Generating Random Numbers of a certain pattern in Swift

I know that Swift can generate random numbers using this code : arc4random_uniform(50). So how can I generate random numbers in a pattern, for example : The computer should generate random numbers in the order 2,8,9,15,16,22... like any number x,x+6,x+6+1,x+6+1+6.... like that. Please help soon..! It would be a great help !

via Chebli Mohamed