PadLeft
PadLeft[list, n] pads a list to length n with zeros on the left. PadLeft[list, n, x] uses element x for padding.
Examples
Pad lists on the left:
PadLeft[{1, 2, 3}, 6]
(* {0, 0, 0, 1, 2, 3} *)
PadLeft[{1, 2, 3}, 6, "x"]
(* {"x", "x", "x", 1, 2, 3} *)
(* Cyclic padding *)
PadLeft[{1, 2}, 7, {a, b, c}]
(* {a, b, c, a, b, 1, 2} *)Please visit the official Wolfram Language Reference for more details.