如何在可变的HashMap中更新值? [英] How can I update a value in a mutable HashMap?

问题描述

这就是我想要做的:

use std::collections::HashMap;

fn main() {
    let mut my_map = HashMap::new();
    my_map.insert("a", 1);
    my_map.insert("b", 3);

    my_map["a"] += 10;
    // I expect my_map becomes {"b": 3, "a": 11}
}

但这会引发错误:

2015年锈蚀

error[E0594]: cannot assign to immutable indexed content
 --> src/main.rs:8:5
  |
8 |     my_map["a"] += 10;
  |     ^^^^^^^^^^^^^^^^^ cannot borrow as mutable
  |
  = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `std::collections::HashMap<&str, i32>`

2018年锈蚀

error[E0594]: cannot assign to data in a `&` reference
 --> src/main.rs:8:5
  |
8 |     my_map["a"] += 10;
  |     ^^^^^^^^^^^^^^^^^ cannot assign

我真的不明白这意味着什么，因为我使HashMap可变了.当我尝试更新vector中的元素时，得到了预期的结果:

I don't really understand what that means, since I made the HashMap mutable. When I try to update an element in a vector, I get the expected result:

let mut my_vec = vec![1, 2, 3];

my_vec[0] += 10;
println! {"{:?}", my_vec};
// [11, 2, 3]

我收到上述错误与HashMap有什么不同?有没有更新值的方法?

What is different about HashMap that I am getting the above error? Is there a way to update a value?

推荐答案

不变的索引和可变的索引由两个不同的特征提供:

Indexing immutably and indexing mutably are provided by two different traits: Index and IndexMut, respectively.

当前， HashMap 未实现，而 Vec可以.

Currently, HashMap does not implement IndexMut, while Vec does.

删除了HashMap的IndexMut实现的提交状态:

The commit that removed HashMap's IndexMut implementation states:

此提交将删除HashMap和BTreeMap上的IndexMut隐式， 为了将来对API进行过时验证，以防最终包含 IndexSet特征.

This commit removes the IndexMut impls on HashMap and BTreeMap, in order to future-proof the API against the eventual inclusion of an IndexSet trait.

我的理解是，假设的IndexSet特征将使您可以为HashMap分配全新的值，而不仅仅是读取或变异现有条目:

It's my understanding that a hypothetical IndexSet trait would allow you to assign brand-new values to a HashMap, and not just read or mutate existing entries:

let mut map = HashMap::new();
map["key"] = "value";

目前，您可以使用 get_mut :

For now, you can use get_mut:

*my_map.get_mut("a").unwrap() += 10;

或 entry API :

Or the entry API:

*my_map.entry("a").or_insert(42) += 10;

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