Solving a system of differential equation

Can someone tell me why the answers are coming as 'NaN'?

Also in the first for loop, by writing dEs in descritized form (line 22 and 23) my aim is to find (dEs/dt) and use the value of Es in line 23. Also,after this loop, the final values of El and Es needs to be carried over to next loop. I have coded this as best as i know. Can someone tell me how i can find d(Es)/dt directly instead of discretizing it?

[Ameer Hamza]

Can you show your ODEs? Also, in hemicellulose.m, you need to output the value of the derivative (dEs), not the output of the function (Es).

[Abhishek Varma]

[Abhishek Varma]

[Abhishek Varma]

why do i need to output dEs? i need to take the value of Es and El alone to the next loop. Can you please explain your point for me sir?

[Ameer Hamza]

The way you wrote your function seems to be confusing. You are also using time as a state?

f(3) = t;

You also seems to be using something like Euler method in the following line

Es = Es + dEs*(t-t_pre);

Also, it seems that you are using X_next to share infomation between iterations of ode45.

Following shows a simple example of how to write function for an ODE

function dydt = odeFun(t, y)
    dydt = -2*y.^2;
end

you don't need to do something like the following

function f = odeFun(t, y)
    Y = y(1);
    t_pre = y(2);
    dydt = -2*y(1).^2;
    Y = Y + dydt(t-t_pre);
    f(1) = Y;
    f(2) = t;
end

[Abhishek Varma]

i am using t as state function because i need to find delta t to find the d(Eendo,s)/dt term in discretized form. I know its not the most effective way, but since im not very well versed in matlab i dont know how to do it otherwise. I was hoping to find a way to code it by using the same ode45 to solve for Es also while solving the concentration terms. But also, once t<300 loop is over, the value of Es needs to be carried over to the next t>=300 loop, where we have the equation to find Es directly, but at this time, the value of Es must be updated from the previous itertation but with a new equation.

[Ameer Hamza]

But a scheme like this will not work as expected. ode45() does not do one calculation in each iteration. It is an adaptive step-size algorithm, and the point moves forward and backward several times in each iteration for ode45 to decide what should be a good step length. So You might be expecting that it is the value from the previous iteration, but is not exactly that.