# Matlab:Evaluating ODE at Points

• MATLAB

## Main Question or Discussion Point

Hello everyone.

I have a second order ODE, which I reduced to two first order ODEs, and solved with ode45. Now I want to evaluate the solution and the derivative at certain points (specifically, I want to plot the Poincare section).

So, what I did is this:

Code:
>> sol=ode45(@...);
>> xint=a:b;
>> [Sxint, Spxint]=deval(sol, xint);
>> A=[Sxint, Spxint];
>> plot(Sxint, Spxint,'o')
Where:
xint is the points at which I want to evaluate the solution.
Sxint is the value of the solution at the points xint.
Spxint is the value of the derivative at those points.

But this is clearly wrong. I know the solution should be periodic, but I get something random. I think it's the plotting I'm doing incorrectly. Any help appreciated.

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Ok, instead, I created a seperate M-file:

Code:
hold on,
for i=1:length(xint)
plot(Sxint(:,i), Spxint(:,1),'.')
end
Is this correct?

Nevermind, I've figured it out, you can close the thread.