在 Rust 中，如何从 HashMap 的键创建 HashSet? [英] In Rust, how do I create a HashSet from the keys of a HashMap?

问题描述

我有两个 HashMap 并且想要计算键的交集.是否可以从 HashMap::keys() 返回的任何内容中构造一个 HashSet ?例如:

I have two HashMaps and want to compute the intersection of the keys. Is it possible to construct a HashSet out of whatever HashMap::keys() returns? For example:

use std::collections::{HashSet, HashMap};

fn main() {
    let mut map1: HashMap<i64, i64> = HashMap::new();
    let mut map2: HashMap<i64, i64> = HashMap::new();

    // Add some values into the HashMaps for demonstration
    map1.insert(1, 10);
    map1.insert(5, 50);
    map2.insert(3, 30);
    map2.insert(5, 50);

    let set1: HashSet<i64> = HashSet::from(map1.keys());  // How to do this?
    let set2: HashSet<i64> = HashSet::from(map2.keys());  // How to do this?

    let set3 = set1.intersection(&set2); // What I'm looking to accomplish
    // set3 should contain [5], as this is the one key shared by the two HashMaps
}

推荐答案

简单的解决方案

您的代码只需要进行一些调整即可实际编译(参见游乐场):

use std::collections::{HashSet, HashMap};

fn main() {
    let mut map1 = HashMap::new();
    let mut map2 = HashMap::new();

    // Add some values into the HashMaps for demonstration
    map1.insert(1, 10);
    map1.insert(5, 50);
    map2.insert(3, 30);
    map2.insert(5, 50);

    let set1: HashSet<i64> = map1.keys().cloned().collect();
    let set2: HashSet<i64> = map2.keys().cloned().collect();

    let set3 = set1.intersection(&set2);
    println!("{:?}", set3);
}

特别注意map1.keys().cloned().collect():

• HashMap::keys() 返回一个 Iterator,
• .cloned() 将其转换为 Iterator,
• .collect() 从中构建一个集合，因为 HashSet 实现了 FromIterator trait.

• HashMap<K, V>::keys() returns an Iterator<Item = &'a K>,
• .cloned() transforms that to an Iterator<Item = K>,
• .collect() builds a collection from that, since HashSet implements the FromIterator trait.

然而，这不是很有效:

• 复杂度明智:O(map1.size() + map2.size()).
• 内存方面:潜在的大量分配.

直接在HashMap的键上实现intersection.

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