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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" class="active"><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="vectors"><a class="header" href="#vectors">Vectors</a></h2>
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<p><strong><a href="https://youtu.be/Eh-DsRnDKmw">See this chapter on YouTube</a></strong></p>
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<p>In the same way that we have <code>&str</code> and <code>String</code>, we have arrays and vectors. Arrays are faster with less functionality, and vectors are slower with more functionality. (Of course, Rust is always very fast so vectors are not slow, just slow<em>er</em> than arrays.) The type is written <code>Vec</code>, and you can also just call it a "vec".</p>
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<p>There are two main ways to declare a vector. One is like with <code>String</code> using <code>new</code>:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let name1 = String::from("Windy");
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let name2 = String::from("Gomesy");
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let mut my_vec = Vec::new();
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// If we run the program now, the compiler will give an error.
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// It doesn't know the type of vec.
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my_vec.push(name1); // Now it knows: it's Vec<String>
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my_vec.push(name2);
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}
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</code></pre></pre>
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<p>You can see that a <code>Vec</code> always has something else inside it, and that's what the <code><></code> (angle brackets) are for. A <code>Vec<String></code> is a vector with one or more <code>String</code>s. You can also have more types inside. For example:</p>
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<ul>
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<li><code>Vec<(i32, i32)></code> this is a <code>Vec</code> where each item is a tuple: <code>(i32, i32)</code>.</li>
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<li><code>Vec<Vec<String>></code> this is a <code>Vec</code> that has <code>Vec</code>s of <code>Strings</code>. Say for example you wanted to save your favourite book as a <code>Vec<String></code>. Then you do it again with another book, and get another <code>Vec<String></code>. To hold both books, you would put them into another <code>Vec</code> and that would be a <code>Vec<Vec<String>></code>.</li>
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</ul>
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<p>Instead of using <code>.push()</code> to make Rust decide the type, you can just declare the type.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let mut my_vec: Vec<String> = Vec::new(); // The compiler knows the type
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// so there is no error.
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}
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</code></pre></pre>
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<p>You can see that items in vectors must have the same type.</p>
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<p>Another easy way to create a vector is with the <code>vec!</code> macro. It looks like an array declaration, but has <code>vec!</code> in front of it.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let mut my_vec = vec![8, 10, 10];
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}
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</code></pre></pre>
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<p>The type is <code>Vec<i32></code>. You call it a "Vec of i32s". And a <code>Vec<String></code> is a "Vec of strings". And a <code>Vec<Vec<String>></code> is a "Vec of a vec of strings".</p>
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<p>You can slice a vector too, just like in an array.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let vec_of_ten = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
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// Everything is the same as above except we added vec!.
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let three_to_five = &vec_of_ten[2..5];
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let start_at_two = &vec_of_ten[1..];
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let end_at_five = &vec_of_ten[..5];
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let everything = &vec_of_ten[..];
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println!("Three to five: {:?},
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start at two: {:?}
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end at five: {:?}
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everything: {:?}", three_to_five, start_at_two, end_at_five, everything);
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}
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</code></pre></pre>
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<p>Because a vec is slower than an array, we can use some methods to make it faster. A vec has a <strong>capacity</strong>, which means the space given to the vector. When you push a new item on the vector, it gets closer and closer to the capacity. Then if you go past the capacity, it will make its capacity double and copy the items into the new space. This is called reallocation. We'll use a method called <code>.capacity()</code> to look at the capacity of a vector as we add items to it.</p>
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<p>For example:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let mut num_vec = Vec::new();
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println!("{}", num_vec.capacity()); // 0 elements: prints 0
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num_vec.push('a'); // add one character
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println!("{}", num_vec.capacity()); // 1 element: prints 4. Vecs with 1 item always start with capacity 4
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num_vec.push('a'); // add one more
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num_vec.push('a'); // add one more
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num_vec.push('a'); // add one more
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println!("{}", num_vec.capacity()); // 4 elements: still prints 4.
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num_vec.push('a'); // add one more
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println!("{}", num_vec.capacity()); // prints 8. We have 5 elements, but it doubled 4 to 8 to make space
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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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4
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4
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8
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</code></pre>
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<p>So this vector has two reallocations: 0 to 4, and 4 to 8. We can make it faster:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let mut num_vec = Vec::with_capacity(8); // Give it capacity 8
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num_vec.push('a'); // add one character
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println!("{}", num_vec.capacity()); // prints 8
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num_vec.push('a'); // add one more
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println!("{}", num_vec.capacity()); // prints 8
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num_vec.push('a'); // add one more
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println!("{}", num_vec.capacity()); // prints 8.
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num_vec.push('a'); // add one more
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num_vec.push('a'); // add one more // Now we have 5 elements
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println!("{}", num_vec.capacity()); // Still 8
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}
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</code></pre></pre>
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<p>This vector has 0 reallocations, which is better. So if you think you know how many elements you need, you can use <code>Vec::with_capacity()</code> to make it faster.</p>
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<p>You remember that you can use <code>.into()</code> to make a <code>&str</code> into a <code>String</code>. You can also use it to make an array into a <code>Vec</code>. You have to tell <code>.into()</code> that you want a <code>Vec</code>, but you don't have to choose the type of <code>Vec</code>. If you don't want to choose, you can write <code>Vec<_></code>.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let my_vec: Vec<u8> = [1, 2, 3].into();
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let my_vec2: Vec<_> = [9, 0, 10].into(); // Vec<_> means "choose the Vec type for me"
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// Rust will choose Vec<i32>
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}
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</code></pre></pre>
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