specify coefficients in pdeeig

I'm working on finding the eigenvalues of a PDE, I'm using: [v,l] = pdeeig(b,p,e,t,1.0,'c*exp(-x)',1.0,[-5 0.9]); the sixth entry related to the potential ? given in the formula of the pde in the Eigenmode.. I would like to add a loop to this code to plot some certain eigenvalues for specific values of the constant c in the pde..

my attempt is

>

for ?=0.1:0.2:1c=0.1:0.2:1

.

.

[v,l] = pdeeig(b,p,e,t,1.0,'c*exp(-x)',1.0,[-5 0.9]);

plot(real(l),c,'*')

hold on

end

'''

the code works when I enter the value of ? manually and without the loop e.g., [v,l] = pdeeig(b,p,e,t,1.0,'0.5*exp(-x)',1.0,[-5 0.9]); but it says there is an error when I treat ? as a symbol with different values in a loop as I mentioned in the code.. Is there any idea how I can fix this .. appreciate any help..