<!DOCTYPE HTML>
<html lang="en" class="sidebar-visible no-js">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>Hash Maps - The Rust Programming Language</title>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<meta name="description" content="">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#ffffff" />
<base href="">
<link rel="stylesheet" href="book.css">
<link href="https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800" rel="stylesheet" type="text/css">
<link href="https://fonts.googleapis.com/css?family=Source+Code+Pro:500" rel="stylesheet" type="text/css">
<link rel="shortcut icon" href="favicon.png">
<!-- Font Awesome -->
<link rel="stylesheet" href="_FontAwesome/css/font-awesome.css">
<link rel="stylesheet" href="highlight.css">
<link rel="stylesheet" href="tomorrow-night.css">
<link rel="stylesheet" href="ayu-highlight.css">
<!-- Custom theme stylesheets -->
</head>
<body class="light">
<!-- Work around some values being stored in localStorage wrapped in quotes -->
<script type="text/javascript">
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 type="text/javascript">
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = 'light'; }
document.body.className = theme;
document.querySelector('html').className = theme + ' js';
</script>
<!-- Hide / unhide sidebar before it is displayed -->
<script type="text/javascript">
var html = document.querySelector('html');
var sidebar = 'hidden';
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
}
html.classList.remove('sidebar-visible');
html.classList.add("sidebar-" + sidebar);
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<ol class="chapter"><li class="affix"><a href="foreword.html">Foreword</a></li><li class="affix"><a href="ch00-00-introduction.html">Introduction</a></li><li><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> How Functions Work</a></li><li><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References & Borrowing</a></li><li><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slices</a></li></ol></li><li><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li><a href="ch07-00-modules.html"><strong aria-hidden="true">7.</strong> Modules</a></li><li><ol class="section"><li><a href="ch07-01-mod-and-the-filesystem.html"><strong aria-hidden="true">7.1.</strong> mod and the Filesystem</a></li><li><a href="ch07-02-controlling-visibility-with-pub.html"><strong aria-hidden="true">7.2.</strong> Controlling Visibility with pub</a></li><li><a href="ch07-03-importing-names-with-use.html"><strong aria-hidden="true">7.3.</strong> Referring to Names in Different Modules</a></li></ol></li><li><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Vectors</a></li><li><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Strings</a></li><li><a href="ch08-03-hash-maps.html" class="active"><strong aria-hidden="true">8.3.</strong> Hash Maps</a></li></ol></li><li><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> To panic! or Not To panic!</a></li></ol></li><li><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Testing</a></li><li><ol class="section"><li><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> Writing tests</a></li><li><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Running tests</a></li><li><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> An I/O Project: Building a Command Line Program</a></li><li><ol class="section"><li><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> Refactoring to Improve Modularity and Error Handling</a></li><li><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> Developing the Library’s Functionality with Test Driven Development</a></li><li><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> Writing Error Messages to Standard Error Instead of Standard Output</a></li></ol></li><li><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Functional Language Features: Iterators and Closures</a></li><li><ol class="section"><li><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> More about Cargo and Crates.io</a></li><li><ol class="section"><li><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> Publishing a Crate to Crates.io</a></li><li><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> Installing Binaries from Crates.io with cargo install</a></li><li><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Box<T> Points to Data on the Heap and Has a Known Size</a></li><li><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> The Deref Trait Allows Access to the Data Through a Reference</a></li><li><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> The Drop Trait Runs Code on Cleanup</a></li><li><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Creating Reference Cycles and Leaking Memory is Safe</a></li></ol></li><li><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Threads</a></li><li><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Message Passing</a></li><li><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared State</a></li><li><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency: Sync and Send</a></li></ol></li><li><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Object Oriented Programming Features of Rust</a></li><li><ol class="section"><li><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> Using Trait Objects that Allow for Values of Different Types</a></li><li><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> Implementing an Object-Oriented Design Pattern</a></li></ol></li><li><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns Match the Structure of Values</a></li><li><ol class="section"><li><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns May be Used</a></li><li><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> All the Pattern Syntax</a></li></ol></li><li><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li><a href="ch19-02-advanced-lifetimes.html"><strong aria-hidden="true">19.2.</strong> Advanced Lifetimes</a></li><li><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.3.</strong> Advanced Traits</a></li><li><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.4.</strong> Advanced Types</a></li><li><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.5.</strong> Advanced Functions & Closures</a></li></ol></li><li><a href="ch20-00-final-project-a-web-server.html"><strong aria-hidden="true">20.</strong> Final Project: Building a Multithreaded Web Server</a></li><li><ol class="section"><li><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> A Single Threaded Web Server</a></li><li><a href="ch20-02-multithreaded.html"><strong aria-hidden="true">20.2.</strong> Turning our Single Threaded Server into a Multithreaded Server</a></li><li><a href="ch20-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.3.</strong> Graceful Shutdown and Cleanup</a></li></ol></li><li><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li><a href="appendix-04-macros.html"><strong aria-hidden="true">21.4.</strong> D - Macros</a></li><li><a href="appendix-05-translation.html"><strong aria-hidden="true">21.5.</strong> E - Translations</a></li><li><a href="appendix-06-nightly-rust.html"><strong aria-hidden="true">21.6.</strong> F - How Rust is Made and “Nightly Rust”</a></li></ol></li></ol>
</nav>
<div id="page-wrapper" class="page-wrapper">
<div class="page">
<div id="menu-bar" class="menu-bar">
<div id="menu-bar-sticky-container">
<div class="left-buttons">
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
<i class="fa fa-bars"></i>
</button>
<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 <span class="default">(default)</span></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">The Rust Programming Language</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>
</div>
</div>
</div>
<div id="search-wrapper" class="hidden">
<form id="searchbar-outer" class="searchbar-outer">
<input type="search" name="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 type="text/javascript">
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>
<a class="header" href="ch08-03-hash-maps.html#storing-keys-with-associated-values-in-hash-maps" id="storing-keys-with-associated-values-in-hash-maps"><h2>Storing Keys with Associated Values in Hash Maps</h2></a>
<p>The last of our common collections is the <em>hash map</em>. The type <code>HashMap<K, V></code>
stores a mapping of keys of type <code>K</code> to values of type <code>V</code>. It does this via a
<em>hashing function</em>, which determines how it places these keys and values into
memory. Many programming languages support this kind of data structure, but
they often use a different name, such as hash, map, object, hash table, or
associative array, just to name a few.</p>
<p>Hash maps are useful when you want to look up data not by using an index, as
you can with vectors, but by using a key that can be of any type. For example,
in a game, you could keep track of each team’s score in a hash map in which
each key is a team’s name and the values are each team’s score. Given a team
name, you can retrieve its score.</p>
<p>We’ll go over the basic API of hash maps in this section, but many more goodies
are hiding in the functions defined on <code>HashMap<K, V></code> by the standard library.
As always, check the standard library documentation for more information.</p>
<a class="header" href="ch08-03-hash-maps.html#creating-a-new-hash-map" id="creating-a-new-hash-map"><h3>Creating a New Hash Map</h3></a>
<p>You can create an empty hash map with <code>new</code> and add elements with <code>insert</code>. In
Listing 8-20, we’re keeping track of the scores of two teams whose names are
Blue and Yellow. The Blue team starts with 10 points, and the Yellow team
starts with 50:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from("Blue"), 10);
scores.insert(String::from("Yellow"), 50);
#}</code></pre></pre>
<p><span class="caption">Listing 8-20: Creating a new hash map and inserting some
keys and values</span></p>
<p>Note that we need to first <code>use</code> the <code>HashMap</code> from the collections portion of
the standard library. Of our three common collections, this one is the least
often used, so it’s not included in the features brought into scope
automatically in the prelude. Hash maps also have less support from the
standard library; there’s no built-in macro to construct them, for example.</p>
<p>Just like vectors, hash maps store their data on the heap. This <code>HashMap</code> has
keys of type <code>String</code> and values of type <code>i32</code>. Like vectors, hash maps are
homogeneous: all of the keys must have the same type, and all of the values
must have the same type.</p>
<p>Another way of constructing a hash map is by using the <code>collect</code> method on a
vector of tuples, where each tuple consists of a key and its value. The
<code>collect</code> method gathers data into a number of collection types, including
<code>HashMap</code>. For example, if we had the team names and initial scores in two
separate vectors, we could use the <code>zip</code> method to create a vector of tuples
where “Blue” is paired with 10, and so forth. Then we could use the <code>collect</code>
method to turn that vector of tuples into a hash map, as shown in Listing 8-21:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
use std::collections::HashMap;
let teams = vec![String::from("Blue"), String::from("Yellow")];
let initial_scores = vec![10, 50];
let scores: HashMap<_, _> = teams.iter().zip(initial_scores.iter()).collect();
#}</code></pre></pre>
<p><span class="caption">Listing 8-21: Creating a hash map from a list of teams
and a list of scores</span></p>
<p>The type annotation <code>HashMap<_, _></code> is needed here because it’s possible to
<code>collect</code> into many different data structures and Rust doesn’t know which you
want unless you specify. For the parameters for the key and value types,
however, we use underscores, and Rust can infer the types that the hash map
contains based on the types of the data in the vectors.</p>
<a class="header" href="ch08-03-hash-maps.html#hash-maps-and-ownership" id="hash-maps-and-ownership"><h3>Hash Maps and Ownership</h3></a>
<p>For types that implement the <code>Copy</code> trait, like <code>i32</code>, the values are copied
into the hash map. For owned values like <code>String</code>, the values will be moved and
the hash map will be the owner of those values, as demonstrated in Listing 8-22:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
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);
// field_name and field_value are invalid at this point, try using them and
// see what compiler error you get!
#}</code></pre></pre>
<p><span class="caption">Listing 8-22: Showing that keys and values are owned by
the hash map once they’re inserted</span></p>
<p>We aren’t able to use the variables <code>field_name</code> and <code>field_value</code> after
they’ve been moved into the hash map with the call to <code>insert</code>.</p>
<p>If we insert references to values into the hash map, the values won’t be moved
into the hash map. The values that the references point to must be valid for at
least as long as the hash map is valid. We’ll talk more about these issues in
the “Validating References with Lifetimes” section in Chapter 10.</p>
<a class="header" href="ch08-03-hash-maps.html#accessing-values-in-a-hash-map" id="accessing-values-in-a-hash-map"><h3>Accessing Values in a Hash Map</h3></a>
<p>We can get a value out of the hash map by providing its key to the <code>get</code>
method, as shown in Listing 8-23:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
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(&team_name);
#}</code></pre></pre>
<p><span class="caption">Listing 8-23: Accessing the score for the Blue team
stored in the hash map</span></p>
<p>Here, <code>score</code> will have the value that’s associated with the Blue team, and the
result will be <code>Some(&10)</code>. The result is wrapped in <code>Some</code> because <code>get</code>
returns an <code>Option<&V></code>; if there’s no value for that key in the hash map,
<code>get</code> will return <code>None</code>. The program will need to handle the <code>Option</code> in one
of the ways that we covered in Chapter 6.</p>
<p>We can iterate over each key/value pair in a hash map in a similar manner as we
do with vectors, using a <code>for</code> loop:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
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 &scores {
println!("{}: {}", key, value);
}
#}</code></pre></pre>
<p>This code will print each pair in an arbitrary order:</p>
<pre><code class="language-text">Yellow: 50
Blue: 10
</code></pre>
<a class="header" href="ch08-03-hash-maps.html#updating-a-hash-map" id="updating-a-hash-map"><h3>Updating a Hash Map</h3></a>
<p>Although the number of keys and values is growable, each key can only have one
value associated with it at a time. When you want to change the data in a hash
map, you have to decide how to handle the case when a key already has a value
assigned. You could replace the old value with the new value, completely
disregarding the old value. You could keep the old value and ignore the new
value, only adding the new value if the key <em>doesn’t</em> already have a value. Or
you could combine the old value and the new value. Let’s look at how to do each
of these!</p>
<a class="header" href="ch08-03-hash-maps.html#overwriting-a-value" id="overwriting-a-value"><h4>Overwriting a Value</h4></a>
<p>If we insert a key and a value into a hash map and then insert that same key
with a different value, the value associated with that key will be replaced.
Even though the code in Listing 8-24 calls <code>insert</code> twice, the hash map will
only contain one key/value pair because we’re inserting the value for the Blue
team’s key both times:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
use std::collections::HashMap;
let mut scores = HashMap::new();
scores.insert(String::from("Blue"), 10);
scores.insert(String::from("Blue"), 25);
println!("{:?}", scores);
#}</code></pre></pre>
<p><span class="caption">Listing 8-24: Replacing a value stored with a particular
key</span></p>
<p>This code will print <code>{"Blue": 25}</code>. The original value of <code>10</code> has been
overwritten.</p>
<a class="header" href="ch08-03-hash-maps.html#only-inserting-a-value-if-the-key-has-no-value" id="only-inserting-a-value-if-the-key-has-no-value"><h4>Only Inserting a Value If the Key Has No Value</h4></a>
<p>It’s common to check whether a particular key has a value and, if it doesn’t,
insert a value for it. Hash maps have a special API for this called <code>entry</code>
that takes the key you want to check as a parameter. The return value of the
<code>entry</code> function is an enum called <code>Entry</code> that represents a value that might
or might not exist. Let’s say we want to check whether the key for the Yellow
team has a value associated with it. If it doesn’t, we want to insert the value
50, and the same for the Blue team. Using the <code>entry</code> API, the code looks like
Listing 8-25:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
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);
#}</code></pre></pre>
<p><span class="caption">Listing 8-25: Using the <code>entry</code> method to only insert if
the key does not already have a value</span></p>
<p>The <code>or_insert</code> method on <code>Entry</code> is defined to return a mutable reference to
the value for the corresponding <code>Entry</code> key if that key exists, and if not,
inserts the parameter as the new value for this key and returns a mutable
reference to the new value. This technique is much cleaner than writing the
logic ourselves and, in addition, plays more nicely with the borrow checker.</p>
<p>Running the code in Listing 8-25 will print <code>{"Yellow": 50, "Blue": 10}</code>. The
first call to <code>entry</code> will insert the key for the Yellow team with the value
<code>50</code> because the Yellow team doesn’t have a value already. The second call to
<code>entry</code> will not change the hash map because the Blue team already has the
value <code>10</code>.</p>
<a class="header" href="ch08-03-hash-maps.html#updating-a-value-based-on-the-old-value" id="updating-a-value-based-on-the-old-value"><h4>Updating a Value Based on the Old Value</h4></a>
<p>Another common use case for hash maps is to look up a key’s value and then
update it based on the old value. For instance, Listing 8-26 shows code that
counts how many times each word appears in some text. We use a hash map with
the words as keys and increment the value to keep track of how many times we’ve
seen that word. If it’s the first time we’ve seen a word, we’ll first insert
the value <code>0</code>:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
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);
#}</code></pre></pre>
<p><span class="caption">Listing 8-26: Counting occurrences of words using a hash
map that stores words and counts</span></p>
<p>This code will print <code>{"world": 2, "hello": 1, "wonderful": 1}</code>. The
<code>or_insert</code> method actually returns a mutable reference (<code>&mut V</code>) to the value
for this key. Here we store that mutable reference in the <code>count</code> variable, so
in order to assign to that value, we must first dereference <code>count</code> using the
asterisk (<code>*</code>). The mutable reference goes out of scope at the end of the <code>for</code>
loop, so all of these changes are safe and allowed by the borrowing rules.</p>
<a class="header" href="ch08-03-hash-maps.html#hashing-functions" id="hashing-functions"><h3>Hashing Functions</h3></a>
<p>By default, <code>HashMap</code> uses a cryptographically secure hashing function that can
provide resistance to Denial of Service (DoS) attacks. This is not the fastest
hashing algorithm available, but the trade-off for better security that comes
with the drop in performance is worth it. If you profile your code and find
that the default hash function is too slow for your purposes, you can switch to
another function by specifying a different <em>hasher</em>. A hasher is a type that
implements the <code>BuildHasher</code> trait. We’ll talk about traits and how to
implement them in Chapter 10. You don’t necessarily have to implement your own
hasher from scratch; <a href="https://crates.io">crates.io</a> has libraries shared by
other Rust users that provide hashers implementing many common hashing
algorithms.</p>
<a class="header" href="ch08-03-hash-maps.html#summary" id="summary"><h2>Summary</h2></a>
<p>Vectors, strings, and hash maps will provide a large amount of functionality
necessary in programs when you need to store, access, and modify data. Here are
some exercises you should now be equipped to solve:</p>
<ul>
<li>Given a list of integers, use a vector and return the mean (the average
value), median (when sorted, the value in the middle position), and mode (the
value that occurs most often; a hash map will be helpful here) of the list.</li>
<li>Convert strings to pig latin. The first consonant of each word is moved to
the end of the word and “ay” is added, so “first” becomes “irst-fay.” Words
that start with a vowel have “hay” added to the end instead (“apple” becomes
“apple-hay”). Keep in mind the details about UTF-8 encoding!</li>
<li>Using a hash map and vectors, create a text interface to allow a user to add
employee names to a department in a company. For example, “Add Sally to
Engineering” or “Add Amir to Sales.” Then let the user retrieve a list of all
people in a department or all people in the company by department, sorted
alphabetically.</li>
</ul>
<p>The standard library API documentation describes methods that vectors, strings,
and hash maps have that will be helpful for these exercises!</p>
<p>We’re getting into more complex programs in which operations can fail, so, it’s
a perfect time to discuss error handling. We’ll do that next!</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" 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 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 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 src="searchindex.js" type="text/javascript" charset="utf-8"></script>
<script src="elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
<script src="mark.min.js" type="text/javascript" charset="utf-8"></script>
<script src="searcher.js" type="text/javascript" charset="utf-8"></script>
<script src="clipboard.min.js" type="text/javascript" charset="utf-8"></script>
<script src="highlight.js" type="text/javascript" charset="utf-8"></script>
<script src="book.js" type="text/javascript" charset="utf-8"></script>
<!-- Custom JS scripts -->
</body>
</html>
distrib
>
Mageia
>
6
>
armv7hl
>
media
>
core-updates
>
by-pkgid
>
4e2dbb669434a7691662cb2f0ad38972
>
files
>
512
