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Rust 02.01 控制流运算符match

2023-02-05 10:10:59 36 0 0

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> match

# 例子
```rust
fn match_n_1(n:i32){
    match n{
        1 => { println!("It is 1");}
        2 => { println!("It is 2");}
        3|4 => { println!("It is 3 or 4");}
        5 ..= 9 => { println!("It is 5 to 9");}
        _ => { println!("others");}  //匹配剩余所有
    }
}

fn main() {
    match_n_1(7);
}
```
```rust
struct Point {x:i32,y:i32,}

fn main() {
    match (Point{x:1,y:0}) {
        Point{x,y:0} => println!("y=0 and x = {}",x),  //匹配y=0 的，抓取x 
        Point{x,..} => println!("x = {}",x),        //无视其它参数，只抓取x
    }
}
```
**match 必须穷举所有的可能**  
***

# 语法糖if let
match 有一个语法糖if let ,以下两段代码效果相同   
```rust
match value {
    some_value => xxx,
    _ => (),
}

if let some_value = value {
    xxx 
}
```
if let 与if 的区别是：  
if 后面必须是布尔值，if let 可以判断各种类型。

# match抓取内容
```rust
fn main() {
    let n = Some(2);
    match n{
            Some(1) => { println!("It is 1");}
            Some(x) => { println!("It is some({})",x);}  // 抓取数值，赋值给x
            _ => { println!("others");}
    }
}
```
```rust
fn main() {
    let n = 100;
    match n{
            the_n@ 1 ..= 1000 => { println!("It is {}",the_n);}
            _ => { println!("others");}
    }
}
```

# match使用匹配守卫添加额外条件
```rust
fn main() {
    let y = 5;
    let n = Some(2);
    match n {
        Some(x) if x < y => println!("x < {} and x = {}",y,x),
        Some(x) => println!("x = {}",x),     
        _ => println!("Others"),
    }
}
```

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