Use more idiomatic Rust in file_io2/file_io3

- file_io2: replace the manual line_number counter with
  .lines().enumerate(), and replace
  buffered_file_writer.write(format!(...).as_bytes()) with writeln!,
  which both formats and guarantees a complete write (write_fmt loops
  internally, unlike a single Write::write call).
- file_io3: replace the manual match on file_path.parent() with
  .ok_or_else(...)? for a more idiomatic Option-to-Result conversion.

Verified locally: both solutions compile under edition 2024, clippy is
silent, and running them still produces identical output (3 lines
processed / 117-byte file, non-readonly permissions) with no leftover
files.

exercises/24_file_io/file_io2.rs

@@ -24,23 +24,22 @@ fn main() -> Result<(), std::io::Error> {
 
     let mut buffered_file_writer = BufWriter::new(output_file);
 
-    let mut line_number = 1;
+    let mut lines_processed = 0;
 
-    for line in buffered_input_file.lines() {
+    for (index, line) in buffered_input_file.lines().enumerate() {
         let line = line.inspect_err(|err| {
             eprintln!("{} line parse error {:?}", TEST_INPUT_FILE_NAME, err);
         })?;
 
-        buffered_file_writer
+        let line_number = index + 1;
-            .write(format!("Line {} : {}\n", line_number, line).as_bytes())
+        writeln!(buffered_file_writer, "Line {} : {}", line_number, line).inspect_err(|err| {
-            .inspect_err(|err| {
             eprintln!("{} line write error {:?}", TEST_INPUT_FILE_NAME, err);
         })?;
 
-        line_number += 1;
+        lines_processed = line_number;
     }
 
-    println!("{} : lines processed", line_number - 1);
+    println!("{} : lines processed", lines_processed);
     file_cleanup()
 }
 

exercises/24_file_io/file_io3.rs

@@ -38,10 +38,9 @@ fn main() -> Result<(), std::io::Error> {
 fn create_required_files() -> Result<(), std::io::Error> {
     let file_path = sample_file_path();
 
-    let dir_path = match file_path.parent() {
+    let dir_path = file_path
-        Some(parent) => parent,
+        .parent()
-        None => return Err(Error::other("Could not get parent path")),
+        .ok_or_else(|| Error::other("Could not get parent path"))?;
-    };
 
     if !dir_path.exists() {
         fs::create_dir_all(dir_path).inspect_err(|x| {

solutions/24_file_io/file_io2.rs

@@ -23,23 +23,22 @@ fn main() -> Result<(), std::io::Error> {
 
     let mut buffered_file_writer = BufWriter::new(output_file);
 
-    let mut line_number = 1;
+    let mut lines_processed = 0;
 
-    for line in buffered_input_file.lines() {
+    for (index, line) in buffered_input_file.lines().enumerate() {
         let line = line.inspect_err(|err| {
             eprintln!("{} line parse error {:?}", TEST_INPUT_FILE_NAME, err);
         })?;
 
-        buffered_file_writer
+        let line_number = index + 1;
-            .write(format!("Line {} : {}\n", line_number, line).as_bytes())
+        writeln!(buffered_file_writer, "Line {} : {}", line_number, line).inspect_err(|err| {
-            .inspect_err(|err| {
             eprintln!("{} line write error {:?}", TEST_INPUT_FILE_NAME, err);
         })?;
 
-        line_number += 1;
+        lines_processed = line_number;
     }
 
-    println!("{} : lines processed", line_number - 1);
+    println!("{} : lines processed", lines_processed);
     file_cleanup()
 }
 

solutions/24_file_io/file_io3.rs

@@ -37,10 +37,9 @@ fn main() -> Result<(), std::io::Error> {
 fn create_required_files() -> Result<(), std::io::Error> {
     let file_path = sample_file_path();
 
-    let dir_path = match file_path.parent() {
+    let dir_path = file_path
-        Some(parent) => parent,
+        .parent()
-        None => return Err(Error::other("Could not get parent path")),
+        .ok_or_else(|| Error::other("Could not get parent path"))?;
-    };
 
     if !dir_path.exists() {
         fs::create_dir_all(dir_path).inspect_err(|x| {