在 Rust 中合并两个 HashMap [英] Merge two HashMaps in Rust

问题描述

所以我有点卡住了，试图合并两个 HashMap.

So I'm a bit stuck, trying to merge two HashMaps.

内联很容易做到:

fn inline() {
    let mut first_context = HashMap::new();
    first_context.insert("Hello", "World");
    let mut second_context = HashMap::new();
    second_context.insert("Hey", "There");

    let mut new_context = HashMap::new();
    for (key, value) in first_context.iter() {
        new_context.insert(*key, *value);
    }
    for (key, value) in second_context.iter() {
        new_context.insert(*key, *value);
    }
    println!("Inline:\t\t{}", new_context);
    println!("Inline:\t\t{}\t{} [Initial Maps Still Usable]", first_context, second_context);
}

创建一个函数很容易:

fn abstracted() {
    fn merge<'a>(first_context: &HashMap<&'a str, &'a str>, second_context: &HashMap<&'a str, &'a str>) -> HashMap<&'a str, &'a str> {
        let mut new_context = HashMap::new();
        for (key, value) in first_context.iter() {
            new_context.insert(*key, *value);
        }
        for (key, value) in second_context.iter() {
            new_context.insert(*key, *value);
        }
        new_context
    }

    let mut first_context = HashMap::new();
    first_context.insert("Hello", "World");
    let mut second_context = HashMap::new();
    second_context.insert("Hey", "There");

    println!("Abstracted:\t{}", merge(&first_context, &second_context));
    println!("Abstracted:\t{}\t{} [Initial Maps Still Usable]", first_context, second_context);
}

但是，我似乎无法让通用版本正常工作:

However, I can't seem to get the generic version to work:

fn generic() {
    fn merge<'a, K: Hash + Eq, V>(first_context: &HashMap<&'a K, &'a V>, second_context: &HashMap<&'a K, &'a V>) -> HashMap<&'a K, &'a V> {
        let mut new_context = HashMap::new();
        for (key, value) in first_context.iter() {
            new_context.insert(*key, *value);
        }
        for (key, value) in second_context.iter() {
            new_context.insert(*key, *value);
        }
        new_context
    }

    let mut first_context = HashMap::new();
    first_context.insert("Hello", "World");
    let mut second_context = HashMap::new();
    second_context.insert("Hey", "There");

    println!("Generic:\t{}", merge(&first_context, &second_context));
    println!("Generic:\t{}\t{} [Initial Maps Still Usable]", first_context, second_context);
}

play.rust-lang.org 上的上述代码.

编译:

error: the trait `core::kinds::Sized` is not implemented for the type `str`

我知道编译器对通用值的大小感到困惑，但我不确定为什么str"没有严格的内存大小?我知道它是一个字符串切片而不是一个类型，但这仍然应该有效，不是吗?这是一个错误吗?

I get that the compiler is confused about the size of the generic value, but I'm not sure why "str" doesn't have a strict memory size? I know its a String slice and not a type, but still this should work, no? Is this a bug?

我认为这将是一个相对微不足道的功能.如果有人有好的解决方案，我很乐意学习.实际上，理想情况下，我希望看到一个带有 trait Mergeable 的解决方案，并为 HashMap<&K, &V> 编写一个装饰器，这样我就可以调用 let new_context = first_context.merge(&second_context); 但这可能是一个不同的问题.

I thought this would be a relatively trivial function. If someone has a good solution, I'd love to learn. Actually ideally, I'd love to see a solution with a trait Mergeable and write a decorator for HashMap<&K, &V>, such that I can call let new_context = first_context.merge(&second_context); but this can be a different question.

推荐答案

来自 this 的最新答案推文:

use std::collections::HashMap;

// Mutating one map
fn merge1(map1: &mut HashMap<(), ()>, map2: HashMap<(), ()>) {
    map1.extend(map2);
}

// Without mutation
fn merge2(map1: HashMap<(), ()>, map2: HashMap<(), ()>) -> HashMap<(), ()> {
    map1.into_iter().chain(map2).collect()
}

// If you only have a reference to the map to be merged in
fn merge_from_ref(map: &mut HashMap<(), ()>, map_ref: &HashMap<(), ()>) {
    map.extend(map_ref.into_iter().map(|(k, v)| (k.clone(), v.clone())));
}

Rust Playground 链接

这篇关于在 Rust 中合并两个 HashMap的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！