Matching Values

The match keyword lets you match a value against one or more patterns. The patterns can be simple values, similarly to switch in C and C++, but they can also be used to express more complex conditions:

#[rustfmt::skip]
fn main() {
    let input = 'x';
    match input {
        'q'                       => println!("Quitting"),
        'a' | 's' | 'w' | 'd'     => println!("Moving around"),
        '0'..='9'                 => println!("Number input"),
        key if key.is_lowercase() => println!("Lowercase: {key}"),
        _                         => println!("Something else"),
    }
}

A variable in the pattern (key in this example) will create a binding that can be used within the match arm. We will learn more about this on the next slide.

A match guard causes the arm to match only if the condition is true. If the condition is false the match will continue checking later cases.

This slide should take about 10 minutes.

Key Points:

  • You might point out how some specific characters are being used when in a pattern

    • | as an or
    • .. can expand as much as it needs to be
    • 1..=5 represents an inclusive range
    • _ is a wild card

  • Match guards as a separate syntax feature are important and necessary when we wish to concisely express more complex ideas than patterns alone would allow.

  • Match guards are different from if expressions after the =>. An if expression is evaluated after the match arm is selected. Failing the if condition inside of that block won’t result in other arms of the original match expression being considered. In the following example, the wildcard pattern _ => is never even attempted.

#[rustfmt::skip]
fn main() {
    let input = 'a';
    match input {
        key if key.is_uppercase() => println!("Uppercase"),
        key => if input == 'q' { println!("Quitting") },
        _   => println!("Bug: this is never printed"),
    }
}

  • The condition defined in the guard applies to every expression in a pattern with an |.

  • Note that you can’t use an existing variable as the condition in a match arm, as it will instead be interpreted as a variable name pattern, which creates a new variable that will shadow the existing one. For example:

    #![allow(unused)]
fn main() {
let expected = 5;
match 123 {
    expected => println!("Expected value is 5, actual is {expected}"),
    _ => println!("Value was something else"),
}
}

    Here we’re trying to match on the number 123, where we want the first case to check if the value is 5. The naive expectation is that the first case won’t match because the value isn’t 5, but instead this is interpreted as a variable pattern which always matches, meaning the first branch will always be taken. If a constant is used instead this will then work as expected.

More To Explore

  • Another piece of pattern syntax you can show students is the @ syntax which binds a part of a pattern to a variable. For example:

    #![allow(unused)]
fn main() {
let opt = Some(123);
match opt {
    outer @ Some(inner) => {
        println!("outer: {outer:?}, inner: {inner}");
    }
    None => {}
}
}

    In this example inner has the value 123 which it pulled from the Option via destructuring, outer captures the entire Some(inner) expression, so it contains the full Option::Some(123). This is rarely used but can be useful in more complex patterns.