Trouble using width for String in for-in cycle

Question

I've write a simple code:

extension String {
    func trailingSpaces (width: Int) -> String {
        var s = "\(self)"
        for i in count(s)..<width {
            s = s + " "
        }
        return s
    }

    func leadingSpaces (width: Int) -> String {
        var s = "\(self)"
        for i in count(s)..<width {
            s = " " + s
        }
        return s
    }
}

class ViewController: UIViewController {

    var users = ["Marco", "Gianni", "Antonio", "Giulio", "Franco"]
    var ages = [29, 45, 17, 33, 37]

    override func viewDidLoad() {
        super.viewDidLoad()

        var merged = [String: Int] ()
        var totalAge = 0.0

        for var i = 0; i < ages.count; i++ {
            merged[users[i]] = ages[i]
        }

        for user in sorted(merged.keys) {
            let age = merged[user]
            totalAge += Double(age!)
            let paddedUser = user.trailingSpaces(10)
            let paddedAge = "\(age)".leadingSpaces(3)
            println("\(paddedUser) \(age!)")
        }

        println("\n\(merged.count) users")
        println("average age: \(totalAge / Double(merged.count))")
    }
}

but I can't make it work the leadingSpaces function and I can't understand the reason, it's quite identical to the other extension func that works.

It give the error

fatal error: Can't form Range with end < start

on runtime

Answer 1

in case you run into this kind of problem, always do a println() of the variable you are using

println("\(age)") right before let paddedAge = "\(age!)".leadingSpaces(3) reveals the problem

age is an optional, meaning that you are trying to do the padding on a String which has this value "Optional(17)"

Thus, your count(s) is higher than 3, and you have an invalid range