Trying to use Stem() - Error message

Hi, I'm trying to plot an output using Stem(). I solved for my transfer function symbolically, from my expected output in the time domain. I'm not sure how to go back to doing things numerically once I have all of my functions figured out.

I keep getting the errors:

   Error using stem (line 40)
  DOUBLE cannot convert the input expression into a double array.

Error in prog (line 30)
stem(t,y)

Ideally this should spit out a dirac comb with a period of omega. I'm still relatively new to Matlab, so any help would be appreciated! This is also a very rough version of the hopefully final code.

close all; clear all; clc;
syms t omega n A s lambda;

func = dirac(t - n*((2*pi)/omega));
y = symsum(func,n,0,10);
Y = laplace(y);

x = A*cos(omega*t)*exp(-lambda*t);

X = laplace(x);

H = piecewise(omega < 0, ((lambda + s)^2 + omega^2)/(A*(lambda + s)), 0 <= omega, (((lambda + s)^2 +...
    omega^2)*(exp(-(2*pi*s)/omega) + exp(-(4*pi*s)/omega) + exp(-(6*pi*s)/omega) + exp(-(8*pi*s)/omega) +...
    exp(-(10*pi*s)/omega) + exp(-(12*pi*s)/omega) + exp(-(14*pi*s)/omega) + exp(-(16*pi*s)/omega) +...
    exp(-(18*pi*s)/omega) + exp(-(20*pi*s)/omega) + 1))/(A*(lambda + s)));

   h = ilaplace(H);

   y = ilaplace(X*H);

   A = 5;
   omega = 5;
   lambda = 1;
   t = linspace(0,20);
  idx = y == Inf;
  y(idx) = 1;

stem(t,y)
ylim([-.02 1.5])
xlabel('time')
ylabel

Answer 1

[Elizabeth Reese]

You can use the subs function in the Symbolic Math Toolbox to get the numerical values of y and t. Here is the link to the documentation for that function.

For example, take the y from the following line:

 y = ilaplace(X*H);

Then you can evaluate:

 tEval = linspace(0,20);
 yEval = subs(y,{A,omega,lambda,t},{5,5,1,tEval})
 yEval(yEval == Inf) = 1
 stem(tEval,yEval)
 ylim([-.02 1.5])
 xlabel('time')
 ylabel('y')