RustBook/ch03-02-data-types.html

555 lines
44 KiB
HTML
Raw Normal View History

2020-01-06 20:57:15 +00:00
<!DOCTYPE HTML>
<html lang="en" class="sidebar-visible no-js light">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>Data Types - 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" />
<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 href="googleFonts/css.css" rel="stylesheet" type="text/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">
</head>
<body>
<!-- Provide site root to javascript -->
<script type="text/javascript">
var path_to_root = "";
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "light" : "light";
</script>
<!-- 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 = default_theme; }
var html = document.querySelector('html');
html.classList.remove('no-js')
html.classList.remove('light')
html.classList.add(theme);
html.classList.add('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">
<div id="sidebar-scrollbox" class="sidebar-scrollbox">
<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html" class="active"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><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 class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><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 class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-modu
</div>
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
</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 (default)</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>
<h2><a class="header" href="#data-types" id="data-types">Data Types</a></h2>
<p>Every value in Rust is of a certain <em>data type</em>, which tells Rust what kind of
data is being specified so it knows how to work with that data. Well look at
two data type subsets: scalar and compound.</p>
<p>Keep in mind that Rust is a <em>statically typed</em> language, which means that it
must know the types of all variables at compile time. The compiler can usually
infer what type we want to use based on the value and how we use it. In cases
when many types are possible, such as when we converted a <code>String</code> to a numeric
type using <code>parse</code> in the <a href="ch02-00-guessing-game-tutorial.html#comparing-the-guess-to-the-secret-number">“Comparing the Guess to the Secret
Number”</a><!-- ignore --> section in
Chapter 2, we must add a type annotation, like this:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>let guess: u32 = &quot;42&quot;.parse().expect(&quot;Not a number!&quot;);
<span class="boring">}
</span></code></pre></pre>
<p>If we dont add the type annotation here, Rust will display the following
error, which means the compiler needs more information from us to know which
type we want to use:</p>
<pre><code class="language-text">error[E0282]: type annotations needed
--&gt; src/main.rs:2:9
|
2 | let guess = &quot;42&quot;.parse().expect(&quot;Not a number!&quot;);
| ^^^^^
| |
| cannot infer type for `_`
| consider giving `guess` a type
</code></pre>
<p>Youll see different type annotations for other data types.</p>
<h3><a class="header" href="#scalar-types" id="scalar-types">Scalar Types</a></h3>
<p>A <em>scalar</em> type represents a single value. Rust has four primary scalar types:
integers, floating-point numbers, Booleans, and characters. You may recognize
these from other programming languages. Lets jump into how they work in Rust.</p>
<h4><a class="header" href="#integer-types" id="integer-types">Integer Types</a></h4>
<p>An <em>integer</em> is a number without a fractional component. We used one integer
type in Chapter 2, the <code>u32</code> type. This type declaration indicates that the
value its associated with should be an unsigned integer (signed integer types
start with <code>i</code>, instead of <code>u</code>) that takes up 32 bits of space. Table 3-1 shows
the built-in integer types in Rust. Each variant in the Signed and Unsigned
columns (for example, <code>i16</code>) can be used to declare the type of an integer
value.</p>
<p><span class="caption">Table 3-1: Integer Types in Rust</span></p>
<table><thead><tr><th>Length</th><th>Signed</th><th>Unsigned</th></tr></thead><tbody>
<tr><td>8-bit</td><td><code>i8</code></td><td><code>u8</code></td></tr>
<tr><td>16-bit</td><td><code>i16</code></td><td><code>u16</code></td></tr>
<tr><td>32-bit</td><td><code>i32</code></td><td><code>u32</code></td></tr>
<tr><td>64-bit</td><td><code>i64</code></td><td><code>u64</code></td></tr>
<tr><td>128-bit</td><td><code>i128</code></td><td><code>u128</code></td></tr>
<tr><td>arch</td><td><code>isize</code></td><td><code>usize</code></td></tr>
</tbody></table>
<p>Each variant can be either signed or unsigned and has an explicit size.
<em>Signed</em> and <em>unsigned</em> refer to whether its possible for the number to be
negative or positive—in other words, whether the number needs to have a sign
with it (signed) or whether it will only ever be positive and can therefore be
represented without a sign (unsigned). Its like writing numbers on paper: when
the sign matters, a number is shown with a plus sign or a minus sign; however,
when its safe to assume the number is positive, its shown with no sign.
Signed numbers are stored using <a href="https://en.wikipedia.org/wiki/Two%27s_complement">twos complement</a> representation.</p>
<p>Each signed variant can store numbers from -(2<sup>n - 1</sup>) to 2<sup>n -
1</sup> - 1 inclusive, where <em>n</em> is the number of bits that variant uses. So an
<code>i8</code> can store numbers from -(2<sup>7</sup>) to 2<sup>7</sup> - 1, which equals
-128 to 127. Unsigned variants can store numbers from 0 to 2<sup>n</sup> - 1,
so a <code>u8</code> can store numbers from 0 to 2<sup>8</sup> - 1, which equals 0 to 255.</p>
<p>Additionally, the <code>isize</code> and <code>usize</code> types depend on the kind of computer your
program is running on: 64 bits if youre on a 64-bit architecture and 32 bits
if youre on a 32-bit architecture.</p>
<p>You can write integer literals in any of the forms shown in Table 3-2. Note
that all number literals except the byte literal allow a type suffix, such as
<code>57u8</code>, and <code>_</code> as a visual separator, such as <code>1_000</code>.</p>
<p><span class="caption">Table 3-2: Integer Literals in Rust</span></p>
<table><thead><tr><th>Number literals</th><th>Example</th></tr></thead><tbody>
<tr><td>Decimal</td><td><code>98_222</code></td></tr>
<tr><td>Hex</td><td><code>0xff</code></td></tr>
<tr><td>Octal</td><td><code>0o77</code></td></tr>
<tr><td>Binary</td><td><code>0b1111_0000</code></td></tr>
<tr><td>Byte (<code>u8</code> only)</td><td><code>b'A'</code></td></tr>
</tbody></table>
<p>So how do you know which type of integer to use? If youre unsure, Rusts
defaults are generally good choices, and integer types default to <code>i32</code>: this
type is generally the fastest, even on 64-bit systems. The primary situation in
which youd use <code>isize</code> or <code>usize</code> is when indexing some sort of collection.</p>
<blockquote>
<h5><a class="header" href="#integer-overflow" id="integer-overflow">Integer Overflow</a></h5>
<p>Lets say you have a variable of type <code>u8</code> that can hold values between 0 and 255.
If you try to change the variable to a value outside of that range, such
as 256, <em>integer overflow</em> will occur. Rust has some interesting rules
involving this behavior. When youre compiling in debug mode, Rust includes
checks for integer overflow that cause your program to <em>panic</em> at runtime if
this behavior occurs. Rust uses the term panicking when a program exits with
an error; well discuss panics in more depth in the <a href="ch09-01-unrecoverable-errors-with-panic.html">“Unrecoverable Errors
with <code>panic!</code></a><!-- ignore --> section in
Chapter 9.</p>
<p>When youre compiling in release mode with the <code>--release</code> flag, Rust does
<em>not</em> include checks for integer overflow that cause panics. Instead, if
overflow occurs, Rust performs <em>twos complement wrapping</em>. In short, values
greater than the maximum value the type can hold “wrap around” to the minimum
of the values the type can hold. In the case of a <code>u8</code>, 256 becomes 0, 257
becomes 1, and so on. The program wont panic, but the variable will have a
value that probably isnt what you were expecting it to have. Relying on
integer overflows wrapping behavior is considered an error. If you want to
wrap explicitly, you can use the standard library type <a href="../std/num/struct.Wrapping.html"><code>Wrapping</code></a>.</p>
</blockquote>
<h4><a class="header" href="#floating-point-types" id="floating-point-types">Floating-Point Types</a></h4>
<p>Rust also has two primitive types for <em>floating-point numbers</em>, which are
numbers with decimal points. Rusts floating-point types are <code>f32</code> and <code>f64</code>,
which are 32 bits and 64 bits in size, respectively. The default type is <code>f64</code>
because on modern CPUs its roughly the same speed as <code>f32</code> but is capable of
more precision.</p>
<p>Heres an example that shows floating-point numbers in action:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let x = 2.0; // f64
let y: f32 = 3.0; // f32
}
</code></pre></pre>
<p>Floating-point numbers are represented according to the IEEE-754 standard. The
<code>f32</code> type is a single-precision float, and <code>f64</code> has double precision.</p>
<h4><a class="header" href="#numeric-operations" id="numeric-operations">Numeric Operations</a></h4>
<p>Rust supports the basic mathematical operations youd expect for all of the
number types: addition, subtraction, multiplication, division, and remainder.
The following code shows how youd use each one in a <code>let</code> statement:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
// addition
let sum = 5 + 10;
// subtraction
let difference = 95.5 - 4.3;
// multiplication
let product = 4 * 30;
// division
let quotient = 56.7 / 32.2;
// remainder
let remainder = 43 % 5;
}
</code></pre></pre>
<p>Each expression in these statements uses a mathematical operator and evaluates
to a single value, which is then bound to a variable. Appendix B contains a
list of all operators that Rust provides.</p>
<h4><a class="header" href="#the-boolean-type" id="the-boolean-type">The Boolean Type</a></h4>
<p>As in most other programming languages, a Boolean type in Rust has two possible
values: <code>true</code> and <code>false</code>. Booleans are one byte in size. The Boolean type in
Rust is specified using <code>bool</code>. For example:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let t = true;
let f: bool = false; // with explicit type annotation
}
</code></pre></pre>
<p>The main way to use Boolean values is through conditionals, such as an <code>if</code>
expression. Well cover how <code>if</code> expressions work in Rust in the <a href="ch03-05-control-flow.html#control-flow">“Control
Flow”</a><!-- ignore --> section.</p>
<h4><a class="header" href="#the-character-type" id="the-character-type">The Character Type</a></h4>
<p>So far weve worked only with numbers, but Rust supports letters too. Rusts
<code>char</code> type is the languages most primitive alphabetic type, and the following
code shows one way to use it. (Note that <code>char</code> literals are specified with
single quotes, as opposed to string literals, which use double quotes.)</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let c = 'z';
let z = '';
let heart_eyed_cat = '😻';
}
</code></pre></pre>
<p>Rusts <code>char</code> type is four bytes in size and represents a Unicode Scalar Value,
which means it can represent a lot more than just ASCII. Accented letters;
Chinese, Japanese, and Korean characters; emoji; and zero-width spaces are all
valid <code>char</code> values in Rust. Unicode Scalar Values range from <code>U+0000</code> to
<code>U+D7FF</code> and <code>U+E000</code> to <code>U+10FFFF</code> inclusive. However, a “character” isnt
really a concept in Unicode, so your human intuition for what a “character” is
may not match up with what a <code>char</code> is in Rust. Well discuss this topic in
detail in <a href="ch08-02-strings.html#storing-utf-8-encoded-text-with-strings">“Storing UTF-8 Encoded Text with Strings”</a><!-- ignore -->
in Chapter 8.</p>
<h3><a class="header" href="#compound-types" id="compound-types">Compound Types</a></h3>
<p><em>Compound types</em> can group multiple values into one type. Rust has two
primitive compound types: tuples and arrays.</p>
<h4><a class="header" href="#the-tuple-type" id="the-tuple-type">The Tuple Type</a></h4>
<p>A tuple is a general way of grouping together a number of values with a variety
of types into one compound type. Tuples have a fixed length: once declared,
they cannot grow or shrink in size.</p>
<p>We create a tuple by writing a comma-separated list of values inside
parentheses. Each position in the tuple has a type, and the types of the
different values in the tuple dont have to be the same. Weve added optional
type annotations in this example:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let tup: (i32, f64, u8) = (500, 6.4, 1);
}
</code></pre></pre>
<p>The variable <code>tup</code> binds to the entire tuple, because a tuple is considered a
single compound element. To get the individual values out of a tuple, we can
use pattern matching to destructure a tuple value, like this:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let tup = (500, 6.4, 1);
let (x, y, z) = tup;
println!(&quot;The value of y is: {}&quot;, y);
}
</code></pre></pre>
<p>This program first creates a tuple and binds it to the variable <code>tup</code>. It then
uses a pattern with <code>let</code> to take <code>tup</code> and turn it into three separate
variables, <code>x</code>, <code>y</code>, and <code>z</code>. This is called <em>destructuring</em>, because it breaks
the single tuple into three parts. Finally, the program prints the value of
<code>y</code>, which is <code>6.4</code>.</p>
<p>In addition to destructuring through pattern matching, we can access a tuple
element directly by using a period (<code>.</code>) followed by the index of the value we
want to access. For example:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let x: (i32, f64, u8) = (500, 6.4, 1);
let five_hundred = x.0;
let six_point_four = x.1;
let one = x.2;
}
</code></pre></pre>
<p>This program creates a tuple, <code>x</code>, and then makes new variables for each
element by using their respective indices. As with most programming languages,
the first index in a tuple is 0.</p>
<h4><a class="header" href="#the-array-type" id="the-array-type">The Array Type</a></h4>
<p>Another way to have a collection of multiple values is with an <em>array</em>. Unlike
a tuple, every element of an array must have the same type. Arrays in Rust are
different from arrays in some other languages because arrays in Rust have a
fixed length, like tuples.</p>
<p>In Rust, the values going into an array are written as a comma-separated list
inside square brackets:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let a = [1, 2, 3, 4, 5];
}
</code></pre></pre>
<p>Arrays are useful when you want your data allocated on the stack rather than
the heap (we will discuss the stack and the heap more in Chapter 4) or when
you want to ensure you always have a fixed number of elements. An array isnt
as flexible as the vector type, though. A vector is a similar collection type
provided by the standard library that <em>is</em> allowed to grow or shrink in size.
If youre unsure whether to use an array or a vector, you should probably use a
vector. Chapter 8 discusses vectors in more detail.</p>
<p>An example of when you might want to use an array rather than a vector is in a
program that needs to know the names of the months of the year. Its very
unlikely that such a program will need to add or remove months, so you can use
an array because you know it will always contain 12 elements:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>let months = [&quot;January&quot;, &quot;February&quot;, &quot;March&quot;, &quot;April&quot;, &quot;May&quot;, &quot;June&quot;, &quot;July&quot;,
&quot;August&quot;, &quot;September&quot;, &quot;October&quot;, &quot;November&quot;, &quot;December&quot;];
<span class="boring">}
</span></code></pre></pre>
<p>You would write an arrays type by using square brackets, and within the
brackets include the type of each element, a semicolon, and then the number of
elements in the array, like so:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>let a: [i32; 5] = [1, 2, 3, 4, 5];
<span class="boring">}
</span></code></pre></pre>
<p>Here, <code>i32</code> is the type of each element. After the semicolon, the number <code>5</code>
indicates the array contains five elements.</p>
<p>Writing an arrays type this way looks similar to an alternative syntax for
initializing an array: if you want to create an array that contains the same
value for each element, you can specify the initial value, followed by a
semicolon, and then the length of the array in square brackets, as shown here:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>let a = [3; 5];
<span class="boring">}
</span></code></pre></pre>
<p>The array named <code>a</code> will contain <code>5</code> elements that will all be set to the value
<code>3</code> initially. This is the same as writing <code>let a = [3, 3, 3, 3, 3];</code> but in a
more concise way.</p>
<h5><a class="header" href="#accessing-array-elements" id="accessing-array-elements">Accessing Array Elements</a></h5>
<p>An array is a single chunk of memory allocated on the stack. You can access
elements of an array using indexing, like this:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">fn main() {
let a = [1, 2, 3, 4, 5];
let first = a[0];
let second = a[1];
}
</code></pre></pre>
<p>In this example, the variable named <code>first</code> will get the value <code>1</code>, because
that is the value at index <code>[0]</code> in the array. The variable named <code>second</code> will
get the value <code>2</code> from index <code>[1]</code> in the array.</p>
<h5><a class="header" href="#invalid-array-element-access" id="invalid-array-element-access">Invalid Array Element Access</a></h5>
<p>What happens if you try to access an element of an array that is past the end
of the array? Say you change the example to the following code, which will
compile but exit with an error when it runs:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><code class="language-rust ignore panics">fn main() {
let a = [1, 2, 3, 4, 5];
let index = 10;
let element = a[index];
println!(&quot;The value of element is: {}&quot;, element);
}
</code></pre>
<p>Running this code using <code>cargo run</code> produces the following result:</p>
<pre><code class="language-text">$ cargo run
Compiling arrays v0.1.0 (file:///projects/arrays)
Finished dev [unoptimized + debuginfo] target(s) in 0.31 secs
Running `target/debug/arrays`
thread 'main' panicked at 'index out of bounds: the len is 5 but the index is
10', src/main.rs:5:19
note: Run with `RUST_BACKTRACE=1` for a backtrace.
</code></pre>
<p>The compilation didnt produce any errors, but the program resulted in a
<em>runtime</em> error and didnt exit successfully. When you attempt to access an
element using indexing, Rust will check that the index youve specified is less
than the array length. If the index is greater than or equal to the array
length, Rust will panic.</p>
<p>This is the first example of Rusts safety principles in action. In many
low-level languages, this kind of check is not done, and when you provide an
incorrect index, invalid memory can be accessed. Rust protects you against this
kind of error by immediately exiting instead of allowing the memory access and
continuing. Chapter 9 discusses more of Rusts error handling.</p>
</main>
<nav class="nav-wrapper" aria-label="Page navigation">
<!-- Mobile navigation buttons -->
<a rel="prev" href="ch03-01-variables-and-mutability.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="ch03-03-how-functions-work.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="ch03-01-variables-and-mutability.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="ch03-03-how-functions-work.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 type="text/javascript">
window.playpen_copyable = true;
</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 -->
<script type="text/javascript" src="ferris.js"></script>
</body>
</html>