# train dynamics undefined variable and ode problem

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sarngon on 25 Jan 2012
hello,
i need help about ode23 and subfunction topic
function xdot=train(t,x) alfa=0.3; beta=0.13; gama=0.16; Ua=1; Ub=2; xdot=[x(2);((2/pi)*(atan((x-4)/0.05)+atan((x-2)/0.05))-alfa-beta*abs(x(2))-gama*(x(2)^2)+Ua-Ub)];
end
and the plot function
[t,x]=ode23('train',[0,0.1],[0;0]); subplot(2,1,1);plot(t,x(:,1)) subplot(2,1,2); plot(t,x(:,2))
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and the error is
??? Error using ==> odearguments at 116 TRAIN returns a vector of length 3, but the length of initial conditions vector is 2. The vector returned by TRAIN and the initial conditions vector must have the same number of elements.
Error in ==> ode23 at 172 [neq, tspan, ntspan, next, t0, tfinal, tdir, y0, f0, odeArgs, odeFcn, ...
Error in ==> main1 at 1 [t,x]=ode23('train',[0,0.1],[0;0]);
##### 2 CommentsShow 1 older commentHide 1 older comment
sarngon on 25 Jan 2012
sorrry

Walter Roberson on 25 Jan 2012
Your expression for xdot has two atan() calls with arguments that are expressions in x -- but x is a vector, so you get out a vector result the same length as x... but you also put x(2) at the front of that vector, resulting in a vector of length 3.
sarngon on 26 Jan 2012
Hi can u help me pls i added a message to my ouestion
thanks

sarngon on 26 Jan 2012
ok i change the matrix variable name as k(2)and kdot but now the prog gives such a error
codes function kdot=train(t,x) alfa=0.3; beta=0.13; gama=0.16; Ua=1; Ub=2; kdot=[k(2);(2/pi)*(atan((x-4)/0.05)+atan((x-2)/0.05))-(alfa+beta*abs(k(2))+gama*(k(2)^2))+Ua-Ub];
end
and the error line
??? Undefined function or method 'k' for input arguments of type 'double'.
Error in ==> train at 7 kdot=[k(2);(2/pi)*(atan((x-4)/0.05)+atan((x-2)/0.05))-(alfa+beta*abs(k(2))+gama*(k(2)^2))+Ua-Ub];
thanks..
Walter Roberson on 26 Jan 2012
The Question is solved then?

Hello I want to know that how to know the ode's in a vehicle train model. we can calculate manually if the degrees of freedom is small like 5,10 . but what is it is lie 41DOF and more than that . is there any possible approach in matlab to directly code a script and get to know the equations for the system by coding anything in matlab. kindly help and suggest anything in this regard.