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<!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="Creates a new unfold source with the specified closure as the “iterator function” and an initial state to eventually pass to the closure"><title>unfold in itertools - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-5bc39a1768837dd0.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="itertools" data-themes="" data-resource-suffix="" data-rustdoc-version="1.77.1 (7cf61ebde 2024-03-27)" data-channel="1.77.1" data-search-js="search-dd67cee4cfa65049.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-4c98445ec4002617.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-48f368f3872407c8.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-04d5337699b92874.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc fn"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle" title="show sidebar"></button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../itertools/index.html">itertools</a><span class="version">0.12.1</span></h2></div><div class="sidebar-elems"></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../itertools/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Function <a href="index.html">itertools</a>::<wbr><a class="fn" href="#">unfold</a><button id="copy-path" title="Copy item path to clipboard"><img src="../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../src/itertools/sources.rs.html#101-109">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>&#x2212;</span>]</button></span></div><pre class="rust item-decl"><code>pub fn unfold&lt;A, St, F&gt;(initial_state: St, f: F) -&gt; <a class="struct" href="structs/struct.Unfold.html" title="struct itertools::structs::Unfold">Unfold</a>&lt;St, F&gt; <a href="#" class="tooltip" data-notable-ty="Unfold&lt;St, F&gt;">ⓘ</a><div class="where">where
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F: <a class="trait" href="https://doc.rust-lang.org/1.77.1/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>(<a class="primitive" href="https://doc.rust-lang.org/1.77.1/std/primitive.reference.html">&amp;mut St</a>) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.77.1/core/option/enum.Option.html" title="enum core::option::Option">Option</a>&lt;A&gt;,</div></code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Creates a new unfold source with the specified closure as the “iterator
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function” and an initial state to eventually pass to the closure</p>
<p><code>unfold</code> is a general iterator builder: it has a mutable state value,
and a closure with access to the state that produces the next value.</p>
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<p>This more or less equivalent to a regular struct with an <a href="https://doc.rust-lang.org/1.77.1/core/iter/traits/iterator/trait.Iterator.html" title="trait core::iter::traits::iterator::Iterator"><code>Iterator</code></a>
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implementation, and is useful for one-off iterators.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// an iterator that yields sequential Fibonacci numbers,
// and stops at the maximum representable value.
</span><span class="kw">use </span>itertools::unfold;
<span class="kw">let </span><span class="kw-2">mut </span>fibonacci = unfold((<span class="number">1u32</span>, <span class="number">1u32</span>), |(x1, x2)| {
<span class="comment">// Attempt to get the next Fibonacci number
</span><span class="kw">let </span>next = x1.saturating_add(<span class="kw-2">*</span>x2);
<span class="comment">// Shift left: ret &lt;- x1 &lt;- x2 &lt;- next
</span><span class="kw">let </span>ret = <span class="kw-2">*</span>x1;
<span class="kw-2">*</span>x1 = <span class="kw-2">*</span>x2;
<span class="kw-2">*</span>x2 = next;
<span class="comment">// If addition has saturated at the maximum, we are finished
</span><span class="kw">if </span>ret == <span class="kw-2">*</span>x1 &amp;&amp; ret &gt; <span class="number">1 </span>{
<span class="prelude-val">None
</span>} <span class="kw">else </span>{
<span class="prelude-val">Some</span>(ret)
}
});
itertools::assert_equal(fibonacci.by_ref().take(<span class="number">8</span>),
<span class="macro">vec!</span>[<span class="number">1</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">5</span>, <span class="number">8</span>, <span class="number">13</span>, <span class="number">21</span>]);
<span class="macro">assert_eq!</span>(fibonacci.last(), <span class="prelude-val">Some</span>(<span class="number">2_971_215_073</span>))</code></pre></div>
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</div></details><script type="text/json" id="notable-traits-data">{"Unfold<St, F>":"<h3>Notable traits for <code><a class=\"struct\" href=\"structs/struct.Unfold.html\" title=\"struct itertools::structs::Unfold\">Unfold</a>&lt;St, F&gt;</code></h3><pre><code><div class=\"where\">impl&lt;A, St, F&gt; <a class=\"trait\" href=\"https://doc.rust-lang.org/1.77.1/core/iter/traits/iterator/trait.Iterator.html\" title=\"trait core::iter::traits::iterator::Iterator\">Iterator</a> for <a class=\"struct\" href=\"structs/struct.Unfold.html\" title=\"struct itertools::structs::Unfold\">Unfold</a>&lt;St, F&gt;<div class=\"where\">where\n F: <a class=\"trait\" href=\"https://doc.rust-lang.org/1.77.1/core/ops/function/trait.FnMut.html\" title=\"trait core::ops::function::FnMut\">FnMut</a>(<a class=\"primitive\" href=\"https://doc.rust-lang.org/1.77.1/std/primitive.reference.html\">&amp;mut St</a>) -&gt; <a class=\"enum\" href=\"https://doc.rust-lang.org/1.77.1/core/option/enum.Option.html\" title=\"enum core::option::Option\">Option</a>&lt;A&gt;,</div></div><div class=\"where\"> type <a href=\"https://doc.rust-lang.org/1.77.1/core/iter/traits/iterator/trait.Iterator.html#associatedtype.Item\" class=\"associatedtype\">Item</a> = A;</div>"}</script></section></div></main></body></html>
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