How to know the step time inside the function that integrated by ode45?

Question

Mohamed Aburakhis 2016-6-24

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https://ww2.mathworks.cn/matlabcentral/answers/292232-how-to-know-the-step-time-inside-the-function-that-integrated-by-ode45

评论： Walter Roberson 2016-6-25

I have

x0=1; 
[t,x] = ode45(@My_Fun,[0:0.01:10],x0);

and My_Fun is

function x_dot =My_Fun(t, x)

Here %%%

%%I want to know the instantaneous step time %%%

What are t(now) and t(next) %%%

x_dot(1)=2/m-x;

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James Tursa 2016-6-25

Why do want this info? I.e., what would you do with it?

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Answer 1

Roger Stafford 2016-6-25

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https://ww2.mathworks.cn/matlabcentral/answers/292232-how-to-know-the-step-time-inside-the-function-that-integrated-by-ode45#answer_226708

编辑：Walter Roberson 2016-6-25

You might be interested in a comment made in

http://www.mathworks.com/help/matlab/ref/ode45.html

“If tspan contains two elements, [t0 tf], then t contains the internal evaluation points used to perform the integration.” It depends on whether you believe this in a literal sense or that it is merely an "approximate" truth.

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Answer 2

Walter Roberson 2016-6-25

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You cannot know the t of the next step. ode45() uses adaptive time steps, so the next time step will depend upon will depend upon the outcome of this step. Also, if your x is not a scalar, then the next ode45 call might have the same time as the current one, but with different boundary conditions (x), so that it can explore the change along the other dimensions.

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Walter Roberson 2016-6-25

None of the ode* routines permit fixed time step. They do allow setting a guideline for the initial step, and they do allow setting the maximum step, but they do not allow the step size to be fixed.

http://www.mathworks.com/help/matlab/ref/odeset.html

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