I created a playground in Xcode 6.3 (6D570) and input these following code:
import UIKit var randum_num = arc4random_uniform(13) + 1 String(format: \"card%i\
You just have to omit "arguments:". Try like this:
let randum_num = arc4random_uniform(13) + 1 String(format: "card%i", randum_num)
or simply
String(format: "card%i", arc4random_uniform(13) + 1)