Using ODE45 on inverted pendulum without external torque

I have this problem, I'm trying to plot my solutions but there is an error not enough input arguments. The question is in the attached file.

function dx = invpen(t,x)
dx = zeros(2,1); % column vector
dx(1) = x(2);
dx(2) = (1/(1.0*1.0^2))*(1.0*9.8*1.0*sin(x(1)-0.5*x(2)));

[T,X] = ode45(@invpen,[0,20],[30,0]);

plot(T,X(:,1),'-',T,X(:,2),'.')

[Jan]

Is this stored in one file and you start it using the "Start" button of the editor? Then invpen is called without inputs.

Answer 1

[Stephan]

Hi,

use:

[T,X] = ode45(@invpen,[0,20],[30,0]);
plot(T,X(:,1),'-',T,X(:,2),'.')

function dx = invpen(~,x)
dx = zeros(2,1); % column vector
dx(1) = x(2);
dx(2) = (1/(1.0*1.0^2))*(1.0*9.8*1.0*sin(x(1)-0.5*x(2)));
end

The code runs so far - i did not check if it is a meaningful result.

Best regards

Stephan

[Jonathan Diaz]

That code does not run! It doesn't work.

[Jonathan Diaz]

Nevermind! Thanks!

[Stephan]

Glad to hear - Please accept helpful answers in order to help other people having similar problems finding helpful answers.

Answer 2

[Jan]

Is this stored in one file and you start it using the "Start" button of the editor? Then invpen is called without inputs.

Stephen's suggestion should work. Create a file called "main.m" (or whatever you like) and insert the code. Then you can start this as a script from the command window or with the "Start" button. Defining subfunctions inside scripts works in modern Matlab versions only. For an older version start the "main.m" file with

function main

[Stephan]

From which version are subfunctions inside scripts supported?

[Jan]

@Stephan: This feature was introduced in R2016b: https://www.mathworks.com/help/matlab/matlab_prog/local-functions-in-scripts.html

By the way: Answers: Search for "subfunctions in scripts"   ;-)

[Stephan]

Thank you!