control_flow.md

# 控制流

Rust 语言的逻辑控制流有： 
- if/else 条件分支
- loop 循环
- while 循环
- for in range 循环
- match 表达式
- if let
- while let

代码表示分别为：

```rust

if condition_a {}
else if condition_b {}
else {}

loop {}

'outer: loop {
    'innter: loop {
        ...
        break;
        // or
        break 'outer;
    }
}

loop {
    ...
    break counter * 2;
}

while condition_a {}

for n in 1..=100 {}

for name in names.iter() {}

match val {
    tuple_a => statement_a ;
    tuple_b => statement_b ;
    (x, y) if x = y => statement_c ;
    (x, y) => statement_d ;
    (Enum::A, Enum::B) => statement_e ;
    (n @ 1..=50, m @ 51..=100) => statement_f ;
    (Some(n), Some(m)) => statement_g ;
}

if let Some(i) = number {}
while let Some(i) = number {}

```

下列代码能正常运行的是：

## 答案

```rust

fn main() {
    let mut count = 0;
    'outer: loop {
        'inner: loop {
            if count >= 20 {
                break 'outer; 
            }
            count += 2;
        }
    }

    println!("Complete!");
}

```


## 选项

###

```rust

fn main() {
    let mut count = 0;
    'outer: loop {
        'inner: loop {
            if count <= 20 {
                count += 2;
                goto: 'inner; 
            }
        }
    }

    println!("Complete!");
}

```


### 

```rust

fn main() {
    for i in 0..=10 {
        if i == 10 {
            panic!("This never gonna happen");
        }
    }
}

```

###

```rust

fn main() {
    let guess = 51;
    let secret_number = 42;
    let result = if guess < secret_number {
        "Too small"
    } else if guess > secret_number {
        "Too big"
    } else {
        42
    };
    
    println!("{}", result);
}

```