任何編程語言都有自己方法來修改控製流的主要部分： if/else, for, 以及其它。讓我們來談談這些在Rust中。

if/else

具有分支的if-else語句類似於其他語言。 不同於其它，布爾條件不需要由括號包圍，並且每個條件後跟一個塊。if-else 條件等都是表達式，以及所有分支必須返還相同種類。

 

fn main() {
    let n = 5;

    if n < 0 {
        print!("{} is negative", n);
    } else if n > 0 {
        print!("{} is positive", n);
    } else {
        print!("{} is zero", n);
    }

    let big_n =
        if n < 10 && n > -10 {
            println!(", and is a small number, increase ten-fold");

            // This expression returns an `i32`.
            10 * n
        } else {
            println!(", and is a big number, reduce by two");

            // This expression must return an `i32` as well.
            n / 2
            // TODO ^ Try suppressing this expression with a semicolon.
        };
    //   ^ Don't forget to put a semicolon here! All `let` bindings need it.

    println!("{} -> {}", n, big_n);
}