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<main>
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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="ch03-02-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">
|
||
<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
|
||
</span> let v = vec![1, 2, 3, 4, 5];
|
||
|
||
let third: &i32 = &v[2];
|
||
println!("The third element is {third}");
|
||
|
||
let third: Option<&i32> = v.get(2);
|
||
match third {
|
||
Some(third) => println!("The third element is {third}"),
|
||
None => println!("There is no third element."),
|
||
}
|
||
<span class="boring">}</span></code></pre>
|
||
<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>
|
||
</figure>
|
||
<p>Note a few details here. We use the index value of <code>2</code> to get the third element
|
||
because vectors are indexed by number, starting at zero. Using <code>&</code> and <code>[]</code>
|
||
gives us a reference to the element at the index value. When we use the <code>get</code>
|
||
method with the index passed as an argument, we get an <code>Option<&T></code> that we can
|
||
use with <code>match</code>.</p>
|
||
<p>Rust provides these two ways to reference an element so that you can choose how
|
||
the program behaves when you try to use an index value outside the range of
|
||
existing elements. As an example, let’s see what happens when we have a vector
|
||
of five elements and then we try to access an element at index 100 with each
|
||
technique, as shown in Listing 8-5.</p>
|
||
<figure class="listing" id="listing-8-5">
|
||
<pre class="playground"><code class="language-rust should_panic panics edition2024"><span class="boring">fn main() {
|
||
</span> let v = vec![1, 2, 3, 4, 5];
|
||
|
||
let does_not_exist = &v[100];
|
||
let does_not_exist = v.get(100);
|
||
<span class="boring">}</span></code></pre>
|
||
<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>
|
||
</figure>
|
||
<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 may happen 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 that 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 8-6, where we hold an immutable
|
||
reference to the first element in a vector and try to add an element to the
|
||
end. This program won’t work if we also try to refer to that element later in
|
||
the function.</p>
|
||
<figure class="listing" id="listing-8-6">
|
||
<pre><code class="language-rust ignore does_not_compile"><span class="boring">fn main() {
|
||
</span> let mut v = vec![1, 2, 3, 4, 5];
|
||
|
||
let first = &v[0];
|
||
|
||
v.push(6);
|
||
|
||
println!("The first element is: {first}");
|
||
<span class="boring">}</span></code></pre>
|
||
<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>
|
||
</figure>
|
||
<p>Compiling this code will result in this error:</p>
|
||
<pre><code class="language-console">$ cargo run
|
||
Compiling collections v0.1.0 (file:///projects/collections)
|
||
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
|
||
|
||
For more information about this error, try `rustc --explain E0502`.
|
||
error: could not compile `collections` (bin "collections") due to 1 previous error
|
||
</code></pre>
|
||
<p>The code in Listing 8-6 might look like it should work: Why should a reference
|
||
to the first element care about changes at the end of the vector? This error is
|
||
due to the way vectors work: Because vectors put the values next to each other
|
||
in memory, 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
|
||
is currently stored. 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>
|
||
<section class="note" aria-role="note">
|
||
<p>Note: For more on the implementation details of the <code>Vec<T></code> type, see <a href="../nomicon/vec/vec.html">“The
|
||
Rustonomicon”</a>.</p>
|
||
</section>
|
||
<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>
|
||
<p>To access each element in a vector in turn, we would iterate through all of the
|
||
elements rather than use indices to access one at a time. Listing 8-7 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>
|
||
<figure class="listing" id="listing-8-7">
|
||
<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
|
||
</span> let v = vec![100, 32, 57];
|
||
for i in &v {
|
||
println!("{i}");
|
||
}
|
||
<span class="boring">}</span></code></pre>
|
||
<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>
|
||
</figure>
|
||
<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 8-8
|
||
will add <code>50</code> to each element.</p>
|
||
<figure class="listing" id="listing-8-8">
|
||
<pre class="playground"><code class="language-rust edition2024"><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>
|
||
<figcaption><a href="#listing-8-8">Listing 8-8</a>: Iterating over mutable references to elements in a vector</figcaption>
|
||
</figure>
|
||
<p>To change the value that the mutable reference refers to, we have to use the
|
||
<code>*</code> dereference operator 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
|
||
Reference to the Value”</a><!-- ignore --> section of Chapter 15.</p>
|
||
<p>Iterating over a vector, whether immutably or mutably, is safe because of the
|
||
borrow checker’s rules. If we attempted to insert or remove items in the <code>for</code>
|
||
loop bodies in Listing 8-7 and Listing 8-8, we would get a compiler error
|
||
similar to the one we got with the code in Listing 8-6. The reference to the
|
||
vector that the <code>for</code> loop holds prevents simultaneous modification of the
|
||
whole vector.</p>
|
||
<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>
|
||
<p>Vectors can only store values that are of 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 one type to represent elements of
|
||
different types, 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 all the enum variants will be considered the same type: that
|
||
of the enum. Then, we can create a vector to hold that enum and so, ultimately,
|
||
hold different types. We’ve demonstrated this in Listing 8-9.</p>
|
||
<figure class="listing" id="listing-8-9">
|
||
<pre class="playground"><code class="language-rust edition2024"><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>
|
||
<figcaption><a href="#listing-8-9">Listing 8-9</a>: Defining an enum to store values of different types in one vector</figcaption>
|
||
</figure>
|
||
<p>Rust needs to know what types will be in the vector at compile time so that it
|
||
knows exactly how much memory on the heap will be needed to store each element.
|
||
We must also 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>If you don’t know the exhaustive set of types a 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 18.</p>
|
||
<p>Now that we’ve discussed some of the most common ways to use vectors, be sure
|
||
to review <a href="../std/vec/struct.Vec.html">the API documentation</a><!-- ignore --> for all of 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.</p>
|
||
<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>
|
||
<p>Like any other <code>struct</code>, a vector is freed when it goes out of scope, as
|
||
annotated in Listing 8-10.</p>
|
||
<figure class="listing" id="listing-8-10">
|
||
<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
|
||
</span> {
|
||
let v = vec![1, 2, 3, 4];
|
||
|
||
// do stuff with v
|
||
} // <- v goes out of scope and is freed here
|
||
<span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-8-10">Listing 8-10</a>: Showing where the vector and its elements are dropped</figcaption>
|
||
</figure>
|
||
<p>When the vector gets dropped, all of its contents are also dropped, meaning the
|
||
integers it holds will be cleaned up. The borrow checker ensures that any
|
||
references to contents of a vector are only used while the vector itself is
|
||
valid.</p>
|
||
<p>Let’s move on to the next collection type: <code>String</code>!</p>
|
||
|
||
</main>
|
||
|
||
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|
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<template id=fa-eye><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 576 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M288 32c-80.8 0-145.5 36.8-192.6 80.6C48.6 156 17.3 208 2.5 243.7c-3.3 7.9-3.3 16.7 0 24.6C17.3 304 48.6 356 95.4 399.4C142.5 443.2 207.2 480 288 480s145.5-36.8 192.6-80.6c46.8-43.5 78.1-95.4 93-131.1c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C433.5 68.8 368.8 32 288 32zM432 256c0 79.5-64.5 144-144 144s-144-64.5-144-144s64.5-144 144-144s144 64.5 144 144zM288 192c0 35.3-28.7 64-64 64c-11.5 0-22.3-3-31.6-8.4c-.2 2.8-.4 5.5-.4 8.4c0 53 43 96 96 96s96-43 96-96s-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6z"/></svg></span></template>
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<template id=fa-eye-slash><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 640 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M38.8 5.1C28.4-3.1 13.3-1.2 5.1 9.2S-1.2 34.7 9.2 42.9l592 464c10.4 8.2 25.5 6.3 33.7-4.1s6.3-25.5-4.1-33.7L525.6 386.7c39.6-40.6 66.4-86.1 79.9-118.4c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C465.5 68.8 400.8 32 320 32c-68.2 0-125 26.3-169.3 60.8L38.8 5.1zM223.1 149.5C248.6 126.2 282.7 112 320 112c79.5 0 144 64.5 144 144c0 24.9-6.3 48.3-17.4 68.7L408 294.5c5.2-11.8 8-24.8 8-38.5c0-53-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6c0 10.2-2.4 19.8-6.6 28.3l-90.3-70.8zm223.1 298L373 389.9c-16.4 6.5-34.3 10.1-53 10.1c-79.5 0-144-64.5-144-144c0-6.9 .5-13.6 1.4-20.2L83.1 161.5C60.3 191.2 44 220.8 34.5 243.7c-3.3 7.9-3.3 16.7 0 24.6c14.9 35.7 46.2 87.7 93 131.1C174.5 443.2 239.2 480 320 480c47.8 0 89.9-12.9 126.2-32.5z"/></svg></span></template>
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<template id=fa-copy><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M502.6 70.63l-61.25-61.25C435.4 3.371 427.2 0 418.7 0H255.1c-35.35 0-64 28.66-64 64l.0195 256C192 355.4 220.7 384 256 384h192c35.2 0 64-28.8 64-64V93.25C512 84.77 508.6 76.63 502.6 70.63zM464 320c0 8.836-7.164 16-16 16H255.1c-8.838 0-16-7.164-16-16L239.1 64.13c0-8.836 7.164-16 16-16h128L384 96c0 17.67 14.33 32 32 32h47.1V320zM272 448c0 8.836-7.164 16-16 16H63.1c-8.838 0-16-7.164-16-16L47.98 192.1c0-8.836 7.164-16 16-16H160V128H63.99c-35.35 0-64 28.65-64 64l.0098 256C.002 483.3 28.66 512 64 512h192c35.2 0 64-28.8 64-64v-32h-47.1L272 448z"/></svg></span></template>
|
||
<template id=fa-play><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 384 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M73 39c-14.8-9.1-33.4-9.4-48.5-.9S0 62.6 0 80V432c0 17.4 9.4 33.4 24.5 41.9s33.7 8.1 48.5-.9L361 297c14.3-8.7 23-24.2 23-41s-8.7-32.2-23-41L73 39z"/></svg></span></template>
|
||
<template id=fa-clock-rotate-left><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M75 75L41 41C25.9 25.9 0 36.6 0 57.9V168c0 13.3 10.7 24 24 24H134.1c21.4 0 32.1-25.9 17-41l-30.8-30.8C155 85.5 203 64 256 64c106 0 192 86 192 192s-86 192-192 192c-40.8 0-78.6-12.7-109.7-34.4c-14.5-10.1-34.4-6.6-44.6 7.9s-6.6 34.4 7.9 44.6C151.2 495 201.7 512 256 512c141.4 0 256-114.6 256-256S397.4 0 256 0C185.3 0 121.3 28.7 75 75zm181 53c-13.3 0-24 10.7-24 24V256c0 6.4 2.5 12.5 7 17l72 72c9.4 9.4 24.6 9.4 33.9 0s9.4-24.6 0-33.9l-65-65V152c0-13.3-10.7-24-24-24z"/></svg></span></template>
|
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