Index exceeds the number of array elements (2)

I am doing homework for school where i have to get a spring oscillation graph. The program keeps reporting an error to me and I don’t know what could be wrong.

function [t,Y]=naloga6

g=9.8;
m=1;
k1=20;
k2=30;
tr=0.4;
L=2;
alfa=pi/6;

tstart=0;
tfinal=10;
y0 = [L*cos(alfa);L*sin(alfa)];

[t,Y] = ode23(@desne_strani, [tstart tfinal],y0,[],g,m,k1,k2,alfa,tr);

figure(16);
% hold on
% plot3(Y(:,1),Y(:,2),Y(:,3),'r')
dimen= [min(Y(:,1)) max(Y(:,1)) min(Y(:,2)) max(Y(:,2))+0.5 ];


casi=tstart:0.01:tfinal;
Yfilm=interp1(t,Y,casi);
for i=1:size(casi,2)
    
    plot(Yfilm(i,1),Yfilm(i,2),'ro')
    axis(dimen)
    
    Film(i)=getframe;
end

function dydt = desne_strani(t,y,g,m,k1,k2,alfa,tr)
dydt = [
    y(4)
    0
    y(6)
    -((k1*y(1)*cos(alfa))/m)-((k2*y(1)*cos(alfa))/m)-(tr*g*cos(alfa))
    0
    -((k1*y(3)*sin(alfa))/m)-((k2*y(3)*sin(alfa))/m)-(tr*g*sin(alfa))
 ];

------------------------------------------------------

Errors:

Error in naloga6>desne_strani (line 35)
    y(4)

Error in odearguments (line 90)
f0 = feval(ode,t0,y0,args{:});   % ODE15I sets args{1} to yp0.

Error in ode23 (line 115)
  odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);

Error in naloga6 (line 15)
[t,Y] = ode23(@desne_strani, [tstart tfinal],y0,[],g,m,k1,k2,alfa,tr);

[Stephen23]

The ode23 documentation states that "y0 must be the same length as the vector output of odefun". In your code y0 has two elements, but your output has six elements. You need to fix this.

The error is caused by you defining y0 with two elements, which then defines the size of y provided to your function handle, but inside the function handle you try to access elements 3, 4, and 6 (which clearly do not exist in a two-element vector). You need to fix this too.

Avoid using undocumented and obsolete ways to pass parameter value to the objective function, much better is to follow the MATLAB documentation:

https://www.mathworks.com/help/matlab/math/parameterizing-functions.html

Answer 1

[Walter Roberson]

    y(6)

Your code expects 6 (or more) boundary conditions are going to be passed in

    y0 = [L*cos(alfa);L*sin(alfa)];

but your boundary conditions, y0, is a vector of length 2, not 6.

[Nejc Jansa]

Working now, thank you very much