Extending Array to check if it is sorted in Swift?

Question

I want to extend Array class so that it can know whether it is sorted (ascending) or not. I want to add a computed property called isSorted. How can I state the elements of the Array to be comparable?

My current implementation in Playground

extension Array {   var isSorted: Bool {     for i in 1..self.count {       if self[i-1] > self[i] { return false }     }     return true   } }  // The way I want to get the computed property [1, 1, 2, 3, 4, 5, 6, 7, 8].isSorted //= true [2, 1, 3, 8, 5, 6, 7, 4, 8].isSorted //= false 

The error Could not find an overload for '>' that accepts the supplied arguments

Of course, I still got an error because Swift doesn't know how to compare the elements. How can I implement this extension in Swift? Or am I doing something wrong here?

Answer 1

The alternative solution to a free function is to do what Swift's built-in Array.sort and Array.sorted methods do, and require that you pass a suitable comparator to the method:

extension Array {     func isSorted(isOrderedBefore: (T, T) -> Bool) -> Bool {         for i in 1..<self.count {             if !isOrderedBefore(self[i-1], self[i]) {                 return false             }         }         return true     } }  [1, 5, 3].isSorted(<) // false [1, 5, 10].isSorted(<) // true [3.5, 2.1, -5.4].isSorted(>) // true 

Answer 2

In Swift 2.0 you can now extend protocols!

extension CollectionType where Generator.Element: Comparable {      public var isSorted: Bool {          var previousIndex = startIndex         var currentIndex = startIndex.successor()          while currentIndex != endIndex {              if self[previousIndex] > self[currentIndex] {                 return false             }              previousIndex = currentIndex             currentIndex = currentIndex.successor()         }          return true     }  }  [1, 2, 3, 4].isSorted // true ["a", "b", "c", "e"].isSorted // true ["b", "a", "c", "e"].isSorted // false [/* Anything not implementing `Comparable` */].isSorted // <~~ Type-error 

Note that because we're using Indexable.Index instead of a simple Int as an index we have to use a while-loop instead, which looks a bit less pretty and clear.