[rustlings]11_hashmaps

HashMap<K, V> 类型储存了一个键类型 K 对应一个值类型 V 的映射。它通过一个哈希函数（hashing function）来实现映射，决定如何将键和值放入内存中。

HashMap

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use std::collections::HashMap;

fn main() {
    // 创建一个新的 HashMap
    let mut my_map: HashMap<String, i32> = HashMap::new();

    // 插入键值对
    my_map.insert(String::from("apple"), 3);
    my_map.insert(String::from("banana"), 2);
    my_map.insert(String::from("orange"), 5);

    // 查询值
    if let Some(value) = my_map.get("banana") {
        println!("The value of 'banana' is: {}", value);
    } else {
        println!("'banana' key not found");
    }

    // 更新值
    my_map.insert(String::from("apple"), 4);

    // 删除键值对
    my_map.remove("orange");

    // 遍历 HashMap
    for (key, value) in my_map.iter() {
        println!("{}: {}", key, value);
    }
}

https://rustwiki.org/zh-CN/book/ch08-03-hash-maps.html

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pub fn or_insert_with<F: FnOnce() -> V>(self, default: F) -> &'a mut V
如果为空，则通过插入默认函数的结果来确保该值在条目中，并返回对条目中值的可变引用。

use std::collections::HashMap;

let mut map: HashMap<&str, String> = HashMap::new();
let s = "hoho".to_string();

map.entry("poneyland").or_insert_with(|| s);

assert_eq!(map["poneyland"], "hoho".to_string());

hashmaps3.rs

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// A list of scores (one per line) of a soccer match is given. Each line is of
// the form "<team_1_name>,<team_2_name>,<team_1_goals>,<team_2_goals>"
// Example: "England,France,4,2" (England scored 4 goals, France 2).
//
// You have to build a scores table containing the name of the team, the total
// number of goals the team scored, and the total number of goals the team
// conceded.

use std::collections::HashMap;

// A structure to store the goal details of a team.
#[derive(Default)]
struct Team {
    goals_scored: u8,
    goals_conceded: u8,
}

fn build_scores_table(results: &str) -> HashMap<&str, Team> {
    // The name of the team is the key and its associated struct is the value.
    let mut scores = HashMap::new();

    for line in results.lines() {
        let mut split_iterator = line.split(',');
        // NOTE: We use `unwrap` because we didn't deal with error handling yet.
        let team_1_name = split_iterator.next().unwrap();
        let team_2_name = split_iterator.next().unwrap();
        let team_1_score: u8 = split_iterator.next().unwrap().parse().unwrap();
        let team_2_score: u8 = split_iterator.next().unwrap().parse().unwrap();

        // TODO: Populate the scores table with the extracted details.
        // Keep in mind that goals scored by team 1 will be the number of goals
        // conceded by team 2. Similarly, goals scored by team 2 will be the
        // number of goals conceded by team 1.
        scores.entry(team_1_name).or_insert_with(Team::default).goals_scored += team_1_score;
        scores.entry(team_1_name).or_insert_with(Team::default).goals_conceded += team_2_score;
        scores.entry(team_2_name).or_insert_with(Team::default).goals_scored += team_2_score;
        scores.entry(team_2_name).or_insert_with(Team::default).goals_conceded += team_1_score;
    }

    scores
}

fn main() {
    // You can optionally experiment here.
}

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

    const RESULTS: &str = "England,France,4,2
France,Italy,3,1
Poland,Spain,2,0
Germany,England,2,1
England,Spain,1,0";

    #[test]
    fn build_scores() {
        let scores = build_scores_table(RESULTS);

        assert!(["England", "France", "Germany", "Italy", "Poland", "Spain"]
            .into_iter()
            .all(|team_name| scores.contains_key(team_name)));
    }

    #[test]
    fn validate_team_score_1() {
        let scores = build_scores_table(RESULTS);
        let team = scores.get("England").unwrap();
        assert_eq!(team.goals_scored, 6);
        assert_eq!(team.goals_conceded, 4);
    }

    #[test]
    fn validate_team_score_2() {
        let scores = build_scores_table(RESULTS);
        let team = scores.get("Spain").unwrap();
        assert_eq!(team.goals_scored, 0);
        assert_eq!(team.goals_conceded, 3);
    }
}

参考：

https://github.com/rust-lang/rustlings

https://rustwiki.org/zh-CN/book/ch08-03-hash-maps.html