How can I put condition in ode solver

I'm trying to put if condition in ode solver.

but it seems that I can't control variables in ode function. (can control just derivative of variables)

for example

function dcdt = exo(t,c)
    dcdt = zeros(3,1)
    dcdt(1) = -c(1)*exp(-1/c(3));
    if c(1) < 0
        c(1) = 0;                               ----->>> this doesn't work!!!
    end
    dcdt(2) = -c(2)*exp(-1/c(3));
    dcdt(3) = - dcdt(1) + dcdt(2);
end

How can I change this function to make c(1) = 0 if c(1) < 0

Answer 1

[Walter Roberson]

https://www.mathworks.com/help/matlab/math/nonnegative-ode-solution.html

The above solution is really only appropriate if the reason that c(1) becomes negative is due to round-off or overshooting (that is, a smaller step size would not have gone negative.)

If you are trying to do the equivalent of bounding off a floor, then it is not appropriate to use this solution, and instead you should use an ode event function to terminate the integration, then change the boundary condition, and then restart the integration.

Answer 2

[Steven Lord]

Use odeset to create an options structure with the NonNegative option set to the elements of the solution that need to remain non-negative and pass that options structure into the ODE solver.

[kkh1897]

I try to use ode23 solver. Sorry I didn't mention the solver.