What's wrong with my code?

function dxdt = pendulum(t,x)

L = 1;

theta = x(1);

gamma = x(2);

dtheta = gamma;

dgamma = -(9.8/L)*sin(theta);

dxdt = zeros(2,1);

dxdt(1)=dtheta;

dxdt(2)=dgamma;

[t,x]=ode45('pendulum',[0 10],[0.9*pi 0]);

plot(t,x(:,1));

hold on;

plot(t,x(:,2),'r');

legend('Position','Velocity');

plot(x(:,1),x(:,2));

xlabel('Position');

yLabel('Velocity');

It says x is undefined

[dpb]

Need EXACT error in context including traceback to be able to tell--too many places where x is used. You may well need to change the internal use of x as the result of ode45 and where it's the passed-in value; didn't try to read the code thoroughly enough to tell for certain.

[Image Analyst]

Well one problem is that the formatting is bad. Read this to correct it: http://www.mathworks.com/matlabcentral/answers/13205-tutorial-how-to-format-your-question-with-markup

Answer 1

[Azzi Abdelmalek]

There is no error in your code, the problem is the way you are using it.

Save the followed code as pendulum.m

function dxdt = pendulum(t,x)
L = 1;
theta = x(1);
gamma = x(2);
dtheta = gamma;
dgamma = -(9.8/L)*sin(theta);
dxdt = zeros(2,1);
dxdt(1)=dtheta;
dxdt(2)=dgamma;

Then in another file write and run this code

[t,x]=ode45('pendulum',[0 10],[0.9*pi 0]);
plot(t,x(:,1));
hold on;
plot(t,x(:,2),'r');
legend('Position','Velocity');
plot(x(:,1),x(:,2));
xlabel('Position');
ylabel('Velocity');

[Andrew Davies]

Something is going wrong. Here is another code i'm actually trying to get to work

 %function dLdt = Lorentz(t,x,y,z)

 %s = 10

 %b = 8/3

 %x = L(1);

 %y = L(2);

 %z = L(3);

 %dx = s*(y - x)

 %dy = r*x - y - x*z

 %dz = x*y - b*z

 %dLdt = zeros(3,2,1);

 %dLdt(1)=dx;

 %dLdt(2)=dy;

 %dLdt(3)=dz;

That is the one .m file i made. The second one is simply

 %[t,x,y,z]=ode45('pro1',0:0.001:1,[1 1 1]);

 %plot3(L(:,1),L(:,2),L(:,3));

Now it says 'Undefined function or method 'L' for input arguments of type 'double' pro1 is the .m file

[Azzi Abdelmalek]

Andrew, what about your original question?

[Andrew Davies]

That was an example from a textbook and i was using it to try to understand how to solve my own one, the second one i described