Question

我一直在使用sort（）函数，但它混合了相对顺序。

这是我的代码的样子。

recipes.sort { $0.skill.value <= $1.skill.value }

Swift API说：

排序算法不稳定。不稳定的排序可能会改变比较相等的元素的相对顺序。

如何更改此设置以使相对顺序与以前保持一致？

Answer 1

下面的实现就像标准库中的sorted方法一样，没有额外的限制。

extension RandomAccessCollection {

    /// return a sorted collection
    /// this use a stable sort algorithm
    ///
    /// - Parameter areInIncreasingOrder: return nil when two element are equal
    /// - Returns: the sorted collection
    public func stableSorted(by areInIncreasingOrder: (Iterator.Element, Iterator.Element) -> Bool?) -> [Iterator.Element] {

        let sorted = self.enumerated().sorted { (one, another) -> Bool in
            if let result = areInIncreasingOrder(one.element, another.element) {
                return result
            } else {
                return one.offset < another.offset
            }
        }
        return sorted.map{ $0.element }
    }
}

稳定排序需要保留原始订单。所以我们给每个元素一个权重除了它的值，索引，然后原始的排序方法将起作用，因为永远不会有2个相等的元素。

Answer 2

我很欣赏leavez's answer的优雅。我将其改编为与Sequence.sorted(by:)具有相同的签名：

extension Sequence {
  func stableSorted(
    by areInIncreasingOrder: (Element, Element) throws -> Bool)
    rethrows -> [Element]
  {
    return try enumerated()
      .sorted { a, b -> Bool in
        try areInIncreasingOrder(a.element, b.element) ||
          (a.offset < b.offset && !areInIncreasingOrder(b.element, a.element))
      }
      .map { $0.element }
  }
}

Answer 3

        let sortedArray = (recipes as NSArray).sortedArray(options: .stable, usingComparator: { (lhs, rhs) -> ComparisonResult in
            let lhs = (lhs as! Recipe)
            let rhs = (rhs as! Recipe)
            if lhs.skill.value == rhs.skill.value {
                return ComparisonResult.orderedSame
            } else if lhs.skill.value < rhs.skill.value {
                return ComparisonResult.orderedAscending
            } else {
                return ComparisonResult.orderedDescending
            }
        })

从这里开始：https://medium.com/@cocotutch/a-swift-sorting-problem-e0ebfc4e46d4

Answer 4

我使用这个包装器

extension Array where Element: Comparable, Element: AnyObject {
    public func stableSorted() -> [Element] {
        let array = self as NSArray
        let result = array.sortedArray(options: .stable) { (left, right) -> ComparisonResult in
            let left = left as! Element
            let right = right as! Element
            if left < right {
                return ComparisonResult.orderedAscending
            }
            if left > right {
                return ComparisonResult.orderedDescending
            }
            return ComparisonResult.orderedSame
        }
        return result as! [Element]
    }
    public func stableReversed() -> [Element] {
        let array = self as NSArray
        let result = array.sortedArray(options: .stable) { (left, right) -> ComparisonResult in
            let left = left as! Element
            let right = right as! Element
            if left > right {
                return ComparisonResult.orderedAscending
            }
            if left < right {
                return ComparisonResult.orderedDescending
            }
            return ComparisonResult.orderedSame
        }
        return result as! [Element]
    }
}

Answer 5

在Swift 5 sort()中使用稳定的实现方式，不久后它将正式保证稳定。

来自Swift forums：

...
另一方面，实际的实现调用

/// Sorts the elements of this buffer according to `areInIncreasingOrder`,
/// using a stable, adaptive merge sort.
///
/// The adaptive algorithm used is Timsort, modified to perform a straight
/// merge of the elements using a temporary buffer.
@inlinable
public mutating func _stableSortImpl(
    by areInIncreasingOrder: (Element, Element) throws -> Bool
) rethrows { ... }

和

如果我记得，sort（）目前是稳定的，但尚不能保证保持稳定（意思是，它目前处于稳定状态实施细节，而Swift的未来版本可能会发布而是使用不稳定算法。

如何在swift中稳定排序数组？

5 个答案: