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