<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="API documentation for the Rust `loop` keyword in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, loop"><title>loop - Rust</title><link rel="stylesheet" type="text/css" href="../normalize1.35.0.css"><link rel="stylesheet" type="text/css" href="../rustdoc1.35.0.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../dark1.35.0.css"><link rel="stylesheet" type="text/css" href="../light1.35.0.css" id="themeStyle"><script src="../storage1.35.0.js"></script><noscript><link rel="stylesheet" href="../noscript1.35.0.css"></noscript><link rel="shortcut icon" href="../favicon1.35.0.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow1.35.0.svg");}</style></head><body class="rustdoc keyword"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../std/index.html'><img src='../rust-logo1.35.0.png' alt='logo' width='100'></a><div class="sidebar-elems"><p class='location'><a href='index.html'>std</a></p><script>window.sidebarCurrent = {name: 'loop', ty: 'keyword', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush1.35.0.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme1.35.0.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../settings.html"><img src="../wheel1.35.0.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><span class='out-of-band'><span id='render-detail'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>−</span>]</a></span></span><span class='in-band'>Keyword <a class="keyword" href=''>loop</a></span></h1><div class='docblock'><p>The loop-defining keyword.</p>
<p><code>loop</code> is used to define the simplest kind of loop supported in Rust. It runs the code inside
it until the code uses <code>break</code> or the program exits.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">loop</span> {
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"hello world forever!"</span>);
}
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">i</span> <span class="op">=</span> <span class="number">0</span>;
<span class="kw">loop</span> {
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"i is {}"</span>, <span class="ident">i</span>);
<span class="kw">if</span> <span class="ident">i</span> <span class="op">></span> <span class="number">10</span> {
<span class="kw">break</span>;
}
<span class="ident">i</span> <span class="op">+=</span> <span class="number">1</span>;
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Aloop%20%7B%0A%20%20%20%20println!(%22hello%20world%20forever!%22)%3B%0Abreak%3B%0A%7D%0A%0Alet%20mut%20i%20%3D%200%3B%0Aloop%20%7B%0A%20%20%20%20println!(%22i%20is%20%7B%7D%22%2C%20i)%3B%0A%20%20%20%20if%20i%20%3E%2010%20%7B%0A%20%20%20%20%20%20%20%20break%3B%0A%20%20%20%20%7D%0A%20%20%20%20i%20%2B%3D%201%3B%0A%7D%0A%7D">Run</a></pre></div>
<p>Unlike the other kinds of loops in Rust (<code>while</code>, <code>while let</code>, and <code>for</code>), loops can be used as
expressions that return values via <code>break</code>.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">i</span> <span class="op">=</span> <span class="number">1</span>;
<span class="kw">let</span> <span class="ident">something</span> <span class="op">=</span> <span class="kw">loop</span> {
<span class="ident">i</span> <span class="op">*=</span> <span class="number">2</span>;
<span class="kw">if</span> <span class="ident">i</span> <span class="op">></span> <span class="number">100</span> {
<span class="kw">break</span> <span class="ident">i</span>;
}
};
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">something</span>, <span class="number">128</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20mut%20i%20%3D%201%3B%0Alet%20something%20%3D%20loop%20%7B%0A%20%20%20%20i%20*%3D%202%3B%0A%20%20%20%20if%20i%20%3E%20100%20%7B%0A%20%20%20%20%20%20%20%20break%20i%3B%0A%20%20%20%20%7D%0A%7D%3B%0Aassert_eq!(something%2C%20128)%3B%0A%7D">Run</a></pre></div>
<p>Every <code>break</code> in a loop has to have the same type. When it's not explicitly giving something,
<code>break;</code> returns <code>()</code>.</p>
<p>For more information on <code>loop</code> and loops in general, see the <a href="https://doc.rust-lang.org/reference/expressions/loop-expr.html">Reference</a>.</p>
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