Swift: Generate a set of (X) random 6 character substrings - arrays

Question

I am trying to generate 5 random substrings of six characters each from the alphabet. For example: ABCDEF, RSTUVW, UVWXYZ, etc. These substrings can be duplicates, so generating ABCDEF twice is a not a problem.
When I have these 5 substrings, I want to generate five arrays containing three characters. One of these characters should be the last letter of the substring and the other two letters should be two random unique letters from the entire alphabet.
Example:
Get five random substrings:
[ABCDEF], [RSTUVW], [CDEFGH], [LMNOPQ], [UVWXYZ]
For [ABCDEF] the system could generate [F, H, S] and for [RSTUVW] it could generate [K, Q, W]. As you can see, the three-character arrays always contain the last letter of its substring and two other randomised unique letters.
The above is part of a game for kids to practice the order of the alphabet. In order to generate possible answers I actually need small sets of characters to assign to buttons.
What do you think is the best way to approach this?

Answer 1

Thanks to #vacawama here's a possible solution.
1. Create the alphabet
let alphabet = Array("ABCDEFGHIJKLMNOPQRSTUVWXYZ".characters)
2. Create the 6 "sequences"
let sequences = (0..<5).map { _ -> String in
let startIndex = Int(arc4random_uniform(UInt32(alphabet.count - 5)))
let endIndex = startIndex + 5
return String(alphabet[startIndex...endIndex])
}
["PQRSTU", "DEFGHI", "JKLMNO", "CDEFGH", "KLMNOP"]
3. Get the last char for each sequence
let lastChars = sequences.flatMap { $0.characters.last }
["U", "I", "O", "H", "P"]
4. Build 5 elms
Here's the code snippet
let elms = lastChars.map { char0 -> String in
var tempAlphabet = alphabet
tempAlphabet.removeAtIndex(tempAlphabet.indexOf(char0)!)
let index1 = Int(arc4random_uniform(UInt32(tempAlphabet.count)))
let char1 = tempAlphabet.removeAtIndex(index1)
let index2 = Int(arc4random_uniform(UInt32(tempAlphabet.count)))
let char2 = tempAlphabet[index2]
return String(Array(Set<Character>([char0, char1, char2])))
}
["DUN", "ZIQ", "ROP", "HSW", "PGS"]
Update
Here's another solution to fix the problem hightligted by #dfri in the comments below. The following code snipped could replace the previous bullet 4.
extension Array {
mutating func removeRandom() -> Element {
let index = Int(arc4random_uniform(UInt32(count)))
return removeAtIndex(index)
}
}
var availableChars = Array(Set(alphabet).subtract(lastChars))
let elms = lastChars.map { String([$0, availableChars.removeRandom(), availableChars.removeRandom()]) }

Answer 2

Random substrings:
func randomString(length: Int) -> String {
let charactersString = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
let n = UInt32(charactersString.characters.count)
var out = ""
for _ in 0..<length {
let index = charactersString.startIndex.advancedBy(Int(arc4random_uniform(n)))
out.append(charactersString[index])
}
return out
}
Get 5 Strings:
func createSubstrings() -> [String] {
var array = [String]()
for _ in 0...5 {
array.append(self.randomString(6))
}
return array
}
Last array:
func createFinalStrings() -> [String] {
let substrings = createSubstrings()
var finalStrings = [String]()
let lastChars = substrings.flatMap { $0.characters.last }
for _ in 0...5 {
var string = ""
while string.characters.count < 3 {
let index = Int(arc4random_uniform(5))
let lastChar = lastChars[index]
if string.rangeOfString(String(lastChar)) == nil{
string = string + String(lastChar)
}
}
finalStrings.append(string)
}
return finalStrings
}