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<ol class="chapter"><li class="chapter-item expanded "><a href="Chapter_0.html"><strong aria-hidden="true">1.</strong> Update</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" class="active"><strong aria-hidden="true">32.</strong> Option and Result</a></li><li class="chapter-item expanded "><a href="Chapter_32.html">
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<h2><a class="header" href="#option-and-result" id="option-and-result">Option and Result</a></h2>
<p>We understand enums and generics now, so we can understand <code>Option</code> and <code>Result</code>. Rust uses these two enums to make code safer.</p>
<p>We will start with <code>Option</code>.</p>
<h3><a class="header" href="#option" id="option">Option</a></h3>
<p>You use <code>Option</code> when you have a value that might exist, or might not exist. When a value exists it is <code>Some(value)</code> and when it doesn't it's just <code>None</code>, Here is an example of bad code that can be improved with <code>Option</code>.</p>
<pre><pre class="playground"><code class="language-rust"> // ⚠️
fn take_fifth(value: Vec&lt;i32&gt;) -&gt; i32 {
value[4]
}
fn main() {
let new_vec = vec![1, 2];
let index = take_fifth(new_vec);
}
</code></pre></pre>
<p>When we run the code, it panics. Here is the message:</p>
<pre><code class="language-text">thread 'main' panicked at 'index out of bounds: the len is 2 but the index is 4', src\main.rs:34:5
</code></pre>
<p>Panic means that the program stops before the problem happens. Rust sees that the function wants something impossible, and stops. It &quot;unwinds the stack&quot; (takes the values off the stack) and tells you &quot;sorry, I can't do that&quot;.</p>
<p>So now we will change the return type from <code>i32</code> to <code>Option&lt;i32&gt;</code>. This means &quot;give me a <code>Some(i32)</code> if it's there, and give me <code>None</code> if it's not&quot;. We say that the <code>i32</code> is &quot;wrapped&quot; in an <code>Option</code>, which means that it's inside an <code>Option</code>. You have to do something to get the value out.</p>
<pre><pre class="playground"><code class="language-rust">fn take_fifth(value: Vec&lt;i32&gt;) -&gt; Option&lt;i32&gt; {
if value.len() &lt; 5 { // .len() gives the length of the vec.
// It must be at least 5.
None
} else {
Some(value[4])
}
}
fn main() {
let new_vec = vec![1, 2];
let bigger_vec = vec![1, 2, 3, 4, 5];
println!(&quot;{:?}, {:?}&quot;, take_fifth(new_vec), take_fifth(bigger_vec));
}
</code></pre></pre>
<p>This prints <code>None, Some(5)</code>. This is good, because now we don't panic anymore. But how do we get the value 5?</p>
<p>We can get the value inside an option with <code>.unwrap()</code>, but be careful with <code>.unwrap()</code>. It's just like unwrapping a present: maybe there's something good inside, or maybe there's an angry snake inside. You only want to <code>.unwrap()</code> if you are sure. If you unwrap a value that is <code>None</code>, the program will panic.</p>
<pre><pre class="playground"><code class="language-rust">// ⚠️
fn take_fifth(value: Vec&lt;i32&gt;) -&gt; Option&lt;i32&gt; {
if value.len() &lt; 4 {
None
} else {
Some(value[4])
}
}
fn main() {
let new_vec = vec![1, 2];
let bigger_vec = vec![1, 2, 3, 4, 5];
println!(&quot;{:?}, {:?}&quot;,
take_fifth(new_vec).unwrap(), // this one is None. .unwrap() will panic!
take_fifth(bigger_vec).unwrap()
);
}
</code></pre></pre>
<p>The message is: &quot;thread 'main' panicked at 'called <code>Option::unwrap()</code> on a <code>None</code> value', src\main.rs:14:9&quot;.</p>
<p>But we don't need to use <code>.unwrap()</code>. We can use a <code>match</code>. Then we can print the value we have <code>Some</code>, and not touch it if we have <code>None</code>. For example:</p>
<pre><pre class="playground"><code class="language-rust">fn take_fifth(value: Vec&lt;i32&gt;) -&gt; Option&lt;i32&gt; {
if value.len() &lt; 4 {
None
} else {
Some(value[4])
}
}
fn handle_option(my_option: Vec&lt;Option&lt;i32&gt;&gt;) {
for item in my_option {
match item {
Some(number) =&gt; println!(&quot;Found a {}!&quot;, number),
None =&gt; println!(&quot;Found a None!&quot;),
}
}
}
fn main() {
let new_vec = vec![1, 2];
let bigger_vec = vec![1, 2, 3, 4, 5];
let mut option_vec = Vec::new(); // Make a new vec to hold our options
// The vec is type: Vec&lt;Option&lt;i32&gt;&gt;. That means a vec of Option&lt;i32&gt;.
option_vec.push(take_fifth(new_vec)); // This pushes &quot;None&quot; into the vec
option_vec.push(take_fifth(bigger_vec)); // This pushes &quot;Some(5)&quot; into the vec
handle_option(option_vec); // handle_option looks at every option in the vec.
// It prints the value if it is Some. It doesn't touch it if it is None.
}
</code></pre></pre>
<p>This prints:</p>
<pre><code class="language-text">Found a None!
Found a 5!
</code></pre>
<p>Because we know generics, we are able to read the code for <code>Option</code>. It looks like this:</p>
<pre><pre class="playground"><code class="language-rust">enum Option&lt;T&gt; {
None,
Some(T),
}
fn main() {}
</code></pre></pre>
<p>The important point to remember: with <code>Some</code>, you have a value of type <code>T</code> (any type). Also note that the angle brackets after the <code>enum</code> name around <code>T</code> is what tells the compiler that it's generic. It has no trait like <code>Display</code> or anything to limit it, so it can be anything. But with <code>None</code>, you don't have anything.</p>
<p>So in a <code>match</code> statement for Option you can't say:</p>
<pre><pre class="playground"><code class="language-rust">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// 🚧
Some(value) =&gt; println!(&quot;The value is {}&quot;, value),
None(value) =&gt; println!(&quot;The value is {}&quot;, value),
<span class="boring">}
</span></code></pre></pre>
<p>because <code>None</code> is just <code>None</code>.</p>
<p>Of course, there are easier ways to use Option. In this code, we will use a method called <code>.is_some()</code> to tell us if it is <code>Some</code>. (Yes, there is also a method called <code>.is_none()</code>.) In this easier way, we don't need <code>handle_option()</code> anymore. We also don't need a vec for the Options.</p>
<pre><pre class="playground"><code class="language-rust">fn take_fifth(value: Vec&lt;i32&gt;) -&gt; Option&lt;i32&gt; {
if value.len() &lt; 4 {
None
} else {
Some(value[4])
}
}
fn main() {
let new_vec = vec![1, 2];
let bigger_vec = vec![1, 2, 3, 4, 5];
let vec_of_vecs = vec![new_vec, bigger_vec];
for vec in vec_of_vecs {
let inside_number = take_fifth(vec);
if inside_number.is_some() {
// .is_some() returns true if we get Some, false if we get None
println!(&quot;We got: {}&quot;, inside_number.unwrap()); // now it is safe to use .unwrap() because we already checked
} else {
println!(&quot;We got nothing.&quot;);
}
}
}
</code></pre></pre>
<p>This prints:</p>
<pre><code class="language-text">We got nothing.
We got: 5
</code></pre>
<h3><a class="header" href="#result" id="result">Result</a></h3>
<p>Result is similar to Option, but here is the difference:</p>
<ul>
<li>Option is about <code>Some</code> or <code>None</code> (value or no value),</li>
<li>Result is about <code>Ok</code> or <code>Err</code> (okay result, or error result).</li>
</ul>
<p>So <code>Option</code> is if you are thinking: &quot;Maybe there will be something, and maybe there won't.&quot; But <code>Result</code> is if you are thinking: &quot;Maybe it will fail.&quot;</p>
<p>To compare, here are the signatures for Option and Result.</p>
<pre><pre class="playground"><code class="language-rust">enum Option&lt;T&gt; {
None,
Some(T),
}
enum Result&lt;T, E&gt; {
Ok(T),
Err(E),
}
fn main() {}
</code></pre></pre>
<p>So Result has a value inside of <code>Ok</code>, and a value inside of <code>Err</code>. That is because errors usually (and should have) have information inside them.</p>
<p><code>Result&lt;T, E&gt;</code> means you need to think of what you want to return for <code>Ok</code>, and what you want to return for <code>Err</code>. Actually, you can decide anything. Even this is okay:</p>
<pre><pre class="playground"><code class="language-rust">fn check_error() -&gt; Result&lt;(), ()&gt; {
Ok(())
}
fn main() {
check_error();
}
</code></pre></pre>
<p><code>check_error</code> says &quot;return <code>()</code> if we get <code>Ok</code>, and return <code>()</code> if we get <code>Err</code>&quot;. Then we return <code>Ok</code> with a <code>()</code>.</p>
<p>The compiler gives us an interesting warning:</p>
<pre><code class="language-text">warning: unused `std::result::Result` that must be used
--&gt; src\main.rs:6:5
|
6 | check_error();
| ^^^^^^^^^^^^^^
|
= note: `#[warn(unused_must_use)]` on by default
= note: this `Result` may be an `Err` variant, which should be handled
</code></pre>
<p>This is true: we only returned the <code>Result</code> but it could have been an <code>Err</code>. So let's handle the error a bit, even though we're still not really doing anything.</p>
<pre><pre class="playground"><code class="language-rust">fn give_result(input: i32) -&gt; Result&lt;(), ()&gt; {
if input % 2 == 0 {
return Ok(())
} else {
return Err(())
}
}
fn main() {
if give_result(5).is_ok() {
println!(&quot;It's okay, guys&quot;)
} else {
println!(&quot;It's an error, guys&quot;)
}
}
</code></pre></pre>
<p>This prints <code>It's an error, guys</code>. So we just handled our first error.</p>
<p>Remember, the four methods to easily check are <code>.is_some()</code>, <code>is_none()</code>, <code>is_ok()</code>, and <code>is_err()</code>.</p>
<p>Sometimes a function with Result will use a <code>String</code> for the <code>Err</code> value. This is not the best method to use, but it is a little better than what we've done so far.</p>
<pre><pre class="playground"><code class="language-rust">fn check_if_five(number: i32) -&gt; Result&lt;i32, String&gt; {
match number {
5 =&gt; Ok(number),
_ =&gt; Err(&quot;Sorry, the number wasn't five.&quot;.to_string()), // This is our error message
}
}
fn main() {
let mut result_vec = Vec::new(); // Create a new vec for the results
for number in 2..7 {
result_vec.push(check_if_five(number)); // push each result into the vec
}
println!(&quot;{:?}&quot;, result_vec);
}
</code></pre></pre>
<p>Our vec prints:</p>
<pre><code class="language-text">[Err(&quot;Sorry, the number wasn\'t five.&quot;), Err(&quot;Sorry, the number wasn\'t five.&quot;), Err(&quot;Sorry, the number wasn\'t five.&quot;), Ok(5),
Err(&quot;Sorry, the number wasn\'t five.&quot;)]
</code></pre>
<p>Just like Option, <code>.unwrap()</code> on <code>Err</code> will panic.</p>
<pre><pre class="playground"><code class="language-rust"> // ⚠️
fn main() {
let error_value: Result&lt;i32, &amp;str&gt; = Err(&quot;There was an error&quot;); // Create a Result that is already an Err
println!(&quot;{}&quot;, error_value.unwrap()); // Unwrap it
}
</code></pre></pre>
<p>The program panics, and prints:</p>
<pre><code class="language-text">thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: &quot;There was an error&quot;', src\main.rs:30:20
</code></pre>
<p>This information helps you fix your code. <code>src\main.rs:30:20</code> means &quot;inside main.rs in directory src, on line 30 and column 20&quot;. So you can go there to look at your code and fix the problem.</p>
<p>You can also create your own error types. Result functions in the standard library and other people's code usually do this. For example, this function from the standard library:</p>
<pre><pre class="playground"><code class="language-rust">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// 🚧
pub fn from_utf8(vec: Vec&lt;u8&gt;) -&gt; Result&lt;String, FromUtf8Error&gt;
<span class="boring">}
</span></code></pre></pre>
<p>This function takes a vector of bytes (<code>u8</code>) and tries to make a <code>String</code>. So the success case for the Result is a <code>String</code> and the error case is <code>FromUtf8Error</code>. You can give your error type any name you want.</p>
<p>Using a <code>match</code> with <code>Option</code> and <code>Result</code> sometimes requires a lot of code. For example, the <code>.get()</code> method returns an <code>Option</code> on a <code>Vec</code>.</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let my_vec = vec![2, 3, 4];
let get_one = my_vec.get(0); // 0 to get the first number
let get_two = my_vec.get(10); // Returns None
println!(&quot;{:?}&quot;, get_one);
println!(&quot;{:?}&quot;, get_two);
}
</code></pre></pre>
<p>This prints</p>
<pre><code class="language-text">Some(2)
None
</code></pre>
<p>So now we can match to get the values. Let's use a range from 0 to 10 to see if it matches the numbers in <code>my_vec</code>.</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let my_vec = vec![2, 3, 4];
for index in 0..10 {
match my_vec.get(index) {
Some(number) =&gt; println!(&quot;The number is: {}&quot;, number),
None =&gt; {}
}
}
}
</code></pre></pre>
<p>This is good, but we don't do anything for <code>None</code> because we don't care. Here we can make the code smaller by using <code>if let</code>. <code>if let</code> means &quot;do something if it matches, and don't do anything if it doesn't&quot;. <code>if let</code> is when you don't care about matching for everything.</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let my_vec = vec![2, 3, 4];
for index in 0..10 {
if let Some(number) = my_vec.get(index) {
println!(&quot;The number is: {}&quot;, number);
}
}
}
</code></pre></pre>
<p><strong>Important to remember</strong>: <code>if let Some(number) = my_vec.get(index)</code> means &quot;if you get <code>Some(number)</code> from <code>my_vec.get(index)</code>&quot;.</p>
<p>Also note: it uses one <code>=</code>. It is not a boolean.</p>
<p><code>while let</code> is like a while loop for <code>if let</code>. Imagine that we have weather station data like this:</p>
<pre><code class="language-text">[&quot;Berlin&quot;, &quot;cloudy&quot;, &quot;5&quot;, &quot;-7&quot;, &quot;78&quot;]
[&quot;Athens&quot;, &quot;sunny&quot;, &quot;not humid&quot;, &quot;20&quot;, &quot;10&quot;, &quot;50&quot;]
</code></pre>
<p>We want to get the numbers, but not the words. For the numbers, we can use a method called <code>parse::&lt;i32&gt;()</code>. <code>parse()</code> is the method, and <code>::&lt;i32&gt;</code> is the type. It will try to turn the <code>&amp;str</code> into an <code>i32</code>, and give it to us if it can. It returns a <code>Result</code>, because it might not work (like if you wanted it to parse &quot;Billybrobby&quot; - that's not a number).</p>
<p>We will also use <code>.pop()</code>. This takes the last item off of the vector.</p>
<pre><pre class="playground"><code class="language-rust">fn main() {
let weather_vec = vec![
vec![&quot;Berlin&quot;, &quot;cloudy&quot;, &quot;5&quot;, &quot;-7&quot;, &quot;78&quot;],
vec![&quot;Athens&quot;, &quot;sunny&quot;, &quot;not humid&quot;, &quot;20&quot;, &quot;10&quot;, &quot;50&quot;],
];
for mut city in weather_vec {
println!(&quot;For the city of {}:&quot;, city[0]); // In our data, every first item is the city name
while let Some(information) = city.pop() {
// This means: keep going until you can't pop anymore
// When the vector reaches 0 items, it will return None
// and it will stop.
if let Ok(number) = information.parse::&lt;i32&gt;() {
// Try to parse the variable we called information
// This returns a result. If it's Ok(number), it will print it
println!(&quot;The number is: {}&quot;, number);
} // We don't write anything here because we do nothing if we get an error. Throw them all away
}
}
}
</code></pre></pre>
<p>This will print:</p>
<pre><code class="language-text">For the city of Berlin:
The number is: 78
The number is: -7
The number is: 5
For the city of Athens:
The number is: 50
The number is: 10
The number is: 20
</code></pre>
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