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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"><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" class="active"><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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<main>
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<h2 id="channels"><a class="header" href="#channels">Channels</a></h2>
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<p>A channel is an easy way to use many threads that send to one place. They are fairly popular because they are pretty simple to put together. You can create a channel in Rust with <code>std::sync::mpsc</code>. <code>mpsc</code> means "multiple producer, single consumer", so "many threads sending to one place". To start a channel, you use <code>channel()</code>. This creates a <code>Sender</code> and a <code>Receiver</code> that are tied together. You can see this in the function signature:</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>// 🚧
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pub fn channel<T>() -> (Sender<T>, Receiver<T>)
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<span class="boring">}
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</span></code></pre></pre>
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<p>So you have to choose one name for the sender and one for the receiver. Usually you see something like <code>let (sender, receiver) = channel();</code> to start. Because it's generic, Rust won't know the type if that is all you write:</p>
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<pre><pre class="playground"><code class="language-rust">use std::sync::mpsc::channel;
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fn main() {
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let (sender, receiver) = channel(); // ⚠️
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}
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</code></pre></pre>
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<p>The compiler says:</p>
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<pre><code class="language-text">error[E0282]: type annotations needed for `(std::sync::mpsc::Sender<T>, std::sync::mpsc::Receiver<T>)`
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--> src\main.rs:30:30
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30 | let (sender, receiver) = channel();
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| ------------------ ^^^^^^^ cannot infer type for type parameter `T` declared on the function `channel`
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| |
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| consider giving this pattern the explicit type `(std::sync::mpsc::Sender<T>, std::sync::mpsc::Receiver<T>)`, where
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the type parameter `T` is specified
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</code></pre>
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<p>It suggests adding a type for the <code>Sender</code> and <code>Receiver</code>. You can do that if you want:</p>
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<pre><pre class="playground"><code class="language-rust">use std::sync::mpsc::{channel, Sender, Receiver}; // Added Sender and Receiver here
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fn main() {
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let (sender, receiver): (Sender<i32>, Receiver<i32>) = channel();
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}
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</code></pre></pre>
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<p>but you don't have to. Once you start using the <code>Sender</code> and <code>Receiver</code>, Rust can guess the type.</p>
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<p>So let's look at the simplest way to use a channel.</p>
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<pre><pre class="playground"><code class="language-rust">use std::sync::mpsc::channel;
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fn main() {
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let (sender, receiver) = channel();
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sender.send(5);
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receiver.recv(); // recv = receive, not "rec v"
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}
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</code></pre></pre>
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<p>Now the compiler knows the type. <code>sender</code> is a <code>Result<(), SendError<i32>></code> and <code>receiver</code> is a <code>Result<i32, RecvError></code>. So you can use <code>.unwrap()</code> to see if the sending works, or use better error handling. Let's add <code>.unwrap()</code> and also <code>println!</code> to see what we get:</p>
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<pre><pre class="playground"><code class="language-rust">use std::sync::mpsc::channel;
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fn main() {
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let (sender, receiver) = channel();
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sender.send(5).unwrap();
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println!("{}", receiver.recv().unwrap());
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}
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</code></pre></pre>
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<p>This prints <code>5</code>.</p>
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<p>A <code>channel</code> is like an <code>Arc</code> because you can clone it and send the clones into other threads. Let's make two threads and send values to <code>receiver</code>. This code will work, but it is not exactly what we want.</p>
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<pre><pre class="playground"><code class="language-rust">use std::sync::mpsc::channel;
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fn main() {
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let (sender, receiver) = channel();
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let sender_clone = sender.clone();
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std::thread::spawn(move|| { // move sender in
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sender.send("Send a &str this time").unwrap();
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});
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std::thread::spawn(move|| { // move sender_clone in
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sender_clone.send("And here is another &str").unwrap();
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});
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println!("{}", receiver.recv().unwrap());
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}
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</code></pre></pre>
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<p>The two threads start sending, and then we <code>println!</code>. It might say <code>Send a &str this time</code> or <code>And here is another &str</code>, depending on which thread finished first. Let's make a join handle to make them wait.</p>
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<pre><pre class="playground"><code class="language-rust">use std::sync::mpsc::channel;
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fn main() {
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let (sender, receiver) = channel();
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let sender_clone = sender.clone();
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let mut handle_vec = vec![]; // Put our handles in here
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handle_vec.push(std::thread::spawn(move|| { // push this into the vec
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sender.send("Send a &str this time").unwrap();
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}));
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handle_vec.push(std::thread::spawn(move|| { // and push this into the vec
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sender_clone.send("And here is another &str").unwrap();
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}));
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for _ in handle_vec { // now handle_vec has 2 items. Let's print them
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println!("{:?}", receiver.recv().unwrap());
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}
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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">"Send a &str this time"
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"And here is another &str"
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</code></pre>
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<p>Now let's make a <code>results_vec</code> instead of printing.</p>
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<pre><pre class="playground"><code class="language-rust">use std::sync::mpsc::channel;
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fn main() {
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let (sender, receiver) = channel();
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let sender_clone = sender.clone();
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let mut handle_vec = vec![];
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let mut results_vec = vec![];
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handle_vec.push(std::thread::spawn(move|| {
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sender.send("Send a &str this time").unwrap();
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}));
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handle_vec.push(std::thread::spawn(move|| {
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sender_clone.send("And here is another &str").unwrap();
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}));
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for _ in handle_vec {
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results_vec.push(receiver.recv().unwrap());
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}
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println!("{:?}", results_vec);
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}
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</code></pre></pre>
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<p>Now the results are in our vec: <code>["Send a &str this time", "And here is another &str"]</code>.</p>
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<p>Now let's pretend that we have a lot of work to do, and want to use threads. We have a big vec with 1 million items, all 0. We want to change each 0 to a 1. We will use ten threads, and each thread will do one tenth of the work. We will create a new vec and use <code>.extend()</code> to put the work in.</p>
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<pre><pre class="playground"><code class="language-rust">use std::sync::mpsc::channel;
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use std::thread::spawn;
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fn main() {
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let (sender, receiver) = channel();
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let hugevec = vec![0; 1_000_000];
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let mut newvec = vec![];
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let mut handle_vec = vec![];
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for i in 0..10 {
|
|
let sender_clone = sender.clone();
|
|
let mut work: Vec<u8> = Vec::with_capacity(hugevec.len() / 10); // new vec to put the work in. 1/10th the size
|
|
work.extend(&hugevec[i*100_000..(i+1)*100_000]); // first part gets 0..100_000, next gets 100_000..200_000, etc.
|
|
let handle =spawn(move || { // make a handle
|
|
|
|
for number in work.iter_mut() { // do the actual work
|
|
*number += 1;
|
|
};
|
|
sender_clone.send(work).unwrap(); // use the sender_clone to send the work to the receiver
|
|
});
|
|
handle_vec.push(handle);
|
|
}
|
|
|
|
for handle in handle_vec { // stop until the threads are done
|
|
handle.join().unwrap();
|
|
}
|
|
|
|
while let Ok(results) = receiver.try_recv() {
|
|
newvec.push(results); // push the results from receiver.recv() into the vec
|
|
}
|
|
|
|
// Now we have a Vec<Vec<u8>>. To put it together we can use .flatten()
|
|
let newvec = newvec.into_iter().flatten().collect::<Vec<u8>>(); // Now it's one vec of 1_000_000 u8 numbers
|
|
|
|
println!("{:?}, {:?}, total length: {}", // Let's print out some numbers to make sure they are all 1
|
|
&newvec[0..10], &newvec[newvec.len()-10..newvec.len()], newvec.len() // And show that the length is 1_000_000 items
|
|
);
|
|
|
|
for number in newvec { // And let's tell Rust that it can panic if even one number is not 1
|
|
if number != 1 {
|
|
panic!();
|
|
}
|
|
}
|
|
}
|
|
</code></pre></pre>
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