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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 href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> 定义模块来控制作用域与私有性</a></li><li class="chapter-item expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> 引用模块项目的路径</a></li><li class="chapter-item expanded "><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> 使用 use 关键字将路径引入作用域</a></li><li class="chapter-item expanded "><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> 将模块拆分成多个文件</a></li></ol></li><li class="chapter-item expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> 常见集合</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> 使用 Vector 储存列表</a></li><li class="chapter-item expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> 使用字符串储存 UTF-8 编码的文本</a></li><li class="chapter-item expanded "><a href="ch08-03-hash-maps.html" class="active"><strong aria-hidden="true">8.3.</strong> 使用 Hash Map 储存键值对</a></li></ol></li><li class="chapter-item expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> 错误处理</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> 用 panic! 处理不可恢复的错误</a></li><li class="chapter-item expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> 用 Result 处理可恢复的错误</a></li><li class="chapter-item expanded "><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> 要不要 panic!</a></li></ol></li><li class="chapter-item expanded "><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> 泛型、Trait 和生命周期</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> 泛型数据类型</a></li><li class="chapter-item expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Trait：定义共同行为</a></li><li class="chapter-item expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> 生命周期确保引用有效</a></li></ol></li><li class="chapter-item expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> 编写自动化测试</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> 如何编写测试</a></li><li class="chapter-item expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> 控制测试如何运行</a></li><li class="chapter-item expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> 测试的组织结构</a></li></ol></li><li class="chapter-item expanded "><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> 一个 I/O 项目：构建命令行程序</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> 接受命令行参数</a></li><li class="chapter-item expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> 读取文件</a></li><li class="chapter-item expanded "><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> 重构以改进模块化与错误处理</a></li><li class="chapter-item expanded "><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> 采用测试驱动开发完善库的功能</a></li><li class="chapter-item expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> 处理环境变量</a></li><li class="chapter-item expanded "><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> 将错误信息输出到标准错误而不是标准输出</a></li></ol></li><li class="chapter-item expanded "><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Rust 中的函数式语言功能：迭代器与闭包</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> 闭包：可以捕获其环境的匿名函数</a></li><li class="chapter-item expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> 使用迭代器处理元素序列</a></li><li class="chapter-item expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> 改进之前的 I/O 项目</a></li><li class="chapter-item expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> 性能比较：循环对迭代器</a></li></ol></li><li class="chapter-item expanded "><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> 更多关于 Cargo 和 Crates.io 的内容</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> 采用发布配置自定义构建</a></li><li class="chapter-item expanded "><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> 将 crate 发布到 Crates.io</a></li><li class="chapter-item expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo 工作空间</a></li><li class="chapter-item expanded "><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> 使用 cargo install 安装二进制文件</a></li><li class="chapter-item expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Cargo 自定义扩展命令</a></li></ol></li><li class="chapter-item expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> 智能指针</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> 使用 Box&lt;T&gt; 指向堆上数据</a></li><li class="chapter-item expanded "><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> 使用 Deref Trait 将智能指针当作常规引用处理</a></li><li class="chapter-item expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> 使用 Drop Trait 运行清理代码</a></li><li class="chapter-item expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc&lt;T&gt; 引用计数智能指针</a></li><li class="chapter-item expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell&lt;T&gt; 与内部可变性模式</a></li><li class="chapter-item expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> 引用循环会导致内存泄漏</a></li></ol></li><li class="chapter-item expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> 无畏并发</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> 使用线程同时地运行代码</a></li><li class="chapter-item expanded "><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> 使用消息传递在线程间通信</a></li><li class="chapter-item expanded "><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> 共享状态并发</a></li><li class="chapter-item expanded "><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> 使用 Sync 与 Send Traits 的可扩展并发</a></li></ol></li><li class="chapter-item expanded "><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Rust 的面向对象编程特性</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> 面向对象语言的特点</a></li><li class="chapter-item expanded "><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> 顾及不同类型值的 trait 对象</a></li><li class="chapter-item expanded "><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> 面向对象设计模式的实现</a></li></ol></li><li class="chapter-item expanded "><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> 模式与模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> 所有可能会用到模式的位置</a></li><li class="chapter-item expanded "><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability（可反驳性）: 模式是否会匹配失效</a></li><li class="chapter-item expanded "><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> 模式语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> 高级特征</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> 不安全的 Rust</a></li><li class="chapter-item expanded "><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.2.</strong> 高级 trait</a></li><li class="chapter-item expanded "><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.3.</strong> 高级类型</a></li><li class="chapter-item expanded "><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.4.</strong> 高级函数与闭包</a></li><li class="chapter-item expanded "><a href="ch19-06-macros.html"><strong aria-hidden="true">19.5.</strong> 宏</a></li></ol></li><li class="chapter-item expanded "><a href="ch20-00-final-project-a-web-server.html"><strong aria-hidden="true">20.</strong> 最后的项目：构建多线程 web server</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> 建立单线程 web server</a></li><li class="chapter-item expanded "><a href="ch20-02-multithreaded.html"><strong aria-hidden="true">20.2.</strong> 将单线程 server 变为多线程 server</a></li><li class="chapter-item expanded "><a href="ch20-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.3.</strong> 优雅停机与清理</a></li></ol></li><li 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href="appendix-07-nightly-rust.html"><strong aria-hidden="true">21.7.</strong> G - Rust 是如何开发的与 “Nightly Rust”</a></li></ol></li></ol>
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<h2 id="使用-hash-map-储存键值对"><a class="header" href="#使用-hash-map-储存键值对">使用 Hash Map 储存键值对</a></h2>
<blockquote>
<p><a href="https://github.com/rust-lang/book/blob/main/src/ch08-03-hash-maps.md">ch08-03-hash-maps.md</a>
<br>
commit 50775360ba3904c41e84176337ff47e6e7d6177c</p>
</blockquote>
<p>最后介绍的常用集合类型是 <strong>哈希 map</strong>（<em>hash map</em>）。<code>HashMap&lt;K, V&gt;</code> 类型储存了一个键类型 <code>K</code> 对应一个值类型 <code>V</code> 的映射。它通过一个 <strong>哈希函数</strong>（<em>hashing function</em>）来实现映射，决定如何将键和值放入内存中。很多编程语言支持这种数据结构，不过通常有不同的名字：哈希、map、对象、哈希表或者关联数组，仅举几例。</p>
<p>哈希 map 可以用于需要任何类型作为键来寻找数据的情况，而不是像 vector 那样通过索引。例如，在一个游戏中，你可以将每个团队的分数记录到哈希 map 中，其中键是队伍的名字而值是每个队伍的分数。给出一个队名，就能得到他们的得分。</p>
<p>本章我们会介绍哈希 map 的基本 API，不过还有更多吸引人的功能隐藏于标准库在 <code>HashMap&lt;K, V&gt;</code> 上定义的函数中。一如既往请查看标准库文档来了解更多信息。</p>
<h3 id="新建一个哈希-map"><a class="header" href="#新建一个哈希-map">新建一个哈希 map</a></h3>
<p>可以使用 <code>new</code> 创建一个空的 <code>HashMap</code>，并使用 <code>insert</code> 增加元素。在示例 8-20 中我们记录两支队伍的分数，分别是蓝队和黄队。蓝队开始有 10 分而黄队开始有 50 分：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from(&quot;Blue&quot;), 10);
scores.insert(String::from(&quot;Yellow&quot;), 50);
<span class="boring">}</span></code></pre></pre>
<p><span class="caption">示例 8-20：新建一个哈希 map 并插入一些键值对</span></p>
<p>注意必须首先 <code>use</code> 标准库中集合部分的 <code>HashMap</code>。在这三个常用集合中，<code>HashMap</code> 是最不常用的，所以并没有被 prelude 自动引用。标准库中对 <code>HashMap</code> 的支持也相对较少，例如，并没有内建的构建宏。</p>
<p>像 vector 一样，哈希 map 将它们的数据储存在堆上，这个 <code>HashMap</code> 的键类型是 <code>String</code> 而值类型是 <code>i32</code>。类似于 vector，哈希 map 是同质的：所有的键必须是相同类型，值也必须都是相同类型。</p>
<h3 id="访问哈希-map-中的值"><a class="header" href="#访问哈希-map-中的值">访问哈希 map 中的值</a></h3>
<p>可以通过 <code>get</code> 方法并提供对应的键来从哈希 map 中获取值，如示例 8-21 所示：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from(&quot;Blue&quot;), 10);
scores.insert(String::from(&quot;Yellow&quot;), 50);
let team_name = String::from(&quot;Blue&quot;);
let score = scores.get(&amp;team_name).copied().unwrap_or(0);
<span class="boring">}</span></code></pre></pre>
<p><span class="caption">示例 8-21：访问哈希 map 中储存的蓝队分数</span></p>
<p>这里，<code>score</code> 是与蓝队分数相关的值，应为 <code>10</code>。<code>get</code> 方法返回 <code>Option&lt;&amp;V&gt;</code>，如果某个键在哈希 map 中没有对应的值，<code>get</code> 会返回 <code>None</code>。程序中通过调用 <code>copied</code> 方法来获取一个 <code>Option&lt;i32&gt;</code> 而不是 <code>Option&lt;&amp;i32&gt;</code>，接着调用 <code>unwrap_or</code> 在 <code>scores</code> 中没有该键所对应的项时将其设置为零。</p>
<p>可以使用与 vector 类似的方式来遍历哈希 map 中的每一个键值对，也就是 <code>for</code> 循环：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from(&quot;Blue&quot;), 10);
scores.insert(String::from(&quot;Yellow&quot;), 50);
for (key, value) in &amp;scores {
println!(&quot;{key}: {value}&quot;);
}
<span class="boring">}</span></code></pre></pre>
<p>这会以任意顺序打印出每一个键值对：</p>
<pre><code class="language-text">Yellow: 50
Blue: 10
</code></pre>
<h3 id="哈希-map-和所有权"><a class="header" href="#哈希-map-和所有权">哈希 map 和所有权</a></h3>
<p>对于像 <code>i32</code> 这样的实现了 <code>Copy</code> trait 的类型，其值可以拷贝进哈希 map。对于像 <code>String</code> 这样拥有所有权的值，其值将被移动而哈希 map 会成为这些值的所有者，如示例 8-22 所示：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let field_name = String::from(&quot;Favorite color&quot;);
let field_value = String::from(&quot;Blue&quot;);
let mut map = HashMap::new();
map.insert(field_name, field_value);
// 这里 field_name 和 field_value 不再有效，
// 尝试使用它们看看会出现什么编译错误！
<span class="boring">}</span></code></pre></pre>
<p><span class="caption">示例 8-22：展示一旦键值对被插入后就为哈希 map 所拥有</span></p>
<p>当 <code>insert</code> 调用将 <code>field_name</code> 和 <code>field_value</code> 移动到哈希 map 中后，将不能使用这两个绑定。</p>
<p>如果将值的引用插入哈希 map，这些值本身将不会被移动进哈希 map。但是这些引用指向的值必须至少在哈希 map 有效时也是有效的。第十章 <a href="ch10-03-lifetime-syntax.html#%E7%94%9F%E5%91%BD%E5%91%A8%E6%9C%9F%E7%A1%AE%E4%BF%9D%E5%BC%95%E7%94%A8%E6%9C%89%E6%95%88">“生命周期确保引用有效”</a> 部分将会更多的讨论这个问题。</p>
<h3 id="更新哈希-map"><a class="header" href="#更新哈希-map">更新哈希 map</a></h3>
<p>尽管键值对的数量是可以增长的，每个唯一的键只能同时关联一个值（反之不一定成立：比如蓝队和黄队的 <code>scores</code> 哈希 map 中都可能存储有 10 这个值）。</p>
<p>当我们想要改变哈希 map 中的数据时，必须决定如何处理一个键已经有值了的情况。可以选择完全无视旧值并用新值代替旧值。可以选择保留旧值而忽略新值，并只在键 <strong>没有</strong> 对应值时增加新值。或者可以结合新旧两值。让我们看看这分别该如何处理！</p>
<h4 id="覆盖一个值"><a class="header" href="#覆盖一个值">覆盖一个值</a></h4>
<p>如果我们插入了一个键值对，接着用相同的键插入一个不同的值，与这个键相关联的旧值将被替换。即便示例 8-23 中的代码调用了两次 <code>insert</code>，哈希 map 也只会包含一个键值对，因为两次都是对蓝队的键插入的值：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from(&quot;Blue&quot;), 10);
scores.insert(String::from(&quot;Blue&quot;), 25);
println!(&quot;{scores:?}&quot;);
<span class="boring">}</span></code></pre></pre>
<p><span class="caption">示例 8-23：替换以特定键储存的值</span></p>
<p>这会打印出 <code>{&quot;Blue&quot;: 25}</code>。原始的值 <code>10</code> 则被覆盖了。</p>
<h4 id="只在键没有对应值时插入键值对"><a class="header" href="#只在键没有对应值时插入键值对">只在键没有对应值时插入键值对</a></h4>
<p>我们经常会检查某个特定的键是否已经存在于哈希 map 中并进行如下操作：如果哈希 map 中键已经存在则不做任何操作。如果不存在则连同值一块插入。</p>
<p>为此哈希 map 有一个特有的 API，叫做 <code>entry</code>，它获取我们想要检查的键作为参数。<code>entry</code> 函数的返回值是一个枚举，<code>Entry</code>，它代表了可能存在也可能不存在的值。比如说我们想要检查黄队的键是否关联了一个值。如果没有，就插入值 50，对于蓝队也是如此。使用 <code>entry</code> API 的代码看起来像示例 8-24 这样：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from(&quot;Blue&quot;), 10);
scores.entry(String::from(&quot;Yellow&quot;)).or_insert(50);
scores.entry(String::from(&quot;Blue&quot;)).or_insert(50);
println!(&quot;{scores:?}&quot;);
<span class="boring">}</span></code></pre></pre>
<p><span class="caption">示例 8-24：使用 <code>entry</code> 方法只在键没有对应一个值时插入</span></p>
<p><code>Entry</code> 的 <code>or_insert</code> 方法在键对应的值存在时就返回这个值的可变引用，如果不存在则将参数作为新值插入并返回新值的可变引用。这比编写自己的逻辑要简明的多，另外也与借用检查器结合得更好。</p>
<p>运行示例 8-24 的代码会打印出 <code>{&quot;Yellow&quot;: 50, &quot;Blue&quot;: 10}</code>。第一个 <code>entry</code> 调用会插入黄队的键和值 <code>50</code>，因为黄队并没有一个值。第二个 <code>entry</code> 调用不会改变哈希 map 因为蓝队已经有了值 <code>10</code>。</p>
<h4 id="根据旧值更新一个值"><a class="header" href="#根据旧值更新一个值">根据旧值更新一个值</a></h4>
<p>另一个常见的哈希 map 的应用场景是找到一个键对应的值并根据旧的值更新它。例如，示例 8-25 中的代码计数一些文本中每一个单词分别出现了多少次。我们使用哈希 map 以单词作为键并递增其值来记录我们遇到过几次这个单词。如果是第一次看到某个单词，就插入值 <code>0</code>。</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let text = &quot;hello world wonderful world&quot;;
let mut map = HashMap::new();
for word in text.split_whitespace() {
let count = map.entry(word).or_insert(0);
*count += 1;
}
println!(&quot;{map:?}&quot;);
<span class="boring">}</span></code></pre></pre>
<p><span class="caption">示例 8-25：通过哈希 map 储存单词和计数来统计出现次数</span></p>
<p>这会打印出 <code>{&quot;world&quot;: 2, &quot;hello&quot;: 1, &quot;wonderful&quot;: 1}</code>。你可能会看到相同的键值对以不同的顺序打印：回忆一下<a href="#%E8%AE%BF%E9%97%AE%E5%93%88%E5%B8%8C-map-%E4%B8%AD%E7%9A%84%E5%80%BC">“访问哈希 map 中的值”</a>部分中遍历哈希 map 会以任意顺序进行。</p>
<p><code>split_whitespace</code> 方法返回一个由空格分隔 <code>text</code> 值子 slice 的迭代器。<code>or_insert</code> 方法返回这个键的值的一个可变引用（<code>&amp;mut V</code>）。这里我们将这个可变引用储存在 <code>count</code> 变量中，所以为了赋值必须首先使用星号（<code>*</code>）解引用 <code>count</code>。这个可变引用在 <code>for</code> 循环的结尾离开作用域，这样所有这些改变都是安全的并符合借用规则。</p>
<h3 id="哈希函数"><a class="header" href="#哈希函数">哈希函数</a></h3>
<p><code>HashMap</code> 默认使用一种叫做 SipHash 的哈希函数，它可以抵御涉及哈希表（hash table）<sup class="footnote-reference"><a href="#siphash">1</a></sup> 的拒绝服务（Denial of Service, DoS）攻击。然而这并不是可用的最快的算法，不过为了更高的安全性值得付出一些性能的代价。如果性能监测显示此哈希函数非常慢，以致于你无法接受，你可以指定一个不同的 <em>hasher</em> 来切换为其它函数。hasher 是一个实现了 <code>BuildHasher</code> trait 的类型。第十章会讨论 trait 和如何实现它们。你并不需要从头开始实现你自己的 hasher；<a href="https://crates.io">crates.io</a> 有其他人分享的实现了许多常用哈希算法的 hasher 的库。</p>
<div class="footnote-definition" id="siphash"><sup class="footnote-definition-label">1</sup>
<p><a href="https://en.wikipedia.org/wiki/SipHash">https://en.wikipedia.org/wiki/SipHash</a></p>
</div>
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
<p>vector、字符串和哈希 map 会在你的程序需要储存、访问和修改数据时帮助你。这里有一些你应该能够解决的练习问题：</p>
<ul>
<li>给定一系列数字，使用 vector 并返回这个列表的中位数（排列数组后位于中间的值）和众数（出现次数最多的值；在这里哈希 map 会很有帮助）。</li>
<li>将字符串转换为 Pig Latin，也就是每一个单词的第一个辅音字母被移动到单词的结尾并增加 “ay”，所以 “first” 会变成 “irst-fay”。元音字母开头的单词则在结尾增加 “hay”（“apple” 会变成 “apple-hay”）。牢记 UTF-8 编码！</li>
<li>使用哈希 map 和 vector，创建一个文本接口来允许用户向公司的部门中增加员工的名字。例如，“Add Sally to Engineering” 或 “Add Amir to Sales”。接着让用户获取一个部门的所有员工的列表，或者公司每个部门的所有员工按照字典序排列的列表。</li>
</ul>
<p>标准库 API 文档中描述的这些类型的方法将有助于你进行这些练习！</p>
<p>我们已经开始接触可能会有失败操作的复杂程序了，这也意味着接下来是一个了解错误处理的绝佳时机！</p>
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