...
substruct1.a1 = 15.2;
substruct1.a2 = int8([1 2;3 4]);

mystruct = struct('ele1',20.5,'ele2',single(100),
                  'ele3',substruct1);

substruct2 = mystruct;
substruct2.ele3.a2 = 2*(substruct1.a2);
...

...
y = X(1).a % Extracts the value of field a 
           % of the first structure in array X
...

...
s.a = 0;
s.b = 0;
X = repmat(s,1,5); 
for i = 1:5   
    X(i).a = i;   
    X(i).b = i+1; 
end