Swift 3: Array to Dictionary?

Is that it (in Swift 4)?

let dict = Dictionary(uniqueKeysWithValues: array.map{ ($0.key, $0) })

Note: As mentioned in the comment, using uniqueKeysWithValues would give a fatal error (Fatal error: Duplicate values for key: 'your_key':) if you have duplicated keys.

If you fear that may be your case, then you can use init(_:uniquingKeysWith:) e.g.

let pairsWithDuplicateKeys = [("a", 1), ("b", 2), ("a", 3), ("b", 4)]
let firstValues = Dictionary(pairsWithDuplicateKeys, uniquingKeysWith: { (first, _) in first })
let lastValues = Dictionary(pairsWithDuplicateKeys, uniquingKeysWith: { (_, last) in last })
print(firstValues)

//prints ["a": 1, "b": 2]

print(lastValues)

//prints ["a": 3, "b": 4]

I think you're looking for something like this:

extension Array {
    public func toDictionary<Key: Hashable>(with selectKey: (Element) -> Key) -> [Key:Element] {
        var dict = [Key:Element]()
        for element in self {
            dict[selectKey(element)] = element
        }
        return dict
    }
}

You can now do:

struct Person {
    var name: String
    var surname: String
    var identifier: String
}

let arr = [Person(name: "John", surname: "Doe", identifier: "JOD"),
           Person(name: "Jane", surname: "Doe", identifier: "JAD")]
let dict = arr.toDictionary { $0.identifier }

print(dict) // Result: ["JAD": Person(name: "Jane", surname: "Doe", identifier: "JAD"), "JOD": Person(name: "John", surname: "Doe", identifier: "JOD")]

If you'd like your code to be more general, you could even add this extension on Sequence instead of Array:

extension Sequence {
    public func toDictionary<Key: Hashable>(with selectKey: (Iterator.Element) -> Key) -> [Key:Iterator.Element] {
        var dict: [Key:Iterator.Element] = [:]
        for element in self {
            dict[selectKey(element)] = element
        }
        return dict
    }
}

Do note, that this causes the Sequence to be iterated over and could have side effects in some cases.

On Swift 4, you can achieve this by using Dictionary's grouping:by: initializer

For ex: You have class named A

class A {

    var name: String

    init(name: String) {
        self.name = name
    }
    // .
    // .
    // .
    // other declations and implementions
}

Next, you have an array of objects of type A

let a1 = A(name: "Joy")
let a2 = A(name: "Ben")
let a3 = A(name: "Boy")
let a4 = A(name: "Toy")
let a5 = A(name: "Tim")

let array = [a1, a2, a3, a4, a5]

Let's say you want to create a Dictionary by grouping all the names by their first letter. You use Swifts Dictionary(grouping:by:) to achieve this

let dictionary = Dictionary(grouping: array, by: { $0.name.first! })
// this will give you a dictionary
// ["J": [a1], "B": [a2, a3], "T": [a4, a5]]

source

Note however that the resulting Dictionary "dictionary" is of type

[String : [A]]

it is not of type

[String : A]

as you may expect. (Use #uniqueKeysWithValues to achieve the latter.)

Swift 3: Array to Dictionary?

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