268 lines
16 KiB
HTML
268 lines
16 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<title>Storing Lists of Values with Vectors</title>
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</head>
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<body>
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<h2 id="storing-lists-of-values-with-vectors"><a class="header" href="#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 vector.
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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 id="creating-a-new-vector"><a class="header" href="#creating-a-new-vector">Creating a New Vector</a></h3>
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<p>To create a new, empty vector, we call the <code>Vec::new</code> function, as shown in
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Listing 8-1.</p>
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<figure class="listing" id="listing-8-1">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> let v: Vec<i32> = Vec::new();
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-1">Listing 8-1</a>: Creating a new, empty vector to hold values of type <code>i32</code></figcaption>
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</figure>
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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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When we create a vector to hold a specific type, we can specify the type 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>More often, you’ll create a <code>Vec<T></code> with initial values, and Rust will infer
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the type of value you want to store, so you rarely need to do this type
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annotation. Rust conveniently provides the <code>vec!</code> macro, which will create a
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new vector that holds the values you give it. Listing 8-2 creates a new
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<code>Vec<i32></code> that holds the values <code>1</code>, <code>2</code>, and <code>3</code>. The integer type is <code>i32</code>
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because that’s the default integer type, as we discussed in the <a href="../ch03/ch03-02-data-types.html#data-types">“Data
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Types”</a><!-- ignore --> section of Chapter 3.</p>
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<figure class="listing" id="listing-8-2">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> let v = vec![1, 2, 3];
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-2">Listing 8-2</a>: Creating a new vector containing values</figcaption>
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</figure>
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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 id="updating-a-vector"><a class="header" href="#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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<figure class="listing" id="listing-8-3">
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<pre class="playground"><code class="language-rust edition2024"><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">}</span></code></pre>
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<figcaption><a href="#listing-8-3">Listing 8-3</a>: Using the <code>push</code> method to add values to a vector</figcaption>
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</figure>
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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 id="reading-elements-of-vectors"><a class="header" href="#reading-elements-of-vectors">Reading Elements of Vectors</a></h3>
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<p>There are two ways to reference a value stored in a vector: via indexing or by
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using the <code>get</code> method. In the following examples, we’ve annotated the types of
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the values that are returned from these functions for extra clarity.</p>
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<p>Listing 8-4 shows both methods of accessing a value in a vector, with indexing
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syntax and the <code>get</code> method.</p>
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<figure class="listing" id="listing-8-4">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> let v = vec![1, 2, 3, 4, 5];
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let third: &i32 = &v[2];
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println!("The third element is {third}");
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let third: Option<&i32> = v.get(2);
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match third {
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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">}</span></code></pre>
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<figcaption><a href="#listing-8-4">Listing 8-4</a>: Using indexing syntax and using the <code>get</code> method to access an item in a vector</figcaption>
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</figure>
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<p>Note a few details here. We use the index value of <code>2</code> to get the third element
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because vectors are indexed by number, starting at zero. Using <code>&</code> and <code>[]</code>
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gives us a reference to the element at the index value. When we use the <code>get</code>
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method with the index passed as an argument, we get an <code>Option<&T></code> that we can
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use with <code>match</code>.</p>
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<p>Rust provides these two ways to reference an element so that you can choose how
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the program behaves when you try to use an index value outside the range of
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existing elements. As an example, let’s see what happens when we have a vector
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of five elements and then we try to access an element at index 100 with each
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technique, as shown in Listing 8-5.</p>
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<figure class="listing" id="listing-8-5">
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<pre class="playground"><code class="language-rust should_panic panics edition2024"><span class="boring">fn main() {
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</span> let v = vec![1, 2, 3, 4, 5];
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let does_not_exist = &v[100];
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let does_not_exist = v.get(100);
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-5">Listing 8-5</a>: Attempting to access the element at index 100 in a vector containing five elements</figcaption>
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</figure>
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<p>When we run this code, the first <code>[]</code> method will cause the program to panic
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because it references a nonexistent element. This method is best used when you
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want your program to crash if there’s an attempt to access an element past the
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end of the vector.</p>
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<p>When the <code>get</code> method is passed an index that is outside the vector, it returns
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<code>None</code> without panicking. You would use this method if accessing an element
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beyond the range of the vector may happen occasionally under normal
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circumstances. Your code will then have logic to handle having either
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<code>Some(&element)</code> or <code>None</code>, as discussed in Chapter 6. For example, the index
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could be coming from a person entering a number. If they accidentally enter a
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number that’s too large and the program gets a <code>None</code> value, you could tell the
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user how many items are in the current vector and give them another chance to
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enter a valid value. That would be more user-friendly than crashing the program
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due to a typo!</p>
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<p>When the program has a valid reference, the borrow checker enforces the
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ownership and borrowing rules (covered in Chapter 4) to ensure that this
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reference and any other references to the contents of the vector remain valid.
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Recall the rule that states you can’t have mutable and immutable references in
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the same scope. That rule applies in Listing 8-6, where we hold an immutable
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reference to the first element in a vector and try to add an element to the
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end. This program won’t work if we also try to refer to that element later in
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the function.</p>
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<figure class="listing" id="listing-8-6">
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<pre><code class="language-rust ignore does_not_compile"><span class="boring">fn main() {
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</span> let mut v = vec![1, 2, 3, 4, 5];
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let first = &v[0];
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v.push(6);
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println!("The first element is: {first}");
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-6">Listing 8-6</a>: Attempting to add an element to a vector while holding a reference to an item</figcaption>
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</figure>
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<p>Compiling this code will result in this error:</p>
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<pre><code class="language-console">$ cargo run
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Compiling collections v0.1.0 (file:///projects/collections)
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error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
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--> src/main.rs:6:5
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4 | let first = &v[0];
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| - immutable borrow occurs here
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5 |
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6 | v.push(6);
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| ^^^^^^^^^ mutable borrow occurs here
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7 |
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8 | println!("The first element is: {first}");
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| ----- immutable borrow later used here
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For more information about this error, try `rustc --explain E0502`.
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error: could not compile `collections` (bin "collections") due to 1 previous error
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</code></pre>
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<p>The code in Listing 8-6 might look like it should work: Why should a reference
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to the first element care about changes at the end of the vector? This error is
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due to the way vectors work: Because vectors put the values next to each other
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in memory, adding a new element onto the end of the vector might require
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allocating new memory and copying the old elements to the new space, if there
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isn’t enough room to put all the elements next to each other where the vector
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is currently stored. In that case, the reference to the first element would be
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pointing to deallocated memory. The borrowing rules prevent programs from
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ending up in that situation.</p>
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<section class="note" aria-role="note">
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<p>Note: For more on the implementation details of the <code>Vec<T></code> type, see <a href="../nomicon/vec/vec.html">“The
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Rustonomicon”</a>.</p>
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</section>
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<h3 id="iterating-over-the-values-in-a-vector"><a class="header" href="#iterating-over-the-values-in-a-vector">Iterating Over the Values in a Vector</a></h3>
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<p>To access each element in a vector in turn, we would iterate through all of the
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elements rather than use indices to access one at a time. Listing 8-7 shows how
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to use a <code>for</code> loop to get immutable references to each element in a vector of
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<code>i32</code> values and print them.</p>
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<figure class="listing" id="listing-8-7">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> let v = vec![100, 32, 57];
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for i in &v {
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println!("{i}");
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}
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-7">Listing 8-7</a>: Printing each element in a vector by iterating over the elements using a <code>for</code> loop</figcaption>
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</figure>
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<p>We can also iterate over mutable references to each element in a mutable vector
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in order to make changes to all the elements. The <code>for</code> loop in Listing 8-8
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will add <code>50</code> to each element.</p>
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<figure class="listing" id="listing-8-8">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> let mut v = vec![100, 32, 57];
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for i in &mut v {
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*i += 50;
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}
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-8">Listing 8-8</a>: Iterating over mutable references to elements in a vector</figcaption>
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</figure>
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<p>To change the value that the mutable reference refers to, we have to use the
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<code>*</code> dereference operator to get to the value in <code>i</code> before we can use the <code>+=</code>
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operator. We’ll talk more about the dereference operator in the <a href="../ch15/ch15-02-deref.html#following-the-pointer-to-the-value-with-the-dereference-operator">“Following the
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Reference to the Value”</a><!-- ignore --> section of Chapter 15.</p>
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<p>Iterating over a vector, whether immutably or mutably, is safe because of the
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borrow checker’s rules. If we attempted to insert or remove items in the <code>for</code>
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loop bodies in Listing 8-7 and Listing 8-8, we would get a compiler error
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similar to the one we got with the code in Listing 8-6. The reference to the
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vector that the <code>for</code> loop holds prevents simultaneous modification of the
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whole vector.</p>
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<h3 id="using-an-enum-to-store-multiple-types"><a class="header" href="#using-an-enum-to-store-multiple-types">Using an Enum to Store Multiple Types</a></h3>
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<p>Vectors can only store values that are of the same type. This can be
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inconvenient; there are definitely use cases for needing to store a list of
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items of different types. Fortunately, the variants of an enum are defined
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under the same enum type, so when we need one type to represent elements of
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different types, we can define and use an enum!</p>
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<p>For example, say we want to get values from a row in a spreadsheet in which
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some of the columns in the row contain integers, some floating-point numbers,
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and some strings. We can define an enum whose variants will hold the different
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value types, and all the enum variants will be considered the same type: that
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of the enum. Then, we can create a vector to hold that enum and so, ultimately,
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hold different types. We’ve demonstrated this in Listing 8-9.</p>
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<figure class="listing" id="listing-8-9">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> enum SpreadsheetCell {
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Int(i32),
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Float(f64),
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Text(String),
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}
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let row = vec![
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SpreadsheetCell::Int(3),
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SpreadsheetCell::Text(String::from("blue")),
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SpreadsheetCell::Float(10.12),
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];
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-9">Listing 8-9</a>: Defining an enum to store values of different types in one vector</figcaption>
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</figure>
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<p>Rust needs to know what types will be in the vector at compile time so that it
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knows exactly how much memory on the heap will be needed to store each element.
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We must also be explicit about what types are allowed in this vector. If Rust
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allowed a vector to hold any type, there would be a chance that one or more of
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the types would cause errors with the operations performed on the elements of
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the vector. Using an enum plus a <code>match</code> expression means that Rust will ensure
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at compile time that every possible case is handled, as discussed in Chapter 6.</p>
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<p>If you don’t know the exhaustive set of types a program will get at runtime to
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store in a vector, the enum technique won’t work. Instead, you can use a trait
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object, which we’ll cover in Chapter 18.</p>
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<p>Now that we’ve discussed some of the most common ways to use vectors, be sure
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to review <a href="../std/vec/struct.Vec.html">the API documentation</a><!-- ignore --> for all of the many
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useful methods defined on <code>Vec<T></code> by the standard library. For example, in
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addition to <code>push</code>, a <code>pop</code> method removes and returns the last element.</p>
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<h3 id="dropping-a-vector-drops-its-elements"><a class="header" href="#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 8-10.</p>
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<figure class="listing" id="listing-8-10">
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<pre class="playground"><code class="language-rust edition2024"><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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// do stuff with v
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} // <- v goes out of scope and is freed here
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-8-10">Listing 8-10</a>: Showing where the vector and its elements are dropped</figcaption>
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</figure>
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<p>When the vector gets dropped, all of its contents are also dropped, meaning the
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integers it holds will be cleaned up. The borrow checker ensures that any
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references to contents of a vector are only used while the vector itself is
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valid.</p>
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<p>Let’s move on to the next collection type: <code>String</code>!</p>
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</body>
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</html>
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