Why do you want to do this by the evil eval? See: TUTORIAL: how and why to avoid Eval . Beginners tend to try to solve problems by eval() not knowing, that this causes more problems than it solves.
But the error message has another reason: If Data is not a struct, you cannot append a field.
Data = 0;
Data.Set = 5
The error is fuxed, if you define Data as a struct before. But the main question remains, why you want to do this by eval?
Data{50}.Set{2} = [15.5 10.2; 0.01 2.0];
This is perfect already.
