Match expressions and patterns

A school can divide valid marks into several bands. Match compares a value with patterns and chooses the first matching arm. A pattern describes a shape or range to recognize, rather than an arbitrary expression that always returns true or false.

Inclusive ranges use ..=. An underscore pattern matches remaining values. Match must cover all possible inputs, and its arms must return compatible types when used as a value. Match guards add a condition after a pattern when recognition needs more than shape alone.

Save the complete source as main.rs in the Rust environment. No standard input is needed.

Rust 2024 · complete program

fn main() {
  let score = 82;
  let band = match score {
    90..=100 => "Excellent",
    75..=89 => "Target met",
    0..=74 => "More practice",
    _ => "Invalid mark",
  };
  println!("{band}");
}

Expected output

Target met

Follow the execution

Eighty-two does not match the first range but matches the second. That arm supplies Target met. The final underscore handles negative values and values above 100, so the match covers the full integer type.

Try the complete program in the Rust compiler. For tests, Cargo commands and local files, use the specified local environment.

Your task: What happens if score becomes 120?

Rust 2024 · complete program

fn main() {
  let score = 82;
  let band = match score {
    90..=100 => "Excellent",
    75..=89 => "Target met",
    0..=74 => "More practice",
    _ => "Invalid mark",
  };
  println!("{band}");
}

Worked answer

The remaining-value arm produces Invalid mark. It is outside every accepted range, so it should not receive a valid grading label.

Order matters when patterns overlap. Exhaustive matching checks structural coverage, but a catch-all arm can hide newly added enum variants from review. Pattern matching is not restricted to numbers; enums and destructuring make it especially useful.

The remaining-value arm produces Invalid mark. It is outside every accepted range, so it should not receive a valid grading label. Order matters when patterns overlap.

Further reading: the official Rust Book on match expressions and patterns.