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模式和匹配

match 分支

所有结果都得匹配到

// Match Expressionfn main() {#[derive(Debug)]enum Language {English,Spanish,Russian,Japanese,}let language = Language::English;match language {Language::English => println!("Hello World!"),Language::Spanish => println!("Hola Mundo!"),Language::Russian => println!("npuBeT, Mnp!"),//_=> println!("Unsupported language!"),lang => println!("Unsupported language! {:?}", lang),}
}

if let表达式

if let表达式缺点：编译期无法强制保证覆盖所有的情况

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

#![allow(unused)]
fn main() {let mut stack = Vec::new();stack.push(1);stack.push(2);stack.push(3);// 一旦其返回 None，while 循环停止。while let Some(top) = stack.pop() {println!("{}", top);}
}

for 循环

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

let 语句

#![allow(unused)]
fn main() {let x = 5;// 将指分别绑定到x, y和z上let (x, y, z) = (1, 2, 3);let (x, y, _) = (1, 2, 3);
}

函数参数

//一个在参数中解构元组的函数
fn print_coordinates(&(x, y): &(i32, i32)) {println!("Current location: ({}, {})", x, y);
}fn main() {let point = (3, 5);print_coordinates(&point);
}

Refutability（可失败）与Irrefutable(不可失败的)

fn main() {// Irrefutable(不可失败的)let x = 5;// Refutable(可失败的),如果x是None,模式则无法匹配let x: Option<&str> = None;if let Some(x) = x {println!("{}", x)}// 只能接受不可失败模式//function parameters// let statements// for loops// 既可以接受可失败,也可以接受不可失败// if let// while let
}

fn main() {let x: Option<&str> = None;let Some(x) = x; //error:`let` bindings require an "irrefutable pattern"//error:这个模式总是匹配,所以这个 `if` 没啥用if let x = 5 {println!("{}", x);};
}

模式语法

匹配字面量

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

匹配命名变量

fn main() {let x = Some(5);let y = 10;match x {Some(50) => println!("Got 50"),Some(y) => println!("Matched, y = {:?}", y),//因为定义了y,会遮蔽外面的y_ => println!("Default case, x = {:?}", x),}println!("at the end: x = {:?}, y = {:?}", x, y);
}

多个模式

#![allow(unused)]
fn main() {
let x = 1;match x {1 | 2 => println!("one or two"),3 => println!("three"),_ => println!("anything"),
}
}

通过 …= 匹配值的范围

只适用于数值和字符

#![allow(unused)]
fn main() {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"),}
}

解构并分解值

解构结构体

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);
}

可以简写如下：

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);
}

使用字面量作为结构体模式的一部分进行解构

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),}
}

解构枚举

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)}}
}

解构嵌套的结构体和枚举

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)}_ => (),}
}

解构结构体和元组

#![allow(unused)]
fn main() {
struct Point {x: i32,y: i32,
}let ((feet, inches), Point {x, y}) = ((3, 10), Point { x: 3, y: -10 });
}

忽略模式中的值

使用 _ 忽略整个值

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

使用嵌套的 _ 忽略部分值

#![allow(unused)]
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);
}

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

通过在名字前以一个下划线开头来忽略未使用的变量

给变量增加下划线前缀和使用_是不同的

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

#![allow(unused)]
fn main() {let s = Some(String::from("Hello!"));//s中的值会被移入到_s中,虽然你用了忽略if let Some(_s) = s {println!("found a string");}println!("{:?}", s);
}

解决办法

#![allow(unused)]
fn main() {let s = Some(String::from("Hello!"));// 然而只使用下划线本身，并不会绑定值。if let Some(_) = s {println!("found a string");}println!("{:?}", s);
}

用 … 忽略剩余值

#![allow(unused)]
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),
}
}

… 会扩展为所需要的值的数量

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

匹配守卫提供的额外条件

#![allow(unused)]
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),//个 y 正是 外部的 y _ => println!("Default case, x = {:?}", x),}println!("at the end: x = {:?}, y = {}", x, y);
}

#![allow(unused)]
fn main() {
let x = 4;
let y = false;match x {4 | 5 | 6 if y => println!("yes"),//且_ => println!("no"),
}
}

@ 绑定

#![allow(unused)]
fn main() {enum Message {Hello { id: i32 },}let msg = Message::Hello { id: 5 };match msg {//  Message::Hello 的 id 字段是否位于 3..=7 范围内，同时也希望能将其值绑定到 id_variable 变量中以便此分支相关联的代码可以使用它。可以将 id_variable 命名为 idMessage::Hello {id: id_variable @ 3..=7,} => {println!("Found an id in range: {}", id_variable)}// 没有将 id 值保存进一个变量。Message::Hello { id: 10..=12 } => {println!("Found an id in another range")}Message::Hello { id } => {println!("Found some other id: {}", id)}}
}

参考

• 第18章～模式和匹配