s-function error

Hello, I have problem with my s-function

The errors are:
Error in 'Hexacopter_1/S-Function' while executing MATLAB S-function 'Trial_2', flag = 3 (output), at time 0.0.

Attempted to access x(2); index out of bounds because numel(x)=1.

And this is my s-function:

function [sys,x0,str,ts] = Trial_2(t,x,u,flag,xini,m,l,Ix,Iy,Iz,IR,g,omegaD)

% The following outlines the general structure of an S-function.
switch flag,

    case 0,
      [sys,x0,str,ts]=mdlInitializeSizes(xini);

    case 1,
      sys=mdlDerivatives(t,x,u,m,l,Ix,Iy,Iz,IR,g,omegaD);
    case 2,
      sys=mdlUpdate(t,x,u);

    case 3,
      sys=mdlOutputs(t,x,u);
    case 4,
      sys=mdlGetTimeOfNextVarHit(t,x,u);

    case 9,
      sys=mdlTerminate(t,x,u);

    otherwise
      error(['Unhandled flag = ',num2str(flag)]);

end
%
%=============================================================================
% mdlInitializeSizes
% Return the sizes, initial conditions, and sample times for the S-function.
%=============================================================================
%
function [sys,x0,str,ts]=mdlInitializeSizes(xini)

sizes = simsizes;

sizes.NumContStates  = 12;   %x(1) bis x(12)
sizes.NumDiscStates  = 0;
sizes.NumOutputs     = 12;    %x,y,z,roll,pitch and yaw
sizes.NumInputs      = 4;    %u(1),u(2),u(3) und u(4)
sizes.DirFeedthrough = 0;
sizes.NumSampleTimes = 1;   % at least one sample time is needed

sys = simsizes(sizes);

%
% initialize the initial conditions
x0  = xini;

%
% str is always an empty matrix
%
str = [];

%
% initialize the array of sample times
%
ts  = [0 0];

% end mdlInitializeSizes

%
%=============================================================================
% mdlDerivatives
% Return the derivatives for the continuous states.
%=============================================================================
%
function sys=mdlDerivatives(t,x,u,m,l,Ix,Iy,Iz,IR,g,omegaD)

%____State variables__%     
x=x(1);                    
xdot=x(2);                  
y=x(3);                         
ydot=x(4);                    
z=x(5);                    
zdot=x(6);                   
roll=x(7);                   
rolldot=x(8);                
pitch=x(9);                 
pitchdot=x(10);              
yaw=x(11);                   
yawdot=x(12);                 

dxdt=[xdot,
     (cos(roll)*sin(pitch)*cos(yaw)+sin(roll)*sin(yaw))*u(1)/m,
      ydot,
      (cos(roll)*sin(pitch)*sin(yaw)-sin(roll)*cos(yaw))*u(1)/m,
      zdot,
      -g+((cos(roll))*cos(pitch)*u(1)/m),
      rolldot,
      (yawdot*pitchdot*(Iy-Iz)/Ix)-(IR/Ix*pitchdot*omegaD)+(l/Ix*u(2)),
      pitchdot,(yawdot*rolldot*(Iz-Ix)/Iy)+(IR/Iy*rolldot*omegaD)+(l/Iy*u(3)),
      yawdot,
      pitchdot*rolldot*(Ix-Iy)/Iz+(l/Iz*u(4))];

sys =dxdt;
% end mdlDerivatives

%
%=============================================================================
% mdlUpdate
% Handle discrete state updates, sample time hits, and major time step
% requirements.
%=============================================================================
%
function sys=mdlUpdate(t,x,u)

sys = [];

% end mdlUpdate

%
%=============================================================================
% mdlOutputs
% Return the block outputs.
%=============================================================================
%
function sys=mdlOutputs(t,x,u)

x=x(1);                    
xdot=x(2);                  
y=x(3);                         
ydot=x(4);                    
z=x(5);                    
zdot=x(6);                   
roll=x(7);                   
rolldot=x(8);                
pitch=x(9);                 
pitchdot=x(10);              
yaw=x(11);                   
yawdot=x(12);

sys = [x xdot y ydot z zdot roll rolldot pitch pitchdot yaw yawdot];

% end mdlOutputs

%
%=============================================================================
% mdlGetTimeOfNextVarHit
% Return the time of the next hit for this block.  Note that the result is
% absolute time.  Note that this function is only used when you specify a
% variable discrete-time sample time [-2 0] in the sample time array in
% mdlInitializeSizes.
%=============================================================================
%
function sys=mdlGetTimeOfNextVarHit(t,x,u)

sampleTime = 1;    %  Example, set the next hit to be one second later.
sys = t + sampleTime;

% end mdlGetTimeOfNextVarHit

%
%=============================================================================
% mdlTerminate
% Perform any end of simulation tasks.
%=============================================================================
%
function sys=mdlTerminate(t,x,u)

sys = [];

% end mdlTerminate

And this is the m-file for my parameters:

m=2.5;
l=0.5;
Ix=0.17;
Iy=0.17;
Iz=0.3;
IR=1;
b=1;
d=1;
omegaD=1;
g=9.8;

xini=[0 0 0 0 0 0 0.1745 0 0.1745 0 0.1745 0];

I have programmed with s-function twice for pendulum model and Gnatry crane model but i have no problem with it. Could you please help me with my problem. TQ

Answer 1

[Kaustubha Govind]

In both mdlDerivatives and mdlOutputs, you have these lines:

   x = x(1);                    
   xdot = x(2);

The first line overwrites the "x" that you receive from the engine, so even if "x" were a vector, you are effectively overwriting it with a single element with:

   x = x(1);

Which is causing the statement "x(2)" to error out.

[Norazuwa]

Thank you so much Kaustubha!Your answer is very helpful ;) I was so careless.