array of structs how to preallocate and assign

I have a struct;

s.a = 1;
s.b=[1 2 3];

and i want a struct array such that:

sa(1).a = 1;
sa(1).b=s.b=[1 2 3];
sa(2).a=3;
sa(2).b=[0 0 0];

and so on. Assignment to sa is done in a loop. How do i preallocate sa? I tried

sa = struct([]);
sa = zeros(1,n);

but none of the above works. If I do not preallocate, matlab warns about speed of the code.

Another question is, how do I assign s to sa(i)? That is how to make the following work:

sa(1) = s;

Answer 1

[Jan]

An easy way is to create the struct array backwards:

sa = struct([]);
for k = 3:-1:1
    sa(k).a = k;
    sa(k).b = rand;
end

Another option:

sb = struct('a', cell(1, 3), 'b', cell(1, 3));
% Now sb is a struct array of size [1, 3] with the empty fields a and b.
sb(1)

ans = struct with fields:
    a: []
    b: []

For the 2nd question: Your code is working, so what exactly are you asking for?

s.a = 1;
s.b = [1 2 3];
sa(1) = s

sa = 1×3 struct array with fields:
    a
    b