hashmap - 使用 Rust HashMap - .find() 没有返回匹配的预期值

Question

我想构建一个函数来计算元素在数组中出现的次数，但我无法解决编译时错误，这是我的代码

fn vec_count_elem(vec: Vec<int>) -> Vec<int> {
    let mut counts: HashMap<int, int> = HashMap::new();;
    let mut result: Vec<int>;

    for el in vec.iter() {
        match counts.find(el) {
            Some(count) => {
                    let new_count: int = count + 1;
                    counts.remove(el);
                    counts.insert(*el, new_count)
                }, 
            None => counts.insert(*el, 0)
        }
    }
    for value in counts.values() {
        result.push(*value);
    }

    return result;   
}

这是编译日志

/sorting/src/main.rs:40:9: 47:10 error: mismatched types: expected `()`, found `bool` (expected (), found bool)
/sorting/src/main.rs:40         match counts.find(el) {
/sorting/src/main.rs:41             Some(count) => {
/sorting/src/main.rs:42                     let new_count: int = count + 1;
/sorting/src/main.rs:43                     counts.remove(el);
/sorting/src/main.rs:44                     counts.insert(*el, new_count)
/sorting/src/main.rs:45                 },

看看这个例子 ( http://doc.rust-lang.org/std/collections/struct.HashMap.html#example )，counts.find(el) 应该为匹配运算符返回正确的数据类型

谢谢!

更新 1:第一个问题已解决(缺少 ;)，谢谢 Arjan!现在我的问题在于我在 match 子句中访问计数 HashMap ，我收到此错误:

sorting/src/main.rs:50:21: 50:27 error: cannot borrow `counts` as mutable because it is also borrowed as immutable
sorting/src/main.rs:50                     counts.remove(el);
                                                                                                                                                               ^~~~~~
sorting/src/main.rs:46:20: 46:26 note: previous borrow of `counts` occurs here; the immutable borrow prevents subsequent moves or mutable borrows of `counts` until the borrow ends

有什么想法可以做到这一点吗？

Answer 1

使用 HashMap 的 .entry() 方法，您可以制作一个非常紧凑的版本:

use std::collections::hashmap::{HashMap, Occupied, Vacant};

fn vec_count_elem(vec: Vec<int>) -> Vec<int> {
    let mut counts: HashMap<int, int> = HashMap::new();

    for el in vec.iter() {
        match counts.entry(*el) {
            Occupied(mut e) => { *e.get_mut() += 1; },
            Vacant(mut e) => { e.set(1); }
        }
    }

    return counts.values().map(|i| *i).collect();   
}