263 lines
16 KiB
HTML
263 lines
16 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<title>Storing Keys with Associated Values in Hash Maps</title>
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</head>
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<body>
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<h2 id="storing-keys-with-associated-values-in-hash-maps"><a class="header" href="#storing-keys-with-associated-values-in-hash-maps">Storing Keys with Associated Values in Hash Maps</a></h2>
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<p>The last of our common collections is the hash map. The type <code>HashMap<K, V></code>
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stores a mapping of keys of type <code>K</code> to values of type <code>V</code> using a <em>hashing
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function</em>, which determines how it places these keys and values into memory.
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Many programming languages support this kind of data structure, but they often
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use a different name, such as <em>hash</em>, <em>map</em>, <em>object</em>, <em>hash table</em>,
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<em>dictionary</em>, or <em>associative array</em>, just to name a few.</p>
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<p>Hash maps are useful when you want to look up data not by using an index, as
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you can with vectors, but by using a key that can be of any type. For example,
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in a game, you could keep track of each team’s score in a hash map in which
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each key is a team’s name and the values are each team’s score. Given a team
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name, you can retrieve its score.</p>
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<p>We’ll go over the basic API of hash maps in this section, but many more goodies
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are hiding in the functions defined on <code>HashMap<K, V></code> by the standard library.
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As always, check the standard library documentation for more information.</p>
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<h3 id="creating-a-new-hash-map"><a class="header" href="#creating-a-new-hash-map">Creating a New Hash Map</a></h3>
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<p>One way to create an empty hash map is to use <code>new</code> and to add elements with
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<code>insert</code>. In Listing 8-20, we’re keeping track of the scores of two teams whose
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names are <em>Blue</em> and <em>Yellow</em>. The Blue team starts with 10 points, and the
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Yellow team starts with 50.</p>
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<figure class="listing" id="listing-8-20">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> use std::collections::HashMap;
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let mut scores = HashMap::new();
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scores.insert(String::from("Blue"), 10);
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scores.insert(String::from("Yellow"), 50);
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-20">Listing 8-20</a>: Creating a new hash map and inserting some keys and values</figcaption>
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</figure>
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<p>Note that we need to first <code>use</code> the <code>HashMap</code> from the collections portion of
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the standard library. Of our three common collections, this one is the least
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often used, so it’s not included in the features brought into scope
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automatically in the prelude. Hash maps also have less support from the
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standard library; there’s no built-in macro to construct them, for example.</p>
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<p>Just like vectors, hash maps store their data on the heap. This <code>HashMap</code> has
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keys of type <code>String</code> and values of type <code>i32</code>. Like vectors, hash maps are
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homogeneous: All of the keys must have the same type, and all of the values
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must have the same type.</p>
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<h3 id="accessing-values-in-a-hash-map"><a class="header" href="#accessing-values-in-a-hash-map">Accessing Values in a Hash Map</a></h3>
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<p>We can get a value out of the hash map by providing its key to the <code>get</code>
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method, as shown in Listing 8-21.</p>
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<figure class="listing" id="listing-8-21">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> use std::collections::HashMap;
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let mut scores = HashMap::new();
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scores.insert(String::from("Blue"), 10);
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scores.insert(String::from("Yellow"), 50);
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let team_name = String::from("Blue");
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let score = scores.get(&team_name).copied().unwrap_or(0);
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-21">Listing 8-21</a>: Accessing the score for the Blue team stored in the hash map</figcaption>
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</figure>
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<p>Here, <code>score</code> will have the value that’s associated with the Blue team, and the
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result will be <code>10</code>. The <code>get</code> method returns an <code>Option<&V></code>; if there’s no
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value for that key in the hash map, <code>get</code> will return <code>None</code>. This program
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handles the <code>Option</code> by calling <code>copied</code> to get an <code>Option<i32></code> rather than an
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<code>Option<&i32></code>, then <code>unwrap_or</code> to set <code>score</code> to zero if <code>scores</code> doesn’t
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have an entry for the key.</p>
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<p>We can iterate over each key-value pair in a hash map in a similar manner as we
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do with vectors, using a <code>for</code> loop:</p>
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> use std::collections::HashMap;
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let mut scores = HashMap::new();
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scores.insert(String::from("Blue"), 10);
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scores.insert(String::from("Yellow"), 50);
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for (key, value) in &scores {
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println!("{key}: {value}");
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}
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<span class="boring">}</span></code></pre>
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<p>This code will print each pair in an arbitrary order:</p>
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<pre><code class="language-text">Yellow: 50
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Blue: 10
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</code></pre>
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<!-- Old headings. Do not remove or links may break. -->
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<p><a id="hash-maps-and-ownership"></a></p>
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<h3 id="managing-ownership-in-hash-maps"><a class="header" href="#managing-ownership-in-hash-maps">Managing Ownership in Hash Maps</a></h3>
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<p>For types that implement the <code>Copy</code> trait, like <code>i32</code>, the values are copied
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into the hash map. For owned values like <code>String</code>, the values will be moved and
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the hash map will be the owner of those values, as demonstrated in Listing 8-22.</p>
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<figure class="listing" id="listing-8-22">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> use std::collections::HashMap;
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let field_name = String::from("Favorite color");
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let field_value = String::from("Blue");
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let mut map = HashMap::new();
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map.insert(field_name, field_value);
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// field_name and field_value are invalid at this point, try using them and
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// see what compiler error you get!
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-22">Listing 8-22</a>: Showing that keys and values are owned by the hash map once they’re inserted</figcaption>
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</figure>
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<p>We aren’t able to use the variables <code>field_name</code> and <code>field_value</code> after
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they’ve been moved into the hash map with the call to <code>insert</code>.</p>
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<p>If we insert references to values into the hash map, the values won’t be moved
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into the hash map. The values that the references point to must be valid for at
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least as long as the hash map is valid. We’ll talk more about these issues in
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<a href="../ch10/ch10-03-lifetime-syntax.html#validating-references-with-lifetimes">“Validating References with
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Lifetimes”</a><!-- ignore --> in Chapter 10.</p>
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<h3 id="updating-a-hash-map"><a class="header" href="#updating-a-hash-map">Updating a Hash Map</a></h3>
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<p>Although the number of key and value pairs is growable, each unique key can
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only have one value associated with it at a time (but not vice versa: For
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example, both the Blue team and the Yellow team could have the value <code>10</code>
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stored in the <code>scores</code> hash map).</p>
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<p>When you want to change the data in a hash map, you have to decide how to
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handle the case when a key already has a value assigned. You could replace the
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old value with the new value, completely disregarding the old value. You could
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keep the old value and ignore the new value, only adding the new value if the
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key <em>doesn’t</em> already have a value. Or you could combine the old value and the
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new value. Let’s look at how to do each of these!</p>
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<h4 id="overwriting-a-value"><a class="header" href="#overwriting-a-value">Overwriting a Value</a></h4>
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<p>If we insert a key and a value into a hash map and then insert that same key
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with a different value, the value associated with that key will be replaced.
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Even though the code in Listing 8-23 calls <code>insert</code> twice, the hash map will
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only contain one key-value pair because we’re inserting the value for the Blue
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team’s key both times.</p>
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<figure class="listing" id="listing-8-23">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> use std::collections::HashMap;
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let mut scores = HashMap::new();
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scores.insert(String::from("Blue"), 10);
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scores.insert(String::from("Blue"), 25);
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println!("{scores:?}");
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-23">Listing 8-23</a>: Replacing a value stored with a particular key</figcaption>
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</figure>
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<p>This code will print <code>{"Blue": 25}</code>. The original value of <code>10</code> has been
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overwritten.</p>
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<!-- Old headings. Do not remove or links may break. -->
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<p><a id="only-inserting-a-value-if-the-key-has-no-value"></a></p>
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<h4 id="adding-a-key-and-value-only-if-a-key-isnt-present"><a class="header" href="#adding-a-key-and-value-only-if-a-key-isnt-present">Adding a Key and Value Only If a Key Isn’t Present</a></h4>
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<p>It’s common to check whether a particular key already exists in the hash map
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with a value and then to take the following actions: If the key does exist in
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the hash map, the existing value should remain the way it is; if the key
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doesn’t exist, insert it and a value for it.</p>
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<p>Hash maps have a special API for this called <code>entry</code> that takes the key you
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want to check as a parameter. The return value of the <code>entry</code> method is an enum
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called <code>Entry</code> that represents a value that might or might not exist. Let’s say
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we want to check whether the key for the Yellow team has a value associated
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with it. If it doesn’t, we want to insert the value <code>50</code>, and the same for the
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Blue team. Using the <code>entry</code> API, the code looks like Listing 8-24.</p>
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<figure class="listing" id="listing-8-24">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> use std::collections::HashMap;
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let mut scores = HashMap::new();
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scores.insert(String::from("Blue"), 10);
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scores.entry(String::from("Yellow")).or_insert(50);
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scores.entry(String::from("Blue")).or_insert(50);
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println!("{scores:?}");
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-24">Listing 8-24</a>: Using the <code>entry</code> method to only insert if the key does not already have a value</figcaption>
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</figure>
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<p>The <code>or_insert</code> method on <code>Entry</code> is defined to return a mutable reference to
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the value for the corresponding <code>Entry</code> key if that key exists, and if not, it
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inserts the parameter as the new value for this key and returns a mutable
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reference to the new value. This technique is much cleaner than writing the
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logic ourselves and, in addition, plays more nicely with the borrow checker.</p>
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<p>Running the code in Listing 8-24 will print <code>{"Yellow": 50, "Blue": 10}</code>. The
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first call to <code>entry</code> will insert the key for the Yellow team with the value
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<code>50</code> because the Yellow team doesn’t have a value already. The second call to
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<code>entry</code> will not change the hash map, because the Blue team already has the
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value <code>10</code>.</p>
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<h4 id="updating-a-value-based-on-the-old-value"><a class="header" href="#updating-a-value-based-on-the-old-value">Updating a Value Based on the Old Value</a></h4>
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<p>Another common use case for hash maps is to look up a key’s value and then
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update it based on the old value. For instance, Listing 8-25 shows code that
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counts how many times each word appears in some text. We use a hash map with
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the words as keys and increment the value to keep track of how many times we’ve
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seen that word. If it’s the first time we’ve seen a word, we’ll first insert
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the value <code>0</code>.</p>
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<figure class="listing" id="listing-8-25">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> use std::collections::HashMap;
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let text = "hello world wonderful world";
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let mut map = HashMap::new();
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for word in text.split_whitespace() {
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let count = map.entry(word).or_insert(0);
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*count += 1;
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}
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println!("{map:?}");
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-25">Listing 8-25</a>: Counting occurrences of words using a hash map that stores words and counts</figcaption>
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</figure>
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<p>This code will print <code>{"world": 2, "hello": 1, "wonderful": 1}</code>. You might see
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the same key-value pairs printed in a different order: Recall from <a href="#accessing-values-in-a-hash-map">“Accessing
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Values in a Hash Map”</a><!-- ignore --> that iterating over a hash map
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happens in an arbitrary order.</p>
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<p>The <code>split_whitespace</code> method returns an iterator over subslices, separated by
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whitespace, of the value in <code>text</code>. The <code>or_insert</code> method returns a mutable
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reference (<code>&mut V</code>) to the value for the specified key. Here, we store that
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mutable reference in the <code>count</code> variable, so in order to assign to that value,
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we must first dereference <code>count</code> using the asterisk (<code>*</code>). The mutable
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reference goes out of scope at the end of the <code>for</code> loop, so all of these
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changes are safe and allowed by the borrowing rules.</p>
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<h3 id="hashing-functions"><a class="header" href="#hashing-functions">Hashing Functions</a></h3>
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<p>By default, <code>HashMap</code> uses a hashing function called <em>SipHash</em> that can provide
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resistance to denial-of-service (DoS) attacks involving hash
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tables<sup class="footnote-reference" id="fr-siphash-1"><a href="#footnote-siphash">1</a></sup><!-- ignore -->. This is not the fastest hashing algorithm
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available, but the trade-off for better security that comes with the drop in
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performance is worth it. If you profile your code and find that the default
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hash function is too slow for your purposes, you can switch to another function
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by specifying a different hasher. A <em>hasher</em> is a type that implements the
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<code>BuildHasher</code> trait. We’ll talk about traits and how to implement them in
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<a href="../ch10/ch10-02-traits.html">Chapter 10</a><!-- ignore -->. You don’t necessarily have to implement
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your own hasher from scratch; <a href="https://crates.io/">crates.io</a><!-- ignore -->
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has libraries shared by other Rust users that provide hashers implementing many
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common hashing algorithms.</p>
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<h2 id="summary"><a class="header" href="#summary">Summary</a></h2>
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<p>Vectors, strings, and hash maps will provide a large amount of functionality
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necessary in programs when you need to store, access, and modify data. Here are
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some exercises you should now be equipped to solve:</p>
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<ol>
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<li>Given a list of integers, use a vector and return the median (when sorted,
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the value in the middle position) and mode (the value that occurs most
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often; a hash map will be helpful here) of the list.</li>
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<li>Convert strings to Pig Latin. The first consonant of each word is moved to
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the end of the word and <em>ay</em> is added, so <em>first</em> becomes <em>irst-fay</em>. Words
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that start with a vowel have <em>hay</em> added to the end instead (<em>apple</em> becomes
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<em>apple-hay</em>). Keep in mind the details about UTF-8 encoding!</li>
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<li>Using a hash map and vectors, create a text interface to allow a user to add
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employee names to a department in a company; for example, “Add Sally to
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Engineering” or “Add Amir to Sales.” Then, let the user retrieve a list of
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all people in a department or all people in the company by department, sorted
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alphabetically.</li>
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</ol>
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<p>The standard library API documentation describes methods that vectors, strings,
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and hash maps have that will be helpful for these exercises!</p>
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<p>We’re getting into more complex programs in which operations can fail, so it’s
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a perfect time to discuss error handling. We’ll do that next!</p>
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<hr>
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<ol class="footnote-definition">
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<li id="footnote-siphash">
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<p><a href="https://en.wikipedia.org/wiki/SipHash">https://en.wikipedia.org/wiki/SipHash</a> <a href="#fr-siphash-1">↩</a></p>
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</li>
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</ol>
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</body>
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</html>
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