vee41 & ArcadEd thanks for the hint. But i have found the problem. Instead of Push back, i should put it at front.
It seems this is what happen when i use "push back"
Array ["","","","","element","element","element"]
So when a randomNumber generates the number 0 - 4 it will returns 0 by default because there is nothing there.
If i use "push front" this is what happen
Array ["element, "element", "element", ....]
Thanks for the hint though.