简单英语学Rust

循环

有了循环，你可以告诉 Rust 继续某事，直到你想让它停止。您使用 loop 来启动一个不会停止的循环，除非您告诉它何时break。

fn main() { // This program will never stop
    loop {

    }
}

所以，我们要告诉编译器什么时候能停止:

fn main() {
    let mut counter = 0; // set a counter to 0
    loop {
        counter +=1; // increase the counter by 1
        println!("The counter is now: {}", counter);
        if counter == 5 { // stop when counter == 5
            break;
        }
    }
}

这将打印:

The counter is now: 1
The counter is now: 2
The counter is now: 3
The counter is now: 4
The counter is now: 5

如果你在一个循环里面有一个循环，你可以给它们命名。有了名字，你可以告诉 Rust 要从哪个循环中 break 出来。使用 ' (称为 "tick") 和 : 来给它命名。

fn main() {
    let mut counter = 0;
    let mut counter2 = 0;
    println!("Now entering the first loop.");

    'first_loop: loop {
        // Give the first loop a name
        counter += 1;
        println!("The counter is now: {}", counter);
        if counter > 9 {
            // Starts a second loop inside this loop
            println!("Now entering the second loop.");

            'second_loop: loop {
                // now we are inside 'second_loop
                println!("The second counter is now: {}", counter2);
                counter2 += 1;
                if counter2 == 3 {
                    break 'first_loop; // Break out of 'first_loop so we can exit the program
                }
            }
        }
    }
}

这将打印:

Now entering the first loop.
The counter is now: 1
The counter is now: 2
The counter is now: 3
The counter is now: 4
The counter is now: 5
The counter is now: 6
The counter is now: 7
The counter is now: 8
The counter is now: 9
The counter is now: 10
Now entering the second loop.
The second counter is now: 0
The second counter is now: 1
The second counter is now: 2

while循环是指在某件事情还在true时继续的循环。每一次循环，Rust 都会检查它是否仍然是 true。如果变成false，Rust会停止循环。

fn main() {
    let mut counter = 0;

    while counter < 5 {
        counter +=1;
        println!("The counter is now: {}", counter);
    }
}

for循环可以让你告诉Rust每次要做什么。但是在 for 循环中，循环会在一定次数后停止。for循环经常使用范围。你使用 .. 和 ..= 来创建一个范围。

• ..创建一个排他的范围:0..3创建了0, 1, 2.
• ..=创建一个包含的范围: 0..=3创建0, 1, 2。0..=3 = 0, 1, 2, 3.

fn main() {
    for number in 0..3 {
        println!("The number is: {}", number);
    }

    for number in 0..=3 {
        println!("The next number is: {}", number);
    }
}

这个将打印:

The number is: 0
The number is: 1
The number is: 2
The next number is: 0
The next number is: 1
The next number is: 2
The next number is: 3

同时注意到，number成为0..3的变量名。我们可以把它叫做 n，或者 ntod_het___hno_f，或者任何名字。然后，我们可以在println!中使用这个名字。

如果你不需要变量名，就用_。

fn main() {
    for _ in 0..3 {
        println!("Printing the same thing three times");
    }
}

这个打印:

Printing the same thing three times
Printing the same thing three times
Printing the same thing three times

因为我们每次都没有给它任何数字来打印。

而实际上，如果你给了一个变量名却不用，Rust会告诉你:

fn main() {
    for number in 0..3 {
        println!("Printing the same thing three times");
    }
}

这打印的内容和上面一样。程序编译正常，但Rust会提醒你没有使用number:

warning: unused variable: `number`
 --> src\main.rs:2:9
  |
2 |     for number in 0..3 {
  |         ^^^^^^ help: if this is intentional, prefix it with an underscore: `_number`

Rust 建议写 _number 而不是 _。在变量名前加上 _ 意味着 "也许我以后会用到它"。但是只用_意味着 "我根本不关心这个变量"。所以，如果你以后会使用它们，并且不想让编译器告诉你，你可以在变量名前面加上_。

你也可以用break来返回一个值。 你把值写在 break 之后，并使用 ;。下面是一个用 loop 和一个断点给出 my_number 值的例子。

fn main() {
    let mut counter = 5;
    let my_number = loop {
        counter +=1;
        if counter % 53 == 3 {
            break counter;
        }
    };
    println!("{}", my_number);
}

这时打印出56。break counter;的意思是 "中断并返回计数器的值"。而且因为整个块以let开始，所以my_number得到值。

现在我们知道了如何使用循环，这里有一个更好的解决方案来解决我们之前的颜色 "匹配"问题。这是一个更好的解决方案，因为我们要比较所有的东西，而 "for"循环会查看每一项。

fn match_colours(rbg: (i32, i32, i32)) {
    println!("Comparing a colour with {} red, {} blue, and {} green:", rbg.0, rbg.1, rbg.2);
    let new_vec = vec![(rbg.0, "red"), (rbg.1, "blue"), (rbg.2, "green")]; // Put the colours in a vec. Inside are tuples with the colour names
    let mut all_have_at_least_10 = true; // Start with true. We will set it to false if one colour is less than 10
    for item in new_vec {
        if item.0 < 10 {
            all_have_at_least_10 = false; // Now it's false
            println!("Not much {}.", item.1) // And we print the colour name.
        }
    }
    if all_have_at_least_10 { // Check if it's still true, and print if true
        println!("Each colour has at least 10.")
    }
    println!(); // Add one more line
}

fn main() {
    let first = (200, 0, 0);
    let second = (50, 50, 50);
    let third = (200, 50, 0);

    match_colours(first);
    match_colours(second);
    match_colours(third);
}

这个打印:

Comparing a colour with 200 red, 0 blue, and 0 green:
Not much blue.
Not much green.

Comparing a colour with 50 red, 50 blue, and 50 green:
Each colour has at least 10.

Comparing a colour with 200 red, 50 blue, and 0 green:
Not much green.