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.

关键点：

  • 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
    • “_”是通配符

  • 有些想法比模式本身所允许的程度更加复杂，如果我们希望简要地表达这些想法，就必须把匹配守卫视为独立的语法功能。

  • 它们与匹配分支中的单独“if”表达式不同。选择匹配分支后，分支块内（在“=>”之后）会出现“if”表达式。如果该分支块内的“if”条件失败，系统不会考虑原始“match”表达式的其他分支。

  • 只要表达式在包含“|”的模式中，就会适用守卫定义的条件。

  • 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.