if let
if let
allows you to combine if and let together to reduce the overhead of certain kinds of pattern matches.
let option = Some(12); if let Some(x) = option { foo(x); } else { bar(); }
while let
In a similar fashion, while let can be used when you want to conditionally loop as long as a value matches a certain pattern.
let mut v = vec![1, 3, 5, 7, 11]; while let Some(x) = v.pop() { println!("{}", x); }
使用 |
来匹配复合模式:
let x = 1; match x { 1 | 2 => println!("one or two"), 3 => println!("three"), _ => println!("anything"), } //打印结果: one or two
如果有一个复杂的数据类型,例如: struct,我们可以使用pattern来解构:
struct Point { x: i32, y: i32, } let origin = Point { x: 0, y: 0 }; match origin { Point { x, y } => println!("({},{})", x, y), }
我们使用 :
来指定不同的名字:
struct Point { x: i32, y: i32, } let origin = Point { x: 0, y: 0 }; match origin { Point { x: x1, y: y1 } => println!("({},{})", x1, y1), }
如果我们只关系其中的某些值,我们不必指定所有的名字:
struct Point { x: i32, y: i32, } let origin = Point { x: 0, y: 0 }; match origin { Point { x, .. } => println!("x is {}", x), }
打印出 x is 0
解构
也完全适用于 tuple
和 enums
match some_value { Ok(value) => println!("got a value: {}", value), Err(_) => println!("an error occurred"), } fn coordinate() -> (i32, i32, i32) { // generate and return some sort of triple tuple } let (x, _, z) = coordinate();
相似的,我们可以使用 ..
来忽略多个值:
enum OptionalTuple { Value(i32, i32, i32), Missing, } let x = OptionalTuple::Value(5, -2, 3); match x { OptionalTuple::Value(..) => println!("Got a tuple!"), OptionalTuple::Missing => println!("No such luck."), }
如果需要获取一个引用,我们可以使用 ref
关键字:
let x = 5; match x { ref r => println!("Got a reference to {}", r), }
这里, r
在 match
中的数据类型为 &i32
,换句话说, ref
在使用patterns中创建了一个引用。如果需要一个可变引用,可以使用 ref mut
let mut x = 5; match x { ref mut mr => println!("Got a mutable reference to {}", mr), }
我们使用 ...
来匹配一个范围的值:
let x = 1; match x { 1 ... 5 => println!("one through five"), _ => println!("anything"), }
我们可以通过 @
绑定值到一个命名变量上:
let x = 1; match x { e @ 1 ... 5 => println!("got a range element {}", e), _ => println!("anything"), }
在匹配复杂的数据结构中是非常有用的,例如:
#[derive(Debug)] struct Person { name: Option<String>, } let name = "Steve".to_string(); let mut x: Option<Person> = Some(Person { name: Some(name) }); match x { Some(Person { name: ref a @ Some(_), .. }) => println!("{:?}", a), _ => {} }
使用 @
和 |
,可以分别匹配不同的部分:
let x = 5; match x { e @ 1 ... 5 | e @ 8 ... 10 => println!("got a range element {}", e), _ => println!("anything"), }
enum OptionalInt { Value(i32), Missing, } let x = OptionalInt::Value(5); match x { OptionalInt::Value(i) if i > 5 => println!("Got an int bigger than five!"), OptionalInt::Value(..) => println!("Got an int!"), OptionalInt::Missing => println!("No such luck."), }