Attempted to access y(5); index out of bounds because numel(y)=2.

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Jeppe Nikolajsen
Jeppe Nikolajsen 2014-9-12
回答： Rushikesh Tade 2014-9-12
I get this message when i try to run my code. I am new in the program. I don't know what to do
my code is following
function [dy,ay_NL,beta_NL,rss_NL]=derivNL(t,y,cons)
% Constants
y=zeros(2,1);
a=cons.a; % Longitudinal distance from COG to front wheels [m]
b=cons.b; % Longitudinal distance from COG to rear wheels [m]
C_alpha_f=cons.c_alpha_f; %[N/rad]-Guess/cornering stiffness front wheel
C_alpha_r=cons.c_alpha_r; %[N/rad]-Guess/Cornering stiffness rear wheel
I_zz=cons.I_zz; % Yaw inertia of mass approximation
M=cons.M; % 1688kg car, 80kg person, 79 liters fuel, 20kg oil u=cons.u; % Vihecle logditudinal speed [m/s]
w_r=cons.w_r;
w_f=cons.w_f;
L=cons.L;
g=cons.g;
my=cons.my; N_f=cons.N_f; N_r=cons.N_r;
% Steering angle at 0.2 g'es
delta_f=0.00395*3; % calculated analytic
% Inputs
alpha_f=((y(2)+a*y(1))/u)-delta_f; % Slip angle front
alpha_r=((y(2)-b*y(1))/u); % Slip angel rear
F_yf_NL=NLtire(alpha_f,2*C_alpha_f,my,N_f); % Lateral force front wheel [N]
F_yr_NL=NLtire(alpha_r,2*C_alpha_r,my,N_r); % Lateral force rear wheel [N]
% Yaw rate and lateral velocity
dy(1)=(1/I_zz)*(F_yf_NL*a-F_yr_NL*b); % Yaw rate [deg/sec]
dy(2)=((1/M)*(F_yf_NL+F_yr_NL))-u*y(1); % Lateral speed [m/sec]
dy(3)=(u*cos(y(5)))-(y(2)*sin(y(5))); % Global coordinat car X-direc
dy(4)=(u*sin(y(5)))+(y(2)*cos(y(5))); % Global coordinat car Y-direc
dy(5)=y(1); % Rotational angle psi

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回答（3 个）

Julia
Julia 2014-9-12
Hi,
Since you declare y to be a zeros-vector, you overwrite your input y.
Your vector y has only 2 entries.
>> y=zeros(2,1)
y =
0
0
But you try to access y(5) when you compute dy.
Mischa Kim
Mischa Kim 2014-9-12
编辑：Mischa Kim 2014-9-12
Jeppe, if I am not mistaken, derivNL defines a system of differential equations. If that's the case, change
y = zeros(2,1);
to
dy = zeros(5,1);
y represents the initial conditions state vector (which should not be resetted to zero). dy is the derivative of the state vector.
Make sure to call the function with an inital conditions state vector of appropriate size, 5-by-1.
Rushikesh Tade
Rushikesh Tade 2014-9-12
Thats because you have declared as:
y=zeros(2,1);
which creates y variable as:
y=[0;0]
which means y variable is only having 2 elements in it and you can access these elements as y(1) and y(2) and not more than that.
now in the following lines
dy(3)=(u*cos(y(5)))-(y(2)*sin(y(5))); % Global coordinat car X-direc
dy(4)=(u*sin(y(5)))+(y(2)*cos(y(5))); % Global coordinat car Y-direc
you have tried to access 5th element in y using y(5) which is not present, resulting in error.
Also I have observed that you are trying to take y as an input from calling function in that case y=zeros(2,1) will overwrite the value of y which you got from calling function.

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