BufReader
- Rust course
- Intermediate
- Lesson 92/115
- 4 min
Read line by line efficiently with BufReader.
BufReader
To read large files line by line instead of loading them fully into memory, BufReader and BufRead::lines are used. Each line is a separate io::Result<String>:
use std::fs::File;
use std::io::{BufRead, BufReader};
let f = File::open("sayilar.txt")?;
for satir in BufReader::new(f).lines() {
let satir = satir?;
println!("{satir}");
}lines() strips line endings (\n, \r\n) automatically. The BufRead trait must be in scope.
Separating bad lines
In real files some lines may be broken. Handling a read error with ? and a content error with match separately is a good habit:
let mut toplam = 0;
let mut hatali = 0;
for satir in BufReader::new(f).lines() {
match satir?.trim().parse::<i32>() {
Ok(n) => toplam += n,
Err(_) => hatali += 1,
}
}Practice Sum the lines
Write a file containing "3\n5\n x \n7\n" in the temp folder. Read it line by line with BufReader; sum the parseable lines, count the others and print toplam 15, hatalı 1.
Starter code
use std::env;
use std::fs::{self, File};
use std::io::{self, BufRead, BufReader};
fn main() {
// buraya yaz
}Sample solution
use std::env;
use std::fs::{self, File};
use std::io::{self, BufRead, BufReader};
fn main() -> io::Result<()> {
let yol = env::temp_dir().join("veri.txt");
fs::write(&yol, "3\n5\n x \n7\n")?;
let f = File::open(&yol)?;
let mut toplam = 0;
let mut hatali = 0;
for s in BufReader::new(f).lines() {
match s?.trim().parse::<i32>() {
Ok(n) => toplam += n,
Err(_) => hatali += 1,
}
}
println!("toplam {}, hatalı {}", toplam, hatali);
Ok(())
}Expected output
toplam 15, hatalı 1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 When is <code>BufReader::lines</code> preferred over <code>read_to_string</code>?
- When the file is very large or lines are processed as a stream Correct answer
- When the file is small
- For binary files
- Never
Explanation: lines keeps one line in memory at a time; even gigabyte logs can be processed with constant memory.
2 Line by line
for satir in BufReader::new(f).____() {
let satir = satir?;
}for satir in BufReader::new(f).lines() {
let satir = satir?;
}Explanation: BufRead::lines yields each line as io::Result<String>.
3 Type and role
| Term | Meaning |
|---|---|
BufReader<File> |
Buffered reading |
BufRead |
The lines() trait |
s? |
Read error |
match on parse |
Content error |