MATLAB: Order of the Matlab Ode Solutions Appearing at Output

Question

When solving higher order differential equations using an ode solver (for example [t,y]=ode15i…), there are many outputs in y such as v1(t), v2(t), v3(t), Dv2t(t), Dv3t(t), Dv4t(t).

How do these signals appear at output (y) of the ode solver? What is their order?

In other words, what should I expect to get at y(:,1), y(:,2),…,y(:,end)?

Answer 1

Hi,

unfortunally Matlab sometimes changes the order of outputs to an order which appears not be logical (at least for me). I dont know why this happens, but it does in some cases. For ODE's it is a good idea to call odeToVectorField with two output arguments like shown here. The order of new_vars is corresponding to the output arguments of y, if you use matlabFunction to create the function handle.

See also this question: https://de.mathworks.com/matlabcentral/answers/287083-problem-with-order-of-substitution-with-odetovectorfield-solving-second-order-ode-with-ode45

In case of DAE the usage of reduceDAEIndex needs two output arguments which are new_eqs and new_vars. The usage of daeFunction needs both inputs, so that i would expect the order of the input vector for new_vars as the order of the output. But to be honest, for DAE i have no experiences.

Best regards

Stephan