Simple random strings generator on Swift

randomstring.swift

func randomString(len:Int) -> String {
    let charSet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    var c = Array(charSet)
    var s:String = ""
    for n in (1...10) {
        s.append(c[Int(arc4random()) % c.count])
    }
    return s
}

Hi, your code doesn't work with Swift 2 anymore, also it crashed when giving a length of 0.

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
} 

You need to use .characters.map{ String($0) } to get an array from string in Swift 2.3 and 3.0. Testing in the playground shows this can be very slow and the longer the salt (charSet) the slower it gets. Not sure if it is the most efficient way to do this, but here goes

func randomString(length:Int) -> String {
        let charSet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
        var c = charSet.characters.map { String($0) }
        var s:String = ""
        for _ in (1...length) {
            s.append(c[Int(arc4random()) % c.count])
        }
        return s
    }

This will crash on 32 bit devices on: Int(arc4random())

Swift4+ version

 func randomString(of length: Int) -> String {
     let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
     var s = ""
     for _ in 0 ..< length {
         s.append(letters.randomElement()!)
     }
     return s
}

converting to Array provides 2x performance improvement when generating long strings

e.g. On my Mac (MacBook Pro (16-inch, 2019) 2.3 GHz 8-Core Intel Core i9)
1M random character string generation takes on Average (10 attempts)

• 2 seconds with String based generation
• 1 second with Array based generation

Probably it has to do with the following Complexity note
Complexity: O(1) if the collection conforms to RandomAccessCollection; otherwise, O(n), where n is the length of the collection.
in https://developer.apple.com/documentation/swift/array/randomelement()

public func randomString(of length: Int) -> String {
    let letters = Array("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")
    var s = ""
    for _ in 0 ..< length {
        s.append(letters.randomElement()!)
    }
    return s
}