假设我有一个数组,我想随机选择一个元素.
最简单的方法是什么?
显而易见的方式是array[random index]
.但也许有类似红宝石的东西array.sample
?或者如果没有,可以使用扩展程序创建这样的方法?
关于Lucas所说的,你可以像这样创建一个Array类的扩展:
extension Array { func randomItem() -> Element? { if isEmpty { return nil } let index = Int(arc4random_uniform(UInt32(self.count))) return self[index] } }
例如:
let myArray = [1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 16] let myItem = myArray.randomItem() // Note: myItem is an Optional<Int>
Swift 4.2及以上版本
新推荐的方法是一种内置方法:randomElement()
.它返回一个可选项以避免我之前假设的空案例.
let array = ["Frodo", "Sam", "Wise", "Gamgee"] print(array.randomElement()!) // Using ! knowing I have array.count > 0
如果您不创建数组并且不保证计数> 0,您应该执行以下操作:
if let randomElement = array.randomElement() { print(randomElement) }
Swift 4.1及以下版本
只是回答你的问题,你可以这样做以实现随机数组选择:
let array = ["Frodo", "sam", "wise", "gamgee"] let randomIndex = Int(arc4random_uniform(UInt32(array.count))) print(array[randomIndex])
铸件是丑陋的,但我相信它们是必需的,除非别人有另一种方式.
Swift 4版本:
extension Collection where Index == Int { /** Picks a random element of the collection. - returns: A random element of the collection. */ func randomElement() -> Iterator.Element? { return isEmpty ? nil : self[Int(arc4random_uniform(UInt32(endIndex)))] } }
在Swift 2.2中,这可以推广,以便我们有:
UInt.random UInt8.random UInt16.random UInt32.random UInt64.random UIntMax.random // closed intervals: (-3...3).random (Int.min...Int.max).random // and collections, which return optionals since they can be empty: (1..<4).sample [1,2,3].sample "abc".characters.sample ["a": 1, "b": 2, "c": 3].sample
首先,random
为UnsignedIntegerType
s 实现静态属性:
import Darwin func sizeof <T> (_: () -> T) -> Int { // sizeof return type without calling return sizeof(T.self) } let ARC4Foot: Int = sizeof(arc4random) extension UnsignedIntegerType { static var max: Self { // sadly `max` is not required by the protocol return ~0 } static var random: Self { let foot = sizeof(Self) guard foot > ARC4Foot else { return numericCast(arc4random() & numericCast(max)) } var r = UIntMax(arc4random()) for i in 1..<(foot / ARC4Foot) { r |= UIntMax(arc4random()) << UIntMax(8 * ARC4Foot * i) } return numericCast(r) } }
那么,对于ClosedInterval
有UnsignedIntegerType
边界的s :
extension ClosedInterval where Bound : UnsignedIntegerType { var random: Bound { guard start > 0 || end < Bound.max else { return Bound.random } return start + (Bound.random % (end - start + 1)) } }
然后(稍微复杂一点),对于ClosedInterval
带有SignedIntegerType
边界的s (使用下面进一步描述的辅助方法):
extension ClosedInterval where Bound : SignedIntegerType { var random: Bound { let foot = sizeof(Bound) let distance = start.unsignedDistanceTo(end) guard foot > 4 else { // optimisation: use UInt32.random if sufficient let off: UInt32 if distance < numericCast(UInt32.max) { off = UInt32.random % numericCast(distance + 1) } else { off = UInt32.random } return numericCast(start.toIntMax() + numericCast(off)) } guard distance < UIntMax.max else { return numericCast(IntMax(bitPattern: UIntMax.random)) } let off = UIntMax.random % (distance + 1) let x = (off + start.unsignedDistanceFromMin).plusMinIntMax return numericCast(x) } }
...其中unsignedDistanceTo
,unsignedDistanceFromMin
并plusMinIntMax
辅助方法可以实现如下:
extension SignedIntegerType { func unsignedDistanceTo(other: Self) -> UIntMax { let _self = self.toIntMax() let other = other.toIntMax() let (start, end) = _self < other ? (_self, other) : (other, _self) if start == IntMax.min && end == IntMax.max { return UIntMax.max } if start < 0 && end >= 0 { let s = start == IntMax.min ? UIntMax(Int.max) + 1 : UIntMax(-start) return s + UIntMax(end) } return UIntMax(end - start) } var unsignedDistanceFromMin: UIntMax { return IntMax.min.unsignedDistanceTo(self.toIntMax()) } } extension UIntMax { var plusMinIntMax: IntMax { if self > UIntMax(IntMax.max) { return IntMax(self - UIntMax(IntMax.max) - 1) } else { return IntMax.min + IntMax(self) } } }
最后,对于所有集合,其中Index.Distance == Int
:
extension CollectionType where Index.Distance == Int { var sample: Generator.Element? { if isEmpty { return nil } let end = UInt(count) - 1 let add = (0...end).random let idx = startIndex.advancedBy(Int(add)) return self[idx] } }
...可以对整数Range
s 进行一点优化:
extension Range where Element : SignedIntegerType { var sample: Element? { guard startIndex < endIndex else { return nil } let i: ClosedInterval = startIndex...endIndex.predecessor() return i.random } } extension Range where Element : UnsignedIntegerType { var sample: Element? { guard startIndex < endIndex else { return nil } let i: ClosedInterval = startIndex...endIndex.predecessor() return i.random } }
你可以使用Swift的内置random()函数来扩展:
extension Array { func sample() -> Element { let randomIndex = Int(rand()) % count return self[randomIndex] } } let array = [1, 2, 3, 4] array.sample() // 2 array.sample() // 2 array.sample() // 3 array.sample() // 3 array.sample() // 1 array.sample() // 1 array.sample() // 3 array.sample() // 1
另一个Swift 3的建议
private extension Array { var randomElement: Element { let index = Int(arc4random_uniform(UInt32(count))) return self[index] } }