# Modeling concentration using ODE

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Stark Volt on 26 Sep 2021
I'm trying to model a dye concentration but I cannot make to run and I'm stuck. It gives an error but it will not tell what is wrong. Can any one help thanks?
clear all;
Unable to find file or directory 'test_data.dat'.
time = size(data,1);
u = data(:,2);
tspan = [1:1:time];
y0 = data(1,3);
f = -0.25;
g = 0.25;
T = 10.0;
[t,y]=ode45(@model,tspan,y0,f,g,T,U);
plot(t, y(:,end), '*-b', 'LineWidth', 1);
function dydt = model(t,y,f,g,T,u)
if t-1 <= T
u = 0;
else
t_back = t-T;
n = floor(t_back); u =(n);
end
dydt = f*y + g*u;
return;
end
Star Strider on 29 Sep 2021
Please provide a symbolic (preferably LaTeX) version of the differential equation system you’re working with, and the parameters you want to estimate. If you have a PDF of a paper describing what you want to do, that would be even better. (Attach / upload the PDF, not a link to it, because thery’re usually behind a paywall, preventing me from accessing them if I don’t subscribe to the journal.)
Also, putting if blocks in the differential equation function is going to cause problems. Numerical differential equation integration functions don’t do well when integrating across the discontinuities that the if blocks cause.
I don’t understand the reason ‘parameter’ is nowhere represented in the differential equation.
I’ll do my best to see if I can get the differential equation (or system of them) to work with my code and your data.
.

Sulaymon Eshkabilov on 29 Sep 2021
You had better use this data importing fcn:
Make sure that the data file (test_data.dat) is present in your current directory or change to the directory where the data file (test_data.dat) is residing.

R2021a

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