Rust Programming Language Book Club – 18. Patterns and Matching

Literals

Usage: respond to specific values

    let x = 1;

    match x {
        1 => println!("one"),
        2 => println!("two"),
        3 => println!("three"),
        _ => println!("anything"),
    }

Named variables

Usage: match any value

    let x = Some(5);
    let y = 10;

    match x {
        Some(50) => println!("Got 50"),
        Some(y) => println!("Matched, y = {y}"),
        _ => println!("Default case, x = {x:?}"),
    }

    println!("at the end: x = {x:?}, y = {y}");

Multiple patterns

Usage: match against multiple patterns

    let x = 1;

    match x {
        1 | 2 => println!("one or two"),
        3 => println!("three"),
        _ => println!("anything"),
    }

Multiple patterns in a range

Usage: match against a range of values

    let x = 5;

    match x {
        1..=5 => println!("one through five"),
        _ => println!("something else"),
    }

    let x = 'c';

    match x {
        'a'..='j' => println!("early ASCII letter"),
        'k'..='z' => println!("late ASCII letter"),
        _ => println!("something else"),
    }

Destructuring - structs

Usage: match against different parts

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let p = Point { x: 0, y: 7 };

    let Point { x: a, y: b } = p;
    assert_eq!(0, a);
    assert_eq!(7, b);
}

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let p = Point { x: 0, y: 7 };

    let Point { x, y } = p;
    assert_eq!(0, x);
    assert_eq!(7, y);
}

Destructuring - enums

Usage: match against different parts

enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(i32, i32, i32),
}

fn main() {
    let msg = Message::ChangeColor(0, 160, 255);

    match msg {
        Message::Quit => {
            println!("The Quit variant has no data to destructure.");
        }
        Message::Move { x, y } => {
            println!("Move in the x direction {x} and in the y direction {y}");
        }
        Message::Write(text) => {
            println!("Text message: {text}");
        }
        Message::ChangeColor(r, g, b) => {
            println!("Change the color to red {r}, green {g}, and blue {b}")
        }
    }
}

Destructuring - nested enums

Usage: match against different parts

enum Color {
    Rgb(i32, i32, i32),
    Hsv(i32, i32, i32),
}

enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(Color),
}

fn main() {
    let msg = Message::ChangeColor(Color::Hsv(0, 160, 255));

    match msg {
        Message::ChangeColor(Color::Rgb(r, g, b)) => {
            println!("Change color to red {r}, green {g}, and blue {b}");
        }
        Message::ChangeColor(Color::Hsv(h, s, v)) => {
            println!("Change color to hue {h}, saturation {s}, value {v}")
        }
        _ => (),
    }
}

Destructuring - structs & tuples

Usage: match against different parts

let ((feet, inches), Point { x, y }) = ((3, 10), Point { x: 3, y: -10 });

Ignoring values in a pattern

Ignore a value

Usage: ignore an entire value

fn foo(_: i32, y: i32) {
    println!("This code only uses the y parameter: {y}");
}

fn main() {
    foo(3, 4);
}

Ignore part of a value

Usage: ignore only part of a value

fn foo(_: i32, y: i32) {
    println!("This code only uses the y parameter: {y}");
}

fn main() {
    foo(3, 4);
}

Ignore a nested value

Usage: ignore a nested value

    let mut setting_value = Some(5);
    let new_setting_value = Some(10);

    match (setting_value, new_setting_value) {
        (Some(_), Some(_)) => {
            println!("Can't overwrite an existing customized value");
        }
        _ => {
            setting_value = new_setting_value;
        }
    }

    println!("setting is {setting_value:?}");

Ignore multiple places

Usage: ignore multiple places in a single pattern

let numbers = (2, 4, 8, 16, 32);

match numbers {
      (first, _, third, _, fifth) => {
          println!("Some numbers: {first}, {third}, {fifth}")
      }
}

Ignore unused variable

Usage: ignore an unused variable

fn main() {
    let _x = 5;
    let y = 10;
}

Ignore remaining parts

Usage: use first part(s) of a value, ignore the rest

struct Point {
    x: i32,
    y: i32,
    z: i32,
}

let origin = Point { x: 0, y: 0, z: 0 };

match origin {
    Point { x, .. } => println!("x is {x}"),
}

fn main() {
    let numbers = (2, 4, 8, 16, 32);

    match numbers {
        (first, .., last) => {
            println!("Some numbers: {first}, {last}");
        }
    }
}

Match guards

A match guard is an additional if condition, specified after the pattern in a match arm, that must also match for that arm to be chosen.

Match guards

let num = Some(4);

match num {
    Some(x) if x % 2 == 0 => println!("The number {x} is even"),
    Some(x) => println!("The number {x} is odd"),
    None => (),
}

Match guards

fn main() {
    let x = Some(5);
    let y = 10;

    match x {
        Some(50) => println!("Got 50"),
        Some(n) if n == y => println!("Matched, n = {n}"),
        _ => println!("Default case, x = {x:?}"),
    }

    println!("at the end: x = {x:?}, y = {y}");
}

Match guard

let x = 4;
let y = false
match x {
    4 | 5 | 6 if y => println!("yes"),
    _ => println!("no"),
}

`@` Bindings

The at operator @ lets us create a variable that holds a value at the same time as we’re testing that value for a pattern match

`@` Bindings

enum Message {
    Hello { id: i32 },

let msg = Message::Hello { id: 5 }
match msg {
    Message::Hello {
        id: id_variable @ 3..=7,
    } => println!("Found an id in range: {id_variable}"),
    Message::Hello { id: 10..=12 } => {
        println!("Found an id in another range")
    }
    Message::Hello { id } => println!("Found some other id: {id}"),
}