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

Answer 1

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

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.)