ios - How to create 2-dimensional array with nil elements -

in chess game use 2-dimensional array track positions of pieces @ chess board.

initially thought create nsmuteablearray , indicate unoccupied squares nil. occupied slots should hold pointer piece object...

however following code:

nsmutablearray* _board; ...  _board = [[nsmutablearray alloc] init]; (int = 0; < 8; i++) {     nsmutablearray *row = [[nsmutablearray alloc] init];     (int j = 0; j < 8; j++) {         [row addobject:nil];     }     [_board addobject:row]; } 

fails runtime error:

*** terminating app due uncaught exception 'nsinvalidargumentexception', reason: '*** -[__nsarraym insertobject:atindex:]: object cannot nil' 

so nil can't passed argument addobject... here instead?

try

nsmutablearray* _board; ...  _board = [[nsmutablearray alloc] init]; (int = 0; < 8; i++) {     nsmutablearray *row = [[nsmutablearray alloc] init];     (int j = 0; j < 8; j++) {         [row addobject:[nsnull null]];     }     [_board addobject:row]; } 

reason using [nsnull null] instead of nil can seen here