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Pattern Matching

Level: intro (score: 1)

🚀 These intro exercises are prep for our Rust Intro Cohort Program.

🎯 Rust’s match goes far beyond switch-style branching. This exercise drills the essentials you’ll use in the parser: destructuring, multiple patterns, ranges, guards, and the catch‑all _.

✅ Your task

Implement these three functions in src/main.rs:

classify_char(c: char) -> &'static str
- Return "digit" for '0'..='9'
- "alpha" for 'a'..='z' | 'A'..='Z' | '_'
- "op" for one of + - * / =
- "ws" for space, tab, or newline
- "other" otherwise
sum_positive_pairs(input: &[(i32, i32)]) -> i32
- Sum x + y only for pairs where both are > 0 using tuple destructuring with a guard. Skip others.
Define the enum:

#[derive(Debug, PartialEq, Eq)]
pub enum Message {
    Ping,
    Echo(String),
    Move { x: i32, y: i32 },
    Quit,
}

Implement route(msg: Message) -> String using pattern matching:

Ping → "pong"
Echo(s) → return s if non-empty, else "silence" (use a guard)
Move { x, y } →
- if x == 0 && y == 0 → "noop"
- else format "move:{x},{y}"
Quit → "bye"
use _ only if you must (exhaustive matches are preferred)

💡 Hints

Ranges: '0'..='9', 'a'..='z'
Multiple patterns: ' ' | ' ' | ' '
Guards: SomeBinding if condition => ...
Destructure tuples: for &(x, y) in input { ... }

Example tests:

assert_eq!(classify_char('8'), "digit");
assert_eq!(classify_char('_'), "alpha");
assert_eq!(classify_char('+'), "op");
assert_eq!(classify_char('\n'), "ws");
assert_eq!(classify_char('?'), "other");

assert_eq!(sum_positive_pairs(&[(1,2), (-1,3), (4,0)]), 3);

use Message::*;
assert_eq!(route(Ping), "pong");
assert_eq!(route(Echo("hi".into())), "hi");
assert_eq!(route(Echo("".into())), "silence");
assert_eq!(route(Move { x: 0, y: 0 }), "noop");
assert_eq!(route(Move { x: 2, y: -1 }), "move:2,-1");
assert_eq!(route(Quit), "bye");

enums match

Code

#[derive(Debug, PartialEq, Eq)]
pub enum Message {
    Ping,
    Echo(String),
    Move { x: i32, y: i32 },
    Quit,
}

pub fn classify_char(c: char) -> &'static str {
    // TODO: use ranges and multiple patterns to classify
    "other"
}

pub fn sum_positive_pairs(input: &[(i32, i32)]) -> i32 {
    // TODO: destructure tuple and use a guard to sum only > 0 pairs
    0
}

pub fn route(msg: Message) -> String {
    // TODO: match on Message, using guards for Echo non-empty and Move (0,0)
    String::new()
}

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

#[test]
    fn test_classify_char() {
        assert_eq!(classify_char('8'), "digit");
        assert_eq!(classify_char('_'), "alpha");
        assert_eq!(classify_char('+'), "op");
        assert_eq!(classify_char('\n'), "ws");
        assert_eq!(classify_char('?'), "other");
    }

#[test]
    fn test_sum_positive_pairs() {
        assert_eq!(sum_positive_pairs(&[]), 0);
        assert_eq!(sum_positive_pairs(&[(1,2), (-1,3), (4,0)]), 3);
        assert_eq!(sum_positive_pairs(&[(2,2), (3,3)]), 10);
    }

#[test]
    fn test_route() {
        assert_eq!(route(Ping), "pong");
        assert_eq!(route(Echo("hi".into())), "hi");
        assert_eq!(route(Echo("".into())), "silence");
        assert_eq!(route(Move { x: 0, y: 0 }), "noop");
        assert_eq!(route(Move { x: 2, y: -1 }), "move:2,-1");
        assert_eq!(route(Quit), "bye");
    }
}