<!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="Source to the Rust file `libstd/collections/mod.rs`."> <meta name="keywords" content="rust, rustlang, rust-lang"> <title>mod.rs.html -- source</title> <link rel="stylesheet" type="text/css" href="../../../normalize.css"> <link rel="stylesheet" type="text/css" href="../../../rustdoc.css" id="mainThemeStyle"> <link rel="stylesheet" type="text/css" href="../../../dark.css"> <link rel="stylesheet" type="text/css" href="../../../main.css" id="themeStyle"> <script src="../../../storage.js"></script> <link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"> </head> <body class="rustdoc source"> <!--[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='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a> </nav> <div class="theme-picker"> <button id="theme-picker" aria-label="Pick another theme!"> <img src="../../../brush.svg" width="18" alt="Pick another theme!"> </button> <div id="theme-choices"></div> </div> <script src="../../../theme.js"></script> <nav class="sub"> <form class="search-form js-only"> <div class="search-container"> <input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"> </div> </form> </nav> <section id='main' class="content"><pre class="line-numbers"><span id="1"> 1</span> <span id="2"> 2</span> <span id="3"> 3</span> <span id="4"> 4</span> <span id="5"> 5</span> <span id="6"> 6</span> <span id="7"> 7</span> <span id="8"> 8</span> <span id="9"> 9</span> <span id="10"> 10</span> <span id="11"> 11</span> <span id="12"> 12</span> <span id="13"> 13</span> <span id="14"> 14</span> <span id="15"> 15</span> <span id="16"> 16</span> <span id="17"> 17</span> <span id="18"> 18</span> <span id="19"> 19</span> <span id="20"> 20</span> <span id="21"> 21</span> <span id="22"> 22</span> <span id="23"> 23</span> <span id="24"> 24</span> <span id="25"> 25</span> <span id="26"> 26</span> <span id="27"> 27</span> <span id="28"> 28</span> <span id="29"> 29</span> <span id="30"> 30</span> <span id="31"> 31</span> <span id="32"> 32</span> <span id="33"> 33</span> <span id="34"> 34</span> <span id="35"> 35</span> <span id="36"> 36</span> <span id="37"> 37</span> <span id="38"> 38</span> <span id="39"> 39</span> <span id="40"> 40</span> <span id="41"> 41</span> <span id="42"> 42</span> <span id="43"> 43</span> <span id="44"> 44</span> <span id="45"> 45</span> <span id="46"> 46</span> <span id="47"> 47</span> <span id="48"> 48</span> <span id="49"> 49</span> <span id="50"> 50</span> <span id="51"> 51</span> <span id="52"> 52</span> <span id="53"> 53</span> <span id="54"> 54</span> <span id="55"> 55</span> <span id="56"> 56</span> <span id="57"> 57</span> <span id="58"> 58</span> <span id="59"> 59</span> <span id="60"> 60</span> <span id="61"> 61</span> <span id="62"> 62</span> <span id="63"> 63</span> <span id="64"> 64</span> <span id="65"> 65</span> <span id="66"> 66</span> <span id="67"> 67</span> <span id="68"> 68</span> <span id="69"> 69</span> <span id="70"> 70</span> <span id="71"> 71</span> <span id="72"> 72</span> <span id="73"> 73</span> <span id="74"> 74</span> <span id="75"> 75</span> <span id="76"> 76</span> <span id="77"> 77</span> <span id="78"> 78</span> <span id="79"> 79</span> <span id="80"> 80</span> <span id="81"> 81</span> <span id="82"> 82</span> <span id="83"> 83</span> <span id="84"> 84</span> <span id="85"> 85</span> <span id="86"> 86</span> <span id="87"> 87</span> <span id="88"> 88</span> <span id="89"> 89</span> <span id="90"> 90</span> <span id="91"> 91</span> <span id="92"> 92</span> <span id="93"> 93</span> <span id="94"> 94</span> <span id="95"> 95</span> <span id="96"> 96</span> <span id="97"> 97</span> <span id="98"> 98</span> <span id="99"> 99</span> <span id="100">100</span> <span id="101">101</span> <span id="102">102</span> <span id="103">103</span> <span id="104">104</span> <span id="105">105</span> <span id="106">106</span> <span id="107">107</span> <span id="108">108</span> <span id="109">109</span> <span id="110">110</span> <span id="111">111</span> <span id="112">112</span> <span id="113">113</span> <span id="114">114</span> <span id="115">115</span> <span id="116">116</span> <span id="117">117</span> <span id="118">118</span> <span id="119">119</span> <span id="120">120</span> <span id="121">121</span> <span id="122">122</span> <span id="123">123</span> <span id="124">124</span> <span id="125">125</span> <span id="126">126</span> <span id="127">127</span> <span id="128">128</span> <span id="129">129</span> <span id="130">130</span> <span id="131">131</span> <span id="132">132</span> <span id="133">133</span> <span id="134">134</span> <span id="135">135</span> <span id="136">136</span> <span id="137">137</span> <span id="138">138</span> <span id="139">139</span> <span id="140">140</span> <span id="141">141</span> <span id="142">142</span> <span id="143">143</span> <span id="144">144</span> <span id="145">145</span> <span id="146">146</span> <span id="147">147</span> <span id="148">148</span> <span id="149">149</span> <span id="150">150</span> <span id="151">151</span> <span id="152">152</span> <span id="153">153</span> <span id="154">154</span> <span id="155">155</span> <span id="156">156</span> <span id="157">157</span> <span id="158">158</span> <span id="159">159</span> <span id="160">160</span> <span id="161">161</span> <span id="162">162</span> <span id="163">163</span> <span id="164">164</span> <span id="165">165</span> <span id="166">166</span> <span id="167">167</span> <span id="168">168</span> <span id="169">169</span> <span id="170">170</span> <span id="171">171</span> <span id="172">172</span> <span id="173">173</span> <span id="174">174</span> <span id="175">175</span> <span id="176">176</span> <span id="177">177</span> <span id="178">178</span> <span id="179">179</span> <span id="180">180</span> <span id="181">181</span> <span id="182">182</span> <span id="183">183</span> <span id="184">184</span> <span id="185">185</span> <span id="186">186</span> <span id="187">187</span> <span id="188">188</span> <span id="189">189</span> <span id="190">190</span> <span id="191">191</span> <span id="192">192</span> <span id="193">193</span> <span id="194">194</span> <span id="195">195</span> <span id="196">196</span> <span id="197">197</span> <span id="198">198</span> <span id="199">199</span> <span id="200">200</span> <span id="201">201</span> <span id="202">202</span> <span id="203">203</span> <span id="204">204</span> <span id="205">205</span> <span id="206">206</span> <span id="207">207</span> <span id="208">208</span> <span id="209">209</span> <span id="210">210</span> <span id="211">211</span> <span id="212">212</span> <span id="213">213</span> <span id="214">214</span> <span id="215">215</span> <span id="216">216</span> <span id="217">217</span> <span id="218">218</span> <span id="219">219</span> <span id="220">220</span> <span id="221">221</span> <span id="222">222</span> <span id="223">223</span> <span id="224">224</span> <span id="225">225</span> <span id="226">226</span> <span id="227">227</span> <span id="228">228</span> <span id="229">229</span> <span id="230">230</span> <span id="231">231</span> <span id="232">232</span> <span id="233">233</span> <span id="234">234</span> <span id="235">235</span> <span id="236">236</span> <span id="237">237</span> <span id="238">238</span> <span id="239">239</span> <span id="240">240</span> <span id="241">241</span> <span id="242">242</span> <span id="243">243</span> <span id="244">244</span> <span id="245">245</span> <span id="246">246</span> <span id="247">247</span> <span id="248">248</span> <span id="249">249</span> <span id="250">250</span> <span id="251">251</span> <span id="252">252</span> <span id="253">253</span> <span id="254">254</span> <span id="255">255</span> <span id="256">256</span> <span id="257">257</span> <span id="258">258</span> <span id="259">259</span> <span id="260">260</span> <span id="261">261</span> <span id="262">262</span> <span id="263">263</span> <span id="264">264</span> <span id="265">265</span> <span id="266">266</span> <span id="267">267</span> <span id="268">268</span> <span id="269">269</span> <span id="270">270</span> <span id="271">271</span> <span id="272">272</span> <span id="273">273</span> <span id="274">274</span> <span id="275">275</span> <span id="276">276</span> <span id="277">277</span> <span id="278">278</span> <span id="279">279</span> <span id="280">280</span> <span id="281">281</span> <span id="282">282</span> <span id="283">283</span> <span id="284">284</span> <span id="285">285</span> <span id="286">286</span> <span id="287">287</span> <span id="288">288</span> <span id="289">289</span> <span id="290">290</span> <span id="291">291</span> <span id="292">292</span> <span id="293">293</span> <span id="294">294</span> <span id="295">295</span> <span id="296">296</span> <span id="297">297</span> <span id="298">298</span> <span id="299">299</span> <span id="300">300</span> <span id="301">301</span> <span id="302">302</span> <span id="303">303</span> <span id="304">304</span> <span id="305">305</span> <span id="306">306</span> <span id="307">307</span> <span id="308">308</span> <span id="309">309</span> <span id="310">310</span> <span id="311">311</span> <span id="312">312</span> <span id="313">313</span> <span id="314">314</span> <span id="315">315</span> <span id="316">316</span> <span id="317">317</span> <span id="318">318</span> <span id="319">319</span> <span id="320">320</span> <span id="321">321</span> <span id="322">322</span> <span id="323">323</span> <span id="324">324</span> <span id="325">325</span> <span id="326">326</span> <span id="327">327</span> <span id="328">328</span> <span id="329">329</span> <span id="330">330</span> <span id="331">331</span> <span id="332">332</span> <span id="333">333</span> <span id="334">334</span> <span id="335">335</span> <span id="336">336</span> <span id="337">337</span> <span id="338">338</span> <span id="339">339</span> <span id="340">340</span> <span id="341">341</span> <span id="342">342</span> <span id="343">343</span> <span id="344">344</span> <span id="345">345</span> <span id="346">346</span> <span id="347">347</span> <span id="348">348</span> <span id="349">349</span> <span id="350">350</span> <span id="351">351</span> <span id="352">352</span> <span id="353">353</span> <span id="354">354</span> <span id="355">355</span> <span id="356">356</span> <span id="357">357</span> <span id="358">358</span> <span id="359">359</span> <span id="360">360</span> <span id="361">361</span> <span id="362">362</span> <span id="363">363</span> <span id="364">364</span> <span id="365">365</span> <span id="366">366</span> <span id="367">367</span> <span id="368">368</span> <span id="369">369</span> <span id="370">370</span> <span id="371">371</span> <span id="372">372</span> <span id="373">373</span> <span id="374">374</span> <span id="375">375</span> <span id="376">376</span> <span id="377">377</span> <span id="378">378</span> <span id="379">379</span> <span id="380">380</span> <span id="381">381</span> <span id="382">382</span> <span id="383">383</span> <span id="384">384</span> <span id="385">385</span> <span id="386">386</span> <span id="387">387</span> <span id="388">388</span> <span id="389">389</span> <span id="390">390</span> <span id="391">391</span> <span id="392">392</span> <span id="393">393</span> <span id="394">394</span> <span id="395">395</span> <span id="396">396</span> <span id="397">397</span> <span id="398">398</span> <span id="399">399</span> <span id="400">400</span> <span id="401">401</span> <span id="402">402</span> <span id="403">403</span> <span id="404">404</span> <span id="405">405</span> <span id="406">406</span> <span id="407">407</span> <span id="408">408</span> <span id="409">409</span> <span id="410">410</span> <span id="411">411</span> <span id="412">412</span> <span id="413">413</span> <span id="414">414</span> <span id="415">415</span> <span id="416">416</span> <span id="417">417</span> <span id="418">418</span> <span id="419">419</span> <span id="420">420</span> <span id="421">421</span> <span id="422">422</span> <span id="423">423</span> <span id="424">424</span> <span id="425">425</span> <span id="426">426</span> <span id="427">427</span> <span id="428">428</span> <span id="429">429</span> <span id="430">430</span> <span id="431">431</span> <span id="432">432</span> <span id="433">433</span> <span id="434">434</span> <span id="435">435</span> <span id="436">436</span> <span id="437">437</span> <span id="438">438</span> <span id="439">439</span> <span id="440">440</span> <span id="441">441</span> <span id="442">442</span> <span id="443">443</span> <span id="444">444</span> <span id="445">445</span> <span id="446">446</span> <span id="447">447</span> <span id="448">448</span> <span id="449">449</span> <span id="450">450</span> <span id="451">451</span> <span id="452">452</span> <span id="453">453</span> <span id="454">454</span> <span id="455">455</span> </pre><pre class="rust "> <span class="comment">// Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT</span> <span class="comment">// file at the top-level directory of this distribution and at</span> <span class="comment">// http://rust-lang.org/COPYRIGHT.</span> <span class="comment">//</span> <span class="comment">// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span> <span class="comment">// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span> <span class="comment">// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span> <span class="comment">// option. This file may not be copied, modified, or distributed</span> <span class="comment">// except according to those terms.</span> <span class="doccomment">//! Collection types.</span> <span class="doccomment">//!</span> <span class="doccomment">//! Rust's standard collection library provides efficient implementations of the</span> <span class="doccomment">//! most common general purpose programming data structures. By using the</span> <span class="doccomment">//! standard implementations, it should be possible for two libraries to</span> <span class="doccomment">//! communicate without significant data conversion.</span> <span class="doccomment">//!</span> <span class="doccomment">//! To get this out of the way: you should probably just use [`Vec`] or [`HashMap`].</span> <span class="doccomment">//! These two collections cover most use cases for generic data storage and</span> <span class="doccomment">//! processing. They are exceptionally good at doing what they do. All the other</span> <span class="doccomment">//! collections in the standard library have specific use cases where they are</span> <span class="doccomment">//! the optimal choice, but these cases are borderline *niche* in comparison.</span> <span class="doccomment">//! Even when `Vec` and `HashMap` are technically suboptimal, they're probably a</span> <span class="doccomment">//! good enough choice to get started.</span> <span class="doccomment">//!</span> <span class="doccomment">//! Rust's collections can be grouped into four major categories:</span> <span class="doccomment">//!</span> <span class="doccomment">//! * Sequences: [`Vec`], [`VecDeque`], [`LinkedList`]</span> <span class="doccomment">//! * Maps: [`HashMap`], [`BTreeMap`]</span> <span class="doccomment">//! * Sets: [`HashSet`], [`BTreeSet`]</span> <span class="doccomment">//! * Misc: [`BinaryHeap`]</span> <span class="doccomment">//!</span> <span class="doccomment">//! # When Should You Use Which Collection?</span> <span class="doccomment">//!</span> <span class="doccomment">//! These are fairly high-level and quick break-downs of when each collection</span> <span class="doccomment">//! should be considered. Detailed discussions of strengths and weaknesses of</span> <span class="doccomment">//! individual collections can be found on their own documentation pages.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ### Use a `Vec` when:</span> <span class="doccomment">//! * You want to collect items up to be processed or sent elsewhere later, and</span> <span class="doccomment">//! don't care about any properties of the actual values being stored.</span> <span class="doccomment">//! * You want a sequence of elements in a particular order, and will only be</span> <span class="doccomment">//! appending to (or near) the end.</span> <span class="doccomment">//! * You want a stack.</span> <span class="doccomment">//! * You want a resizable array.</span> <span class="doccomment">//! * You want a heap-allocated array.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ### Use a `VecDeque` when:</span> <span class="doccomment">//! * You want a [`Vec`] that supports efficient insertion at both ends of the</span> <span class="doccomment">//! sequence.</span> <span class="doccomment">//! * You want a queue.</span> <span class="doccomment">//! * You want a double-ended queue (deque).</span> <span class="doccomment">//!</span> <span class="doccomment">//! ### Use a `LinkedList` when:</span> <span class="doccomment">//! * You want a [`Vec`] or [`VecDeque`] of unknown size, and can't tolerate</span> <span class="doccomment">//! amortization.</span> <span class="doccomment">//! * You want to efficiently split and append lists.</span> <span class="doccomment">//! * You are *absolutely* certain you *really*, *truly*, want a doubly linked</span> <span class="doccomment">//! list.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ### Use a `HashMap` when:</span> <span class="doccomment">//! * You want to associate arbitrary keys with an arbitrary value.</span> <span class="doccomment">//! * You want a cache.</span> <span class="doccomment">//! * You want a map, with no extra functionality.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ### Use a `BTreeMap` when:</span> <span class="doccomment">//! * You want a map sorted by its keys.</span> <span class="doccomment">//! * You want to be able to get a range of entries on-demand.</span> <span class="doccomment">//! * You're interested in what the smallest or largest key-value pair is.</span> <span class="doccomment">//! * You want to find the largest or smallest key that is smaller or larger</span> <span class="doccomment">//! than something.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ### Use the `Set` variant of any of these `Map`s when:</span> <span class="doccomment">//! * You just want to remember which keys you've seen.</span> <span class="doccomment">//! * There is no meaningful value to associate with your keys.</span> <span class="doccomment">//! * You just want a set.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ### Use a `BinaryHeap` when:</span> <span class="doccomment">//!</span> <span class="doccomment">//! * You want to store a bunch of elements, but only ever want to process the</span> <span class="doccomment">//! "biggest" or "most important" one at any given time.</span> <span class="doccomment">//! * You want a priority queue.</span> <span class="doccomment">//!</span> <span class="doccomment">//! # Performance</span> <span class="doccomment">//!</span> <span class="doccomment">//! Choosing the right collection for the job requires an understanding of what</span> <span class="doccomment">//! each collection is good at. Here we briefly summarize the performance of</span> <span class="doccomment">//! different collections for certain important operations. For further details,</span> <span class="doccomment">//! see each type's documentation, and note that the names of actual methods may</span> <span class="doccomment">//! differ from the tables below on certain collections.</span> <span class="doccomment">//!</span> <span class="doccomment">//! Throughout the documentation, we will follow a few conventions. For all</span> <span class="doccomment">//! operations, the collection's size is denoted by n. If another collection is</span> <span class="doccomment">//! involved in the operation, it contains m elements. Operations which have an</span> <span class="doccomment">//! *amortized* cost are suffixed with a `*`. Operations with an *expected*</span> <span class="doccomment">//! cost are suffixed with a `~`.</span> <span class="doccomment">//!</span> <span class="doccomment">//! All amortized costs are for the potential need to resize when capacity is</span> <span class="doccomment">//! exhausted. If a resize occurs it will take O(n) time. Our collections never</span> <span class="doccomment">//! automatically shrink, so removal operations aren't amortized. Over a</span> <span class="doccomment">//! sufficiently large series of operations, the average cost per operation will</span> <span class="doccomment">//! deterministically equal the given cost.</span> <span class="doccomment">//!</span> <span class="doccomment">//! Only [`HashMap`] has expected costs, due to the probabilistic nature of hashing.</span> <span class="doccomment">//! It is theoretically possible, though very unlikely, for [`HashMap`] to</span> <span class="doccomment">//! experience worse performance.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ## Sequences</span> <span class="doccomment">//!</span> <span class="doccomment">//! | | get(i) | insert(i) | remove(i) | append | split_off(i) |</span> <span class="doccomment">//! |----------------|----------------|-----------------|----------------|--------|----------------|</span> <span class="doccomment">//! | [`Vec`] | O(1) | O(n-i)* | O(n-i) | O(m)* | O(n-i) |</span> <span class="doccomment">//! | [`VecDeque`] | O(1) | O(min(i, n-i))* | O(min(i, n-i)) | O(m)* | O(min(i, n-i)) |</span> <span class="doccomment">//! | [`LinkedList`] | O(min(i, n-i)) | O(min(i, n-i)) | O(min(i, n-i)) | O(1) | O(min(i, n-i)) |</span> <span class="doccomment">//!</span> <span class="doccomment">//! Note that where ties occur, [`Vec`] is generally going to be faster than [`VecDeque`], and</span> <span class="doccomment">//! [`VecDeque`] is generally going to be faster than [`LinkedList`].</span> <span class="doccomment">//!</span> <span class="doccomment">//! ## Maps</span> <span class="doccomment">//!</span> <span class="doccomment">//! For Sets, all operations have the cost of the equivalent Map operation.</span> <span class="doccomment">//!</span> <span class="doccomment">//! | | get | insert | remove | predecessor | append |</span> <span class="doccomment">//! |--------------|-----------|----------|----------|-------------|--------|</span> <span class="doccomment">//! | [`HashMap`] | O(1)~ | O(1)~* | O(1)~ | N/A | N/A |</span> <span class="doccomment">//! | [`BTreeMap`] | O(log n) | O(log n) | O(log n) | O(log n) | O(n+m) |</span> <span class="doccomment">//!</span> <span class="doccomment">//! # Correct and Efficient Usage of Collections</span> <span class="doccomment">//!</span> <span class="doccomment">//! Of course, knowing which collection is the right one for the job doesn't</span> <span class="doccomment">//! instantly permit you to use it correctly. Here are some quick tips for</span> <span class="doccomment">//! efficient and correct usage of the standard collections in general. If</span> <span class="doccomment">//! you're interested in how to use a specific collection in particular, consult</span> <span class="doccomment">//! its documentation for detailed discussion and code examples.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ## Capacity Management</span> <span class="doccomment">//!</span> <span class="doccomment">//! Many collections provide several constructors and methods that refer to</span> <span class="doccomment">//! "capacity". These collections are generally built on top of an array.</span> <span class="doccomment">//! Optimally, this array would be exactly the right size to fit only the</span> <span class="doccomment">//! elements stored in the collection, but for the collection to do this would</span> <span class="doccomment">//! be very inefficient. If the backing array was exactly the right size at all</span> <span class="doccomment">//! times, then every time an element is inserted, the collection would have to</span> <span class="doccomment">//! grow the array to fit it. Due to the way memory is allocated and managed on</span> <span class="doccomment">//! most computers, this would almost surely require allocating an entirely new</span> <span class="doccomment">//! array and copying every single element from the old one into the new one.</span> <span class="doccomment">//! Hopefully you can see that this wouldn't be very efficient to do on every</span> <span class="doccomment">//! operation.</span> <span class="doccomment">//!</span> <span class="doccomment">//! Most collections therefore use an *amortized* allocation strategy. They</span> <span class="doccomment">//! generally let themselves have a fair amount of unoccupied space so that they</span> <span class="doccomment">//! only have to grow on occasion. When they do grow, they allocate a</span> <span class="doccomment">//! substantially larger array to move the elements into so that it will take a</span> <span class="doccomment">//! while for another grow to be required. While this strategy is great in</span> <span class="doccomment">//! general, it would be even better if the collection *never* had to resize its</span> <span class="doccomment">//! backing array. Unfortunately, the collection itself doesn't have enough</span> <span class="doccomment">//! information to do this itself. Therefore, it is up to us programmers to give</span> <span class="doccomment">//! it hints.</span> <span class="doccomment">//!</span> <span class="doccomment">//! Any `with_capacity` constructor will instruct the collection to allocate</span> <span class="doccomment">//! enough space for the specified number of elements. Ideally this will be for</span> <span class="doccomment">//! exactly that many elements, but some implementation details may prevent</span> <span class="doccomment">//! this. [`Vec`] and [`VecDeque`] can be relied on to allocate exactly the</span> <span class="doccomment">//! requested amount, though. Use `with_capacity` when you know exactly how many</span> <span class="doccomment">//! elements will be inserted, or at least have a reasonable upper-bound on that</span> <span class="doccomment">//! number.</span> <span class="doccomment">//!</span> <span class="doccomment">//! When anticipating a large influx of elements, the `reserve` family of</span> <span class="doccomment">//! methods can be used to hint to the collection how much room it should make</span> <span class="doccomment">//! for the coming items. As with `with_capacity`, the precise behavior of</span> <span class="doccomment">//! these methods will be specific to the collection of interest.</span> <span class="doccomment">//!</span> <span class="doccomment">//! For optimal performance, collections will generally avoid shrinking</span> <span class="doccomment">//! themselves. If you believe that a collection will not soon contain any more</span> <span class="doccomment">//! elements, or just really need the memory, the `shrink_to_fit` method prompts</span> <span class="doccomment">//! the collection to shrink the backing array to the minimum size capable of</span> <span class="doccomment">//! holding its elements.</span> <span class="doccomment">//!</span> <span class="doccomment">//! Finally, if ever you're interested in what the actual capacity of the</span> <span class="doccomment">//! collection is, most collections provide a `capacity` method to query this</span> <span class="doccomment">//! information on demand. This can be useful for debugging purposes, or for</span> <span class="doccomment">//! use with the `reserve` methods.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ## Iterators</span> <span class="doccomment">//!</span> <span class="doccomment">//! Iterators are a powerful and robust mechanism used throughout Rust's</span> <span class="doccomment">//! standard libraries. Iterators provide a sequence of values in a generic,</span> <span class="doccomment">//! safe, efficient and convenient way. The contents of an iterator are usually</span> <span class="doccomment">//! *lazily* evaluated, so that only the values that are actually needed are</span> <span class="doccomment">//! ever actually produced, and no allocation need be done to temporarily store</span> <span class="doccomment">//! them. Iterators are primarily consumed using a `for` loop, although many</span> <span class="doccomment">//! functions also take iterators where a collection or sequence of values is</span> <span class="doccomment">//! desired.</span> <span class="doccomment">//!</span> <span class="doccomment">//! All of the standard collections provide several iterators for performing</span> <span class="doccomment">//! bulk manipulation of their contents. The three primary iterators almost</span> <span class="doccomment">//! every collection should provide are `iter`, `iter_mut`, and `into_iter`.</span> <span class="doccomment">//! Some of these are not provided on collections where it would be unsound or</span> <span class="doccomment">//! unreasonable to provide them.</span> <span class="doccomment">//!</span> <span class="doccomment">//! `iter` provides an iterator of immutable references to all the contents of a</span> <span class="doccomment">//! collection in the most "natural" order. For sequence collections like [`Vec`],</span> <span class="doccomment">//! this means the items will be yielded in increasing order of index starting</span> <span class="doccomment">//! at 0. For ordered collections like [`BTreeMap`], this means that the items</span> <span class="doccomment">//! will be yielded in sorted order. For unordered collections like [`HashMap`],</span> <span class="doccomment">//! the items will be yielded in whatever order the internal representation made</span> <span class="doccomment">//! most convenient. This is great for reading through all the contents of the</span> <span class="doccomment">//! collection.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ```</span> <span class="doccomment">//! let vec = vec![1, 2, 3, 4];</span> <span class="doccomment">//! for x in vec.iter() {</span> <span class="doccomment">//! println!("vec contained {}", x);</span> <span class="doccomment">//! }</span> <span class="doccomment">//! ```</span> <span class="doccomment">//!</span> <span class="doccomment">//! `iter_mut` provides an iterator of *mutable* references in the same order as</span> <span class="doccomment">//! `iter`. This is great for mutating all the contents of the collection.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ```</span> <span class="doccomment">//! let mut vec = vec![1, 2, 3, 4];</span> <span class="doccomment">//! for x in vec.iter_mut() {</span> <span class="doccomment">//! *x += 1;</span> <span class="doccomment">//! }</span> <span class="doccomment">//! ```</span> <span class="doccomment">//!</span> <span class="doccomment">//! `into_iter` transforms the actual collection into an iterator over its</span> <span class="doccomment">//! contents by-value. This is great when the collection itself is no longer</span> <span class="doccomment">//! needed, and the values are needed elsewhere. Using `extend` with `into_iter`</span> <span class="doccomment">//! is the main way that contents of one collection are moved into another.</span> <span class="doccomment">//! `extend` automatically calls `into_iter`, and takes any `T: `[`IntoIterator`].</span> <span class="doccomment">//! Calling `collect` on an iterator itself is also a great way to convert one</span> <span class="doccomment">//! collection into another. Both of these methods should internally use the</span> <span class="doccomment">//! capacity management tools discussed in the previous section to do this as</span> <span class="doccomment">//! efficiently as possible.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ```</span> <span class="doccomment">//! let mut vec1 = vec![1, 2, 3, 4];</span> <span class="doccomment">//! let vec2 = vec![10, 20, 30, 40];</span> <span class="doccomment">//! vec1.extend(vec2);</span> <span class="doccomment">//! ```</span> <span class="doccomment">//!</span> <span class="doccomment">//! ```</span> <span class="doccomment">//! use std::collections::VecDeque;</span> <span class="doccomment">//!</span> <span class="doccomment">//! let vec = vec![1, 2, 3, 4];</span> <span class="doccomment">//! let buf: VecDeque<_> = vec.into_iter().collect();</span> <span class="doccomment">//! ```</span> <span class="doccomment">//!</span> <span class="doccomment">//! Iterators also provide a series of *adapter* methods for performing common</span> <span class="doccomment">//! threads to sequences. Among the adapters are functional favorites like `map`,</span> <span class="doccomment">//! `fold`, `skip` and `take`. Of particular interest to collections is the</span> <span class="doccomment">//! `rev` adapter, that reverses any iterator that supports this operation. Most</span> <span class="doccomment">//! collections provide reversible iterators as the way to iterate over them in</span> <span class="doccomment">//! reverse order.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ```</span> <span class="doccomment">//! let vec = vec![1, 2, 3, 4];</span> <span class="doccomment">//! for x in vec.iter().rev() {</span> <span class="doccomment">//! println!("vec contained {}", x);</span> <span class="doccomment">//! }</span> <span class="doccomment">//! ```</span> <span class="doccomment">//!</span> <span class="doccomment">//! Several other collection methods also return iterators to yield a sequence</span> <span class="doccomment">//! of results but avoid allocating an entire collection to store the result in.</span> <span class="doccomment">//! This provides maximum flexibility as `collect` or `extend` can be called to</span> <span class="doccomment">//! "pipe" the sequence into any collection if desired. Otherwise, the sequence</span> <span class="doccomment">//! can be looped over with a `for` loop. The iterator can also be discarded</span> <span class="doccomment">//! after partial use, preventing the computation of the unused items.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ## Entries</span> <span class="doccomment">//!</span> <span class="doccomment">//! The `entry` API is intended to provide an efficient mechanism for</span> <span class="doccomment">//! manipulating the contents of a map conditionally on the presence of a key or</span> <span class="doccomment">//! not. The primary motivating use case for this is to provide efficient</span> <span class="doccomment">//! accumulator maps. For instance, if one wishes to maintain a count of the</span> <span class="doccomment">//! number of times each key has been seen, they will have to perform some</span> <span class="doccomment">//! conditional logic on whether this is the first time the key has been seen or</span> <span class="doccomment">//! not. Normally, this would require a `find` followed by an `insert`,</span> <span class="doccomment">//! effectively duplicating the search effort on each insertion.</span> <span class="doccomment">//!</span> <span class="doccomment">//! When a user calls `map.entry(&key)`, the map will search for the key and</span> <span class="doccomment">//! then yield a variant of the `Entry` enum.</span> <span class="doccomment">//!</span> <span class="doccomment">//! If a `Vacant(entry)` is yielded, then the key *was not* found. In this case</span> <span class="doccomment">//! the only valid operation is to `insert` a value into the entry. When this is</span> <span class="doccomment">//! done, the vacant entry is consumed and converted into a mutable reference to</span> <span class="doccomment">//! the value that was inserted. This allows for further manipulation of the</span> <span class="doccomment">//! value beyond the lifetime of the search itself. This is useful if complex</span> <span class="doccomment">//! logic needs to be performed on the value regardless of whether the value was</span> <span class="doccomment">//! just inserted.</span> <span class="doccomment">//!</span> <span class="doccomment">//! If an `Occupied(entry)` is yielded, then the key *was* found. In this case,</span> <span class="doccomment">//! the user has several options: they can `get`, `insert` or `remove` the</span> <span class="doccomment">//! value of the occupied entry. Additionally, they can convert the occupied</span> <span class="doccomment">//! entry into a mutable reference to its value, providing symmetry to the</span> <span class="doccomment">//! vacant `insert` case.</span> <span class="doccomment">//!</span> <span class="doccomment">//! ### Examples</span> <span class="doccomment">//!</span> <span class="doccomment">//! Here are the two primary ways in which `entry` is used. First, a simple</span> <span class="doccomment">//! example where the logic performed on the values is trivial.</span> <span class="doccomment">//!</span> <span class="doccomment">//! #### Counting the number of times each character in a string occurs</span> <span class="doccomment">//!</span> <span class="doccomment">//! ```</span> <span class="doccomment">//! use std::collections::btree_map::BTreeMap;</span> <span class="doccomment">//!</span> <span class="doccomment">//! let mut count = BTreeMap::new();</span> <span class="doccomment">//! let message = "she sells sea shells by the sea shore";</span> <span class="doccomment">//!</span> <span class="doccomment">//! for c in message.chars() {</span> <span class="doccomment">//! *count.entry(c).or_insert(0) += 1;</span> <span class="doccomment">//! }</span> <span class="doccomment">//!</span> <span class="doccomment">//! assert_eq!(count.get(&'s'), Some(&8));</span> <span class="doccomment">//!</span> <span class="doccomment">//! println!("Number of occurrences of each character");</span> <span class="doccomment">//! for (char, count) in &count {</span> <span class="doccomment">//! println!("{}: {}", char, count);</span> <span class="doccomment">//! }</span> <span class="doccomment">//! ```</span> <span class="doccomment">//!</span> <span class="doccomment">//! When the logic to be performed on the value is more complex, we may simply</span> <span class="doccomment">//! use the `entry` API to ensure that the value is initialized and perform the</span> <span class="doccomment">//! logic afterwards.</span> <span class="doccomment">//!</span> <span class="doccomment">//! #### Tracking the inebriation of customers at a bar</span> <span class="doccomment">//!</span> <span class="doccomment">//! ```</span> <span class="doccomment">//! use std::collections::btree_map::BTreeMap;</span> <span class="doccomment">//!</span> <span class="doccomment">//! // A client of the bar. They have a blood alcohol level.</span> <span class="doccomment">//! struct Person { blood_alcohol: f32 }</span> <span class="doccomment">//!</span> <span class="doccomment">//! // All the orders made to the bar, by client id.</span> <span class="doccomment">//! let orders = vec![1,2,1,2,3,4,1,2,2,3,4,1,1,1];</span> <span class="doccomment">//!</span> <span class="doccomment">//! // Our clients.</span> <span class="doccomment">//! let mut blood_alcohol = BTreeMap::new();</span> <span class="doccomment">//!</span> <span class="doccomment">//! for id in orders {</span> <span class="doccomment">//! // If this is the first time we've seen this customer, initialize them</span> <span class="doccomment">//! // with no blood alcohol. Otherwise, just retrieve them.</span> <span class="doccomment">//! let person = blood_alcohol.entry(id).or_insert(Person { blood_alcohol: 0.0 });</span> <span class="doccomment">//!</span> <span class="doccomment">//! // Reduce their blood alcohol level. It takes time to order and drink a beer!</span> <span class="doccomment">//! person.blood_alcohol *= 0.9;</span> <span class="doccomment">//!</span> <span class="doccomment">//! // Check if they're sober enough to have another beer.</span> <span class="doccomment">//! if person.blood_alcohol > 0.3 {</span> <span class="doccomment">//! // Too drunk... for now.</span> <span class="doccomment">//! println!("Sorry {}, I have to cut you off", id);</span> <span class="doccomment">//! } else {</span> <span class="doccomment">//! // Have another!</span> <span class="doccomment">//! person.blood_alcohol += 0.1;</span> <span class="doccomment">//! }</span> <span class="doccomment">//! }</span> <span class="doccomment">//! ```</span> <span class="doccomment">//!</span> <span class="doccomment">//! # Insert and complex keys</span> <span class="doccomment">//!</span> <span class="doccomment">//! If we have a more complex key, calls to `insert` will</span> <span class="doccomment">//! not update the value of the key. For example:</span> <span class="doccomment">//!</span> <span class="doccomment">//! ```</span> <span class="doccomment">//! use std::cmp::Ordering;</span> <span class="doccomment">//! use std::collections::BTreeMap;</span> <span class="doccomment">//! use std::hash::{Hash, Hasher};</span> <span class="doccomment">//!</span> <span class="doccomment">//! #[derive(Debug)]</span> <span class="doccomment">//! struct Foo {</span> <span class="doccomment">//! a: u32,</span> <span class="doccomment">//! b: &'static str,</span> <span class="doccomment">//! }</span> <span class="doccomment">//!</span> <span class="doccomment">//! // we will compare `Foo`s by their `a` value only.</span> <span class="doccomment">//! impl PartialEq for Foo {</span> <span class="doccomment">//! fn eq(&self, other: &Self) -> bool { self.a == other.a }</span> <span class="doccomment">//! }</span> <span class="doccomment">//!</span> <span class="doccomment">//! impl Eq for Foo {}</span> <span class="doccomment">//!</span> <span class="doccomment">//! // we will hash `Foo`s by their `a` value only.</span> <span class="doccomment">//! impl Hash for Foo {</span> <span class="doccomment">//! fn hash<H: Hasher>(&self, h: &mut H) { self.a.hash(h); }</span> <span class="doccomment">//! }</span> <span class="doccomment">//!</span> <span class="doccomment">//! impl PartialOrd for Foo {</span> <span class="doccomment">//! fn partial_cmp(&self, other: &Self) -> Option<Ordering> { self.a.partial_cmp(&other.a) }</span> <span class="doccomment">//! }</span> <span class="doccomment">//!</span> <span class="doccomment">//! impl Ord for Foo {</span> <span class="doccomment">//! fn cmp(&self, other: &Self) -> Ordering { self.a.cmp(&other.a) }</span> <span class="doccomment">//! }</span> <span class="doccomment">//!</span> <span class="doccomment">//! let mut map = BTreeMap::new();</span> <span class="doccomment">//! map.insert(Foo { a: 1, b: "baz" }, 99);</span> <span class="doccomment">//!</span> <span class="doccomment">//! // We already have a Foo with an a of 1, so this will be updating the value.</span> <span class="doccomment">//! map.insert(Foo { a: 1, b: "xyz" }, 100);</span> <span class="doccomment">//!</span> <span class="doccomment">//! // The value has been updated...</span> <span class="doccomment">//! assert_eq!(map.values().next().unwrap(), &100);</span> <span class="doccomment">//!</span> <span class="doccomment">//! // ...but the key hasn't changed. b is still "baz", not "xyz".</span> <span class="doccomment">//! assert_eq!(map.keys().next().unwrap().b, "baz");</span> <span class="doccomment">//! ```</span> <span class="doccomment">//!</span> <span class="doccomment">//! [`Vec`]: ../../std/vec/struct.Vec.html</span> <span class="doccomment">//! [`HashMap`]: ../../std/collections/struct.HashMap.html</span> <span class="doccomment">//! [`VecDeque`]: ../../std/collections/struct.VecDeque.html</span> <span class="doccomment">//! [`LinkedList`]: ../../std/collections/struct.LinkedList.html</span> <span class="doccomment">//! [`BTreeMap`]: ../../std/collections/struct.BTreeMap.html</span> <span class="doccomment">//! [`HashSet`]: ../../std/collections/struct.HashSet.html</span> <span class="doccomment">//! [`BTreeSet`]: ../../std/collections/struct.BTreeSet.html</span> <span class="doccomment">//! [`BinaryHeap`]: ../../std/collections/struct.BinaryHeap.html</span> <span class="doccomment">//! [`IntoIterator`]: ../../std/iter/trait.IntoIterator.html</span> <span class="attribute">#![<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">use</span> <span class="ident">alloc</span>::<span class="ident">Bound</span>; <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">use</span> <span class="ident">alloc</span>::{<span class="ident">BinaryHeap</span>, <span class="ident">BTreeMap</span>, <span class="ident">BTreeSet</span>}; <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">use</span> <span class="ident">alloc</span>::{<span class="ident">LinkedList</span>, <span class="ident">VecDeque</span>}; <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">use</span> <span class="ident">alloc</span>::{<span class="ident">binary_heap</span>, <span class="ident">btree_map</span>, <span class="ident">btree_set</span>}; <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">use</span> <span class="ident">alloc</span>::{<span class="ident">linked_list</span>, <span class="ident">vec_deque</span>}; <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">use</span> <span class="self">self</span>::<span class="ident">hash_map</span>::<span class="ident">HashMap</span>; <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">use</span> <span class="self">self</span>::<span class="ident">hash_set</span>::<span class="ident">HashSet</span>; <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">use</span> <span class="ident">alloc</span>::<span class="ident">range</span>; <span class="kw">mod</span> <span class="ident">hash</span>; <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">mod</span> <span class="ident">hash_map</span> { <span class="doccomment">//! A hash map implemented with linear probing and Robin Hood bucket stealing.</span> <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">use</span> <span class="kw">super</span>::<span class="ident">hash</span>::<span class="ident">map</span>::<span class="kw-2">*</span>; } <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">mod</span> <span class="ident">hash_set</span> { <span class="doccomment">//! A hash set implemented as a `HashMap` where the value is `()`.</span> <span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span> <span class="kw">pub</span> <span class="kw">use</span> <span class="kw">super</span>::<span class="ident">hash</span>::<span class="ident">set</span>::<span class="kw-2">*</span>; } </pre> </section> <section id='search' class="content hidden"></section> <section class="footer"></section> <aside id="help" class="hidden"> <div> <h1 class="hidden">Help</h1> <div class="shortcuts"> <h2>Keyboard Shortcuts</h2> <dl> <dt><kbd>?</kbd></dt> <dd>Show this help dialog</dd> <dt><kbd>S</kbd></dt> <dd>Focus the search field</dd> <dt><kbd>↑</kbd></dt> <dd>Move up in search results</dd> <dt><kbd>↓</kbd></dt> <dd>Move down in search results</dd> <dt><kbd>↹</kbd></dt> <dd>Switch tab</dd> <dt><kbd>⏎</kbd></dt> <dd>Go to active search result</dd> <dt><kbd>+</kbd></dt> <dd>Expand all sections</dd> <dt><kbd>-</kbd></dt> <dd>Collapse all sections</dd> </dl> </div> <div class="infos"> <h2>Search Tricks</h2> <p> Prefix searches with a type followed by a colon (e.g. <code>fn:</code>) to restrict the search to a given type. </p> <p> Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>. </p> <p> Search functions by type signature (e.g. <code>vec -> usize</code> or <code>* -> vec</code>) </p> </div> </div> </aside> <script> window.rootPath = "../../../"; window.currentCrate = "std"; </script> <script src="../../../main.js"></script> <script defer src="../../../search-index.js"></script> </body> </html>