Is there any way I can ensure that the numbers generated in ode45 are positive?

Here is my code. Many numbers generated in the solution are negative and there are a lot of NaNs too.

%setting up the values of constants
N=1000; %number of systems
%m=0.3*ones(N,1)
%S=0.7;%coupling streangth
tspan=linspace(0,10000,10000)';%time steps

%solving the equation
ystart=zeros(N,1);
for i=1:N
   ystart(i)=unifrnd(0,1);%Initial conditions; (9 by 1) column vector 
                          %indicating that each oscillator starts with 
                          %random initil voltage between 0 and 1
end
%options=odeset('NonNegative',1);
[t,y]=ode45(@CLIF,tspan,ystart);%options);

%Plotting
%rescale(y(11,:))
plot(y(5,:),'.')
%axis([1 11 0 1])
xlabel('ith oscillator')
ylabel('voltage')
title('Coupled LIF')

%Defining the system of equations

function dydt=CLIF(t,y,m,N,S)
N=1000;
m=0.3;
S=15;
dydt=zeros(N,1);
for i=1:N
    if i<N
        dydt(i)=-y(i)+m+(S).*((y(i)-y(i+1)));
    else
        break
    end
end    
end

[madhan ravi]

upload your code

[KSSV]

It depends on your ODE. Confirm that your code is correct. If not post your code here.

[Vipin Padinjarath]

Code added to the question. Kindly check.

[Torsten]

What makes you think the solution should be non-negative ?

[Vipin Padinjarath]

They are oscillator states. There is no physical meaning to negagive states.

[Torsten]

Choosing N=2 and starting values y10, y20 as initial values for y(1) and y(2) such that 7*y10-9*y20 < 0 will give you a strong negative solution for y(1) after some time.

[Vipin Padinjarath]

Thats true. Let me see again. I hope the code is fine.

[Jan]

What is the purpose of this strange construction:

for i=1:N
  if i<N
      dydt(i)=-y(i)+m+(S).*((y(i)-y(i+1)));
  else
      break
  end
end

Why not:

dydt(N, 1) = 0;
for i = 1:N-1
  dydt(i)=-y(i)+m+(S).*((y(i)-y(i+1)));
end

or

dydt = [-y(1:N-1) - S .* diff(y); 0];

[Vipin Padinjarath]

My intention was to implement nearest neighbor coupling. The system is a set of nodes arranged in one dimension.

[Vipin Padinjarath]

Oh... I think your code would be better. I am still learning. Thank you.

[Vipin Padinjarath]

But then will it take the Nth value? I mean y(N)? Because loop runs only upti N-1.

[Jan]

You need y(i+1) to construct the N'th value. Does y has N+1 elements? If so, use N instead of N-1.

[Vipin Padinjarath]

Oh... okay. I got it. Thank you..