Attempted to access x(2); index out of bounds because numel(x)=1 for an ODE function

I have written a code for a 2 DOF equation transformation which follows like wise,

function:

function [dx] = dx(m1,m2,k1,k2,c1,c2,x,t,time,P_1,P_2)

dx = zeros(4,1);

dx(1) = x(2);

dx(3) = x(4);

dx(2) = (interp1(time,P_1/m1,t,'spline'))+(1/m1)*(-(c1+c2)*x(2)+(c2*x(4))-(k1+k2)*x(1)+(k2)*x(3));

dx(4) = (interp1(time,P_2/m2,t,'spline'))+(1/m1)*((c2)*x(2)-(c2*x(4))+(k2)*x(1)-(k2)*x(3));

end

Main Script:

clear all; clc;

% For 1st Mass system

W1 = 53000;

k1 = 31000;

c1 = 300;

Po_1 = 0;

W_bar = 20;

phi_1 = 0;

% For 2nd Mass system

W2 = 26000;

k2 = 45000;

c2 = 450;

Po_2 = 10000;

phi_2 = pi/2;

grav =386;

m1 = W1/grav;

m2 = W2/grav;

fprintf('The Mass calculated for 1st system m1=%4.4f lb*s^2/in \n', m1);

fprintf('The Mass calculated for 1st system m2=%4.4f lb*s^2/in \n', m2);

% TIME SPAN

N = 101;

time = linspace(0,50*pi,N);

t_span = [0 4*pi];

% INITIAL DISPLACEMENT

x_o =[0;0;0;0];

%FORCING FUNCTION

P_1 =Po_1*sin(W_bar*time-phi_1);

P_2 =Po_2*sin(W_bar*time-phi_2);

% USING ODE45

[t,x] =ode45(@dx,t_span,x_o,[],m1,m2,k1,k2,c1,c2,time,P_1,P_2);

When I execute it, an error pops up showing as "Attempted to access x(2); index out of bounds because numel(x)=1".

I need to get all the x(1), x(2), x(3), x(4) values. Could anyone help me in this regard..?

Answer 1

[Titus Edelhofer]

Hi,

you will need to change the order of variables: your function dx should look like

function dx = dx(t, x, m1, m2, ...)

Titus

PS: Personally I would not recommend to name the output like the function name, but it's not wrong.

[Vashish]

Hi Titus, Would you be able to give more information on why changing the variables' order would give such a result? I'm having similar problems and I'd like to know in what way should I rearrange my variables.

Thanks, VashishS

[Titus Edelhofer]

That's a requirement from the ode solver. The ode solver does not "look" into the function to find out, which parameter does what. It always assumes the function to be of kind

function dx = myfun(t, x, param1, param2, ...)

where of course dx, t, and x may have names as they like, I just named the variables to express what they represent.

Titus