The syntax
(-5 < u1 < 5)
does not do what you think it does. You mean
(-5 < u1) & (u1 < 5).
MATLAB evaluates left to right so that the first part (-5 < u1) is either true or false, 1 or 0. Then you would have either (0 < 5) or (1 < 5), which is always true, and your switch will never change. See also Why is Answer to 3 < A < 7 Unexpected?
(Also, your model is aliasing. You need to set your max step size based on the frequency of your sign wave. Perhaps you have learned how to calculate the Nyquist rate?)
