Rust猜数字之循环

循环

     loop关键字给我们一个无限循环。让我们添加它：

     extern crate rand;

     use std::io;
     use std::cmp::Ordering;
     use rand::Rng;

     fn main() {
         println!("Guess the number!");

         let secret_number = rand::thread_rng().gen_range(1, 101);

         println!("The secret number is: {}", secret_number);

         loop {
             println!("Please input your guess.");

             let mut guess = String::new();

             io::stdin().read_line(&mut guess)
                 .ok()
                 .expect("failed to read line");

             let guess: u32 = guess.trim().parse()
                 .ok()
                 .expect("Please type a number!");

             println!("You guessed: {}", guess);

             match guess.cmp(&secret_number) {
                 Ordering::Less    => println!("Too small!"),
                 Ordering::Greater => println!("Too big!"),
                 Ordering::Equal   => println!("You win!"),
             }
         }
     }

     试一下。但是等等，我们不是仅仅添加了一个死循环吗？是的，但是记住我们讨论的parse()了吗?如果我们给一个非数字答案，我们就爱那个会返回并推出。观察：

     $ cargo run
        Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
          Running `target/guessing_game`
     Guess the number!
     The secret number is: 59
     Please input your guess.
     45
     You guessed: 45
     Too small!
     Please input your guess.
     60
     You guessed: 60
     Too big!
     Please input your guess.
     59
     You guessed: 59
     You win!
     Please input your guess.
     quit
     thread '
' panicked at 'Please type a number!'

     哈！quit确实退出了。首先，当你赢的时候实现真正的退出。

     extern crate rand;

     use std::io;
     use std::cmp::Ordering;
     use rand::Rng;

     fn main() {
         println!("Guess the number!");

         let secret_number = rand::thread_rng().gen_range(1, 101);

         println!("The secret number is: {}", secret_number);

         loop {
             println!("Please input your guess.");

             let mut guess = String::new();

             io::stdin().read_line(&mut guess)
                 .ok()
                 .expect("failed to read line");

             let guess: u32 = guess.trim().parse()
                 .ok()
                 .expect("Please type a number!");

             println!("You guessed: {}", guess);

             match guess.cmp(&secret_number) {
                 Ordering::Less    => println!("Too small!"),
                 Ordering::Greater => println!("Too big!"),
                 Ordering::Equal   => {
                     println!("You win!");
                     break;
                 }
             }
         }
     }

     通过在You win!后面添加break，当我们胜利时将会退出循环。退出循环也意味着退出程序，因为它是main()中最后的事情。我们还要做一个修正：当有人输入的是非数字的时候，我们不想退出，我们只想忽略它。我们可以这样做：

     extern crate rand;

     use std::io;
     use std::cmp::Ordering;
     use rand::Rng;

     fn main() {
         println!("Guess the number!");

         let secret_number = rand::thread_rng().gen_range(1, 101);

         println!("The secret number is: {}", secret_number);

         loop {
             println!("Please input your guess.");

             let mut guess = String::new();

             io::stdin().read_line(&mut guess)
                 .ok()
                 .expect("failed to read line");

             let guess: u32 = match guess.trim().parse() {
                 Ok(num) => num,
                 Err(_) => continue,
             };

             println!("You guessed: {}", guess);

             match guess.cmp(&secret_number) {
                 Ordering::Less    => println!("Too small!"),
                 Ordering::Greater => println!("Too big!"),
                 Ordering::Equal   => {
                     println!("You win!");
                     break;
                 }
             }
         }
     }

     这些是改变的行：

     let guess: u32 = match guess.trim().parse() {
           Ok(num) => num,
           Err(_) => continue,
     };

     这是你通常从‘遇到错误退出’变成‘实际处理错误’，通过从ok().expect()转换成一个match声明。通过parse()返回的Result是一个枚举类型类似于Ordering，但是在这种情况下，每一中变化都有一些数据与之关联：Ok是成功，Err是失败。每个包含更多信息：正确解析整数或者一个错误类型。在这种情况下，我们匹配Ok(num)，将Ok的内部值设置成num，并且我们仅仅在右边返回它即可。在Err的情况下，我们不关心是什么样的错误，所以我们使用_代替一个名字。忽略错误，continue将会进入下一次循环迭代。

     现在应该是好使的！让我们试试：

     $ cargo run
        Compiling guessing_game v0.0.1 (file:///home/you/projects/guessing_game)
          Running `target/guessing_game`
     Guess the number!
     The secret number is: 61
     Please input your guess.
     10
     You guessed: 10
     Too small!
     Please input your guess.
     99
     You guessed: 99
     Too big!
     Please input your guess.
     foo
     Please input your guess.
     61
     You guessed: 61
     You win!

     非常了不起！还有一处需要修正，我们已经完成了猜数字的游戏。你能想出这一处是什么吗？对的，我们不想打印出神秘数字。对于测试它是好的，但是它毁掉了游戏。这是我们的最终代码：

     extern crate rand;

     use std::io;
     use std::cmp::Ordering;
     use rand::Rng;

     fn main() {
         println!("Guess the number!");

         let secret_number = rand::thread_rng().gen_range(1, 101);

         loop {
             println!("Please input your guess.");

             let mut guess = String::new();

             io::stdin().read_line(&mut guess)
                 .ok()
                 .expect("failed to read line");

             let guess: u32 = match guess.trim().parse() {
                 Ok(num) => num,
                 Err(_) => continue,
             };

             println!("You guessed: {}", guess);

             match guess.cmp(&secret_number) {
                 Ordering::Less    => println!("Too small!"),
                 Ordering::Greater => println!("Too big!"),
                 Ordering::Equal   => {
                     println!("You win!");
                     break;
                 }
             }
         }
     }

结束语

     到这里，你已经成功构建了猜数字游戏！恭喜！

     第一个项目展示了很多东西：let、match、方法、关联函数、使用外部crate等等。我们下一个项目将会展示更多。