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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" class="active"><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="iterators"><a class="header" href="#iterators">Iterators</a></h2>
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<p>An iterator is a construct that can give you the items in the collection, one at a time. Actually, we have already used iterators a lot: the <code>for</code> loop gives you an iterator. When you want to use an iterator other times, you have to choose what kind:</p>
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<ul>
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<li><code>.iter()</code> for an iterator of references</li>
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<li><code>.iter_mut()</code> for an iterator of mutable references</li>
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<li><code>.into_iter()</code> for an iterator of values (not references)</li>
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</ul>
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<p>A <code>for</code> loop is actually just an iterator that owns its values. That's why it can make it mutable and then you can change the values when you use it.</p>
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<p>We can use iterators like this:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let vector1 = vec![1, 2, 3]; // we will use .iter() and .into_iter() on this one
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let vector1_a = vector1.iter().map(|x| x + 1).collect::<Vec<i32>>();
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let vector1_b = vector1.into_iter().map(|x| x * 10).collect::<Vec<i32>>();
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let mut vector2 = vec![10, 20, 30]; // we will use .iter_mut() on this one
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vector2.iter_mut().for_each(|x| *x +=100);
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println!("{:?}", vector1_a);
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println!("{:?}", vector2);
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println!("{:?}", vector1_b);
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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">[2, 3, 4]
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[110, 120, 130]
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[10, 20, 30]
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</code></pre>
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<p>The first two we used a method called <code>.map()</code>. This method lets you do something to every item, then pass it on. The last one we used is one called <code>.for_each()</code>. This method just lets you do something to every item. <code>.iter_mut()</code> plus <code>for_each()</code> is basically just a <code>for</code> loop. Inside each method we can give a name to every item (we just called it <code>x</code>) and use that to change it. These are called closures and we will learn about them in the next section.</p>
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<p>Let's go over them again, one at a time.</p>
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<p>First we used <code>.iter()</code> on <code>vector1</code> to get references. We added 1 to each, and made it into a new Vec. <code>vector1</code> is still alive because we only used references: we didn't take by value. Now we have <code>vector1</code>, and a new Vec called <code>vector1_a</code>. Because <code>.map()</code> just passes it on, we needed to use <code>.collect()</code> to make it into a <code>Vec</code>.</p>
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<p>Then we used <code>into_iter</code> to get an iterator by value from <code>vector1</code>. This destroys <code>vector1</code>, because that's what <code>into_iter()</code> does. So after we make <code>vector1_b</code> we can't use <code>vector1</code> again.</p>
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<p>Finally we used <code>.iter_mut()</code> for <code>vector2</code>. It is mutable, so we don't need to use <code>.collect()</code> to create a new Vec. Instead, we change the values in the same Vec with mutable references. So <code>vector2</code> is still there. Because we don't need a new Vec, we use <code>for_each</code>: it's just like a <code>for</code> loop.</p>
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<h3 id="how-an-iterator-works"><a class="header" href="#how-an-iterator-works">How an iterator works</a></h3>
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<p>An iterator works by using a method called <code>.next()</code>, which gives an <code>Option</code>. When you use an iterator, Rust calls <code>next()</code> over and over again. If it gets <code>Some</code>, it keeps going. If it gets <code>None</code>, it stops.</p>
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<p>Do you remember the <code>assert_eq!</code> macro? In documentation, you see it all the time. Here it is showing how an iterator works.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let my_vec = vec!['a', 'b', '거', '柳']; // Just a regular Vec
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let mut my_vec_iter = my_vec.iter(); // This is an Iterator type now, but we haven't called it yet
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assert_eq!(my_vec_iter.next(), Some(&'a')); // Call the first item with .next()
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assert_eq!(my_vec_iter.next(), Some(&'b')); // Call the next
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assert_eq!(my_vec_iter.next(), Some(&'거')); // Again
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assert_eq!(my_vec_iter.next(), Some(&'柳')); // Again
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assert_eq!(my_vec_iter.next(), None); // Nothing is left: just None
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assert_eq!(my_vec_iter.next(), None); // You can keep calling .next() but it will always be None
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}
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</code></pre></pre>
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<p>Implementing <code>Iterator</code> for your own struct or enum is not too hard. First let's make a book library and think about it.</p>
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<pre><pre class="playground"><code class="language-rust">#[derive(Debug)] // we want to print it with {:?}
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struct Library {
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library_type: LibraryType, // this is our enum
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books: Vec<String>, // list of books
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}
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#[derive(Debug)]
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enum LibraryType { // libraries can be city libraries or country libraries
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City,
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Country,
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}
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impl Library {
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fn add_book(&mut self, book: &str) { // we use add_book to add new books
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self.books.push(book.to_string()); // we take a &str and turn it into a String, then add it to the Vec
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}
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fn new() -> Self { // this creates a new Library
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Self {
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library_type: LibraryType::City, // most are in the city so we'll choose City
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// most of the time
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books: Vec::new(),
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}
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}
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}
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fn main() {
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let mut my_library = Library::new(); // make a new library
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my_library.add_book("The Doom of the Darksword"); // add some books
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my_library.add_book("Demian - die Geschichte einer Jugend");
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my_library.add_book("구운몽");
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my_library.add_book("吾輩は猫である");
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println!("{:?}", my_library.books); // we can print our list of books
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}
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</code></pre></pre>
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<p>That works well. Now we want to implement <code>Iterator</code> for the library so we can use it in a <code>for</code> loop. Right now if we try a <code>for</code> loop, it doesn't work:</p>
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<pre><pre class="playground"><code class="language-rust">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>for item in my_library {
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println!("{}", item); // ⚠️
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>It says:</p>
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<pre><code class="language-text">error[E0277]: `Library` is not an iterator
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--> src\main.rs:47:16
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47 | for item in my_library {
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| ^^^^^^^^^^ `Library` is not an iterator
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|
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= help: the trait `std::iter::Iterator` is not implemented for `Library`
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= note: required by `std::iter::IntoIterator::into_iter`
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</code></pre>
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<p>But we can make library into an iterator with <code>impl Iterator for Library</code>. Information on the <code>Iterator</code> trait is here in the standard library: <a href="https://doc.rust-lang.org/std/iter/trait.Iterator.html">https://doc.rust-lang.org/std/iter/trait.Iterator.html</a></p>
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<p>On the top left of the page it says: <code>Associated Types: Item</code> and <code>Required Methods: next</code>. An "associated type" means "a type that goes together". Our associated type will be <code>String</code>, because we want the iterator to give us Strings.</p>
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<p>In the page it has an example that looks like this:</p>
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<pre><pre class="playground"><code class="language-rust">// an iterator which alternates between Some and None
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struct Alternate {
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state: i32,
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}
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impl Iterator for Alternate {
|
|
type Item = i32;
|
|
|
|
fn next(&mut self) -> Option<i32> {
|
|
let val = self.state;
|
|
self.state = self.state + 1;
|
|
|
|
// if it's even, Some(i32), else None
|
|
if val % 2 == 0 {
|
|
Some(val)
|
|
} else {
|
|
None
|
|
}
|
|
}
|
|
}
|
|
|
|
fn main() {}
|
|
</code></pre></pre>
|
|
<p>You can see that under <code>impl Iterator for Alternate</code> it says <code>type Item = i32</code>. This is the associated type. Our iterator will be for our list of books, which is a <code>Vec<String></code>. When we call next, it will give us a <code>String</code>. So we will write <code>type Item = String;</code>. That is the associated item.</p>
|
|
<p>To implement <code>Iterator</code>, you need to write the <code>fn next()</code> function. This is where you decide what the iterator should do. For our <code>Library</code>, we want it to give us the last books first. So we will <code>match</code> with <code>.pop()</code> which takes the last item off if it is <code>Some</code>. We also want to print " is found!" for each item. Now it looks like this:</p>
|
|
<pre><pre class="playground"><code class="language-rust">#[derive(Debug, Clone)]
|
|
struct Library {
|
|
library_type: LibraryType,
|
|
books: Vec<String>,
|
|
}
|
|
|
|
#[derive(Debug, Clone)]
|
|
enum LibraryType {
|
|
City,
|
|
Country,
|
|
}
|
|
|
|
impl Library {
|
|
fn add_book(&mut self, book: &str) {
|
|
self.books.push(book.to_string());
|
|
}
|
|
|
|
fn new() -> Self {
|
|
Self {
|
|
library_type: LibraryType::City,
|
|
// most of the time
|
|
books: Vec::new(),
|
|
}
|
|
}
|
|
}
|
|
|
|
impl Iterator for Library {
|
|
type Item = String;
|
|
|
|
fn next(&mut self) -> Option<String> {
|
|
match self.books.pop() {
|
|
Some(book) => Some(book + " is found!"), // Rust allows String + &str
|
|
None => None,
|
|
}
|
|
}
|
|
}
|
|
|
|
fn main() {
|
|
let mut my_library = Library::new();
|
|
my_library.add_book("The Doom of the Darksword");
|
|
my_library.add_book("Demian - die Geschichte einer Jugend");
|
|
my_library.add_book("구운몽");
|
|
my_library.add_book("吾輩は猫である");
|
|
|
|
for item in my_library.clone() { // we can use a for loop now. Give it a clone so Library won't be destroyed
|
|
println!("{}", item);
|
|
}
|
|
}
|
|
</code></pre></pre>
|
|
<p>This prints:</p>
|
|
<pre><code class="language-text">吾輩は猫である is found!
|
|
구운몽 is found!
|
|
Demian - die Geschichte einer Jugend is found!
|
|
The Doom of the Darksword is found!
|
|
</code></pre>
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