Creates a random alphanumeric string with a defined length.

RandomString.swift

// Swift 2 code

func randomString(length: Int) -> String {
    let charactersString = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    let charactersArray = Array(arrayLiteral: charactersString)
    
    var string = ""
    for _ in 0..<length {
        string += charactersArray[Int(arc4random()) % charactersArray.count]
    }
        
    return string
} 

let charactersArray = Array(arrayLiteral: charactersString) returns an array with a single string element containing the contents of charactersString (i.e ["abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"]

To get an array of characters of charactersString, use Array(charactersString.characters).

Then, when adding the character to the output string, append the character.

func randomString(length: Int) -> String {
    let charactersString = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    let charactersArray : [Character] = Array(charactersString.characters)

    var string = ""
    for _ in 0..<length {
        string.append(charactersArray[Int(arc4random()) % charactersArray.count])
    }

    return string
}