What's wrong with my code?

2014 5 18
1 个回答
回答已采纳
更新时间：2014 5 18
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0 个投票

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

2 个评论
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dpb
dpb 2014-5-18
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.

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 采纳的回答

Azzi Abdelmalek
Azzi Abdelmalek 2014-5-18
编辑：Azzi Abdelmalek 2014-5-18

0 个投票

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');

3 个评论
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Andrew Davies
Andrew Davies 2014-5-18
编辑：Andrew Davies 2014-5-18
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
Azzi Abdelmalek 2014-5-18
Andrew, what about your original question?
Andrew Davies
Andrew Davies 2014-5-18
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

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