Rust语言学习笔记（12）

模式匹配的应用

// if let, else if let
fn main() {
    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, {}, as the background", color);
    } else if is_tuesday {
        println!("Tuesday is green day!");
    } else if let Ok(age) = age {
        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 循环
let v = vec!['a', 'b', 'c'];
for (index, value) in v.iter().enumerate() {
    println!("{} is at index {}", value, index);
}

// let 语句
let (x, y, z) = (1, 2, 3);
// let (x, y) = (1, 2, 3); // error

// 参数
fn print_coordinates(&(x, y): &(i32, i32)) {
    println!("Current location: ({}, {})", x, y);
}
fn main() {
    let point = (3, 5);
    print_coordinates(&point);
}

模式匹配的语法

// 匹配字面量
let x = 1;
match x {
    1 => println!("one"),
    2 => println!("two"),
    3 => println!("three"),
    _ => println!("anything"),
}

// 匹配具名变量
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);
}

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

// 匹配区间
let x = 5;
match x {
    1..=5 => println!("one through five"),
    _ => println!("something else"),
}
// 匹配区间
let x = 'c';
match x {
    'a'..='j' => println!("early ASCII letter"),
    'k'..='z' => println!("late ASCII letter"),
    _ => println!("something else"),
}

// 结构体的解构
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);
}
// 结构体的解构
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);
}
// 结构体的解构
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),
    }
}

// 枚举的解构
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
            )
        }
    }
}

// 嵌套类型的解构
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
            )
        }
        _ => ()
    }
}

// 元组的解构
let ((feet, inches), Point {x, y}) = ((3, 10), Point { x: 3, y: -10 });

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

// 忽略部分值
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);

// 忽略部分值
let numbers = (2, 4, 8, 16, 32);
match numbers {
    (first, _, third, _, fifth) => {
        println!("Some numbers: {}, {}, {}", first, third, fifth)
    },
}

// 以下划线打头的变量即便不使用也没有警告
fn main() {
    let _x = 5;
    let y = 10;
}

// 忽略结构体的一部分
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),
}

// 忽略元组的一部分
fn main() {
    let numbers = (2, 4, 8, 16, 32);
    match numbers {
        (first, .., last) => {
            println!("Some numbers: {}, {}", first, last);
        },
    }
}

// 匹配守卫
let num = Some(4);
match num {
    Some(x) if x < 5 => println!("less than five: {}", x),
    Some(x) => println!("{}", x),
    None => (),
}
// 匹配守卫
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);
}
// 匹配守卫
let x = 4;
let y = false;
match x {
    4 | 5 | 6 if y => println!("yes"), // (4 | 5 | 6) if y => ...
    _ => println!("no"),
}

// 绑定
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)
    },
}