how to handle data incrementally is an important skill for working with large input files or streaming data. Now, you're prepared to apply these concepts in practice exercises, reinforcing your new skills and continuing to build your confidence as a Rust developer.

Keep experimenting, have fun, and explore different ways to parse and manipulate your data. You're on the path to mastering file I/O and text processing in Rust — great job so far!