HashMap §

Init:

use std::collections::HasMap;
 
let mut my_gems = HashMap::new();
 
my_gems.insert("Ruby", 1);
my_gems.insert("Obsidian", 2);
my_gems.insert("Stone", 18);

Init hashmap with capacity：

let capacity = 10;
let hash_map = HashMap::with_capacity(capacity);

Vector to Hashmap:

use std::collections::HashMap;
let teams_list = vec![ 
    ("中国队".to_string(), 100), 
    ("美国队".to_string(), 10), 
    ("日本队".to_string(), 50), 
];
let teams_map: HashMap<_,_> = tems_list.into_iter().collect();
println!("{:?}",teams_map)

Get elements from Hashmap:

let mut scores = HashMap::new();
scores.insert(String::from("Blue"), 10);
scores.insert(String::from("Yellow"), 50);
 
let team_name = String::from("Blue");
let score: Option<&i32> = scores.get(&tema_name);

If we want to get value directly, instead of Option:

let score: i32 = scores.get(&team_name).copied().unwrap_or(0);
// or
if scores.contains_key("Blue") {
    let value = scores["Blue"];
}

For-loop:

for (key, value) in &scores {
    println!("{}: {}", key, value);
}

Update:

fn main() {
    use std::collections::HashMap;
 
    let mut scores = HashMap::new();
    scores.insert("Blue", 10);
 
    // 覆盖已有的值
    let old = scores.insert("Blue", 20);
    assert_eq!(old, Some(10));
 
    // 查询Yellow对应的值，若不存在则插入新值
    let v = scores.entry("Yellow").or_insert(5);
    assert_eq!(*v, 5); // 不存在，插入5
 
    // 查询Yellow对应的值，若不存在则插入新值
    let v = scores.entry("Yellow").or_insert(50);
    assert_eq!(*v, 5); // 已经存在，因此50没有插入
}