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<ol class="chapter"><li class="chapter-item expanded affix "><a href="title-page.html">Rust 程序设计语言</a></li><li class="chapter-item expanded affix "><a href="foreword.html">前言</a></li><li class="chapter-item expanded affix "><a href="ch00-00-introduction.html">简介</a></li><li class="chapter-item expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> 入门指南</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> 安装</a></li><li class="chapter-item expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="chapter-item expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="chapter-item expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> 写个猜数字游戏</a></li><li class="chapter-item expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> 常见编程概念</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> 变量与可变性</a></li><li class="chapter-item expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> 数据类型</a></li><li class="chapter-item expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> 函数</a></li><li class="chapter-item expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> 注释</a></li><li class="chapter-item expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> 控制流</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> 认识所有权</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> 什么是所有权？</a></li><li class="chapter-item expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> 引用与借用</a></li><li class="chapter-item expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slice 类型</a></li></ol></li><li class="chapter-item expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> 使用结构体组织相关联的数据</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> 结构体的定义和实例化</a></li><li class="chapter-item expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> 结构体示例程序</a></li><li class="chapter-item expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> 方法语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> 枚举和模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> 枚举的定义</a></li><li class="chapter-item expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> match 控制流结构</a></li><li class="chapter-item expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> if let 简洁控制流</a></li></ol></li><li class="chapter-item expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> 使用包、Crate 和模块管理不断增长的项目</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> 包和 Crate</a></li><li class="chapter-item expanded "><a h
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<main>
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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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<blockquote>
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch12-02-reading-a-file.md">ch12-02-reading-a-file.md</a>
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<br>
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commit 02a168ed346042f07010f8b65b4eeed623dd31d1</p>
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</blockquote>
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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 <code>poem.txt</code>，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>创建完这个文件之后，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，将会越来越难以用简单的方法修复它们。在开发程序时，及早开始重构是一个最佳实践，因为重构少量代码时要容易的多，所以让我们现在就开始吧。</p>
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