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<h1 id="enum"><a class="header" href="#enum">Enum</a></h1>
<ol>
<li>🌟🌟 Enums can be created with explicit discriminator.</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">
// Fix the errors
enum Number {
Zero,
One,
Two,
}
enum Number1 {
Zero = 0,
One,
Two,
}
// C-like enum
enum Number2 {
Zero = 0.0,
One = 1.0,
Two = 2.0,
}
fn main() {
// An enum variant can be converted to a integer by `as`
assert_eq!(Number::One, Number1::One);
assert_eq!(Number1::One, Number2::One);
println!("Success!");
} </code></pre></pre>
<ol start="2">
<li>🌟 Each enum variant can hold its own data.</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">
// Fill in the blank
enum Message {
Quit,
Move { x: i32, y: i32 },
Write(String),
ChangeColor(i32, i32, i32),
}
fn main() {
let msg1 = Message::Move{__}; // Instantiating with x = 1, y = 2
let msg2 = Message::Write(__); // Instantiating with "hello, world!"
println!("Success!");
} </code></pre></pre>
<ol start="3">
<li>🌟🌟 We can get the data which an enum variant is holding by pattern match.</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">
// Fill in the blank and fix the error
enum Message {
Quit,
Move { x: i32, y: i32 },
Write(String),
ChangeColor(i32, i32, i32),
}
fn main() {
let msg = Message::Move{x: 1, y: 2};
if let Message::Move{__} = msg {
assert_eq!(a, b);
} else {
panic!("NEVER LET THIS RUN！");
}
println!("Success!");
} </code></pre></pre>
<ol start="4">
<li>🌟🌟</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">
// Fill in the blank and fix the errors
enum Message {
Quit,
Move { x: i32, y: i32 },
Write(String),
ChangeColor(i32, i32, i32),
}
fn main() {
let msgs: __ = [
Message::Quit,
Message::Move{x:1, y:3},
Message::ChangeColor(255,255,0)
];
for msg in msgs {
show_message(msg)
}
}
fn show_message(msg: Message) {
println!("{}", msg);
}</code></pre></pre>
<ol start="5">
<li>🌟🌟 Since there is no <code>null</code> in Rust, we have to use enum <code>Option&lt;T&gt;</code> to deal with the cases when the value is absent.</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">
// Fill in the blank to make the `println` work.
// Also add some code to prevent the `panic` from running.
fn main() {
let five = Some(5);
let six = plus_one(five);
let none = plus_one(None);
if let __ = six {
println!("{}", n);
println!("Success!");
}
panic!("NEVER LET THIS RUN！");
}
fn plus_one(x: Option&lt;i32&gt;) -&gt; Option&lt;i32&gt; {
match x {
__ =&gt; None,
__ =&gt; Some(i + 1),
}
}</code></pre></pre>
<ol start="6">
<li>🌟🌟🌟🌟 Implement a <code>linked-list</code> via enums.</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">
use crate::List::*;
enum List {
// Cons: Tuple struct that wraps an element and a pointer to the next node
Cons(u32, Box&lt;List&gt;),
// Nil: A node that signifies the end of the linked list
Nil,
}
// Methods can be attached to an enum
impl List {
// Create an empty list
fn new() -&gt; List {
// `Nil` has type `List`
Nil
}
// Consume a list, and return the same list with a new element at its front
fn prepend(self, elem: u32) -&gt; __ {
// `Cons` also has type List
Cons(elem, Box::new(self))
}
// Return the length of the list
fn len(&amp;self) -&gt; u32 {
// `self` has to be matched, because the behavior of this method
// depends on the variant of `self`
// `self` has type `&amp;List`, and `*self` has type `List`, matching on a
// concrete type `T` is preferred over a match on a reference `&amp;T`
// After Rust 2018 you can use self here and tail (with no ref) below as well,
// rust will infer &amp;s and ref tail.
// See https://doc.rust-lang.org/edition-guide/rust-2018/ownership-and-lifetimes/default-match-bindings.html
match *self {
// Can't take ownership of the tail, because `self` is borrowed;
// Instead take a reference to the tail
Cons(_, ref tail) =&gt; 1 + tail.len(),
// Base Case: An empty list has zero length
Nil =&gt; 0
}
}
// Return representation of the list as a (heap allocated) string
fn stringify(&amp;self) -&gt; String {
match *self {
Cons(head, __ tail) =&gt; {
// `format!` is similar to `print!`, but returns a heap
// allocated string instead of printing to the console
format!("{}, {}", head, tail.__())
},
Nil =&gt; {
format!("Nil")
},
}
}
}
fn main() {
// Create an empty linked list
let mut list = List::new();
// Prepend some elements
list = list.prepend(1);
list = list.prepend(2);
list = list.prepend(3);
// Show the final state of the list
println!("linked list has length: {}", list.len());
println!("{}", list.stringify());
}</code></pre></pre>
<blockquote>
<p>You can find the solutions <a href="https://github.com/sunface/rust-by-practice/blob/master/solutions/compound-types/enum.md">here</a>(under the solutions path), but only use it when you need it</p>
</blockquote>
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