2 | 3 | 5 | 7 | 11 => println!("This is a prime"),
// TODO ^ Try adding 13 to the list of prime values
// Match an inclusive range
13..=19 => println!("A teen"),
// Handle the rest of cases
_ => println!("Ain't special"),
// TODO ^ Try commenting out this catch-all arm
}
let boolean = true;
// Match is an expression too
let binary = match boolean {
// The arms of a match must cover all the possible values
false => 0,
true => 1,
// TODO ^ Try commenting out one of these arms
};
println!("{} -> {}", boolean, binary);
}
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Destructuring
A match block can destructure items in a variety of ways.
• Destructuring Tuples
• Destructuring Enums
• Destructuring Pointers
• Destructuring Structures
tuples
Tuples can be destructured in a match as follows:
fn main() {
let triple = (0, -2, 3);
// TODO ^ Try different values for `triple`
println!("Tell me about {:?}", triple);
// Match can be used to destructure a tuple
match triple {
// Destructure the second and third elements
(0, y, z) => println!("First is `0`, `y` is {:?}, and `z` is {:?}", y, z),
(1, ..) => println!("First is `1` and the rest doesn't matter"),
// `..` can be the used ignore the rest of the tuple
_ => println!("It doesn't matter what they are"),
// `_` means don't bind the value to a variable
}
}
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See also:
Tuples
enums
An enum is destructured similarly:
// `allow` required to silence warnings because only
// one variant is used.
#[allow(dead_code)]
enum Color {
// These 3 are specified solely by their name.
Red,
Blue,
Green,
// These likewise tie `u32` tuples to different names: color models.
RGB(u32, u32, u32),
HSV(u32, u32, u32),
HSL(u32, u32, u32),
CMY(u32, u32, u32),
CMYK(u32, u32, u32, u32),
}
fn main() {
let color = Color::RGB(122, 17, 40);
// TODO ^ Try different variants for `color`
println!("What color is it?");
// An `enum` can be destructured using a `match`.
match color {
Color::Red => println!("The color is Red!"),
Color::Blue => println!("The color is Blue!"),
Color::Green => println!("The color is Green!"),
Color::RGB(r, g, b) =>
println!("Red: {}, green: {}, and blue: {}!", r, g, b),
Color::HSV(h, s, v) =>
println!("Hue: {}, saturation: {}, value: {}!", h, s, v),
Color::HSL(h, s, l) =>
println!("Hue: {}, saturation: {}, lightness: {}!", h, s, l),
Color::CMY(c, m, y) =>
println!("Cyan: {}, magenta: {}, yellow: {}!", c, m, y),
Color::CMYK(c, m, y, k) =>
println!("Cyan: {}, magenta: {}, yellow: {}, key (black): {}!",
c, m, y, k),
// Don't need another arm because all variants have been examined
}
}
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See also:
#[allow(...)], color models and enum
pointers/ref
For pointers, a distinction needs to be made between destructuring and dereferencing as they are different concepts which are used differently from a language like C.
• Dereferencing uses *
• Destructuring uses &, ref, and ref mut
fn main() {
// Assign a reference of type `i32`. The `&` signifies there
// is a reference being assigned.
let reference = &4;
match reference {
// If `reference` is pattern matched against `&val`, it results
// in a comparison like:
// `&i32`
// `&val`
// ^ We see that if the matching `&`s are dropped, then the `i32`
// should be assigned to `val`.
&val => println!("Got a value via destructuring: {:?}", val),
}
// To avoid the `&`, you dereference before matching.
match *reference {
val => println!("Got a value via dereferencing: {:?}", val),
}
// What if you don't start with a reference? `reference` was a `&`
// because the right side was already a reference. This is not
// a reference because the right side is not one.
let _not_a_reference = 3;
// Rust provides `ref` for exactly this purpose. It modifies the
// assignment so that a reference is created for the element; this
// reference is assigned.
let ref _is_a_reference = 3;
// Accordingly, by defining 2 values without references, references
// can be retrieved via `ref` and `ref mut`.