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添加 Rust 语言圣经（镜像）
2 years ago
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<h1 id="使用迭代器来改进我们的程序"><a class="header" href="#使用迭代器来改进我们的程序">使用迭代器来改进我们的程序</a></h1>
<blockquote>
<p>本章节是可选内容，请大家在看完<a href="https://course.rs/advance/functional-programing/iterator.html">迭代器章节</a>后，再来阅读</p>
</blockquote>
<p>在之前的 <code>minigrep</code> 中，功能虽然已经 ok，但是一些细节上还值得打磨下，下面一起看看如何使用迭代器来改进 <code>Config::build</code> 和 <code>serach</code> 的实现。</p>
<h2 id="移除-clone-的使用"><a class="header" href="#移除-clone-的使用">移除 <code>clone</code> 的使用</a></h2>
<p>虽然之前有讲过为什么这里可以使用 <code>clone</code>，但是也许总有同学心有芥蒂，毕竟程序员嘛，都希望代码处处完美，而不是丑陋的处处妥协。</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl Config {
pub fn build(args: &amp;[String]) -&gt; Result&lt;Config, &amp;'static str&gt; {
if args.len() &lt; 3 {
return Err(&quot;not enough arguments&quot;);
}
let query = args[1].clone();
let file_path = args[2].clone();
let ignore_case = env::var(&quot;IGNORE_CASE&quot;).is_ok();
Ok(Config {
query,
file_path,
ignore_case,
})
}
}
<span class="boring">}
</span></code></pre></pre>
<p>之前的代码大致长这样，两行 <code>clone</code> 确实有点啰嗦，好在，在学习完迭代器后，我们知道了 <code>build</code> 函数实际上可以<strong>直接拿走迭代器的所有权</strong>，而不是去借用一个数组切 <code>&amp;[String]</code>。</p>
<p>这里先不给出代码，下面统一给出。</p>
<h2 id="直接使用返回的迭代器"><a class="header" href="#直接使用返回的迭代器">直接使用返回的迭代器</a></h2>
<p>在之前的实现中，我们的 <code>args</code> 是一个动态数组:</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
let args: Vec&lt;String&gt; = env::args().collect();
let config = Config::build(&amp;args).unwrap_or_else(|err| {
eprintln!(&quot;Problem parsing arguments: {err}&quot;);
process::exit(1);
});
// --snip--
}
</code></pre></pre>
<p>当时还提到了 <code>collect</code> 方法的使用，相信大家学完迭代器后，对这个方法会有更加深入的认识。</p>
<p>现在呢，无需数组了，直接传入迭代器即可：</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
let config = Config::build(env::args()).unwrap_or_else(|err| {
eprintln!(&quot;Problem parsing arguments: {err}&quot;);
process::exit(1);
});
// --snip--
}
</code></pre></pre>
<p>如上所示，我们甚至省去了一行代码，原因是 <code>env::args</code> 可以直接返回一个迭代器，再作为 <code>Config::build</code> 的参数传入，下面再来改写 <code>build</code> 方法。</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl Config {
pub fn build(
mut args: impl Iterator&lt;Item = String&gt;,
) -&gt; Result&lt;Config, &amp;'static str&gt; {
// --snip--
<span class="boring">}
</span></code></pre></pre>
<p>为了可读性和更好的通用性，这里的 <code>args</code> 类型并没有使用本身的 <code>std::env::Args</code> ，而是使用了特征约束的方式来描述 <code>impl Iterator&lt;Item = String&gt;</code>，这样意味着 <code>arg</code> 可以是任何实现了 <code>String</code> 迭代器的类型。</p>
<p>还有一点值得注意，由于迭代器的所有权已经转移到 <code>build</code> 内，因此可以直接对其进行修改，这里加上了 <code>mut</code> 关键字。</p>
<h2 id="移除数组索引的使用"><a class="header" href="#移除数组索引的使用">移除数组索引的使用</a></h2>
<p>数组索引会越界，为了安全性和简洁性，使用 <code>Iterator</code> 特征自带的 <code>next</code> 方法是一个更好的选择:</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl Config {
pub fn build(
mut args: impl Iterator&lt;Item = String&gt;,
) -&gt; Result&lt;Config, &amp;'static str&gt; {
// 第一个参数是程序名，由于无需使用，因此这里直接空调用一次
args.next();
let query = match args.next() {
Some(arg) =&gt; arg,
None =&gt; return Err(&quot;Didn't get a query string&quot;),
};
let file_path = match args.next() {
Some(arg) =&gt; arg,
None =&gt; return Err(&quot;Didn't get a file path&quot;),
};
let ignore_case = env::var(&quot;IGNORE_CASE&quot;).is_ok();
Ok(Config {
query,
file_path,
ignore_case,
})
}
}
<span class="boring">}
</span></code></pre></pre>
<p>喔，上面使用了迭代器和模式匹配的代码，看上去是不是很 Rust？我想我们已经走在了正确的道路上。</p>
<h2 id="使用迭代器适配器让代码更简洁"><a class="header" href="#使用迭代器适配器让代码更简洁">使用迭代器适配器让代码更简洁</a></h2>
<p>为了帮大家更好的回忆和对比，之前的 <code>search</code> 长这样：</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// in lib.rs
pub fn search&lt;'a&gt;(query: &amp;str, contents: &amp;'a str) -&gt; Vec&lt;&amp;'a str&gt; {
let mut results = Vec::new();
for line in contents.lines() {
if line.contains(query) {
results.push(line);
}
}
results
}
<span class="boring">}
</span></code></pre></pre>
<p>引入了迭代器后，就连古板的 <code>search</code> 函数也可以变得更 rusty 些:</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub fn search&lt;'a&gt;(query: &amp;str, contents: &amp;'a str) -&gt; Vec&lt;&amp;'a str&gt; {
contents
.lines()
.filter(|line| line.contains(query))
.collect()
}
<span class="boring">}
</span></code></pre></pre>
<p>Rock，让我们的函数编程 Style rock 起来，这种一行到底的写法有时真的让人沉迷。</p>
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
<p>至此，整个大章节全部结束，本章没有试图覆盖已学的方方面面( 也许未来会 )，而是聚焦于 Rust 的一些核心知识：所有权、生命周期、借用、模式匹配等等。</p>
<p>强烈推荐大家忘记已有的一切，自己重新实现一遍 <code>minigrep</code>，甚至可以根据自己的想法和喜好，来完善一些，也欢迎在评论中附上自己的练习项目，供其它人学习参考( 提个小建议，项目主页写清楚新增的功能、亮点等 )。</p>
<p>从下一章开始，我们将正式开始 Rust 进阶学习，请深呼吸一口，然后问自己：你..准备好了吗？</p>
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