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Foundation iOS Swift

Sorting arrays in Swift: multi-criteria

Swift’s foundation library provides a sorted(by:) function for sorting arrays. The areInIncreasingOrder closure needs to return true if the closure’s arguments are increasing, false otherwise. How to use the closure for sorting by multiple criteria? Let’s take a look at an example of sorting an array of Player structs.

  1. Sort by score in descending order
  2. Sort by name in ascending order
  3. Sort by id in ascending order
struct Player {
let id: Int
let name: String
let score: Int
}
extension Player: CustomDebugStringConvertible {
var debugDescription: String {
"id=\(id) name=\(name) score=\(score)"
}
}
let players: [Player] = [
Player(id: 0, name: "April", score: 7),
Player(id: 1, name: "Nora", score: 8),
Player(id: 2, name: "Joe", score: 5),
Player(id: 3, name: "Lisa", score: 4),
Player(id: 4, name: "Michelle", score: 6),
Player(id: 5, name: "Joe", score: 5),
Player(id: 6, name: "John", score: 7)
]
view raw Sort.swift hosted with ❤ by GitHub

As said before, the closure should return true if the left element should be ordered before the right element. If they happen to be equal, we should use the next sorting criteria. For comparing strings, we’ll go for case-insensitive sorting using Foundation’s built-in localizedCaseInsensitiveCompare.

let sorted = players.sorted { lhs, rhs in
if lhs.score == rhs.score {
let nameOrdering = lhs.name.localizedCaseInsensitiveCompare(rhs.name)
if nameOrdering == .orderedSame {
return lhs.id < rhs.id
} else {
return nameOrdering == .orderedAscending
}
} else {
return lhs.score > rhs.score
}
}
print(sorted.map(\.debugDescription).joined(separator: "\n"))
// id=1 name=Nora score=8
// id=0 name=April score=7
// id=6 name=John score=7
// id=4 name=Michelle score=6
// id=2 name=Joe score=5
// id=5 name=Joe score=5
// id=3 name=Lisa score=4
view raw Sort.swift hosted with ❤ by GitHub

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