An iteration Code causing error

Thanks guys for the response so far

function []=model3()
%parameters
h=0.0006944444; V=10; 
tfinal=30;
Qe = 10; Qo = Qe; Y=0.4;
Xo=0; Xe=Xo; u1=0.1; S=500; Snoe1=100; Ks=10; Kno=0.5; b=0.02;
%initial conditions
X1(i)=200; Sse(i)=5; Snoe(i)=2;
t(i)=0;

%define the ODE function hundle
f1=@(t,X1)    ((Qo*Xo-Qe*Xe-V*X1/10)/V)+u1*(Snoe/(Ks+Snoe))*(Snoe/(Kno+Snoe))*X1-b*X1;

f2=@(t,Snoe) (Qo*Snoe1-Qe*Snoe)/V-(1-Y)/(2.86*Y)*u1*(Sse/(Ks+Sse))*(Snoe/(Kno+Snoe))*X1;

f3=@(t,Sse)   (Qo*S-Qe*Sse)/V-u1/Y*(Sse/(Ks+Sse))*(Snoe/(Kno+Snoe))*X1;


for i=1:ceil(tfinal/h)
    
    %update t
    t(i+1)=t(i)+h;
    
    %update x
    kX11=f1(t(i), X1(i));
    kX12=f1(t(i)+0.5*h, X1(i)+0.5*kX11*h);
    kX13=f1(t(i)+0.5*h, X1(i)+0.5*kX12*h);
    kX14=f1(t(i)+    h, X1(i)+       kX13*h);
    X1(i+1)=X1(i)+(h/6)*(kX11+2*kX12+2*kX13+kX14);
    
    %update Snoe
    kn1=f3(t(i), Snoe(i));
    kn2=f3(t(i)+0.5*h, Snoe(i)+0.5*kn1*h);
    kn3=f3(t(i)+0.5*h, Snoe(i)+0.5*kn2*h);
    kn4=f3(t(i)+    h, Snoe(i)+       kn3*h);
    Snoe(i+1)=Snoe(i)+(h/6)*(kn1+2*kn2+2*kn3+kn4);
    
    %update Sse
    ks1=f3(t(i), Sse(i));
    ks2=f3(t(i)+0.5*h, Sse(i)+0.5*ks1*h);
    ks3=f3(t(i)+0.5*h, Sse(i)+0.5*ks2*h);
    ks4=f3(t(i)+    h, Sse(i)+       ks3*h);
    Sse(i+1)=Sse(i)+(h/6)*(ks1+2*ks2+2*ks3+ks4);
end
end

[Jan]

You forgot to post the error message. It is much easier to fix a problem than to guess, what the problem is.

Answer 1

[Jan]

The code stops in:

X1(i)=200; Sse(i)=5; Snoe(i)=2;
t(i)=0;

While i is not defined yet, it is the imaginary unit. I guess you meant 1 instead of i.

X1(1)=200; Sse(1)=5; Snoe(1)=2;
t(1)=0;

Answer 2

[YT]

You say you're getting an error, but you didnt specify it in the description of your question. I ran the script and I assume you meant the following error

Unable to perform assignment because the left and right sides have a different number of elements.

which first occured at

Snoe(i+1)=Snoe(i)+(h/6)*(kn1+2*kn2+2*kn3+kn4);

In this case Snoe will be a 1-by-X array, but you are trying to add an vector to it Snoe(i)+(h/6)*(kn1+2*kn2+2*kn3+kn4).

I think the easiest way to fix it, is by doing

Snoe(i+1,:) %also for X1/Sse

Extra:

I would initialize these matrices before the loop so

X1 = zeros([ceil(tfinal/h)+1 V]);
Snoe = zeros([ceil(tfinal/h)+1 V]);
Sse = zeros([ceil(tfinal/h)+1 V]);
for i=1:ceil(tfinal/h)
    ...
end

[Jan]

After I fixed the 'i' to '1' to define the inital values, the code ran fine. The pre-allocation is an important advice.