Control Flow

Match

Selecting by Value

match compares a value against patterns and runs the first arm that fits. _ is the catch-all.

Program

Play the program to map a numeric code to a label.

match_value.rs
Replay: real traced execution (multi-file project)
fn main() {
    let code = 2;
    let label = match code {
        1 => "one",
        2 => "two",
        _ => "many",
    };
    println!("{label}");
}

  1. code ← 2, label ← "two"

    1fn main() {2    let cod→ 2e = 2;3    let labe→ "two"l = match cod2e {4        1 => "one",5        2 => "two",6        _ => "many",7    };8    println!("{label}");9}
    outputtwo

Follow the Match

  1. code starts at 2.
  2. match compares code against its arms.
  3. The selected arm is 2 => "two".
  4. label becomes two.
  5. The program prints two. | code | selected arm | label | | --- | --- | --- | | 2 | 2 => "two" | two |
match `match` picks one arm based on the value.
arm Each `pattern => value` is one arm.
wildcard `_` matches anything not already handled.

Exercise: match_value.rs

Reproduce the output two, then identify the match arm that creates the label.