How can I add rabdom variables to an ode

So i have these two differential equations that I can solve but I want to add a noisy term to one of the variables, the options are to either have a random number between -1 and 1 or just a sin/cos term that varies between this, the code runs fine until i attempt to add this noise then it gets errors can anyone help i am just not sure how to add this noisy term

h=0.01; 
ti=0;
tf=5;
N=floor(tf/h); 
t=linspace(ti, tf, N); 
b = 2; 
a = 0.5; 
x(1)=0.1;
y(1)=0.1;
noise=rand(-1,1);
for i=1:(N-1)
x_tmp = x(i)+h*a*x(i)*(8-x(i))*(x(i)*x(i)-16);
y_tmp = y(i)+h*(6+b^2) +h*noise;
x(i+1) = x(i)+h/2*(a*x(i)*(8-x(i))*(x(i)*x(i)-16))+a*x_tmp*(8-x_tmp)*(x_tmp*x_tmp-16);
y(i+1) = y(i)+h/2*(6+b*b+y_tmp);
end

[Mischa Kim]

Without having gone through your code, what makes you think that the code does not work "fine" anymore when adding noise?

[Azzi Abdelmalek]

Post your ode function

Answer 1

[Paul]

rand gives a random value between 0 and 1. The arguments of rand are not the boundaries but the size (of the matrix) of the output. change

     noise=rand(-1,1);

to:

     noise = -1 + 2*rand(1)

Answer 2

[Bill]

Thanks it runs now, but it doesn't seem to actually do anything i.e add noise, is it changing with iteration or is it because the amplitude h is so small that any effect is removed?