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【rCore OS 开源操作系统】Rust 枚举与模式匹配

article 2024/10/7 20:21:01

【rCore OS 开源操作系统】Rust 枚举与模式匹配

前言

目前虽然人在大阪，但是还是把笔记本是从来都带身上的。

每天玩了之后，晚上还可以学一学rust写写代码。

环球影城-鬼灭之刃
言归正传，本章节涉及到的知识点有：

枚举的应用：这里不再是基本语法，而是要学会用。
模式匹配实战：使用 match来处理问题。
语法糖：使用where let和if let来处理简化代码。
引用与借用：复习一下。

知识点

练习题

option1

题目

// options1.rs
//
// Execute `rustlings hint options1` or use the `hint` watch subcommand for a
// hint.

// I AM NOT DONE

// This function returns how much icecream there is left in the fridge.
// If it's before 10PM, there's 5 pieces left. At 10PM, someone eats them
// all, so there'll be no more left :(
fn maybe_icecream(time_of_day: u16) -> Option<u16> {
    // We use the 24-hour system here, so 10PM is a value of 22 and 12AM is a
    // value of 0 The Option output should gracefully handle cases where
    // time_of_day > 23.
    // TODO: Complete the function body - remember to return an Option!
    ???
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn check_icecream() {
        assert_eq!(maybe_icecream(9), Some(5));
        assert_eq!(maybe_icecream(10), Some(5));
        assert_eq!(maybe_icecream(23), Some(0));
        assert_eq!(maybe_icecream(22), Some(0));
        assert_eq!(maybe_icecream(25), None);
    }

    #[test]
    fn raw_value() {
        // TODO: Fix this test. How do you get at the value contained in the
        // Option?
        let icecreams = maybe_icecream(12);
        assert_eq!(icecreams, 5);
    }
}

题解

没有特别难的点，主要是搞懂题目的意思：

22 点之前，冰箱里都有 5 个冰淇淋🍦；
22 点及之后，冰箱里的冰淇淋都被吃掉🍽️；
0～24点之外的情况不存在，需要处理🙅；

fn maybe_icecream(time_of_day: u16) -> Option<u16> {
    if time_of_day > 24 || time_of_day < 0 {
        None
    } else if time_of_day < 22 {
        Some(5)
    } else {
        Some(0)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn check_icecream() {
        assert_eq!(maybe_icecream(9), Some(5));
        assert_eq!(maybe_icecream(10), Some(5));
        assert_eq!(maybe_icecream(23), Some(0));
        assert_eq!(maybe_icecream(22), Some(0));
        assert_eq!(maybe_icecream(25), None);
    }

    #[test]
    fn raw_value() {
        let icecreams = maybe_icecream(12);
        assert_eq!(icecreams.unwrap_or(0), 5);
    }
}

option2

题目

这里主要是要求把题目中涉及的模式匹配代码改成if let表达式。

// options2.rs
//
// Execute `rustlings hint options2` or use the `hint` watch subcommand for a
// hint.

// I AM NOT DONE

#[cfg(test)]
mod tests {
    #[test]
    fn simple_option() {
        let target = "rustlings";
        let optional_target = Some(target);

        // TODO: Make this an if let statement whose value is "Some" type
        word = optional_target {
            assert_eq!(word, target);
        }
    }

    #[test]
    fn layered_option() {
        let range = 10;
        let mut optional_integers: Vec<Option<i8>> = vec![None];

        for i in 1..(range + 1) {
            optional_integers.push(Some(i));
        }

        let mut cursor = range;

        // TODO: make this a while let statement - remember that vector.pop also
        // adds another layer of Option<T>. You can stack `Option<T>`s into
        // while let and if let.
        integer = optional_integers.pop() {
            assert_eq!(integer, cursor);
            cursor -= 1;
        }

        assert_eq!(cursor, 0);
    }
}

题解

#[cfg(test)]
mod tests {
    #[test]
    fn simple_option() {
        let target = "rustlings";
        let optional_target = Some(target);

        if let optional_target = target {
            let word = optional_target;
            assert_eq!(word, target)
        }
    }

    #[test]
    fn layered_option() {
        let range = 10;
        let mut optional_integers: Vec<Option<i8>> = vec![None];

        for i in 1..(range + 1) {
            optional_integers.push(Some(i));
        }

        let mut cursor = range;

		// where let 能依次匹配每一个 vec item
        while let Some(maybe_integer) = optional_integers.pop() {
        	// 由于 optional_integers 的 item 每个都是 Option
        	// 所以这还得用 if let 再比较一次
            if let Some(integer) = maybe_integer {
                assert_eq!(integer, cursor);
                cursor -= 1;
            } else {
                // 如果 maybe_integer 是 None，跳过这一轮循环
                continue;
            }
        }

        assert_eq!(cursor, 0);
    }
}

option3

题目

// options3.rs
//
// Execute `rustlings hint options3` or use the `hint` watch subcommand for a
// hint.

// I AM NOT DONE

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let y: Option<Point> = Some(Point { x: 100, y: 200 });

    match y {
        Some(p) => println!("Co-ordinates are {},{} ", p.x, p.y),
        _ => panic!("no match!"),
    }
    y; // Fix without deleting this line.
}

题解

题目代码的报错信息中已经说得很明白了：

⚠️  Compiling of exercises/options/options3.rs failed! Please try again. Here's the output:
error[E0382]: use of partially moved value: `y`
  --> exercises/options/options3.rs:20:5
   |
17 |         Some(p) => println!("Co-ordinates are {},{} ", p.x, p.y),
   |              - value partially moved here
...
20 |     y; // Fix without deleting this line.
   |     ^ value used here after partial move
   |
   = note: partial move occurs because value has type `Point`, which does not implement the `Copy` trait
help: borrow this binding in the pattern to avoid moving the value
   |
17 |         Some(ref p) => println!("Co-ordinates are {},{} ", p.x, p.y),
   |              +++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0382`.

注意其中这一段：
Rust报错 E0382 help
所以问题就应迎刃而解了，使用引用实现借用即可。

// options3.rs
//
// Execute `rustlings hint options3` or use the `hint` watch subcommand for a
// hint.

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let y: Option<Point> = Some(Point { x: 100, y: 200 });
	// 这里改成引用，这样所有权就不会转移而是被借用
    match &y {
        Some(p) => println!("Co-ordinates are {},{} ", p.x, p.y),
        _ => panic!("no match!"),
    }
    y; // Fix without deleting this line.
}