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< h1 id = "不太勤快的迭代器" > < a class = "header" href = "#不太勤快的迭代器" > 不太勤快的迭代器< / a > < / h1 >
< p > 迭代器，在 Rust 中是一个非常耀眼的存在，它光鲜亮丽，它让 Rust 大道至简，它备受用户的喜爱。可是，它也是懒惰的，不信？一起来看看。< / p >
< h2 id = "for-循环-vs-迭代器" > < a class = "header" href = "#for-循环-vs-迭代器" > for 循环 vs 迭代器< / a > < / h2 >
< p > 在迭代器学习中，我们提到过迭代器在功能上可以替代循环，性能上略微优于循环(避免边界检查),安全性上优于循环，因此在 Rust 中，迭代器往往都是更优的选择，前提是迭代器得发挥作用。< / p >
< p > 在下面代码中，分别是使用< code > for< / code > 循环和迭代器去生成一个< code > HashMap< / code > 。< / p >
< p > 使用循环:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::collections::HashMap;
#[derive(Debug)]
struct Account {
id: u32,
}
fn main() {
let accounts = [Account { id: 1 }, Account { id: 2 }, Account { id: 3 }];
let mut resolvers = HashMap::new();
for a in accounts {
resolvers.entry(a.id).or_insert(Vec::new()).push(a);
}
println!("{:?}",resolvers);
}< / code > < / pre >
< p > 使用迭代器:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let mut resolvers = HashMap::new();
accounts.into_iter().map(|a| {
resolvers
.entry(a.id)
.or_insert(Vec::new())
.push(a);
});
println!("{:?}",resolvers);
< span class = "boring" > }< / span > < / code > < / pre >
< h4 id = "预料之外的结果" > < a class = "header" href = "#预料之外的结果" > 预料之外的结果< / a > < / h4 >
< p > 两端代码乍一看(很多时候我们快速浏览代码的时候，不会去细看)都很正常, 运行下试试:< / p >
< ul >
< li > < code > for< / code > 循环很正常，输出< code > {2: [Account { id: 2 }], 1: [Account { id: 1 }], 3: [Account { id: 3 }]}< / code > < / li >
< li > 迭代器很。。。不正常，输出了一个< code > {}< / code > , 黑人问号< code > ? ?< / code > < strong > ?< / strong > < / li >
< / ul >
< p > 在继续深挖之前，我们先来简单回顾下迭代器。< / p >
< h2 id = "回顾下迭代器" > < a class = "header" href = "#回顾下迭代器" > 回顾下迭代器< / a > < / h2 >
< p > 在迭代器章节中，我们曾经提到过，迭代器的< a href = "https://course.rs/advance/functional-programing/iterator.html#消费者与适配器" > 适配器< / a > 分为两种：消费者适配器和迭代器适配器，前者用来将一个迭代器变为指定的集合类型，往往通过< code > collect< / code > 实现；后者用于生成一个新的迭代器，例如上例中的< code > map< / code > 。< / p >
< p > 还提到过非常重要的一点: < strong > 迭代器适配器都是懒惰的，只有配合消费者适配器使用时，才会进行求值< / strong > .< / p >
< h2 id = "懒惰是根因" > < a class = "header" href = "#懒惰是根因" > 懒惰是根因< / a > < / h2 >
< p > 在我们之前的迭代器示例中，只有一个迭代器适配器< code > map< / code > :< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > accounts.into_iter().map(|a| {
resolvers
.entry(a.id)
.or_insert(Vec::new())
.push(a);
});
< span class = "boring" > }< / span > < / code > < / pre >
< p > 首先, < code > accounts< / code > 被拿走所有权后转换成一个迭代器，其次该迭代器通过< code > map< / code > 方法生成一个新的迭代器，最后，在此过程中没有以类如< code > collect< / code > 的消费者适配器收尾。< / p >
< p > 因此在上述过程中，< code > map< / code > 完全是懒惰的，它没有做任何事情，它在等一个消费者适配器告诉它：赶紧起床，任务可以开始了，它才会开始行动。< / p >
< p > 自然，我们的插值计划也失败了。< / p >
< blockquote >
< p > 事实上， ： < / p >
< / blockquote >
< h2 id = "解决办法" > < a class = "header" href = "#解决办法" > 解决办法< / a > < / h2 >
< p > 原因非常清晰，如果读者还有疑惑，建议深度了解下上面给出的迭代器链接，我们这里就不再赘述。< / p >
< p > 下面列出三种合理的解决办法：< / p >
< ol >
< li > 不再使用迭代器适配器< code > map< / code > ，改成< code > for_each< / code > :< / li >
< / ol >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let mut resolvers = HashMap::new();
accounts.into_iter().for_each(|a| {
resolvers
.entry(a.id)
.or_insert(Vec::new())
.push(a);
});
< span class = "boring" > }< / span > < / code > < / pre >
< p > 但是，相关的文档也友善的提示了我们，除非作为链式调用的收尾，否则更建议使用< code > for< / code > 循环来处理这种情况。哎，忙忙碌碌，又回到了原点，不禁让人感叹：天道有轮回。< / p >
< ol start = "2" >
< li > 使用消费者适配器< code > collect< / code > 来收尾，将< code > map< / code > 产生的迭代器收集成一个集合类型:< / li >
< / ol >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let resolvers: HashMap< _, _> = accounts
.into_iter()
.map(|a| (a.id, a))
.collect();
< span class = "boring" > }< / span > < / code > < / pre >
< p > 嗯，还挺简洁，挺< code > rusty< / code > .< / p >
< ol start = "3" >
< li > 使用< code > fold< / code > ，语义表达更强:< / li >
< / ol >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let resolvers = account.into_iter().fold(HashMap::new(), |mut resolvers, a|{
resolvers.entry(a.id).or_insert(Vec::new()).push(a);
resolvers
});
< span class = "boring" > }< / span > < / code > < / pre >
< h2 id = "总结" > < a class = "header" href = "#总结" > 总结< / a > < / h2 >
< p > 在使用迭代器时，要清晰的认识到需要用到的方法是迭代型还是消费型适配器，如果一个调用链中没有以消费型适配器结尾，就需要打起精神了，也许，不远处就是一个陷阱在等你跳:)< / p >
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