4.7 KiB
4.7 KiB
Enum
- ππ Enums can be created with explicit discriminator.
// 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() {
// a enum variant can be converted to a integer by `as`
assert_eq!(Number::One, Number1::One);
assert_eq!(Number1::One, Number2::One);
println!("Success!")
}
- π each enum variant can hold its own data.
// 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!")
}
- ππ we can get the data which a enum variant is holding by pattern match
// 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 RUNοΌ");
}
println!("Success!")
}
- ππ
// 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);
}
- ππ As there is no
null
in Rust, we have to use enumOption<T>
to deal the cases when value is absent.
// 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 RUNοΌ");
}
fn plus_one(x: Option<i32>) -> Option<i32> {
match x {
__ => None,
__ => Some(i + 1),
}
}
- ππππ implement a
linked-list
via enums.
use crate::List::*;
enum List {
// Cons: Tuple struct that wraps an element and a pointer to the next node
Cons(u32, Box<List>),
// 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() -> 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) -> __ {
// `Cons` also has type List
Cons(elem, Box::new(self))
}
// Return the length of the list
fn len(&self) -> u32 {
// `self` has to be matched, because the behavior of this method
// depends on the variant of `self`
// `self` has type `&List`, and `*self` has type `List`, matching on a
// concrete type `T` is preferred over a match on a reference `&T`
// after Rust 2018 you can use self here and tail (with no ref) below as well,
// rust will infer &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) => 1 + tail.len(),
// Base Case: An empty list has zero length
Nil => 0
}
}
// Return representation of the list as a (heap allocated) string
fn stringify(&self) -> String {
match *self {
Cons(head, __ tail) => {
// `format!` is similar to `print!`, but returns a heap
// allocated string instead of printing to the console
format!("{}, {}", head, tail.__())
},
Nil => {
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());
}
You can find the solutions here(under the solutions path), but only use it when you need it