Read file character-by-character in Rust - io

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

I cannot use lines() because my file could be a single line that is gigabytes in size. This an improvement on #malbarbo's recommendation of copying Read::chars from the an old version of Rust. The utf8-chars crate already adds .chars() to BufRead for you.
Inspecting their repository, it doesn't look like they load more than 4 bytes at a time.
Your code will look the same as it did before Rust removed Read::chars:
use std::io::stdin;
use utf8_chars::BufReadCharsExt;
fn main() {
for c in stdin().lock().chars().map(|x| x.unwrap()) {
println!("{}", c);
}
}
Add the following to your Cargo.toml:
[dependencies]
utf8-chars = "1.0.0"

Answer 3

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().