如何插入结构内的哈希映射？ [复制]答案

【问题标题】：How do I insert into a hashmap inside a struct? [duplicate]如何插入结构内的哈希映射？ [复制]
【发布时间】：2020-09-30 03:29:19
【问题描述】：

我有几个状态要存储在各自的哈希图中。看起来像这样的东西：

let mut directory_hash: HashMap<String, String> = HashMap::new();
let mut component_hash: HashMap<String, String> = HashMap::new();
let mut directive_hash: HashMap<String, String> = HashMap::new();

我希望所有这些切片都保存在一个 app_state 对象中，我将像这样初始化它：

pub fn init_app_state() -> HashMap<&'static str, HashMap<String, String>> {
    let mut directory_hash: HashMap<String, String> = HashMap::new();
    let mut component_hash: HashMap<String, String> = HashMap::new();
    let mut directive_hash: HashMap<String, String> = HashMap::new();

    let mut tree_state: HashMap<&str, HashMap<String, String>> = HashMap::new();
    tree_state.insert("directory_hash", directory_hash);
    tree_state.insert("component_hash", component_hash);
    tree_state.insert("module_hash", module_hash);

    tree_state
}

let mut app_state = state::app_state::init_app_state();

现在假设我想插入其中一个嵌套的哈希图。我该怎么做？

以下是一些无效示例的 sn-p：

let mut app_state = state::app_state::init_app_state();
let mut dir_hash = &mut app_state["directory_hash"];
dir_hash.insert(String::from("1"), String::from("first entry"));

上面的错误在第二行(app_state["directory_hash"])

cannot borrow data in an index of `std::collections::HashMap<&str, std::collections::HashMap<std::string::String, std::string::String>>` as mutable

cannot borrow as mutable

help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `std::collections::HashMap<&str, std::collections::HashMap<std::string::String, std::string::String>>`rustc(E0596)

let mut app_state = state::app_state::init_app_state();
let mut dir_hash = &mut app_state.entry("directory_hash");
dir_hash.insert(String::from("1"), String::from("first entry"));

这里的错误在第 3 行 (dir_hash.insert(.....))

use of unstable library feature 'entry_insert'

note: see issue #65225 <https://github.com/rust-lang/rust/issues/65225> for more information

我查看了entry API，但我不确定这是否是要采取的方法。

如何更新这个嵌套的状态？

我可能会将这些 HashMap 实例中的每一个更改为 State 结构上的属性。但我仍然对上述方法在 Rust 中的工作方式感到好奇。

这是我目前采用的方法：

pub struct State {
    pub directory_hash: HashMap<String, String>,
    pub component_hash: HashMap<String, String>,
    pub module_hash: HashMap<String, String>,
}

impl State {
    pub fn new() -> State {
        let directory_hash: HashMap<String, String> = HashMap::new();
        let component_hash: HashMap<String, String> = HashMap::new();
        let module_hash: HashMap<String, String> = HashMap::new();

        State {
            directory_hash,
            component_hash,
            module_hash,
        }
    }
}


let app_state = state::app_state::State::new();
let mut dir_hash = app_state.directory_hash;
dir_hash.insert(String::from("1"), String::from("first value!"));

【问题讨论】：

标签： data-structures rust hashmap

【解决方案1】：

正如错误消息所说，HashMap doesn't implement IndexMut。使用get_mut 获取Option<&mut V>：

let mut app_state = state::app_state::init_app_state();
let dir_hash = app_state.get_mut("directory_hash").unwrap();
dir_hash.insert(String::from("1"), String::from("first entry"));

^(Playground)

【讨论】：