Rust中文翻译7

Page 38

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.但是请稍等一下,我们刚才不是加了一个无限循环了么?是的.还记得我们之前讨论过parse()么?如果我们传入了一个非数字,我们会退出循环.看一下:

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哈哈!quit确实退出了.就像其他非数字的输入一样.这是我们次最优的方法.

现在让我们来真正退出游戏:

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;

             }
        }
    }
}

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在You win这一行结束的时候添加一个break,我们可以在赢得游戏的时候退出.退出循环也就意味着退出程序,因为那是我们main()函数的最后一行.我们还剩下最后一件事情纠结:当某人输入了非数字的时候,我们不想退出,而是想要忽略它.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;
             }
        }
    }
}

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修改的行是这里:

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

通过把ok().expect()语句转为match陈述式,我们可以实现将"错误时崩溃"转换成"错误时处理",这是我们的通用性做法.parse()方法的返回值是一个枚举类型,就像Ordering一样,但是在这里,每一个结果都有一些和他关联的数据:Ok是成功,Ree是失败.两者都有更多的信息:也就是一个成功整型转换,或者一个错误的类型.在此例中,当我们命中了Ok(num)分支,它会把Ok的值赋值给num变量,然后我们会在操作符右侧返回这个值.在Err分支里,我们不关心错误是什么,我们就只用了一个_来代替错误的名字.这将忽略这个错误,然后continue会让我们开始loop的下一次迭代.

现在,我们应该可以成功运行了!让我们试试吧:

Page 42

漂亮!还有最后一个小问题,我们就完成了猜数字游戏.你可以想想是什么问题么?对了,就是我们不要打印出那个秘密数字.它在测试时很好用,但是会毁了游戏.这里是最终版本:

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;
             }
        }
    }
}

Page 43

3.1.6 完成了!

终于,在这里我们成功完成了猜数字游戏!恭喜你!

我们的第一个工程像你展示了很多:let,match,方法,关联方法,使用外部crates等等.我们的下一个工程会展示更多东西.