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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" class="active"><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"><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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<main>
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<h2 id="mutability-changing"><a class="header" href="#mutability-changing">Mutability (changing)</a></h2>
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<p><strong><a href="https://youtu.be/Nyyd6qn7dZY">See this chapter on YouTube</a></strong></p>
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<p>When you declare a variable with <code>let</code>, it is immutable (cannot be changed).</p>
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<p>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 = 8;
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my_number = 10; // ⚠️
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}
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</code></pre></pre>
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<p>The compiler says: <code>error[E0384]: cannot assign twice to immutable variable my_number</code>. This is because variables are immutable if you only write <code>let</code>.</p>
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<p>But sometimes you want to change your variable. To make a variable that you can change, add <code>mut</code> after <code>let</code>:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let mut my_number = 8;
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my_number = 10;
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}
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</code></pre></pre>
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<p>Now there is no problem.</p>
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<p>However, you cannot change the type: even <code>mut</code> doesn't let you do that. This will not work:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let mut my_variable = 8; // it is now an i32. That can't be changed
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my_variable = "Hello, world!"; // ⚠️
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}
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</code></pre></pre>
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<p>You will see the same "expected" message from the compiler: <code>expected integer, found &str</code>. <code>&str</code> is a string type that we will learn soon.</p>
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<h3 id="shadowing"><a class="header" href="#shadowing">Shadowing</a></h3>
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<p><strong><a href="https://youtu.be/InULHyRGw7g">See this chapter on YouTube</a></strong></p>
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<p>Shadowing means using <code>let</code> to declare a new variable with the same name as another variable. It looks like mutability, but it is completely different. Shadowing looks like this:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let my_number = 8; // This is an i32
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println!("{}", my_number); // prints 8
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let my_number = 9.2; // This is an f64 with the same name. But it's not the first my_number - it is completely different!
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println!("{}", my_number) // Prints 9.2
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}
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</code></pre></pre>
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<p>Here we say that we "shadowed" <code>my_number</code> with a new "let binding".</p>
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<p>So is the first <code>my_number</code> destroyed? No, but when we call <code>my_number</code> we now get <code>my_number</code> the <code>f64</code>. And because they are in the same scope block (the same <code>{}</code>), we can't see the first <code>my_number</code> anymore.</p>
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<p>But if they are in different blocks, we can see both. For example:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let my_number = 8; // This is an i32
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println!("{}", my_number); // prints 8
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{
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let my_number = 9.2; // This is an f64. It is not my_number - it is completely different!
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println!("{}", my_number) // Prints 9.2
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// But the shadowed my_number only lives until here.
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// The first my_number is still alive!
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}
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println!("{}", my_number); // prints 8
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}
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</code></pre></pre>
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<p>So when you shadow a variable, you don't destroy it. You <strong>block</strong> it.</p>
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<p>So what is the advantage of shadowing? Shadowing is good when you need to change a variable a lot. Imagine that you want to do a lot of simple math with a variable:</p>
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<pre><pre class="playground"><code class="language-rust">fn times_two(number: i32) -> i32 {
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number * 2
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}
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fn main() {
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let final_number = {
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let y = 10;
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let x = 9; // x starts at 9
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let x = times_two(x); // shadow with new x: 18
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let x = x + y; // shadow with new x: 28
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x // return x: final_number is now the value of x
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};
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println!("The number is now: {}", final_number)
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}
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</code></pre></pre>
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<p>Without shadowing you would have to think of different names, even though you don't care about x:</p>
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<pre><pre class="playground"><code class="language-rust">fn times_two(number: i32) -> i32 {
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number * 2
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}
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fn main() {
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// Pretending we are using Rust without shadowing
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let final_number = {
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let y = 10;
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let x = 9; // x starts at 9
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let x_twice = times_two(x); // second name for x
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let x_twice_and_y = x_twice + y; // third name for x!
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x_twice_and_y // too bad we didn't have shadowing - we could have just used x
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};
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println!("The number is now: {}", final_number)
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}
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</code></pre></pre>
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<p>In general, you see shadowing in Rust in this case. It happens where you want to quickly take variable, do something to it, and do something else again. And you usually use it for quick variables that you don't care too much about.</p>
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