Array indices must be positive integers or logical values.

I am trying to represent to functions in matlab app designer but when I run the interface and click the buttom the app send me this mesage:'Array indices must be positive integers or logical values.'. How can I solve? this is my code:

 P = app.P.Value;
           Q = app.Q.Value;
           a = app.a.Value;
           b = app.b.Value;
           Xo = app.Xo.Value;
           Yo =app.Yo.Value;
           K = b.*Xo-a.*Yo;
           C3 =  (log(a.*Yo)-log(Xo))/- K;
           C4 =  (log(b.*Xo)-log(Yo))/- K;
           t = 0:0.2:100; 
           y(t) = K./(exp((K*t-K*C3))-b);
           y (0) = app.Yo.Value;
           plot (app.Axes,x,y3,'r');
           hold (app.Axes,'on')
           x(t) = - K./(exp(K*t-K*C4)-a);
           x(0) = app.Xo.Value;
           plot (app.Axes,x,y4,'b');
            hold (app.Axes,'off')

[Stephen23]

t = 0:0.2:100; 
y(t) = .. % t is not positive integer
y(0) = .. % 0 is not positive integer

[Alvaro Mª Zumalacarregui Delgado]

thanks!