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<ol class="chapter"><li class="chapter-item expanded "><a href="Chapter_0.html"><strong aria-hidden="true">1.</strong> Update</a></li><li class="chapter-item expanded "><a href="Chapter_1.html"><strong aria-hidden="true">2.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="Chapter_2.html"><strong aria-hidden="true">3.</strong> Who am I?</a></li><li class="chapter-item expanded "><a href="Chapter_3.html"><strong aria-hidden="true">4.</strong> Writing Rust in Easy English</a></li><li class="chapter-item expanded "><a href="Chapter_4.html"><strong aria-hidden="true">5.</strong> Rust Playground</a></li><li class="chapter-item expanded "><a href="Chapter_5.html"><strong aria-hidden="true">6.</strong> 🚧 and ⚠️</a></li><li class="chapter-item expanded "><a href="Chapter_6.html"><strong aria-hidden="true">7.</strong> Comments</a></li><li class="chapter-item expanded "><a href="Chapter_7.html"><strong aria-hidden="true">8.</strong> Types</a></li><li class="chapter-item expanded "><a href="Chapter_8.html"><strong aria-hidden="true">9.</strong> Type inference</a></li><li class="chapter-item expanded "><a href="Chapter_9.html"><strong aria-hidden="true">10.</strong> Printing 'hello, world!'</a></li><li class="chapter-item expanded "><a href="Chapter_10.html"><strong aria-hidden="true">11.</strong> Display and debug</a></li><li class="chapter-item expanded "><a href="Chapter_11.html"><strong aria-hidden="true">12.</strong> Mutability (changing)</a></li><li class="chapter-item expanded "><a href="Chapter_12.html"><strong aria-hidden="true">13.</strong> The stack, the heap, and pointers</a></li><li class="chapter-item expanded "><a href="Chapter_13.html"><strong aria-hidden="true">14.</strong> More about printing</a></li><li class="chapter-item expanded "><a href="Chapter_14.html"><strong aria-hidden="true">15.</strong> Strings</a></li><li class="chapter-item expanded "><a href="Chapter_15.html"><strong aria-hidden="true">16.</strong> const and static</a></li><li class="chapter-item expanded "><a href="Chapter_16.html"><strong aria-hidden="true">17.</strong> More on references</a></li><li class="chapter-item expanded "><a href="Chapter_17.html"><strong aria-hidden="true">18.</strong> Mutable references</a></li><li class="chapter-item expanded "><a href="Chapter_18.html"><strong aria-hidden="true">19.</strong> Giving references to functions</a></li><li class="chapter-item expanded "><a href="Chapter_19.html"><strong aria-hidden="true">20.</strong> Copy types</a></li><li class="chapter-item expanded "><a href="Chapter_20.html"><strong aria-hidden="true">21.</strong> Collection types</a></li><li class="chapter-item expanded "><a href="Chapter_21.html"><strong aria-hidden="true">22.</strong> Vectors</a></li><li class="chapter-item expanded "><a href="Chapter_22.html"><strong aria-hidden="true">23.</strong> Tuples</a></li><li class="chapter-item expanded "><a href="Chapter_23.html"><strong aria-hidden="true">24.</strong> Control flow</a></li><li class="chapter-item expanded "><a href="Chapter_24.html"><strong aria-hidden="true">25.</strong> Structs</a></li><li class="chapter-item expanded "><a href="Chapter_25.html" class="active"><strong aria-hidden="true">26.</strong> Enums</a></li><li class="chapter-item expanded "><a href="Chapter_26.html"><strong aria-hidden="true">27.</strong> Loops</a></li><li class="chapter-item expanded "><a href="Chapter_27.html"><strong aria-hidden="true">28.</strong> Implementing structs and enums</a></li><li class="chapter-item expanded "><a href="Chapter_28.html"><strong aria-hidden="true">29.</strong> Destructuring</a></li><li class="chapter-item expanded "><a href="Chapter_29.html"><strong aria-hidden="true">30.</strong> References and the dot operator</a></li><li class="chapter-item expanded "><a href="Chapter_30.html"><strong aria-hidden="true">31.</strong> Generics</a></li><li class="chapter-item expanded "><a href="Chapter_31.html"><strong aria-hidden="true">32.</strong> Option and Result</a></li><li class="chapter-item expanded "><a href="Chapter_32.html"><strong aria-hidden="true">33.</strong> Other collections</a></li><li class="chapter-item expanded "><a href="Chapter_33.html"><strong aria-hidden="true">34.</strong> The ? operator</a></li><li class="chapter-item expanded "><a href="Chapter_34.html"><strong aria-hidden="true">35.</strong> Traits</a></li><li class="chapter-item expanded "><a href="Chapter_35.html"><strong aria-hidden="true">36.</strong> Chaining methods</a></li><li class="chapter-item expanded "><a href="Chapter_36.html"><strong aria-hidden="true">37.</strong> Iterators</a></li><li class="chapter-item expanded "><a href="Chapter_37.html"><strong aria-hidden="true">38.</strong> Closures</a></li><li class="chapter-item expanded "><a href="Chapter_38.html"><strong aria-hidden="true">39.</strong> The dbg! macro and .inspect</a></li><li class="chapter-item expanded "><a href="Chapter_39.html"><strong aria-hidden="true">40.</strong> Types of &str</a></li><li class="chapter-item expanded "><a href="Chapter_40.html"><strong aria-hidden="true">41.</strong> Lifetimes</a></li><li class="chapter-item expanded "><a href="Chapter_41.html"><strong aria-hidden="true">42.</strong> Interior mutability</a></li><li class="chapter-item expanded "><a href="Chapter_42.html"><strong aria-hidden="true">43.</strong> RefCell</a></li><li class="chapter-item expanded "><a href="Chapter_43.html"><strong aria-hidden="true">44.</strong> Mutex</a></li><li class="chapter-item expanded "><a href="Chapter_44.html"><strong aria-hidden="true">45.</strong> RwLock</a></li><li class="chapter-item expanded "><a href="Chapter_45.html"><strong aria-hidden="true">46.</strong> Cow</a></li><li class="chapter-item expanded "><a href="Chapter_46.html"><strong aria-hidden="true">47.</strong> Type aliases</a></li><li class="chapter-item expanded "><a href="Chapter_47.html"><strong aria-hidden="true">48.</strong> The todo! macro</a></li><li class="chapter-item expanded "><a href="Chapter_48.html"><strong aria-hidden="true">49.</strong> Rc</a></li><li class="chapter-item expanded "><a href="Chapter_49.html"><strong aria-hidden="true">50.</strong> Multiple threads</a></li><li class="chapter-item expanded "><a href="Chapter_50.html"><strong aria-hidden="true">51.</strong> Closures in functions</a></li><li class="chapter-item expanded "><a href="Chapter_51.html"><strong aria-hidden="true">52.</strong> impl Trait</a></li><li class="chapter-item expanded "><a href="Chapter_52.html"><strong aria-hidden="true">53.</strong> Arc</a></li><li class="chapter-item expanded "><a href="Chapter_53.html"><strong aria-hidden="true">54.</strong> Channels</a></li><li class="chapter-item expanded "><a href="Chapter_54.html"><strong aria-hidden="true">55.</strong> Reading Rust documentation</a></li><li class="chapter-item expanded "><a href="Chapter_55.html"><strong aria-hidden="true">56.</strong> Attributes</a></li><li class="chapter-item expanded "><a href="Chapter_56.html"><strong aria-hidden="true">57.</strong> Box</a></li><li class="chapter-item expanded "><a href="Chapter_57.html"><strong aria-hidden="true">58.</strong> Box around traits</a></li><li class="chapter-item expanded "><a href="Chapter_58.html"><strong aria-hidden="true">59.</strong> Default and the builder pattern</a></li><li class="chapter-item expanded "><a href="Chapter_59.html"><strong aria-hidden="true">60.</strong> Deref and DerefMut</a></li><li class="chapter-item expanded "><a href="Chapter_60.html"><strong aria-hidden="true">61.</strong> Crates and modules</a></li><li class="chapter-item expanded "><a href="Chapter_61.html"><strong aria-hidden="true">62.</strong> Testing</a></li><li class="chapter-item expanded "><a href="Chapter_62.html"><strong aria-hidden="true">63.</strong> External crates</a></li><li class="chapter-item expanded "><a href="Chapter_63.html"><strong aria-hidden="true">64.</strong> A tour of the standard library</a></li><li class="chapter-item expanded "><a href="Chapter_64.html"><strong aria-hidden="true">65.</strong> Writing macros</a></li><li class="chapter-item expanded "><a href="Chapter_65.html"><strong aria-hidden="true">66.</strong> cargo</a></li><li class="chapter-item expanded "><a href="Chapter_66.html"><strong aria-hidden="true">67.</strong> Taking user input</a></li><li class="chapter-item expanded "><a href="Chapter_67.html"><strong aria-hidden="true">68.</strong> Using files</a></li><li class="chapter-item expanded "><a href="Chapter_68.html"><strong aria-hidden="true">69.</strong> cargo doc</a></li><li class="chapter-item expanded "><a href="Chapter_69.html"><strong aria-hidden="true">70.</strong> The end?</a></li></ol>
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<main>
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<h2><a class="header" href="#enums" id="enums">Enums</a></h2>
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<p>An <code>enum</code> is short for enumerations. They look very similar to a struct, but are different. Here is the difference:</p>
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<ul>
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<li>Use a <code>struct</code> when you want one thing <strong>AND</strong> another thing.</li>
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<li>Use an <code>enum</code> when you want one thing <strong>OR</strong> another thing.</li>
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</ul>
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<p>So structs are for <strong>many things</strong> together, while enums are for <strong>many choices</strong> together.</p>
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<p>To declare an enum, write <code>enum</code> and use a code block with the options, separated by commas. Just like a <code>struct</code>, the last part can have a comma or not. We will create an enum called <code>ThingsInTheSky</code>:</p>
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<pre><pre class="playground"><code class="language-rust">enum ThingsInTheSky {
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Sun,
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Stars,
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}
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fn main() {}
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</code></pre></pre>
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<p>This is an enum because you can either see the sun, <strong>or</strong> the stars: you have to choose one. These are called <strong>variants</strong>.</p>
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<pre><pre class="playground"><code class="language-rust">// create the enum with two choices
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enum ThingsInTheSky {
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Sun,
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Stars,
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}
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// With this function we can use an i32 to create ThingsInTheSky.
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fn create_skystate(time: i32) -> ThingsInTheSky {
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match time {
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6..=18 => ThingsInTheSky::Sun, // Between 6 and 18 hours we can see the sun
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_ => ThingsInTheSky::Stars, // Otherwise, we can see stars
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}
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}
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// With this function we can match against the two choices in ThingsInTheSky.
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fn check_skystate(state: &ThingsInTheSky) {
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match state {
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ThingsInTheSky::Sun => println!("I can see the sun!"),
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ThingsInTheSky::Stars => println!("I can see the stars!")
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}
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}
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fn main() {
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let time = 8; // it's 8 o'clock
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let skystate = create_skystate(time); // create_skystate returns a ThingsInTheSky
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check_skystate(&skystate); // Give it a reference so it can read the variable skystate
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}
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</code></pre></pre>
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<p>This prints <code>I can see the sun!</code>.</p>
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<p>You can add data to an enum too.</p>
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<pre><pre class="playground"><code class="language-rust">enum ThingsInTheSky {
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Sun(String), // Now each variant has a string
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Stars(String),
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}
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fn create_skystate(time: i32) -> ThingsInTheSky {
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match time {
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6..=18 => ThingsInTheSky::Sun(String::from("I can see the sun!")), // Write the strings here
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_ => ThingsInTheSky::Stars(String::from("I can see the stars!")),
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}
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}
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fn check_skystate(state: &ThingsInTheSky) {
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match state {
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ThingsInTheSky::Sun(description) => println!("{}", description), // Give the string the name description so we can use it
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ThingsInTheSky::Stars(n) => println!("{}", n), // Or you can name it n. Or anything else - it doesn't matter
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}
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}
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fn main() {
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let time = 8; // it's 8 o'clock
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let skystate = create_skystate(time); // create_skystate returns a ThingsInTheSky
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check_skystate(&skystate); // Give it a reference so it can read the variable skystate
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}
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</code></pre></pre>
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<p>This prints the same thing: <code>I can see the sun!</code></p>
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<p>You can also "import" an enum so you don't have to type so much. Here's an example where we have to type <code>Mood::</code> every time we match on our mood:</p>
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<pre><pre class="playground"><code class="language-rust">enum Mood {
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Happy,
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Sleepy,
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NotBad,
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Angry,
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}
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fn match_mood(mood: &Mood) -> i32 {
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let happiness_level = match mood {
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Mood::Happy => 10, // Here we type Mood:: every time
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Mood::Sleepy => 6,
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Mood::NotBad => 7,
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Mood::Angry => 2,
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};
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happiness_level
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}
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fn main() {
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let my_mood = Mood::NotBad;
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let happiness_level = match_mood(&my_mood);
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println!("Out of 1 to 10, my happiness is {}", happiness_level);
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}
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</code></pre></pre>
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<p>It prints <code>Out of 1 to 10, my happiness is 7</code>. Let's import so we can type less. To import everything, write <code>*</code>. Note: it's the same key as <code>*</code> for dereferencing but is completely different.</p>
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<pre><pre class="playground"><code class="language-rust">enum Mood {
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Happy,
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Sleepy,
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NotBad,
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Angry,
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}
|
|
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fn match_mood(mood: &Mood) -> i32 {
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use Mood::*; // We imported everything in Mood. Now we can just write Happy, Sleepy, etc.
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let happiness_level = match mood {
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Happy => 10, // We don't have to write Mood:: anymore
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Sleepy => 6,
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NotBad => 7,
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Angry => 2,
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};
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happiness_level
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}
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fn main() {
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let my_mood = Mood::Happy;
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let happiness_level = match_mood(&my_mood);
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println!("Out of 1 to 10, my happiness is {}", happiness_level);
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}
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</code></pre></pre>
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<p>Parts of an <code>enum</code> can also be turned into an integer. That's because Rust gives each arm of an <code>enum</code> a number that starts with 0 for its own use. You can do things with it if your enum doesn't have any other data in it.</p>
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<pre><pre class="playground"><code class="language-rust">enum Season {
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Spring, // If this was Spring(String) or something it wouldn't work
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Summer,
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Autumn,
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Winter,
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}
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fn main() {
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use Season::*;
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let four_seasons = vec![Spring, Summer, Autumn, Winter];
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for season in four_seasons {
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println!("{}", season as u32);
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}
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}
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</code></pre></pre>
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<p>This prints:</p>
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<pre><code class="language-text">0
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1
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2
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3
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</code></pre>
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|
<p>You can give it a different number though if you want - Rust doesn't care and can use it in the same way. Just add an <code>=</code> and your number to the variant that you want to have a number. You don't have to give all of them a number. But if you don't, Rust will just add 1 from the arm before to give it a number.</p>
|
|
<pre><pre class="playground"><code class="language-rust">enum Star {
|
|
BrownDwarf = 10,
|
|
RedDwarf = 50,
|
|
YellowStar = 100,
|
|
RedGiant = 1000,
|
|
DeadStar, // Think about this one. What number will it have?
|
|
}
|
|
|
|
fn main() {
|
|
use Star::*;
|
|
let starvec = vec![BrownDwarf, RedDwarf, YellowStar, RedGiant];
|
|
for star in starvec {
|
|
match star as u32 {
|
|
size if size <= 80 => println!("Not the biggest star."), // Remember: size doesn't mean anything. It's just a name we chose so we can print it
|
|
size if size >= 80 => println!("This is a good-sized star."),
|
|
_ => println!("That star is pretty big!"),
|
|
}
|
|
}
|
|
println!("What about DeadStar? It's the number {}.", DeadStar as u32);
|
|
}
|
|
</code></pre></pre>
|
|
<p>This prints:</p>
|
|
<pre><code class="language-text">Not the biggest star.
|
|
Not the biggest star.
|
|
This is a good-sized star.
|
|
This is a good-sized star.
|
|
What about DeadStar? It's the number 1001.
|
|
</code></pre>
|
|
<p><code>DeadStar</code> would have been number 4, but now it's 1001.</p>
|
|
<h3><a class="header" href="#enums-to-use-multiple-types" id="enums-to-use-multiple-types">Enums to use multiple types</a></h3>
|
|
<p>You know that items in a <code>Vec</code>, array, etc. all need the same type (only tuples are different). But you can actually use an enum to put different types in. Imagine we want to have a <code>Vec</code> with <code>u32</code>s or <code>i32</code>s. Of course, you can make a <code>Vec<(u32, i32)></code> (a vec with <code>(u32, i32)</code> tuples) but we only want one each time. So here you can use an enum. Here is a simple example:</p>
|
|
<pre><pre class="playground"><code class="language-rust">enum Number {
|
|
U32(u32),
|
|
I32(i32),
|
|
}
|
|
|
|
fn main() {}
|
|
</code></pre></pre>
|
|
<p>So there are two variants: the <code>U32</code> variant with a <code>u32</code> inside, and the <code>I32</code> variant with <code>i32</code> inside. <code>U32</code> and <code>I32</code> are just names we made. They could have been <code>UThirtyTwo</code> or <code>IThirtyTwo</code> or anything else.</p>
|
|
<p>Now, if we put them into a <code>Vec</code> we just have a <code>Vec<Number></code>, and the compiler is happy because it's all the same type. The compiler doesn't care that we have either <code>u32</code> or <code>i32</code> because they are all inside a single type called <code>Number</code>. And because it's an enum, you have to pick one, which is what we want. We will use the <code>.is_positive()</code> method to pick. If it's <code>true</code> then we will choose <code>U32</code>, and if it's <code>false</code> then we will choose <code>I32</code>.</p>
|
|
<p>Now the code looks like this:</p>
|
|
<pre><pre class="playground"><code class="language-rust">enum Number {
|
|
U32(u32),
|
|
I32(i32),
|
|
}
|
|
|
|
fn get_number(input: i32) -> Number {
|
|
let number = match input.is_positive() {
|
|
true => Number::U32(input as u32), // change it to u32 if it's positive
|
|
false => Number::I32(input), // otherwise just give the number because it's already i32
|
|
};
|
|
number
|
|
}
|
|
|
|
|
|
fn main() {
|
|
let my_vec = vec![get_number(-800), get_number(8)];
|
|
|
|
for item in my_vec {
|
|
match item {
|
|
Number::U32(number) => println!("It's a u32 with the value {}", number),
|
|
Number::I32(number) => println!("It's an i32 with the value {}", number),
|
|
}
|
|
}
|
|
}
|
|
</code></pre></pre>
|
|
<p>This prints what we wanted to see:</p>
|
|
<pre><code class="language-text">It's an i32 with the value -800
|
|
It's a u32 with the value 8
|
|
</code></pre>
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