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<title>接受命令行参数 - Rust 程序设计语言 简体中文版</title>
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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" class="active"><strong>12.1.</strong> 接受命令行参数</a></li><li><a href="ch12-02-reading-a-file.html"><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></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="#接受命令行参数" name="接受命令行参数"><h2>接受命令行参数</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/ch12-01-accepting-command-line-arguments.md">ch12-01-accepting-command-line-arguments.md</a>
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<br>
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commit 4f2dc564851dc04b271a2260c834643dfd86c724</p>
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</blockquote>
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<p>第一个任务是让<code>greprs</code>接受两个命令行参数。crates.io 上有一些现存的库可以帮助我们，不过因为我们正在学习，我们将自己实现一个。</p>
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<p>我们需要调用一个 Rust 标准库提供的函数：<code>std::env::args</code>。这个函数返回一个传递给程序的命令行参数的<strong>迭代器</strong>（<em>iterator</em>）。我们还未讨论到迭代器，第十三章会全面的介绍他们。但是对于我们的目的来说，使用他们并不需要知道多少技术细节。我们只需要明白两点：</p>
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<ol>
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<li>迭代器生成一系列的值。</li>
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<li>在迭代器上调用<code>collect</code>方法可以将其生成的元素转换为一个 vector。</li>
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</ol>
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<p>让我们试试列表 12-1 中的代码：</p>
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<figure>
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<span class="filename">Filename: src/main.rs</span>
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<pre><code class="language-rust">use std::env;
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fn main() {
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let args: Vec<String> = env::args().collect();
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println!("{:?}", args);
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}
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</code></pre>
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<figcaption>
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<p>Listing 12-1: Collect the command line arguments into a vector and print them out</p>
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</figcaption>
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</figure>
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<!-- Will add wingdings in libreoffice /Carol -->
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<p>首先使用<code>use</code>语句来将<code>std::env</code>模块引入作用域。当函数嵌套了多于一层模块时，比如说<code>std::env::args</code>，通常使用<code>use</code>将父模块引入作用域，而不是引入其本身。<code>env::args</code>比单独的<code>args</code>要明确一些。当然，如果使用了多余一个<code>std::env</code>中的函数，我们也只需要一个<code>use</code>语句。</p>
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<p>在<code>main</code>函数的第一行，我们调用了<code>env::args</code>，并立即使用<code>collect</code>来创建了一个 vector。这里我们也显式的注明了<code>args</code>的类型：<code>collect</code>可以被用来创建很多类型的集合。Rust 并不能推断出我们需要什么类型，所以类型注解是必须的。在 Rust 中我们很少会需要注明类型，不过<code>collect</code>是就一个通常需要这么做的函数。</p>
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<p>最后，我们使用调试格式<code>:?</code>打印出 vector。让我们尝试不用参数运行代码，接着用两个参数：</p>
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<pre><code>$ cargo run
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["target/debug/greprs"]
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$ cargo run needle haystack
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...snip...
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["target/debug/greprs", "needle", "haystack"]
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</code></pre>
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<p>你会注意一个有趣的事情：二进制文件的名字是第一个参数。其原因超出了本章介绍的范围，不过这是我们必须记住的。</p>
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<p>现在我们有了一个访问所有参数的方法，让我们如列表 12-2 中所示将需要的变量存放到变量中：</p>
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<figure>
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<span class="filename">Filename: src/main.rs</span>
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<pre><code class="language-rust">use std::env;
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fn main() {
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let args: Vec<String> = env::args().collect();
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let search = &args[1];
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let filename = &args[2];
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println!("Searching for {}", search);
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println!("In file {}", filename);
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}
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</code></pre>
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<figcaption>
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<p>Listing 12-2: Create variables to hold the search argument and filename argument</p>
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</figcaption>
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</figure>
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<!-- Will add ghosting and wingdings in libreoffice /Carol -->
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<p>记住，程序名称是是第一个参数，所以并不需要<code>args[0]</code>。我们决定从第一个参数将是需要搜索的字符串，所以将第一个参数的引用放入变量<code>search</code>中。第二个参数将是文件名，将其放入变量<code>filename</code>中。再次尝试运行程序：</p>
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<pre><code>$ cargo run test sample.txt
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Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs
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Running `target\debug\greprs.exe test sample.txt`
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Searching for test
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In file sample.txt
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</code></pre>
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<p>很棒！不过有一个问题。让我们不带参数运行：</p>
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<pre><code>$ cargo run
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Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs
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Running `target\debug\greprs.exe`
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thread 'main' panicked at 'index out of bounds: the len is 1
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but the index is 1', ../src/libcollections\vec.rs:1307
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note: Run with `RUST_BACKTRACE=1` for a backtrace.
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</code></pre>
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<p>因为 vector 中只有一个元素，就是程序名称，不过我们尝试访问第二元素，程序 panic 并提示越界访问。虽然这个错误信息是_准确的_，不过它对程序的用户来说就没有意义了。现在就可以修复这个问题，不过我先继续学习别的内容：在程序结束前我们会改善这个情况。</p>
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