def null_matrix(nplayers):
    """
    A null n by n matrix for n players

    Parameters
    ----------
    nplayers : integer
        The number of players in the tournament.
    Returns
    -------
    list
        A null n by n matrix where n is the number of players.
    """
    plist = list(range(nplayers))
    return [[0 for j in plist] for i in plist]