Swift 3: Array to Dictionary? - arrays

Question

I have a large array and need to access it by a key (a lookup) so I need to create Dictionary. Is there a built in function in Swift 3.0 to do so, or do I need to write it myself?
First I will need it for a class with key "String" and later on maybe I will be able to write a template version for general purpose (all types of data and key).
Note for 2019. This is now simply built-in to Swift 5, uniqueKeysWithValues and similar calls.

Answer 1

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)] // or `let pairsWithDuplicateKeys = array.map{ ($0.key, $0) }`
let firstValues = Dictionary(pairsWithDuplicateKeys, uniquingKeysWith: { (first, _) in first })
print(firstValues)
//prints ["a": 1, "b": 2]
let lastValues = Dictionary(pairsWithDuplicateKeys, uniquingKeysWith: { (_, last) in last })
print(lastValues)
//prints ["a": 3, "b": 4]

Answer 2

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.)

Answer 3

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.

Answer 4

As others already said, we need to understand which are the keys.
However I am trying to provide a solution to my interpretation of your question.
struct User {
let id: String
let firstName: String
let lastName: String
}
Here I am assuming that 2 users with the same id cannot exist
let users: [User] = ...
let dict = users.reduce([String:User]()) { (result, user) -> [String:User] in
var result = result
result[user.id] = user
return result
}
Now dict is a dictionary where the key is the user id and the value is the user value.
To access a user via its id you can now simply write
let user = dict["123"]
Update #1: General approach
Given an array of a given type Element, and a closure that determine the key of an Element, the following generic function will generate a Dictionary of type [Key:Element]
func createIndex<Key, Element>(elms:[Element], extractKey:(Element) -> Key) -> [Key:Element] where Key : Hashable {
return elms.reduce([Key:Element]()) { (dict, elm) -> [Key:Element] in
var dict = dict
dict[extractKey(elm)] = elm
return dict
}
}
Example
let users: [User] = [
User(id: "a0", firstName: "a1", lastName: "a2"),
User(id: "b0", firstName: "b1", lastName: "b2"),
User(id: "c0", firstName: "c1", lastName: "c2")
]
let dict = createIndex(elms: users) { $0.id }
// ["b0": {id "b0", firstName "b1", lastName "b2"}, "c0": {id "c0", firstName "c1", lastName "c2"}, "a0": {id "a0", firstName "a1", lastName "a2"}]
Update #2
As noted by Martin R the reduce will create a new dictionary for each iteration of the related closure. This could lead to huge memory consumption.
Here's another version of the createIndex function where the space requirement is O(n) where n is the length of elms.
func createIndex<Key, Element>(elms:[Element], extractKey:(Element) -> Key) -> [Key:Element] where Key : Hashable {
var dict = [Key:Element]()
for elm in elms {
dict[extractKey(elm)] = elm
}
return dict
}

Answer 5

let pills = ["12", "34", "45", "67"]
let kk = Dictionary(uniqueKeysWithValues: pills.map{ ($0, "number") })
["12": "number", "67": "number", "34": "number", "45": "number"]
swift5 swift4

Answer 6

The following converts an array to a dictionary.
let firstArray = [2,3,4,5,5]
let dict = Dictionary(firstArray.map { ($0, 1) } , uniquingKeysWith: +)

Answer 7

Swift 5
extension Array {
func toDictionary() -> [Int: Element] {
self.enumerated().reduce(into: [Int: Element]()) { $0[$1.offset] = $1.element }
}
}

Answer 8

This extension works for all sequences (including arrays) and lets you select both key and value:
extension Sequence {
public func toDictionary<K: Hashable, V>(_ selector: (Iterator.Element) throws -> (K, V)?) rethrows -> [K: V] {
var dict = [K: V]()
for element in self {
if let (key, value) = try selector(element) {
dict[key] = value
}
}
return dict
}
}
Example:
let nameLookup = persons.toDictionary{($0.name, $0)}

Answer 9

Just do it simply,
let items = URLComponents(string: "https://im.qq.com?q=13&id=23")!.queryItems!
var dic = [String: Any?]()
items.foreach {
dic[$0.name] = $0.value
}
reduce is not very suitable,
let dic: [String: Any?] = items.reduce([:]) { (result: [String: Any?], item: URLQueryItem) -> [String: Any?] in
var r = result
r[item.name] = item.value // will create an copy of result!!!!!!
return r
}

Answer 10

As i understand from you're question you would like to convert to Array to Dictionary.
In my case i create extension for the Array and keys for the dictionary will be indexes of the Array.
Example:
var intArray = [2, 3, 5, 3, 2, 1]
extension Array where Element: Any {
var toDictionary: [Int:Element] {
var dictionary: [Int:Element] = [:]
for (index, element) in enumerate() {
dictionary[index] = element
}
return dictionary
}
}
let dic = intArray.toDictionary

Answer 11

Compatible with Swift 5 Standard Library (Xcode 10.2+ , iOS 12.2).
Here's an example of usage of an initializer init(uniqueKeysWithValues:)
The input let array: [String] = Locale.isoRegionCodes is an array of ISO31661-2 codes represented by a string.
let countryCodeAndName: [String: String] = Dictionary(uniqueKeysWithValues: Locale.isoRegionCodes.map { ($0, Locale.current.localizedString(forRegionCode: $0) ?? "")} )
Returned dictionary, will list all regions with ISO31661-2 code as a key and a localized region name as a value.
Output:
...
"PL":"Poland"
"DE":"Germany"
"FR":"France"
"ES":"Spain"
...
Example 2:
let dictionary: [String: String] = Dictionary(uniqueKeysWithValues: [ ("key1", "value1"), ("key2", "value2")] )
Output:
["key1": "value1", "key2": "value2"]
Important:
Precondition: The sequence must not have duplicate keys.
Code below will crash an app:
let digitWords = ["one", "two", "three", "four", "five", "five"]
let wordToValue = Dictionary(uniqueKeysWithValues: zip(digitWords, 1...6))
with:
Fatal error: Duplicate values for key: 'five'

Answer 12

If you want to follow the pattern set out by map and reduce in swift you could do something nice and functional like this:
extension Array {
func keyBy<Key: Hashable>(_ keyFor: (Element) -> Key) -> [Key: Element] {
var ret = [Key: Element]()
for item in self{
ret[keyFor(item)] = item
}
return ret
}
}
Usage:
struct Dog {
let id: Int
}
let dogs = [Dog(id: 1), Dog(id: 2), Dog(id: 3), Dog(id: 4)]
let dogsById = dogs.keyBy({ $0.id })
// [4: Dog(id: 4), 1: Dog(id: 1), 3: Dog(id: 3), 2: Dog(id: 2)]

Answer 13

Swift way:
extension Sequence {
func toDictionary<Key: Hashable>(with selectKey: (Element) -> Key) -> [Key: Element] {
reduce(into: [:]) { $0[selectKey($1)] = $1 }
}
}
// let arr = [Person(id: 1, name: "Alan")]
// arr.toDictionary { $0.id }
// ==
// [1: Person(id: 1, name: "Alan")]