trpl-zh-cn/ch08-03-hash-maps.html
2024-11-08 08:29:11 +00:00

376 lines
42 KiB
HTML
Raw Permalink Blame History

This file contains ambiguous Unicode characters

This file contains Unicode characters that might be confused with other characters. If you think that this is intentional, you can safely ignore this warning. Use the Escape button to reveal them.

<!DOCTYPE HTML>
<html lang="en" class="light" dir="ltr">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>使用 Hash Map 储存键值对 - Rust 程序设计语言 简体中文版</title>
<!-- Custom HTML head -->
<meta name="description" content="">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#ffffff">
<link rel="icon" href="favicon.svg">
<link rel="shortcut icon" href="favicon.png">
<link rel="stylesheet" href="css/variables.css">
<link rel="stylesheet" href="css/general.css">
<link rel="stylesheet" href="css/chrome.css">
<link rel="stylesheet" href="css/print.css" media="print">
<!-- Fonts -->
<link rel="stylesheet" href="FontAwesome/css/font-awesome.css">
<link rel="stylesheet" href="fonts/fonts.css">
<!-- Highlight.js Stylesheets -->
<link rel="stylesheet" href="highlight.css">
<link rel="stylesheet" href="tomorrow-night.css">
<link rel="stylesheet" href="ayu-highlight.css">
<!-- Custom theme stylesheets -->
<link rel="stylesheet" href="ferris.css">
<link rel="stylesheet" href="theme/2018-edition.css">
<link rel="stylesheet" href="theme/semantic-notes.css">
<link rel="stylesheet" href="theme/listing.css">
</head>
<body class="sidebar-visible no-js">
<div id="body-container">
<!-- Provide site root to javascript -->
<script>
var path_to_root = "";
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
</script>
<!-- Work around some values being stored in localStorage wrapped in quotes -->
<script>
try {
var theme = localStorage.getItem('mdbook-theme');
var sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') && theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
</script>
<!-- Set the theme before any content is loaded, prevents flash -->
<script>
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
var html = document.querySelector('html');
html.classList.remove('light')
html.classList.add(theme);
var body = document.querySelector('body');
body.classList.remove('no-js')
body.classList.add('js');
</script>
<input type="checkbox" id="sidebar-toggle-anchor" class="hidden">
<!-- Hide / unhide sidebar before it is displayed -->
<script>
var body = document.querySelector('body');
var sidebar = null;
var sidebar_toggle = document.getElementById("sidebar-toggle-anchor");
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
} else {
sidebar = 'hidden';
}
sidebar_toggle.checked = sidebar === 'visible';
body.classList.remove('sidebar-visible');
body.classList.add("sidebar-" + sidebar);
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<div class="sidebar-scrollbox">
<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-async-await.html"><strong aria-hidden="true">17.</strong> Async 和 await</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch17-01-futures-and-syntax.html"><strong aria-hidden="true">17.1.</strong> Futures 和 async 语法</a></li><li class="chapter-item expanded "><a href="ch17-02-concurrency-with-async.html"><strong aria-hidden="true">17.2.</strong> 并发与 async</a></li><li class="chapter-item expanded "><a href="ch17-03-more-futures.html"><strong aria-hidden="true">17.3.</strong> 使用任意数量的 futures</a></li><li class="chapter-item expanded "><a href="ch17-04-streams.html"><strong aria-hidden="true">17.4.</strong>Streams</a></li><li class="chapter-item expanded "><a href="ch17-05-traits-for-async.html"><strong aria-hidden="true">17.5.</strong> 深入理解 async 相关的 traits</a></li><li class="chapter-item expanded "><a href="ch17-06-futures-tasks-threads.html"><strong aria-hidden="true">17.6.</strong> Futures任务tasks和线程threads</a></li></ol></li><li class="chapter-item expanded "><a href="ch18-00-oop.html"><strong aria-hidden="true">18.</strong> Rust 的面向对象编程特性</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch18-01-what-is-oo.html"><strong aria-hidden="true">18.1.</strong> 面向对象语言的特点</a></li><li class="chapter-item expanded "><a href="ch18-02-trait-objects.html"><strong aria-hidden="true">18.2.</strong> 顾及不同类型值的 trait 对象</a></li><li class="chapter-item expanded "><a href="ch18-03-oo-design-patterns.html"><strong aria-hidden="true">18.3.</strong> 面向对象设计模式的实现</a></li></ol></li><li class="chapter-item expanded "><a href="ch19-00-patterns.html"><strong aria-hidden="true">19.</strong> 模式与模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch19-01-all-the-places-for-patterns.html"><strong aria-hidden="true">19.1.</strong> 所有可能会用到模式的位置</a></li><li class="chapter-item expanded "><a href="ch19-02-refutability.html"><strong aria-hidden="true">19.2.</strong> Refutability可反驳性: 模式是否会匹配失效</a></li><li class="chapter-item expanded "><a href="ch19-03-pattern-syntax.html"><strong aria-hidden="true">19.3.</strong> 模式语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch20-00-advanced-features.html"><strong aria-hidden="true">20.</strong> 高级特征</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch20-01-unsafe-rust.html"><strong aria-hidden="true">20.1.</strong> 不安全的 Rust</a></li><li class="chapter-item expanded "><a href="ch20-03-advanced-traits.html"><strong aria-hidden="true">20.2.</strong> 高级 trait</a></li><li class="chapter-item expanded "><a href="ch20-04-advanced-types.html"><strong aria-hidden="true">20.3.</strong> 高级类型</a></li><li class="chapter-item expanded "><a href="ch20-05-advanced-functions-and-closures.html"><strong aria-hidden="true">20.4.</strong> 高级函数与闭包</a></li><li class="chapter-item expanded "><a href="ch20-06-macros.html"><strong aria-hidden="true">20.5.</strong></a></li></ol></li><li class="chapter-item expanded "><a href="ch21-00-final-project-a-web-server.html"><strong aria-hidden="true">21.</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="ch21-01-single-threaded.html"><strong aria-hidden="true">21.1.</strong> 建立单线程 web server</a></li><li class="chapter-item expanded "><a href="ch21-02-multithreaded.html"><strong aria-hidden="true">21.2.</strong> 将单线程 server 变为多线程 server</a></li><li class="chapter-item expanded "><a href="ch21-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">21.3.</strong> 优雅停机与清理</a></li></ol></li><li class="chapter-item expanded "><a href="appendix-00.html"><strong aria-hidden="true">22.</strong> 附录</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">22.1.</strong> A - 关键字</a></li><li class="chapter-item expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">22.2.</strong> B - 运算符与符号</a></li><li class="chapter-item expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">22.3.</strong> C - 可派生的 trait</a></li><li class="chapter-item expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">22.4.</strong> D - 实用开发工具</a></li><li class="chapter-item expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">22.5.</strong> E - 版本</a></li><li class="chapter-item expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">22.6.</strong> F - 本书译本</a></li><li class="chapter-item expanded "><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">22.7.</strong> G - Rust 是如何开发的与 “Nightly Rust”</a></li></ol></li></ol>
</div>
<div id="sidebar-resize-handle" class="sidebar-resize-handle">
<div class="sidebar-resize-indicator"></div>
</div>
</nav>
<!-- Track and set sidebar scroll position -->
<script>
var sidebarScrollbox = document.querySelector('#sidebar .sidebar-scrollbox');
sidebarScrollbox.addEventListener('click', function(e) {
if (e.target.tagName === 'A') {
sessionStorage.setItem('sidebar-scroll', sidebarScrollbox.scrollTop);
}
}, { passive: true });
var sidebarScrollTop = sessionStorage.getItem('sidebar-scroll');
sessionStorage.removeItem('sidebar-scroll');
if (sidebarScrollTop) {
// preserve sidebar scroll position when navigating via links within sidebar
sidebarScrollbox.scrollTop = sidebarScrollTop;
} else {
// scroll sidebar to current active section when navigating via "next/previous chapter" buttons
var activeSection = document.querySelector('#sidebar .active');
if (activeSection) {
activeSection.scrollIntoView({ block: 'center' });
}
}
</script>
<div id="page-wrapper" class="page-wrapper">
<div class="page">
<div id="menu-bar-hover-placeholder"></div>
<div id="menu-bar" class="menu-bar sticky">
<div class="left-buttons">
<label id="sidebar-toggle" class="icon-button" for="sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
<i class="fa fa-bars"></i>
</label>
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
<i class="fa fa-paint-brush"></i>
</button>
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
<li role="none"><button role="menuitem" class="theme" id="light">Light</button></li>
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
</ul>
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
<i class="fa fa-search"></i>
</button>
</div>
<h1 class="menu-title">Rust 程序设计语言 简体中文版</h1>
<div class="right-buttons">
<a href="print.html" title="Print this book" aria-label="Print this book">
<i id="print-button" class="fa fa-print"></i>
</a>
<a href="https://github.com/KaiserY/trpl-zh-cn/tree/main" title="Git repository" aria-label="Git repository">
<i id="git-repository-button" class="fa fa-github"></i>
</a>
<a href="https://github.com/KaiserY/trpl-zh-cn/edit/main/src/ch08-03-hash-maps.md" title="Suggest an edit" aria-label="Suggest an edit">
<i id="git-edit-button" class="fa fa-edit"></i>
</a>
</div>
</div>
<div id="search-wrapper" class="hidden">
<form id="searchbar-outer" class="searchbar-outer">
<input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
</form>
<div id="searchresults-outer" class="searchresults-outer hidden">
<div id="searchresults-header" class="searchresults-header"></div>
<ul id="searchresults">
</ul>
</div>
</div>
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
<script>
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
</script>
<div id="content" class="content">
<main>
<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 edition2021"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from("Blue"), 10);
scores.insert(String::from("Yellow"), 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 edition2021"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from("Blue"), 10);
scores.insert(String::from("Yellow"), 50);
let team_name = String::from("Blue");
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><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 edition2021"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from("Blue"), 10);
scores.insert(String::from("Yellow"), 50);
for (key, value) in &amp;scores {
println!("{key}: {value}");
}
<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 edition2021"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let field_name = String::from("Favorite color");
let field_value = String::from("Blue");
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><code>insert</code> 调用将 <code>field_name</code><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 edition2021"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from("Blue"), 10);
scores.insert(String::from("Blue"), 25);
println!("{scores:?}");
<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>{"Blue": 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 edition2021"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from("Blue"), 10);
scores.entry(String::from("Yellow")).or_insert(50);
scores.entry(String::from("Blue")).or_insert(50);
println!("{scores:?}");
<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><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>{"Yellow": 50, "Blue": 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 edition2021"><span class="boring">fn main() {
</span> use std::collections::HashMap;
let text = "hello world wonderful world";
let mut map = HashMap::new();
for word in text.split_whitespace() {
let count = map.entry(word).or_insert(0);
*count += 1;
}
println!("{map:?}");
<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>{"world": 2, "hello": 1, "wonderful": 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>
</main>
<nav class="nav-wrapper" aria-label="Page navigation">
<!-- Mobile navigation buttons -->
<a rel="prev" href="ch08-02-strings.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next prefetch" href="ch09-00-error-handling.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
<div style="clear: both"></div>
</nav>
</div>
</div>
<nav class="nav-wide-wrapper" aria-label="Page navigation">
<a rel="prev" href="ch08-02-strings.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next prefetch" href="ch09-00-error-handling.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
</nav>
</div>
<script>
window.playground_copyable = true;
</script>
<script src="elasticlunr.min.js"></script>
<script src="mark.min.js"></script>
<script src="searcher.js"></script>
<script src="clipboard.min.js"></script>
<script src="highlight.js"></script>
<script src="book.js"></script>
<!-- Custom JS scripts -->
<script src="ferris.js"></script>
</div>
</body>
</html>