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<section id='main' class="content struct">
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<h1 class='fqn'><span class='in-band'>Struct <a href='../../index.html'>bitflags</a>::<wbr><a href='../index.html'>__core</a>::<wbr><a href='index.html'>collections</a>::<wbr><a class='struct' href=''>HashMap</a></span><span class='out-of-band'><span id='render-detail'>
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[<span class='inner'>−</span>]
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</span><a id='src-1704' class='srclink' href='https://doc.rust-lang.org/nightly/std/collections/hash/map/struct.HashMap.html?gotosrc=1704' title='goto source code'>[src]</a></span></h1>
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<pre class='rust struct'>pub struct HashMap<K, V, S = <a class='struct' href='../../../bitflags/__core/collections/hash_map/struct.RandomState.html' title='bitflags::__core::collections::hash_map::RandomState'>RandomState</a>> {
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// some fields omitted
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}</pre><span class="since">1.0.0</span><div class='docblock'><p>A hash map implementation which uses linear probing with Robin
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Hood bucket stealing.</p>
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<p>The hashes are all keyed by the thread-local random number generator
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on creation by default. This means that the ordering of the keys is
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randomized, but makes the tables more resistant to
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denial-of-service attacks (Hash DoS). This behavior can be
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overridden with one of the constructors.</p>
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<p>It is required that the keys implement the <code>Eq</code> and <code>Hash</code> traits, although
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this can frequently be achieved by using <code>#[derive(PartialEq, Eq, Hash)]</code>.
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If you implement these yourself, it is important that the following
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property holds:</p>
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<pre><code class="language-text">k1 == k2 -> hash(k1) == hash(k2)
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</code></pre>
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<p>In other words, if two keys are equal, their hashes must be equal.</p>
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<p>It is a logic error for a key to be modified in such a way that the key's
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hash, as determined by the <code>Hash</code> trait, or its equality, as determined by
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the <code>Eq</code> trait, changes while it is in the map. This is normally only
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possible through <code>Cell</code>, <code>RefCell</code>, global state, I/O, or unsafe code.</p>
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<p>Relevant papers/articles:</p>
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<ol>
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<li>Pedro Celis. <a href="https://cs.uwaterloo.ca/research/tr/1986/CS-86-14.pdf">"Robin Hood Hashing"</a></li>
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<li>Emmanuel Goossaert. <a href="http://codecapsule.com/2013/11/11/robin-hood-hashing/">"Robin Hood
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hashing"</a></li>
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<li>Emmanuel Goossaert. <a href="http://codecapsule.com/2013/11/17/robin-hood-hashing-backward-shift-deletion/">"Robin Hood hashing: backward shift
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deletion"</a></li>
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</ol>
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<h1 id='examples' class='section-header'><a href='#examples'>Examples</a></h1>
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<pre class='rust rust-example-rendered'>
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>collections</span>::<span class='ident'>HashMap</span>;
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<span class='comment'>// type inference lets us omit an explicit type signature (which</span>
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<span class='comment'>// would be `HashMap<&str, &str>` in this example).</span>
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<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>book_reviews</span> <span class='op'>=</span> <span class='ident'>HashMap</span>::<span class='ident'>new</span>();
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<span class='comment'>// review some books.</span>
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<span class='ident'>book_reviews</span>.<span class='ident'>insert</span>(<span class='string'>"Adventures of Huckleberry Finn"</span>, <span class='string'>"My favorite book."</span>);
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<span class='ident'>book_reviews</span>.<span class='ident'>insert</span>(<span class='string'>"Grimms' Fairy Tales"</span>, <span class='string'>"Masterpiece."</span>);
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<span class='ident'>book_reviews</span>.<span class='ident'>insert</span>(<span class='string'>"Pride and Prejudice"</span>, <span class='string'>"Very enjoyable."</span>);
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<span class='ident'>book_reviews</span>.<span class='ident'>insert</span>(<span class='string'>"The Adventures of Sherlock Holmes"</span>, <span class='string'>"Eye lyked it alot."</span>);
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<span class='comment'>// check for a specific one.</span>
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<span class='kw'>if</span> <span class='op'>!</span><span class='ident'>book_reviews</span>.<span class='ident'>contains_key</span>(<span class='string'>"Les Misérables"</span>) {
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<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"We've got {} reviews, but Les Misérables ain't one."</span>,
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<span class='ident'>book_reviews</span>.<span class='ident'>len</span>());
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}
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<span class='comment'>// oops, this review has a lot of spelling mistakes, let's delete it.</span>
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<span class='ident'>book_reviews</span>.<span class='ident'>remove</span>(<span class='string'>"The Adventures of Sherlock Holmes"</span>);
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<span class='comment'>// look up the values associated with some keys.</span>
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<span class='kw'>let</span> <span class='ident'>to_find</span> <span class='op'>=</span> [<span class='string'>"Pride and Prejudice"</span>, <span class='string'>"Alice's Adventure in Wonderland"</span>];
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<span class='kw'>for</span> <span class='ident'>book</span> <span class='kw'>in</span> <span class='kw-2'>&</span><span class='ident'>to_find</span> {
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<span class='kw'>match</span> <span class='ident'>book_reviews</span>.<span class='ident'>get</span>(<span class='ident'>book</span>) {
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<span class='prelude-val'>Some</span>(<span class='ident'>review</span>) <span class='op'>=></span> <span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{}: {}"</span>, <span class='ident'>book</span>, <span class='ident'>review</span>),
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<span class='prelude-val'>None</span> <span class='op'>=></span> <span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{} is unreviewed."</span>, <span class='ident'>book</span>)
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}
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}
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<span class='comment'>// iterate over everything.</span>
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<span class='kw'>for</span> (<span class='ident'>book</span>, <span class='ident'>review</span>) <span class='kw'>in</span> <span class='kw-2'>&</span><span class='ident'>book_reviews</span> {
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<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{}: \"{}\""</span>, <span class='ident'>book</span>, <span class='ident'>review</span>);
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}</pre>
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<p><code>HashMap</code> also implements an <a href="#method.entry"><code>Entry API</code></a>, which allows
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for more complex methods of getting, setting, updating and removing keys and
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their values:</p>
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<pre class='rust rust-example-rendered'>
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>collections</span>::<span class='ident'>HashMap</span>;
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<span class='comment'>// type inference lets us omit an explicit type signature (which</span>
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<span class='comment'>// would be `HashMap<&str, u8>` in this example).</span>
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<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>player_stats</span> <span class='op'>=</span> <span class='ident'>HashMap</span>::<span class='ident'>new</span>();
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<span class='kw'>fn</span> <span class='ident'>random_stat_buff</span>() <span class='op'>-></span> <span class='ident'>u8</span> {
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<span class='comment'>// could actually return some random value here - let's just return</span>
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<span class='comment'>// some fixed value for now</span>
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<span class='number'>42</span>
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}
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<span class='comment'>// insert a key only if it doesn't already exist</span>
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<span class='ident'>player_stats</span>.<span class='ident'>entry</span>(<span class='string'>"health"</span>).<span class='ident'>or_insert</span>(<span class='number'>100</span>);
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<span class='comment'>// insert a key using a function that provides a new value only if it</span>
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<span class='comment'>// doesn't already exist</span>
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<span class='ident'>player_stats</span>.<span class='ident'>entry</span>(<span class='string'>"defence"</span>).<span class='ident'>or_insert_with</span>(<span class='ident'>random_stat_buff</span>);
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<span class='comment'>// update a key, guarding against the key possibly not being set</span>
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<span class='kw'>let</span> <span class='ident'>stat</span> <span class='op'>=</span> <span class='ident'>player_stats</span>.<span class='ident'>entry</span>(<span class='string'>"attack"</span>).<span class='ident'>or_insert</span>(<span class='number'>100</span>);
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<span class='op'>*</span><span class='ident'>stat</span> <span class='op'>+=</span> <span class='ident'>random_stat_buff</span>();</pre>
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<p>The easiest way to use <code>HashMap</code> with a custom type as key is to derive <code>Eq</code> and <code>Hash</code>.
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We must also derive <code>PartialEq</code>.</p>
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<pre class='rust rust-example-rendered'>
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>collections</span>::<span class='ident'>HashMap</span>;
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<span class='attribute'>#[<span class='ident'>derive</span>(<span class='ident'>Hash</span>, <span class='ident'>Eq</span>, <span class='ident'>PartialEq</span>, <span class='ident'>Debug</span>)]</span>
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<span class='kw'>struct</span> <span class='ident'>Viking</span> {
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<span class='ident'>name</span>: <span class='ident'>String</span>,
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<span class='ident'>country</span>: <span class='ident'>String</span>,
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}
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<span class='kw'>impl</span> <span class='ident'>Viking</span> {
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<span class='doccomment'>/// Create a new Viking.</span>
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<span class='kw'>fn</span> <span class='ident'>new</span>(<span class='ident'>name</span>: <span class='kw-2'>&</span><span class='ident'>str</span>, <span class='ident'>country</span>: <span class='kw-2'>&</span><span class='ident'>str</span>) <span class='op'>-></span> <span class='ident'>Viking</span> {
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<span class='ident'>Viking</span> { <span class='ident'>name</span>: <span class='ident'>name</span>.<span class='ident'>to_string</span>(), <span class='ident'>country</span>: <span class='ident'>country</span>.<span class='ident'>to_string</span>() }
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}
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}
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<span class='comment'>// Use a HashMap to store the vikings' health points.</span>
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<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>vikings</span> <span class='op'>=</span> <span class='ident'>HashMap</span>::<span class='ident'>new</span>();
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<span class='ident'>vikings</span>.<span class='ident'>insert</span>(<span class='ident'>Viking</span>::<span class='ident'>new</span>(<span class='string'>"Einar"</span>, <span class='string'>"Norway"</span>), <span class='number'>25</span>);
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<span class='ident'>vikings</span>.<span class='ident'>insert</span>(<span class='ident'>Viking</span>::<span class='ident'>new</span>(<span class='string'>"Olaf"</span>, <span class='string'>"Denmark"</span>), <span class='number'>24</span>);
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<span class='ident'>vikings</span>.<span class='ident'>insert</span>(<span class='ident'>Viking</span>::<span class='ident'>new</span>(<span class='string'>"Harald"</span>, <span class='string'>"Iceland"</span>), <span class='number'>12</span>);
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<span class='comment'>// Use derived implementation to print the status of the vikings.</span>
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<span class='kw'>for</span> (<span class='ident'>viking</span>, <span class='ident'>health</span>) <span class='kw'>in</span> <span class='kw-2'>&</span><span class='ident'>vikings</span> {
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<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{:?} has {} hp"</span>, <span class='ident'>viking</span>, <span class='ident'>health</span>);
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}</pre>
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