How do I create dictionary from array of tuples?

Question

Let’s say I have array of objects that can be identified and I want to create dictionary from it. I can easily get tuples from my array like so:

let tuples = myArray.map { return ($0.id, $0) }

But I can’t see initializer for dictionary to take array of tuples. Am I missing something? Do I have create extension for dictionary for this functionality (in fact it’s not hard but I thought it would be supplied by default) or there is easier way to do that?

  There is code for extension

extension Dictionary
{
    public init (_ arrayOfTuples : Array<(Key, Value)>)
    {
        self.init(minimumCapacity: arrayOfTuples.count)

        for tuple in arrayOfTuples
        {
            self[tuple.0] = tuple.1
        }
    }
}

Why should it be by default? There's no general way to map array of tuples to the Dictionary. In Dictionary each key must be unique, in array of tuples any number of repeating `keys` is allowed. What to do with them? Rewrite? Ignore? — user28434'mstep, Dec 13 '16 at 14:34
Yes, @user28434 is right. Btw have a look at this http://stackoverflow.com/a/31447400/4272498 — Irfan, Dec 13 '16 at 14:35
Yeah, You're right @user28434 , I wasn't thinking about that. — Adamsor, Dec 13 '16 at 14:43
Why are you creating an array of tuples in the first place? You can created the dictionary with the data in myArray directly instead. — gnasher729, Apr 25 '17 at 12:37

score 64 · Accepted Answer · answered Jul 03 '18 at 13:40

64

Swift 4

If your tuples is (Hashable, String) you can use:

let array = [("key1", "value1"), ("key2", "value2"), ("key3", "value3")]
let dictionary = array.reduce(into: [:]) { $0[$1.0] = $1.1 }
print(dictionary) // ["key1": "value1", "key2": "value2", "key3": "value3"]

answered Jul 03 '18 at 13:40

Serhii Didanov

2,200
1
16
31

3

I don't know why this was downvoted as this is the most appropriate approach to solve the question OP asked. – Ilias Karim Jul 12 '18 at 18:31
1

I ❤️ this answer! – Ashley Mills Sep 25 '18 at 09:54
This solution does not work by-default. `$0` as accumulative dictionary comes immutable on each iteration of `reduce` function so you need a kind of mutable new value to handle that. – hamsternik May 04 '22 at 18:32

dimpiax · Answer 2 · 2020-05-08T21:26:14.450

Swift 4

For creation you can use native Dictionary's init functions:

Dictionary(uniqueKeysWithValues: [("a", 0), ("b", 1)]) 
// ["b": 1, "a": 0]

Dictionary(uniqueKeysWithValues: [("a", 0), ("b", 1), ("b", 2)])
// Fatal error: Duplicate values for key: 'b'

// takes the first match
Dictionary([("a", 0), ("b", 1), ("a", 2)], uniquingKeysWith: { old, _ in old })
// ["b": 1, "a": 0]

// takes the latest match
Dictionary([("a", 0), ("b", 1), ("a", 2)], uniquingKeysWith: { $1 }) 
// ["b": 1, "a": 2]

Also if you want to have the shortcut:

Dictionary([("a", 0), ("b", 1), ("a", 2)]) { $1 } 
// ["b": 1, "a": 2]

Daniel · Answer 3 · 2016-12-13T20:32:23.630

Depending on what you want to do, you could:

let tuples = [(0, "0"), (1, "1"), (1, "2")]
var dictionary = [Int: String]()

Option 1: replace existing keys

tuples.forEach {
    dictionary[$0.0] = $0.1
}    
print(dictionary) //prints [0: "0", 1: "2"]

Option 2: Don't allow repeting keys

enum Errors: Error {
    case DuplicatedKeyError
}

do {
    try tuples.forEach {
        guard dictionary.updateValue($0.1, forKey:$0.0) == nil else { throw Errors.DuplicatedKeyError }
    }
    print(dictionary)
} catch {
    print("Error") // prints Error
}

Sentry.co · Answer 4 · 2017-09-19T20:26:39.390

6

A generic approach:

/**
 * Converts tuples to dict
 */
func dict<K,V>(_ tuples:[(K,V)])->[K:V]{
    var dict:[K:V] = [K:V]()
    tuples.forEach {dict[$0.0] = $0.1}
    return dict
}

Functional programming update:

func dict<K,V>(tuples:[(K,V)])->[K:V]{
    return tuples.reduce([:]) {
       var dict:[K:V] = $0
       dict[$1.0] = $1.1   
       return dict
    }
}

edited Sep 19 '17 at 20:26

answered Apr 25 '17 at 11:59

Sentry.co

5,355
43
38

1

@hasen Thats debatable. Since you pass the tuples as an argument it definitely shouldn't be just func. Making it static explicitly says that it can't change the state of the class it resides in – Sentry.co Sep 17 '17 at 11:26
1

It doesn't need to reside in a class. It's just a function. Swift is not Java. – hasen Sep 19 '17 at 01:07
1

@hasen I guess it's Opinionated I'll change it to func for the sake of brevity. Thanks for your input. – Sentry.co Sep 19 '17 at 20:26
I put this function as a Dictionary extension init. – Daniel T. Oct 06 '17 at 13:02
4

@DanielT. There is already a native one: init(uniqueKeysWithValues:) (swift 4) – Sentry.co Oct 09 '17 at 18:30

score 3 · Answer 5 · answered Jan 05 '18 at 13:17

Improved @GitSync answer using an extension.

extension Array {
    func toDictionary<K,V>() -> [K:V] where Iterator.Element == (K,V) {
        return self.reduce([:]) {
            var dict:[K:V] = $0
            dict[$1.0] = $1.1
            return dict
        }
    }
}

score 1 · Answer 6 · answered Nov 21 '19 at 11:03

1

Update to @Stefan answer's.

extension Array {

    func toDictionary<K, V>() -> [K: V] where Iterator.Element == (K, V) {
        return Dictionary(uniqueKeysWithValues: self)
    }
}

answered Nov 21 '19 at 11:03

matteo2191

39
3

How do I create dictionary from array of tuples?

6 Answers6

Swift 4

A generic approach:

Linked