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< h1 id = "match-和-if-let" > < a class = "header" href = "#match-和-if-let" > match 和 if let< / a > < / h1 >
< p > 在 Rust 中，模式匹配最常用的就是 < code > match< / code > 和 < code > if let< / code > ，本章节将对两者及相关的概念进行详尽介绍。< / p >
< p > 先来看一个关于 < code > match< / code > 的简单例子：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > enum Direction {
East,
West,
North,
South,
}
fn main() {
let dire = Direction::South;
match dire {
Direction::East => println!("East"),
Direction::North | Direction::South => {
println!("South or North");
},
_ => println!("West"),
};
}< / code > < / pre >
< p > 这里我们想去匹配 < code > dire< / code > 对应的枚举类型，因此在 < code > match< / code > 中用三个匹配分支来完全覆盖枚举变量 < code > Direction< / code > 的所有成员类型，有以下几点值得注意：< / p >
< ul >
< li > < code > match< / code > 的匹配必须要穷举出所有可能，因此这里用 < code > _< / code > 来代表未列出的所有可能性< / li >
< li > < code > match< / code > 的每一个分支都必须是一个表达式，且所有分支的表达式最终返回值的类型必须相同< / li >
< li > < strong > X | Y< / strong > ，类似逻辑运算符 < code > 或< / code > ，代表该分支可以匹配 < code > X< / code > 也可以匹配 < code > Y< / code > ，只要满足一个即可< / li >
< / ul >
< p > 其实 < code > match< / code > 跟其他语言中的 < code > switch< / code > 非常像，< code > _< / code > 类似于 < code > switch< / code > 中的 < code > default< / code > 。< / p >
< h2 id = "match-匹配" > < a class = "header" href = "#match-匹配" > < code > match< / code > 匹配< / a > < / h2 >
< p > 首先来看看 < code > match< / code > 的通用形式：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > match target {
模式1 => 表达式1,
模式2 => {
语句1;
语句2;
表达式2
},
_ => 表达式3
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 该形式清晰的说明了何为模式，何为模式匹配：将模式与 < code > target< / code > 进行匹配，即为模式匹配，而模式匹配不仅仅局限于 < code > match< / code > ，后面我们会详细阐述。< / p >
< p > < code > match< / code > 允许我们将一个值与一系列的模式相比较，并根据相匹配的模式执行对应的代码，下面让我们来一一详解，先看一个例子：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > enum Coin {
Penny,
Nickel,
Dime,
Quarter,
}
fn value_in_cents(coin: Coin) -> u8 {
match coin {
Coin::Penny => {
println!("Lucky penny!");
1
},
Coin::Nickel => 5,
Coin::Dime => 10,
Coin::Quarter => 25,
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > value_in_cents< / code > 函数根据匹配到的硬币，返回对应的美分数值。< code > match< / code > 后紧跟着的是一个表达式，跟 < code > if< / code > 很像，但是 < code > if< / code > 后的表达式必须是一个布尔值，而 < code > match< / code > 后的表达式返回值可以是任意类型，只要能跟后面的分支中的模式匹配起来即可，这里的 < code > coin< / code > 是枚举 < code > Coin< / code > 类型。< / p >
< p > 接下来是 < code > match< / code > 的分支。一个分支有两个部分：< strong > 一个模式和针对该模式的处理代码< / strong > 。第一个分支的模式是 < code > Coin::Penny< / code > ，其后的 < code > => < / code > 运算符将模式和将要运行的代码分开。这里的代码就仅仅是表达式 < code > 1< / code > ，不同分支之间使用逗号分隔。< / p >
< p > 当 < code > match< / code > 表达式执行时，它将目标值 < code > coin< / code > 按顺序依次与每一个分支的模式相比较，如果模式匹配了这个值，那么模式之后的代码将被执行。如果模式并不匹配这个值，将继续执行下一个分支。< / p >
< p > 每个分支相关联的代码是一个表达式，而表达式的结果值将作为整个 < code > match< / code > 表达式的返回值。如果分支有多行代码，那么需要用 < code > {}< / code > 包裹，同时最后一行代码需要是一个表达式。< / p >
< h4 id = "使用-match-表达式赋值" > < a class = "header" href = "#使用-match-表达式赋值" > 使用 < code > match< / code > 表达式赋值< / a > < / h4 >
< p > 还有一点很重要，< code > match< / code > 本身也是一个表达式，因此可以用它来赋值：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > enum IpAddr {
Ipv4,
Ipv6
}
fn main() {
let ip1 = IpAddr::Ipv6;
let ip_str = match ip1 {
IpAddr::Ipv4 => "127.0.0.1",
_ => "::1",
};
println!("{}", ip_str);
}< / code > < / pre >
< p > 因为这里匹配到 < code > _< / code > 分支，所以将 < code > "::1"< / code > 赋值给了 < code > ip_str< / code > 。< / p >
< h4 id = "模式绑定" > < a class = "header" href = "#模式绑定" > 模式绑定< / a > < / h4 >
< p > 模式匹配的另外一个重要功能是从模式中取出绑定的值，例如：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > #[derive(Debug)]
enum UsState {
Alabama,
Alaska,
// --snip--
}
enum Coin {
Penny,
Nickel,
Dime,
Quarter(UsState), // 25美分硬币
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 其中 < code > Coin::Quarter< / code > 成员还存放了一个值：美国的某个州（因为在 1999 年到 2008 年间，美国在 25 美分(Quarter)硬币的背后为 50 个州印刷了不同的标记，其它硬币都没有这样的设计）。< / p >
< p > 接下来，我们希望在模式匹配中，获取到 25 美分硬币上刻印的州的名称：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn value_in_cents(coin: Coin) -> u8 {
match coin {
Coin::Penny => 1,
Coin::Nickel => 5,
Coin::Dime => 10,
Coin::Quarter(state) => {
println!("State quarter from {:?}!", state);
25
},
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 上面代码中，在匹配 < code > Coin::Quarter(state)< / code > 模式时，我们把它内部存储的值绑定到了 < code > state< / code > 变量上，因此 < code > state< / code > 变量就是对应的 < code > UsState< / code > 枚举类型。< / p >
< p > 例如有一个印了阿拉斯加州标记的 25 分硬币：< code > Coin::Quarter(UsState::Alaska)< / code > ，它在匹配时，< code > state< / code > 变量将被绑定 < code > UsState::Alaska< / code > 的枚举值。< / p >
< p > 再来看一个更复杂的例子：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > enum Action {
Say(String),
MoveTo(i32, i32),
ChangeColorRGB(u16, u16, u16),
}
fn main() {
let actions = [
Action::Say("Hello Rust".to_string()),
Action::MoveTo(1,2),
Action::ChangeColorRGB(255,255,0),
];
for action in actions {
match action {
Action::Say(s) => {
println!("{}", s);
},
Action::MoveTo(x, y) => {
println!("point from (0, 0) move to ({}, {})", x, y);
},
Action::ChangeColorRGB(r, g, _) => {
println!("change color into '(r:{}, g:{}, b:0)', 'b' has been ignored",
r, g,
);
}
}
}
}< / code > < / pre >
< p > 运行后输出：< / p >
< pre > < code class = "language-console" > $ cargo run
Compiling world_hello v0.1.0 (/Users/sunfei/development/rust/world_hello)
Finished dev [unoptimized + debuginfo] target(s) in 0.16s
Running `target/debug/world_hello`
Hello Rust
point from (0, 0) move to (1, 2)
change color into '(r:255, g:255, b:0)', 'b' has been ignored
< / code > < / pre >
< h4 id = "穷尽匹配" > < a class = "header" href = "#穷尽匹配" > 穷尽匹配< / a > < / h4 >
< p > 在文章的开头，我们简单总结过 < code > match< / code > 的匹配必须穷尽所有情况，下面来举例说明，例如：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > enum Direction {
East,
West,
North,
South,
}
fn main() {
let dire = Direction::South;
match dire {
Direction::East => println!("East"),
Direction::North | Direction::South => {
println!("South or North");
},
};
}< / code > < / pre >
< p > 我们没有处理 < code > Direction::West< / code > 的情况，因此会报错：< / p >
< pre > < code class = "language-console" > error[E0004]: non-exhaustive patterns: `West` not covered // 非穷尽匹配，`West` 没有被覆盖
--> src/main.rs:10:11
|
1 | / enum Direction {
2 | | East,
3 | | West,
| | ---- not covered
4 | | North,
5 | | South,
6 | | }
| |_- `Direction` defined here
...
10 | match dire {
| ^^^^ pattern `West` not covered // 模式 `West` 没有被覆盖
|
= help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
= note: the matched value is of type `Direction`
< / code > < / pre >
< p > 不禁想感叹， < strong > 真强大< / strong > ， ， ， < code > match< / code > 中有哪些分支没有被覆盖，这种行为能强制我们处理所有的可能性，有效避免传说中价值< strong > 十亿美金< / strong > 的 < code > null< / code > 陷阱。< / p >
< h4 id = "_-通配符" > < a class = "header" href = "#_-通配符" > < code > _< / code > 通配符< / a > < / h4 >
< p > 当我们不想在匹配时列出所有值的时候，可以使用 Rust 提供的一个特殊< strong > 模式< / strong > ，例如，< code > u8< / code > 可以拥有 0 到 255 的有效的值，但是我们只关心 < code > 1、3、5 和 7< / code > 这几个值，不想列出其它的 < code > 0、2、4、6、8、9 一直到 255< / code > 的值。那么, 我们不必一个一个列出所有值, 因为可以使用特殊的模式 < code > _< / code > 替代：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let some_u8_value = 0u8;
match some_u8_value {
1 => println!("one"),
3 => println!("three"),
5 => println!("five"),
7 => println!("seven"),
_ => (),
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 通过将 < code > _< / code > 其放置于其他分支后，< code > _< / code > 将会匹配所有遗漏的值。< code > ()< / code > 表示返回< strong > 单元类型< / strong > 与所有分支返回值的类型相同，所以当匹配到 < code > _< / code > 后，什么也不会发生。< / p >
< p > 除了< code > _< / code > 通配符，用一个变量来承载其他情况也是可以的。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > #[derive(Debug)]
enum Direction {
East,
West,
North,
South,
}
fn main() {
let dire = Direction::South;
match dire {
Direction::East => println!("East"),
other => println!("other direction: {:?}", other),
};
}< / code > < / pre >
< p > 然而，在某些场景下，我们其实只关心< strong > 某一个值是否存在< / strong > ，此时 < code > match< / code > 就显得过于啰嗦。< / p >
< h2 id = "if-let-匹配" > < a class = "header" href = "#if-let-匹配" > < code > if let< / code > 匹配< / a > < / h2 >
< p > 有时会遇到只有一个模式的值需要被处理，其它值直接忽略的场景，如果用 < code > match< / code > 来处理就要写成下面这样：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let v = Some(3u8);
match v {
Some(3) => println!("three"),
_ => (),
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 我们只想要对 < code > Some(3)< / code > 模式进行匹配, 不想处理任何其他 < code > Some< u8> < / code > 值或 < code > None< / code > 值。但是为了满足 < code > match< / code > 表达式（穷尽性）的要求，写代码时必须在处理完这唯一的成员后加上 < code > _ => ()< / code > ，这样会增加不少无用的代码。< / p >
< p > 俗话说“杀鸡焉用牛刀”，我们完全可以用 < code > if let< / code > 的方式来实现：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > if let Some(3) = v {
println!("three");
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这两种匹配对于新手来说，可能有些难以抉择，但是只要记住一点就好：< strong > 当你只要匹配一个条件，且忽略其他条件时就用 < code > if let< / code > ，否则都用 < code > match< / code > < / strong > 。< / p >
< h2 id = "matches宏" > < a class = "header" href = "#matches宏" > matches!宏< / a > < / h2 >
< p > Rust 标准库中提供了一个非常实用的宏：< code > matches!< / code > ，它可以将一个表达式跟模式进行匹配，然后返回匹配的结果 < code > true< / code > or < code > false< / code > 。< / p >
< p > 例如，有一个动态数组，里面存有以下枚举：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > enum MyEnum {
Foo,
Bar
}
fn main() {
let v = vec![MyEnum::Foo,MyEnum::Bar,MyEnum::Foo];
}< / code > < / pre >
< p > 现在如果想对 < code > v< / code > 进行过滤，只保留类型是 < code > MyEnum::Foo< / code > 的元素，你可能想这么写：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > v.iter().filter(|x| x == MyEnum::Foo);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 但是，实际上这行代码会报错，因为你无法将 < code > x< / code > 直接跟一个枚举成员进行比较。好在，你可以使用 < code > match< / code > 来完成，但是会导致代码更为啰嗦，是否有更简洁的方式？答案是使用 < code > matches!< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > v.iter().filter(|x| matches!(x, MyEnum::Foo));
< span class = "boring" > }< / span > < / code > < / pre >
< p > 很简单也很简洁，再来看看更多的例子：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let foo = 'f';
assert!(matches!(foo, 'A'..='Z' | 'a'..='z'));
let bar = Some(4);
assert!(matches!(bar, Some(x) if x > 2));
< span class = "boring" > }< / span > < / code > < / pre >
< h2 id = "变量遮蔽" > < a class = "header" href = "#变量遮蔽" > 变量遮蔽< / a > < / h2 >
< p > 无论是 < code > match< / code > 还是 < code > if let< / code > ，这里都是一个新的代码块，而且这里的绑定相当于新变量，如果你使用同名变量，会发生变量遮蔽：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let age = Some(30);
println!("在匹配前，
if let Some(age) = age {
println!("匹配出来的age是{}",age);
}
println!("在匹配后，
}< / code > < / pre >
< p > < code > cargo run < / code > 运行后输出如下：< / p >
< pre > < code class = "language-console" > 在匹配前，
匹配出来的age是30
在匹配后，
< / code > < / pre >
< p > 可以看出在 < code > if let< / code > 中，< code > =< / code > 右边 < code > Some(i32)< / code > 类型的 < code > age< / code > 被左边 < code > i32< / code > 类型的新 < code > age< / code > 遮蔽了，该遮蔽一直持续到 < code > if let< / code > 语句块的结束。因此第三个 < code > println!< / code > 输出的 < code > age< / code > 依然是 < code > Some(i32)< / code > 类型。< / p >
< p > 对于 < code > match< / code > 类型也是如此:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let age = Some(30);
println!("在匹配前，
match age {
Some(age) => println!("匹配出来的age是{}",age),
_ => ()
}
println!("在匹配后，
}< / code > < / pre >
< p > 需要注意的是，< strong > < code > match< / code > 中的变量遮蔽其实不是那么的容易看出< / strong > ，因此要小心！其实这里最好不要使用同名，避免难以理解，如下。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let age = Some(30);
println!("在匹配前，
match age {
Some(x) => println!("匹配出来的age是{}", x),
_ => ()
}
println!("在匹配后，
}< / code > < / pre >
< h2 id = "课后练习" > < a class = "header" href = "#课后练习" > 课后练习< / a > < / h2 >
< blockquote >
< p > < a href = "https://practice-zh.course.rs/pattern-match/match-iflet.html" > Rust By Practice< / a > ，支持代码在线编辑和运行，并提供详细的< a href = "https://github.com/sunface/rust-by-practice/blob/master/solutions/pattern-match/match.md" > 习题解答< / a > 。< / p >
< / blockquote >
< / main >
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