<!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 `for` keyword in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, for"><title>for - 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: 'for', 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=''>for</a></span></h1><div class='docblock'><p>The <code>for</code> keyword.</p>
<p>The <code>for</code> keyword is used in many syntactic locations:</p>
<ul>
<li><code>for</code> is used in for-in-loops (see below).</li>
<li><code>for</code> is used when implementing traits as in <code>impl Trait for Type</code> (see <a href="keyword.impl.html"><code>impl</code></a> for more info
on that).</li>
<li><code>for</code> is also used for <a href="https://doc.rust-lang.org/nightly/reference/trait-bounds.html#higher-ranked-trait-bounds">higher-ranked trait bounds</a> as in <code>for<'a> &'a T: PartialEq<i32></code>.</li>
</ul>
<p>for-in-loops, or to be more precise, iterator loops, are a simple syntactic sugar over a common
practice within Rust, which is to loop over an iterator until that iterator returns <code>None</code> (or
<code>break</code> is called).</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">for</span> <span class="ident">i</span> <span class="kw">in</span> <span class="number">0</span>..<span class="number">5</span> {
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"{}"</span>, <span class="ident">i</span> <span class="op">*</span> <span class="number">2</span>);
}
<span class="kw">for</span> <span class="ident">i</span> <span class="kw">in</span> <span class="ident">std</span>::<span class="ident">iter</span>::<span class="ident">repeat</span>(<span class="number">5</span>) {
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"turns out {} never stops being 5"</span>, <span class="ident">i</span>);
<span class="kw">break</span>; <span class="comment">// would loop forever otherwise</span>
}
<span class="lifetime">'outer</span>: <span class="kw">for</span> <span class="ident">x</span> <span class="kw">in</span> <span class="number">5</span>..<span class="number">50</span> {
<span class="kw">for</span> <span class="ident">y</span> <span class="kw">in</span> <span class="number">0</span>..<span class="number">10</span> {
<span class="kw">if</span> <span class="ident">x</span> <span class="op">==</span> <span class="ident">y</span> {
<span class="kw">break</span> <span class="lifetime">'outer</span>;
}
}
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Afor%20i%20in%200..5%20%7B%0A%20%20%20%20println!(%22%7B%7D%22%2C%20i%20*%202)%3B%0A%7D%0A%0Afor%20i%20in%20std%3A%3Aiter%3A%3Arepeat(5)%20%7B%0A%20%20%20%20println!(%22turns%20out%20%7B%7D%20never%20stops%20being%205%22%2C%20i)%3B%0A%20%20%20%20break%3B%20%2F%2F%20would%20loop%20forever%20otherwise%0A%7D%0A%0A'outer%3A%20for%20x%20in%205..50%20%7B%0A%20%20%20%20for%20y%20in%200..10%20%7B%0A%20%20%20%20%20%20%20%20if%20x%20%3D%3D%20y%20%7B%0A%20%20%20%20%20%20%20%20%20%20%20%20break%20'outer%3B%0A%20%20%20%20%20%20%20%20%7D%0A%20%20%20%20%7D%0A%7D%0A%7D">Run</a></pre></div>
<p>As shown in the example above, <code>for</code> loops (along with all other loops) can be tagged, using
similar syntax to lifetimes (only visually similar, entirely distinct in practice). Giving the
same tag to <code>break</code> breaks the tagged loop, which is useful for inner loops. It is definitely
not a goto.</p>
<p>A <code>for</code> loop expands as shown:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">for</span> <span class="ident">loop_variable</span> <span class="kw">in</span> <span class="ident">iterator</span> {
<span class="ident">code</span>()
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Afn%20code()%20%7B%20%7D%0Alet%20iterator%20%3D%200..2%3B%0Afor%20loop_variable%20in%20iterator%20%7B%0A%20%20%20%20code()%0A%7D%0A%7D">Run</a></pre></div>
<div class="example-wrap"><pre class="rust rust-example-rendered">
{
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">_iter</span> <span class="op">=</span> <span class="ident">std</span>::<span class="ident">iter</span>::<span class="ident">IntoIterator</span>::<span class="ident">into_iter</span>(<span class="ident">iterator</span>);
<span class="kw">loop</span> {
<span class="kw">match</span> <span class="ident">_iter</span>.<span class="ident">next</span>() {
<span class="prelude-val">Some</span>(<span class="ident">loop_variable</span>) <span class="op">=></span> {
<span class="ident">code</span>()
},
<span class="prelude-val">None</span> <span class="op">=></span> <span class="kw">break</span>,
}
}
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Afn%20code()%20%7B%20%7D%0Alet%20iterator%20%3D%200..2%3B%0A%7B%0A%20%20%20%20let%20mut%20_iter%20%3D%20std%3A%3Aiter%3A%3AIntoIterator%3A%3Ainto_iter(iterator)%3B%0A%20%20%20%20loop%20%7B%0A%20%20%20%20%20%20%20%20match%20_iter.next()%20%7B%0A%20%20%20%20%20%20%20%20%20%20%20%20Some(loop_variable)%20%3D%3E%20%7B%0A%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20code()%0A%20%20%20%20%20%20%20%20%20%20%20%20%7D%2C%0A%20%20%20%20%20%20%20%20%20%20%20%20None%20%3D%3E%20break%2C%0A%20%20%20%20%20%20%20%20%7D%0A%20%20%20%20%7D%0A%7D%0A%7D">Run</a></pre></div>
<p>More details on the functionality shown can be seen at the <a href="iter/trait.IntoIterator.html"><code>IntoIterator</code></a> docs.</p>
<p>For more information on for-loops, see the <a href="https://doc.rust-lang.org/book/2018-edition/ch03-05-control-flow.html#looping-through-a-collection-with-for">Rust book</a> or the <a href="https://doc.rust-lang.org/reference/expressions/loop-expr.html#iterator-loops">Reference</a>.</p>
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