How to read file line by line

Use std::fs::File with BufReader and the lines() iterator to read a file line by line efficiently.

use std::fs::File;
use std::io::{self, BufRead, BufReader};

fn main() -> io::Result<()> {
    let file = File::open("input.txt")?;
    let reader = BufReader::new(file);

    for line in reader.lines() {
        let line = line?;
        println!("{line}");
    }
    Ok(())
}

Reading a file line by line opens the file and processes it one line at a time instead of loading the whole thing into memory. It is like reading a book page by page rather than trying to hold the entire book in your hands at once. You use this when processing large text files to save memory and improve speed.