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</style>
## Patterns and Matching
#### @wusyong, 吳昱緯
----
### Elements of pattern:
* Literals
* Destructured arrays, enums, structs, or tuples
* Variables
* Wildcards
* Placeholders
---
## All the Places Patterns Can Be Used
Literally everywhere
----
### match Arms
```rust
match VALUE {
PATTERN => EXPRESSION,
PATTERN => EXPRESSION,
PATTERN => EXPRESSION,
}
```
----
### Conditional if let Expressions
```rust
fn main() {
let favorite_color: Option<&str> = None;
let is_tuesday = false;
let age: Result<u8, _> = "34".parse();
if let Some(color) = favorite_color {
println!("Using your favorite color, {}, as the background", color);
} else if is_tuesday {
println!("Tuesday is green day!");
} else if let Ok(age) = age {
if age > 30 {
println!("Using purple as the background color");
} else {
println!("Using orange as the background color");
}
} else {
println!("Using blue as the background color");
}
}
```
----
### while let Conditional Loops
```rust
let mut stack = Vec::new();
stack.push(1);
stack.push(2);
stack.push(3);
while let Some(top) = stack.pop() {
println!("{}", top);
}
```
----
### for Loops
```rust
let v = vec!['a', 'b', 'c'];
for (index, value) in v.iter().enumerate() {
println!("{} is at index {}", value, index);
}
```
----
### let Statements
```rust
let PATTERN = EXPRESSION;
let (x, y, z) = (1, 2, 3);
```
----
### Function Parameters
```rust
fn print_coordinates(&(x, y): &(i32, i32)) {
println!("Current location: ({}, {})", x, y);
}
fn main() {
let point = (3, 5);
print_coordinates(&point);
}
```
---
## Refutability: Whether a Pattern Might Fail to Match
- Refutable: if let, while let
- Irrefutable: let, for Loops
---
## Refutable & Irrefutable
```rust
let Some(x) = some_option_value;
error[E0005]: refutable pattern in local binding: `None` not covered
-->
|
3 | let Some(x) = some_option_value;
| ^^^^^^^ pattern `None` not covered
```
---
## Pattern Syntax
----
### Matching Literals
```rust
let x = 1;
match x {
1 => println!("one"),
2 => println!("two"),
3 => println!("three"),
_ => println!("anything"),
}
```
----
### Matching Named Variables
```rust
fn main() {
let x = Some(5);
let y = 10;
match x {
Some(50) => println!("Got 50"),
Some(y) => println!("Matched, y = {:?}", y),
_ => println!("Default case, x = {:?}", x),
}
println!("at the end: x = {:?}, y = {:?}", x, y);
}
```
----
### Multiple Patterns
```rust
let x = 1;
match x {
1 | 2 => println!("one or two"),
3 => println!("three"),
_ => println!("anything"),
}
```
----
### Matching Ranges of Values with ...
```rust
let x = 5;
match x {
1...5 => println!("one through five"),
_ => println!("something else"),
}
let x = 'c';
match x {
'a'...'j' => println!("early ASCII letter"),
'k'...'z' => println!("late ASCII letter"),
_ => println!("something else"),
}
```
----
## Destructuring to Break Apart Values
----
### Destructuring Structs
```rust
struct Point {
x: i32,
y: i32,
}
fn main() {
let p = Point { x: 0, y: 7 };
let Point { x: a, y: b } = p;
assert_eq!(0, a);
assert_eq!(7, b);
}
```
----
### Destructuring Structs
```rust
struct Point {
x: i32,
y: i32,
}
fn main() {
let p = Point { x: 0, y: 7 };
let Point { x, y } = p;
assert_eq!(0, x);
assert_eq!(7, y);
}
```
----
### Destructuring Structs
```rust
fn main() {
let p = Point { x: 0, y: 7 };
match p {
Point { x, y: 0 } => println!("On the x axis at {}", x),
Point { x: 0, y } => println!("On the y axis at {}", y),
Point { x, y } => println!("On neither axis: ({}, {})", x, y),
}
}
```
----
### Destructuring Enums
```rust
enum Message {
Quit,
Move { x: i32, y: i32 },
Write(String),
ChangeColor(i32, i32, i32),
}
fn main() {
let msg = Message::ChangeColor(0, 160, 255);
match msg {
Message::Quit => {
println!("The Quit variant has no data to destructure.")
},
Message::Move { x, y } => {
println!(
"Move in the x direction {} and in the y direction {}",
x, y
);
}
Message::Write(text) => println!("Text message: {}", text),
Message::ChangeColor(r, g, b) => {
println!(
"Change the color to red {}, green {}, and blue {}",
r, g, b
)
}
}
}
```
----
### Destructuring Nested Structs and Enums
```rust
enum Color {
Rgb(i32, i32, i32),
Hsv(i32, i32, i32),
}
enum Message {
Quit,
Move { x: i32, y: i32 },
Write(String),
ChangeColor(Color),
}
fn main() {
let msg = Message::ChangeColor(Color::Hsv(0, 160, 255));
match msg {
Message::ChangeColor(Color::Rgb(r, g, b)) => {
println!(
"Change the color to red {}, green {}, and blue {}",
r, g, b
)
},
Message::ChangeColor(Color::Hsv(h, s, v)) => {
println!(
"Change the color to hue {}, saturation {}, and value {}",
h, s, v
)
}
_ => ()
}
}
```
----
### Destructuring Structs and Tuples
```rust
let ((feet, inches), Point {x, y}) = ((3, 10), Point { x: 3, y: -10 });
```
----
## Ignoring Values in a Pattern
----
### Ignoring an Entire Value with _
```rust
fn foo(_: i32, y: i32) {
println!("This code only uses the y parameter: {}", y);
}
fn main() {
foo(3, 4);
}
```
----
### Ignoring Parts of a Value with a Nested _
```rust
let mut setting_value = Some(5);
let new_setting_value = Some(10);
match (setting_value, new_setting_value) {
(Some(_), Some(_)) => {
println!("Can't overwrite an existing customized value");
}
_ => {
setting_value = new_setting_value;
}
}
println!("setting is {:?}", setting_value);
```
----
### Ignoring Parts of a Value with a Nested _
```rust
let numbers = (2, 4, 8, 16, 32);
match numbers {
(first, _, third, _, fifth) => {
println!("Some numbers: {}, {}, {}", first, third, fifth)
},
}
```
----
### Ignoring an Unused Variable by Starting Its Name with _
```rust
fn main() {
let _x = 5;
let y = 10;
}
```
----
### Ignoring an Unused Variable by Starting Its Name with _
The syntax _x still binds the value to the variable, whereas _ doesn’t bind at all.
```rust
let s = Some(String::from("Hello!"));
if let Some(_s) = s {
println!("found a string");
}
println!("{:?}", s);
```
----
### Ignoring Remaining Parts of a Value with ..
```rust
struct Point {
x: i32,
y: i32,
z: i32,
}
let origin = Point { x: 0, y: 0, z: 0 };
match origin {
Point { x, .. } => println!("x is {}", x),
}
```
----
### Ignoring Remaining Parts of a Value with ..
```rust
fn main() {
let numbers = (2, 4, 8, 16, 32);
match numbers {
(first, .., last) => {
println!("Some numbers: {}, {}", first, last);
},
}
}
```
----
### Ignoring Remaining Parts of a Value with ..
```rust
fn main() {
let numbers = (2, 4, 8, 16, 32);
match numbers {
(.., second, ..) => {
println!("Some numbers: {}", second)
},
}
}
error: `..` can only be used once per tuple or tuple struct pattern
--> src/main.rs:5:22
|
5 | (.., second, ..) => {
|
```
----
### Extra Conditionals with Match Guards
```rust
let num = Some(4);
match num {
Some(x) if x < 5 => println!("less than five: {}", x),
Some(x) => println!("{}", x),
None => (),
}
```
----
### Extra Conditionals with Match Guards
```rust
fn main() {
let x = Some(5);
let y = 10;
match x {
Some(50) => println!("Got 50"),
Some(n) if n == y => println!("Matched, n = {:?}", n),
_ => println!("Default case, x = {:?}", x),
}
println!("at the end: x = {:?}, y = {:?}", x, y);
}
```
----
### Extra Conditionals with Match Guards
```rust
let x = 4;
let y = false;
match x {
4 | 5 | 6 if y => println!("yes"),
_ => println!("no"),
}
```
----
### @ Bindings
```rust
enum Message {
Hello { id: i32 },
}
let msg = Message::Hello { id: 5 };
match msg {
Message::Hello { id: id_variable @ 3...7 } => {
println!("Found an id in range: {}", id_variable)
},
Message::Hello { id: 10...12 } => {
println!("Found an id in another range")
},
Message::Hello { id } => {
println!("Found some other id: {}", id)
},
}
```
---
## Thanks for listening!
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