collections/hashmap

[giscus[bot]]

collections/hashmap

Learning Rust By Practice, narrowing the gap between beginner and skilled-dev with challenging examples, exercises and projects.

https://zh.practice.rs/collections/hashmap.html

[quarksb]

题目2 的答案有误
let teams_map2: HashMap<,> = teams.into_iter().collect();
collect 会报错， value of type HashMap<&str, i32> cannot be built from std::iter::Iterator<Item=&(&str, i32)>
需改成 let teams_map2: HashMap<,> = teams.iter().cloned().collect();
原理我还在研究

[quarksb]

个人猜测：
collect 会涉及到所有权的转换问题，需要先 cloned()/copid()，加完 clone(),会有个 warning
warning: this method call resolves to <&[T; N] as IntoIterator>::into_iter (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
into_iter -> iter 这 warning 没看懂

[quarksb]

…… 这是edition = "2018" 才有的问题，edition = "2021" 就没这问题了

[lemonbiscuits-ljw]

第五题从哪里可以体现出需要自反性呢？

[ZLD1819858]

第五题不是所有权的问题吗？自反性是啥

[roushanyyzz]

自反性：自己 == 自己，如果是浮点数的话就不满足自反性

[wangshankun]

有点奇怪，
vec使用into_iter()时候是会消耗vec的，为什么下面这句话没有把teams所有权给消耗掉？
let teams_map2 = teams.into_iter().collect(); //

而使用下面iter()代替into_iter()时候反而出现了，上面@quarksb
说的edition = "2018" 才有的问题
let teams_map2 = teams.iter().collect(); //

[twandylue]

Here are the reasons:

1. teams is a array, not a vector.
2. array is primitive type, and it implements Copy and Clone traits in default.

Thus, teams.into_iter() would not consume the ownership. In fact, it copy the array and convert the copied array into a new iterator.

If I missed anything, please let me know. THX!

References:

1. array

[lxhzzy06]

@twandylue Thanks your answer!

[3517277]

楼上是对的，into_iter()确实会取得所有权，但如果每个元素都实现了copy,那它会直接复制，而不会移动。

[Bruce012356]

mark

[XU0618]

lala

[Ghy0202]

1、HashMap的基本操作

use std::collections::HashMap;
fn main() {
    let mut scores = HashMap::new();
    //需要保持<key,value>类型一致
    scores.insert("Sunface", 98);
    scores.insert("Daniel", 95);
    scores.insert("Ashley", 69);
    scores.insert("Katie", 58);

    // get 返回一个 Option<&V> 枚举值
    let score = scores.get("Sunface");
    assert_eq!(score, Some(&98));

    if scores.contains_key("Daniel") {
        // 索引返回一个值 V
        let score = scores["Daniel"];
        assert_eq!(score, 95);
        scores.remove("Daniel");
    }

    assert_eq!(scores.len(), 3);

    for (name, score) in scores {
        println!("The score of {} is {}", name, score)
    }
}

2、

(1)collect的用法

use std::collections::HashMap;
fn main() {
    let teams = [
        ("Chinese Team", 100),
        ("American Team", 10),
        ("France Team", 50),
    ];

    let mut teams_map1 = HashMap::new();
    for team in &teams {
        teams_map1.insert(team.0, team.1);
    }

    // 使用两种方法实现 team_map2
    // 提示:其中一种方法是使用 `collect` 方法
    let teams_map2:HashMap<_,_>=teams.into_iter().collect();

    assert_eq!(teams_map1, teams_map2);

    println!("Success!")
}

(2)暴力复制

use std::collections::HashMap;
fn main() {
    let teams = [
        ("Chinese Team", 100),
        ("American Team", 10),
        ("France Team", 50),
    ];

    let mut teams_map1 = HashMap::new();
    for team in &teams {
        teams_map1.insert(team.0, team.1);
    }

    // 使用两种方法实现 team_map2
    // 提示:其中一种方法是使用 `collect` 方法
    let teams_map2=teams_map1.clone();

    assert_eq!(teams_map1, teams_map2);

    println!("Success!")
}

3、理解

// 填空
use std::collections::HashMap;
fn main() {
    // 编译器可以根据后续的使用情况帮我自动推断出 HashMap 的类型，当然你也可以显式地标注类型：HashMap<&str, u8>
    let mut player_stats = HashMap::new();

    // 查询指定的 key, 若不存在时，则插入新的 kv 值
    player_stats.entry("health").or_insert(100);

    assert_eq!(player_stats["health"], 100);

    // 通过函数来返回新的值
    player_stats.entry("health").or_insert_with(random_stat_buff);
    assert_eq!(player_stats["health"], 100);

    let health = player_stats.entry("health").or_insert(50);
    assert_eq!(*health, 100);
    *health -= 50;
    assert_eq!(*health, 50);

    println!("Success!")
}

fn random_stat_buff() -> u8 {
    // 为了简单，我们没有使用随机，而是返回一个固定的值
    42
}

4、

// 修复错误
// 提示: `derive` 是实现一些常用特征的好办法
use std::collections::HashMap;
#[derive(Eq,Hash,PartialEq,Debug)]
struct Viking {
    name: String,
    country: String,
}

impl Viking {
    fn new(name: &str, country: &str) -> Viking {
        Viking {
            name: name.to_string(),
            country: country.to_string(),
        }
    }
}

fn main() {
    // 使用 HashMap 来存储 viking 的生命值
    let vikings = HashMap::from([
        (Viking::new("Einar", "Norway"), 25),
        (Viking::new("Olaf", "Denmark"), 24),
        (Viking::new("Harald", "Iceland"), 12),
    ]);

    // 使用 derive 的方式来打印 viking 的当前状态
    for (viking, health) in &vikings {
        println!("{:?} has {} hp", viking, health);
    }
}

5、所有权会被 HashMap调用

// 修复错误，尽可能少的去修改代码
// 不要移除任何代码行！
use std::collections::HashMap;
fn main() {
  let v1 = 10;
  let mut m1 = HashMap::new();
  m1.insert(v1, v1);
  println!("v1 is still usable after inserting to hashmap : {}", v1);

  let v2 = "hello".to_string();
  let mut m2 = HashMap::new();
  // 所有权在这里发生了转移
  m2.insert(v2.clone(), v1);

  assert_eq!(v2, "hello");

   println!("Success!")
}

[3517277]

第四题不明白，struct Viking {
name: String,
country: String,
}内都是String,String原来就有实现Hash和Eq，那这个结构体应该也是自动实现了Viking才对。为什么仍然要加#[derive(Hash, Eq, PartialEq, Debug)]才不报错呢？

[urnotsukilee]

应该是结构体不会自动实现，而集合会

[chenhai998]

done

[felipe300]

• done

[karapy]

three way to solve question number 2:

use std::collections::HashMap;

fn main() {
let teams : [(&str, i32); 3] = [
("Chinese Team", 100),
("American Team", 10),
("France Team", 50),
];

let mut teams_map1 = HashMap::new();
for team in &teams {
    teams_map1.insert(team.0, team.1);
}

// IMPLEMENT team_map2 in two ways
// Tips: one of the approaches is to use `collect` method
// let teams_map2 : HashMap<&str, i32> = teams.into();
// let teams_map2 : HashMap<&str, i32> = teams.into_iter().collect();
let teams_map2 : HashMap<&str, i32> = HashMap::from(teams);

assert_eq!(teams_map1, teams_map2);

println!("Success!");

}

[DylanJinx]

done~~~

[AllenWu233]

fn random_stat_buff() -> u8 {
    // 为了简单，我们没有使用随机，而是返回一个固定的值
    42
}

宇宙的终极答案（

[TDAkory]

mark finished

[bansal-harsh-2504]

Day 10
Done✅