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<h2 id="读取文件"><a class="header" href="#读取文件">读取文件</a></h2>
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<!-- https://github.com/rust-lang/book/blob/main/src/ch12-02-reading-a-file.md -->
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<!-- commit 3a30e4c1fbe641afc066b3af9eb01dcdf5ed8b24 -->
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<p>现在我们要增加读取由 <code>file_path</code> 命令行参数指定的文件的功能。首先，需要一个用来测试的示例文件：我们会用一个拥有多行少量文本且有一些重复单词的文件。示例 12-3 是一首艾米莉·狄金森（Emily Dickinson）的诗，它正适合这个工作！在项目根目录创建一个文件 <em>poem.txt</em>，并输入诗 "I'm nobody! Who are you?"：</p>
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<p><span class="filename">文件名：poem.txt</span></p>
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<pre><code class="language-text">I'm nobody! Who are you?
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Are you nobody, too?
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Then there's a pair of us - don't tell!
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They'd banish us, you know.
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How dreary to be somebody!
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How public, like a frog
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To tell your name the livelong day
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To an admiring bog!
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</code></pre>
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<p><span class="caption">示例 12-3：艾米莉·狄金森的诗 “I’m nobody! Who are you?”，一个好的测试用例</span></p>
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<p>有了文本后，修改 <em>src/main.rs</em> 并增加如示例 12-4 所示的打开文件的代码：</p>
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<p><span class="filename">文件名：src/main.rs</span></p>
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<pre><code class="language-rust should_panic noplayground">use std::env;
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use std::fs;
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fn main() {
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// --snip--
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<span class="boring"> let args: Vec<String> = env::args().collect();
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</span><span class="boring">
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</span><span class="boring"> let query = &args[1];
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</span><span class="boring"> let file_path = &args[2];
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</span><span class="boring">
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</span><span class="boring"> println!("Searching for {query}");
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</span> println!("In file {file_path}");
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let contents = fs::read_to_string(file_path)
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.expect("Should have been able to read the file");
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println!("With text:\n{contents}");
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}</code></pre>
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<p><span class="caption">示例 12-4：读取第二个参数所指定的文件内容</span></p>
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<p>首先，我们增加了一个 <code>use</code> 语句来引入标准库中的相关部分：我们需要 <code>std::fs</code> 来处理文件。</p>
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<p>在 <code>main</code> 中新增了一行语句：<code>fs::read_to_string</code> 接受 <code>file_path</code>，打开文件，接着返回包含其内容的 <code>std::io::Result<String></code>。</p>
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<p>在这些代码之后，我们再次增加了临时的 <code>println!</code> 打印出读取文件之后 <code>contents</code> 的值，这样就可以检查目前为止的程序能否工作。</p>
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<p>尝试运行这些代码，随意指定一个字符串作为第一个命令行参数（因为还未实现搜索功能的部分）而将 <em>poem.txt</em> 文件将作为第二个参数：</p>
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<pre><code class="language-console">$ cargo run -- the poem.txt
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Compiling minigrep v0.1.0 (file:///projects/minigrep)
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Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.0s
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Running `target/debug/minigrep the poem.txt`
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Searching for the
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In file poem.txt
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With text:
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I'm nobody! Who are you?
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Are you nobody, too?
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Then there's a pair of us - don't tell!
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They'd banish us, you know.
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How dreary to be somebody!
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How public, like a frog
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To tell your name the livelong day
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To an admiring bog!
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</code></pre>
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<p>好的！代码读取并打印出了文件的内容。虽然它还有一些瑕疵：此时 <code>main</code> 函数有着多个职能，通常函数只负责一个功能的话会更简洁并易于维护。另一个问题是没有尽可能的处理错误。虽然我们的程序还很小，这些瑕疵并不是什么大问题，不过随着程序功能的丰富，要干净地修复它们就会越来越困难。在开发程序时，及早开始重构是一个良好实践，因为重构少量代码时要容易的多。接下来我们就来进行重构。</p>
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