Read file character-by-character in Rust

Question

Is there an idiomatic way to process a file one character at a time in Rust?

This seems to be roughly what I'm after:

let mut f = io::BufReader::new(try!(fs::File::open("input.txt")));

for c in f.chars() {
    println!("Character: {}", c.unwrap());
}

But Read::chars is still unstable as of Rust v1.6.0.

I considered using Read::read_to_string, but the file may be large and I don't want to read it all into memory.

Answer 1

Let's compare 4 approaches.

1. Read::chars

You could copy Read::chars implementation, but it is marked unstable with

the semantics of a partial read/write of where errors happen is currently unclear and may change

so some care must be taken. Anyway, this seems to be the best approach.

2. flat_map

The flat_map alternative does not compile:

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

pub fn main() {
    let mut f = BufReader::new(File::open("input.txt").expect("open failed"));

    for c in f.lines().flat_map(|l| l.expect("lines failed").chars()) {
        println!("Character: {}", c);
    }
}

The problems is that chars borrows from the string, but l.expect("lines failed") lives only inside the closure, so compiler gives the error borrowed value does not live long enough.

3. Nested for

This code

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

pub fn main() {
    let mut f = BufReader::new(File::open("input.txt").expect("open failed"));

    for line in f.lines() {
        for c in line.expect("lines failed").chars() {
            println!("Character: {}", c);
        }
    }
}

works, but it keeps allocation a string for each line. Besides, if there is no line break on the input file, the whole file would be load to the memory.

4. BufRead::read_until

A memory efficient alternative to approach 3 is to use Read::read_until, and use a single string to read each line:

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

pub fn main() {
    let mut f = BufReader::new(File::open("input.txt").expect("open failed"));

    let mut buf = Vec::<u8>::new();
    while f.read_until(b'\n', &mut buf).expect("read_until failed") != 0 {
        // this moves the ownership of the read data to s
        // there is no allocation
        let s = String::from_utf8(buf).expect("from_utf8 failed");
        for c in s.chars() {
            println!("Character: {}", c);
        }
        // this returns the ownership of the read data to buf
        // there is no allocation
        buf = s.into_bytes();
        buf.clear();
    }
}

Answer 2

There are two solutions that make sense here.

First, you could copy the implementation of Read::chars() and use it; that would make it completely trivial to move your code over to the standard library implementation if/when it stabilizes.

On the other hand, you could simply iterate line by line (using f.lines()) and then use line.chars() on each line to get the chars. This is a little more hacky, but it will definitely work.

If you only wanted one loop, you could use flat_map() with a lambda like |line| line.chars().