Generating an array of keyed arrays: More efficient way? - ios

Question

I want to sort an array of performers so that they are grouped by the first character of their first name. So for example 'A' in the following output, is for a collection of performers who's first name starts with 'A'.
[
"A"[Performer,Performer,Performer,Performer]
"B"[Performer,Performer,Performer]
"C"[Performer,Performer,Performer]
"D"[Performer,Performer,Performer]
"F"[Performer,Performer,Performer]
"M"[Performer,Performer,Performer]
... etc
]
I have achieved it but I'm hoping there is a more trivial way to do it. The following is how I achieved it.
class Performer {
let firstName: String
let lastName: String
let dateOfBirth: NSDate
init(firstName: String, lastName: String, dateOfBirth: NSDate) {
self.firstName = firstName
self.lastName = lastName
self.dateOfBirth = dateOfBirth
}
}
private var keyedPerformers: [[String: [Performer]]]?
init() {
super.init()
let performers = generatePerformers()
let sortedPerformers = performers.sort { $0.0.firstName < $0.1.firstName }
keyedPerformers = generateKeyedPerformers(sortedPerformers)
}
//'sortedPerformers' param must be sorted alphabetically by first name
private func generateKeyedPerformers(sortedPerformers: [Performer]) -> [[String: [Performer]]] {
var collectionKeyedPerformers = [[String: [Performer]]]()
var keyedPerformers = [String: [Performer]]()
var lastLetter: String?
var collection = [Performer]()
for performer in sortedPerformers {
let letter = String(performer.firstName.characters.first!)
if lastLetter == nil {
lastLetter = letter
}
if letter == lastLetter {
collection.append(performer)
} else {
keyedPerformers[lastLetter!] = collection
collectionKeyedPerformers.append(keyedPerformers)
keyedPerformers = [String: [Performer]]()
collection = [Performer]()
}
lastLetter = letter
}
return collectionKeyedPerformers.sort { $0.0.keys.first! < $0.1.keys.first! }
}

Answer 1

First of all, since you have an (sorted) array of section index titles it's not necessary to use an array of dictionaries. The most efficient way is to retrieve the array of Performers from the dictionary by key.
Add a lazy instantiated variable initialLetter in the Performer class. I added also the description property to get a more descriptive string representation.
class Performer : CustomStringConvertible{
let firstName: String
let lastName: String
let dateOfBirth: NSDate
init(firstName: String, lastName: String, dateOfBirth: NSDate) {
self.firstName = firstName
self.lastName = lastName
self.dateOfBirth = dateOfBirth
}
lazy var initialLetter : String = {
return self.firstName.substringToIndex(self.firstName.startIndex.successor())
}()
var description : String {
return "\(firstName) \(lastName)"
}
To create the performers use an array rather than a simple string. The "missing" letters are already omitted.
private let createIndexTitles = ["A","B","C","D","E","F","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z"]
The function to create the dummy instances pre-sorts the arrays of performers by initialLetter and then by lastName
private func generatePerformers() -> [Performer] {
var performers = [Performer]()
for i in 0...99 {
let x = i % createIndexTitles.count
let letter = createIndexTitles[x]
performers.append(Performer(firstName: "\(letter)", lastName: "Doe", dateOfBirth: NSDate()))
}
return performers.sort({ (p1, p2) -> Bool in
if p1.initialLetter < p2.initialLetter { return true }
return p1.lastName < p2.lastName
})
}
Now create an empty array of strings for the section title indexes
private var sectionIndexTitles = [String]()
The function to create the keyed performers follows a simple algorithm : If the key for initialLetter doesn't exist, create it. Then append the performer to the array. At the end assign the sorted keys of the dictionary to sectionIndexTitles, the elements of the keyed arrays are already sorted.
private func generateKeyedPerformers(sortedPerformers: [Performer]) -> [String: [Performer]] {
var keyedPerformers = [String: [Performer]]()
for performer in sortedPerformers {
let letter = performer.initialLetter
var keyedPerformer = keyedPerformers[letter]
if keyedPerformer == nil {
keyedPerformer = [Performer]()
}
keyedPerformer!.append(performer)
keyedPerformers[letter] = keyedPerformer!
}
sectionIndexTitles = Array(keyedPerformers.keys).sort { $0 < $1 }
return keyedPerformers
}
Now test it
let performers = generatePerformers()
let keyedPerformers = generateKeyedPerformers(performers)
print(sectionIndexTitles , keyedPerformers)
In the (presumably) table view use sectionIndexTitles as the section array and get the performer arrays by key respectively.