rust - 如何在插入或更新值后获取 HashMap 中的键数？

Question

我想在map中插入或更新一个值，然后获取键的个数。

 use std::collections::HashMap;

fn main() {
    let mut map = HashMap::new();
    let count = map.entry("Tom").or_insert(0);
    *count += 1;

    let size = map.keys().len();
    println!("{} men found", size);
}

编译错误:

error[E0502]: cannot borrow `map` as immutable because it is also borrowed as mutable
  --> src/main.rs:8:16
   |
5  |     let count = map.entry("Tom").or_insert(0);
   |                 --- mutable borrow occurs here
...
8  |     let size = map.keys().len();
   |                ^^^ immutable borrow occurs here
9  |     println!("{} men found", size);
10 | }
   | - mutable borrow ends here

有什么办法可以解决这个问题吗？是我写错了吗？

Answer 1

选择以下之一:

1. 将 Rust 2018 或其他版本的 Rust 与 non-lexical lifetimes 结合使用:

   use std::collections::HashMap;
   
   fn main() {
       let mut map = HashMap::new();
       let count = map.entry("Tom").or_insert(0);
       *count += 1;
   
       let size = map.keys().len();
       println!("{} men found", size);
   }
   

2. 不要创建临时值:

   *map.entry("Tom").or_insert(0) += 1;
   

3. 添加一个 block 来限制借用:

   {
       let count = map.entry("Tom").or_insert(0);
       *count += 1;
   }