模式匹配
模式
- 模式是Rust中的一种特殊语法,用于匹配复杂和简单类型的结构
- 将模式与匹配表达式和其他构造结合使用,可以更好地控制程序的控制流
- 模式由以下元素(的一些组合)组成:
- 字面值
- 解构的数组、enum、struct 和 tuple
- 变量
- 通配符
- 占位符
- 想要使用模式,需要将其与某个值进行比较:
一、用到模式(匹配)的地方
match 的 Arm
- match VALUE {
- PATTERN => EXPRESSION,
- PATTERN => EXPRESSION,
- PATTERN => EXPRESSION,
- }
复制代码
- match 表达式的要求:
- 一个特殊的模式:_(下划线):
- 它会匹配任何东西
- 不会绑定到变量
- 通常用于 match 的最后一个 arm;或用于忽略某些值。
条件 if let 表达式
- if let 表达式主要是作为一种简短的方式来等价的代替只有一个匹配项的 match
- if let 可选的可以拥有 else,包括:
- 但,if let 不会检查穷尽性
- 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 if_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");
- }
- }
复制代码 While let 条件循环
- 只要模式继续满足匹配的条件,那它允许 while 循环一直运行
- fn main() {
- let mut stack = Vec::new();
-
- stack.push(1);
- stack.push(2);
- stack.push(3);
-
- while let Some(top) = stack.pop() {
- println!("{}", top);
- }
- }
复制代码 for 循环
- for 循环是Rust 中最常见的循环
- for 循环中,模式就是紧随 for 关键字后的值
- fn main() {
- let v = vec!['a', 'b', 'c'];
-
- for (index, value) in v.iter().enumerate() {
- println!("{} is at index {}", value , index);
- }
- }
复制代码 Let 语句
- let 语句也是模式
- let PATTERN = EXPRESSION
- fn main() {
- let a = 5;
-
- let (x, y, z) = (1, 2, 3);
-
- let (q, w) = (4, 5, 6); // 报错 类型不匹配 3 2
- }
复制代码 函数参数
- fn foo(x: i32) {
- // code goes here
- }
- fn print_coordinates(&(x, y): &(i32, i32)) {
- println!("Current location: ({}, {})", x, y);
- }
- fn main() {
- let point = (3, 5);
- print_coordinates(&point);
- }
复制代码 二、可辩驳性:模式是否会无法匹配
模式的两种形式
- 模式有两种形式:可辨驳的、无可辩驳的
- 能匹配任何可能传递的值的模式:无可辩驳的
- 对某些可能得值,无法进行匹配的模式:可辩驳的
- 例如:if let Some(x) = a_value
- 函数参数、let 语句、for 循环只接受无可辩驳的模式
- if let 和 while let 接受可辨驳和无可辩驳的模式
- fn main() {
- let a: Option<i32> = Some(5);
- let Some(x) = a: // 报错 None
- if let Some(x) = a {}
- if let x = 5 {} // 警告
- }
复制代码 三、模式语法
匹配字面值
- fn main() {
- let x = 1;
-
- match x {
- 1 => println!("one"),
- 2 => println!("two"),
- 3 => println!("three"),
- _ => println!("anything"),
- }
- }
复制代码 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),
- _ => println!("Default case, x = {:?}", x),
- }
-
- println!("at the end: x = {:?}, y = {:?}", x, y);
- }
复制代码 多重模式
- 在match 表达式中,使用 | 语法(就是或的意思),可以匹配多种模式
- fn main() {
- let x = 1;
-
- match x {
- 1 | 2 => println!("one or two"),
- 3 => println!("three"),
- _ => println!("anything"),
- }
- }
复制代码 使用 ..= 来匹配某个范围的值
- 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"),
- }
- }
复制代码 解构以分解值
- 可以使用模式来解构 struct、enum、tuple,从而引用这些类型值的不同部分
- 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);
-
- let Point {x, y} = p;
- assert_eq!(0, x);
- assert_eq!(7, y);
-
- 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),
- }
- }
复制代码 解构 enum
- 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);
- }
- }
- }
复制代码 解构嵌套的 struct 和 enum
- 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::ChangeClolr(Color::Rgb(r, g, b)) => {
- println!("Change the color to red {}, green {}, and blur {}", r, g, b)
- }
- Message::ChangeColor(Color::Hsv(h, s, v)) => {
- println!("Change the color to hue {}, saturation {}, and value {}", h, s, v)
- }
- _ => (),
- }
- }
复制代码 解构 struct 和 tuple
- struct Point {
- x: i32,
- y: i32,
- }
- fn main() {
- let ((feet, inches), Point {x, y}) = ((3, 10), Point {x: 3, y: -10});
- }
复制代码 在模式中忽略值
- 有几种方式可以在模式中忽略整个值或部分值:
- _
- _ 配合其它模式
- 使用以 _ 开头的名称
- .. (忽略值的剩余部分)
使用 _ 来忽略整个值
- fn foo(_: i32, y: i32) {
- println!("This code only uses the y parameter: {}", y);
- }
- fn main() {
- foo(3, 4);
- }
复制代码 使用嵌套的 _ 来忽略值的一部分
- 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, 6, 8, 16, 32);
-
- match numbers {
- (first, _, third, _, fifth) => {
- println!("Some numbers: {}, {}, {}", first, third, fifth)
- }
- }
- }
复制代码 通过使用 _ 开头命名来忽略未使用的变量
- fn main() {
- let _x = 5;
- let y = 10; // 创建未使用 警告
-
- let s = Some(String::from("Hello"));
-
- if let Some(_s) = s { // if let Some(_) = s {
- println!("found a string");
- }
-
- println!("{:?}", s); // 报错
- }
复制代码 使用 .. 来忽略值的剩余部分
- struct Point {
- x: i32,
- y: i32,
- z: i32,
- }
- fn main() {
- let origin = Point {x: 0, y: 0, z: 0};
- match origin {
- Point {x, ..} => println!("x is {}", x),
- }
-
- let numbers = (2, 4, 8, 16, 32);
- match numbers {
- (first, .., last) => {
- println!("Some numbers: {}, {}", first, last);
- }
- }
-
- match numbers {
- (.., second, ..) => { // 报错
- println!("Some numbers: {}", second)
- },
- }
- }
复制代码 使用 match 守卫来提供额外的条件
- match 守卫就是 match arm 模式后额外的 if 条件,想要匹配该条件也必须满足
- match 守卫适用于比单独的模式更复杂的场景
例子一:- fn main() {
- let num = Some(4);
-
- match num {
- Some(x) if x < 5 => println!("less than five: {}", x),
- Some(x) => println!("{}", x),
- None => (),
- }
- }
复制代码 例子二:- 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);
- }
复制代码 例子三:- fn main() {
- let x = 4;
- let y = false;
-
- match x {
- 4 | 5 | 6 if y => println!("yes"),
- _ => println!("no"),
- }
- }
复制代码 @绑定
- @ 符号让我们可以创建一个变量,该变量可以在测试某个值是否与模式匹配的同时保存该值
- enum Message {
- Hello {id: i32},
- }
- fn main() {
- 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)
- }
- }
- }
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