You can use this syntax of zeros() to preallocate the data as a complex array -https://in.mathworks.com/help/matlab/ref/zeros.html#d126e1907954
N = 1e5;
%syntax
x = zeros(1, N, 'like', 1i);
for k = 2:N
Real = k*5;
Img = k*8;
x(k) = complex(Real, Img);
end
x(2:5)
