Euler integration for an ode

I have the following system:

m(dx2/dt2)=-C(dx/dt)-kx+mgsin(phi)+u

du/dt=-au+mu

with tracking error=e=xd-x.

I have assigned 100 random values to xd,x,m,C,k,phi (within -pi and pi). I am trying to use Euler integration to plot e(t) for the 100 random variable assignments and my code is posted below. I am getting a weird output for my plot and I don't know if its a problem with my code or me not using the euler integration formula correctly. I am expecting the plot of r(t) to exponentially decay to zero based on my analysis of the system. Any help on this issue is greatly appreciated.

low=0;

high=5;

m=(high-low).*rand(100,1) + low;

c=(high-low).*rand(100,1) + low;

k=(high-low).*rand(100,1) + low;

a=(high-low).*rand(100,1) + low;

alpha=(high-low).*rand(100,1) + low;

lowphi=-1*pi;

highphi=pi;

phi=(highphi-lowphi).*rand(100,1) + lowphi;

xd=(high-low).*rand(100,1) + low;

x=(high-low).*rand(100,1) + low;

dt=0.1;

Xxd=xd(1):dt:xd(100);

e=zeros(size(Xxd));

e(1)=xd(1)-x(1);

n=numel(e);

for i=1:n-1

e(i+1)=e(i)+dt*(xd(i)-x(i));

plot(e)

xlabel('Time')

ylabel('x1(t)')

hold on

end

hold off