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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"><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" class="active"><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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<h2 id="the-dbg-macro-and-inspect"><a class="header" href="#the-dbg-macro-and-inspect">The dbg! macro and .inspect</a></h2>
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<p><code>dbg!</code> is a very useful macro that prints quick information. It is a good alternative to <code>println!</code> because it is faster to type and gives more information:</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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dbg!(my_number);
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}
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</code></pre></pre>
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<p>This prints <code>[src\main.rs:4] my_number = 8</code>.</p>
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<p>But actually, you can put <code>dbg!</code> in many other places, and even wrap code in it. Look at this code for example:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let mut my_number = 9;
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my_number += 10;
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let new_vec = vec![8, 9, 10];
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let double_vec = new_vec.iter().map(|x| x * 2).collect::<Vec<i32>>();
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}
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</code></pre></pre>
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<p>This code creates a new mutable number and changes it. Then it creates a vec, and uses <code>iter</code> and <code>map</code> and <code>collect</code> to create a new vec. We can put <code>dbg!</code> almost everywhere in this code. <code>dbg!</code> asks the compiler: "What are you doing at this moment?" and tells you.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let mut my_number = dbg!(9);
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dbg!(my_number += 10);
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let new_vec = dbg!(vec![8, 9, 10]);
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let double_vec = dbg!(new_vec.iter().map(|x| x * 2).collect::<Vec<i32>>());
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dbg!(double_vec);
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}
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</code></pre></pre>
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<p>So this prints:</p>
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<pre><code class="language-text">[src\main.rs:3] 9 = 9
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</code></pre>
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<p>and:</p>
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<pre><code class="language-text">[src\main.rs:4] my_number += 10 = ()
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</code></pre>
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<p>and:</p>
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<pre><code class="language-text">[src\main.rs:6] vec![8, 9, 10] = [
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8,
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9,
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10,
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]
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</code></pre>
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<p>and this one, which even shows you the value of the expression:</p>
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<pre><code class="language-text">[src\main.rs:8] new_vec.iter().map(|x| x * 2).collect::<Vec<i32>>() = [
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16,
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18,
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20,
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]
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</code></pre>
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<p>and:</p>
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<pre><code class="language-text">[src\main.rs:10] double_vec = [
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16,
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18,
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20,
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]
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</code></pre>
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<p><code>.inspect</code> is a bit similar to <code>dbg!</code> but you use it like <code>map</code> in an iterator. It gives you the iterator item and you can print it or do whatever you want. For example, let's look at our <code>double_vec</code> again.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let new_vec = vec![8, 9, 10];
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let double_vec = new_vec
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.iter()
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.map(|x| x * 2)
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.collect::<Vec<i32>>();
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}
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</code></pre></pre>
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<p>We want to know more information about what the code is doing. So we add <code>inspect()</code> in two places:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let new_vec = vec![8, 9, 10];
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let double_vec = new_vec
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.iter()
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.inspect(|first_item| println!("The item is: {}", first_item))
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.map(|x| x * 2)
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.inspect(|next_item| println!("Then it is: {}", next_item))
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.collect::<Vec<i32>>();
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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">The item is: 8
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Then it is: 16
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The item is: 9
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Then it is: 18
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The item is: 10
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Then it is: 20
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</code></pre>
|
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<p>And because <code>.inspect</code> takes a closure, we can write as much as we want:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let new_vec = vec![8, 9, 10];
|
|
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let double_vec = new_vec
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.iter()
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.inspect(|first_item| {
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println!("The item is: {}", first_item);
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match **first_item % 2 { // first item is a &&i32 so we use **
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0 => println!("It is even."),
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_ => println!("It is odd."),
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}
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println!("In binary it is {:b}.", first_item);
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})
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.map(|x| x * 2)
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.collect::<Vec<i32>>();
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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">The item is: 8
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It is even.
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In binary it is 1000.
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The item is: 9
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It is odd.
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In binary it is 1001.
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The item is: 10
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It is even.
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In binary it is 1010.
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</code></pre>
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</html>
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