Pattern Matching

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Advanced Pattern Matching: The match Expression (10 min)

The match expression is one of Rust's most powerful control flow constructs. It allows you to compare a value against a series of patterns and then execute code based on which pattern matches. It's often a more robust and readable alternative to long if-else if chains.

• Exhaustiveness: A key feature of match is that it must be exhaustive. This means you have to cover every possible value that the data could take. If you don't, Rust's compiler will give you an error, which helps prevent bugs!

Let's look at a simple example with numbers:

fn main() {
    let number = 3;

    match number { // Match the 'number' against these patterns
        1 => println!("One!"), // If number is 1, do this
        2 => println!("Two!"), // If number is 2, do this
        3 | 4 => println!("Three or Four!"), // If number is 3 OR 4, do this (multiple patterns)
        5..=10 => println!("Between 5 and 10, inclusive!"), // If number is in this range
        _ => println!("Something else!"), // The underscore '_' is a catch-all pattern (like 'default' in switch)
    }

    let result = match number { // 'match' can also be an expression, returning a value
        1 => "It's one",
        _ => "It's not one", // All branches must return the same type!
    };
    println!("Result: {}", result);
}

• When to use match vs. if:
  • Use if for simple true/false conditions or a few distinct branches.
  • Use match when you have many possible values or complex patterns to handle, especially when working with enums (which we'll cover in Lesson 3) or Result types (which you saw in Lesson 1's I/O).

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