Vowel Counter

🎯 In Python, you can iterate directly over characters in a string: for c in "hello". You can also use a generator expression like sum(1 for c in s if c.isdigit()).

In Rust, strings are UTF-8 encoded bytes, not arrays of characters. You can't index into them with s[0] like in Python — that would be a byte offset, not necessarily a character. Instead, you use .chars() to get an iterator over Unicode characters:

for c in "hello".chars() {
    println!("{c}");
}

This iterator pattern is central to Rust. Instead of building up a count variable in a loop, Rust encourages chaining iterator methods — similar to how you'd chain operations in a Python generator expression:

let digits = "abc123".chars()
    .filter(|c| c.is_ascii_digit())
    .count();
// 3

Notice the closure syntax: |c| defines an inline function (like Python's lambda c: ...). The .filter() method keeps only elements matching the condition, and .count() consumes the iterator and returns how many items passed through.

Your Task

Implement count_vowels(s: &str) -> usize that returns the number of vowels (a, e, i, o, u — both lowercase and uppercase) in the given string.

The function takes a &str (a borrowed string slice) — meaning it reads the string without taking ownership. The caller keeps their string.

Example

assert_eq!(count_vowels("hello"), 2);
assert_eq!(count_vowels("RUST"), 1);
assert_eq!(count_vowels("bcdfgh"), 0);

Your Task

Example

Further Reading