Indexing Error with Ode45

I am trying to simulate a self driving car before I implement a controller. I am getting an indexing error however.

Here is the code I am working on

start_loc = [0; 0];

end_loc = [10; 10]

obstacles = [5, 5, 1; 7, 8, 0.5];

[t, X] = simulate_self_driving_car(start_loc, end_loc, obstacles);

function [t, y] = simulate_self_driving_car(start_loc, end_loc, obstacles)

% Simulation parameters

tspan = [0, 20]; % Simulation time span

y0 = [start_loc(1); start_loc(2); atan2(end_loc(2)-start_loc(2), end_loc(1)-start_loc(1)); 0; 0]; % Initial conditions

disp(y0)

% Model parameters

L = 2.5; % Wheelbase (m)

m = 1000; % Mass (kg)

Cf = 80000; % Front tire stiffness (N/rad)

Cr = 120000; % Rear tire stiffness (N/rad)

Iz = 2000; % Moment of inertia (kg*m^2)

% ODE function

odefun = @(t, y) self_driving_car_ode(t, y, L, m, Cf, Cr, Iz, obstacles);

% Solve ODE

[t, y] = ode45(odefun, tspan, y0);

end

function dydt = self_driving_car_ode(t, y, L, m, Cf, Cr, Iz, obstacles)

% State variables

x = y(1);

y = y(2);

theta = y(3); % this is where the error begins

v = y(4);

phi = y(5);

%Removed rest of code for simplicity

end

The error being given is:

Index exceeds the number of array elements. Index must not exceed 1.

Error in main>self_driving_car_ode (line 55)

theta = y(3);

Error in main>@(t,y)self_driving_car_ode(t,y,L,m,Cf,Cr,Iz,obstacles) (line 36)

odefun = @(t, y) self_driving_car_ode(t, y, L, m, Cf, Cr, Iz, obstacles);

Error in odearguments (line 92)

f0 = ode(t0,y0,args{:}); % ODE15I sets args{1} to yp0.

Error in ode45 (line 107)

odearguments(odeIsFuncHandle,odeTreatAsMFile, solver_name, ode, tspan, y0, options, varargin);

Error in main>simulate_self_driving_car (line 39)

[t, y] = ode45(odefun, tspan, y0);

Error in main (line 18)

[t, X] = simulate_self_driving_car(start_loc, end_loc, obstacles);

Answer 1

[Jack]

The error is occurring because you are overwriting the variable y in your self_driving_car_ode function. In the function definition, you have y as an input argument and then you are using it as a local variable. Therefore, when you try to access y(3) to get the value of theta, you are actually accessing the local variable y, which only has one element, causing the indexing error.

To fix this, you can change the name of the local variable in the self_driving_car_ode function. For example, you can change the line:

y = y(2); 

to:

pos_y = y(2); 

And then access the value of theta using y(3).

Alternatively, you can change the name of the input argument y in the function definition to avoid the conflict with the local variable. For example:

function dydt = self_driving_car_ode(t, state, L, m, Cf, Cr, Iz, obstacles)
% State variables 
x = state(1);
y = state(2); 
theta = state(3); 
v = state(4); 
phi = state(5); 
% rest of the codeend 
end

Then you can use state(3) to get the value of theta without any conflicts.

[Matthew Lancaster]

Thank you so much! I can't believe I overlooked that.