Saving the derivative during solving ode functions

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Hiep Dang 2012-12-11

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https://ww2.mathworks.cn/matlabcentral/answers/56239-saving-the-derivative-during-solving-ode-functions

I have a question and hope that somebody came across this earlier. Your advice will be of great help. I have to solve a set of differential equations. They are coupled altogether, and look like this:

dw(1) = f(w(1),w(2),dw(2));
dw(2) = g(w(1),w(2),dw(1));

I don't want to get into details of f and g since they're quite long. The solution can be generated. However, besides w(1) and w(2) as the outputs, I also want to get dw(1) or dw(2), Is there anyway that I can use to achieve this. For some reasons, calculating the derivatives of w(1) and w(2) (after getting the outputs) is not desirable for this problem. What I want is somehow saving dw(1) and dw(2) in every time instant.

Thanks a lot for your help,

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Answer 1

Hiep Dang 2012-12-11

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Thanks Walter for your answer. However, I don't understand much about your recommendation.

I came across with the method in this page: http://www.mathworks.co.uk/matlabcentral/newsreader/view_thread/279302

It will involve with a "for" loop to call the dw after getting the output of t and w. It works for me at the moment, but will not be efficient because of the loop, especially when the ode function system is complicated.

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Answer 2

Walter Roberson 2012-12-11

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You could use an options structure with an OutputFcn specified. Exactly how you do the saving depends on what is convenient. For example, you could use a "global" cell array.

Note: time steps do not always go strictly forward. Sometimes, especially near boundary conditions, the ode solvers need to step backwards for greater accuracy.