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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="Extension trait on iterators, providing random sampling methods."><title>IteratorRandom in rand::seq - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Italic-81dc35de.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-MediumItalic-ccf7e434.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2"href="../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../../static.files/rustdoc-ca0dd0c4.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="rand" data-themes="" data-resource-suffix="" data-rustdoc-version="1.93.1 (01f6ddf75 2026-02-11) (Arch Linux rust 1:1.93.1-1)" data-channel="1.93.1" data-search-js="search-9e2438ea.js" data-stringdex-js="stringdex-a3946164.js" data-settings-js="settings-c38705f0.js" ><script src="../../static.files/storage-e2aeef58.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../static.files/main-a410ff4d.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-263c88ec.css"></noscript><link rel="icon" href="https://www.rust-lang.org/favicon.ico"></head><body class="rustdoc trait"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><rustdoc-topbar><h2><a href="#">IteratorRandom</a></h2></rustdoc-topbar><nav class="sidebar"><div class="sidebar-crate"><a class="logo-container" href="../../rand/index.html"><img src="https://www.rust-lang.org/logos/rust-logo-128x128-blk.png" alt="logo"></a><h2><a href="../../rand/index.html">rand</a><span class="version">0.9.2</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">Iterator<wbr>Random</a></h2><h3><a href="#provided-methods">Provided Methods</a></h3><ul class="block"><li><a href="#method.choose" title="choose">choose</a></li><li><a href="#method.choose_multiple" title="choose_multiple">choose_multiple</a></li><li><a href="#method.choose_multiple_fill" title="choose_multiple_fill">choose_multiple_fill</a></li><li><a href="#method.choose_stable" title="choose_stable">choose_stable</a></li></ul><h3><a href="#dyn-compatibility">Dyn Compatibility</a></h3><h3><a href="#implementors">Implementors</a></h3></section><div id="rustdoc-modnav"><h2><a href="index.html">In rand::<wbr>seq</a></h2></div></div></nav><div class="sidebar-resizer" title="Drag to resize sidebar"></div><main><div class="width-limiter"><section id="main-content" class="content"><div class="main-heading"><div class="rustdoc-breadcrumbs"><a href="../index.html">rand</a>::<wbr><a href="index.html">seq</a></div><h1>Trait <span class="trait">Iterator<wbr>Random</span> <button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../src/rand/seq/iterator.rs.html#34-269">Source</a> </span></div><pre class="rust item-decl"><code>pub trait IteratorRandom: <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html" title="trait core::iter::traits::iterator::Iterator">Iterator</a> + <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> {
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// Provided methods
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fn <a href="#method.choose" class="fn">choose</a><R>(self, rng: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.reference.html">&mut R</a>) -> <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/option/enum.Option.html" title="enum core::option::Option">Option</a><Self::<a class="associatedtype" href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#associatedtype.Item" title="type core::iter::traits::iterator::Iterator::Item">Item</a>>
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<span class="where">where R: <a class="trait" href="../trait.Rng.html" title="trait rand::Rng">Rng</a> + ?<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a></span> { ... }
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<span class="item-spacer"></span> fn <a href="#method.choose_stable" class="fn">choose_stable</a><R>(self, rng: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.reference.html">&mut R</a>) -> <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/option/enum.Option.html" title="enum core::option::Option">Option</a><Self::<a class="associatedtype" href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#associatedtype.Item" title="type core::iter::traits::iterator::Iterator::Item">Item</a>>
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<span class="where">where R: <a class="trait" href="../trait.Rng.html" title="trait rand::Rng">Rng</a> + ?<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a></span> { ... }
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<span class="item-spacer"></span> fn <a href="#method.choose_multiple_fill" class="fn">choose_multiple_fill</a><R>(
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self,
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rng: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.reference.html">&mut R</a>,
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buf: &mut [Self::<a class="associatedtype" href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#associatedtype.Item" title="type core::iter::traits::iterator::Iterator::Item">Item</a>],
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) -> <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.usize.html">usize</a>
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<span class="where">where R: <a class="trait" href="../trait.Rng.html" title="trait rand::Rng">Rng</a> + ?<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a></span> { ... }
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<span class="item-spacer"></span> fn <a href="#method.choose_multiple" class="fn">choose_multiple</a><R>(self, rng: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.reference.html">&mut R</a>, amount: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.usize.html">usize</a>) -> <a class="struct" href="https://doc.rust-lang.org/1.93.1/alloc/vec/struct.Vec.html" title="struct alloc::vec::Vec">Vec</a><Self::<a class="associatedtype" href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#associatedtype.Item" title="type core::iter::traits::iterator::Iterator::Item">Item</a>>
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<span class="where">where R: <a class="trait" href="../trait.Rng.html" title="trait rand::Rng">Rng</a> + ?<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a></span> { ... }
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}</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Extension trait on iterators, providing random sampling methods.</p>
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<p>This trait is implemented on all iterators <code>I</code> where <code>I: Iterator + Sized</code>
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and provides methods for
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choosing one or more elements. You must <code>use</code> this trait:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>rand::seq::IteratorRandom;
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<span class="kw">let </span>faces = <span class="string">"😀😎😐😕😠😢"</span>;
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<span class="macro">println!</span>(<span class="string">"I am {}!"</span>, faces.chars().choose(<span class="kw-2">&mut </span>rand::rng()).unwrap());</code></pre></div>
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<p>Example output (non-deterministic):</p>
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<div class="example-wrap"><pre class="language-none"><code>I am 😀!</code></pre></div></div></details><h2 id="provided-methods" class="section-header">Provided Methods<a href="#provided-methods" class="anchor">§</a></h2><div class="methods"><details class="toggle method-toggle" open><summary><section id="method.choose" class="method"><a class="src rightside" href="../../src/rand/seq/iterator.rs.html#66-119">Source</a><h4 class="code-header">fn <a href="#method.choose" class="fn">choose</a><R>(self, rng: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.reference.html">&mut R</a>) -> <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/option/enum.Option.html" title="enum core::option::Option">Option</a><Self::<a class="associatedtype" href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#associatedtype.Item" title="type core::iter::traits::iterator::Iterator::Item">Item</a>><div class="where">where
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R: <a class="trait" href="../trait.Rng.html" title="trait rand::Rng">Rng</a> + ?<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,</div></h4></section></summary><div class="docblock"><p>Uniformly sample one element</p>
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<p>Assuming that the <a href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#method.size_hint" title="method core::iter::traits::iterator::Iterator::size_hint"><code>Iterator::size_hint</code></a> is correct, this method
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returns one uniformly-sampled random element of the slice, or <code>None</code>
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only if the slice is empty. Incorrect bounds on the <code>size_hint</code> may
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cause this method to incorrectly return <code>None</code> if fewer elements than
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the advertised <code>lower</code> bound are present and may prevent sampling of
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elements beyond an advertised <code>upper</code> bound (i.e. incorrect <code>size_hint</code>
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is memory-safe, but may result in unexpected <code>None</code> result and
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non-uniform distribution).</p>
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<p>With an accurate <a href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#method.size_hint" title="method core::iter::traits::iterator::Iterator::size_hint"><code>Iterator::size_hint</code></a> and where <a href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#method.nth" title="method core::iter::traits::iterator::Iterator::nth"><code>Iterator::nth</code></a> is
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a constant-time operation, this method can offer <code>O(1)</code> performance.
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Where no size hint is
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available, complexity is <code>O(n)</code> where <code>n</code> is the iterator length.
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Partial hints (where <code>lower > 0</code>) also improve performance.</p>
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<p>Note further that <a href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#method.size_hint" title="method core::iter::traits::iterator::Iterator::size_hint"><code>Iterator::size_hint</code></a> may affect the number of RNG
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samples used as well as the result (while remaining uniform sampling).
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Consider instead using <a href="trait.IteratorRandom.html#method.choose_stable" title="method rand::seq::IteratorRandom::choose_stable"><code>IteratorRandom::choose_stable</code></a> to avoid
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<a href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html" title="trait core::iter::traits::iterator::Iterator"><code>Iterator</code></a> combinators which only change size hints from affecting the
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results.</p>
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<h5 id="example"><a class="doc-anchor" href="#example">§</a>Example</h5>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>rand::seq::IteratorRandom;
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<span class="kw">let </span>words = <span class="string">"Mary had a little lamb"</span>.split(<span class="string">' '</span>);
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<span class="macro">println!</span>(<span class="string">"{}"</span>, words.choose(<span class="kw-2">&mut </span>rand::rng()).unwrap());</code></pre></div></div></details><details class="toggle method-toggle" open><summary><section id="method.choose_stable" class="method"><a class="src rightside" href="../../src/rand/seq/iterator.rs.html#145-186">Source</a><h4 class="code-header">fn <a href="#method.choose_stable" class="fn">choose_stable</a><R>(self, rng: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.reference.html">&mut R</a>) -> <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/option/enum.Option.html" title="enum core::option::Option">Option</a><Self::<a class="associatedtype" href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#associatedtype.Item" title="type core::iter::traits::iterator::Iterator::Item">Item</a>><div class="where">where
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R: <a class="trait" href="../trait.Rng.html" title="trait rand::Rng">Rng</a> + ?<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,</div></h4></section></summary><div class="docblock"><p>Uniformly sample one element (stable)</p>
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<p>This method is very similar to <a href="trait.IteratorRandom.html#method.choose" title="method rand::seq::IteratorRandom::choose"><code>choose</code></a> except that the result
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only depends on the length of the iterator and the values produced by
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<code>rng</code>. Notably for any iterator of a given length this will make the
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same requests to <code>rng</code> and if the same sequence of values are produced
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the same index will be selected from <code>self</code>. This may be useful if you
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need consistent results no matter what type of iterator you are working
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with. If you do not need this stability prefer <a href="trait.IteratorRandom.html#method.choose" title="method rand::seq::IteratorRandom::choose"><code>choose</code></a>.</p>
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<p>Note that this method still uses <a href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#method.size_hint" title="method core::iter::traits::iterator::Iterator::size_hint"><code>Iterator::size_hint</code></a> to skip
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constructing elements where possible, however the selection and <code>rng</code>
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calls are the same in the face of this optimization. If you want to
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force every element to be created regardless call <code>.inspect(|e| ())</code>.</p>
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</div></details><details class="toggle method-toggle" open><summary><section id="method.choose_multiple_fill" class="method"><a class="src rightside" href="../../src/rand/seq/iterator.rs.html#203-227">Source</a><h4 class="code-header">fn <a href="#method.choose_multiple_fill" class="fn">choose_multiple_fill</a><R>(self, rng: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.reference.html">&mut R</a>, buf: &mut [Self::<a class="associatedtype" href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#associatedtype.Item" title="type core::iter::traits::iterator::Iterator::Item">Item</a>]) -> <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.usize.html">usize</a><div class="where">where
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R: <a class="trait" href="../trait.Rng.html" title="trait rand::Rng">Rng</a> + ?<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,</div></h4></section></summary><div class="docblock"><p>Uniformly sample <code>amount</code> distinct elements into a buffer</p>
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<p>Collects values at random from the iterator into a supplied buffer
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until that buffer is filled.</p>
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<p>Although the elements are selected randomly, the order of elements in
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the buffer is neither stable nor fully random. If random ordering is
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desired, shuffle the result.</p>
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<p>Returns the number of elements added to the buffer. This equals the length
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of the buffer unless the iterator contains insufficient elements, in which
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case this equals the number of elements available.</p>
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<p>Complexity is <code>O(n)</code> where <code>n</code> is the length of the iterator.
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For slices, prefer <a href="trait.IndexedRandom.html#method.choose_multiple" title="method rand::seq::IndexedRandom::choose_multiple"><code>IndexedRandom::choose_multiple</code></a>.</p>
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</div></details><details class="toggle method-toggle" open><summary><section id="method.choose_multiple" class="method"><a class="src rightside" href="../../src/rand/seq/iterator.rs.html#244-268">Source</a><h4 class="code-header">fn <a href="#method.choose_multiple" class="fn">choose_multiple</a><R>(self, rng: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.reference.html">&mut R</a>, amount: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.usize.html">usize</a>) -> <a class="struct" href="https://doc.rust-lang.org/1.93.1/alloc/vec/struct.Vec.html" title="struct alloc::vec::Vec">Vec</a><Self::<a class="associatedtype" href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html#associatedtype.Item" title="type core::iter::traits::iterator::Iterator::Item">Item</a>><div class="where">where
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R: <a class="trait" href="../trait.Rng.html" title="trait rand::Rng">Rng</a> + ?<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,</div></h4></section></summary><div class="docblock"><p>Uniformly sample <code>amount</code> distinct elements into a <a href="https://doc.rust-lang.org/1.93.1/alloc/vec/struct.Vec.html" title="struct alloc::vec::Vec"><code>Vec</code></a></p>
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<p>This is equivalent to <code>choose_multiple_fill</code> except for the result type.</p>
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<p>Although the elements are selected randomly, the order of elements in
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the buffer is neither stable nor fully random. If random ordering is
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desired, shuffle the result.</p>
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<p>The length of the returned vector equals <code>amount</code> unless the iterator
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contains insufficient elements, in which case it equals the number of
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elements available.</p>
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<p>Complexity is <code>O(n)</code> where <code>n</code> is the length of the iterator.
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For slices, prefer <a href="trait.IndexedRandom.html#method.choose_multiple" title="method rand::seq::IndexedRandom::choose_multiple"><code>IndexedRandom::choose_multiple</code></a>.</p>
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</div></details></div><h2 id="dyn-compatibility" class="section-header">Dyn Compatibility<a href="#dyn-compatibility" class="anchor">§</a></h2><div class="dyn-compatibility-info"><p>This trait is <b>not</b> <a href="https://doc.rust-lang.org/1.93.1/reference/items/traits.html#dyn-compatibility">dyn compatible</a>.</p><p><i>In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.</i></p></div><h2 id="implementors" class="section-header">Implementors<a href="#implementors" class="anchor">§</a></h2><div id="implementors-list"><section id="impl-IteratorRandom-for-I" class="impl"><a class="src rightside" href="../../src/rand/seq/iterator.rs.html#271">Source</a><a href="#impl-IteratorRandom-for-I" class="anchor">§</a><h3 class="code-header">impl<I> <a class="trait" href="trait.IteratorRandom.html" title="trait rand::seq::IteratorRandom">IteratorRandom</a> for I<div class="where">where
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I: <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/iter/traits/iterator/trait.Iterator.html" title="trait core::iter::traits::iterator::Iterator">Iterator</a> + <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,</div></h3></section></div><script src="../../trait.impl/rand/seq/iterator/trait.IteratorRandom.js" async></script></section></div></main></body></html> |