Swift - Convert Array to Dictionary

Question

I just want convert an array of Player Names into a dictionary Scoreboard, giving everyone an initial score of 0.

Meaning...

var playerNames =          


        

Answer 1

Using reduce(into:_:):

var playerNames = ["Harry", "Ron", "Hermione"]

let playerScore = playerNames.reduce(into: [:]) { counts, playerNames in
    counts[playerNames, default: 0] += 0 
}

print(playerScore)

To keep a count of the players names (eg. duplicate names):

counts[myArray, default: 0] += 1

So for example if Ron had two entries before the game started (score > 0) then you would know.

Without using reduce(into:_:) method and as an extension:

var playerNames = ["Harry", "Ron", "Hermione"]

extension Sequence where Self.Iterator.Element: Hashable {

func freq() -> [Self.Iterator.Element: Int] {
    return reduce([:]) {
            ( iter: [Self.Iterator.Element: Int], element) in
            var dict = iter
            dict[element] = 0
            return dict
        }
    }
}

print(playerNames.freq())
// ["Harry": 0, "Hermione": 0, "Ron": 0]

keep a count (eg. duplicate names):

dict[element, default: -1 ] += 1

Answer 2

You can use reduce(into:) as you suspected. You simply need to declare the initial value as [String:Int]() to be an empty Dictionary of the required type, then simply set the value of all keys in playerNames to 0.

var playerNames = ["Harry", "Ron", "Hermione"]
var scoreBoard = playerNames.reduce(into: [String:Int](), { currentScores,player in
    currentScores[player] = 0
})

Answer 3

Here is how you can do that:

var playerNames = ["Harry", "Ron", "Hermione"]

var dictionary = [String: Int]()
for player in playerNames {
    dictionary[player] = 0
}

Answer 4

reduce is definitely one of the more difficult builtin functions to use correctly, but it is what you want here.

let names = ["Harry", "Ron", "Hermione"]
let scoreboard: [String: Int] = names.reduce(into: [:], { result, next in
    result[next] = 0
})

It takes 2 parameters: the initial value (in our case, an empty dictionary [:]), and a closure that updates the result with each element in the array. This closure has 2 parameters, result and next. We want to update result based on the next element. Our closure does this by setting result[next] to 0.

Answer 5

If the player names are known to be all different then you can do

let playerNames = ["Harry", "Ron", "Hermione", "Ron"]

var scoreBoard = Dictionary(uniqueKeysWithValues: zip(playerNames,
                                                      repeatElement(0, count: playerNames.count)))

print(scoreBoard) // ["Harry": 0, "Ron": 0, "Hermione": 0]

Here zip is used to create a sequence of player/score pairs, from which the dictionary is created.

Remark: Originally I had used AnySequence { 0 } to generate the zeros. Using repeatElement() instead was suggested by Alexander and has the advantage that the correct required capacity is passed to the dictionary intializer.

Answer 6

Here's a quick one liner that I like to use:

let scoreboard = playerNames.reduce(into: [String: Int]()) { $0[$1] = 0 }