How to set iteration results as columns in matrix?

Hello,

I'm trying to solve an ode of a second order response for my homework in dynamics.

my function is:

%SORODE_Z%
function [dX]=SORODE_Z(t,X,z,varargin)
tau=0.5;
Xa=1;
C=(1/tau^2)*[0, Xa]';
A=[0, 1; -1/tau^2 , -2.*z./tau];
dX=A*X+C;
end

my script is:

    %second_main%
  clear
  t0=0;
  tfinal=10; 
  tspan=[t0 tfinal];
  X0=[0,0]';
  zv=input('input zv as [n1,n2...nn]:');
  nn=numel(zv);
  options=[];
  for i=1:nn
    v(i)=zv(i);
    z=zv(i);
    [t,X]=ode45(@SORODE_Z,tspan,X0,options,z);
    X1(:,i)=X(:,1);
    X2(:,i)=X(:,2);
  end
  figure(1)
  clf
  plot (t1(:,1),X1(:,1),t1(:,1),X1(:,2),t1(:,1),X1(:,3),t1(:,1),X1(:,4))
  title('Step response by second order')
  xlabel('t [sec]')
  ylabel('X')
  axis([0,10,0,2])
  legend('z=o','z=0.5','z=1','z=2')

my input is: [0,0.5,1,2]

when I run the script I get this error:

Subscripted assignment dimension mismatch.
Error in second_main (line 14)
X1(:,i)=X(:,1);

Is there a solution for this?

Answer 1

[Walter Roberson]

You should not be assuming that the number of timesteps used internally will be exactly the same each time through. For your calculations use

X1{i} = X(:,1);
X2{i} = X(:,2);
T{i} = t;

and then the plot at the end should be

plot (T{1},X1{1},T{2},X1{2},T{3},X1{3},T{4},X1{4})

[Achiad]

finally! thank you very very much! :)