Rust if keyword
Rust 的 if 关键字

What you’ll learn: 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.
将学到什么： Rust 的控制流结构，包括作为表达式的 if/else、loop/while/for、match，以及它们与 C/C++ 对应写法的差异。最重要的一点是：Rust 中的大多数控制流都能返回值。

• 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
  在 Rust 中，if 实际上是表达式，也就是说它可以参与赋值；但与此同时，它也具备语句的行为。▶ 亲自试试

fn main() {
    let x = 42;
    if x < 42 {
        println!("Smaller than the secret of life");
    } else if x == 42 {
        println!("Is equal to the secret of life");
    } else {
        println!("Larger than the secret of life");
    }
    let is_secret_of_life = if x == 42 {true} else {false};
    println!("{}", is_secret_of_life);
}

Rust loops using while and for
使用 while 和 for 的 Rust 循环

• The while keyword can be used to loop while an expression is true
  while 关键字可以在条件表达式为真时持续循环

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

• The for keyword can be used to iterate over ranges
  for 关键字可以用于遍历区间

fn main() {
    // Will not print 43; use 40..=43 to include last element
    for x in 40..43 {
        println!("{}", x);
    } 
}

Rust loops using loop
使用 loop 的 Rust 循环

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

fn main() {
    let mut x = 40;
    // Change the below to 'here: loop to specify optional label for the loop
    loop {
        if x == 42 {
            break; // Use break x; to return the value of x
        }
        x += 1;
    }
}

• The break statement can include an optional expression that can be used to assign the value of a loop expression
  break 语句可以附带一个表达式，用来作为整个 loop 表达式的返回值
• The continue keyword can be used to return to the top of the loop
continue 关键字可以让流程直接回到 loop 的开头
• Loop labels can be used with break or continue and are useful when dealing with nested loops
  循环标签可以和 break 或 continue 一起使用，在处理嵌套循环时尤其有用

Rust expression blocks
Rust 表达式代码块

• Rust expression blocks are simply a sequence of expressions enclosed in {}. The evaluated value is simply the last expression in the block
  Rust 的表达式代码块就是一串被 {} 包裹起来的表达式，其求值结果就是代码块中的最后一个表达式

fn main() {
    let x = {
        let y = 40;
        y + 2 // Note: ; must be omitted
    };
    // Notice the Python style printing
    println!("{x}");
}

• Rust style is to use this to omit the return keyword in functions
  Rust 的惯用写法经常利用这一点，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}
    x == 42 // Note: ; must be omitted 
}
fn main() {
    println!("{}", is_secret_of_life(42));
}