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<ul class="chapter"><li><a href="ch01-00-introduction.html"><strong>1.</strong> 介绍</a></li><li><ul class="section"><li><a href="ch01-01-installation.html"><strong>1.1.</strong> 安装</a></li><li><a href="ch01-02-hello-world.html"><strong>1.2.</strong> Hello, World!</a></li></ul></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong>2.</strong> 猜猜看教程</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong>3.</strong> 通用编程概念</a></li><li><ul class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong>3.1.</strong> 变量和可变性</a></li><li><a href="ch03-02-data-types.html"><strong>3.2.</strong> 数据类型</a></li><li><a href="ch03-03-how-functions-work.html"><strong>3.3.</strong> 函数如何工作</a></li><li><a href="ch03-04-comments.html"><strong>3.4.</strong> 注释</a></li><li><a href="ch03-05-control-flow.html"><strong>3.5.</strong> 控制流</a></li></ul></li><li><a href="ch04-00-understanding-ownership.html"><strong>4.</strong> 认识所有权</a></li><li><ul class="section"><li><a href="ch04-01-what-is-ownership.html"><strong>4.1.</strong> 什么是所有权</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong>4.2.</strong> 引用 & 借用</a></li><li><a href="ch04-03-slices.html"><strong>4.3.</strong> Slices</a></li></ul></li><li><a href="ch05-00-structs.html"><strong>5.</strong> 结构体</a></li><li><ul class="section"><li><a href="ch05-01-method-syntax.html"><strong>5.1.</strong> 方法语法</a></li></ul></li><li><a href="ch06-00-enums.html"><strong>6.</strong> 枚举和模式匹配</a></li><li><ul class="section"><li><a href="ch06-01-defining-an-enum.html"><strong>6.1.</strong> 定义枚举</a></li><li><a href="ch06-02-match.html"><strong>6.2.</strong> <code>match</code>控制流运算符</a></li><li><a href="ch06-03-if-let.html"><strong>6.3.</strong> <code>if let</code>简单控制流</a></li></ul></li><li><a href="ch07-00-modules.html"><strong>7.</strong> 模块</a></li><li><ul class="section"><li><a href="ch07-01-mod-and-the-filesystem.html"><strong>7.1.</strong> <code>mod</code>和文件系统</a></li><li><a href="ch07-02-controlling-visibility-with-pub.html"><strong>7.2.</strong> 使用<code>pub</code>控制可见性</a></li><li><a href="ch07-03-importing-names-with-use.html"><strong>7.3.</strong> 使用<code>use</code>导入命名</a></li></ul></li><li><a href="ch08-00-common-collections.html"><strong>8.</strong> 通用集合类型</a></li><li><ul class="section"><li><a href="ch08-01-vectors.html"><strong>8.1.</strong> vector</a></li><li><a href="ch08-02-strings.html"><strong>8.2.</strong> 字符串</a></li><li><a href="ch08-03-hash-maps.html"><strong>8.3.</strong> 哈希 map</a></li></ul></li><li><a href="ch09-00-error-handling.html"><strong>9.</strong> 错误处理</a></li><li><ul class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong>9.1.</strong> <code>panic!</code>与不可恢复的错误</a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong>9.2.</strong> <code>Result</code>与可恢复的错误</a></li><li><a href="ch09-03-to-panic-or-not-to-panic.html"><strong>9.3.</strong> <code>panic!</code>还是不<code>panic!</code></a></li></ul></li><li><a href="ch10-00-generics.html"><strong>10.</strong> 泛型、trait 和生命周期</a></li><li><ul class="section"><li><a href="ch10-01-syntax.html"><strong>10.1.</strong> 泛型数据类型</a></li><li><a href="ch10-02-traits.html"><strong>10.2.</strong> trait：定义共享的行为</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong>10.3.</strong> 生命周期与引用有效性</a></li></ul></li><li><a href="ch11-00-testing.html"><strong>11.</strong> 测试</a></li><li><ul class="section"><li><a href="ch11-01-writing-tests.html"><strong>11.1.</strong> 编写测试</a></li><li><a href="ch11-02-running-tests.html"><strong>11.2.</strong> 运行测试</a></li><li><a href="ch11-03-test-organization.html"><strong>11.3.</strong> 测试的组织结构</a></li></ul></li><li><a href="ch12-00-an-io-project.html"><strong>12.</strong> 一个 I/O 项目</a></li><li><ul class="section"><li><a href="ch12-01-accepting-command-line-arguments.html"><strong>12.1.</strong> 接受命令行参数</a></li><li><a href="ch12-02-reading-a-file.html" class="active"><strong>12.2.</strong> 读取文件</a></li><li><a href="ch12-03-improving-error-handling-and-modularity.html"><strong>12.3.</strong> 增强错误处理和模块化</a></li><li><a href="ch12-04-testing-the-librarys-functionality.html"><strong>12.4.</strong> 测试库的功能</a></li><li><a href="ch12-05-working-with-environment-variables.html"><strong>12.5.</strong> 处理环境变量</a></li><li><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong>12.6.</strong> 输出到<code>stderr</code>而不是<code>stdout</code></a></li></ul></li><li><a href="ch13-00-functional-features.html"><strong>13.</strong> Rust 中的函数式语言功能</a></li><li><ul class="section"><li><a href="ch13-01-closures.html"><strong>13.1.</strong> 闭包</a></li><li><a href="ch13-02-iterators.html"><strong>13.2.</strong> 迭代器</a></li><li><a href="ch13-03-improving-our-io-project.html"><strong>13.3.</strong> 改进 I/O 项目</a></li><li><a href="ch13-04-performance.html"><strong>13.4.</strong> 性能</a></li></ul></li><li><a href="ch14-00-more-about-cargo.html"><strong>14.</strong> 更多关于 Cargo 和 Crates.io</a></li><li><ul class="section"><li><a href="ch14-01-release-profiles.html"><strong>14.1.</strong> 发布配置</a></li><li><a href="ch14-02-publishing-to-crates-io.html"><strong>14.2.</strong> 将 crate 发布到 Crates.io</a></li><li><a href="ch14-03-cargo-workspaces.html"><strong>14.3.</strong> Cargo 工作空间</a></li><li><a href="ch14-04-installing-binaries.html"><strong>14.4.</strong> 使用<code>cargo install</code>从 Crates.io 安装文件</a></li><li><a href="ch14-05-extending-cargo.html"><strong>14.5.</strong> Cargo 自定义扩展命令</a></li></ul></li><li><a href="ch15-00-smart-pointers.html"><strong>15.</strong> 智能指针</a></li><li><ul class="section"><li><a href="ch15-01-box.html"><strong>15.1.</strong> <code>Box<T></code>用于已知大小的堆上数据</a></li><li><a href="ch15-02-deref.html"><strong>15.2.</strong> <code>Deref</code> Trait 允许通过引用访问数据</a></li><li><a href="ch15-03-drop.html"><strong>15.3.</strong> <code>Drop</code> Trait 运行清理代码</a></li><li><a href="ch15-04-rc.html"><strong>15.4.</strong> <code>Rc<T></code> 引用计数智能指针</a></li><li><a href="ch15-05-interior-mutability.html"><strong>15.5.</strong> <code>RefCell<T></code>和内部可变性模式</a></li><li><a href="ch15-06-reference-cycles.html"><strong>15.6.</strong> 引用循环和内存泄漏是安全的</a></li></ul></li><li><a href="ch16-00-concurrency.html"><strong>16.</strong> 无畏并发</a></li><li><ul class="section"><li><a href="ch16-01-threads.html"><strong>16.1.</strong> 线程</a></li><li><a href="ch16-02-message-passing.html"><strong>16.2.</strong> 消息传递</a></li><li><a href="ch16-03-shared-state.html"><strong>16.3.</strong> 共享状态</a></li><li><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong>16.4.</strong> 可扩展的并发：<code>Sync</code>和<code>Send</code></a></li></ul></li><li><a href="ch17-00-oop.html"><strong>17.</strong> 面向对象</a></li><li><ul class="section"><li><a href="ch17-01-what-is-oo.html"><strong>17.1.</strong> 什么是面向对象</a></li></ul></li></ul>
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<h1 class="menu-title">Rust 程序设计语言 简体中文版</h1>
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<a class="header" href="#读取文件" name="读取文件"><h2>读取文件</h2></a>
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<blockquote>
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<p><a href="https://github.com/rust-lang/book/blob/master/second-edition/src/ch12-02-reading-a-file.md">ch12-02-reading-a-file.md</a>
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<br>
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commit b8e4fcbf289b82c12121b282747ce05180afb1fb</p>
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</blockquote>
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<p>接下来我们将读取由命令行文件名参数指定的文件。首先，需要一个用来测试的示例文件——用来确保<code>greprs</code>正常工作的最好的文件是拥有少量文本和多个行且有一些重复单词的文件。列表 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">Filename: 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">Listing 12-3: The poem "I'm nobody! Who are you?" by
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Emily Dickinson that will make a good test case</span></p>
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<!-- Public domain Emily Dickinson poem. This will work best with something
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short, but that has multiple lines and some repetition. We could search through
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code; that gets a bit meta and possibly confusing... Changes to this are most
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welcome. /Carol -->
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<!-- :D I like it! I'm all for keeping -->
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<!-- Great! /Carol -->
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<p>创建完这个文件之后，修改 <em>src/main.rs</em> 并增加如列表 12-4 所示的打开文件的代码：</p>
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<p><span class="filename">Filename: src/main.rs</span></p>
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<pre><code class="language-rust,should_panic">use std::env;
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use std::fs::File;
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use std::io::prelude::*;
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fn main() {
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let args: Vec<String> = env::args().collect();
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let query = &args[1];
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let filename = &args[2];
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println!("Searching for {}", query);
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println!("In file {}", filename);
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let mut f = File::open(filename).expect("file not found");
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let mut contents = String::new();
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f.read_to_string(&mut contents).expect("something went wrong reading the file");
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println!("With text:\n{}", contents);
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}
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</code></pre>
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<p>Listing 12-4: Reading the contents of the file specified by the second argument</p>
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<!-- Will add ghosting and wingdings in libreoffice /Carol -->
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<p>首先，增加了更多的<code>use</code>语句来引入标准库中的相关部分：需要<code>std::fs::File</code>来处理文件，而<code>std::io::prelude::*</code>则包含许多对于 I/O 包括文件 I/O 有帮助的 trait。类似于 Rust 有一个通用的 prelude 来自动引入特定内容，<code>std::io</code>也有其自己的 prelude 来引入处理 I/O 时所需的通用内容。不同于默认的 prelude，必须显式<code>use</code>位于<code>std::io</code>中的 prelude。</p>
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<p>在<code>main</code>中，我们增加了三点内容：第一，通过传递变量<code>filename</code>的值调用<code>File::open</code>函数的值来获取文件的可变句柄。创建了叫做<code>contents</code>的变量并将其设置为一个可变的，空的<code>String</code>。它将会存放之后读取的文件的内容。第三，对文件句柄调用<code>read_to_string</code>并传递<code>contents</code>的可变引用作为参数。</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>$ cargo run the poem.txt
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Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs
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Running `target/debug/greprs 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：<code>main</code>函数有着多个功能，同时也没有处理可能出现的错误。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他们。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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