read_lines

A naive approach

This might be a reasonable first attempt for a beginner’s first implementation for reading lines from a file.

#![allow(unused)]
fn main() {
use std::fs::read_to_string;

fn read_lines(filename: &str) -> Vec<String> {
    let mut result = Vec::new();

    for line in read_to_string(filename).unwrap().lines() {
        result.push(line.to_string())
    }

    result
}
}

Since the method lines() returns an iterator over the lines in the file, we can also perform a map inline and collect the results, yielding a more concise and fluent expression.

#![allow(unused)]
fn main() {
use std::fs::read_to_string;

fn read_lines(filename: &str) -> Vec<String> {
    read_to_string(filename)
        .unwrap()  // panic on possible file-reading errors
        .lines()  // split the string into an iterator of string slices
        .map(String::from)  // make each slice into a string
        .collect()  // gather them together into a vector
}
}

Note that in both examples above, we must convert the &str reference returned from lines() to the owned type String, using .to_string() and String::from respectively.

A more efficient approach

Here we pass ownership of the open File to a BufReader struct. BufReader uses an internal buffer to reduce intermediate allocations.

We also update read_lines to return an iterator instead of allocating new String objects in memory for each line.

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

fn main() {
    // File hosts.txt must exist in the current path
    if let Ok(lines) = read_lines("./hosts.txt") {
        // Consumes the iterator, returns an (Optional) String
        for line in lines.map_while(Result::ok) {
            println!("{}", line);
        }
    }
}

// The output is wrapped in a Result to allow matching on errors.
// Returns an Iterator to the Reader of the lines of the file.
fn read_lines<P>(filename: P) -> io::Result<io::Lines<io::BufReader<File>>>
where P: AsRef<Path>, {
    let file = File::open(filename)?;
    Ok(io::BufReader::new(file).lines())
}

Running this program simply prints the lines individually.

$ echo -e "127.0.0.1\n192.168.0.1\n" > hosts.txt
$ rustc read_lines.rs && ./read_lines
127.0.0.1
192.168.0.1

(Note that since File::open expects a generic AsRef<Path> as argument, we define our generic read_lines() method with the same generic constraint, using the where keyword.)

This process is more efficient than creating a String in memory with all of the file’s contents. This can especially cause performance issues when working with larger files.