4. Control Flow / 4. 控制流

What you’ll learn / 你将学到： Rust’s control flow constructs — if/else as expressions, loop/while/for, match, and how they differ from C/C++ counterparts. The key insight: most Rust control flow returns values.

Rust 的控制流结构 —— 作为表达式的 if/else、loop/while/for、match，以及它们与 C/C++ 对应部分的区别。核心见解：大多数 Rust 控制流都会返回值。

• In Rust, if is actually an expression, i.e., it can be used to assign values, but it also behaves like a statement. ▶ Try it / 尝试运行

fn main() {
    let x = 42;
    if x < 42 {
        println!("Smaller than the secret of life / 小于生命之秘");
    } else if x == 42 {
        println!("Is equal to the secret of life / 等于生命之秘");
    } else {
        println!("Larger than the secret of life / 大于生命之秘");
    }
    let is_secret_of_life = if x == 42 {true} else {false};
    println!("{}", is_secret_of_life);
}

Rust loops using while and for / Rust 循环：while 与 for

• The while keyword can be used to loop while an expression is true / while 关键字用于在表达式为真时进行循环

fn main() {
    let mut x = 40;
    while x != 42 {
        x += 1;
    }
}

• The for keyword can be used to iterate over ranges / for 关键字可用于遍历范围

fn main() {
    // Will not print 43; use 40..=43 to include last element
    // 不会打印 43；使用 40..=43 来包含最后一个元素
    for x in 40..43 {
        println!("{}", x);
    } 
}

Rust loops using loop / Rust 循环：loop

• The loop keyword creates an infinite loop until a break is encountered / loop 关键字创建一个无限循环，直到遇到 break

fn main() {
    let mut x = 40;
    // Change the below to 'here: loop to specify optional label for the loop
    // 将下面改为 'here: loop 可以为循环指定可选标签
    loop {
        if x == 42 {
            break; // Use break x; to return the value of x / 使用 break x; 可以返回 x 的值
        }
        x += 1;
    }
}

• The break statement can include an optional expression that can be used to assign the value of a loop expression / break 语句可以包含一个可选表达式，用于为 loop 表达式赋值
• The continue keyword can be used to return to the top of the loop / continue 关键字可以用于返回 loop 的顶部
• Loop labels can be used with break or continue and are useful when dealing with nested loops / 循环标签可以配合 break 或 continue 使用，在处理嵌套循环时非常有用

Rust expression blocks / Rust 表达式块

• Rust expression blocks are simply a sequence of expressions enclosed in {}. The evaluated value is simply the last expression in the block / Rust 表达式块只是用 {} 包裹的一系列表达式。其求值结果就是块中的最后一个表达式

fn main() {
    let x = {
        let y = 40;
        y + 2 // Note: ; must be omitted / 注意：必须省略分号 ;
    };
    // Notice the Python style printing / 注意这种 Python 风格的打印方式
    println!("{x}");
}

• Rust style is to use this to omit the return keyword in functions / Rust 的风格是利用这一点在函数中省略 return 关键字

fn is_secret_of_life(x: u32) -> bool {
    // Same as if x == 42 {true} else {false}
    // 等同于 if x == 42 {true} else {false}
    x == 42 // Note: ; must be omitted / 注意：必须省略分号 ;
}
fn main() {
    println!("{}", is_secret_of_life(42));
}