What is the most elegant way to get yp from solution of ODE15i?

4 views (last 30 days)
Hi,
I'm solving an ODE using ODE15i. As per the usual use case of 15i, the function I'm integrating is a function of t,y,and prime (the time derivative of y):
function sol = myFunc
% Set up ICs and shared variables
sol = ode15i(@nested_integrand,tspan,y0,yp0)
function f = nested_integrand(t,y,yp)
% some code
end
end
However, the solution sol only contains values of t and y at the integration points. Not yp! This seems like an oversight to me, since the consistent values of yp have already been calculated within ode15i.
So my question is in two parts:
1. What is the most elegant way of getting the yp values that correspond with sol.t and sol.y?
2. Does anyone else have a use for this output? If so I'll make a feature request.
Thanks all,
Tom

Accepted Answer

Torsten
Torsten on 13 May 2015
[Y,YP] = deval(sol,tspan)
will give you the time-derivatives of your solution variables at times tspan in the matrix YP if your call to ode15i was
sol = ode15i(@nested_integrand,tspan,y0,yp0)
Best wishes
Torsten.
  1 Comment
Tom Clark
Tom Clark on 13 May 2015
Perfect, thanks Torsten. I'd studied the documentation of deval() but my eyes must be swimming, as I'd missed that!
What a hero :)
Tom

Sign in to comment.

More Answers (1)

Torsten
Torsten on 13 May 2015
If you solve for n unknowns y1,...,yn,
add n additional algebraic equations as
y(n+1)-dy(1)=0,y(n+2)-dy(2)=0,...,y(2*n)-dy(n)=0.
y(n+1),...,y(2*n) will then give you the derivatives of the variables you solved for.
Best wishes
Torsten.
  2 Comments
Tom Clark
Tom Clark on 13 May 2015
Thanks Torsten, nice implementation. Would this affect the performance of the solver though?
Torsten
Torsten on 13 May 2015
Maybe if n is large ...
But I think it's less time-consuming than to solve f(t,y,yp)=0 (with fsolve,e.g.) for each output time.
Best wishes
Torsten.

Sign in to comment.

Products

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!