Rust 9.2 Patterns and Matching

Rust学习笔记

Rust编程语言入门教程课程笔记

参考教材: The Rust Programming Language (by Steve Klabnik and Carol Nichols, with contributions from the Rust Community)

Lecture 18: Patterns and Matching

src/main.rs

fn main() {
    //match arms
    // match VALUE {
    //     PATTERN => EXPRESSION,
    //     PATTERN => EXPRESSION,
    //     PATTERN => EXPRESSION,
    //     _ => EXPRESSION,
    // }

    let x = 1;

    match x {
        1 => println!("one"),
        2 => println!("two"),
        3 => println!("three"),
        _ => println!("anything"),
    }
    
    //if let
    //else if
    //else if let
    let favorite_color: Option<&str> = None;
    let is_tuesday = false;
    let age: Result<u8, _> = "34".parse();

    if let Some(color) = favorite_color {
        println!("Using your favorite color, {color}, as the background");
    } else if is_tuesday {
        println!("Tuesday is green day!");
    } else if let Ok(age) = age {//use the shadowed age variable
        if age > 30 {
            println!("Using purple as the background color");
        } else {
            println!("Using orange as the background color");
        }
    } else {
        println!("Using blue as the background color");
    } 

    //while let
    let mut stack = Vec::new();
    stack.push(1);
    stack.push(2);
    stack.push(3);

    while let Some(top) = stack.pop(){
        println!("{}", top);
    }

    //for loops
    let v = vec!['a', 'b', 'c'];
    for (index, value) in v.iter().enumerate(){
        println!("{} is at index {}", value, index);
    }

    //let statements
    //let Pattern = Expression;
    let (x, y, z) = (1, 2, 3);

    //function parameters
    foo(3);
    let point = (3, 5);
    print_coordinates(&point);

    //Refutability: Whether a Pattern Might Fail to Match
    let some_option_value: Option<i32> = None;
    // let Some(x) = some_option_value; //error: refutable pattern in local binding: `None` not covered
    if let Some(x) = some_option_value {
        println!("{}", x);
    }

    //Pattern Syntax

    //Matching Named Variables
    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);//x = Some(5), y = 10

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

    //Matching Ranges of Values with ...
    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 to Break Apart Values
    let p = Point { x: 0, y: 7 };

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

    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
    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 Structs and Enums
    let msg = Message3::ChangeColor(Color::Hsv(0, 160, 255));
 
    match msg {
        Message3::ChangeColor(Color::Rgb(r, g, b)) => {
            println!("Change color to red {r}, green {g}, and blue {b}");
        }
        Message3::ChangeColor(Color::Hsv(h, s, v)) => {
            println!("Change color to hue {h}, saturation {s}, value {v}")
        }
        _ => (),
    }

    //Destructuring Structs and Tuples
    let ((feet, inches), Point { x, y }) = ((3, 10), Point { x: 3, y: -10 });

    //Ignoring Values in a Pattern
    foo2(3, 4);

    //Ignoring Parts of a Value with a Nested _
    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);

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

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

    //Ignoring an Unused Variable by Starting Its Name with _

    let _x = 5;
    let y = 10;
    println!("{}", y);

    let s = Some(String::from("Hello!"));

    //if let Some(_s) = s {
    //    println!("found a string");
    //}

    //println!("{:?}", s);

    if let Some(_) = s {
        println!("found a string");
    }

    println!("{:?}", s);

    //Ignoring Remaining Parts of a Value with ..
    let origin = Point2 { x: 0, y: 0, z: 0 };

    match origin {
        Point2 { x, .. } => println!("x is {}", x),
    }

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

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

    //match numbers {
    //     (.., second, ..) => { //error: It’s impossible for Rust to determine how many values in the tuple to ignore before matching a value with second and then how many further values to ignore thereafter.
    //         println!("Some numbers: {}", second)
    //     },
    // }


    //Extra Conditionals with Match Guards
    let num = Some(4);
    match num {
        Some(x) if x % 2 == 0 => println!("The number {} is even", x),
        Some(x) => println!("The number {} is odd", x),
        None => (),
    }

    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 = {y}", x);

    let x = 4;
    let y = false;

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

    // @ Bindings
    enum Message2 {
        Hello { id: i32 },
    }

    let msg = Message2::Hello { id: 5 };
    match msg {
        Message2::Hello {
            id: id_variable @ 3..=7,
        } => println!("Found an id in range: {id_variable}"),
        Message2::Hello { id: 10..=12 } => {
            println!("Found an id in another range")
        }
        Message2::Hello { id } => println!("Found some other id: {id}"),
    }
}

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

struct Point2 {
    x: i32,
    y: i32,
    z: i32,
}

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

enum Message3 {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(Color),
}

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

fn foo(x: i32) {
   // code goes here
}

fn foo2(_: i32, y: i32) {
    println!("This code only uses the y parameter: {}", y);
}

fn print_coordinates(&(x, y): &(i32, i32)) {
    println!("Current location: ({}, {})", x, y);
}