Exercise 3: Palindrome Checker

Difficulty: Beginner Concepts: String iteration, chars vs bytes, reversal, borrowing

Problem Statement

Write a function is_palindrome that checks if a string reads the same forwards and backwards.

Ignore case and non-alphanumeric characters. For example:

• "racecar" → true
• "A man, a plan, a canal: Panama" → true
• "hello" → false

Learning Goals

• Iterate over characters with .chars()
• Understand the difference between chars and bytes
• Filter and transform characters
• Compare strings/iterators
• Handle Unicode properly

Examples

#![allow(unused)]
fn main() {
assert_eq!(is_palindrome("racecar"), true);
assert_eq!(is_palindrome("hello"), false);
assert_eq!(is_palindrome("A man, a plan, a canal: Panama"), true);
assert_eq!(is_palindrome(""), true);
}

Starter Code

#![allow(unused)]
fn main() {
pub fn is_palindrome(s: &str) -> bool {
    // TODO: implement this function
    todo!()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_simple_palindrome() {
        assert_eq!(is_palindrome("racecar"), true);
    }

    #[test]
    fn test_not_palindrome() {
        assert_eq!(is_palindrome("hello"), false);
    }

    #[test]
    fn test_with_spaces_and_punctuation() {
        assert_eq!(is_palindrome("A man, a plan, a canal: Panama"), true);
    }

    #[test]
    fn test_empty() {
        assert_eq!(is_palindrome(""), true);
    }

    #[test]
    fn test_single_char() {
        assert_eq!(is_palindrome("a"), true);
    }

    #[test]
    fn test_mixed_case() {
        assert_eq!(is_palindrome("RaceCar"), true);
    }

    #[test]
    fn test_numbers() {
        assert_eq!(is_palindrome("12321"), true);
        assert_eq!(is_palindrome("12345"), false);
    }
}
}

Follow-up Challenges

1. Implement without allocating any String or Vec
2. Make a case-sensitive version
3. Create a function that returns the longest palindromic substring
4. Handle grapheme clusters (combined Unicode characters) correctly using the unicode-segmentation crate

Key Takeaways

• chars() iterates over Unicode scalar values (characters)
• bytes() iterates over raw bytes (not Unicode-aware)
• filter() keeps elements matching a predicate
• flat_map() is useful when a function returns an iterator (like to_lowercase())
• collect() gathers iterator elements into a collection
• rev() reverses an iterator
• Always consider Unicode when working with text

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