wiki/content/20201006102934-pattern_syntax.md

date	id	title
2020-10-06	8c351588-1dc2-4955-86b5-a13181367cf9	Rust Pattern Syntax

Examples

Matching Literals

fn main() {
    let x = 1;

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

Matching Named Variables

fn main() {
    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 = {:?}, y = {:?}", x, y);
}

Multiple Patterns

fn main() {
    let x = 1;

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

Matching Ranges of Values with ..=

fn main() {
    let x = 5;

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

fn main() {
    let x = 'c';

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

Destructuring

Destructuring to break apart Values

1. Destructuring Structs

   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);
   }
   

   1. Shorthand version

      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);
      }
      

      #+RESULTS:
      

Destructuring and matching literal values in one pattern

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

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

    match p {
        Point { x, y: 0 } => println!("On the x axis at {}", x),
        Point { x: 0, y } => println!("On the y axis at {}", y),
        Point { x, y } => println!("On neither axis: ({}, {})", x, y),
    }
}

Destructuring Enums

Enums can also be deconstructed:

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 {} and in the y direction {}", x, y);
        }
        Message::Write(text) => println!("Text message: {}", text),
        Message::ChangeColor(r, g, b) => {
            println!("Change the color to red {}, green {}, and blue {}", r, g, b)
        }
    }
}

Destructuring nested Structs and Enums

Nested structs and enums can also be destructured:

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

Destructuring Structs and Tuples

fn main() {
    struct Point {
        x: i32,
        y: i32,
    }

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

Ignoring Values in a Pattern

_

1. Ignoring an entire Value with _

   fn foo(_: i32, y: i32) {
       println!("This code only uses the y parameter: {}", y);
   }
   
   fn main() {
       foo(3, 4);
   }
   

2. Ignoring parts of a Value with a nested _

   fn main() {
       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);
   }
   

   fn main() {
       let numbers = (2, 4, 8, 16, 32);
   
       match numbers {
           (first, _, third, _, fifth) => println!("Some numbers: {}, {}, {}", first, third, fifth),
       }
   }
   

3. Ignoring an unused Variable by starting its name with _

   fn main() {
       let _x = 5;
   }
   

   #+RESULTS:
   

…

1. Ignoring remaining parts of a Value with …

   fn main() {
       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);
           }
       }
   }
   

Extra conditionals with Match Guards

fn main() {
    let num = Some(4);

    match num {
        Some(x) if x < 5 => println!("less than five: {}", x),
        Some(x) => println!("{}", x),
        None => (),
    }
}

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 = {:?}, y = {}", x, y);
}

fn main() {
    let x = 4;
    let y = false;

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

@ Bindings

fn main() {
    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),
    }
}