[英]How can I make an extension for extracting keys of a Dictionary in Swift?
I am working on this code for making a extension for making key array, but I am unable to give generic type to my array, what I should do for fixing?我正在编写此代码来制作用于制作键数组的扩展,但我无法为我的数组提供泛型类型,我应该怎么做才能修复?
extension Dictionary {
func extractKeys() -> Array<T> {
return self.map({ $0.key }).sorted(by: { $0 < $1 } )
}
}
update:更新:
extension Dictionary {
var extractSortedKeysV2: [Key] where Key: Comparable {
return self.map({ $0.key }).sorted(by: { $0 < $1 } )
}
}
struct Dictionary<Key, Value>
is a generic type where Key
is the type of the keys, and Value
the type of the values. struct Dictionary<Key, Value>
是一个泛型类型,其中Key
是键的类型, Value
是值的类型。 So you'll want to return Array<Key>
(or just [Key]
).所以你会想要返回
Array<Key>
(或者只是[Key]
)。
In addition, in order to sort the keys, you have to require that Key
conforms to the Comparable
protocol:此外,为了对键进行排序,您必须要求
Key
符合Comparable
协议:
extension Dictionary where Key: Comparable {
func sortedKeys() -> [Key] {
return self.map({ $0.key }).sorted(by: { $0 < $1 } )
}
}
This can be simplified to这可以简化为
extension Dictionary where Key: Comparable {
func sortedKeys() -> [Key] {
keys.sorted()
}
}
Or as a computed property:或作为计算属性:
extension Dictionary where Key: Comparable {
var sortedKeys: [Key] { keys.sorted() }
}
In the case of the function/method, the constraint can be attached to the function declaration instead:在函数/方法的情况下,可以将约束附加到 function 声明中:
extension Dictionary {
func sortedKeys() -> [Key] where Key: Comparable {
keys.sorted()
}
}
That is not possible with computed properties.这对于计算属性是不可能的。
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