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<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><
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<div id="content" class="content">
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<main>
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<h2><a class="header" href="#storing-lists-of-values-with-vectors" id="storing-lists-of-values-with-vectors">Storing Lists of Values with Vectors</a></h2>
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<p>The first collection type we’ll look at is <code>Vec<T></code>, also known as a <em>vector</em>.
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Vectors allow you to store more than one value in a single data structure that
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puts all the values next to each other in memory. Vectors can only store values
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of the same type. They are useful when you have a list of items, such as the
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lines of text in a file or the prices of items in a shopping cart.</p>
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<h3><a class="header" href="#creating-a-new-vector" id="creating-a-new-vector">Creating a New Vector</a></h3>
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<p>To create a new, empty vector, we can call the <code>Vec::new</code> function, as shown in
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Listing 8-1.</p>
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<pre><pre class="playpen"><code class="language-rust">
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<span class="boring">#![allow(unused_variables)]
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</span><span class="boring">fn main() {
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</span>let v: Vec<i32> = Vec::new();
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<span class="boring">}
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</span></code></pre></pre>
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<p><span class="caption">Listing 8-1: Creating a new, empty vector to hold values
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of type <code>i32</code></span></p>
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<p>Note that we added a type annotation here. Because we aren’t inserting any
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values into this vector, Rust doesn’t know what kind of elements we intend to
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store. This is an important point. Vectors are implemented using generics;
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we’ll cover how to use generics with your own types in Chapter 10. For now,
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know that the <code>Vec<T></code> type provided by the standard library can hold any type,
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and when a specific vector holds a specific type, the type is specified within
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angle brackets. In Listing 8-1, we’ve told Rust that the <code>Vec<T></code> in <code>v</code> will
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hold elements of the <code>i32</code> type.</p>
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<p>In more realistic code, Rust can often infer the type of value you want to
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store once you insert values, so you rarely need to do this type annotation.
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It’s more common to create a <code>Vec<T></code> that has initial values, and Rust
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provides the <code>vec!</code> macro for convenience. The macro will create a new vector
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that holds the values you give it. Listing 8-2 creates a new <code>Vec<i32></code> that
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holds the values <code>1</code>, <code>2</code>, and <code>3</code>.</p>
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<pre><pre class="playpen"><code class="language-rust">
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<span class="boring">#![allow(unused_variables)]
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</span><span class="boring">fn main() {
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</span>let v = vec![1, 2, 3];
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<span class="boring">}
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</span></code></pre></pre>
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<p><span class="caption">Listing 8-2: Creating a new vector containing
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values</span></p>
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<p>Because we’ve given initial <code>i32</code> values, Rust can infer that the type of <code>v</code>
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is <code>Vec<i32></code>, and the type annotation isn’t necessary. Next, we’ll look at how
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to modify a vector.</p>
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<h3><a class="header" href="#updating-a-vector" id="updating-a-vector">Updating a Vector</a></h3>
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<p>To create a vector and then add elements to it, we can use the <code>push</code> method,
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as shown in Listing 8-3.</p>
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<pre><pre class="playpen"><code class="language-rust">
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<span class="boring">#![allow(unused_variables)]
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</span><span class="boring">fn main() {
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</span>let mut v = Vec::new();
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v.push(5);
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v.push(6);
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v.push(7);
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v.push(8);
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<span class="boring">}
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</span></code></pre></pre>
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<p><span class="caption">Listing 8-3: Using the <code>push</code> method to add values to a
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vector</span></p>
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<p>As with any variable, if we want to be able to change its value, we need to
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make it mutable using the <code>mut</code> keyword, as discussed in Chapter 3. The numbers
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we place inside are all of type <code>i32</code>, and Rust infers this from the data, so
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we don’t need the <code>Vec<i32></code> annotation.</p>
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<h3><a class="header" href="#dropping-a-vector-drops-its-elements" id="dropping-a-vector-drops-its-elements">Dropping a Vector Drops Its Elements</a></h3>
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<p>Like any other <code>struct</code>, a vector is freed when it goes out of scope, as
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annotated in Listing 8-4.</p>
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<pre><pre class="playpen"><code class="language-rust">
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<span class="boring">#![allow(unused_variables)]
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</span><span class="boring">fn main() {
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</span>{
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let v = vec![1, 2, 3, 4];
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|
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// do stuff with v
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} // <- v goes out of scope and is freed here
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<span class="boring">}
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</span></code></pre></pre>
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<p><span class="caption">Listing 8-4: Showing where the vector and its elements
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are dropped</span></p>
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<p>When the vector gets dropped, all of its contents are also dropped, meaning
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those integers it holds will be cleaned up. This may seem like a
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straightforward point but can get a bit more complicated when you start to
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introduce references to the elements of the vector. Let’s tackle that next!</p>
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<h3><a class="header" href="#reading-elements-of-vectors" id="reading-elements-of-vectors">Reading Elements of Vectors</a></h3>
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<p>Now that you know how to create, update, and destroy vectors, knowing how to
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read their contents is a good next step. There are two ways to reference a
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value stored in a vector. In the examples, we’ve annotated the types of the
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values that are returned from these functions for extra clarity.</p>
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<p>Listing 8-5 shows both methods of accessing a value in a vector, either with
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indexing syntax or the <code>get</code> method.</p>
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<pre><pre class="playpen"><code class="language-rust">
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<span class="boring">#![allow(unused_variables)]
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</span><span class="boring">fn main() {
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</span>let v = vec![1, 2, 3, 4, 5];
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|
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let third: &i32 = &v[2];
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println!("The third element is {}", third);
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match v.get(2) {
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Some(third) => println!("The third element is {}", third),
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None => println!("There is no third element."),
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p><span class="caption">Listing 8-5: Using indexing syntax or the <code>get</code> method to
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access an item in a vector</span></p>
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<p>Note two details here. First, we use the index value of <code>2</code> to get the third
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element: vectors are indexed by number, starting at zero. Second, the two ways
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to get the third element are by using <code>&</code> and <code>[]</code>, which gives us a reference,
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or by using the <code>get</code> method with the index passed as an argument, which gives
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us an <code>Option<&T></code>.</p>
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<p>Rust has two ways to reference an element so you can choose how the program
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behaves when you try to use an index value that the vector doesn’t have an
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element for. As an example, let’s see what a program will do if it has a vector
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that holds five elements and then tries to access an element at index 100, as
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shown in Listing 8-6.</p>
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<pre><pre class="playpen"><code class="language-rust should_panic panics">
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|
<span class="boring">#![allow(unused_variables)]
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</span><span class="boring">fn main() {
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</span>let v = vec![1, 2, 3, 4, 5];
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|
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let does_not_exist = &v[100];
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|||
|
let does_not_exist = v.get(100);
|
|||
|
<span class="boring">}
|
|||
|
</span></code></pre></pre>
|
|||
|
<p><span class="caption">Listing 8-6: Attempting to access the element at index
|
|||
|
100 in a vector containing five elements</span></p>
|
|||
|
<p>When we run this code, the first <code>[]</code> method will cause the program to panic
|
|||
|
because it references a nonexistent element. This method is best used when you
|
|||
|
want your program to crash if there’s an attempt to access an element past the
|
|||
|
end of the vector.</p>
|
|||
|
<p>When the <code>get</code> method is passed an index that is outside the vector, it returns
|
|||
|
<code>None</code> without panicking. You would use this method if accessing an element
|
|||
|
beyond the range of the vector happens occasionally under normal circumstances.
|
|||
|
Your code will then have logic to handle having either <code>Some(&element)</code> or
|
|||
|
<code>None</code>, as discussed in Chapter 6. For example, the index could be coming from
|
|||
|
a person entering a number. If they accidentally enter a number that’s too
|
|||
|
large and the program gets a <code>None</code> value, you could tell the user how many
|
|||
|
items are in the current vector and give them another chance to enter a valid
|
|||
|
value. That would be more user-friendly than crashing the program due to a typo!</p>
|
|||
|
<p>When the program has a valid reference, the borrow checker enforces the
|
|||
|
ownership and borrowing rules (covered in Chapter 4) to ensure this reference
|
|||
|
and any other references to the contents of the vector remain valid. Recall the
|
|||
|
rule that states you can’t have mutable and immutable references in the same
|
|||
|
scope. That rule applies in Listing 8-7, where we hold an immutable reference to
|
|||
|
the first element in a vector and try to add an element to the end, which won’t
|
|||
|
work.</p>
|
|||
|
<pre><code class="language-rust ignore does_not_compile">let mut v = vec![1, 2, 3, 4, 5];
|
|||
|
|
|||
|
let first = &v[0];
|
|||
|
|
|||
|
v.push(6);
|
|||
|
|
|||
|
println!("The first element is: {}", first);
|
|||
|
</code></pre>
|
|||
|
<p><span class="caption">Listing 8-7: Attempting to add an element to a vector
|
|||
|
while holding a reference to an item</span></p>
|
|||
|
<p>Compiling this code will result in this error:</p>
|
|||
|
<pre><code class="language-text">error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
|
|||
|
--> src/main.rs:6:5
|
|||
|
|
|
|||
|
4 | let first = &v[0];
|
|||
|
| - immutable borrow occurs here
|
|||
|
5 |
|
|||
|
6 | v.push(6);
|
|||
|
| ^^^^^^^^^ mutable borrow occurs here
|
|||
|
7 |
|
|||
|
8 | println!("The first element is: {}", first);
|
|||
|
| ----- immutable borrow later used here
|
|||
|
</code></pre>
|
|||
|
<p>The code in Listing 8-7 might look like it should work: why should a reference
|
|||
|
to the first element care about what changes at the end of the vector? This
|
|||
|
error is due to the way vectors work: adding a new element onto the end of the
|
|||
|
vector might require allocating new memory and copying the old elements to the
|
|||
|
new space, if there isn’t enough room to put all the elements next to each
|
|||
|
other where the vector currently is. In that case, the reference to the first
|
|||
|
element would be pointing to deallocated memory. The borrowing rules prevent
|
|||
|
programs from ending up in that situation.</p>
|
|||
|
<blockquote>
|
|||
|
<p>Note: For more on the implementation details of the <code>Vec<T></code> type, see “The
|
|||
|
Rustonomicon” at https://doc.rust-lang.org/stable/nomicon/vec.html.</p>
|
|||
|
</blockquote>
|
|||
|
<h3><a class="header" href="#iterating-over-the-values-in-a-vector" id="iterating-over-the-values-in-a-vector">Iterating over the Values in a Vector</a></h3>
|
|||
|
<p>If we want to access each element in a vector in turn, we can iterate through
|
|||
|
all of the elements rather than use indices to access one at a time. Listing
|
|||
|
8-8 shows how to use a <code>for</code> loop to get immutable references to each element
|
|||
|
in a vector of <code>i32</code> values and print them.</p>
|
|||
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
</span><span class="boring">fn main() {
|
|||
|
</span>let v = vec![100, 32, 57];
|
|||
|
for i in &v {
|
|||
|
println!("{}", i);
|
|||
|
}
|
|||
|
<span class="boring">}
|
|||
|
</span></code></pre></pre>
|
|||
|
<p><span class="caption">Listing 8-8: Printing each element in a vector by
|
|||
|
iterating over the elements using a <code>for</code> loop</span></p>
|
|||
|
<p>We can also iterate over mutable references to each element in a mutable vector
|
|||
|
in order to make changes to all the elements. The <code>for</code> loop in Listing 8-9
|
|||
|
will add <code>50</code> to each element.</p>
|
|||
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
</span><span class="boring">fn main() {
|
|||
|
</span>let mut v = vec![100, 32, 57];
|
|||
|
for i in &mut v {
|
|||
|
*i += 50;
|
|||
|
}
|
|||
|
<span class="boring">}
|
|||
|
</span></code></pre></pre>
|
|||
|
<p><span class="caption">Listing 8-9: Iterating over mutable references to
|
|||
|
elements in a vector</span></p>
|
|||
|
<p>To change the value that the mutable reference refers to, we have to use the
|
|||
|
dereference operator (<code>*</code>) to get to the value in <code>i</code> before we can use the
|
|||
|
<code>+=</code> operator. We’ll talk more about the dereference operator in the
|
|||
|
<a href="ch15-02-deref.html#following-the-pointer-to-the-value-with-the-dereference-operator">“Following the Pointer to the Value with the Dereference Operator”</a>
|
|||
|
section of Chapter 15.</p>
|
|||
|
<h3><a class="header" href="#using-an-enum-to-store-multiple-types" id="using-an-enum-to-store-multiple-types">Using an Enum to Store Multiple Types</a></h3>
|
|||
|
<p>At the beginning of this chapter, we said that vectors can only store values
|
|||
|
that are the same type. This can be inconvenient; there are definitely use
|
|||
|
cases for needing to store a list of items of different types. Fortunately, the
|
|||
|
variants of an enum are defined under the same enum type, so when we need to
|
|||
|
store elements of a different type in a vector, we can define and use an enum!</p>
|
|||
|
<p>For example, say we want to get values from a row in a spreadsheet in which
|
|||
|
some of the columns in the row contain integers, some floating-point numbers,
|
|||
|
and some strings. We can define an enum whose variants will hold the different
|
|||
|
value types, and then all the enum variants will be considered the same type:
|
|||
|
that of the enum. Then we can create a vector that holds that enum and so,
|
|||
|
ultimately, holds different types. We’ve demonstrated this in Listing 8-10.</p>
|
|||
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
</span><span class="boring">fn main() {
|
|||
|
</span>enum SpreadsheetCell {
|
|||
|
Int(i32),
|
|||
|
Float(f64),
|
|||
|
Text(String),
|
|||
|
}
|
|||
|
|
|||
|
let row = vec![
|
|||
|
SpreadsheetCell::Int(3),
|
|||
|
SpreadsheetCell::Text(String::from("blue")),
|
|||
|
SpreadsheetCell::Float(10.12),
|
|||
|
];
|
|||
|
<span class="boring">}
|
|||
|
</span></code></pre></pre>
|
|||
|
<p><span class="caption">Listing 8-10: Defining an <code>enum</code> to store values of
|
|||
|
different types in one vector</span></p>
|
|||
|
<p>Rust needs to know what types will be in the vector at compile time so it knows
|
|||
|
exactly how much memory on the heap will be needed to store each element. A
|
|||
|
secondary advantage is that we can be explicit about what types are allowed in
|
|||
|
this vector. If Rust allowed a vector to hold any type, there would be a chance
|
|||
|
that one or more of the types would cause errors with the operations performed
|
|||
|
on the elements of the vector. Using an enum plus a <code>match</code> expression means
|
|||
|
that Rust will ensure at compile time that every possible case is handled, as
|
|||
|
discussed in Chapter 6.</p>
|
|||
|
<p>When you’re writing a program, if you don’t know the exhaustive set of types
|
|||
|
the program will get at runtime to store in a vector, the enum technique won’t
|
|||
|
work. Instead, you can use a trait object, which we’ll cover in Chapter 17.</p>
|
|||
|
<p>Now that we’ve discussed some of the most common ways to use vectors, be sure
|
|||
|
to review the API documentation for all the many useful methods defined on
|
|||
|
<code>Vec<T></code> by the standard library. For example, in addition to <code>push</code>, a <code>pop</code>
|
|||
|
method removes and returns the last element. Let’s move on to the next
|
|||
|
collection type: <code>String</code>!</p>
|
|||
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