通过与其他HashSet交叉更新原始HashSet，而不是创建新的HashSet

Question

Rust的HashSet::intersection函数创建了一个新的HashSet。如何通过两个HashSet交集的结果来更新原始HashSets？

fn main() {
    // intersecting two HashSets, creating a new HashSet: OK
    use std::collections::HashSet;
    let a = HashSet::from(["first", "second", "third"]);
    let b = HashSet::from(["first", "third", "fifth", "ninth"]);
    let intersection: HashSet<_> = a.intersection(&b).collect();
    println!("{:?}", intersection);

    
    // intersecting two HashSets, updating the original HashSet: Failed
    let mut a = HashSet::from(["first", "second", "third"]);
    let b = HashSet::from(["first", "third", "fifth", "ninth"]);
    println!("{:?}", a.intersection(&b).collect::<HashSet<_>>());
    println!("{:?}", a);
    a = a.intersection(&b).collect::<HashSet<_>>();  // doesn't work

    // filter: verbose
    let mut a = HashSet::from(["first", "second", "third"]);
    let b = HashSet::from(["first", "third", "fifth", "ninth"]);
    a = a.iter().filter(|&s| b.contains(s)).map(|s| *s).collect::<HashSet<_>>();
    println!("{:?}", a);
}

Answer 1

要做到这一点而不分配新的HashSet，您可以使用HashSet::retain，并且只保留第二个HashSet中存在的元素。

fn main() {
    use std::collections::HashSet;
    let mut a = HashSet::from(["first", "second", "third"]);
    let b = HashSet::from(["first", "third", "fifth", "ninth"]);
    println!("{:?}", a.intersection(&b).collect::<HashSet<_>>());
    a.retain(|x| b.contains(x));
    println!("{:?}", a);
}

输出：

{"third", "first"}
{"first", "third"}

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