% Direct indexing:
x = [1 2 3]
x(4) = 4

% Direct indexing other method:
x(end+1) = 4;    % Note that end is a keyword and you do not need a variable called end

% Concatination
x = [x newval]

% Concatination other method
x = [x, newval]

% Concatination column vector
x = [x; newval]