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<ol class="chapter"><li class="chapter-item expanded "><a href="Chapter_0.html"><strong aria-hidden="true">1.</strong> Updates</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" class="active"><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"><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> Cow</a></li><li class="chapter-item expanded "><a href="Chapter_43.html"><strong aria-hidden="true">44.</strong> Type aliases</a></li><li class="chapter-item expanded "><a href="Chapter_44.html"><strong aria-hidden="true">45.</strong> The todo! macro</a></li><li class="chapter-item expanded "><a href="Chapter_45.html"><strong aria-hidden="true">46.</strong> Rc</a></li><li class="chapter-item expanded "><a href="Chapter_46.html"><strong aria-hidden="true">47.</strong> Multiple threads</a></li><li class="chapter-item expanded "><a href="Chapter_47.html"><strong aria-hidden="true">48.</strong> Closures in functions</a></li><li class="chapter-item expanded "><a href="Chapter_48.html"><strong aria-hidden="true">49.</strong> impl Trait</a></li><li class="chapter-item expanded "><a href="Chapter_49.html"><strong aria-hidden="true">50.</strong> Arc</a></li><li class="chapter-item expanded "><a href="Chapter_50.html"><strong aria-hidden="true">51.</strong> Channels</a></li><li class="chapter-item expanded "><a href="Chapter_51.html"><strong aria-hidden="true">52.</strong> Reading Rust documentation</a></li><li class="chapter-item expanded "><a href="Chapter_52.html"><strong aria-hidden="true">53.</strong> Attributes</a></li><li class="chapter-item expanded "><a href="Chapter_53.html"><strong aria-hidden="true">54.</strong> Box</a></li><li class="chapter-item expanded "><a href="Chapter_54.html"><strong aria-hidden="true">55.</strong> Box around traits</a></li><li class="chapter-item expanded "><a href="Chapter_55.html"><strong aria-hidden="true">56.</strong> Default and the builder pattern</a></li><li class="chapter-item expanded "><a href="Chapter_56.html"><strong aria-hidden="true">57.</strong> Deref and DerefMut</a></li><li class="chapter-item expanded "><a href="Chapter_57.html"><strong aria-hidden="true">58.</strong> Crates and modules</a></li><li class="chapter-item expanded "><a href="Chapter_58.html"><strong aria-hidden="true">59.</strong> Testing</a></li><li class="chapter-item expanded "><a href="Chapter_59.html"><strong aria-hidden="true">60.</strong> External crates</a></li><li class="chapter-item expanded "><a href="Chapter_60.html"><strong aria-hidden="true">61.</strong> A tour of the standard library</a></li><li class="chapter-item expanded "><a href="Chapter_61.html"><strong aria-hidden="true">62.</strong> Writing macros</a></li><li class="chapter-item expanded "><a href="Chapter_62.html"><strong aria-hidden="true">63.</strong> cargo</a></li><li class="chapter-item expanded "><a href="Chapter_63.html"><strong aria-hidden="true">64.</strong> Taking user input</a></li><li class="chapter-item expanded "><a href="Chapter_64.html"><strong aria-hidden="true">65.</strong> Using files</a></li><li class="chapter-item expanded "><a href="Chapter_65.html"><strong aria-hidden="true">66.</strong> cargo doc</a></li><li class="chapter-item expanded "><a href="Chapter_66.html"><strong aria-hidden="true">67.</strong> The end?</a></li></ol>
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<h1 class="menu-title">Easy Rust</h1>
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<main>
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<h2 id="control-flow"><a class="header" href="#control-flow">Control flow</a></h2>
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<p><strong>See this chapter on YouTube: <a href="https://youtu.be/UAymDOpv_us">Part 1</a> and <a href="https://youtu.be/eqysTfiiQZs">Part 2</a></strong></p>
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<p>Control flow means telling your code what to do in different situations. The simplest control flow is <code>if</code>.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let my_number = 5;
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if my_number == 7 {
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println!("It's seven");
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}
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}
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</code></pre></pre>
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<p>Also note that you use <code>==</code> and not <code>=</code>. <code>==</code> is to compare, <code>=</code> is to <em>assign</em> (to give a value). Also note that we wrote <code>if my_number == 7</code> and not <code>if (my_number == 7)</code>. You don't need brackets with <code>if</code> in Rust.</p>
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<p><code>else if</code> and <code>else</code> give you more control:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let my_number = 5;
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if my_number == 7 {
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println!("It's seven");
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} else if my_number == 6 {
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println!("It's six")
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} else {
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println!("It's a different number")
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}
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}
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</code></pre></pre>
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<p>This prints <code>It's a different number</code> because it's not equal to 7 or 6.</p>
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<p>You can add more conditions with <code>&&</code> (and) and <code>||</code> (or).</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let my_number = 5;
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if my_number % 2 == 1 && my_number > 0 { // % 2 means the number that remains after diving by two
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println!("It's a positive odd number");
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} else if my_number == 6 {
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println!("It's six")
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} else {
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println!("It's a different number")
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}
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}
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</code></pre></pre>
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<p>This prints <code>It's a positive odd number</code> because when you divide it by 2 you have a remainder of 1, and it's greater than 0.</p>
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<p>You can see that too much <code>if</code>, <code>else</code>, and <code>else if</code> can be difficult to read. In this case you can use <code>match</code> instead, which looks much cleaner. But you must match for every possible result. For example, this will not work:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let my_number: u8 = 5;
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match my_number {
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0 => println!("it's zero"),
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1 => println!("it's one"),
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2 => println!("it's two"),
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// ⚠️
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}
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}
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</code></pre></pre>
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<p>The compiler says:</p>
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<pre><code class="language-text">error[E0004]: non-exhaustive patterns: `3u8..=std::u8::MAX` not covered
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--> src\main.rs:3:11
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|
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3 | match my_number {
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| ^^^^^^^^^ pattern `3u8..=std::u8::MAX` not covered
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</code></pre>
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<p>This means "you told me about 0 to 2, but <code>u8</code>s can go up to 255. What about 3? What about 4? What about 5?" And so on. So you can add <code>_</code> which means "anything else".</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let my_number: u8 = 5;
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match my_number {
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0 => println!("it's zero"),
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1 => println!("it's one"),
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2 => println!("it's two"),
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_ => println!("It's some other number"),
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}
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}
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</code></pre></pre>
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<p>That prints <code>It's some other number</code>.</p>
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<p>Remember this for match:</p>
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<ul>
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<li>You write <code>match</code> and then make a <code>{}</code> code block.</li>
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<li>Write the <em>pattern</em> on the left and use a <code>=></code> fat arrow to say what to do when it matches.</li>
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<li>Each line is called an "arm".</li>
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<li>Put a comma between the arms (not a semicolon).</li>
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</ul>
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<p>You can declare a value with a match:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let my_number = 5;
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let second_number = match my_number {
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0 => 0,
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5 => 10,
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_ => 2,
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};
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}
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</code></pre></pre>
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<p><code>second_number</code> will be 10. Do you see the semicolon at the end? That is because, after the match is over, we actually told the compiler this: <code>let second_number = 10;</code></p>
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<p>You can match on more complicated things too. You use a tuple to do it.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let sky = "cloudy";
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let temperature = "warm";
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match (sky, temperature) {
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("cloudy", "cold") => println!("It's dark and unpleasant today"),
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("clear", "warm") => println!("It's a nice day"),
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("cloudy", "warm") => println!("It's dark but not bad"),
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_ => println!("Not sure what the weather is."),
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}
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}
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</code></pre></pre>
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<p>This prints <code>It's dark but not bad</code> because it matches "cloudy" and "warm" for <code>sky</code> and <code>temperature</code>.</p>
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<p>You can even put <code>if</code> inside of <code>match</code>. This is called a "match guard":</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let children = 5;
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let married = true;
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match (children, married) {
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(children, married) if married == false => println!("Not married with {} children", children),
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(children, married) if children == 0 && married == true => println!("Married but no children"),
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_ => println!("Married? {}. Number of children: {}.", married, children),
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}
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}
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</code></pre></pre>
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<p>This will print <code>Married? true. Number of children: 5.</code></p>
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<p>You can use _ as many times as you want in a match. In this match on colours, we have three but only check one at a time.</p>
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<pre><pre class="playground"><code class="language-rust">fn match_colours(rbg: (i32, i32, i32)) {
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match rbg {
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(r, _, _) if r < 10 => println!("Not much red"),
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(_, b, _) if b < 10 => println!("Not much blue"),
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(_, _, g) if g < 10 => println!("Not much green"),
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_ => println!("Each colour has at least 10"),
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}
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}
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fn main() {
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let first = (200, 0, 0);
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let second = (50, 50, 50);
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let third = (200, 50, 0);
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match_colours(first);
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match_colours(second);
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match_colours(third);
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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">Not much blue
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Each colour has at least 10
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Not much green
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</code></pre>
|
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<p>This also shows how <code>match</code> statements work, because in the first example it only printed <code>Not much blue</code>. But <code>first</code> also has not much green. A <code>match</code> statement always stops when it finds a match, and doesn't check the rest. This is a good example of code that compiles well but is not the code you want.</p>
|
|
<p>You can make a really big <code>match</code> statement to fix it, but it is probably better to use a <code>for</code> loop. We will talk about loops soon.</p>
|
|
<p>A match has to return the same type. So you can't do this:</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn main() {
|
|
let my_number = 10;
|
|
let some_variable = match my_number {
|
|
10 => 8,
|
|
_ => "Not ten", // ⚠️
|
|
};
|
|
}
|
|
</code></pre></pre>
|
|
<p>The compiler tells you that:</p>
|
|
<pre><code class="language-text">error[E0308]: `match` arms have incompatible types
|
|
--> src\main.rs:17:14
|
|
|
|
|
15 | let some_variable = match my_number {
|
|
| _________________________-
|
|
16 | | 10 => 8,
|
|
| | - this is found to be of type `{integer}`
|
|
17 | | _ => "Not ten",
|
|
| | ^^^^^^^^^ expected integer, found `&str`
|
|
18 | | };
|
|
| |_____- `match` arms have incompatible types
|
|
</code></pre>
|
|
<p>This will also not work, for the same reason:</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn main() {
|
|
let some_variable = if my_number == 10 { 8 } else { "something else "}; // ⚠️
|
|
}
|
|
</code></pre></pre>
|
|
<p>But this works, because it's not a <code>match</code> so you have a different <code>let</code> statement each time:</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn main() {
|
|
let my_number = 10;
|
|
|
|
if my_number == 10 {
|
|
let some_variable = 8;
|
|
} else {
|
|
let some_variable = "Something else";
|
|
}
|
|
}
|
|
</code></pre></pre>
|
|
<p>You can also use <code>@</code> to give a name to the value of a <code>match</code> expression, and then you can use it. In this example we match an <code>i32</code> input in a function. If it's 4 or 13 we want to use that number in a <code>println!</code> statement. Otherwise, we don't need to use it.</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn match_number(input: i32) {
|
|
match input {
|
|
number @ 4 => println!("{} is an unlucky number in China (sounds close to 死)!", number),
|
|
number @ 13 => println!("{} is unlucky in North America, lucky in Italy! In bocca al lupo!", number),
|
|
_ => println!("Looks like a normal number"),
|
|
}
|
|
}
|
|
|
|
fn main() {
|
|
match_number(50);
|
|
match_number(13);
|
|
match_number(4);
|
|
}
|
|
</code></pre></pre>
|
|
<p>This prints:</p>
|
|
<pre><code class="language-text">Looks like a normal number
|
|
13 is unlucky in North America, lucky in Italy! In bocca al lupo!
|
|
4 is an unlucky number in China (sounds close to 死)!
|
|
</code></pre>
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