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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" class="active"><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"><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 &amp;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="printing-hello-world"><a class="header" href="#printing-hello-world">Printing 'hello, world!'</a></h2>
<p><strong>See this chapter on YouTube: <a href="https://youtu.be/yYlPHRl2geQ">Video 1</a>, <a href="https://youtu.be/DTCSfBJJZb8">Video 2</a></strong></p>
<p>When you start a new Rust program, it always has this code:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
println!(&quot;Hello, world!&quot;);
}
</code></pre></pre>
<ul>
<li><code>fn</code> means function,</li>
<li><code>main</code> is the function that starts the program,</li>
<li><code>()</code> means that we didn't give the function any variables to start.</li>
</ul>
<p><code>{}</code> is called a <strong>code block</strong>. This is the space where code lives.</p>
<p><code>println!</code> is a <strong>macro</strong> that prints to the console. A <strong>macro</strong> is like a function that writes code for you. Macros have a <code>!</code> after them. We will learn about making macros later. For now, remember that <code>!</code> means that it is a macro.</p>
<p>To learn about the <code>;</code>, we will create another function. First, in <code>main</code> we will print a number 8:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
println!(&quot;Hello, world number {}!&quot;, 8);
}
</code></pre></pre>
<p>The <code>{}</code> in <code>println!</code> means &quot;put the variable inside here&quot;. This prints <code>Hello, world number 8!</code>.</p>
<p>We can put more in, just like we did before:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
println!(&quot;Hello, worlds number {} and {}!&quot;, 8, 9);
}
</code></pre></pre>
<p>This prints <code>Hello, worlds number 8 and 9!</code>.</p>
<p>Now let's create the function.</p>
<pre><pre class="playground"><code class="language-rust">fn number() -&gt; i32 {
8
}
fn main() {
println!(&quot;Hello, world number {}!&quot;, number());
}
</code></pre></pre>
<p>This also prints <code>Hello, world number 8!</code>. When Rust looks at <code>number()</code> it sees a function. This function:</p>
<ul>
<li>Does not take anything (because it has <code>()</code>)</li>
<li>Returns an <code>i32</code>. The <code>-&gt;</code> (called a &quot;skinny arrow&quot;) shows what the function returns.</li>
</ul>
<p>Inside the function is just <code>8</code>. Because there is no <code>;</code>, this is the value it returns. If it had a <code>;</code>, it would not return anything (it would return a <code>()</code>). Rust will not compile this if it has a <code>;</code>, because the return is <code>i32</code> and <code>;</code> returns <code>()</code>, not <code>i32</code>:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
println!(&quot;Hello, world number {}&quot;, number());
}
fn number() -&gt; i32 {
8; // ⚠️
}
</code></pre></pre>
<pre><code class="language-text">5 | fn number() -&gt; i32 {
| ------ ^^^ expected `i32`, found `()`
| |
| implicitly returns `()` as its body has no tail or `return` expression
6 | 8;
| - help: consider removing this semicolon
</code></pre>
<p>This means &quot;you told me that <code>number()</code> returns an <code>i32</code>, but you added a <code>;</code> so it doesn't return anything&quot;. So the compiler suggests removing the semicolon.</p>
<p>You can also write <code>return 8;</code> but in Rust it is normal to just remove the <code>;</code> to <code>return</code>.</p>
<p>When you want to give variables to a function, put them inside the <code>()</code>. You have to give them a name and write the type.</p>
<pre><pre class="playground"><code class="language-rust">fn multiply(number_one: i32, number_two: i32) { // Two i32s will enter the function. We will call them number_one and number_two.
let result = number_one * number_two;
println!(&quot;{} times {} is {}&quot;, number_one, number_two, result);
}
fn main() {
multiply(8, 9); // We can give the numbers directly
let some_number = 10; // Or we can declare two variables
let some_other_number = 2;
multiply(some_number, some_other_number); // and put them in the function
}
</code></pre></pre>
<p>We can also return an <code>i32</code>. Just take out the semicolon at the end:</p>
<pre><pre class="playground"><code class="language-rust">fn multiply(number_one: i32, number_two: i32) -&gt; i32 {
let result = number_one * number_two;
println!(&quot;{} times {} is {}&quot;, number_one, number_two, result);
result // this is the i32 that we return
}
fn main() {
let multiply_result = multiply(8, 9); // We used multiply() to print and to give the result to multiply_result
}
</code></pre></pre>
<h3 id="declaring-variables-and-code-blocks"><a class="header" href="#declaring-variables-and-code-blocks">Declaring variables and code blocks</a></h3>
<p>Use <code>let</code> to declare a variable (declare a variable = tell Rust to make a variable).</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let my_number = 8;
println!(&quot;Hello, number {}&quot;, my_number);
}
</code></pre></pre>
<p>Variables start and end inside a code block <code>{}</code>. In this example, <code>my_number</code> ends before we call <code>println!</code>, because it is inside its own code block.</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
{
let my_number = 8; // my_number starts here
// my_number ends here!
}
println!(&quot;Hello, number {}&quot;, my_number); // ⚠️ there is no my_number and
// println!() can't find it
}
</code></pre></pre>
<p>You can use a code block to return a value:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let my_number = {
let second_number = 8;
second_number + 9 // No semicolon, so the code block returns 8 + 9.
// It works just like a function
};
println!(&quot;My number is: {}&quot;, my_number);
}
</code></pre></pre>
<p>If you add a semicolon inside the block, it will return <code>()</code> (nothing):</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let my_number = {
let second_number = 8; // declare second_number,
second_number + 9; // add 9 to second_number
// but we didn't return it!
// second_number dies now
};
println!(&quot;My number is: {:?}&quot;, my_number); // my_number is ()
}
</code></pre></pre>
<p>So why did we write <code>{:?}</code> and not <code>{}</code>? We will talk about that now.</p>
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