rust - 借用 HashMap 而不借用他的内容

Question

我是新手，我已经阅读了有关所有权和借用的文档。显然，我开始为这个概念而苦苦挣扎...

这是我的代码:

#[derive(Serialize, Deserialize, Debug)]
pub struct City {
    name: String,
    dep: String,
    dynamic: String,
    population: u64, 
    cp: String,
    price_max: u8,
    price_mid: u8,
    price_min: u8,
    lng: f64,
    lat: f64,
    dist_near_city: Vec<(String, u64)>
}

fn load_cities<P: AsRef<Path>>(path: P) -> HashMap<String, City> {
    // Open the file in read-only mode with buffer.
    let file = File::open(path).expect("Unable to open file");
    let reader = BufReader::new(file);
    let retval: HashMap<String, City> = serde_json::from_reader(reader).expect("Unable to parse json");

    return retval;
}

fn update_nearby_cities(pin_city: &mut City, cities : &HashMap<String, City>, min_pop: u64, distKm: f64){
    pin_city.dist_near_city = Vec::new();
    for (id, city) in cities {
        if city.population > min_pop && is_nearby(&pin_city, &city, distKm){
            pin_city.dist_near_city.push((id.to_string(), city.population));
        }
    }
}


fn main(){
    let path  = "/home/marcel/Documents/immobilier/cities_export2.json";
    let mut cities = load_cities(path);

    for (cp, mut current_city) in &cities {
        update_nearby_cities(&mut current_city, &cities, 2000, 10.);
    }
}

我在main()循环中收到arg &mut current_city的错误:
cannot borrow data in a和 reference as mutable
首先，我不明白为什么我不能借用可变的引用，因为在我的代码的其他部分和文档( https://doc.rust-lang.org/rust-by-example/scope/borrow/mut.html)中都有可能

我认为这是因为 &cities上的循环为我提供了 current_city: &City，并且通过提供 &mut current_city我传递了一个 &mut &City，这是一个问题(我不知道如何处理这种类型，我什至不认为我理解这是什么。 ..)。

因此，如果我尝试仅传递 mut current_city，我会得到 Exepted expression。

我预见到的另一个问题是，我试图在循环中借用hashmap cite，然后在循环内重新借用它。

怎样正确地做这种事情，我对错误的看法是否正确？

Answer 1

for entry in &cities给出了对(＆K，＆V)的迭代器。如果您需要对(＆K，＆mut V)进行迭代的迭代器，请使用`＆mut city'

Even with this change code will not work. You are trying to iterate & modify cities at the same time.

一种无需进行大量重构的简单方法是一次计算所有结果，然后将它们稍后添加到哈希图中。

fn get_nearby_cities(pin_city: &City, cities : &HashMap<String, City>, min_pop: u64, distKm: f64) -> Vec<(String, u64)> {
    let mut dist_near_city = Vec::new();
    for (id, city) in cities {
        if city.population > min_pop && is_nearby(&pin_city, &city, distKm){
            dist_near_city.push((id.to_string(), city.population));
        }
    }

    dist_near_city
}


   # Compute all the required results. Unfortunately, I ended up clone the string. May be this could be avoided.
   let mut result = HashMap::new();
    for (cp, current_city) in &cities {
        result.insert(cp.clone(), get_nearby_cities(current_city, &cities, 2000, 10.));
    }

    for (city, nearby_cities) in result {
        cities.get_mut(&city).unwrap().dist_near_city.extend(nearby_cities);
    }