Random Alphanumeric String Linux Swift 3

Question

Im having 2 problems when trying to generate a random string in Linux with Swift 3.

1. arc4random_uniform is not available in Linux only on BSD. SO i was able

Answer 1

Swift 4.2, Ubuntu 16.04

let letters : String = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
let len = letters.count
var randomString:String = ""
for _ in 0 ..< length {
   let rand = Int.random(in: 0..<len)
   randomString += letters.map { String($0) }[rand]
}

Answer 2

I copied and pasted your code exactly, and it doesn't compile.

fatal error: Can't form a Character from an empty String

Here's an alternative method:

// Keep at top of your code (outside of functions)
#if os(Linux)
    srandom(UInt32(time(nil)))
#endif


func getRandomNumber(_ min: Int, _ max: Int) -> Int {
    #if os(Linux)
        return Int(random() % max) + min
    #else
        return Int(arc4random_uniform(UInt32(max)) + UInt32(min))
    #endif
}


func getRandomString(_ chars: String, _ length: Int) -> String {
    var str = ""

    for _ in 1...length {
        str.append(chars.itemOnStartIndex(advancedBy: getRandomNumber(0, chars.count - 1)))
    }

    return str
}


// Best practice to define this outside of the function itself
let chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
print(getRandomString(chars, 10))

This works for me on Ubuntu.

Answer 3

1) Always the same number

You have to set a seed once to get "random" numbers from random():

randomSeed(Int(Date().timeIntervalSince1970)

Man page:

If no seed value is provided, the random() function is automatically seeded with a value of 1.

As the seed is always the same (1), you always get the same sequence of "random" numbers.

2) Alphanumeric string

To create your string without using NSString:

func randomString(length: Int) -> String {

    let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    let len = UInt32(letters.characters.count)

    var randomString = ""

    for _ in 0 ..< length {
        let rand = myCustomRandom(len)
        let randIndex = letters.index(letters.startIndex, offsetBy: Int(rand))
        let nextChar = letters[randIndex]
        randomString += String(nextChar)
    }

    return randomString
}

Answer 4

Figured it out.

So the answer to the repeating random number/string was to just add this line before i called the random() function

srand(UInt32(time(nil)))

and I'm assuming thats what fixed the illegal instruction also. Because i don't recall changing anything else.

Needless to say here is my final result

 func generateRandomNumber() -> Int
 {
    var place = 1

    var finalNumber = 0;

    #if os(Linux)

    srand(UInt32(time(nil)))

    for _ in 0..<5
    {
        place *= 10

        let randomNumber = Int(random() % 10) + 1

        finalNumber += randomNumber * place
    }
    #else
    for _ in 0..<5
    {
        place *= 10

        let randomNumber = Int(arc4random_uniform(10))

        finalNumber += randomNumber * place
    }
    #endif

     return finalNumber
 }



 func randomString(_ length: Int) -> String
 {

    let letters : String = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    let len = letters.characters.count

    var randomString = ""

    #if os(Linux)

    srand(UInt32(time(nil)))

   for _ in 0..<length
   {
     let randomValue = (random() % len) + 1

     randomString += "\(letters[letters.index(letters.startIndex, offsetBy: Int(randomValue))])"
   }

   #else
  for _ in 0 ..< length
  {
     let rand = arc4random_uniform(UInt32(len))

     randomString += "\(letters[letters.index(letters.startIndex, offsetBy: Int(rand))])"
  }
  #endif

   return randomString
 }