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添加 Rust 语言圣经（镜像）
2 years ago
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<h1 id="实现基本功能"><a class="header" href="#实现基本功能">实现基本功能</a></h1>
<p>无论功能设计的再怎么花里胡哨，对于一个文件查找命令而言，首先得指定文件和待查找的字符串，它们需要用户从命令行给予输入，然后我们在程序内进行读取。</p>
<h2 id="接收命令行参数"><a class="header" href="#接收命令行参数">接收命令行参数</a></h2>
<p>国际惯例，先创建一个新的项目 <code>minigrep</code> ，该名字充分体现了我们的自信：就是不如 <code>grep</code>。</p>
<pre><code class="language-shell">cargo new minigrep
Created binary (application) `minigrep` project
$ cd minigrep
</code></pre>
<p>首先来思考下，如果要传入文件路径和待搜索的字符串，那这个命令该长啥样，我觉得大概率是这样:</p>
<pre><code class="language-shell">cargo run -- searchstring example-filename.txt
</code></pre>
<p><code>--</code> 告诉 <code>cargo</code> 后面的参数是给我们的程序使用的，而不是给 <code>cargo</code> 自己使用，例如 <code>--</code> 前的 <code>run</code> 就是给它用的。</p>
<p>接下来就是在程序中读取传入的参数，这个很简单，下面代码就可以:</p>
<pre><pre class="playground"><code class="language-rust edition2021">// in main.rs
use std::env;
fn main() {
let args: Vec&lt;String&gt; = env::args().collect();
dbg!(args);
}
</code></pre></pre>
<p>首先通过 <code>use</code> 引入标准库中的 <code>env</code> 包，然后 <code>env::args</code> 方法会读取并分析传入的命令行参数，最终通过 <code>collect</code> 方法输出一个集合类型 <code>Vector</code>。</p>
<p>可能有同学疑惑，为啥不直接引入 <code>args</code> ，例如 <code>use std::env::args</code> ，这样就无需 <code>env::args</code> 来繁琐调用，直接<code>args.collect()</code> 即可。原因很简单，<code>args</code> 方法只会使用一次，啰嗦就啰嗦点吧，把相同的好名字让给 <code>let args..</code> 这位大哥不好吗？毕竟人家要出场多次的。</p>
<blockquote>
<h3 id="不可信的输入"><a class="header" href="#不可信的输入">不可信的输入</a></h3>
<p>所有的用户输入都不可信！不可信！不可信！</p>
<p>重要的话说三遍，我们的命令行程序也是，用户会输入什么你根本就不知道，例如他输入了一个非 Unicode 字符，你能阻止吗？显然不能，但是这种输入会直接让我们的程序崩溃！</p>
<p>原因是当传入的命令行参数包含非 Unicode 字符时， <code>std::env::args</code> 会直接崩溃，如果有这种特殊需求，建议大家使用 <code>std::env::args_os</code>，该方法产生的数组将包含 <code>OsString</code> 类型，而不是之前的 <code>String</code> 类型，前者对于非 Unicode 字符会有更好的处理。</p>
<p>至于为啥我们不用，两个理由，你信哪个：1. 用户爱输入啥输入啥，反正崩溃了，他就知道自己错了 2. <code>args_os</code> 会引入额外的跨平台复杂性 </p>
</blockquote>
<p><code>collect</code> 方法其实并不是<code>std::env</code>包提供的，而是迭代器自带的方法(<code>env::args()</code> 会返回一个迭代器)，它会将迭代器消费后转换成我们想要的集合类型，关于迭代器和 <code>collect</code> 的具体介绍，请参加<a href="https://course.rs/advance/functional-programing/iterator.html">这里</a>。</p>
<p>最后，代码中使用 <code>dbg!</code> 宏来输出读取到的数组内容，来看看长啥样：</p>
<pre><code class="language-shell">$ cargo run
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 0.61s
Running `target/debug/minigrep`
[src/main.rs:5] args = [
&quot;target/debug/minigrep&quot;,
]
</code></pre>
<pre><code class="language-shell">$ cargo run -- needle haystack
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 1.57s
Running `target/debug/minigrep needle haystack`
[src/main.rs:5] args = [
&quot;target/debug/minigrep&quot;,
&quot;needle&quot;,
&quot;haystack&quot;,
]
</code></pre>
<p>上面两个版本分别是无参数和两个参数，其中无参数版本实际上也会读取到一个字符串，仔细看，是不是长得很像我们的程序名，Bingo! <code>env::args</code> 读取到的参数中第一个就是程序的可执行路径名。</p>
<h2 id="存储读取到的参数"><a class="header" href="#存储读取到的参数">存储读取到的参数</a></h2>
<p>在编程中，给予清晰合理的变量名是一项基本功，咱总不能到处都是 <code>args[1]</code> 、<code>args[2]</code> 这样的糟糕代码吧。</p>
<p>因此我们需要两个变量来存储文件路径和待搜索的字符串:</p>
<pre><pre class="playground"><code class="language-rust edition2021">use std::env;
fn main() {
let args: Vec&lt;String&gt; = env::args().collect();
let query = &amp;args[1];
let file_path = &amp;args[2];
println!(&quot;Searching for {}&quot;, query);
println!(&quot;In file {}&quot;, file_path);
}
</code></pre></pre>
<p>很简单的代码，来运行下:</p>
<pre><code class="language-shell">$ cargo run -- test sample.txt
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 0.0s
Running `target/debug/minigrep test sample.txt`
Searching for test
In file sample.txt
</code></pre>
<p>输出结果很清晰的说明了我们的目标：在文件 <code>sample.txt</code> 中搜索包含 <code>test</code> 字符串的内容。</p>
<p>事实上，就算作为一个简单的程序，它也太过于简单了，例如用户不提供任何参数怎么办？因此，错误处理显然是不可少的，但是在添加之前，先来看看如何读取文件内容。</p>
<h2 id="文件读取"><a class="header" href="#文件读取">文件读取</a></h2>
<p>既然读取文件，那么首先我们需要创建一个文件并给予一些内容，来首诗歌如何？&quot;我啥也不是，你呢?&quot;</p>
<pre><code class="language-text">I'm nobody! Who are you?
我啥也不是，你呢？
Are you nobody, too?
牛逼如你也是无名之辈吗？
Then there's a pair of us - don't tell!
那我们就是天生一对，嘘！别说话！
They'd banish us, you know.
你知道，我们不属于这里。
How dreary to be somebody!
因为这里属于没劲的大人物！
How public, like a frog
他们就像青蛙一样呱噪，
To tell your name the livelong day
成天将自己的大名
To an admiring bog!
传遍整个无聊的沼泽！
</code></pre>
<p>在项目根目录创建 <code>poem.txt</code> 文件，并写入如上的优美诗歌(可能翻译的很烂，别打我，哈哈，事实上大家写入英文内容就够了)。</p>
<p>接下来修改 <code>main.rs</code> 来读取文件内容：</p>
<pre><pre class="playground"><code class="language-rust edition2021">use std::env;
use std::fs;
fn main() {
// --省略之前的内容--
println!(&quot;In file {}&quot;, file_path);
let contents = fs::read_to_string(file_path)
.expect(&quot;Should have been able to read the file&quot;);
println!(&quot;With text:\n{contents}&quot;);
}
</code></pre></pre>
<p>首先，通过 <code>use std::fs</code> 引入文件操作包，然后通过 <code>fs::read_to_string</code> 读取指定的文件内容，最后返回的 <code>contents</code> 是 <code>std::io::Result&lt;String&gt;</code> 类型。</p>
<p>运行下试试，这里无需输入第二个参数，因为我们还没有实现查询功能:</p>
<pre><code class="language-shell">$ cargo run -- the poem.txt
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 0.0s
Running `target/debug/minigrep the poem.txt`
Searching for the
In file poem.txt
With text:
I'm nobody! Who are you?
Are you nobody, too?
Then there's a pair of us - don't tell!
They'd banish us, you know.
How dreary to be somebody!
How public, like a frog
To tell your name the livelong day
To an admiring bog!
</code></pre>
<p>完美，虽然代码还有很多瑕疵，例如所有内容都在 <code>main</code> 函数，这个不符合软件工程，没有错误处理，功能不完善等。不过没关系，万事开头难，好歹我们成功迈开了第一步。</p>
<p>好了，是时候重构赚波 KPI 了，读者：are you serious? 这就开始重构了？</p>
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