sort(by:)

Declaration

mutating func sort(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows

Parameters

• areInIncreasingOrder:

  A predicate that returns true if its first argument should be ordered before its second argument; otherwise, false. If areInIncreasingOrder throws an error during the sort, the elements may be in a different order, but none will be lost.

Discussion

When you want to sort a collection of elements that don’t conform to the Comparable protocol, pass a closure to this method that returns true when the first element should be ordered before the second.

In the following example, the closure provides an ordering for an array of a custom enumeration that describes an HTTP response. The predicate orders errors before successes and sorts the error responses by their error code.

enum HTTPResponse {
    case ok
    case error(Int)
}

var responses: [HTTPResponse] = [.error(500), .ok, .ok, .error(404), .error(403)]
responses.sort {
    switch ($0, $1) {
    // Order errors by code
    case let (.error(aCode), .error(bCode)):
        return aCode < bCode

    // All successes are equivalent, so none is before any other
    case (.ok, .ok): return false

    // Order errors before successes
    case (.error, .ok): return true
    case (.ok, .error): return false
    }
}
print(responses)
// Prints "[.error(403), .error(404), .error(500), .ok, .ok]"

Alternatively, use this method to sort a collection of elements that do conform to Comparable when you want the sort to be descending instead of ascending. Pass the greater-than operator (>) operator as the predicate.

var students = ["Kofi", "Abena", "Peter", "Kweku", "Akosua"]
students.sort(by: >)
print(students)
// Prints "["Peter", "Kweku", "Kofi", "Akosua", "Abena"]"

areInIncreasingOrder must be a strict weak ordering over the elements. That is, for any elements a, b, and c, the following conditions must hold:

• areInIncreasingOrder(a, a) is always false. (Irreflexivity)

• If areInIncreasingOrder(a, b) and areInIncreasingOrder(b, c) are both true, then areInIncreasingOrder(a, c) is also true. (Transitive comparability)

• Two elements are incomparable if neither is ordered before the other according to the predicate. If a and b are incomparable, and b and c are incomparable, then a and c are also incomparable. (Transitive incomparability)

The sorting algorithm is guaranteed to be stable. A stable sort preserves the relative order of elements for which areInIncreasingOrder does not establish an order.