<!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 `src/libcore/iter/traits/double_ended.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>double_ended.rs.html -- source</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 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='../../../../core/index.html'><img src='../../../../rust-logo1.35.0.png' alt='logo' width='100'></a></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"><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>
</pre><div class="example-wrap"><pre class="rust ">
<span class="kw">use</span> <span class="ident">ops</span>::<span class="ident">Try</span>;
<span class="kw">use</span> <span class="ident">iter</span>::<span class="ident">LoopState</span>;
<span class="doccomment">/// An iterator able to yield elements from both ends.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Something that implements `DoubleEndedIterator` has one extra capability</span>
<span class="doccomment">/// over something that implements [`Iterator`]: the ability to also take</span>
<span class="doccomment">/// `Item`s from the back, as well as the front.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// It is important to note that both back and forth work on the same range,</span>
<span class="doccomment">/// and do not cross: iteration is over when they meet in the middle.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// In a similar fashion to the [`Iterator`] protocol, once a</span>
<span class="doccomment">/// `DoubleEndedIterator` returns `None` from a `next_back()`, calling it again</span>
<span class="doccomment">/// may or may not ever return `Some` again. `next()` and `next_back()` are</span>
<span class="doccomment">/// interchangeable for this purpose.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Iterator`]: trait.Iterator.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Basic usage:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let numbers = vec![1, 2, 3, 4, 5, 6];</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let mut iter = numbers.iter();</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(Some(&1), iter.next());</span>
<span class="doccomment">/// assert_eq!(Some(&6), iter.next_back());</span>
<span class="doccomment">/// assert_eq!(Some(&5), iter.next_back());</span>
<span class="doccomment">/// assert_eq!(Some(&2), iter.next());</span>
<span class="doccomment">/// assert_eq!(Some(&3), iter.next());</span>
<span class="doccomment">/// assert_eq!(Some(&4), iter.next());</span>
<span class="doccomment">/// assert_eq!(None, iter.next());</span>
<span class="doccomment">/// assert_eq!(None, iter.next_back());</span>
<span class="doccomment">/// ```</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">trait</span> <span class="ident">DoubleEndedIterator</span>: <span class="ident">Iterator</span> {
<span class="doccomment">/// Removes and returns an element from the end of the iterator.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Returns `None` when there are no more elements.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The [trait-level] docs contain more details.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [trait-level]: trait.DoubleEndedIterator.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Basic usage:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let numbers = vec![1, 2, 3, 4, 5, 6];</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let mut iter = numbers.iter();</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(Some(&1), iter.next());</span>
<span class="doccomment">/// assert_eq!(Some(&6), iter.next_back());</span>
<span class="doccomment">/// assert_eq!(Some(&5), iter.next_back());</span>
<span class="doccomment">/// assert_eq!(Some(&2), iter.next());</span>
<span class="doccomment">/// assert_eq!(Some(&3), iter.next());</span>
<span class="doccomment">/// assert_eq!(Some(&4), iter.next());</span>
<span class="doccomment">/// assert_eq!(None, iter.next());</span>
<span class="doccomment">/// assert_eq!(None, iter.next_back());</span>
<span class="doccomment">/// ```</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">fn</span> <span class="ident">next_back</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="self">Self</span>::<span class="ident">Item</span><span class="op">></span>;
<span class="doccomment">/// Returns the `n`th element from the end of the iterator.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This is essentially the reversed version of [`nth`]. Although like most indexing</span>
<span class="doccomment">/// operations, the count starts from zero, so `nth_back(0)` returns the first value fro</span>
<span class="doccomment">/// the end, `nth_back(1)` the second, and so on.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Note that all elements between the end and the returned element will be</span>
<span class="doccomment">/// consumed, including the returned element. This also means that calling</span>
<span class="doccomment">/// `nth_back(0)` multiple times on the same iterator will return different</span>
<span class="doccomment">/// elements.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// `nth_back()` will return [`None`] if `n` is greater than or equal to the length of the</span>
<span class="doccomment">/// iterator.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`None`]: ../../std/option/enum.Option.html#variant.None</span>
<span class="doccomment">/// [`nth`]: ../../std/iter/trait.Iterator.html#method.nth</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Basic usage:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// #![feature(iter_nth_back)]</span>
<span class="doccomment">/// let a = [1, 2, 3];</span>
<span class="doccomment">/// assert_eq!(a.iter().nth_back(2), Some(&1));</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Calling `nth_back()` multiple times doesn't rewind the iterator:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// #![feature(iter_nth_back)]</span>
<span class="doccomment">/// let a = [1, 2, 3];</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let mut iter = a.iter();</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(iter.nth_back(1), Some(&2));</span>
<span class="doccomment">/// assert_eq!(iter.nth_back(1), None);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Returning `None` if there are less than `n + 1` elements:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// #![feature(iter_nth_back)]</span>
<span class="doccomment">/// let a = [1, 2, 3];</span>
<span class="doccomment">/// assert_eq!(a.iter().nth_back(10), None);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_nth_back"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"56995"</span>)]</span>
<span class="kw">fn</span> <span class="ident">nth_back</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="kw-2">mut</span> <span class="ident">n</span>: <span class="ident">usize</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="self">Self</span>::<span class="ident">Item</span><span class="op">></span> {
<span class="kw">for</span> <span class="ident">x</span> <span class="kw">in</span> <span class="self">self</span>.<span class="ident">rev</span>() {
<span class="kw">if</span> <span class="ident">n</span> <span class="op">==</span> <span class="number">0</span> { <span class="kw">return</span> <span class="prelude-val">Some</span>(<span class="ident">x</span>) }
<span class="ident">n</span> <span class="op">-=</span> <span class="number">1</span>;
}
<span class="prelude-val">None</span>
}
<span class="doccomment">/// This is the reverse version of [`try_fold()`]: it takes elements</span>
<span class="doccomment">/// starting from the back of the iterator.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`try_fold()`]: trait.Iterator.html#method.try_fold</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Basic usage:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let a = ["1", "2", "3"];</span>
<span class="doccomment">/// let sum = a.iter()</span>
<span class="doccomment">/// .map(|&s| s.parse::<i32>())</span>
<span class="doccomment">/// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));</span>
<span class="doccomment">/// assert_eq!(sum, Ok(6));</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Short-circuiting:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let a = ["1", "rust", "3"];</span>
<span class="doccomment">/// let mut it = a.iter();</span>
<span class="doccomment">/// let sum = it</span>
<span class="doccomment">/// .by_ref()</span>
<span class="doccomment">/// .map(|&s| s.parse::<i32>())</span>
<span class="doccomment">/// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));</span>
<span class="doccomment">/// assert!(sum.is_err());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // Because it short-circuited, the remaining elements are still</span>
<span class="doccomment">/// // available through the iterator.</span>
<span class="doccomment">/// assert_eq!(it.next_back(), Some(&"1"));</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iterator_try_fold"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.27.0"</span>)]</span>
<span class="kw">fn</span> <span class="ident">try_rfold</span><span class="op"><</span><span class="ident">B</span>, <span class="ident">F</span>, <span class="ident">R</span><span class="op">></span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">init</span>: <span class="ident">B</span>, <span class="kw-2">mut</span> <span class="ident">f</span>: <span class="ident">F</span>) <span class="op">-></span> <span class="ident">R</span>
<span class="kw">where</span>
<span class="self">Self</span>: <span class="ident">Sized</span>,
<span class="ident">F</span>: <span class="ident">FnMut</span>(<span class="ident">B</span>, <span class="self">Self</span>::<span class="ident">Item</span>) <span class="op">-></span> <span class="ident">R</span>,
<span class="ident">R</span>: <span class="ident">Try</span><span class="op"><</span><span class="prelude-val">Ok</span><span class="op">=</span><span class="ident">B</span><span class="op">></span>
{
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">accum</span> <span class="op">=</span> <span class="ident">init</span>;
<span class="kw">while</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">x</span>) <span class="op">=</span> <span class="self">self</span>.<span class="ident">next_back</span>() {
<span class="ident">accum</span> <span class="op">=</span> <span class="ident">f</span>(<span class="ident">accum</span>, <span class="ident">x</span>)<span class="question-mark">?</span>;
}
<span class="ident">Try</span>::<span class="ident">from_ok</span>(<span class="ident">accum</span>)
}
<span class="doccomment">/// An iterator method that reduces the iterator's elements to a single,</span>
<span class="doccomment">/// final value, starting from the back.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This is the reverse version of [`fold()`]: it takes elements starting from</span>
<span class="doccomment">/// the back of the iterator.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// `rfold()` takes two arguments: an initial value, and a closure with two</span>
<span class="doccomment">/// arguments: an 'accumulator', and an element. The closure returns the value that</span>
<span class="doccomment">/// the accumulator should have for the next iteration.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The initial value is the value the accumulator will have on the first</span>
<span class="doccomment">/// call.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// After applying this closure to every element of the iterator, `rfold()`</span>
<span class="doccomment">/// returns the accumulator.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This operation is sometimes called 'reduce' or 'inject'.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Folding is useful whenever you have a collection of something, and want</span>
<span class="doccomment">/// to produce a single value from it.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`fold()`]: trait.Iterator.html#method.fold</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Basic usage:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let a = [1, 2, 3];</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // the sum of all of the elements of a</span>
<span class="doccomment">/// let sum = a.iter()</span>
<span class="doccomment">/// .rfold(0, |acc, &x| acc + x);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(sum, 6);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This example builds a string, starting with an initial value</span>
<span class="doccomment">/// and continuing with each element from the back until the front:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let numbers = [1, 2, 3, 4, 5];</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let zero = "0".to_string();</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let result = numbers.iter().rfold(zero, |acc, &x| {</span>
<span class="doccomment">/// format!("({} + {})", x, acc)</span>
<span class="doccomment">/// });</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))");</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_rfold"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.27.0"</span>)]</span>
<span class="kw">fn</span> <span class="ident">rfold</span><span class="op"><</span><span class="ident">B</span>, <span class="ident">F</span><span class="op">></span>(<span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">accum</span>: <span class="ident">B</span>, <span class="kw-2">mut</span> <span class="ident">f</span>: <span class="ident">F</span>) <span class="op">-></span> <span class="ident">B</span>
<span class="kw">where</span>
<span class="self">Self</span>: <span class="ident">Sized</span>,
<span class="ident">F</span>: <span class="ident">FnMut</span>(<span class="ident">B</span>, <span class="self">Self</span>::<span class="ident">Item</span>) <span class="op">-></span> <span class="ident">B</span>,
{
<span class="self">self</span>.<span class="ident">try_rfold</span>(<span class="ident">accum</span>, <span class="kw">move</span> <span class="op">|</span><span class="ident">acc</span>, <span class="ident">x</span><span class="op">|</span> <span class="prelude-val">Ok</span>::<span class="op"><</span><span class="ident">B</span>, <span class="op">!</span><span class="op">></span>(<span class="ident">f</span>(<span class="ident">acc</span>, <span class="ident">x</span>))).<span class="ident">unwrap</span>()
}
<span class="doccomment">/// Searches for an element of an iterator from the back that satisfies a predicate.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// `rfind()` takes a closure that returns `true` or `false`. It applies</span>
<span class="doccomment">/// this closure to each element of the iterator, starting at the end, and if any</span>
<span class="doccomment">/// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return</span>
<span class="doccomment">/// `false`, it returns [`None`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// `rfind()` is short-circuiting; in other words, it will stop processing</span>
<span class="doccomment">/// as soon as the closure returns `true`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Because `rfind()` takes a reference, and many iterators iterate over</span>
<span class="doccomment">/// references, this leads to a possibly confusing situation where the</span>
<span class="doccomment">/// argument is a double reference. You can see this effect in the</span>
<span class="doccomment">/// examples below, with `&&x`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Some(element)`]: ../../std/option/enum.Option.html#variant.Some</span>
<span class="doccomment">/// [`None`]: ../../std/option/enum.Option.html#variant.None</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Basic usage:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let a = [1, 2, 3];</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(a.iter().rfind(|&&x| x == 5), None);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Stopping at the first `true`:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let a = [1, 2, 3];</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let mut iter = a.iter();</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // we can still use `iter`, as there are more elements.</span>
<span class="doccomment">/// assert_eq!(iter.next_back(), Some(&1));</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"iter_rfind"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.27.0"</span>)]</span>
<span class="kw">fn</span> <span class="ident">rfind</span><span class="op"><</span><span class="ident">P</span><span class="op">></span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="kw-2">mut</span> <span class="ident">predicate</span>: <span class="ident">P</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="self">Self</span>::<span class="ident">Item</span><span class="op">></span>
<span class="kw">where</span>
<span class="self">Self</span>: <span class="ident">Sized</span>,
<span class="ident">P</span>: <span class="ident">FnMut</span>(<span class="kw-2">&</span><span class="self">Self</span>::<span class="ident">Item</span>) <span class="op">-></span> <span class="ident">bool</span>
{
<span class="self">self</span>.<span class="ident">try_rfold</span>((), <span class="kw">move</span> <span class="op">|</span>(), <span class="ident">x</span><span class="op">|</span> {
<span class="kw">if</span> <span class="ident">predicate</span>(<span class="kw-2">&</span><span class="ident">x</span>) { <span class="ident">LoopState</span>::<span class="ident">Break</span>(<span class="ident">x</span>) }
<span class="kw">else</span> { <span class="ident">LoopState</span>::<span class="ident">Continue</span>(()) }
}).<span class="ident">break_value</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">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">I</span>: <span class="ident">DoubleEndedIterator</span> <span class="op">+</span> <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">DoubleEndedIterator</span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">I</span> {
<span class="kw">fn</span> <span class="ident">next_back</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">I</span>::<span class="ident">Item</span><span class="op">></span> {
(<span class="kw-2">*</span><span class="kw-2">*</span><span class="self">self</span>).<span class="ident">next_back</span>()
}
<span class="kw">fn</span> <span class="ident">nth_back</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">n</span>: <span class="ident">usize</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">I</span>::<span class="ident">Item</span><span class="op">></span> {
(<span class="kw-2">*</span><span class="kw-2">*</span><span class="self">self</span>).<span class="ident">nth_back</span>(<span class="ident">n</span>)
}
}
</pre></div>
</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><p>Search multiple things at once by splitting your query with comma (e.g., <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../../../../";window.currentCrate = "core";</script><script src="../../../../aliases.js"></script><script src="../../../../main1.35.0.js"></script><script src="../../../../source-script1.35.0.js"></script><script src="../../../../source-files.js"></script><script defer src="../../../../search-index.js"></script></body></html>
distrib
>
Mageia
>
7
>
armv7hl
>
media
>
core-release
>
by-pkgid
>
0c2243f8a1696816431e7210e991fa52
>
files
>
13871
