ArrayPad

ArrayPad[array, m] gives an array with m zeros of padding on every side.

ArrayPad[array, m, padding] uses the specified padding.

ArrayPad[array, {m, n}] pads with m elements at the beginning and n at the end.

Examples

Pad a list with zeros:

ArrayPad[{1, 2, 3}, 2]
(* {0, 0, 1, 2, 3, 0, 0} *)

Pad with a specific value:

ArrayPad[{1, 2, 3}, 1, x]
(* {x, 1, 2, 3, x} *)

Asymmetric padding:

ArrayPad[{1, 2, 3}, {1, 3}]
(* {0, 1, 2, 3, 0, 0, 0} *)

Please visit the official Wolfram Language Reference for more details.