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<main>
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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>
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|
<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>
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<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>
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|
<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>
|
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<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>
|
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<p>使用循环:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::collections::HashMap;
|
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|
#[derive(Debug)]
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struct Account {
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id: u32,
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}
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fn main() {
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let accounts = [Account { id: 1 }, Account { id: 2 }, Account { id: 3 }];
|
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let mut resolvers = HashMap::new();
|
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for a in accounts {
|
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resolvers.entry(a.id).or_insert(Vec::new()).push(a);
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}
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println!("{:?}",resolvers);
|
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|
}</code></pre>
|
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<p>使用迭代器:</p>
|
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<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
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</span><span class="boring">fn main() {
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|
|
</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，IDE 和编译器都会对这种代码给出警告：iterators are lazy and do nothing unless consumed</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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