Read lines from file, iterate over each line and each character in that line

Question

I need to read a file, get each line, iterate over each line and check if that line contains any character from "aeiuo" and if it contains at least 2 of the characters "äüö".

Is this code idiomatic Rust? How do I check for several characters in a String?

My attempt so far with some Google and code stealing:

use std::error::Error;
use std::fs::File;
use std::io::BufReader;
use std::io::prelude::*;
use std::path::Path;

fn main() {
    // Create a path to the desired file
    let path = Path::new("foo.txt");
    let display = path.display();

    // Open the path in read-only mode, returns `io::Result<File>`
    let file = match File::open(&path) {
        // The `description` method of `io::Error` returns a string that describes the error
        Err(why) => panic!("couldn't open {}: {}", display, Error::to_string(&why)),
        Ok(file) => file,
    };

    // Collect all lines into a vector
    let reader = BufReader::new(file);
    let lines: Vec<_> = reader.lines().collect();

    for l in lines {
        if (l.unwrap().contains("a")) {
            println!("here is a");
        }
    }
}

(Playground link)

Answer 1

This works:

use std::error::Error;
use std::fs::File;
use std::io::BufReader;
use std::io::prelude::*;
use std::path::Path;

fn is_vowel(x: &char) -> bool {
    "aAeEiIoOuU".chars().any(|y| y == *x)
}

fn is_umlaut(x: &char) -> bool {
    "äÄüÜöÖ".chars().any(|y| y == *x)
}

fn valid(line: &str) -> bool {
    line.chars().all(|c| !is_vowel(&c)) && line.chars().filter(is_umlaut).fuse().nth(1).is_some()
}

fn main() {
    // Create a path to the desired file
    let path = Path::new("c.txt");
    let display = path.display();
    // Open the path in read-only mode, returns `io::Result<File>`
    let file = match File::open(&path) {
        Err(why) => panic!("couldn't open {}: {}", display, Error::description(&why)),
        Ok(file) => file,
    };
    let reader = BufReader::new(file);
    for line in reader.lines() {
        match line {
            Ok(line) => {
                if valid(&line) {
                    println!("{}", line)
                }
            }
            Err(e) => println!("ERROR: {}", e),
        }
    }
}

Answer 2

1) "Is this code idiomatic Rust?"

Overall yes, it seems good. There is one minor point that you probably want to improve: you don't need to collect the lines into a vector to iterate on them. This is unwanted because it triggers unneeded memory allocations. Just reading the lines() iterator directly will work. (If you come from C++, you can forget about collecting things into intermediary vectors: think functional, think iterators!)

let reader = BufReader::new(file);
let lines: Vec<_> = reader.lines().collect();

for l in lines {
    ...
}

becomes

let reader = BufReader::new(file);
let lines = reader.lines(); 
// lines is a instance of some type which implements Iterator<Item=&str>

for l in lines {
    ...
}

2) "How do I check for several characters in a string?"

I suggest a simple approach based on .any():

fn is_aeiou(x: &char) -> bool {
    "aeiou".chars().any(|y| y == *x)
}

fn is_weird_auo(x: &char) -> bool {
    "äüö".chars().any(|y| y == *x)
}

fn valid(line: &str) -> bool {
    line.chars().any(|c| is_aeiou(&c)) &&
    line.chars().filter(is_weird_auo).fuse().nth(1).is_some()
}

Then you can go iterators all the way and write your main test as follows:

let reader = BufReader::new(file);
let lines = reader.lines();

let bad_line = lines.map(|l| l.unwrap()).filter(|line| !valid(line)).next();
match bad_line {
    Some(line_n) => println!("Line {} doesn't pass the test", line_n),
    None => println!("All lines are good!"),
}

// Alternate way if you don't need the line number. More readable
//let all_good = lines.map(|l| l.unwrap()).all(valid);

(Full code on the playground.)