In
a = zeros(1,3);
a = [ 1 2 3 ];
Matlab reserves memory for 3 doubles and fills then with zeros in the first line. In the second line this memory is freed and a new vector is created. This needs about the double work compared to creating [1, 2, 3] directly and in consequence is a waste of time only.
This is not a pre-allocation. A pre-allocation would mean, that the reserved memory is re-used later:
a = zeros(1, 3);
a(1) = 1;
a(2) = 2;
a(3) = 3;
or
a = zeros(1, 3);
a(:) = 1:3;
The "(:)" is essential here, because it tells Matlab to overwrite the contents of the formerly existing memory.
