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<ol class="chapter"><li class="chapter-item expanded "><a href="motivation.html"><strong aria-hidden="true">1.</strong> Motivation</a></li><li class="chapter-item expanded "><a href="rust-basics.html"><strong aria-hidden="true">2.</strong> Rust Basics</a></li><li class="chapter-item expanded "><a href="ownership.html"><strong aria-hidden="true">3.</strong> Ownership</a></li><li class="chapter-item expanded "><a href="borrowing.html"><strong aria-hidden="true">4.</strong> Borrowing</a></li><li class="chapter-item expanded "><a href="vectors.html" class="active"><strong aria-hidden="true">5.</strong> Vectors</a></li><li class="chapter-item expanded "><a href="structs.html"><strong aria-hidden="true">6.</strong> Optional: Structures</a></li><li class="chapter-item expanded "><a href="additional_resources.html"><strong aria-hidden="true">7.</strong> Additional Resources</a></li></ol>
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<h1 id="vectors-in-rust"><a class="header" href="#vectors-in-rust">Vectors in Rust</a></h1>
<p>The previous sections cover everything you need to know about ownership and borrowing in Rust! This section introduces another interesting data structure: vectors.</p>
<p>Like with other languages, the Rust standard library contains many useful
<em>collection</em> types. One of the most useful and common ones are <em>vectors</em>, which
have type <code>Vec<T></code>, where <code>T</code> is the type the that vector holds.</p>
<p>Vectors are heap-allocated, mutable collections that store multiple values of
the same type contiguously in memory. In many ways, they are similar C++
<code>vector</code>s and serve similar purposes.</p>
<p>Vectors are implemented with <em>generics</em>, which allow them to hold any type.
For example, we can have <code>Vec<i32></code> and <code>Vec<String></code> which are the types of
<code>i32</code> vectors and <code>String</code> vectors, respectively. Vectors can hold any
<code>struct</code> or <code>enum</code> type as well. </p>
<h2 id="creating-a-vector"><a class="header" href="#creating-a-vector">Creating A Vector</a></h2>
<h3 id="empty-vectors"><a class="header" href="#empty-vectors">Empty Vectors</a></h3>
<p>To make a new empty vector, we can use the <code>Vec::new()</code> function as follows:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let v: Vec<i32> = Vec::new();
}
</code></pre></pre>
<p>Here, <code>Vec::new()</code> creates an empty vector of <code>i32</code>s and moves ownership to <code>v</code>.
Note that we included a type annotation to <code>v</code>. This is necessary here because
otherwise, Rust won't know which type of vector to create. </p>
<h3 id="creating-vectors-from-initial-values"><a class="header" href="#creating-vectors-from-initial-values">Creating Vectors from Initial Values</a></h3>
<p>We can also create new vectors with initial values using the <code>vec!</code> macro:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let v = vec![1, 2 ,3];
}
</code></pre></pre>
<p>Here, we create a new <code>Vec<i32></code> containing the values <code>1</code>, <code>2</code>, and <code>3</code> in
that order. Note that in this case, we did not need to include a type annotation
for <code>v</code>. This is because we are creating the vector with initial values of a
specific type, so Rust can figure out the type of <code>v</code> in this case.</p>
<h2 id="reading-elements-of-vectors"><a class="header" href="#reading-elements-of-vectors">Reading Elements of Vectors</a></h2>
<h3 id="accessing-an-element-at-a-particular-index"><a class="header" href="#accessing-an-element-at-a-particular-index">Accessing an Element at a Particular Index</a></h3>
<p>We can use the indexing syntax or the <code>get()</code> method to get the value at a
particular index of the vector:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let v = vec![1, 2, 3];
let third: &i32 = &v[2];
println!("The third element is {}", third);
match v.get(2) {
Some(third) => println!("The third element is {}", third),
None => println!("There is no third element."),
}
}
</code></pre></pre>
<p>Here, we use both ways of getting a particular element. The first way is using
the indexing syntax (square brackets), which gives us an immutable reference to
the element. The second way is using the <code>get()</code> method, which returns an
<code>Option</code> type. </p>
<p>With the indexing syntax, if we performed an out-of-bounds access in the vector,
the program would <em>panic</em> (i.e. cause an unrecoverable error.) With the <code>get()</code>
method, an out-of-bounds access would result in the method returning <code>None</code>.
With the <code>get()</code> method, we can handle out-of-bounds accesses gracefully rather
than causing the program to crash. </p>
<h3 id="iterating-over-elements"><a class="header" href="#iterating-over-elements">Iterating over Elements</a></h3>
<p>We can iterate over elements in a vector with a <code>for</code> loop to read the values:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let v = vec![1, 2, 3];
for i in &v {
println!("{}", i);
}
}
</code></pre></pre>
<p>Here, we simply read the values of the vector and print them to the terminal.
Note that the <code>for</code> loop is immutably borrowing <code>v</code>, as shown by the <code>&v</code>.</p>
<h2 id="mutating-vectors"><a class="header" href="#mutating-vectors">Mutating Vectors</a></h2>
<h3 id="push"><a class="header" href="#push">Push</a></h3>
<p>We can add elements to the back of a vector using the <code>push()</code> method:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let mut v = Vec::new();
v.push(1);
v.push(2);
v.push(3);
}
</code></pre></pre>
<p>This creates an empty vector and adds the values <code>1</code>, <code>2</code>, and <code>3</code> to the back
of the vector in that order. In this case, we did not need a type annotation
because the type is inferred from the values we pushed to it. Note that we made
<code>v</code> a mutable variable here. If we didn't, the borrow checker would not allow
us to make calls to <code>push()</code>.</p>
<h3 id="writing-elements-at-a-particular-index"><a class="header" href="#writing-elements-at-a-particular-index">Writing Elements at a Particular Index</a></h3>
<p>We can also write to elements at a particular index in a similar way to how
we read elements at a particular index. We can use the indexing syntax or the
<code>get_mut()</code> method:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let mut v = vec![1, 2, 3];
let second: &mut i32 = &mut v[1];
*second = 3;
match v.get_mut(2) {
Some(third) => *third = 9,
None => println!("There is no third element."),
}
}
</code></pre></pre>
<p>Here, we use the indexing syntax to get a mutable reference to the second
element and change its value to <code>3</code>. We then use the <code>get_mut()</code> method to get
a mutable reference to the third element and change its value to <code>9</code>.</p>
<p>As with the example for reading elements at a particular index, an out-of-bounds
access with the indexing sytanx can cause a <code>panic</code> while an out-of-bounds
access with the <code>get_mut()</code> method returns <code>None</code>.</p>
<h3 id="iterating-over-elements-1"><a class="header" href="#iterating-over-elements-1">Iterating Over Elements</a></h3>
<p>We can iterate over elements in a vector with a <code>for</code> loop to mutate the values:</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let mut v = vec![1, 2, 3];
for i in &mut v {
*i = *i + 1
}
}
</code></pre></pre>
<p>Here, we add <code>1</code> to each of the values in the vector. Note that the <code>for</code> loop
is mutably borrowing <code>v</code>, as shown by the <code>&mut v</code>.</p>
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