ODE Solvers how to check conditions in each time step

To whom it may concern,

Good afternoon and I hope to reach out to you for a help in MATLAB's ode solvers.

I would like to check if a certain condition holds for the right hand side function, during the numerical integration. I am basically printing a variable inside my right hand side function and seeing the work space output to make sense of the condition.

However, from my rudimentry understanding in solving ODEs, and numerical solvers for the ODEs in MATLAB, I understand that the solver will make a guess(?) in its step size and before it can eventually pick the right step size that ensures the error criteria, for a given time step.

If this is true, I would like to see if the conditon that I want to impose can only be evaluated at the correct steps that the solver choose as it integrates the right hand side function, numerically.

Do you have any insights as to how can I check a specific condition during the numerical intergration of a given right hand side function, specifically for the steps that are chosen by the solver?

Thanks a lot!

Tae

[Torsten]

To evaluate your condition after a "correct" step the solver has chosen use the "OutputFcn" function.

If you want to stop the integration or change integration parameters depending on your condition, use the "Events" function.

[Taehun Kim]

Dear Torsten, Thanks a lot for your prompt reply back! I am familiar with the event function feature in ode solvers in MATLAB. Thanks to your suggestion, I was able to think about the usage of the event function to check my condition. Will the event function be evaluated only at the "correct" solutions? I guess the answer is yes...? Also, I am slightly not familiar with OutputFcn function... I opened up ode45.m and look through the code but I am having a bit trouble understanding what it does... Could you please help me to better understand the function? Otherwise, thanks a lot for your replay back! It means a lot to me!

[Torsten]

Will the event function be evaluated only at the "correct" solutions?

No, also at test points for t and y.

Could you please help me to better understand the function?

Read about OutputFcn under

https://de.mathworks.com/help/matlab/ref/odeset.html

[Taehun Kim]

Thanks for your help! I will look into the link!