u = 
Dynamically defining variables is not a good idea, as you have done here -
syms u10 u9 u8 u7 u6 u5 u4 u3 u2 u1;
syms x10 x9 x8 x7 x6 x5 x4 x3 x2 x1;
syms y10 y9 y8 y7 y6 y5 y4 y3 y2 y1;
and here -
u=[];x=[];y=[];
for j=1:1:t+1
u = [u; sym(['u' num2str(j)])];
x = [x; sym(['x' num2str(j)])];
y = [y; sym(['y' num2str(j)])];
end
A better method is to define a symbolic array like this, so that you can define all of them in a single command (as I have done below) -
syms u [1 10]
u
And use indexing to access the individual variables -
u(1)
%Define them collectively in a single command
syms u x y [1 10]
As for the error occuring in the 2nd for loop, it is not clear to me what the objective is or what you want to do, thus I can not suggest anything other than correcting the size of preallocated variable.
