Substitution of initial numerical values before odefunction
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Tony Cheng
am 17 Jul. 2019
Kommentiert: Torsten
am 19 Jul. 2019
Hi guys,
I am using ode45 to solve a system of ODEs. According to the syntax of ode45, the system is formatted into the form of
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/230083/image.png)
where x(t) is a 32*1 symbolic vector in our project whose elements are to be solved in ode45.
Then odefunction is applied to build the handle for mass matrix like
Mass_handle = odefunction( M( t, x(t) ) )
However, the size of M( t, x(t) ) is very huge and the elements inside it are really complicated, then it is quite time-consuming to return Mass_handle: by now it has taken over 96 hours and consumes over 45GB in RAM in our PC and it is still running at this moment.
Here I have an idea: since we have the initial numerical values of x(t), are we able to substitute the 32 symbolic variables in x(t) by their initial numerical values before running the code
Mass_handle = odefunction( M( t, x(t) ) )
Because we notice that in ode45, it is necessary to input the initial values of the symbolic variables, according to its syntax like ode45(odefun, tspan, y0), where y0 is the initial value vector.
If we substitute the initial numerical values of x(t) before running the code odefunction( M( t, x(t) ) ), then M( t, x(t) ) will be numerical rather than symbolic. It will just take a second to return Mass_handle.
Here we want to know: is this way feasible ? if not, are there any methods to deal with this problem?
Many thanks!
Cheers
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Torsten
am 17 Jul. 2019
Use "matlabFunction" to convert every symbolic input to ODE45 into numerical function handles before calling ODE45.
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Torsten
am 19 Jul. 2019
Yes, that's what I meant.
Before calling ode45 with the function handle, you should output the handle into a file to see the list of numerical parameters and the order in which they have to be supplied.
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