Unable to convert expression containing symbolic variables into double array error

I am trying to define a vector xdd in terms of another vector x but when I define elements of the vector xdd in terms of the corresponding elements of x I get the error

Unable to convert expression containing symbolic variables into double array. Apply 'subs' function first

to substitute values for variables.

Can someone please suggest a way to solve this problem?

syms x1 x2 s t
x = [x1; x2];  
xdd = [];
for i = 1:length(x0)
    xdd(i) = s^2*x(i) + 2*sin(t)   
end 

Answer 1

[Star Strider]

Create ‘xdd’ as a cell array instead:

xdd{i} = s^2*x(i) + 2*sin(t)   

That works.

[Aleem Andrew]

Thank you for your answer

[Star Strider]

As always, my pleasure!