Strange return value after generating random strin

Question

I've created a method which generates and returns a random string of both letters and numbers, but for some reason I only get a string with numbers and the length of the string doesn't come close to what I asked it to be. A few examples of strings that have been returned: "478388299949939566" (inserted 18 as the length), "3772919388584334" (inserted 9 as the length), "2293010089409293945" (inserted 6 as the length). Anyone can see what I'm missing here?

func generateRandomStringWithLength(length:Int) -> String {

    let randomString:NSMutableString = NSMutableString(capacity: length)

    let letters:NSMutableString = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"

    for index in 0...length {

        let randomIndex:Int = Int(arc4random_uniform(UInt32(62)))
        randomString.append("\(letters.character(at: randomIndex))")

    }

    return String(randomString)
}

Answer 1

Your problem is here:

letters.character(at: randomIndex)

it's function returns the character at a given UTF-16 code unit index, not not just a character

Here is my version, I guess its more swiftly.

func generateRandomStringWithLength(length: Int) -> String {

    var randomString = ""
    let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"

    for _ in 1...length {
        let randomIndex  = Int(arc4random_uniform(UInt32(letters.characters.count)))
        let a = letters.index(letters.startIndex, offsetBy: randomIndex)
        randomString +=  String(letters[a])
    }

    return randomString
}

generateRandomStringWithLength(length: 5)

Answer 2

Use this:

func generateRandomStringWithLength(length:Int) -> String {

        let randomString:NSMutableString = NSMutableString(capacity: length)

        let letters:NSMutableString = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"

        var i: Int = 0

        while i < length {

            let randomIndex:Int = Int(arc4random_uniform(UInt32(letters.length)))
            randomString.appendString("\(Character( UnicodeScalar( letters.characterAtIndex(randomIndex))))")
            i += 1
        }

        return String(randomString)
    }

Calling generateRandomStringWithLength method:

print(generateRandomStringWithLength(5))

print(generateRandomStringWithLength(10))

print(generateRandomStringWithLength(20))

print(generateRandomStringWithLength(7))

print(generateRandomStringWithLength(14))

Sample Output:

GIrqb
nWmieQRVdk
r0It9V1xkGFRa2HVwtCw
RLIRuVI
nXnFGV2LQ3CjbD